Integrated circuit device having optically coupled layers

[0001] This patent specification relates to coupling signals between different layers of an integrated circuit device.

[0002] Integrated circuit devices have become essential components in a wide variety of products ranging from computers and robotic devices to household appliances and automobile control systems. New applications continue to be found as integrated circuit devices become increasingly capable and fast while continuing to shrink in physical size and power consumption. As more integrated circuit chips performing at least one electrical and/or optical function, and includes both single-chip and multi-chip devices. In multi-chip devices, each integrated circuit chip is usually separately fabricated or “built up” from a substrate, and the resultant chips are bonded together or otherwise coupled into a common physical arrangement.

[0003] Advances in integrated circuit technology continue toward reducing the size of electrical circuits to smaller and smaller sizes, such that an entire local electrical circuit (e.g., a group of memory cells, a shift register, an adder, etc.) can be reduced to the order of hundreds of nanometers in linear dimension, and eventually even to tens of nanometers or less. At these physical scales and in view of ever-increasing clock rates, limitations arise in the data rates achievable between different parts of the integrated circuit device, with local electrical circuits having difficulty communicating with “distant” electrical circuits over electrical interconnection lines that may be only a few hundred or a few thousand microns long.

[0004] To address these issues, proposals have been made for optically interconnecting different electrical circuits in an integrated circuit device. For example, in the commonly assigned U.S. 2005/0078902A1, a photonic interconnect system is described that avoids high capacitance electric interconnects by using optical signals to communicate data between devices.

[0005] As part of such optical interconnection schemes, optical coupling between planar waveguides located on different integrated circuit layers is often needed. In a simplest proposal applicable to inter-chip coupling, two chips are mounted side-by-side such that an edge facet of a first waveguide on the first chip directly abuts an edge facet of a second waveguide on the second chip. In another proposal, an optical fiber is used to transfer optical signals between the two edge facets of the different chips. In yet another proposal, an optical fiber is used to couple between a surface-emitting source on the first chip and a detector on the second chip, each chip having electrical-to-optical (E-O) and optical-to-electrical (O-E) converter(s) as necessary. However, issues arise for such proposals that limit their operational scalability (e.g., the number of optical interconnections achievable between chips) and/or the amount of achievable device compactness.

[0006] Vertical optical coupling schemes have also been proposed in which optical signals are transferred between facing layers of vertically arranged chips, the vertical arrangement providing for a smaller footprint while also accommodating a larger number of optical interconnections between the facing chips. Proposals include the use of angled reflecting structures and/or grating structures for urging vertical projection through an aperture on one layer and corresponding vertical collection into an aperture on the other layer.

[0007] Issues arise, however, in relation to one or more of optical coupling efficiency and optical crosstalk between different aperture pairs, especially as the vertical spacing between the facing layers is increased. Other issues arise in relation to one or more of electrical crosstalk among facing electrical elements, lateral area requirements of the vertical optical coupling subsystems, accommodation of vertical surface features on the facing surfaces, device complexity, alignment issues, and fabrication cost. Still other issues arise as would be apparent to one skilled in the art upon reading the present disclosure. It would be desired to provide for optical coupling between different layers of an integrated circuit device, whether such layers be all-optical or electro-optical, in a manner that addresses one or more of these issues.

[0008] In one embodiment, in an integrated circuit device comprising a vertical arrangement of integrated circuit layers, a method for coupling an optical signal between a first integrated circuit layer thereof and a second integrated circuit layer thereof is provided. The optical signal is evanescently coupled between a photonic crystal defect waveguide and a photonic crystal defect cavity in the first integrated circuit layer and projectably coupled between the photonic crystal defect cavity and an optical aperture on the second integrated circuit layer.

[0009] Also provided is an integrated circuit device, comprising a first integrated circuit layer having an optical aperture and a second integrated circuit layer having a photonic crystal defect waveguide and a photonic crystal defect cavity. An optical signal propagating along the photonic crystal defect waveguide is evanescently coupled between the photonic crystal defect waveguide and the photonic crystal defect cavity and projectably coupled between the photonic crystal defect cavity and the optical aperture.

