Semiconductor and optoelectronic devices

[0001] This application is a continuation application of co-pending U.S. patent application Ser. No. 13/422,057, filed on Mar. 16, 2012, which is a continuation of U.S. patent application Ser. No. 12/904,103, filed on Oct. 13, 2010, now U.S. Pat. No. 8,163,581, the entire contents of the foregoing applications are incorporated by reference. Furthermore, priority is claimed to U.S. patent application Ser. No. 12/900,379, filed on Oct. 7, 2010, now U.S. Pat. No. 8,395,191 and U.S. patent application Ser. No. 13/273,712, filed on Oct. 14, 2011, now U.S. Pat. No. 8,273,610, the entire contents of the foregoing applications are incorporated by reference.

[0002] (A) Field of the Invention

[0003] This invention describes applications of monolithic 3D integration to various disciplines, including but not limited to, for example, light-emitting diodes, displays, image-sensors and solar cells.

[0004] (B) Discussion of Background Art

[0005] Semiconductor and optoelectronic devices often require thin monocrystalline (or single-crystal) films deposited on a certain wafer. To enable this deposition, many techniques, generally referred to as layer transfer technologies, have been developed. These include:

• • [0006] Ion-cut, variations of which are referred to as smart-cut, nano-cleave and smart-cleave: Further information on ion-cut technology is given in “Frontiers of silicon-on-insulator,” J. Appl. Phys. 93, 4955-4978 (2003) by G. K. Celler and S. Cristolovean (“Celler”) and also in “Mechanically induced Si layer transfer in hydrogen-implanted Si wafers,” Appl. Phys. Lett., vol. 76, pp. 2370-2372, 2000 by K. Henttinen, I. Suni, and S. S. Lau (“Hentinnen”).
  • [0007] Porous silicon approaches such as ELTRAN: These are described in “Eltran, Novel SOI Wafer Technology”, JSAP International, Number 4, July 2001 by T. Yonehara and K. Sakaguchi (“Yonehara”).
  • [0008] Lift-off with a temporary substrate, also referred to as epitaxial lift-off: This is described in “Epitaxial lift-off and its applications”, 1993 Semicond. Sci. Technol. 8 1124 by P. Demeester, et al (“Demeester”).
  • [0009] Bonding a substrate with single crystal layers followed by Polishing, Time-controlled etch-back or Etch-stop layer controlled etch-back to thin the bonded substrate: These are described in U.S. Pat. No. 6,806,171 by A. Ulyashin and A. Usenko (“Ulyashin”) and “Enabling SOI-Based Assembly Technology for Three-Dimensional (3D) Integrated Circuits (ICs),” IEDM Tech. Digest, p. 363 (2005) by A. W. Topol, D. C. La Tulipe, L. Shi, S. M. Alam, D. J. Frank, S. E. Steen, J. Vichiconti, D. Posillico, M. Cobb, S. Medd, J. Patel, S. Goma, D. DiMilia, M. T. Robson, E. Duch, M. Farinelli, C. Wang, R. A. Conti, D. M. Canaperi, L. Deligianni, A. Kumar, K. T. Kwietniak, C. D'Emic, J. Ott, A. M. Young, K. W. Guarini, and M. Ieong (“Topol”).
  • [0010] Bonding a wafer with a Gallium Nitride film epitaxially grown on a sapphire substrate followed by laser lift-off for removing the transparent sapphire substrate: This method may be suitable for deposition of Gallium Nitride thin films, and is described in U.S. Pat. No. 6,071,795 by Nathan W. Cheung, Timothy D. Sands and William S. Wong (“Cheung”).
  • [0011] Rubber stamp layer transfer: This is described in “Solar cells sliced and diced”, 19 May 2010, Nature News.
    With novel applications of these methods and recognition of their individual strengths and weaknesses, one can significantly enhance today's light-emitting diode (LED), display, image-sensor and solar cell technologies.
    Background on LEDs

[0012] Light emitting diodes (LEDs) are used in many applications, including automotive lighting, incandescent bulb replacements, and as backlights for displays. Red LEDs are typically made on Gallium Arsenide (GaAs) substrates, and include quantum wells constructed of various materials such as AlInGaP and GaInP. Blue and green LEDs are typically made on Sapphire or Silicon Carbide (SiC) or bulk Gallium Nitride (GaN) substrates, and include quantum wells constructed of various materials such as GaN and InGaN.

[0013] A white LED for lighting and display applications can be constructed by either using a blue LED coated with phosphor (called phosphor-coated LED or pcLED) or by combining light from red, blue, and green LEDs (called RGB LED). RGB LEDs are typically constructed by placing red, blue, and green LEDs side-by-side. While RGB LEDs are more energy-efficient than pcLEDs, they are less efficient in mixing red, blue and green colors to form white light. They also are much more costly than pcLEDs. To tackle issues with RGB LEDs, several proposals have been made.

[0014] One RGB LED proposal from Hong Kong University is described in “Design of vertically stacked polychromatic light emitting diodes”, Optics Express, June 2009 by K. Hui, X. Wang, et al (“Hui”). It involves stacking red, blue, and green LEDs on top of each other after individually packaging each of these LEDs. While this solves light mixing problems, this RGB-LED is still much more costly than a pcLED solution since three LEDs for red, blue, and green color need to be packaged. A pcLED, on the other hand, requires just one LED to be packaged and coated with phosphor.

[0015] Another RGB LED proposal from Nichia Corporation is described in “Phosphor Free High-Luminous-Efficiency White Light-Emitting Diodes Composed of InGaN Multi-Quantum Well”, Japanese Journal of Applied Physics, 2002 by M. Yamada, Y. Narukawa, et al. (“Yamada”). It involves constructing and stacking red, blue and green LEDs of GaN-based materials on a sapphire or SiC substrate. However, red LEDs are not efficient when constructed with GaN-based material systems, and that hampers usefulness of this implementation. It is not possible to deposit defect-free AlInGaP/InGaP for red LEDs on the same substrate as GaN based blue and green LEDs, due to a mismatch in thermal expansion co-efficient between the various material systems.

[0016] Yet another RGB-LED proposal is described in “Cascade Single chip phosphor-free while light emitting diodes”, Applied Physics Letters, 2008 by X. Guo, G. Shen, et al. (“Guo”). It involves bonding GaAs based red LEDs with GaN based blue-green LEDs to produce white light. Unfortunately, this bonding process requires 600° C. temperatures, causing issues with mismatch of thermal expansion co-efficients and cracking. Another publication on this topic is “A trichromatic phosphor-free white light-emitting diode by using adhesive bonding scheme”, Proc. SPIE, Vol. 7635, 2009 by D. Chuai, X. Guo, et al. (“Chuai”). It involves bonding red LEDs with green-blue LED stacks. Bonding is done at the die level after dicing, which is more costly than a wafer-based approach.

[0017] U.S. patent application Ser. No. 12/130,824 describes various stacked RGB LED devices. It also briefly mentions a method for construction of a stacked LED where all layers of the stacked LED are transferred using lift-off with a temporary carrier and Indium Tin Oxide (ITO) to semiconductor bonding. This method has several issues for constructing a RGB LED stack. First, it is difficult to manufacture a lift-off with a temporary carrier of red LEDs for producing a RGB LED stack, especially for substrates larger than 2 inch. This is because red LEDs are typically constructed on non-transparent GaAs substrates, and lift-off with a temporary carrier is done by using an epitaxial lift-off process. Here, the thin film to be transferred typically sits atop a “release-layer” (eg. AlAs), this release layer is removed by etch procedures after the thin film is attached to a temporary substrate. Scaling this process to 4 inch wafers and bigger is difficult. Second, it is very difficult to perform the bonding of ITO to semiconductor materials of a LED layer at reasonable temperatures, as described in the patent application Ser. No. 12/130,824.

[0018] It is therefore clear that a better method for constructing RGB LEDs will be helpful. Since RGB LEDs are significantly more efficient than pcLEDs, they can be used as replacements of today's phosphor-based LEDs for many applications, provided a cheap and effective method of constructing RGB LEDs can be invented.

Background on Image-Sensors:

[0019] Image sensors are used in applications such as cameras. Red, blue, and green components of the incident light are sensed and stored in digital format. CMOS image sensors typically contain a photodetector and sensing circuitry. Almost all image sensors today have both the photodetector and sensing circuitry on the same chip. Since the area consumed by the sensing circuits is high, the photodetector cannot see the entire incident light, and image capture is not as efficient.

[0020] To tackle this problem, several researchers have proposed building the photodetectors and the sensing circuitry on separate chips and stacking them on top of each other. A publication that describes this method is “Megapixel CMOS image sensor fabricated in three-dimensional integrated circuit technology”, Intl. Solid State Circuits Conference 2005 by Suntharalingam, V., Berger, R., et al. (“Suntharalingam”). These proposals use through-silicon via (TSV) technology where alignment is done in conjunction with bonding. However, pixel size is reaching the 1 μm range, and successfully processing TSVs in the 1 μm range or below is very difficult. This is due to alignment issues while bonding. For example, the International Technology Roadmap for Semiconductors (ITRS) suggests that the 2-4 um TSV pitch will be the industry standard until 2012. A 2-4 μm pitch TSV will be too big for a sub-1 μm pixel. Therefore, novel techniques of stacking photodetectors and sensing circuitry are required.

[0021] A possible solution to this problem is given in “Setting up 3D Sequential Integration for Back-Illuminated CMOS Image Sensors with Highly Miniaturized Pixels with Low Temperature Fully-depleted SOI Transistors,” IEDM, p. 1-4 (2008) by P. Coudrain et al. (“Coudrain”). In the publication, transistors are monolithically integrated on top of photodetectors. Unfortunately, transistor process temperatures reach 600° C. or more. This is not ideal for transistors (that require a higher thermal budget) and photodetectors (that may prefer a lower thermal budget).

Background on Displays:

[0022] Liquid Crystal Displays (LCDs) can be classified into two types based on manufacturing technology utilized: (1) Large-size displays that are made of amorphous/polycrystalline silicon thin-film-transistors (TFTs), and (2) Microdisplays that utilize single-crystal silicon transistors. Microdisplays are typically used where very high resolution is needed, such as camera/camcorder view-finders, projectors and wearable computers.

[0023] Microdisplays are made in semiconductor fabs with 200 mm or 300 mm wafers. They are typically constructed with LCOS (Liquid-Crystal-on-Silicon) Technology and are reflective in nature. An exception to this trend of reflective microdisplays is technology from Kopin Corporation (U.S. Pat. No. 5,317,236, filed December 1991). This company utilizes transmittive displays with a lift-off layer transfer scheme. Transmittive displays may be generally preferred for various applications.

[0024] While lift-off layer transfer schemes are viable for transmittive displays, they are frequently not used for semiconductor manufacturing due to yield issues. Therefore, other layer transfer schemes will be helpful. However, it is not easy to utilize other layer transfer schemes for making transistors in microdisplays. For example, application of “smart-cut” layer transfer to attach monocrystalline silicon transistors to glass is described in “Integration of Single Crystal Si TFTs and Circuits on a Large Glass Substrate”, IEDM 2009 by Y. Takafuji, Y. Fukushima, K. Tomiyasu, et al. (“Takafuji”). Unfortunately, hydrogen is implanted through the gate oxide of transferred transistors in the process, and this degrades performance. Process temperatures are as high as 600° C. in this paper, and this requires costly glass substrates. Several challenges therefore need to be overcome for efficient layer transfer, and require innovation.

Background on Solar Cells:

[0025] Solar cells can be constructed of several materials such as, for example, silicon and compound semiconductors. The highest efficiency solar cells are typically multi-junction solar cells that are constructed of compound semiconductor materials. These multi-junction solar cells are typically constructed on a germanium substrate, and semiconductors with various band-gaps are epitaxially grown atop this substrate to capture different portions of the solar spectrum.

[0026] There are a few issues with standard multi-junction solar cells. Since multiple junctions are grown epitaxially above a single substrate (such as Germanium) at high temperature, materials used for different junctions are restricted to those that have lattice constants and thermal expansion co-efficients close to those of the substrate. Therefore, the choice of materials used to build junctions for multi-junction solar cells is limited. As a result, mostmulti-junction solar cells commercially available today cannot capture the full solar spectrum. Efficiency of the solar cell can be improved if a large band of the solar spectrum is captured. Furthermore, multi-junction solar cells today suffer from high cost of the substrate above which multiple junctions are epitaxially grown. Methods to build multi-junction solar cells that tackle both these issues will be helpful.

[0027] A method of making multi-junction solar cells by mechanically bonding two solar cells, one with a Germanium junction and another with a compound semiconductor junction is described in “Towards highly efficient 4-terminal mechanical photovoltaic stacks”, III-Vs Review, Volume 19, Issue 7, September-October 2006 by Giovanni Flamand, Jef Poortmans (“Flamand”). In this work, the authors make the compound semiconductor junctions on a Germanium substrate epitaxially. They then etch away the entire Germanium substrate after bonding to the other substrate with the Germanium junction. The process uses two Germanium substrates, and is therefore expensive.

[0028] Techniques to create multi-junction solar cells with layer transfer have been described in “Wafer bonding and layer transfer processes for 4-junction high efficiency solar cells,” Photovoltaic Specialists Conference, 2002. Conference Record of the Twenty-Ninth IEEE, vol., no., pp. 1039-1042, 19-24 May 2002 by Zahler, J. M.; Fontcuberta i Morral, A.; Chang-Geun Ahn; Atwater, H. A.; Wanlass, M. W.; Chu, C. and Iles, P. A. An anneal is used for ion-cut purposes, and this anneal is typically done at temperatures higher than 350-400° C. (if high bond strength is desired). When that happens, cracking and defects can be produced due to mismatch of co-efficients of thermal expansion between various layers in the stack. Furthermore, semiconductor layers are bonded together, and the quality of this bond not as good as oxide-to-oxide bonding, especially for lower process temperatures.

[0029] Techniques to utilize layer transfer schemes such as ion-cut to form novel light emitting diodes (LEDs), CMOS image sensors, displays, microdisplays and solar cells are discussed.

[0030] In one aspect, an integrated device, the integrated device including a first crystalline layer covered by an oxide layer, a second crystalline layer overlying the oxide layer, wherein the first and second crystalline layers are image sensor layers, and the device includes a third crystalline layer, wherein the third crystalline layer includes single crystal transistors.

[0031] In another aspect, an integrated image sensor, the integrated image sensor including a first mono-crystal layer including a plurality of image sensor pixels and alignment marks, and an oxide layer overlaying and on top of the first mono-crystal layer, and a second mono-crystal layer including a plurality of second image sensor pixels aligned to the alignment marks, and the second mono-crystal layer overlaying the oxide layer, and a third mono-crystal layer, wherein the third mono-crystal layer includes a plurality of single crystal transistors aligned to the alignment marks.

[0032] In another aspect, an integrated device, the integrated device including a first mono-crystal layer including a plurality of single crystal transistors and alignment marks, and an overlaying oxide on top of the first mono-crystal layer, and a second mono-crystal layer overlaying the oxide, and wherein the second mono-crystal layer includes a plurality of image sensor pixels aligned to the alignment marks.

[0065] Embodiments of the present invention are now described with reference to FIGS. 1-30, it being appreciated that the figures illustrate the subject matter not to scale or to measure.

