Maskless photolithography using plasma displays

[0001] This application claims the benefit of U.S. Provisional Application No. 60/301,218, filed Jun. 27, 2001, and incorporated herein by reference.

[0002] The present invention relates to photolithography systems and methods, specifically, to maskless photolithography devices and methods for creating 2-D and 3-D patterns using a plasma display.

[0003] Photolithography systems are known in the art that direct light beams onto a photosensitive surface covered by a mask, etching a desired pattern on the substrate corresponding to the void areas of the mask. Maskless photolithography systems are also known in the art as described in Singh-Gasson, Sangeet et al., Nature Biotechnology 17,974-78, 1999. The system described in that article uses an off-axis light source coupled with a digital mirror device to fabricate DNA chips containing probes for genes or other solid phase combinatorial chemistry to be performed in high-density microarrays.

[0004] A number of patents also exist which relate to maskless photolithography systems, including U.S. Pat. Nos. 5,870,176; 6,060,224; 6,177,980; and 6,251,550; all of which are incorporated herein by reference.

[0005] Plasma displays, as known in the art, generally consist of two glass plates, each containing a network of parallel electrodes and intersecting address electrodes, sealed to form a discharge envelope filled with a neon and xenon gas mixture. A gas discharge plasma is created by applying an electric field between the electrodes. The plasma generates ultraviolet light, which in turn excites a phosphor coating inside the glass envelope to generate a pixel of light.

[0006] A number of patents directed to plasma display devices exist, including U.S. Pat. Nos. 6,376,986, 6,362,799, 6,344,715, and 5,661,500; all of which are incorporated herein by reference.

[0007] While the previously described maskless photolithography systems address several of the problems associated with mask based photolithography systems, such as distortion and uniformity of images, problems still arise. Notably, maskless photolithography systems are complex and require complex optical systems and micromirror arrays.

[0008] Typically, maskless photolithography systems require two sets of complex optics: one set of optics to condition light emanating from a light source and directed to a spatial light modulator and a second set of optics to further conditions and direct the light reflected from the spatial light modulator. Consequently, two sets of complex optics and a light source must be mechanically aligned and maintained for a maskless system. For the digital mirror device, digital shifting of a mask pattern has been proposed to resolve micromirror to object alignment, this technique cannot be used to align the light source in relation to the micromirror array.

[0009] In addition, the use of micromirror arrays in maskless photolithography systems poses other problems. Micromirror arrays are specialized, custom devices that require complex driving circuitry and are subject to a phenomenon known as stiction. Stiction occurs when the individual mirrors in a micromirror array stick in a specific orientation if left in that position for an extended period. Consequently, a higher voltage needs to be applied to the mirror drive to point the mirror in another desired direction. Thus, the micromirror array consumes more power than normal and the reliability of the mirror can be affected.

[0010] Accordingly, there is a need in the art for a method and system for maskless photolithography to eliminate unnecessary complexity and provide a direct radiation means to provide a more effective way to fabricate custom devices. This system needs to eliminate redundant optical fixtures required by micromirror array based maskless photolithography systems that rely on reflection of light generated by a separated light source. The system needs to provide a single device for generating patterned light and directing the patterned light to an object, thereby eliminating the need for a separate light source and associated optics. In addition, the system needs to combine ease of use, reconfigurability, and the ability to provide coarse manual alignment and automated fine alignment of mask patterns. In summary, the system needs to combine all the advantages of a maskless photolithography system with the advantages of pixel addressable, UV emitting plasma display to eliminate complexity and provide a simpler, more cost effective maskless photography system.

[0011] In view of the foregoing deficiencies of the prior art, it is an object of the present invention to provide a maskless photolithography system using a plasma display for creating patterns on objects.

[0012] It is another object of the present invention to provide a method of maskless photolithography that uses a plasma display device to generate and direct patterned light on an object.

[0013] It is still another object of the present invention to provide a plasma device display that lacks phosphor coatings, whereby the plasma device generates ultraviolet (UV) light directly.

[0014] It is still another object of the present invention to provide a positioning fixture; selectively movable in a three dimensions to accurately position a substrate for maskless photolithography using a plasma device as a source of patterned light.

[0015] It is another object of the present invention to provide plasma based, maskless photolithography system for creating micro and macro three-dimensional structures.