[0010] Also provided is an apparatus, comprising a first integrated circuit layer having an optical aperture and a second integrated circuit layer having a photonic crystal defect waveguide and a photonic crystal defect cavity. The apparatus further comprises means for evanescently coupling an optical signal propagating along the photonic crystal defect waveguide between the photonic crystal defect waveguide and the photonic crystal defect cavity, and means for projectably coupling the optical signal between the photonic crystal defect cavity and the optical aperture.

[0014] FIG. 1 illustrates a perspective view of an integrated circuit device 102 according to an embodiment, comprising an upper layer 106 and a lower layer 104 separated by an intervening layer 105, a height of the intervening layer 105 being exaggerated for clarity of presentation. The layers 104 and 106 may be from two different integrated circuit chips that have been glued together, bonded together, or otherwise assembled into a vertical arrangement. Alternatively, the layers 104 and 106 may in a common integrated circuit chip. As used herein, a layer of an integrated circuit device refers to a vertically contiguous slab-like subvolume of the integrated circuit device. It is to be appreciated that a layer may itself comprise a plurality of material sub-layers having relatively complex structures and functionalities. Thus, for example, the layers 104 and 106 may each comprise several adjacent sub-layers of differing materials formed, processed, patterned, or otherwise fabricated to achieve various electrical, electrooptical, and/or optical functionalities.

[0015] The layers 104 and 106 may both be all-optical or may both be electrooptical. Alternatively, one of the layers 104 and 106 may be all-optical and the other may be electrooptical. In one embodiment, the upper layer 106 may comprise densely-packed arrays of electrical circuits, each being laterally adjacent to a nearby optical communications port having O-E and E-O conversion elements, while the lower layer 104 may comprise an “optical LAN” facilitating information transfer of information among “distant” electrical circuits on the upper layer 106. In another embodiment, the layers 104 and 106 may be used for facilitating the photonic interconnect system described in the commonly assigned U.S. 2005/0078902A1, supra.

[0016] Integrated circuit device 102 further comprises a photonic crystal structure 108 formed within the lower layer 104. Photonic crystal structure 108 comprises an artificially engineered periodic dielectric array having at least one photonic bandgap, i.e., a range of frequencies in which ordinary electromagnetic wave propagation is strictly forbidden. The presence of these photonic bandgaps can be used to confine and guide electromagnetic waves along paths of missing or differently-shaped elements of the periodic array, termed defect waveguides herein, for frequencies within the at least one photonic bandgap, those frequencies being permitted to exist along the defect waveguides but not being permitted to exist in the non-defect areas. One such defect waveguide is indicated as element 110 in FIG. 1 and that guides an optical signal S1 having a frequency within the at least one photonic bandgap. As indicated in FIG. 1, the optical signal S1 is received from an outlying section 109 of the lower layer 104 representing any of a broad variety of electrooptical or all-optical circuits that may, or may not, comprise photonic crystal structures.

[0017] According to an embodiment, a photonic crystal defect cavity 112 is formed within the photonic crystal structure 108. The defect cavity 112 is configured and positioned to be evanescently coupled with the defect waveguide 110 at a frequency of the optical signal S1 and to radiate light vertically out of the plane of the lower layer 104 at that frequency. Such evanescent coupling can be achieved where an evanescent field of a resonant mode of the defect cavity 112 overlaps with an evanescent field of the optical signal S1 traveling in the defect waveguide 110 and is phase-matched therewith.

[0018] The defect cavity 112 is projectably coupled with an optical aperture 116 contained on the upper layer 106. As used herein, projectably coupled refers broadly to any of a variety of scenarios in which light is radiated from a first aperture and collected at a second aperture. Usually, the first and second apertures are separated by some non-zero distance, although the scope of the present teachings includes scenarios in which the first and second apertures are in direct contact. Projectable coupling includes, but is not limited to, free-space radiation across air or a homogeneous material volume, as well as projection through a lensing element that images, or partially images, the first aperture onto the second aperture (and vice versa).

[0019] Generally speaking, light radiates out of the defect cavity 112 in a cone-shaped pattern at an angle that is dependent on the particular geometries and materials of the defect cavity 112 and the refractive index of the intervening layer 105. For embodiments in which lensing elements are not used, it is preferable to keep the separation between the layers 104 and 106 less than a lateral dimension of the defect cavity 112 to achieve a reasonable degree of coupling, although the scope of the present teachings is not so limited.