NuLED Technology:

[0066] FIG. 1A illustrates a cross-section of prior art red LEDs. Red LEDs are typically constructed on a Gallium Arsenide substrate 100. Alternatively, Gallium Phosphide or some other material can be used for the substrate. Since Gallium Arsenide 100 is opaque, a Bragg Reflector 101 is added to ensure light moves in the upward direction. Red light is produced by a p-n junction with multiple quantum wells (MQW). A p-type confinement layer 104, a n-type confinement layer 102 and a multiple quantum well 103 form this part of the device. A current spreading region 105 ensures current flows throughout the whole device and not just close to the contacts. Indium Tin Oxide (ITO) could be used for the current spreading region 105. A top contact 106 and a bottom contact 107 are used for making connections to the LED. It will be obvious to one skilled in the art based on the present disclosure that many configurations and material combinations for making red LEDs are possible. This invention is not limited to one particular configuration or set of materials.

[0067] FIG. 1B also illustrates green and blue LED cross-sections. These are typically constructed on a sapphire, SiC or bulk-GaN substrate, indicated by 108. Light is produced by a p-n junction with multiple quantum wells made of In_xGa_1-xN/GaN. A p-type confinement layer 111, a n-type confinement layer 109 and a multiple quantum well 110 form this part of the device. The value of subscript x in In_xGa_1-xN determines whether blue light or green light is produced. For example, blue light typically corresponds to x ranging from 10% to 20% while green light typically corresponds to x ranging from 20% to 30%. A current spreader 112 is typically used as well. ITO could be a material used for the current spreader 112. An alternative material for current spreading could be ZnO. A top contact 113 and a bottom contact 114 are used for making connections to the LED. It will be obvious to one skilled in the art based on the present disclosure that many configurations and material combinations for making blue and green LEDs are possible. This invention is not limited to one particular configuration or set of materials.

[0068] White LEDs for various applications can be constructed in two ways. Method 1 is described in FIG. 2 which shows Red LED 201, blue LED 202, and green LED 203 that are constructed separately and placed side-by-side. Red light 204, blue light 205 and green light 206 are mixed to form white light 207. While these “RGB LEDs” are efficient, they suffer from cost issues and have problems related to light mixing. Method 2 is described in FIG. 3 which shows a blue LED 301 constructed and coated with a phosphor layer 302. The yellow phosphor layer converts blue light into white light 303. These “Phosphor-based LEDs” or “pcLEDs” are cheaper than RGB LEDs but are typically not as efficient.

[0069] FIG. 4A-S illustrate an embodiment of this invention where Red, Blue, and Green LEDs are stacked on top of each other with smart layer transfer techniques. A smart layer transfer may be defined as one or more of the following processes:

• • [0070] Ion-cut, variations of which are referred to as smart-cut, nano-cleave and smart-cleave: Further information on ion-cut technology is given in “Frontiers of silicon-on-insulator,” J. Appl. Phys. 93, 4955-4978 (2003) by G. K. Celler and S. Cristolovean (“Celler”) and also in “Mechanically induced Si layer transfer in hydrogen-implanted Si wafers,” Appl. Phys. Lett., vol. 76, pp. 2370-2372, 2000 by K. Henttinen, I. Suni, and S. S. Lau (“Hentinnen”).
  • [0071] Porous silicon approaches such as ELTRAN: These are described in “Eltran, Novel SOI Wafer Technology,” JSAP International, Number 4, July 2001 by T. Yonehara and K. Sakaguchi (“Yonehara”).
  • [0072] Bonding a substrate with single crystal layers followed by Polishing, Time-controlled etch-back or Etch-stop layer controlled etch-back to thin the bonded substrate: These are described in U.S. Pat. No. 6,806,171 by A. Ulyashin and A. Usenko (“Ulyashin”) and “Enabling SOI-Based Assembly Technology for Three-Dimensional (3D) Integrated Circuits (ICs),” IEDM Tech. Digest, p. 363 (2005) by A. W. Topol, D. C. La Tulipe, L. Shi, S. M. Alam, D. J. Frank, S. E. Steen, J. Vichiconti, D. Posillico, M. Cobb, S. Medd, J. Patel, S. Goma, D. DiMilia, M. T. Robson, E. Duch, M. Farinelli, C. Wang, R. A. Conti, D. M. Canaperi, L. Deligianni, A. Kumar, K. T. Kwietniak, C. D'Emic, J. Ott, A. M. Young, K. W. Guarini, and M. Ieong (“Topol”).
  • [0073] Bonding a wafer with a Gallium Nitride film epitaxially grown on a sapphire substrate followed by laser lift-off for removing the transparent sapphire substrate: This method may be suitable for deposition of Gallium Nitride thin films, and is described in U.S. Pat. No. 6,071,795 by Nathan W. Cheung, Timothy D. Sands and William S. Wong (“Cheung”).
  • [0074] Rubber stamp layer transfer: This is described in “Solar cells sliced and diced,” 19 May 2010, Nature News.

[0075] This process of constructing RGB LEDs could include several steps that occur in a sequence from Step (A) to Step (S). Many of them share common characteristics, features, modes of operation, etc. When the same reference numbers are used in different drawing figures, they are used to indicate analogous, similar or identical structures to enhance the understanding of the present invention by clarifying the relationships between the structures and embodiments presented in the various diagrams—particularly in relating analogous, similar or identical functionality to different physical structures.

[0076] Step (A) is illustrated in FIG. 4A. A red LED wafer 436 is constructed on a GaAs substrate 402 and includes a N-type confinement layer 404, a multiple quantum well (MQW) 406, a P-type confinement layer 408, an optional reflector 409 and an ITO current spreader 410. Examples of materials used to construct these layers, include, but are not limited to, doped AlInGaP for the N-type confinement layer 404 and P-type confinement layer 408, the multiple quantum well layer 406 could be of AlInGaP and GaInP and the optional reflector 409 could be a distributed Bragg Reflector. A double heterostructure configuration or single quantum well configuration could be used instead of a multiple quantum well configuration. Various other material types and configurations could be used for constructing the red LEDs for this process. Yet another wafer is constructed with a green LED. The green LED wafer 438 is constructed on a sapphire or SiC or bulk-GaN substrate 412 and includes a N-type confinement layer 414, a multiple quantum well (MQW) 416, a buffer layer 418, a P-type confinement layer 420, an optional reflector 421 and an ITO current spreader 422. Yet another wafer is constructed with a blue LED. The blue LED wafer 440 is constructed on a sapphire or SiC or bulk-GaN substrate 424 and includes a N-type confinement layer 426, a multiple quantum well (MQW) 428, a buffer layer 430, a P-type confinement layer 432, an optional reflector 433 and an ITO current spreader 434. Examples of materials used to construct these blue and green LED layers, include, but are not limited to, doped GaN for the N-type and P-type confinement layers 414, 420, 426 and 432, AlGaN for the buffer layers 430 and 418 and InGaN/GaN for the multiple quantum wells 416 and 428. The optional reflectors 421 and 433 could be distributed Bragg Reflectors or some other type of reflectors. Various other material types and configurations could be used for constructing blue and green LEDs for this process.
Step (B) is illustrated in FIG. 4B. The blue LED wafer 440 from FIG. 4A is used for this step. Various elements in FIG. 4B such as, for example, 424, 426, 428, 430, 432, 433, and 434 have been previously described. Hydrogen is implanted into the wafer at a certain depth indicated by dotted lines 442. Alternatively, helium could be used for this step.
Step (C) is illustrated in FIG. 4C. A glass substrate 446 is taken and an ITO layer 444 is deposited atop it.
Step (D) is illustrated in FIG. 4D. The wafer shown in FIG. 4B is flipped and bonded atop the wafer shown in FIG. 4C using ITO-ITO bonding. Various elements in FIG. 4D such as 424, 426, 428, 430, 432, 433, 434, 442, 446, and 444 have been previously described. The ITO layer 444 is essentially bonded to the ITO layer 434 using an oxide-to-oxide bonding process.
Step (E) is illustrated in FIG. 4E. Various elements in FIG. 4E such as 424, 426, 428, 430, 432, 433, 434, 442, 446, and 444 have been previously described. An ion-cut process is conducted to cleave the structure shown in FIG. 4D at the hydrogen implant plane 442. This ion-cut process may use a mechanical cleave. An anneal process could be utilized for the cleave as well. After the cleave, a chemical mechanical polish (CMP) process is conducted to planarize the surface. The N-type confinement layer present after this cleave and CMP process is indicated as 427.
Step (F) is illustrated in FIG. 4F. Various elements in FIG. 4F such as 446, 444, 434, 433, 432, 430, 428, and 427 have been previously described. An ITO layer 448 is deposited atop the N-type confinement layer 427.
Step (G) is illustrated in FIG. 4G. The green LED wafer 438 shown in Step (A) is used for this step. Various elements in FIG. 4G such as 412, 414, 416, 418, 420, 421, and 422 have been described previously. Hydrogen is implanted into the wafer at a certain depth indicated by dotted lines 450. Alternatively, helium could be used for this step.
Step (H) is illustrated in FIG. 4H. The structure shown in FIG. 4G is flipped and bonded atop the structure shown in FIG. 4F using ITO-ITO bonding. Various elements in FIG. 4H such as 446, 444, 434, 433, 432, 430, 428, 427, 448, 412, 414, 416, 418, 420, 421, 422, and 450 have been described previously.
Step (I) is illustrated in FIG. 4I. The structure shown in FIG. 4H is cleaved at the hydrogen plane indicated by 450. This cleave process may be preferably done with a mechanical force. Alternatively, an anneal could be used. A CMP process is conducted to planarize the surface. Various elements in FIG. 4I such as 446, 444, 434, 433, 432, 430, 428, 427, 448, 416, 418, 420, 421, and 422 have been described previously. The N-type confinement layer present after this cleave and CMP process is indicated as 415.
Step (J) is illustrated in FIG. 4J. An ITO layer 452 is deposited atop the structure shown in FIG. 4I. Various elements in FIG. 4J such as 446, 444, 434, 433, 432, 430, 428, 427, 448, 416, 418, 420, 421, 415, and 422 have been described previously.
Step (K) is illustrated in FIG. 4K. The red LED wafer 436 shown in Step (A) is used for this step. Various elements in FIG. 4K such as 402, 404, 406, 408, 409, and 410 have been described previously. Hydrogen is implanted into the wafer at a certain depth indicated by dotted lines 454. Alternatively, helium could be used for this step.
Step (L) is illustrated in FIG. 4L. The structure shown in FIG. 4K is flipped and bonded atop the structure shown in FIG. 4J using ITO-ITO bonding. Various elements in FIG. 4L such as 446, 444, 434, 433, 432, 430, 428, 427, 448, 416, 418, 420, 421, 415, 422, 452, 402, 404, 406, 408, 409, 410, and 454 have been described previously.
Step (M) is illustrated in FIG. 4M. The structure shown in FIG. 4L is cleaved at the hydrogen plane 454. A mechanical force could be used for this cleave. Alternatively, an anneal could be used. A CMP process is then conducted to planarize the surface. The N-type confinement layer present after this process is indicated as 405. Various elements in FIG. 4M such as 446, 444, 434, 433, 432, 430, 428, 427, 448, 416, 418, 420, 421, 415, 422, 452, 406, 408, 409, and 410 have been described previously.
Step (N) is illustrated in FIG. 4N. An ITO layer 456 is deposited atop the structure shown in FIG. 4M. Various elements in FIG. 4M such as 446, 444, 434, 433, 432, 430, 428, 427, 448, 416, 418, 420, 421, 415, 422, 452, 406, 408, 409, 410, and 405 have been described previously.
Step (O) is illustrated in FIG. 4O. A reflecting material layer 458, constructed for example with Aluminum or Silver, is deposited atop the structure shown in FIG. 4N. Various elements in FIG. 4O such as 446, 444, 434, 433, 432, 430, 428, 427, 448, 416, 418, 420, 421, 415, 422, 452, 406, 408, 409, 410, 456, and 405 have been described previously.
Step (P) is illustrated in FIG. 4P. The process of making contacts to various layers and packaging begins with this step. A contact and bonding process similar to the one used in “High-power AlGaInN flip-chip light-emitting diodes,” Applied Physics Letters, vol. 78, no. 22, pp. 3379-3381, May 2001, by Wierer, J. J.; Steigerwald, D. A.; Krames, M. R.; OShea, J. J.; Ludowise, M. J.; Christenson, G.; Shen, Y.-C.; Lowery, C.; Martin, P. S.; Subramanya, S.; Gotz, W.; Gardner, N. F.; Kern, R. S.; Stockman, S. A. is used. Vias 460 are etched to different layers of the LED stack. Various elements in FIG. 4P such as 446, 444, 434, 433, 432, 430, 428, 427, 448, 416, 418, 420, 421, 415, 422, 452, 406, 408, 409, 410, 456, 405, and 458 have been described previously. After the via holes 460 are etched, they may optionally be filled with an oxide layer and polished with CMP. This fill with oxide may be optional, and the preferred process may be to leave the via holes as such without fill. Note that the term contact holes could be used instead of the term via holes. Similarly, the term contacts could be used instead of the term vias.
Step (Q) is illustrated in FIG. 4Q. Aluminum is deposited to fill via holes 460 from FIG. 4P. Following this deposition, a lithography and etch process is utilized to define the aluminum metal to form vias 462. The vias 462 are smaller in diameter than the via holes 460 shown in FIG. 4P. Various elements in FIG. 4Q such as 446, 444, 434, 433, 432, 430, 428, 427, 448, 416, 418, 420, 421, 415, 422, 452, 406, 408, 409, 410, 456, 405, 460, and 458 have been described previously.
Step (R) is illustrated in FIG. 4R. A nickel layer 464 and a solder layer 466 are formed using standard procedures. Various elements in FIG. 4R such as 446, 444, 434, 433, 432, 430, 428, 427, 448, 416, 418, 420, 421, 415, 422, 452, 406, 408, 409, 410, 456, 405, 460, 462, and 458 have been described previously.
Step (S) is illustrated in FIG. 4S. The solder layer 466 is then bonded to pads on a silicon sub-mount 468. Various elements in FIG. 4S such as 446, 444, 434, 433, 432, 430, 428, 427, 448, 416, 418, 420, 421, 415, 422, 452, 406, 408, 409, 410, 456, 405, 460, 462, 458, 464, and 466 have been described previously. The configuration of optional reflectors 433, 421, and 409 determines light output coming from the LED. A preferred embodiment of this invention may not have a reflector 433, and may have the reflector 421 (reflecting only the blue light produced by multiple quantum well 428) and the reflector 409 (reflecting only the green light produced by multiple quantum well 416). In the process described in FIG. 4A-FIG. 4S, the original substrates in FIG. 4A, namely 402, 412 and 424, can be reused after ion-cut. This reuse may make the process more cost-effective.

[0077] FIGS. 5A-Q describe an embodiment of this invention, where RGB LEDs are stacked with ion-cut technology, wire bond packaging and conductive oxide bonding. Essentially, smart-layer transfer is utilized to construct this embodiment of the invention. This process of constructing RGB LEDs could include several steps that occur in a sequence from Step (A) to Step (Q). Many of the steps share common characteristics, features, modes of operation, etc. When the same reference numbers are used in different drawing figures, they are used to indicate analogous, similar or identical structures to enhance the understanding of the present invention by clarifying the relationships between the structures and embodiments presented in the various diagrams—particularly in relating analogous, similar or identical functionality to different physical structures.