[0016] To achieve these objects, a system and method are provided to create two dimensional and three dimensional structures using a maskless photolithography system that is directly reconfigurable and does not require masks, templates or stencils to create each of the planes or layers on a multi layer two-dimensional or three dimensional structure. In an embodiment, the invention uses a plasma display device comprising ultraviolet (UV) light emitting pixels to illuminate a substrate that has photoreactive compounds applied to the exposed surface. The plasma display comprises an array of miniature plasma discharge cells, or pixels, that combine light generation and modulation to create individually addressable UV light pixels. Each element of an array of UV light pixels in a plasma display is activated by a network of electrodes and can be selectively turned on and off to create a desired two-dimensional light pattern. The emitted light beam, comprising light from each individual UV light pixel, can be directed towards a target substrate coated with a photoreactive compound. The UV light reacts with the photoreactive compound to create a pattern on the substrate corresponding to the illumination pattern.

[0017] The desired pattern is designed and stored using conventional computer aided drawing techniques and is used to control the firing of the individual pixels of the plasma display to create the corresponding desired mask pattern.

[0018] In addition, an alignment fixture, movable in three dimensions, for mounting of the substrate is provided. The alignment fixture allows the affixed substrate to be moved in three dimensions, providing alignment in two, coplanar dimensions and a third dimension perpendicular to the two coplanar dimensions. By providing alignment in the third dimensional direction, the invention advantageously provides the capability to produce three-dimensional structures on a substrate.

[0019] The advantages of the invention are numerous. One significant advantage is that the tasks of light generation and patterning in a maskless photolithography system can be combined into a single plasma display device. Thus, need for a separate light source and optics stage as in conventional micromirror array-based systems is eliminated, resulting in a simpler, more cost efficient system. Another advantage is that a plasma display device is inherently more reliable than a micromirror device. Yet another advantage is that plasma devices are readily available as off-the-shelf devices, complete with readily adaptable control electronics.

[0020] Still another advantage is the ability to use the invention as a reconfigurable, rapid prototyping tool for creating two-dimensional and three-dimensional micro and macroscopic objects. Yet another advantage of the invention is that it provides the ability to reduce prototyping costs and enable devices to be fabricated more quickly with less risk. Still another advantage of the current invention is a reduction in cost for prototyping activities realized by the elimination of physical masks. Yet another advantage of the current invention is that pattern generation can be performed optically without having to use expensive vacuum system required by conventional mask-based photolithography. A particular advantage of the current invention is the ability to create three-dimensional devices using an alignment stage to selectively expose successive layers in a substrate.

[0021] As a result, any arbitrary micro or macroscopic structure can easily and quickly be created in substrates such as polymers, metals, or ceramics. Patterns such as microfluidic networks, thin film devices, hybrid material devices, micro electromechanical machines (MEMs), and combinations of the above mentioned devices can be created using the reconfigurable, application specific photolithography system disclosed. In addition, because plasma arrays are available in large formats, large-scale objects can be patterned in one exposure, obviating the need to sequentially expose smaller areas until the entire surface is exposed.

[0022] All patents, patent applications, provisional applications, and publications referred to or cited herein, or from which a claim for benefit of priority has been made, are incorporated herein by reference in their entirety to the extent they are not inconsistent with the explicit teachings of this specification.

[0023] Other aspects and advantages of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings, illustrating, by way of example, the principles of the invention.

[0024] In order that the manner in which the above recited and other advantages and objects of the invention are obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof, which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:

[0025] FIG. 1 illustrates a maskless photolithography system using a plasma display device.

[0026] FIG. 2 illustrates a cross sectional view of a plasma display device.

[0027] FIG. 3 is a flow chart illustrating a maskless photolithography method according to an embodiment of the present invention.

[0028] It should be understood that in certain situations for reasons of computational efficiency or ease of maintenance, the ordering and relationships of the blocks of the illustrated flow charts could be rearranged or re-associated by one skilled in the art. While the present invention will be described with reference to the details of the embodiments of the invention shown in the drawings, these details are not intended to limit the scope of the invention.

[0029] Reference will now be made in detail to the embodiments consistent with the invention, examples of which are illustrated in the accompanying drawings. First, briefly, the invention is a system and method to create two dimensional and three dimensional structures using a maskless photolithography system that is directly reconfigurable and does not require masks, templates or stencils to create each of the planes or layers on a multi layer two-dimensional or three dimensional structure. In an embodiment, the invention uses a plasma display device comprising ultraviolet (UV) light emitting pixels to illuminate a substrate that has photoreactive compounds applied to the exposed surface. The plasma display comprises an array of miniature plasma discharge cells, or pixels, that combine light generation and modulation to create individually addressable UV light pixels. Each element of an array of UV light pixels in a plasma display is activated by a network of electrodes and can be selectively turned on and off to create a desired two-dimensional light pattern. The emitted light beam, comprising light from each individual UV light pixel, can be directed towards a target substrate coated with a photoreactive compound. The UV light reacts with photoreactive compound to create a pattern on the substrate corresponding to the illumination pattern. In addition, an alignment fixture for mounting of the substrate allows the substrate to be moved in three dimensions, providing alignment in two, coplanar dimensions and the capability to produce three dimensional structures by aligning the substrate in a third dimension perpendicular to the two coplanar dimensions.

