Patent Publication Number: US-6984816-B2

Title: Vertically integrated photosensor for CMOS imagers

Description:
FIELD OF INVENTION 
     The present invention generally relates to semiconductor and imaging devices; and more particularly, to systems and methods for vertically integrating photosensing elements for use in CMOS imaging applications. 
     BACKGROUND 
     CMOS imagers have begun to challenge CCDs in many electronic imaging applications and are gaining in popularity. The primary advantages of CMOS imagers are their relatively low cost generally resulting from the use of standard, high-volume CMOS processes and their ability to be integrated with native CMOS electronics for control and image processing; this in contrast to CCDs, which typically employ specialized processing optimized for image capturing operations that are not generally amenable to large-scale integration of electronics. 
     Because the photodiodes (PDs) of CMOS imagers are usually fabricated within the same material layer and with similar processes as the electronics, and these materials and processes are often dominated by and optimized for electronic circuitry, the optical and optoelectronic design of the photosensor are generally compromised. Representative limitations involve the Si material itself wherein a fixed band gap generally obstructs the use of band gap engineering as an effective design tool. The material may be responsive to light at visible wavelengths enabling use for many imaging/camera applications, but very weak in the near-IR spectral region where additional applications may exist. Due to the indirect band gap, light absorption may be relatively weak, even at visible wavelengths. Accordingly, thick absorbing layers and deep junctions may be needed to achieve high efficiency; these same design criteria generally being in conflict with those for high performance electronic applications. 
     Incompatibility between the optoelectronic and electronic requirements may be further exacerbated by current scaling trends—in terms of both the pixel pitch and the progression to smaller critical dimension CMOS process technologies (i.e., technology scaling). As the size of the pixel shrinks to reduce the cost of imaging chips, the illuminated area of the photodetector is also typically reduced thereby decreasing the captured signal level. Moreover, as the photodetector active area is reduced, the dark current becomes dominated by the perimeter causing inter alia the noise current density to increase. Together, these effects generally operate to degrade the signal to noise performance of the sensor. 
     There is a current trend that involves increasing the amount of electronic circuitry within each pixel from about the 3–4 transistor level for active pixel sensors (APS) to more than about 100 transistors for digital pixel sensors (DPS). This desire for increased signal processing functionality within the pixel escalates competition for ‘real estate’ within the pixel area creating additional problems generally associated with shrinking the PD active area, reducing the fill factor within the pixel, and applying advanced process technologies. 
     Migration to advanced process technologies may further compromise PD device design and performance. 0.25 μ technologies and beyond typically employ silicides that are optically opaque and potentially leaky, as well as shallow trench isolation (STI) which may often lead to higher perimeter-generated dark current levels. Furthermore, reduced lateral dimensions are generally attended by reduced vertical dimensions leading to thinner layers and shallower junctions as well as higher doping levels and reduced carrier diffusion lengths. Reduced supply voltages also may negatively impact charge collection and storage capability of the PD. Moreover, an ever-increasing number of interconnect levels and corresponding isolating dielectric layers generally decreases the optical throughput and coupling efficiency to the photodetector and introduces additional sources of reflection and scattering that can lead to elevated levels of optical crosstalk. 
     Efforts to improve the level of PD performance can limit flexibility and performance in the CMOS electronics as well. Minimizing the number of interconnect levels to improve the optical coupling to the PD, for example, may make interconnect routing more difficult and less area-efficient. 
     The co-integration of the photosensor with the CMOS electronics (i.e., within the same material layer, using the same processes, etc.) within the pixel of CMOS imagers may severely limit both the performance of the photosensor and the flexibility of the system architectural design. This problem may be further complicated by progression to more advanced CMOS technology generations which generally utilize trench isolation, non-transparent silicides, and increasing numbers of interconnect levels. The current trend toward increase functionality within the pixel (as with digital pixel sensors—DPS) adds more electronic circuitry within the pixel creating competition for pixel area, decreasing fill factor and further compromising of performance. To allow enhanced optimization of the photosensing and the electronics functions, as well as greater flexibility in architectural design, a new imager platform is needed. 