[0078] Step (A): This is illustrated using FIG. 5A. A red LED wafer 536 is constructed on a GaAs substrate 502 and includes a N-type confinement layer 504, a multiple quantum well (MQW) 506, a P-type confinement layer 508, an optional reflector 509 and an ITO current spreader 510. Examples of materials used to construct these layers, include, but are not limited to, doped AlInGaP for the N-type confinement layer 504 and P-type confinement layer 508, the multiple quantum well layer 506 could be of AlInGaP and GaInP and the optional reflector 509 could be a distributed Bragg Reflector. A double heterostructure configuration or single quantum well configuration could be used instead of a multiple quantum well configuration. Various other material types and configurations could be used for constructing the red LEDs for this process. Yet another wafer is constructed with a green LED. The green LED wafer 538 is constructed on a sapphire or SiC or bulk-GaN substrate 512 and includes a N-type confinement layer 514, a multiple quantum well (MQW) 516, a buffer layer 518, a P-type confinement layer 520, an optional reflector 521 and an ITO current spreader 522. Yet another wafer is constructed with a blue LED. The blue LED wafer 540 is constructed on a sapphire or SiC or bulk-GaN substrate 524 and includes a N-type confinement layer 526, a multiple quantum well (MQW) 528, a buffer layer 530, a P-type confinement layer 532, an optional reflector 533 and an ITO current spreader 534. Examples of materials used to construct these blue and green LED layers, include, but are not limited to, doped GaN (for the N-type and P-type confinement layers 514, 520, 526, and 532), AlGaN (for the buffer layers 530 and 518), and InGaN/GaN (for the multiple quantum wells 516 and 528). The optional reflectors 521 and 533 could be distributed Bragg Reflectors or some other type of reflectors. Various other material types and configurations could be used for constructing blue and green LEDs for this process.
Step (B) is illustrated in FIG. 5B. The red LED wafer 536 from FIG. 5A is used for this step. Various elements in FIG. 5B such as 502, 504, 506, 508, 509, and 510 have been previously described. Hydrogen is implanted into the wafer at a certain depth indicated by dotted lines 542. Alternatively, helium could be used for this step.
Step (C) is illustrated in FIG. 5C. A silicon substrate 546 is taken and an ITO layer 544 is deposited atop it.
Step (D) is illustrated in FIG. 5D. The wafer shown in FIG. 5B is flipped and bonded atop the wafer shown in FIG. 5C using ITO-ITO bonding. Various elements in FIG. 5D such as 502, 504, 506, 508, 509, 510, 542, 544, and 546 have been previously described. The ITO layer 544 is essentially bonded to the ITO layer 510 using an oxide-to-oxide bonding process.
Step (E) is illustrated in FIG. 5E. Various elements in FIG. 5E such as 506, 508, 509, 510, 544 and 546 have been previously described. An ion-cut process is conducted to cleave the structure shown in FIG. 5D at the hydrogen implant plane 542. This ion-cut process could preferably use a mechanical cleave. An anneal process could be utilized for the cleave as well. After the cleave, a chemical mechanical polish (CMP) process is conducted to planarize the surface. The N-type confinement layer present after this cleave and CMP process is indicated as 505.
Step (F) is illustrated in FIG. 5F. Various elements in FIG. 5F such as 505, 506, 508, 509, 510, 544, and 546 have been previously described. An ITO layer 548 is deposited atop the N-type confinement layer 505.
Step (G) is illustrated in FIG. 5G. The green LED wafer 538 shown in Step (A) is used for this step. Various elements in FIG. 5G such as 512, 514, 516, 518, 520, 521, and 522 have been described previously. Hydrogen is implanted into the wafer at a certain depth indicated by dotted lines 550. Alternatively, helium could be used for this step.
Step (H) is illustrated in FIG. 5H. The structure shown in FIG. 5G is flipped and bonded atop the structure shown in FIG. 5F using ITO-ITO bonding. Various elements in FIG. 5H such as 505, 506, 508, 509, 510, 544, 546, 548, 512, 514, 516, 518, 520, 521, 550, and 522 have been described previously.
Step (I) is illustrated in FIG. 5I. The structure shown in FIG. 5H is cleaved at the hydrogen plane indicated by 550. This cleave process may be preferably done with a mechanical force. Alternatively, an anneal could be used. A CMP process is conducted to planarize the surface. Various elements in FIG. 5I such as 505, 506, 508, 509, 510, 544, 546, 548, 516, 518, 520, 521, and 522 have been described previously. The N-type confinement layer present after this cleave and CMP process is indicated as 515.
Step (J) is illustrated using FIG. 5J. An ITO layer 552 is deposited atop the structure shown in FIG. 5I. Various elements in FIG. 5J such as 505, 506, 508, 509, 510, 544, 546, 548, 516, 518, 520, 521, 515, and 522 have been described previously.
Step (K) is illustrated using FIG. 5K. The blue LED wafer 540 from FIG. 5A is used for this step. Various elements in FIG. 5K such as 524, 526, 528, 530, 532, 533, and 534 have been previously described. Hydrogen is implanted into the wafer at a certain depth indicated by dotted lines 554. Alternatively, helium could be used for this step.
Step (L) is illustrated in FIG. 5L. The structure shown in FIG. 5K is flipped and bonded atop the structure shown in FIG. 5J using ITO-ITO bonding. Various elements in FIG. 4L such as 505, 506, 508, 509, 510, 544, 546, 548, 516, 518, 520, 521, 515, 522, 552, 524, 526, 528, 530, 532, 533, 554, and 534 have been described previously.
Step (M) is illustrated in FIG. 5M. The structure shown in FIG. 5L is cleaved at the hydrogen plane 554. A mechanical force could be used for this cleave. Alternatively, an anneal could be used. A CMP process is then conducted to planarize the surface. The N-type confinement layer present after this process is indicated as 527. Various elements in FIG. 5M such as 505, 506, 508, 509, 510, 544, 546, 548, 516, 518, 520, 521, 515, 522, 552, 528, 530, 532, 533, and 534 have been described previously.
Step (N) is illustrated in FIG. 5N. An ITO layer 556 is deposited atop the structure shown in FIG. 5M. Various elements in FIG. 5N such as 505, 506, 508, 509, 510, 544, 546, 548, 516, 518, 520, 521, 515, 522, 552, 528, 530, 532, 533, and 534 have been described previously.
Step (O) is illustrated in FIG. 5O. The process of making contacts to various layers and packaging begins with this step. Various elements in FIG. 5O such as 505, 506, 508, 509, 510, 544, 546, 548, 516, 518, 520, 521, 515, 522, 552, 528, 530, 532, 533, 556, and 534 have been described previously. Via holes 560 are etched to different layers of the LED stack. After the via holes 560 are etched, they may optionally be filled with an oxide layer and polished with CMP. This fill with oxide may be optional, and the preferred process may be to leave the via holes as such without fill.
Step (P) is illustrated in FIG. 5P. Aluminum is deposited to fill via holes 560 from FIG. 5O. Following this deposition, a lithography and etch process is utilized to define the aluminum metal to form via holes 562. Various elements in FIG. 5P such as 505, 506, 508, 509, 510, 544, 546, 548, 516, 518, 520, 521, 515, 522, 552, 528, 530, 532, 533, 556, 560, and 534 have been described previously.
Step (Q) is illustrated in FIG. 5Q. Bond pads 564 are constructed and wire bonds are attached to these bond pads following this step. Various elements in FIG. 5Q such as 505, 506, 508, 509, 510, 544, 546, 548, 516, 518, 520, 521, 515, 522, 552, 528, 530, 532, 533, 556, 560, 562, and 534 have been described previously. The configuration of optional reflectors 533, 521 and 509 determines light output coming from the LED. The preferred embodiment of this invention is to have reflector 533 reflect only blue light produced by multiple quantum well 528, to have the reflector 521 reflecting only green light produced by multiple quantum well 516 and to have the reflector 509 reflect light produced by multiple quantum well 506. In the process described in FIG. 5A-FIG. 5Q, the original substrates in FIG. 5A, namely 502, 512 and 524, can be re-used after ion-cut. This may make the process more cost-effective.

[0079] FIGS. 6A-L show an alternative embodiment of this invention, where stacked RGB LEDs are formed with ion-cut technology, flip-chip packaging and aligned bonding. A smart layer transfer process, ion-cut, is therefore utilized. This process of constructing RGB LEDs could include several steps that occur in a sequence from Step (A) to Step (K). Many of the steps share common characteristics, features, modes of operation, etc. When identical reference numbers are used in different drawing figures, they are used to indicate analogous, similar or identical structures to enhance the understanding of the present invention by clarifying the relationships between the structures and embodiments presented in the various diagrams—particularly in relating analogous, similar or identical functionality to different physical structures.

[0080] Step (A) is illustrated in FIG. 6A. A red LED wafer 636 is constructed on a GaAs substrate 602 and includes a N-type confinement layer 604, a multiple quantum well (MQW) 606, a P-type confinement layer 608, an optional reflector 609 and an ITO current spreader 610. Above the ITO current spreader 610, a layer of silicon oxide 692 is deposited, patterned, etched and filled with a metal 690 (e.g., tungsten) which is then CMPed. Examples of materials used to construct these layers, include, but are not limited to, doped AlInGaP for the N-type confinement layer 604 and P-type confinement layer 608, the multiple quantum well layer 606 could be of AlInGaP and GaInP and the optional reflector 609 could be a distributed Bragg Reflector. A double heterostructure configuration or single quantum well configuration could be used instead of a multiple quantum well configuration. Various other material types and configurations could be used for constructing the red LEDs for this process. Yet another wafer is constructed with a green LED. The green LED wafer 638 is constructed on a sapphire or SiC or bulk-GaN substrate 612 and includes a N-type confinement layer 614, a multiple quantum well (MQW) 616, a buffer layer 618, a P-type confinement layer 620, an optional reflector 621 and an ITO current spreader 622. Above the ITO current spreader 622, a layer of silicon oxide 696 is deposited, patterned, etched and filled with a metal 694 (e.g., tungsten) which is then CMPed. Yet another wafer is constructed with a blue LED. The blue LED wafer 640 is constructed on a sapphire or SiC or bulk-GaN substrate 624 and includes a N-type confinement layer 626, a multiple quantum well (MQW) 628, a buffer layer 630, a P-type confinement layer 632, an optional reflector 633 and an ITO current spreader 634. Above the ITO current spreader 634, a layer of silicon dioxide 698 is deposited. Examples of materials used to construct these blue and green LED layers, include, but are not limited to, doped GaN for the N-type and P-type confinement layers 614, 620, 626 and 632, AlGaN for the buffer layers 630 and 618 and InGaN/GaN for the multiple quantum wells 616 and 628. The optional reflectors 621 and 633 could be distributed Bragg Reflectors or some other type of reflectors. Various other material types and configurations could be used for constructing blue and green LEDs for this process.
Step (B) is illustrated in FIG. 6B. The blue LED wafer 640 from FIG. 6A is used for this step. Various elements in FIG. 6B such as 624, 626, 628, 630, 632, 633, 698, and 634 have been previously described. Hydrogen is implanted into the wafer at a certain depth indicated by dotted lines 642. Alternately, helium could be used for this step.
Step (C) is illustrated in FIG. 6C. A glass substrate 646 is taken and a silicon dioxide layer 688 is deposited atop it.
Step (D) is illustrated in FIG. 6D. The wafer shown in FIG. 6B is flipped and bonded atop the wafer shown in FIG. 6C using oxide-oxide bonding. Various elements in FIG. 6D such as 624, 626, 628, 630, 632, 633, 698, 642, 646, 688, and 634 have been previously described. The oxide layer 688 is essentially bonded to the oxide layer 698 using an oxide-to-oxide bonding process.
Step (E) is illustrated in FIG. 6E. Various elements in FIG. 6E such as 628, 630, 632, 633, 698, 646, 688, and 634 have been previously described. An ion-cut process is conducted to cleave the structure shown in FIG. 6D at the hydrogen implant plane 642. This ion-cut process may be preferably using a mechanical cleave. An anneal process could be utilized for the cleave as well. After the cleave, a chemical mechanical polish (CMP) process is conducted to planarize the surface. The N-type confinement layer present after this cleave and CMP process is indicated as 627.
Step (F) is illustrated in FIG. 6F. Various elements in FIG. 6F such as 628, 630, 632, 633, 698, 646, 688, 627, and 634 have been previously described. An ITO layer 648 is deposited atop the N-type confinement layer 627. Above the ITO layer 648, a layer of silicon oxide 686 is deposited, patterned, etched and filled with a metal 684 (e.g., tungsten) which is then CMPed.
Step (G) is illustrated in FIG. 6G. The green LED wafer 638 shown in Step (A) is used for this step. Various elements in FIG. 6G such as 612, 614, 616, 618, 620, 621, 696, 694, and 622 have been described previously. Hydrogen is implanted into the wafer at a certain depth indicated by dotted lines 650. Alternatively, helium could be used for this step.
Step (H) is illustrated in FIG. 6H. The structure shown in FIG. 6G is flipped and bonded atop the structure shown in FIG. 6F using oxide-oxide bonding. The metal regions 694 and 684 on the bonded wafers are aligned to each other. Various elements in FIG. 6H such as 628, 630, 632, 633, 698, 646, 688, 627, 634, 648, 686, 684, 612, 614, 616, 618, 620, 621, 696, 694, 650, and 622 have been described previously.
Step (I) is illustrated in FIG. 6I. The structure shown in FIG. 6H is cleaved at the hydrogen plane indicated by 650. This cleave process may be preferably done with a mechanical force. Alternatively, an anneal could be used. A CMP process is conducted to planarize the surface. Various elements in FIG. 6I such as 628, 630, 632, 633, 698, 646, 688, 627, 634, 648, 686, 684, 616, 618, 620, 621, 696, 694, and 622 have been described previously. The N-type confinement layer present after this cleave and CMP process is indicated as 615.
Step (J) is illustrated in FIG. 6J. An ITO layer 652 is deposited atop the structure shown in FIG. 6I. Above the ITO layer 652, a layer of silicon oxide 682 is deposited, patterned, etched and filled with a metal 680 (e.g., tungsten) which is then CMPed.
Various elements in FIG. 6J such as 628, 630, 632, 633, 698, 646, 688, 627, 634, 648, 686, 684, 616, 618, 620, 621, 696, 694, 615, and 622 have been described previously.
Step (K) is illustrated in FIG. 6K. Using procedures similar to Step (G)-Step (J), the red LED layer is transferred atop the structure shown in FIG. 6J. The N-type confinement layer after ion-cut is indicated by 605. An ITO layer 656 is deposited atop the N-type confinement layer 605. Various elements in FIG. 6K such as 628, 630, 632, 633, 698, 646, 688, 627, 634, 648, 686, 684, 616, 618, 620, 621, 696, 694, 615, 690, 692, 610, 609, 608, 606, and 622 have been described previously.
Step (L) is illustrated in FIG. 6L. Using flip-chip packaging procedures similar to those described in FIG. 4A-FIG. 4S, the RGB LED stack shown in FIG. 6K is attached to a silicon sub-mount 668. 658 indicates a reflecting material, 664 is a nickel layer, 666 represents solder bumps, 670 is an aluminum via, and 672 is either an oxide layer or an air gap. Various elements in FIG. 6K such as 628, 630, 632, 633, 698, 646, 688, 627, 634, 648, 686, 684, 616, 618, 620, 621, 696, 694, 615, 690, 692, 610, 609, 608, 606, 605, 656, and 622 have been described previously. The configuration of optional reflectors 633, 621 and 609 determines light output coming from the LED. A preferred embodiment of this invention may not have a reflector 633, but may have the reflector 621 (reflecting only the blue light produced by multiple quantum well 628) and the reflector 609 (reflecting only the green light produced by multiple quantum well 616). In the process described in FIG. 6A-FIG. 6L, the original substrates in FIG. 6A, namely 602, 612, and 624, can be re-used after ion-cut. This may make the process more cost-effective.