[0030] I. Maskless Photolithography System

[0031] Referring now to FIG. 1, an embodiment of the current invention includes a plasma display device 10, a computer system 12, a lens system 16, a substrate 20, mounted on a movable alignment fixture 22, and an optical viewer 26. A layer of photoreactive chemicals 21 is disposed on the substrate 20.

[0032] As shown, plasma display device 10 generates a beam of light, or patterned light beam 26, wherein each pixel of the plasma display corresponds to a pixel of the mask pattern. Plasma display device 10 is controlled by computer system 16 over signal line(s) 14 to generate light according to a desired mask pattern stored in memory. In addition, computer system 12 can shift the desired mask pattern in two dimensions to align the pattern with the substrate 20 mounted on movable alignment fixture 22. Precise pattern alignments are made electronically by shifting the mask pattern information provided to the plasma display device 10 such that the image directed to the substrate is translated to correct for misalignment. For example, if the mask pattern needs to be shifted to the right one pixel width to be properly aligned on the substrate, the computer compensates for the misalignment by shifting the mask pattern one pixel width to the right.

[0033] The patterned light beam radiated from plasma display device 10 can be selectively filtered by inserting or removing filter 18 from patterned light beam 24. Filtering can take place at any point along the light beam path to prevent exposure during alignment. A lens system 16 can collimate and condition the light beam as desired. After passing through lens system 16, patterned light beam 24 impinges on a layer of photoreactive chemicals 21 applied to substrate 20, thereby creating a pattern on substrate 20 by producing a reaction between the layer of photoreactive chemicals 21 and substrate 20. Alternatively, a photoresist chemical could be applied to substrate 20 to etch areas of substrate 20 not masked off by the mask pattern during an exposure.

[0034] The mask pattern described above is a programmable mask pattern generated with the use of computer aided design and is resident on computer system 12. Accordingly, the mask pattern to be transferred to the layer of photoreactive chemicals 21 and substrate 20 is a selectively programmable mask pattern. Thus, with a programmable mask pattern, any portion of the pattern on the substrate 20 can be manipulated and/or changed as desired for rendering of desired changes as maybe needed, furthermore, on a significantly reduced cycle time.

[0035] Advantageously, the present system optionally allows an image to be shifted electronically to provide fine alignment of the pattern on substrate 20. The mask pattern is digitally shifted according to alignment information in one or more directions for achieving a desired mask alignment on substrate 20. Adjustments in alignment are carried out electronically in the mask bit pattern information provided to the pixel array. As a result, fine adjustments in pattern alignment can thus be easily accomplished.

[0036] Movable alignment fixture 22, in conjunction with optical viewer 26, provides the capability to initially align substrate 20 under patterned light beam 24 using mechanical alignment mechanisms (not shown) to align substrate 20 in three dimensions. The mechanical alignment system may include gears, pulleys, belts, chains, rods, screws, hydraulics, pneumatics, piezo movements, or combinations thereof as known in the art to support and move an object in three dimensions. While performing alignment procedures, filter 11 is inserted in patterned light beam 24 to filter out the wavelengths of light that react with the layer of photoreactive chemicals 21 on substrate 20. Optical viewer 26, provides a means to monitor the positioning of substrate during manual alignment. While providing alignment in coplanar first and second dimensions, alignment fixture 22 advantageously provides alignment in a direction perpendicular to the coplanar first and second dimensions, allowing fabrication of three-dimensional objects. For example, to gain more control over sidewall profiles or to produce complicated structures, multiple layers of substrate 20 can be sequentially exposed by aligning substrate 20 in the third dimension to create three-dimensional features. Coupled with optional computer controlled alignment of the desired pattern, the invention provides the capability to quickly manually align substrate 20 under patterned light beam 24 and allows computer system 16 to automatically finely tune the alignment before exposing layer of photoreactive chemicals 21 on substrate 20.