[0020] The optical aperture 116 broadly represents any kind of optical element capable of collecting and/or transmitting light. Examples include optical emitters, optical detectors, lenses, waveguide facets, etc. In one embodiment, the upper layer 106 comprises a mirror configuration of the lower layer 104 including a second photonic crystal defect cavity and a second photonic crystal defect waveguide, the optical aperture 116 corresponding to a downward-facing surface of the second photonic crystal defect cavity.

[0021] The defect cavity 112 and defect waveguide 110 can also be configured such that a downward-projected optical signal is collected by the defect cavity 112 and evanescently coupled into the defect waveguide 110. In one embodiment, the defect cavity 112 and defect waveguide 110 are configured to accommodate bidirectional vertical coupling out of and into the lower layer 104. In another embodiment, the defect cavity 112 and defect waveguide 110 are optimized for one-way vertical coupling out of the lower layer 104, usually at the expense of coupling efficiency in the other direction into the lower layer 104. In another embodiment, the defect cavity 112 and defect waveguide 110 are optimized for one-way vertical coupling into the lower layer 104, usually at the expense of coupling efficiency in the other direction out of the lower layer 104.

[0022] Particular parameters for achieving the above-described evanescent coupling and vertical projection/collection will be highly dependent on the particular wavelengths, refractive indices, loss coefficients, polarizations, coupling geometries, and physical dimensions used. In view of the present disclosure, one skilled in the art would be readily able to mathematically and/or empirically determine suitable combinations of such parameters providing sufficient vertical optical coupling. By way of example only and not by way of limitation, in one configuration suitable for a typical optical communications wavelength of 1550 nm, the photonic crystal structure 108 may comprise a 250 nm-thick Si slab with a triangular lattice pattern of 120-nm-wide cylindrical air holes having a lattice constant of 420 nm. The defect waveguide 110 may be formed by a single missing line of air holes, and the defect cavity 112 may be formed by an enlarged air hole point defect with a diameter of 445 nm having a center 1090 nm away (i.e., three rows away) from a center of the defect waveguide 110.

[0023] However, a wide variety of other materials and device parameters for the photonic crystal structure 108 can be used for accommodating a wide range of optical wavelengths. Suitable slab materials for the photonic crystal structure 108 include, but are not limited to, relatively high-index materials such as Si (n=3.42), InP (n=3.1), GaAs (n=3.3), and SiN/Si₃N₄ (n=2.2). More generally, suitable photonic crystal slab materials include, but are not limited to, Group IV materials (e.g., Si, Ge, SiC), Group III-V materials (e.g., GaN, GaP, InP, InAs, AlN), and Group II-VI materials (e.g., ZnO, CdS).

[0024] Suitable hole materials for the photonic crystal structure 108 include, but are not limited to air, various low-index liquids or gels, and various other low-index materials such as SiO₂ (n=1.44), poly-dimethyl siloxane (PDMS, n=1.4), and polymethyl methacrylate (PMMA, n=1.49). The intervening layer 105 may comprise air or another low-index material.

[0025] FIG. 2 illustrates a side cut-away view of an integrated circuit device 202 according to an embodiment, comprising a lower layer 204 having a photonic crystal structure into which is formed a defect waveguide 210 (passing into and out of the page) evanescently coupled with a defect cavity 212, the defect cavity 212 projectably emitting light (and/or collecting light) at an optical aperture 218. Integrated circuit device 202 further comprises an upper layer 206 substantially mirroring the lower layer 204, having a photonic crystal structure into which is formed a defect waveguide 211 (also passing into and out of the page) evanescently coupled with a defect cavity 213, the defect cavity 213 collecting light (and/or projectably emitting light) at an optical aperture 219.

[0026] In accordance with an embodiment, a lensing element 222 is positioned in an intervening layer 205 between the lower layer 204 and the upper layer 206, the lensing element 222 being configured and positioned to image the optical apertures 218 and 219 onto each other for increasing optical coupling efficiency and allowing for increased separation between the lower layer 204 and the upper layer 206. The position of the lensing element 222 can be maintained by a low-index solid material 224 in the intervening layer 205, or by using any of a variety of other positioning/mounting techniques.