[0081] FIGS. 7A-L illustrate an embodiment of this invention, where stacked RGB LEDs are formed with laser lift-off, substrate etch, flip-chip packaging and conductive oxide bonding. Essentially, smart layer transfer techniques are used. This process could include several steps that occur in a sequence from Step (A) to Step (M). Many of the steps share common characteristics, features, modes of operation, etc. When identical reference numbers are used in different drawing figures, they are used to indicate analogous, similar or identical structures to enhance the understanding of the present invention by clarifying the relationships between the structures and embodiments presented in the various diagrams—particularly in relating analogous, similar or identical functionality to different physical structures.

[0082] Step (A): This is illustrated using FIG. 7A. A red LED wafer 736 is constructed on a GaAs substrate 702 and includes a N-type confinement layer 704, a multiple quantum well (MQW) 706, a P-type confinement layer 708, an optional reflector 709 and an ITO current spreader 710. Examples of materials used to construct these layers, include, but are not limited to, doped AlInGaP for the N-type confinement layer 704 and P-type confinement layer 708, the multiple quantum well layer 706 could be of AlInGaP and GaInP and the optional reflector 409 could be a distributed Bragg Reflector. A double heterostructure configuration or single quantum well configuration could be used instead of a multiple quantum well configuration. Various other material types and configurations could be used for constructing the red LEDs for this process. Yet another wafer is constructed with a green LED. The green LED wafer 738 is constructed on a sapphire substrate 712 (or some other transparent substrate) and includes a N-type confinement layer 714, a multiple quantum well (MQW) 716, a buffer layer 718, a P-type confinement layer 720, an optional reflector 721 and an ITO current spreader 722. Yet another wafer is constructed with a blue LED. The blue LED wafer 740 is constructed on a sapphire substrate 724 (or some other transparent substrate) and includes a N-type confinement layer 726, a multiple quantum well (MQW) 728, a buffer layer 730, a P-type confinement layer 732, an optional reflector 733 and an ITO current spreader 734. Examples of materials used to construct these blue and green LED layers, include, but are not limited to, doped GaN for the N-type and P-type confinement layers 714, 720, 726 and 732, AlGaN for the buffer layers 730 and 718 and InGaN/GaN for the multiple quantum wells 716 and 728. The optional reflectors 721 and 733 could be distributed Bragg Reflectors or some other type of reflectors. Various other material types and configurations could be used for constructing blue and green LEDs for this process.
Step (B) is illustrated in FIG. 7B. A glass substrate 746 is taken and an ITO layer 744 is deposited atop it.
Step (C) is illustrated in FIG. 7C. The blue LED wafer 740 shown in FIG. 7A is flipped and bonded atop the wafer shown in FIG. 7B using ITO-ITO bonding. Various elements in FIG. 7C such as 724, 726, 728, 730, 732, 733, 734, 746, and 744 have been previously described. The ITO layer 744 is essentially bonded to the ITO layer 734 using an oxide-to-oxide bonding process.
Step (D) is illustrated in FIG. 7D. A laser is used to shine radiation through the sapphire substrate 724 of FIG. 7C and a laser lift-off process is conducted. The sapphire substrate 724 of FIG. 7C is removed with the laser lift-off process. Further details of the laser lift-off process are described in U.S. Pat. No. 6,071,795 by Nathan W. Cheung, Timothy D. Sands and William S. Wong (“Cheung”). A CMP process is conducted to planarize the surface of the N confinement layer 727 after laser lift-off of the sapphire substrate. Various elements in FIG. 7D such as 728, 730, 732, 733, 734, 746, and 744 have been previously described.
Step (E) is illustrated in FIG. 7E. Various elements in FIG. 7E such as 728, 730, 732, 733, 734, 746, 727, and 744 have been previously described. An ITO layer 748 is deposited atop the N confinement layer 727.
Step (F) is illustrated in FIG. 7F. The green LED wafer 738 is flipped and bonded atop the structure shown in FIG. 7E using ITO-ITO bonding of layers 722 and 748. Various elements in FIG. 7F such as 728, 730, 732, 733, 734, 746, 727, 748, 722, 721, 720, 718, 716, 714, 712 and 744 have been previously described.
Step (G) is illustrated in FIG. 7G. A laser is used to shine radiation through the sapphire substrate 712 of FIG. 7F and a laser lift-off process is conducted. The sapphire substrate 712 of FIG. 7F is removed with the laser lift-off process. A CMP process is conducted to planarize the surface of the N-type confinement layer 715 after laser lift-off of the sapphire substrate. Various elements in FIG. 7G such as 728, 730, 732, 733, 734, 746, 727, 748, 722, 721, 720, 718, 716, and 744 have been previously described.
Step (H) is illustrated in FIG. 7H. An ITO layer 752 is deposited atop the N-type confinement layer 715. Various elements in FIG. 7H such as 728, 730, 732, 733, 734, 746, 727, 748, 722, 721, 720, 718, 716, 715, and 744 have been previously described.
Step (I) is illustrated in FIG. 7I. The red LED wafer 736 from FIG. 7A is flipped and bonded atop the structure shown in FIG. 7H using ITO-ITO bonding of layers 710 and 752. Various elements in FIG. 7I such as 728, 730, 732, 733, 734, 746, 727, 748, 722, 721, 720, 718, 716, 715, 752, 710, 709, 708, 706, 704, 702, and 744 have been previously described.
Step (J) is illustrated in FIG. 7J. The GaAs substrate 702 from FIG. 7I is removed using etch and/or CMP. Following this etch and/or CMP process, the N-type confinement layer 704 of FIG. 7I is planarized using CMP to form the N-type confinement layer 705. Various elements in FIG. 7J such as 728, 730, 732, 733, 734, 746, 727, 748, 722, 721, 720, 718, 716, 715, 752, 710, 709, 708, 706, and 744 have been previously described.
Step (K) is illustrated in FIG. 7K. An ITO layer 756 is deposited atop the N confinement layer 705 of FIG. 7J. Various elements in FIG. 7K such as 728, 730, 732, 733, 734, 746, 727, 748, 722, 721, 720, 718, 716, 715, 752, 710, 709, 708, 706, 705, and 744 have been previously described.
Step (L) is illustrated in FIG. 7L. Using flip-chip packaging procedures similar to those described in FIG. 4A-FIG. 4S, the RGB LED stack shown in FIG. 7K is attached to a silicon sub-mount 768. 758 indicates a reflecting material, 764 is a nickel layer, 766 represents solder bumps, 762 is an aluminum via, and 772 is either an oxide layer or an air gap. Various elements in FIG. 7L such as 728, 730, 732, 733, 734, 746, 727, 748, 722, 721, 720, 718, 716, 715, 752, 710, 709, 708, 706, 705, and 756 have been described previously. The configuration of optional reflectors 733, 721 and 709 determines light output coming from the LED. The preferred embodiment of this invention may not have a reflector 733, but may have the reflector 721 (reflecting only the blue light produced by multiple quantum well 728) and the reflector 709 (reflecting only the green light produced by multiple quantum well 716).

[0083] FIGS. 8A-B show an embodiment of this invention, where stacked RGB LEDs are formed from a wafer having red LED layers and another wafer having both green and blue LED layers. Therefore, a smart layer transfer process is used to form the stacked RGB LED. FIG. 8A shows that a red LED wafer 836 and another wafer called a blue-green LED wafer 836 are used. The red LED wafer 836 is constructed on a GaAs substrate 802 and includes a N-type confinement layer 804, a multiple quantum well (MQW) 806, a P-type confinement layer 808, an optional reflector 809 and an ITO current spreader 810. Examples of materials used to construct these layers, include, but are not limited to, doped AlInGaP for the N-type confinement layer 804 and P-type confinement layer 808, the multiple quantum well layer 806 could be of AlInGaP and GaInP and the optional reflector 809 could be a distributed Bragg Reflector. A double heterostructure configuration or single quantum well configuration could be used instead of a multiple quantum well configuration. Various other material types and configurations could be used for constructing the red LEDs for this process. The blue-green LED wafer 838 is constructed on a sapphire or bulk GaN or SiC substrate 812 (or some other transparent substrate) and includes a N-type confinement layer 814, a green multiple quantum well (MQW) 816, a blue multiple quantum well 817, a buffer layer 818, a P-type confinement layer 820, an optional reflector 821, and an ITO current spreader 822. Examples of materials used to construct the blue-green LED wafers, include, but are not limited to, doped GaN for the N-type and P-type confinement layers 814, 820, AlGaN for the buffer layer 818 and InGaN/GaN for the multiple quantum wells 816 and 817. The optional reflector 821 could be a distributed Bragg Reflector or some other type of reflector. The optional reflector 821 could alternatively be built between the N-type confinement layer 814 or below it, and this is valid for all LEDs discussed in the patent application. Various other material types and configurations could be used for constructing blue-green LED wafers for this process. Using smart layer transfer procedures similar to those shown in FIG. 4-FIG. 7, the stacked RGB LED structure shown in FIG. 8B is constructed. Various elements in FIG. 8B such as 806, 808, 809, 810, 816, 817, 818, 820, 821, and 822 have been described previously. 846 is a glass substrate, 844 is an ITO layer, 815 is a N-type confinement layer for a blue-green LED, 852 is an ITO layer, 805 is a N-type confinement layer for a red LED, 856 is an ITO layer, 858 is a reflecting material such as, for example, silver or aluminum, 864 is a nickel layer, 866 is a solder layer, 862 is a contact layer constructed of aluminum or some other metal, 860 may be preferably an air gap but could be an oxide layer and 868 is a silicon sub-mount. The configuration of optional reflectors 821 and 809 determines light produced by the LED. For the configuration shown in FIG. 8B, the preferred embodiment may not have the optional reflector 821 and may have the optional reflector 809 reflecting light produced by the blue and green quantum wells 816 and 817.

[0084] FIG. 9 illustrates an embodiment of this invention, where stacked RGB LEDs are formed with control and driver circuits for the LED built on the silicon sub-mount. Procedures similar to those described in FIG. 4-FIG. 7 are utilized for constructing and packaging the LED. Control and driver circuits are integrated on the silicon sub-mount 968 and can be used for controlling and driving the stacked RGB LED. 946 is a glass substrate, 944 and 934 are ITO layers, 933 is an optional reflector, 932 is a P-type confinement layer for a blue LED, 930 is a buffer layer for a blue LED, 928 is a blue multiple quantum well, 927 is a N-type confinement layer for a blue LED, 948 and 922 are ITO layers, 921 is an optional reflector, 920 is a P-type confinement layer for a green LED, 918 is a buffer layer for a green LED, 916 is a multiple quantum well for a green LED, 915 is a N-type confinement layer for a green LED, 952 and 910 are ITO layers, 909 is a reflector, 908 is a P-type confinement layer for a red LED, 906 is a red multiple quantum well, 905 is a N-type confinement layer for a red LED, 956 is an ITO layer, 958 is a reflecting layer such as aluminum or silver, 962 is a metal via constructed, for example, out of aluminum, 960 is an air-gap or an oxide layer, 964 is a nickel layer, and 966 is a solder bump.

[0085] FIG. 10 illustrates an embodiment of this invention, where stacked RGB LEDs are formed with control and driver circuits as well as image sensors for the LED built on the silicon sub-mount 1068. Image sensors essentially monitor the light coming out of the LED and tune the voltage and current given by control and driver circuits such that light output of the LED is the right color and intensity. 1046 is a glass substrate, 1044 and 1034 are ITO layers, 1033 is an optional reflector, 1032 is a P-type confinement layer for a blue LED, 1030 is a buffer layer for a blue LED, 1028 is a blue multiple quantum well, 1027 is a N-type confinement layer for a blue LED, 1048 and 1022 are ITO layers, 1021 is an optional reflector, 1020 is a P-type confinement layer for a green LED, 1018 is a buffer layer for a green LED, 1016 is a multiple quantum well for a green LED, 1015 is a N-type confinement layer for a green LED, 1052 and 1010 are ITO layers, 1009 is a reflector, 1008 is a P-type confinement layer for a red LED, 1006 is a red multiple quantum well, 1005 is a N-type confinement layer for a red LED, 1056 is an ITO layer, 1058 is a reflecting layer such as aluminum or silver, 1062 is a metal via constructed for example out of aluminum, an air-gap or an oxide layer between silicon sub-mount 1068 and reflecting layer 1058, 1064 is a nickel layer and 1066 is a solder bump. The via hole 1074 helps transfer light produced by the blue multiple quantum well 1028 reach an image sensor on the silicon sub-mount 1068. The via hole 1072 helps transfer light produced by the green multiple quantum well 1016 to an image sensor on the silicon sub-mount 1068. The via hole 1070 helps transfer light produced by the red multiple quantum well 1006 reach an image sensor on the silicon sub-mount 1068. By sampling the light produced by each of the quantum wells on the LED, voltage and current drive levels to different terminals of the LED can be determined. Color tunability, temperature compensation, better color stability, and many other features can be obtained with this scheme. Furthermore, circuits to communicate wirelessly with the LED can be constructed on the silicon sub-mount. Light output of the LED can be modulated by a signal from the user delivered wirelessly to the light.

[0086] While three LED layers, namely, red, green, and blue, are shown as stacked in various embodiments of this invention, it will be clear to one skilled in the art based on the present disclosure that more than three LED layers can also be stacked. For example, red, green, blue and yellow LED layers can be stacked.

[0087] The embodiments of this invention described in FIG. 4-FIG. 10 share a few common features. They have multiple stacked (or overlying) layers, they are constructed using smart layer transfer techniques and at least one of the stacked layers has a thickness less than 50 microns. When cleave is done using ion-cut, substrate layers that are removed using cleave can be reused after a process flow that often includes a CMP.

[0088] FIGS. 11A-F show a prior art illustration of phosphor-coated LEDs (pcLEDs) constructed with ion-cut processes. The process begins in FIG. 11A with a bulk-GaN substrate 1102, and an oxide layer 1104 is deposited atop it. The oxide layer 1104 is an oxide compatible with GaN. FIG. 11B depicts hydrogen being implanted into the structure shown in FIG. 11A at a certain depth (for ion-cut purposes). 1102 and 1104 have been described previously with respect to FIG. 11A. Dotted lines 1106 indicate the plane of hydrogen ions. Alternatively, helium can be implanted instead of hydrogen or hydrogen and helium can be co-implanted. FIG. 11C shows a silicon wafer 1108 with an oxide layer 1110 atop it. The structure shown in FIG. 11B is flipped and bonded atop the structure shown in FIG. 11C using oxide-to-oxide bonding of layers 1104 and 1110. This is depicted in FIG. 11D. 1108, 1110 and 1106 have been described previously. FIG. 11E shows the next step in the process. Using an anneal, a cleave is conducted at the plane of hydrogen atoms 1106 shown in FIG. 11D, and a CMP is done to form GaN layer 1112. 1104, 1110 and 1108 have been described previously. FIG. 11F shows the following step in the process. A blue LED 1114 is grown epitaxially above the GaN layer 1112. 1104, 1108 and 1110 have been described previously. A phosphor layer can be coated atop the blue LED 1114 to form a white phosphor coated LED.