[0037] II. Plasma Display

[0038] The plasma display 10 of FIG. 1 will now be described in more detail with reference to FIG. 2. FIG. 2 illustrates the structure of a conventional AC memory-type plasma display panel in cross section, as is known in the art. As illustrated, this type of plasma display panel has a front substrate 31 and a rear substrate 30 which face each other and which are made of insulating material such as glass. A plurality of transparent surface discharge electrodes 32, formed from an ITO (Indium Tin Oxide) or Nesa film, are provided on the front substrate 31. In order to reduce the resistances of the surface discharge electrodes 32, trace electrodes 33 are formed on each surface discharge electrodes 32. Normally, Cr/Cu/Cr (chrome/copper/chrome) stacked thin film electrodes or Ag (silver) thick film electrodes are use as the trace electrodes 33.

[0039] A dielectric layer 34 is formed on the surface discharge electrodes 32 and the trace electrodes 33. In general, lead glass having a low melting point is used to form the dielectric layer 34. A magnesium oxide (MgO) film 35, having a thickness of approximately 0.5 micrometer (um) to 1.0 um is formed on the dielectric layer 34 by vacuum vapor deposition. The MgO film prevents damage to the dielectric layer 34 from ions generated by the plasma and allows the plasma display to operate at lower voltages. A plurality of data electrodes 36, which face the surface discharge electrodes 32 and which are substantially perpendicular to the surface discharge electrodes 32, are formed on the rear substrate 30. Silver (Ag) thick film electrodes are employed as the data electrodes 36. In addition partitions 38, extending upwardly from the rear substrate 30 are provided to separate each data electrode 36 and define a discharge space 39. The discharge space 39 is filled with a two-component gas, such as neon and xenon.

[0040] A white dielectric layer 37 is formed over the data electrodes 36, the rear substrate 30, and the sides of the partitions 38. The white dielectric layer 37 is formed by printing and sintering glass paste prepared by mixing a powder of white oxide (alumna, titanium oxide, or the like), and a powder of lead glass having a low melting point. The white dielectric layer 37 has the function of reflecting light and directing the light toward the front substrate 41.

[0041] Typically, phosphor layers (not shown) are formed on the white dielectric layer 37 to provide visible light. The phosphor layers are separate coatings of three phosphor materials applied onto the white dielectric layer 37 by thick film printing techniques. When excited by ultraviolet (UV) light, the phosphors emit red, green and blue visible light according the their respective radiation characteristic.

[0042] Plasma displays 37 generate light by applying a voltage to the surface discharge electrodes 32 at the position indicated by a corresponding data electrode 36. The surface electrodes 32 and data electrodes 36 are oriented at right angles to each other, forming a grid that provides the capability to create a plasma pixel of light at the intersections of the surface electrodes 32 and the data electrodes 36. When the voltage reaches a firing level, a plasma discharge occurs on the surface of the dielectric layer 34, at the intersection of the surface electrode 32 and the addressed data electrode 36, resulting in the emission of UV light. In turn, the UV light excites the phosphor coating causing the phosphors to emit visible light. The intensity of the UV light, and the resulting light emitted by the phosphors is controlled by varying the number and width of voltage pulses to the surface discharge electrodes 32.

[0043] For conventional applications, such as television monitors, plasma displays are specifically designed to emit visible light. However, according to the current invention, the UV light produced by a plasma device is used directly to expose substrates in maskless lithography systems. By eliminating the phosphor coating normally used in plasma displays, a display can be designed to emit only UV radiation, which is then used to expose a substrate having UV photoreactive compounds applied.

[0044] III. Method for Maskless Photolithography

[0045] A method of using the current invention to create patterned objects will now be described. It should be understood that in certain situations for reasons of computational efficiency or ease of maintenance, the ordering and relationships of the blocks of the illustrated flow charts could be rearranged or re-associated by one skilled in the art. Starting from step 50, a desired mask pattern is designed and stored on computer system 12 in step 52. Alternatively, mask pattern designs can be designed on other computer systems and imported into computer system 12. Next, in step 54, a substrate 20 is placed on alignment fixture 22 and coated with a layer of photoreactive chemicals 21 in step 56. Once the substrate is mounted in alignment fixture 22, computer system 12 can be instructed to provide the resident mask pattern information to plasma display device 10 as shown in step 58, and the plasma display device 10 responds by applying a firing voltage to each individual pixel to direct a patterned light beam 24 towards the substrate 20 according to the desired pattern. Next, alignment of the substrate is performed by placing filter 18 in patterned light beam 24 path according to step 60 and projecting patterned light beam 24 through lens system 16, and onto layer of photoreactive chemicals 21 and substrate 20.