     SUMMARY OF THE INVENTION 
     In various representative aspects, the present invention provides vertically integrated photosensors for CMOS imagers using 3D integration schemes in which inter alia a monocrystalline active layer is incorporated over and in close proximity to a processed CMOS wafer. The monocrystalline layer may be transferred to the CMOS wafer by means of, for example, wafer to wafer (or die to wafer) bonding followed by substrate removal. The vertically integrated photosensing layer is sufficiently proximate to the final metallization of the CMOS wafer to provide inter alia a high density of interconnects for electrical contact within each pixel of the photosensitive material. An exemplary method for fabricating such a device is disclosed as comprising the steps of inter alia: a processed CMOS layer; and a photosensing element fabricated in a vertically integrated optically active layer, where the optically active layer is bonded to the CMOS layer and the optically active layer is positioned near a metalization surface of the CMOS layer to allow an interconnect for electrical contact within the optically active layer to be made. 
     Additional advantages of the present invention will be set forth in the Detailed Description which follows and may be obvious from the Detailed Description or may be learned by practice of exemplary embodiments of the invention. Still other advantages of the invention may be realized by means of any of the instrumentalities, methods or combinations particularly pointed out in the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       Representative elements, operational features, applications and/or advantages of the present invention reside inter alia in the details of construction and operation as more fully hereafter depicted, described and claimed—reference being made to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout. Other elements, operational features, applications and/or advantages will become apparent to skilled artisans in light of certain exemplary embodiments recited in the Detailed Description, wherein: 
         FIG. 1  depicts a plan view of a CMOS photodiode in accordance with the prior art; 
         FIG. 2  depicts an elevation cross-section view of the prior art CMOS photodiode generally depicted in  FIG. 1 ; 
         FIG. 3  representatively illustrates a plan view of a vertically integrated CMOS photosensing element in accordance with one exemplary aspect of the present invention; 
         FIG. 4  representatively illustrates an elevation cross-section view of the vertically integrated CMOS photosensing element representatively illustrated, for example, in  FIG. 3 ; 
         FIG. 5  representatively illustrates an elevation cross-section view of a precursor structure for use in an exemplary method of fabricating a vertically integrated photosensing device in accordance with the present invention; 
         FIG. 6  representatively illustrates an elevation cross-section view of an intermediate structure for use in an exemplary method of fabricating a vertically integrated photosensing device in accordance with the present invention; 
         FIG. 7  representatively illustrates an elevation cross-section view of an intermediate structure for use in an exemplary method of fabricating a vertically integrated photosensing device in accordance with the present invention; 
         FIG. 8  representatively illustrates an elevation cross-section view of an intermediate structure for use in an exemplary method of fabricating a vertically integrated photosensing device in accordance with the present invention; 
         FIG. 9  representatively illustrates an elevation cross-section view of an intermediate structure for use in an exemplary method of fabricating a vertically integrated photosensing device in accordance with the present invention; 
         FIG. 10  representatively illustrates an elevation cross-section view of an intermediate structure for use in an exemplary method of fabricating a vertically integrated photosensing device in accordance with the present invention; 
         FIG. 11  representatively illustrates an elevation cross-section view of an intermediate structure for use in an exemplary method of fabricating a vertically integrated photosensing device in accordance with the present invention; 
         FIG. 12  representatively illustrates an elevation cross-section view of an intermediate structure for use in an exemplary method of fabricating a vertically integrated photosensing device in accordance with the present invention; 
         FIG. 13  representatively illustrates an elevation cross-section view of an intermediate structure for use in an exemplary method of fabricating a vertically integrated photosensing device in accordance with the present invention; 
         FIG. 14  generally illustrates an elevation cross-section view of an exemplary vertically integrated photosensing device in accordance with a representative aspect of the present invention; and 
         FIG. 15  generally illustrates a plan view of an exemplary vertically integrated photosensing device in accordance with a representative aspect of the present invention. 
     
    
    
     Those skilled in the art will appreciate that elements in the Figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the Figures may be exaggerated relative to other elements to help improve understanding of various embodiments of the present invention. Furthermore, the terms ‘first’, ‘second’, and the like herein, if any, are used inter alia for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. Moreover, the terms front, back, top, bottom, over, under, and the like in the Description and/or in the claims, if any, are generally employed for descriptive purposes and not necessarily for comprehensively describing exclusive relative position. Skilled artisans will therefore understand that any of the preceding terms so used may be interchanged under appropriate circumstances such that various embodiments of the invention described herein, for example, are capable of operation in other orientations than those explicitly illustrated or otherwise described. 
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The following descriptions are of exemplary embodiments of the invention and the inventors&#39; conceptions of the best mode and are not intended to limit the scope, applicability or configuration of the invention in any way. Rather, the following Description is intended to provide convenient illustrations for implementing various embodiments of the invention. As will become apparent, changes may be made in the function and/or arrangement of any of the elements described in the disclosed exemplary embodiments without departing from the spirit and scope of the invention. 