[0089] There may be some severe challenges with the prior art process shown in FIGS. 11A-F. The thermal expansion coefficients for GaN layers 1112 in FIG. 11F are very different from that for silicon layers 1108. This difference can cause cracks and defects while growing the blue LED layer 1114 at high temperatures (>600° C.), which usually occurs. These cracks and defects, in turn, cause bad efficiency and can in turn cause the phosphor coated LED process in FIG. 11A-F to be difficult to manufacture. Furthermore, an anneal (typically >400° C.) is typically used in FIG. 11E to cleave the bulk GaN layers. This can again cause issues with mismatch of thermal expansion co-efficients and cause cracking and defects.

[0090] FIGS. 12A-F describe an embodiment of this invention, where phosphor coated LEDs are formed with an ion-cut process (i.e. a smart layer transfer process). It minimizes the problem with mismatch of thermal expansion co-efficients that is inherent to the process described in FIGS. 11A-F. This process could include several steps as described in the following sequence:

[0091] Step (A): FIG. 12A illustrates this step. A blue LED wafer is constructed on a bulk-GaN substrate 1216. For discussions within this document, the bulk-GaN substrate could be semi-polar or non-polar or polar. The blue LED wafer includes a N-type confinement layer 1214, a multiple quantum well (MQW) 1212, a buffer layer 1210, a P-type confinement layer 1208, an optional reflector 1204 and an ITO current spreader 1206. Examples of materials used to construct these blue LED layers, include, but are not limited to, doped GaN for the N-type and P-type confinement layers 1214 and 1208, AlGaN for the buffer layer 1210 and InGaN/GaN for the multiple quantum wells 1212. The optional reflector 1204 could be distributed Bragg Reflector, an Aluminum or silver layer or some other type of reflectors. A silicon dioxide layer 1202 is deposited atop the optional reflector 1204.
Step (B): FIG. 12B illustrates this step. The blue LED wafer described in FIG. 12A has hydrogen implanted into it at a certain depth. The dotted lines 1218 depict the hydrogen implant. Alternatively, helium can be implanted. Various elements in FIG. 12B such as 1216, 1214, 1212, 1210, 1208, 1206, 1204, and 1202 have been described previously.
Step (C): FIG. 12C illustrates this step. A wafer 1220, preferably of silicon, having the same wafer size as the structure in FIG. 12B is taken and an oxide layer 1222 is grown or deposited atop it.
Step (D): FIG. 12D illustrates this step. The structure shown in FIG. 12B is flipped and bonded atop the structure shown in FIG. 12C using oxide-to-oxide bonding of layers 1202 and 1222. Various elements in FIG. 12D such as 1216, 1214, 1212, 1210, 1208, 1206, 1204, 1220, 1222, 1218 and 1202 have been described previously.
Step (E): FIG. 12E illustrates this step. The structure shown in FIG. 12D is cleaved at its hydrogen plane 1218. A mechanical cleave may be preferably used for this process. However, an anneal could be used as well. The mechanical cleave process typically happens at room temperatures, and therefore can avoid issues with thermal expansion co-efficients mismatch. After cleave, the wafer is planarized and the N-type confinement layer 1215 is formed. Various elements in FIG. 12E such as 1212, 1210, 1208, 1206, 1204, 1220, 1222, and 1202 have been described previously. The bulk GaN substrate 1216 from FIG. 12D that has been cleaved away can be reused. This may be attractive from a cost perspective, since bulk GaN substrates are quite costly.
Step (F): This is illustrated in FIG. 12F. An ITO layer 1224 is deposited atop the structure shown in FIG. 12E. Various elements in FIG. 12F such as 1212, 1210, 1208, 1206, 1204, 1220, 1222, 1215, 1224, and 1202 have been described previously.
A phosphor coating can be applied over the structure shown in FIG. 12F to produce a phosphor-coated LED. The advantage of the process shown in FIG. 12A-F over the process shown in FIG. 11A-F may include low process temperatures, even less than 250° C. Therefore, issues with thermal expansion co-efficients mismatch are substantially mitigated. While the description in FIG. 12A-F is for a LED, many other devices, such as, for example, laser diodes, high power transistors, high frequencies transistors, special transmitter circuits and many other devices can be constructed, according to a similar description, with bulk-GaN.

[0092] In the description of FIG. 12A-F, silicon is described as a preferred material for the substrate 1220. Silicon has a co-efficient of thermal expansion of about 2.6 ppm/° C., while bulk-GaN, which is the substrate 1216 on which the LED is epitaxially grown, has a co-efficient of thermal expansion of 5.6 ppm/° C. In an alternate embodiment of this invention, the substrate 1220 used in FIG. 12A-F could be constructed of a material that has a co-efficient of thermal expansion (CTE) fairly close to bulk-GaN. Preferably, the CTE of the substrate 1220 could be any value in between (the CTE of bulk GaN −2 ppm/° C.) and (the CTE of bulk GaN+2 ppm/° C.). Examples of materials that could be used for the substrate 1220 could include, but are not limited to, Germanium, that has a CTE of 5.8 ppm/° C., and various ceramic materials. Having CTE for the substrate 1220 close to bulk-GaN prevents defects and cracks being formed due to issues with mismatch of CTE, even if higher temperature processing (>250° C.) is used.

[0093] In an alternative embodiment of this invention, the flow in FIG. 11A-F can be used with the substrate 1108 having a CTE fairly close to the CTE of bulk GaN. Preferably, the CTE of the substrate 1108 could be any value in between (the CTE of bulk GaN−2 ppm/° C.) and (the CTE of bulk GaN+2 ppm/° C.). Examples of materials that could be used for the substrate 1108 could include, but are not limited to, Germanium, that has a CTE of 5.8 ppm/° C., and various ceramic materials.

NuImager Technology:

[0094] Layer transfer technology can also be advantageously utilized for constructing image sensors. Image sensors typically include photodetectors on each pixel to convert light energy to electrical signals. These electrical signals are sensed, amplified and stored as digital signals using transistor circuits.

[0095] FIG. 13 shows prior art where through-silicon via (TSV) technology is utilized to connect photodetectors 1302 on one layer (tier) of silicon to transistor read-out circuits 1304 on another layer (tier) of silicon. Unfortunately, pixel sizes in today's image sensors are 1.1 μm or so. It is difficult to get through-silicon vias with size <1 μm due to alignment problems, leading to a diminished ability to utilize through-silicon via technology for future image sensors. In FIG. 13, essentially, transistors can be made for read-out circuits in one wafer, photodetectors can be made on another wafer, and then these wafers can be bonded together with connections made with through-silicon vias.

[0096] FIGS. 14-21 describe some embodiments of this invention, where photodetector and read-out circuits are stacked monolithically with layer transfer. FIG. 14 shows two configurations for stacking photodetectors and read-out circuits. In one configuration, denoted as 1402, a photodetector layer 1406 may be formed above read-out circuit layer 1408 with connections 1404 between these two layers. In another configuration, denoted as 1410, photodetectors 1412 may have read-out circuits 1414 formed above them, with connections 1416 between these two layers.

[0097] FIGS. 15A-H describe an embodiment of this invention, where an image sensor includes a photodetector layer formed atop a read-out circuit layer using layer transfer. In this document, the photodetector layer is denoted as a p-n junction layer. However, any type of photodetector layer, such as a pin layer or some other type of photodetector can be used. The thickness of the photodetector layer is typically less than 5 μm. The process of forming the image sensor could include several steps that occur in a sequence from Step (A) to Step (H). Many of these steps share common characteristics, features, modes of operation, etc. When identical reference numbers are used in different drawing figures, they are used to indicate analogous, similar or identical structures to enhance the understanding of the present invention by clarifying the relationships between the structures and embodiments presented in the various diagrams—particularly in relating analogous, similar or identical functionality to different physical structures.

[0098] Step (A) is illustrated in FIG. 15A. A silicon wafer 1502 may be taken and a n+ Silicon layer 1504 may be formed by ion implantion. Following this, n layer 1506, p layer 1508 and p+ layer 1510 may be formed epitaxially. It will be appreciated by one skilled in the art based on the present disclosure that there are various other procedures to form the structure shown in FIG. 15A. An anneal may then be performed to activate dopants in the various layers.
Step (B) is illustrated in FIG. 15B. Various elements in FIG. 15B such as 1502, 1504, 1506, 1508 and 1510 have been described previously. Using lithography and etch, a via may be etched into the structure shown in FIG. 15A, then may be filled with oxide and then polished with CMP. The regions formed are the oxide filled via 1512 and the oxide layer 1514. The oxide filled via 1512 may also be referred to as an oxide via or an oxide window region or oxide aperture. A cross-section of the structure is indicated by 1598 and a top view is indicated by 1596. 1516 indicates alignment marks and the oxide filled via 1512 may be formed in place of some of the alignment marks printed on the wafer.
Step (C) is illustrated in FIG. 15C. Various elements in FIG. 15C such as 1502, 1504, 1506, 1508, 1510, 1512, 1514, and 1516 have been described previously. Hydrogen may be implanted into the structure indicated in FIG. 15B at a certain depth indicated by dotted lines 1518 of FIG. 15C. Alternatively, Helium can be used as the implanted species. A cross-sectional view 1594 and a top view 1592 are shown.
Step (D) is illustrated in FIG. 15D. A silicon wafer 1520 with read-out circuits (which includes wiring) processed on it is taken, and an oxide layer 1522 may be deposited above it.
Step (E) is illustrated in FIG. 15E. The structure shown in FIG. 15C is flipped and bonded to the structure shown in FIG. 15D using oxide-to-oxide bonding of oxide layers 1514 and 1522. During this bonding procedure, alignment may be done such that oxide vias 1512 (shown in the top view 1526 of the photodetector wafer) are above alignment marks (such as 1530) on the top view 1528 of the read-out circuit wafer. A cross-sectional view of the structure is shown with 1524. Various elements in FIG. 15E such as 1502, 1504, 1506, 1508, 1510, 1512, 1514, 1516, 1518, 1520, and 1522 have been described previously.
Step (F) is illustrated in FIG. 15F. The structure shown in FIG. 15E may be cleaved at its hydrogen plane 1518 preferably using a mechanical process. Alternatively, an anneal could be used for this purpose. A CMP process may be then done to planarize the surface resulting in a final n+ silicon layer indicated as 1534. 1525 depicts a cross-sectional view of the structure after the cleave and CMP process. Various elements in FIG. 15F such as 1506, 1508, 1510, 1512, 1514, 1516, 1520, 1526, 1530, 1528, 1530 and 1522 have been described previously.
Step (G) is illustrated using FIG. 15G. Various elements in FIG. 15G such as 1506, 1508, 1510, 1512, 1514, 1516, 1520, 1526, 1530, 1528, 1530, 1534 and 1522 have been described previously. An oxide layer 1540 may be deposited. Connections between the photodetector and read-out circuit wafers may be formed with metal 1538 and an insulator covering 1536. These connections may be formed well aligned to the read-out circuit layer 1520 by aligning to alignment marks 1530 on the read-out circuit layer 1520 through oxide vias 1512. 1527 depicts a cross-sectional view of the structure.
Step (H) is illustrated in FIG. 15H. Connections are made to the terminals of the photodetector and are indicated as 1542 and 1544. Various elements of FIG. 15H such as 1520, 1522, 1512, 1514, 1510, 1508, 1506, 1534, 1536, 1538, 1540, 1542, and 1544 have been described previously. Contacts and interconnects for connecting terminals of the photodetector to read-out circuits may then be done, following which a packaging process is conducted.

[0099] The thinner the transferred layer, the smaller the through layer via (TLV) diameter obtainable, due to the potential limitations of manufacturable via aspect ratios. Thus, the transferred layer may be, for example, less than about 2 microns thick, less than about 1 micron thick, less than about 0.4 microns thick, less than about 200 nm thick, or less than about 100 nm thick. The vertical connections, or Through Layer Via (TLV) diameter may be less than about 400 nm, less than about 200 nm, less than about 80 nm, less than about 40 nm, or less than about 20 nm. The thickness of the layer or layers transferred according to some embodiments of the present invention may be designed as such to match and enable the best obtainable lithographic resolution capability of the manufacturing process employed to create the through layer vias or any other structures on the transferred layer or layers.

[0100] In many of the embodiments of the invention, the layer or layers transferred may be of a crystalline material, for example, mono-crystalline silicon, and after layer transfer, further processing, such as, for example, plasma/RIE or wet etching, may be done on the layer or layers that may create islands or mesas of the transferred layer or layers of crystalline material, for example, mono-crystalline silicon, the crystal orientation of which has not changed. Thus, a mono-crystalline layer or layers of a certain specific crystal orientation may be layer transferred and then processed whereby the resultant islands or mesas of mono-crystalline silicon have the same crystal specific orientation as the layer or layers before the processing. After this processing, the resultant islands or mesas of crystalline material, for example, mono-crystalline silicon, may be still referred to herein as a layer, for example, mono-crystalline layer, layer of mono-crystalline silicon, and so on

[0101] FIGS. 15A-G show a process where oxide vias may be used to look through photodetector layers to observe alignment marks on the read-out circuit wafer below it. However, if the thickness of the silicon on the photodetector layer is <100-400 nm, the silicon wafer is thin enough that one can look through it without requiring oxide vias. A process similar to FIG. 15A-G where the silicon thickness for the photodetector is <100-400 nm represents another embodiment of this invention. In that embodiment, oxide vias may not be constructed and one could look right through the photodetector layer to observe alignment marks of the read-out circuit layer. This may help making well-aligned through-silicon connections between various layers.

[0102] As mentioned previously, FIGS. 15A-G illustrate a process where oxide vias constructed before layer transfer are used to look through photodetector layers to observe alignment marks on the read-out circuit wafer below it. However, an alternative embodiment of this invention may involve constructing oxide vias after layer transfer. Essentially, after layer transfer of structures without oxide vias, oxide vias whose diameters are larger than the maximum misalignment of the bonding/alignment scheme are formed. This order of sequences may enable observation of alignment marks on the bottom read-out circuit wafer by looking through the photodetector wafer.

[0103] While Silicon has been suggested as the material for the photodetector layer of FIG. 15A-G, Germanium could be used in an alternative embodiment. The advantage of Germanium is that it is sensitive to infra-red wavelengths as well. However, Germanium also suffers from high dark current.

[0104] While FIG. 15A-G described a single p-n junction as the photodetector, it will be obvious to one skilled in the art based on the present disclosure that multiple p-n junctions can be formed one on top of each other, as described in “Color Separation in an Active Pixel Cell Imaging Array Using a Triple-Well Structure,” U.S. Pat. No. 5,965,875, 1999 by R. Merrill and in “Trends in CMOS Image Sensor Technology and Design,” International Electron Devices Meeting Digest of Technical Papers, 2002 by A. El-Gamal. This concept relies on the fact that different wavelengths of light penetrate to different thicknesses of silicon, as described in FIG. 16. It can be observed in FIG. 16 that near the surface 400 nm wavelength light has much higher absorption per unit depth than 450 nm-650 nm wavelength light. On the other hand, at a depth of 0.5 μm, 500 nm light has a higher absorption per unit depth than 400 nm light. An advantage of this approach is that one does not require separate filters (and area) for green, red and blue light; all these different colors/wavelengths of light can be detected with different p-n junctions stacked atop each other. So, the net area required for detecting three different colors of light is reduced, leading to an improvement of resolution.