[0027] FIG. 3 illustrates a perspective view of an integrated circuit device 302 according to an embodiment, comprising a lower layer 304 and an upper layer 306, the vertical separation between them in FIG. 3 being greatly exaggerated for clarity of presentation. Lower layer 304 comprises a photonic crystal structure into which is formed a first defect waveguide 308 evanescently coupled with each of a first defect cavity 310 and a second defect cavity 312. Also formed into the photonic crystal structure is a second defect waveguide 314 evanescently coupled with each of a third defect cavity 316 and a fourth defect cavity 318. Upper layer 306 comprises first and second electrooptical circuits 320 and 322, which may each comprise highly dense electrical circuits 321 and 323 and which may be very “distant” from each other from an electrical signal propagation perspective. Upper layer 306 further comprises first, second, third, and fourth optical apertures 324, 326, 328, and 330 as shown in FIG. 3, wherein the first and second optical apertures 324 and 326 may be an optical emitter/optical receiver pair for one direction of communication between the dense electrical circuits 321 and 323, and wherein the third and fourth optical apertures 328 and 330 may be an optical emitter/optical receiver pair for the other direction.

[0028] In operation, a first optical signal S1 is projectably coupled from the first optical aperture 324 into the first defect cavity 310, evanescently coupled from the first defect cavity 310 into the first defect waveguide 308, propagated along the first defect waveguide 308 and evanescently coupled into the second defect cavity 312, and projectably coupled from the second defect cavity 312 into the second optical aperture 326. For the other direction, a second optical signal S2 is projectably coupled from the third optical aperture 328 into the third defect cavity 316, evanescently coupled from the third defect cavity 316 into the second defect waveguide 314, propagated along the second defect waveguide 314 and evanescently coupled into the fourth defect cavity 318, and projectably coupled from the fourth defect cavity 318 into the fourth optical aperture 330.

[0029] Fabrication of integrated circuit devices according to one or more of the embodiments can be achieved using known integrated circuit fabrication methods including, but not limited to: deposition methods such as chemical vapor deposition (CVD), metal-organic CVD (MOCVD), plasma enhanced CVD (PECVD), chemical solution deposition (CSD), sol-gel based CSD, metal-organic decomposition (MOD), Langmuir-Blodgett (LB) techniques, thermal evaporation/molecular beam epitaxy (MBE), sputtering (DC, magnetron, RF), and pulsed laser deposition (PLD); lithographic methods such as optical lithography, extreme ultraviolet (EUV) lithography, x-ray lithography, electron beam lithography, focused ion beam (FIB) lithography, and nanoimprint lithography; removal methods such as wet etching (isotropic, anisotropic), dry etching, reactive ion etching (RIE), ion beam etching (IBE), reactive IBE (RIBE), chemical-assisted IBE (CAIBE), and chemical-mechanical polishing (CMP); modifying methods such as radiative treatment, thermal annealing, ion beam treatment, and mechanical modification; and assembly methods such as wafer bonding, surface mount, and other wiring and bonding methods.

[0030] Whereas many alterations and modifications of the embodiments will no doubt become apparent to a person of ordinary skill in the art after having read the foregoing description, it is to be understood that the particular embodiments shown and described by way of illustration are in no way intended to be considered limiting. By way of example, resonance conditions associated with the photonic crystal defect cavities can be manipulated to implement various optical filtering or multiplexing/demultiplexing schemes for wavelength-division multiplexed (WDM) optical signals.

[0031] By way of further example, while in one or more of the above embodiments the photonic crystal structures comprise optically passive materials, in other embodiments active materials may be used that, responsive to one or more electrical and/or optical control signals, serve to modulate, amplify, filter, multiplex/demultiplex, or otherwise control lateral propagation properties and/or vertical coupling properties. By way of still further example, although the present teachings are particularly advantageous in the context of ever-shrinking hybrid optoelectronic devices, they are readily applicable to all-optical integrated circuit devices (e.g., as used in all-optical computing devices), as well as to larger-sized devices.

[0032] By way of even further example, although one or more of the embodiments is particularly useful for obviating the need for optical fiber connections between chips, optical fibers may still be used for various other purposes in the integrated circuit device (e.g., importing higher-power optical carrier signals from off-chip lasers) without departing from the scope of the present teachings. By way of still further example, although one or more of the embodiments is particularly useful where the layers are each contained on integrated circuit chips, the scope of the present teachings includes scenarios where one layer is on an integrated circuit chip, and the other layer is on a printed-circuit board or other type of back-plane/packaging assembly. Thus, reference to the details of the described embodiments are not intended to limit their scope.