     A detailed description of an exemplary application, namely a system and method for providing a vertically integrated photosensing device suitably adapted for use in CMOS imaging applications is presented as a specific enabling disclosure that may be readily generalized by skilled artisans to any application of the disclosed system and method in accordance with various embodiments of the present invention. 
     As previously addressed, co-integration of photosensors with CMOS electronics may operate to substantially limit both the performance of the photosensor and the flexibility of the system architectural design. This may be additionally complicated by progression to more advanced CMOS technology generations which generally utilize trench isolation, non-transparent silicides and increasing numbers of interconnect levels. These factors, as well as others, generally degrade performance of photodetectors  100  co-integrated with CMOS electronics  120 . In effect, the photodetector  100  becomes ‘trapped’ within the CMOS technology ‘box’, as generally depicted, for example, in  FIGS. 1 and 2 , with a lateral interconnect  110  further reducing available imaging ‘real estate’. One way to address this problem, in accordance with various representative aspects of the present invention, is to release the PD  200  from the ‘box’ and place a PD  200  array in its own separate active layer positioned over a dielectric layer  115  and a wafer surface containing the electronics  220  with a vertical interconnect  210  disposed therebetween—also generally depicted, for example, in  FIGS. 14 and 15 . Such a three-dimensional, vertically stacked configuration of the optically active layer over the electronically active layer has been termed by the Inventors herein as a “Vertically Integrated PhotoSensor” or VIPS. 
     Removal of the photosensors from the CMOS electronics design and fabrication ‘box’ allows independent optimization of photosensor device performance and system architectural design. For the photosensor, this may include, for example: choice of materials; pixel design (optical and electronic); fabrication processes resulting in detectors with much higher fill factors (on the order of about 3–4×greater); and/or the like. The utilization of the third dimension substantially obviates competition for lateral pixel area and also allows greater functionality to be incorporated into any given 2D form factor. The underlying electronics may then be designed effectively without compromise from the optical coupling requirements of the photosensor. This generally allows for the use of much needed additional layers of interconnect metallization for more efficient layout of chips and also opens the door for the incorporation of greater image processing functions on-chip with more creative architectural partitioning of the imaging system potentially leading to single-chip camera applications. In summary, vertical integration generally provides inter alia improved performance, lower system cost, increased functionality, lower power consumption and smaller form factors. 
     VIPS configurations additionally provide many advantages for both the active layers. In the electronic active layer, not only does the removal of the PD free up space within each pixel element for electronic circuits, but the photo-absorbing layer above it may also be suitably adapted to operate as a natural light shield. Since excellent optical coupling is a given in this arrangement, there is generally no need to limit the number of interconnect levels used. This could enable some innovative architectural partitioning of the imager (e.g., co-integration of the pixel electronics together with the image processing circuitry, for example) that could lead to improved performance and lower cost imagers. 
     Most of the benefits and advantages for the optically active layer generally relate to greater freedom in the design and processing of the PD devices. Since it is removed from the Si, the optically active layer may be composed of more optimal materials; i.e., materials that have higher absorption coefficients for any given spectral range of interest. These might include other semiconductor materials such as, for example: GaAs; InP; GaN; HgCdTe and their related materials and/or combinations thereof; or SiC; SiGe; or even a-Si or p-Si or x-Si. 
     The positioning of the optically active layer over the electronic active layer offers optical advantages including a near 100% optical fill factor for the pixel (as generally illustrated, for example, in  FIG. 3 ), and virtual elimination of optical throughput loss and optical crosstalk generated by reflections and scattering. Additionally, a dedicated optical active layer is more amenable to advanced optical designs including backside reflecting mirrors and resonant cavities. These attributes each help boost the amount of signal captured by the PD, but the near 100% fill factor also leads to an effective reduction of the perimeter-to-area ratio and a corresponding decrease in the normalized dark current. Accordingly, both the signal and noise characteristics are greatly improved by vertical integration in accordance with various representative and exemplary aspects of the present invention. 
     Moreover, since the active layers are generally distinct and isolated, there is the potential for separate optimization in their fabrication processing. This factor is largely dependent on how the two layers may be brought into close proximity and will be discussed vide infra. 