[0105] FIGS. 17A-B illustrate an embodiment of this invention, where red, green, and blue photodetectors are stacked monolithically atop read-out circuits using ion-cut technology (for an image sensor). Therefore, a smart layer transfer technique is utilized. FIG. 17A shows the first step for constructing this image sensor. 1724 shows a cross-sectional view of 1708, a silicon wafer with read-out circuits constructed on it, above which an oxide layer 1710 is deposited. 1726 shows the cross-sectional view of another wafer which may include silicon substrate 1712, a p+ Silicon layer 1714, a p Silicon layer 1716, a n Silicon layer 1718, a n+ Silicon layer 1720, and an oxide layer 1722. These layers may be formed using procedures similar to those described in FIG. 15A-G. An anneal may then be performed to activate dopants in various layers. Hydrogen may be implanted in the wafer at a certain depth depicted by 1798, shown as dashed line. FIG. 17B shows the structure of the image sensor before contact formation. Three layers of p+pnn+ silicon (each corresponding to a color band and similar to the one depicted in 1726 in FIG. 17A) are layer transferred sequentially atop the silicon wafer with read-out circuits (depicted by 1724 in FIG. 17A). Three different layer transfer steps may be used for this purpose. Procedures for layer transfer and alignment for forming the image sensor in FIG. 17B are similar to procedures used for constructing the image sensor shown in FIGS. 15A-G. Each of the three layers of p+pnn+ silicon senses a different wavelength of light. For example, blue light is detected by blue photodetector 1702, green light is detected by green photodetector 1704, and red light is detected by red photodetector 1706. Contacts, metallization, packaging and other steps are done to the structure shown in FIG. 17B to form an image sensor. The oxides 1730 and 1732 could be either transparent conducting oxides or silicon dioxide. Use of transparent conducting oxides could allow fewer contacts to be formed.

[0106] FIG. 18A-B show another embodiment of this invention, where red, green and blue photodetectors are stacked monolithically atop read-out circuits using ion-cut technology (for an image sensor) using a different configuration. Therefore, a smart layer transfer technique is utilized. FIG. 18A shows the first step for constructing this image sensor. 1824 shows a cross-section of 1808, a silicon wafer with read-out circuits constructed on it, above which an oxide layer 1810 is deposited. 1826 shows the cross-sectional view of another wafer which has silicon substrate 1812, a p+ Silicon layer 1814, a p Silicon layer 1816, a n Silicon layer 1818, a p Silicon layer 1820, a n Silicon layer 1822, a n+ Silicon layer 1828 and an oxide layer 1830. These layers may be formed using procedures similar to those described in FIG. 15A-G. An anneal may then be performed to activate dopants in various layers. Hydrogen may implanted in the wafer at a certain depth depicted by 1898, shown as dashed line. FIG. 18B shows the structure of the image sensor before contact formation. A layer of p+pnpnn+ (similar to the one depicted in 1826 in FIG. 18A) is layer transferred sequentially atop the silicon wafer with read-out circuits (depicted by 1824 in FIG. 18A). Procedures for layer transfer and alignment for forming the image sensor in FIG. 18B are similar to procedures used for constructing the image sensor shown in FIG. 15A-G. Contacts, metallization, packaging and other steps are done to the structure shown in FIG. 18B to form an image sensor. Three different pn junctions, denoted by 1802, 1804 and 1806 may be formed in the image sensor to detect different wavelengths of light.

[0107] FIGS. 19A-B show another embodiment of this invention, where an image sensor that can detect both visible and infra-red light is depicted. Such image sensors could be useful for taking photographs in both day and night settings (without necessarily requiring a flash). This embodiment makes use of the fact that while silicon is not sensitive to infra-red light, other materials such as Germanium and Indium Gallium Arsenide are. A smart layer transfer technique is utilized for this embodiment. FIG. 19A shows the first step for constructing this image sensor. 1902 shows a cross-sectional view of 1904, a silicon wafer with read-out circuits constructed on it, above which an oxide layer 1906 is deposited. 1908 shows the cross-sectional view of another wafer which has silicon 1910, a p+ Silicon layer 1912, a p Silicon layer 1914, a n Silicon layer 1916, a n+ Silicon layer 1918 and an oxide layer 1720. These layers may be formed using procedures similar to those described in FIGS. 15A-G. An anneal may then be performed to activate dopants in various layers. Hydrogen may be implanted in the wafer at a certain depth depicted by 1998, shown as dashed line. 1922 shows the cross-sectional view of another wafer which has a substrate 1924, an optional buffer layer 1936, a p+ Germanium layer 1926, a p Germanium layer 1928, a n Germanium layer 1930, a n+ Germanium layer 1932 and an oxide layer 1934. These layers may be formed using procedures similar to those described in FIGS. 15A-G. An anneal may then be performed to activate dopants in various layers. Hydrogen may be implanted in the wafer at a certain depth depicted by 1996, shown as dashed line. Examples of materials used for the structure 1922 may include a Germanium substrate for 1924, no buffer layer and multiple Germanium layers. Alternatively, an Indium Phosphide substrate could be used for 1924 when the layers 1926, 1924, 1922 and 1920 are constructed of InGaAs instead of Germanium. FIG. 19B shows the structure of this embodiment of the invention before contacts and metallization are constructed. The p+pnn+ Germanium layers of structure 1922 of FIG. 19A are layer transferred atop the read-out circuit layer of structure 1902. This is done using smart layer transfer procedures similar to those described in respect to FIG. 15A-G. Following this, multiple p+pnn+ layers similar to those used in structure 1908 may be layer transferred atop the read-out circuit layer and Germanium photodetector layer (using three different layer transfer steps). This, again, is done using procedures similar to those described in FIGS. 15A-G. The structure shown in FIG. 19B therefore has a layer of read-out circuits 1904, above which an infra-red photodetector 1944, a red photodetector 1942, a green photodetector 1940 and a blue photodetector 1938 are present. Procedures for layer transfer and alignment for forming the image sensor in FIG. 19B are similar to procedures used for constructing the image sensor shown in FIG. 15A-G. Each of the p+pnn+ layers senses a different wavelength of light. Contacts, metallization, packaging and other steps are done to the structure shown in FIG. 19B to form an image sensor. The oxides 1946, 1948, and 1950 could be either transparent conducting oxides or silicon dioxide. Use of transparent conducting oxides could allow fewer contacts to be formed.

[0108] FIG. 20A describes another embodiment of this invention, where polarization of incoming light can be detected. The p-n junction photodetector 2006 detects light that has passed through a wire grid polarizer 2004. Details of wire grid polarizers are described in “Fabrication of a 50 nm half-pitch wire grid polarizer using nanoimprint lithography.” Nanotechnology 16 (9): 1874-1877, 2005 by Ahn, S. W.; K. D. Lee, J. S. Kim, S. H. Kim, J. D. Park, S. H. Lee, P. W. Yoon. The wire grid polarizer 2004 absorbs one plane of polarization of the incident light, and may enable detection of other planes of polarization by the p-n junction photodetector 2006. The p-n junction photodetector 2002 detects all planes of polarization for the incident light, while 2006 detects the planes of polarization that are not absorbed by the wire grid polarizer 2004. One can thereby determine polarization information from incoming light by combining results from photodetectors 2002 and 2006. The device described in FIG. 20A can be fabricated by first constructing a silicon wafer with transistor circuits 2008, following which the p-n junction photodetector 2006 can be constructed with the low-temperature layer transfer techniques described in FIG. 15A-G. Following this construction of p-n junction photodetector 2006, the wire grid polarizer 2004 may be constructed using standard integrated circuit metallization methods. The photodetector 2002 can then be constructed by another low-temperature layer transfer process as described in FIG. 15A-G. One skilled in the art, based on the present disclosure, can appreciate that low-temperature layer transfer techniques are critical to build this device, since semiconductor layers in 2002 are built atop metallization layers required for the wire grid polarizer 2004. Thickness of the photodetector layers 2002 and 2006 may be preferably less than 5 μm. An example with polarization detection where the photodetector has other pre-processed optical interaction layers (such as a wire grid polarizer) has been described herein. However, other devices for determining parameters of incoming light (such as phase) may be constructed with layer transfer techniques.

[0109] One of the common issues with taking photographs with image sensors is that in scenes with both bright and dark areas, while the exposure duration or shutter time could be set high enough to get enough photons in the dark areas to reduce noise, picture quality in bright areas degrades due to saturation of the photodetectors' characteristics. This issue is with the dynamic range of the image sensor, i.e. there is a tradeoff between picture quality in dark and bright areas. FIG. 20B shows an embodiment of this invention, where higher dynamic range can be reached. According the embodiment of FIG. 20B, two layers of photodetectors 2032 and 2040, could be stacked atop a read-out circuit layer 2028. 2026 is a schematic of the architecture. Connections 2030 run between the photodetector layers 2032 and 2040 and the read-out circuit layer 2028. 2024 are reflective metal lines that block light from reaching part of the bottom photodetector layer 2032. 2042 is a top view of the photodetector layer 2040. Photodetectors 2036 could be present, with isolation regions 2038 between them. 2044 is a top view of the photodetector layer 2032 and the metal lines 2024. Photodetectors 2048 are present, with isolation regions 2046 between them. A portion of the photodetectors 2048 can be seen to be blocked by metal lines 2024. Brighter portions of an image can be captured with photodetectors 2048, while darker portions of an image can be captured with photodetectors 2036. The metal lines 2024 positioned in the stack may substantially reduce the number of photons (from brighter portions of the image) reaching the bottom photodetectors 2048. This reduction in number of photons reaching the bottom photodetectors 2048 helps keep the dynamic range high. Read-out signals coming from both dark and bright portions of the photodetectors could be used to get the final picture from the image sensor.

[0110] FIG. 21 illustrates another embodiment of this invention where a read-out circuit layer 2104 is monolithically stacked above the photodetector layer 2102 at a temperature approximately less than 400° C. Connections 2106 are formed between these two layers. Procedures for stacking high-quality monocrystalline transistor circuits and wires at temperatures approximately less than 400° C. using layer transfer are described in pending U.S. patent application Ser. No. 12/901,890 by the inventors of this patent application, the content of which is incorporated by reference. The stacked layers could use junction-less transistors, recessed channel transistors, repeating layouts or other devices/techniques described in U.S. patent application Ser. No. 12/901,890 the content of which is incorporated by reference. The embodiments of this invention described in FIG. 14-FIG. 21 may share a few common features. They can have multiple stacked (or overlying) layers, use one or more photodetector layers (terms photodetector layers and image sensor layers are often used interchangeably), thickness of at least one of the stacked layers is less than 5 microns and construction can be done with smart layer transfer techniques and stacking is done at temperatures approximately less than 450° C.

NuDisplay Technology:

[0111] In displays and microdisplays (small size displays where optical magnification is needed), transistors need to be formed on glass or plastic substrates. These substrates typically cannot withstand high process temperatures (e.g., >400° C.). Layer transfer can be advantageously used for constructing displays and microdisplays as well, since it may enable transistors to be processed on these substrates at <400° C. Various embodiments of transistors constructed on glass substrates are described in this patent application. These transistors constructed on glass substrates could form part of liquid crystal displays (LCDs) or other types of displays. It will be clear to those skilled in the art based on the present disclosure that these techniques can also be applied to plastic substrates.

[0112] FIGS. 22A-G describe a process for forming recessed channel single crystal (or monocrystalline) transistors on glass substrates at a temperature approximately less than 400° C. for display and microdisplay applications. This process could include several steps that occur in a sequence from Step (A) to Step (G). Many of these steps share common characteristics, features, modes of operation, etc. When identical reference numbers are used in different drawing figures, they are used to indicate analogous, similar or identical structures to enhance the understanding of the present invention by clarifying the relationships between the structures and embodiments presented in the various diagrams—particularly in relating analogous, similar or identical functionality to different physical structures.

[0113] Step (A) is illustrated in FIG. 22A. A silicon wafer 2202 is taken and a n+ region 2204 is formed by ion implantation. Following this formation, a layer of p− Silicon 2206 is epitaxially grown. An oxide layer 2210 is then deposited. Following this deposition, an anneal is performed to activate dopants in various layers. It will be clear to one skilled in the art based on the present disclosure that various other procedures can be used to get the structure shown in FIG. 22A.
Step (B) is illustrated in FIG. 22B. Hydrogen is implanted into the structure shown in FIG. 22A at a certain depth indicated by 2212. Alternatively, Helium can be used for this purpose. Various elements in FIG. 22B, such as 2202, 2204, 2006, and 2210 have been described previously.
Step (C) is illustrated in FIG. 22C. A glass substrate 2214 is taken and a silicon oxide layer 2216 is deposited atop it at compatible temperatures.
Step (D) is illustrated in FIG. 22D. Various elements in FIG. 22D, such as 2202, 2204, 2206, 2210, 2214, and 2216 have been described previously. The structure shown in FIG. 22B is flipped and bonded to the structure shown in FIG. 22C using oxide-to-oxide bonding of layers 2210 and 2216.
Step (E) is illustrated in FIG. 22E. The structure shown in FIG. 22D is cleaved at the hydrogen plane 2212 of FIG. 22D. A CMP is then done to planarize the surface and yield the n+ Si layer 2218. Various other elements in FIG. 22E, such as 2214, 2216, 2210 and 2206 have been described previously.
Step (F) is illustrated in FIG. 22F. Various elements in FIG. 22F such as 2214, 2216, 2210, and 2206 have been described previously. An oxide layer 2220 is formed using a shallow trench isolation (STI) process. This helps isolate transistors.
Step (G) is illustrated in FIG. 22G. Various elements in FIG. 22G such as 2210, 2216, 2220 and 2214 have been described previously. Using etch techniques, part of the n+ Silicon layer from FIG. 22F and optionally p− Silicon layer from FIG. 22F are etched. After this a thin gate dielectric is deposited, after which a gate dielectrode is deposited. The gate dielectric and gate electrode are then polished away to form the gate dielectric layer 2224 and gate electrode layer 2222. The n+ Silicon layers 2228 and 2226 form the source and drain regions of the transistors while the p− Silicon region after this step is indicated by 2230. Contacts and other parts of the display/microdisplay are then fabricated. It can be observed that during the whole process, the glass substrate substantially always experiences temperatures less than 400° C., or even lower. This is because the crystalline silicon can be transferred atop the glass substrate at a temperature less than 400° C., and dopants are pre-activated before layer transfer to glass.

[0114] FIG. 23A-H describes a process of forming both nMOS and pMOS transistors with single-crystal silicon on a glass substrate at temperatures less than 400° C., and even lower. Ion-cut technology (which is a smart layer transfer technology) is used. While the process flow described is shown for both nMOS and pMOS on a glass substrate, it could also be used for just constructing nMOS devices or for just constructing pMOS devices. This process could include several steps that occur in a sequence from Step (A) to Step (H). Many of these steps share common characteristics, features, modes of operation, etc. When identical reference numbers are used in different drawing figures, they are used to indicate analogous, similar or identical structures to enhance the understanding of the present invention by clarifying the relationships between the structures and embodiments presented in the various diagrams—particularly in relating analogous, similar or identical functionality to different physical structures.