[0046] With the desired pattern projected on substrate 20, alignment fixture 22 can be manually aligned in three dimensions according to step 62 by moving alignment fixture 22 to ensure that substrate 20 is properly located in patterned light beam 24. Proper alignment is verified by viewing the projected pattern on substrate 20 through optical viewer 26. Once substrate 20 is manually aligned, alignment information is optionally provided to computer system 12 and computer system 12 automatically adjusts the plasma display device 10 by shifting the pattern in two dimensions to attain proper alignment in optional step 64. Having aligned substrate 20, the layer of photoreactive chemicals 21 on substrate 20 is exposed in step 68 by removing filter 18 from patterned light beam 24 in step 66 and allowing the light to cause a reaction between layer of photoreactive chemicals 21 and substrate 20 for a required reaction time depending on the photoreactive chemicals used. For example, using standard Novolac positive photoresist, an exposure time of 60 seconds is used.

[0047] If further exposures are desired in step 70, such as required when creating three-dimensional objects, the above method is repeated by returning to step 52 until the desired object is fabricated. A new digital mask pattern is provided, another photoreactive coat is applied, and the substrate is realigned and re-exposed. Once the desired object has been created, the process ends in step 72.

[0048] IV Exemplary Embodiment

[0049] An example of the current invention using the system and method described above will now be presented. In an embodiment, the current invention is designed to be an integrated, reconfigurable, rapid prototyping maskless photography system. The system provides a computer, optics, a plasma display device, and integrated electronic components used to directly generate patterns for the exposure of photoresist and other photoimagable materials. This pattern is then transferred to the substrate surface and used to expose the photo-sensitive material required in the user's fabrication process.

[0050] A personal computer, operably connected to a plasma display device, is used to provide mask patterns. The mask patterns are generated in the computer and then transferred to the plasma display to provide the optical pattern for exposure. The pattern is illuminated on a substrate and is observed using an optical microscope. The microscope is then used to monitor alignment of the pattern on the substrate. Alignment is controlled through the use of a course alignment stage provided by a mechanically movable substrate mounting alignment fixture, combined with a fine, electronic alignment stage. This fine alignment stage is computer controlled and aligns the mask pattern radiated from the plasma display instead of moving the alignment fixture, thereby offering exceptional accuracy and repeatability. Once alignment is complete, the substrate can be exposed. Multiple layers can be created using an alignment stage movable in a direction parallel to the light beam.

[0051] In addition, according to the invention, three-dimensional patterns can be created using the three-dimensional alignment capabilities disclosed above. For example, patterning using thick photo resist or multiple layer patterning of individual photoresist layers. These techniques can be use to provide either a photomask for subsequent etching of substrate materials, or, if the photopolymer is compatible with the chemistry used in the device, the fabricated features can be used as part of the device itself.

[0052] Furthermore, many other variations are possible using the present inventive system and method. For example, the invention can be used for rapidly creating micro electro-mechanical (MEMs) devices, creating artificial receptors chips, creating integrated circuit patterns of conducting polymers, photo-induced etching of semiconductor material, creating patterns in photosensitve or photo-chromic glass, photoselective deposition of metal onto a substrate, creating integrated microsensor arrays and fluid delivery networks, chemical vapor deposition, thin film fabrication, gray scale photolithography, large area pattern expression, and creating molecular based detection devices.

[0053] Based on the foregoing specification, the computer system of the disclosed invention may be implemented using computer programming or engineering techniques including computer software, firmware, hardware or any combination or subset thereof. Any such resulting program, having computer-readable code means, may be embodied or provided within one or more computer-readable media, thereby making a computer program product, i.e., an article of manufacture, according to the invention. The computer readable media may be, for instance, a fixed (hard) drive, diskette, optical disk, magnetic tape, semiconductor memory such as read-only memory (ROM), etc., or any transmitting/receiving medium such as the Internet or other communication network or link. The article of manufacture containing the computer code may be made and/or used by executing the code directly from one medium, by copying the code from one medium to another medium, or by transmitting the code over a network.

[0054] One skilled in the art of computer science will easily be able to combine the software created as described with appropriate general purpose or special purpose computer hardware to create a computer system or computer sub-system embodying the method of the invention. An apparatus for making, using or selling the invention may be one or more processing systems including, but not limited to, a central processing unit (CPU), memory, storage devices, communication links and devices, servers, I/O devices, or any sub-components of one or more processing systems, including software, firmware, hardware or any combination or subset thereof, which embody the invention. User input may be received from the keyboard, mouse, pen, voice, touch screen, or any other means by which a human can input data into a computer, including through other programs such as application programs.

[0055] It should be understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of the claims application.