[0115] Step (A) is illustrated in FIG. 23A. A p− Silicon wafer 2302 is taken and a n well 2304 is formed on the p− Silicon wafer 2302. Various additional implants to optimize dopant profiles can also be done. Following this formation, an isolation process is conducted to form isolation regions 2306. A dummy gate dielectric 2310 made of silicon dioxide and a dummy gate electrode 2308 made of polysilicon are constructed.
Step (B) is illustrated in FIG. 23B. Various elements of FIG. 23B, such as 2302, 2304, 2306, 2308 and 2310 have been described previously. Implants are done to form source-drain regions 2312 and 2314 for both nMOS and pMOS transistors. A rapid thermal anneal (RTA) is then done to activate dopants. Alternatively, a spike anneal or a laser anneal could be done.
Step (C) is illustrated in FIG. 23C. Various elements of FIG. 23C such as 2302, 2304, 2306, 2308, 2310, 2312 and 2314 have been described previously. An oxide layer 2316 is deposited and planarized with CMP.
Step (D) is described in FIG. 23D. Various elements of FIG. 23D such as 2302, 2304, 2306, 2308, 2310, 2312, 2314, and 2316 have been described previously. Hydrogen is implanted into the wafer at a certain depth indicated by 2318. Alternatively, helium can be implanted.
Step (E) is illustrated in FIG. 23E. Various elements of FIG. 23E such as 2302, 2304, 2306, 2308, 2310, 2312, 2314, 2316, and 2318 have been described previously. Using a temporary bonding adhesive, the oxide layer is bonded to a temporary carrier wafer 2320. An example of a temporary bonding adhesive is a polyimide that can be removed by shining a laser. An example of a temporary carrier wafer is glass.
Step (F) is described in FIG. 23F. The structure shown in FIG. 23E is cleaved at the hydrogen plane using a mechanical force. Alternatively, an anneal could be used. Following this cleave, a CMP is done to planarize the surface. An oxide layer is then deposited. FIG. 23F shows the structure after all these steps are done, with the deposited oxide layer indicated as 2328. After the cleave, the p− Silicon region is indicated as 2322, the n− Silicon region is indicated as 2324, and the oxide isolation regions are indicated as 2326. Various other elements in FIG. 23F such as 2308, 2320, 2312, 2314, 2310, and 2316 have been described previously.
Step (G) is described in FIG. 23G. The structure shown in FIG. 23F is bonded to a glass substrate 2332 with an oxide layer 2330 using oxide-to-oxide bonding. Various elements in FIG. 23G such as 2308, 2326, 2322, 2324, 2312, 2314, and 2310 have been described previously. Oxide regions 2328 and 2330 are bonded together. The temporary carrier wafer from FIG. 23F is removed by shining a laser through it. A CMP process is then conducted to reach the surface of the gate electrode 2308. The oxide layer remaining is denoted as 2334.
Step (H) is described in FIG. 23H. Various elements in FIG. 23H such as 2312, 2314, 2328, 2330, 2332, 2334, 2326, 2324, and 2322 have been described previously. The dummy gate dielectric and dummy gate electrode are etched away in this step and a replacement gate dielectric 2336 and a replacement gate electrode 2338 are deposited and planarized with CMP. Examples of replacement gate dielectrics could be hafnium oxide or aluminum oxide while examples of replacement gate electrodes could be TiN or TaN or some other material. Contact formation, metallization and other steps for building a display/microdisplay are then conducted. It can be observed that after attachment to the glass substrate, no process step requires a processing temperature above 400° C.

[0116] FIGS. 24A-F describe an embodiment of this invention, where single-crystal Silicon junction-less transistors are constructed above glass substrates at a temperature approximately less than 400° C. An ion-cut process (which is a smart layer transfer process) is utilized for this purpose. This process could include several steps that occur in a sequence from Step (A) to Step (F). Many of these steps share common characteristics, features, modes of operation, etc. When identical reference numbers are used in different drawing figures, they are used to indicate analogous, similar or identical structures to enhance the understanding of the present invention by clarifying the relationships between the structures and embodiments presented in the various diagrams—particularly in relating analogous, similar or identical functionality to different physical structures.

Step (A) is illustrated in FIG. 24A. A glass substrate 2402 is taken and a layer of silicon oxide 2404 is deposited on the glass substrate 2402.

[0117] Step (B) is illustrated in FIG. 24B. A p− Silicon wafer 2406 is implanted with a n+ Silicon layer 2408 above which an oxide layer 2410 is deposited. A RTA or spike anneal or laser anneal is conducted to activate dopants. Following this, hydrogen is implanted into the wafer at a certain depth indicated by 2412. Alternatively, helium can be implanted.
Step (C) is illustrated in FIG. 24C. The structure shown in FIG. 24B is flipped and bonded onto the structure shown in FIG. 24A using oxide-to-oxide bonding. This bonded structure is cleaved at its hydrogen plane, after which a CMP is done. FIG. 24C shows the structure after all these processes are completed. 2414 indicates the n+ Si layer, while 2402, 2404, and 2410 have been described previously.
Step (D) is illustrated in FIG. 24D. A lithography and etch process is conducted to pattern the n+ Silicon layer 2414 in FIG. 24C to form n+ Silicon regions 2418 in FIG. 24D. The glass substrate is indicated as 2402 and the bonded oxide layers 2404 and 2410 are shown as well.
Step (E) is illustrated in FIG. 24E. A gate dielectric 2420 and gate electrode 2422 are deposited, following which a CMP is done. 2402 is as described previously. The n+ Si regions 2418 are not visible in this figure, since they are covered by the gate electrode 2422. Oxide regions 2404 and 2410 have been described previously.
Step (F) is illustrated in FIG. 24F. The gate dielectric 2420 and gate electrode 2422 from FIG. 24E are patterned and etched to form the structure shown in FIG. 24F. The gate dielectric after the etch process is indicated as 2424 while the gate electrode after the etch process is indicated as 2426. n+ Si regions are indicated as 2418 while the glass substrate is indicated as 2402. Oxide regions 2404 and 2410 have been described previously. It can be observed that a three-side gated junction-less transistor is formed at the end of the process described with respect of FIGS. 24A-F. Contacts, metallization and other steps for constructing a display/microdisplay are performed after the steps indicated by FIGS. 24A-F. It can be seen that the glass substrate is not exposed to temperatures greater than approximately 400° C. during any step of the above process for forming the junction-less transistor.

[0118] FIGS. 25A-D describe an embodiment of this invention, where amorphous Si or polysilicon junction-less transistors are constructed above glass substrates at a temperature less than 400° C. This process could include several steps that occur in a sequence from Step (A) to Step (D). Many of these steps share common characteristics, features, modes of operation, etc. When identical reference numbers are used in different drawing figures, they are used to indicate analogous, similar or identical structures to enhance the understanding of the present invention by clarifying the relationships between the structures and embodiments presented in the various diagrams—particularly in relating analogous, similar or identical functionality to different physical structures.

[0119] Step (A) is illustrated in FIG. 25A. A glass substrate 2502 is taken and a layer of silicon oxide 2504 is deposited on the glass substrate 2502. Following this deposition, a layer of n+ Si 2506 is deposited using low-pressure chemical vapor deposition (LPCVD) or plasma enhanced chemical vapor deposition (PECVD). This layer of n+ Si could optionally be hydrogenated.
Step (B) is illustrated in FIG. 25B. A lithography and etch process is conducted to pattern the n+ Silicon layer 2506 in FIG. 25A to form n+ Silicon regions 2518 in FIG. 25B. 2502 and 2504 have been described previously.
Step (C) is illustrated in FIG. 25C. A gate dielectric 2520 and gate electrode 2522 are deposited, following which a CMP is optionally done. 2502 is as described previously. The n+ Si regions 2518 are not visible in this figure, since they are covered by the gate electrode 2522.
Step (D) is illustrated in FIG. 25D. The gate dielectric 2520 and gate electrode 2522 from FIG. 25C are patterned and etched to form the structure shown in FIG. 25D. The gate dielectric after the etch process is indicated as 2524 while the gate electrode after the etch process is indicated as 2526. n+ Si regions are indicated as 2518 while the glass substrate is indicated as 2502. It can be observed that a three-side gated junction-less transistor is formed at the end of the process described with respect of FIGS. 25A-D. Contacts, metallization and other steps for constructing a display/microdisplay are performed after the steps indicated by FIGS. 25A-D. It can be seen that the glass substrate is not exposed to temperatures greater than 400° C. during any step of the above process for forming the junction-less transistor.

[0120] FIGS. 26A-C illustrate an embodiment of this invention, where a microdisplay is constructed using stacked RGB LEDs and control circuits are connected to each pixel with solder bumps. This process could include several steps that occur in a sequence from Step (A) to Step (C). Many of these steps share common characteristics, features, modes of operation, etc. When identical reference numbers are used in different drawing figures, they are used to indicate analogous, similar or identical structures to enhance the understanding of the present invention by clarifying the relationships between the structures and embodiments presented in the various diagrams—particularly in relating analogous, similar or identical functionality to different physical structures.

Step (A) is illustrated in FIG. 26A. Using procedures similar to FIG. 4A-S, the structure shown in FIG. 26A is constructed. Various elements of FIG. 26A are as follows:

2646—a glass substrate,

2644—an oxide layer, could be a conductive oxide such as ITO,

2634—an oxide layer, could be a conductive oxide such as ITO

2633—a an optional reflector, could be a Distributed Bragg Reflector or some other type of reflector,

2632—a P-type confinement layer that is used for a Blue LED (One example of a material for this region is GaN),

2630—a buffer layer that is typically used for a Blue LED (One example of a material for this region is AlGaN),

2628—a multiple quantum well used for a Blue LED (One example of materials for this region are InGaN/GaN),

2627—a N-type confinement layer that is used for a Blue LED (One example of a material for this region is GaN).

2648—an oxide layer, may be preferably a conductive metal oxide such as ITO,

2622—an oxide layer, may be preferably a conductive metal oxide such as ITO,

2621—an optional reflector (for example, a Distributed Bragg Reflector),

2620—a P-type confinement layer that is used for a Green LED (One example of a material for this region is GaN),

2618—a buffer layer that is typically used for a Green LED (One example of a material for this region is AlGaN),

2616—a multiple quantum well used for a Green LED (One example of materials for this region are InGaN/GaN),

2615—a N-type confinement layer that is used for a Green LED (One example of a material for this region is GaN),

2652—an oxide layer, may be preferably a conductive metal oxide such as ITO,

2610—an oxide layer, may be preferably a conductive metal oxide such as ITO,

2609—an optional reflector (for example, a Distributed Bragg Reflector),

2608—a P-type confinement layer used for a Red LED (One example of a material for this region is AlInGaP),

2606—a multiple quantum well used for a Red LED (One example of materials for this region are AlInGaP/GaInP),

2604—a P-type confinement layer used for a Red LED (One example of a material for this region is AlInGaP),

2656—an oxide layer, may be preferably a transparent conductive metal oxide such as ITO, and

2658—a reflector (for example, aluminum or silver).

[0121] Step (B) is illustrated in FIG. 26B. Via holes 2662 are etched to the substrate layer 2646 to isolate different pixels in the microdisplay/display. Also, via holes 2660 are etched to make contacts to various layers of the stack. These via holes may be preferably not filled. An alternative is to fill the via holes with a compatible oxide and planarize the surface with CMP. Various elements in FIG. 26B such as 2646, 2644, 2634, 2633, 2632, 2630, 2628, 2627, 2648, 2622, 2621, 2620, 2618, 2616, 2615, 2652, 2610, 2609, 2608, 2606, 2604, 2656 and 2658 have been described previously.
Step (C) is illustrated in FIG. 26C. Using procedures similar to those described in respect to FIGS. 4A-S, the via holes 2660 have contacts 2664 (for example, with Aluminum) made to them. Also, using procedures similar to those described in FIGS. 4A-S, nickel layers 2666, solder layers 2668, and a silicon sub-mount 2670 with circuits integrated on them are constructed. The silicon sub-mount 2670 has transistors to control each pixel in the microdisplay/display. Various elements in FIG. 26C such as 2646, 2644, 2634, 2633, 2632, 2630, 2628, 2627, 2648, 2622, 2621, 2620, 2618, 2616, 2615, 2652, 2610, 2609, 2608, 2606, 2604, 2656, 2660, 2662, and 2658 have been described previously.
It can be seen that the structure shown in FIG. 26C can have each pixel emit a certain color of light by tuning the voltage given to the red, green and blue layers within each pixel. This microdisplay may be constructed using the ion-cut technology, a smart layer transfer technique.

[0122] FIGS. 27A-D illustrate an embodiment of this invention, where a microdisplay is constructed using stacked RGB LEDs and control circuits are integrated with the RGB LED stack. This process could include several steps that occur in a sequence from Step (A) to Step (D). Many of these steps share common characteristics, features, modes of operation, etc. When identical reference numbers are used in different drawing figures, they are used to indicate analogous, similar or identical structures to enhance the understanding of the present invention by clarifying the relationships between the structures and embodiments presented in the various diagrams—particularly in relating analogous, similar or identical functionality to different physical structures.

Step (A) is illustrated in FIG. 27A. Using procedures similar to those illustrated in FIGS. 4A-S, the structure shown in FIG. 27A is constructed. Various elements of FIG. 27A are as follows:

2746—a glass substrate,

2744—an oxide layer, could be a conductive oxide such as ITO,

2734—an oxide layer, could be a conductive oxide such as ITO,

2733—a an optional reflector (e.g., a Distributed Bragg Reflector or some other type of reflector),

2732—a P-type confinement layer that is used for a Blue LED (One example of a material for this region is GaN),

2730—a buffer layer that is typically used for a Blue LED (One example of a material for this region is AlGaN),

2728—a multiple quantum well used for a Blue LED (One example of materials for this region are InGaN/GaN),

2727—a N-type confinement layer that is used for a Blue LED (One example of a material for this region is GaN),

2748—an oxide layer, may be preferably a conductive metal oxide such as ITO,

2722—an oxide layer, may be preferably a conductive metal oxide such as ITO,

2721—an optional reflector (e.g., a Distributed Bragg Reflector),

2720—a P-type confinement layer that is used for a Green LED (One example of a material for this region is GaN),

2718—a buffer layer that is typically used for a Green LED (One example of a material for this region is AlGaN),

2716—a multiple quantum well used for a Green LED (One example of materials for this region are InGaN/GaN),

2715—a N-type confinement layer that is used for a Green LED (One example of a material for this region is GaN),

2752—an oxide layer, may be preferably a conductive metal oxide such as ITO,

2710—an oxide layer, may be preferably a conductive metal oxide such as ITO,

2709—an optional reflector (e.g., a Distributed Bragg Reflector),

2708—a P-type confinement layer used for a Red LED (One example of a material for this region is AlInGaP),

2706—a multiple quantum well used for a Red LED (One example of materials for this region are AlInGaP/GaInP),

2704—a P-type confinement layer used for a Red LED (One example of a material for this region is AlInGaP),

2756—an oxide layer, may be preferably a transparent conductive metal oxide such as ITO,

2758—a reflector (e.g., aluminum or silver).

[0123] Step (B) is illustrated in FIG. 27B. Via holes 2762 are etched to the substrate layer 2746 to isolate different pixels in the microdisplay/display. Also, via holes 2760 are etched to make contacts to various layers of the stack. These via holes may be preferably filled with a compatible oxide and the surface can be planarized with CMP. Various elements of FIG. 27B such as 2746, 2744, 2734, 2733, 2732, 2730, 2728, 2727, 2748, 2722, 2721, 2720, 2718, 2716, 2715, 2752, 2710, 2709, 2708, 2706, 2704, 2756 and 2758 have been described previously.
Step (C) is illustrated in FIG. 27C. Metal 2764 (for example) is constructed within the via holes 2760 using procedures similar to those described in respect to FIGS. 4A-S. Following this construction, an oxide layer 2766 is deposited. Various elements of FIG. 27C such as 2746, 2744, 2734, 2733, 2732, 2730, 2728, 2727, 2748, 2722, 2721, 2720, 2718, 2716, 2715, 2752, 2710, 2709, 2708, 2706, 2704, 2756, 2760, 2762 and 2758 have been described previously.
Step (D) is illustrated in FIG. 27D. Using procedures described in co-pending U.S. patent application Ser. No. 12/901,890, the content of which is incorporated herein by reference, a single crystal silicon transistor layer 2768 can be monolithically integrated using ion-cut technology atop the structure shown in FIG. 27C. This transistor layer 2768 is connected to various contacts of the stacked LED layers (not shown in the figure for simplicity). Following this connection, nickel layer 2770 is constructed and solder layer 2772 is constructed. The packaging process then is conducted where the structure shown in FIG. 27D is connected to a silicon sub-mount.
It can be seen that the structure shown in FIG. 27D can have each pixel emit a certain color of light by tuning the voltage given to the red, green and blue layers within each pixel. This microdisplay is constructed using the ion-cut technology, a smart layer transfer technique. This process where transistors are integrated monolithically atop the stacked RGB display can be applied to the LED concepts disclosed in association with FIGS. 4-10.

[0124] The embodiments of this invention described in FIGS. 26-27 may enable novel implementations of “smart-lighting concepts” (also known as visible light communications) that are described in “Switching LEDs on and off to enlighten wireless communications”, EETimes, June 2010 by R. Colin Johnson. For these prior art smart lighting concepts, LED lights could be turned on and off faster than the eye can react, so signaling or communication of information with these LED lights is possible. An embodiment of this invention involves designing the displays/microdisplays described in FIGS. 26-27 to transmit information, by modulating wavelength of each pixel and frequency of switching each pixel on or off. One could thus transmit a high bandwidth through the visible light communication link compared to a LED, since each pixel could emit its own information stream, compared to just one information stream for a standard LED. The stacked RGB LED embodiment described in FIGS. 4A-S could also provide a improved smart-light than prior art since it allows wavelength tunability besides the ability to turn the LED on and off faster than the eye can react.

NuSolar Technology:

[0125] Multijunction solar cells are constructed of multiple p-n junctions stacked atop each other. Multi-junction solar cells are often constructed today as shown in FIG. 18A. A Germanium substrate 2800 is taken and multiple layers are grown epitaxially atop it. The first epitaxial layer is a p-type doped Ge back-surface field (BSF) layer, indicated as 2802. Above it, a n-type doped Ge base layer 2804 is epitaxially grown. A InGaP hetero layer 2806 is grown above this. Following this growth, a n-type InGaAs buffer layer 2808 is grown. A tunnel junction 2810 is grown atop it. The layers 2802, 2804, 2806, and 2808 form the bottom Ge cell 2838 of the multi-junction solar cell described in FIG. 18A. Above this bottom cell and the tunnel junction 2810, a middle cell constructed of InGaAs is epitaxially grown, and is indicated as 2836. The InGaAs middle cell has the following 4 layers: a p+ doped back surface field (BSF) layer 2812 of InGaP, a p doped base layer 2814 of InGaAs, a n doped emitter layer 2816 of InGaAs, and a n+ doped window layer 2818 of InGaP. Above this InGaAs middle cell 2836, a tunnel junction 2820 is grown epitaxially and above this, another cell, constructed of InGaP, and called a top cell 2834 is epitaxially grown. This top cell 2834 has the following layers: a p+ doped back-surface field (BSF) layer of AlInGaP 2822, a p doped base layer of InGaP 2824, a n doped emitter layer of InGaP 2826 and a n+ doped window layer of AlInP 2828. Above this layer of AlInP 2828, a GaAs layer 2830 is epitaxially grown, Aluminum contacts 2840 are deposited and an anti-reflection (AR) coating 2832 is formed. The purpose of back-surface field (BSF) layers in the multi-junction solar cell depicted in FIG. 18A is to reduce scattering of carriers towards the tunnel junctions. The purpose of the window layers is to reduce surface recombination velocity. Both the BSF layers and window layers are heterojunctions that help achieve the above mentioned purposes. Tunnel junctions help achieve good ohmic contact between various junctions in the multi-junction cell. It can be observed that the bottom, middle and top cells in the multi-junction cell are arranged in the order of increasing band-gap and help capture different wavelengths of the sun's spectrum.

[0126] FIG. 28B shows the power spectrum of the sun vs. photon energy. It can be seen that the sun's radiation has energies in between 0.6 eV and 3.5 eV. Unfortunately though, the multi-junction solar cell shown in FIG. 28A has band-gaps not covering the solar spectrum (band-gap of cells varies from 0.65 eV to 1.86 eV).

[0127] FIG. 28C shows the solar spectrum and indicates the fraction of solar power converted to electricity by the multi-junction solar cell from FIG. 28A. It can be observed from FIG. 28C that a good portion of the solar spectrum is not converted to electricity. This is largely because the band-gap of various cells of the multi-junction solar cell does not cover the entire solar spectrum.

[0128] FIGS. 29A-H show a process flow for constructing multijunction solar cells using a layer transfer flow. Although FIGS. 29A-H show a process flow for stacking two cells with two different bandgaps, it is fairly general, and can be extended to processes involving more than two cells as well. This process could include several steps that occur in a sequence from Step (A) to Step (H). Many of these steps share common characteristics, features, modes of operation, etc. When identical reference numbers are used in different drawing figures, they are used to indicate analogous, similar or identical structures to enhance the understanding of the present invention by clarifying the relationships between the structures and embodiments presented in the various diagrams—particularly in relating analogous, similar or identical functionality to different physical structures.

[0129] Step (A) is illustrated in FIG. 29A. Three wafers 2920, 2940 and 2946 have different materials grown or deposited above them. Materials from these three wafers 2920, 2940 and 2946 are stacked using layer transfer to construct the multi-junction solar cell described in this embodiment of the invention. The wafer 2946 includes a substrate C denoted as 2942 above which an oxide layer C, denoted as 2944, is deposited. Examples of materials for 2942 include heavily doped silicon and the oxide layer C 2944 could preferably be a conductive metal oxide such as ITO. The wafer 2940 includes a substrate for material system B, also called substrate B 2938 (e.g., InP or GaAs), a buffer layer 2936, a p++ contact layer B (e.g., InGaP) 2934, a p+ back-surface field (BSF) layer B (e.g., InGaP) 2932, a p base layer B (eg. InGaAs) 2930, a n emitter layer B (e.g., InGaAs) 2928, a n+ window layer B (e.g., InGaP) 2926, a n++ contact layer B (e.g., InGaP) 2924 and an oxide layer B (e.g., ITO) 2922. The wafer 2920 includes a substrate for material system A, also called substrate A 2918 (e.g., InP or GaAs), a buffer layer 2916, a p++ contact layer A (eg. AlInGaP) 2914, a p+ back-surface field (BSF) layer A (e.g., AlInGaP) 2912, a p-base layer A (e.g., InGaP) 2910, a n-emitter layer A (e.g., InGaP) 2918, a n+ window layer A (e.g., AlInP) 2916, a n++ contact layer A (e.g., AlInP) 2914 and an oxide layer A (e.g., ITO) 2912. Various other materials and material systems can be used instead of the examples of materials listed above.
Step (B) is illustrated in FIG. 29B. Hydrogen is implanted into the structure 2920 of FIG. 29A at a certain depth indicated by 2948. Various other elements of FIG. 29B such as 2902, 2904, 2906, 2908, 2910, 2912, 2914, 2916, and 2918 have been described previously. Alternatively, Helium can be implanted instead of hydrogen. Various other atomic species can be implanted.
Step (C) is illustrated in FIG. 29C. The structure shown in FIG. 29B is flipped and bonded atop the structure indicated as 2946 in FIG. 29A. Various elements in FIG. 29C such as 2902, 2904, 2906, 2908, 2910, 2912, 2914, 2916, 2944, 2942, and 2918 have been described previously.
Step (D) is illustrated in FIG. 29D. The structure shown in FIG. 29C may be cleaved at its hydrogen plane 2948 preferably using a sideways mechanical force. Alternatively, an anneal could be used. A CMP is then done to planarize the surface to produce p++ contact layer A 2915. Various other elements in FIG. 29D such as 2942, 2944, 2902, 2904, 2906, 2908, 2910, and 2912 have been described previously. The substrate 2918 from FIG. 29C removed by cleaving may be reused.
Step (E) is illustrated in FIG. 29E. An oxide layer 2950 is deposited atop the structure shown in FIG. 29D. This oxide layer 2950 may be preferably a conductive metal oxide such as ITO, although an insulating oxide could also be used. Various elements in FIG. 29E such as 2942, 2944, 2902, 2904, 2906, 2908, 2910, 2915, and 2912 have been described previously.
Step (F) is illustrated using FIG. 29F. The structure indicated as 2940 in FIG. 29A is implanted with hydrogen at a certain depth 2952. Alternatively, Helium or some other atomic species can be used. Various elements of FIG. 29F such as 2922, 2924, 2926, 2928, 2930, 2932, 2934, 2936, and 2938 have been indicated previously.
Step (G) is illustrated in FIG. 29G. The structure shown in FIG. 29F is flipped and bonded onto the structure shown in FIG. 29E using oxide-to-oxide bonding. Various elements in FIG. 29G such as 2942, 2944, 2902, 2904, 2906, 2908, 2910, 2912, 2915, 2950, 2922, 2924, 2926, 2928, 2930, 2932, 2934, 2936, 2952, and 2938 have been indicated previously.
Step (H) is illustrated in FIG. 29H. The structure shown in FIG. 29G is cleaved at its hydrogen plane 2952. A CMP is then done to planarize the surface and produces the p++ contact layer B indicated as 2935 in FIG. 29H. Above this, an oxide layer 2952 (e.g., ITO) is deposited. The substrate B indicated as 2938 in FIG. 29G can be reused after cleave. Various other elements in FIG. 29H such as 2942, 2944, 2902, 2904, 2906, 2908, 2910, 2912, 2915, 2950, 2922, 2924, 2926, 2928, 2930, and 2932 have been indicated previously.
After completing steps (A) to (H), contacts and packaging are then done. One could make contacts to the top and bottom of the stack shown in FIG. 29H using one front contact to ITO layer 2954 and one back contact to the heavily doped Si substrate 2942. Alternatively, contacts could be made to each cell of the stack shown in FIG. 29H as described in respect to FIG. 4A-S. While FIGS. 29A-H show two cells in series for the multijunction solar cell, the steps shown in the above description can be repeated for stacking more cells that could be constructed of various band gaps. The advantage of the process shown in FIG. 29A-H is that all processes for stacking are done at temperatures less than 400° C., and could even be done at less than 250° C. Therefore, thermal expansion co-efficient mismatch may be substantially mitigated. Likewise, lattice mismatch may be substantially mitigated as well. Therefore, various materials such as GaN, Ge, InGaP and others which have widely different thermal expansion co-efficients and lattice constant can be stacked atop each other. This flexibility in use of different materials may enable a full spectrum solar cell or a solar cell that covers a increased band within the solar spectrum than the prior art cell shown in FIG. 28A.

[0130] FIGS. 30A-D show a process flow for constructing another embodiment of this invention, a multi-junction solar cell using a smart layer transfer technique (ion-cut). This process may include several steps that occur in a sequence from Step (A) to Step (D). Many of these steps share common characteristics, features, modes of operation, etc. When identical reference numbers are used in different drawing figures, they are used to indicate analogous, similar or identical structures to enhance the understanding of the present invention by clarifying the relationships between the structures and embodiments presented in the various diagrams—particularly in relating analogous, similar or identical functionality to different physical structures.

[0131] Step (A) is illustrated in FIG. 30A. It shows a multi-junction solar cell constructed using epitaxial growth on a heavily doped Ge substrate, as described in the prior art multi-junction solar cell of FIG. 28A. The structure shown in FIG. 30A includes the following components:
3002—a Ge substrate,
3004—a p-type Ge BSF layer,
3006—a n-type Ge base layer,
3008—a InGaP hetero layer,
3010—a n-type InGaAs buffer layer,
3012—a tunnel junction,
3014—a p+ InGaP BSF layer,
3016—a p-type InGaAs base layer,
3018—a n-type InGaAs emitter layer,
3020—a n+ InGaP window layer,
3022—a tunnel junction,
3024—a p+ AlInGaP BSF layer,
3026—a p-type InGaP BSF layer,
3028—a n-type InGaP emitter layer,
3030—a n+-type AlInP window layer, and
3032—an oxide layer, may be preferably of a conductive metal oxide such as ITO. Further details of each of these layers is provided in the description of FIG. 28A.
Step (B) is illustrated in FIG. 30B. Above a sapphire or SiC or bulk GaN substrate 3034, various layers such as buffer layer 3036, a n+ GaN layer 3038, a n InGaN layer 3040, a p-type InGaN layer 3042 and a p+ GaN layer 3044 are epitaxially grown. Following this growth, an oxide layer 3046 may be constructed preferably of a transparent conducting oxide such as, for example, ITO is deposited. Hydrogen is implanted into this structure at a certain depth indicated as 3048. Alternatively, Helium or some other atomic species can be implanted.
Step (C) is illustrated in FIG. 30C. The structure shown in FIG. 30B is flipped and bonded atop the structure shown in FIG. 30A using oxide-to-oxide bonding. Various elements in FIG. 30C such as 3002, 3004, 3006, 3008, 3010, 3012, 3014, 3016, 3018, 3020, 3022, 3024, 3026, 3028, 3030, 3032, 3048, 3046, 3044, 3042, 3040, 3038, 3036, and 3034 have been described previously.
Step (D) is illustrated using FIG. 30D. The structure shown in FIG. 30C is cleaved at its hydrogen plane 3048. A CMP process is then conducted to result in the n+ GaN layer 3041. Various elements in FIG. 30D such as 3002, 3004, 3006, 3008, 3010, 3012, 3014, 3016, 3018, 3020, 3022, 3024, 3026, 3028, 3030, 3032, 3046, 3044, 3042, and 3038 have been described previously.
After completing steps (A) to (D), contacts and packaging are then done. Contacts may be made to the top and bottom of the stack shown in FIG. 30D, for example, one front contact to the n+ GaN layer 3041 and one back contact to the heavily doped Ge substrate 3002. Alternatively, contacts could be made to each cell of the stack shown in FIG. 30D as described in FIGS. 4A-S.

[0132] FIGS. 29-30 described solar cells with layer transfer processes. Although not shown in FIG. 29-30, it will be clear to those skilled in the art based on the present disclosure that front and back reflectors could be used to increase optical path length of the solar cell and harness more energy. Various other light-trapping approaches could be utilized to boost efficiency as well.

[0133] An aspect of various embodiments of this invention is the ability to cleave wafers and bond wafers at lower temperatures (e.g., less than 400° C. or even less than 250° C.). In co-pending U.S. patent application Ser. No. 12/901,890 the content of which is incorporated by reference, several techniques to reduce temperatures for cleave and bond processes are described. These techniques are herein incorporated in this document by reference.

[0134] Several material systems have been quoted as examples for various embodiments of this invention in this patent application. It will be clear to one skilled in the art based on the present disclosure that various other material systems and configurations can also be used without violating the concepts described. It will also be appreciated by persons of ordinary skill in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and sub-combinations of the various features described hereinabove as well as modifications and variations which would occur to such skilled persons upon reading the foregoing description. Thus the invention is to be limited only by the appended claims.