     Although there are many possible means to realize the vertically integrated optical active layer and integrate photosensors within this layer, as generally described above, one exemplary method is detailed here for clarity. A representative process begins with a substantially fully processed CMOS “host” wafer onto which the “donor” wafer (generally depicted, for example, in  FIG. 5 ) will be bonded. The host wafer typically contains electronic circuitry for in-pixel signal operations such as amplification, readout, ADC, etc. and potentially other image processing and/or peripheral circuitry. The top level of interconnect metallization on the host wafer comprises metal landing pads for the various types of interconnects to be made between the host and donor wafers. These may include, for example: the pads for intra-pixel connections between the individual photodiodes in the optically active layer and their corresponding pixel circuits in the electronic active layer on the host wafer; pads for interconnects between the common side of the photodiodes and the host wafer; and pads for I/O connections off-chip. Alignment keys for the wafer-to-wafer bonding process may also be included in this layer. These pads may be encapsulated with, for example, SiO 2  and the wafer planarized with CMP processing. 
     In this embodiment, the donor wafer starts as an SOI wafer:  FIG. 5 ; Si layer  300  and buried oxide (‘BOX’) layer  310 . The SOI wafer also may be configured with a relatively thin, lowly doped p −  top Si layer  320  (on the order of about 2000–5000 Å). An intrinsic or p − -epitaxial layer  330  is then grown to a thickness on the order of about 2–4 μm to better accommodate the low absorption coefficient of the Si material. A thermal oxide  340  is next grown on the top surface which provides a high quality interface for inter alia minimization of photodiode dark current levels. At this point, alignment keys may be formed in the Si layer to mate to the corresponding keys on the host wafer during the bonding process, and to serve as base keys for alignment of subsequent (post wafer bond) process steps. A photosensor region  320 ,  350  (see  FIG. 6 ) may be defined by further processing of the donor wafer, which may include patterning of via features  370  followed by deposition of thermal oxide (e.g., SiO 2 )  360  over the processed top surface of the donor wafer. 
     Photodiodes may then fabricated within this donor wafer using a series of ion implant steps. As generally illustrated in  FIG. 7 , a patterned, multiple-energy, n-type implant (e.g., p + ) may be used to convert the thickness of the Si layer to form a column of n-type material  380  at the center of each pixel forming the anode of the photodiodes. A portion of this column, most generally the region near the implanted surface, may be implanted to a sufficiently high concentration to allow ohmic contact to this region. This forms a vertical pn junction in which the depletion region extends laterally outward from the n-type columns  380  to between the p-type columns  395 ,  390 . This configuration results in photogenerated charge carriers being drawn toward the axis of the PN junction and the interconnect  517 . As the surrounding regions are intrinsic or nearly so, These depletion regions can generally extend laterally for many microns even to the point of overlapping with neighboring SCRs. In this case, any non-uniformities in doping level and/or thickness may result in non-uniform photocarrier collection regions and “ill defined” pixels. To alleviate this situation, a patterned, multiple-energy, p-type implant (e.g., B+) may be employed to form relatively narrow, moderately doped walls that act to limit the lateral extent of the depletion regions and define the shape of the pixels. This column may also contain a highly doped portion, again most likely near the implanted surface, for ohmic contacting purposes. Through this ohmic contact, the implanted p-type grid can then be used to uniformly apply a bias (typically ground) to the common cathodes of the array of photodiodes. 
     After the formation of the photodiodes, an optically reflective structure (comprising, for example: thermal oxide layer  360 ; a dielectric layer  400 , such as, for example, SiO 2 , Si 3 N 4 , etc.; and a bond interface layer  410 ) may optionally be formed over the top of the wafer, as generally depicted, for example, in  FIG. 8 . This structure may ultimately become a backside reflector once the wafer is flipped over, wafer bonded and the substrate removed. The reflector may alternatively, conjunctively or sequentially comprise a single metallic layer (e.g., Al, Ag, Au, etc.) or multiple, alternating layers of high and low refractive index dielectric materials forming a distributed Bragg reflector (DBR). Blanket metallic reflectors generally offer the advantage of high reflectivity in thin films, but pose challenges for the subsequent etching of the contact vias. Alternatively, the metal layer may be deposited, prior to bonding, and patterned with a hole for the via, but at the cost of an additional masking step. Dielectric-based DBRs are generally easier to etch through, but multiple layer pairs are typically required to achieve sufficient reflectivity.  FIG. 8  shows the application of a single layer pair DBR made from SiO 2 /Si 3 N 4    400 . Finally, even without an optimized reflector integral to the transferred layer structure, some level of reflectance could likely be realized from the interconnect landing pads residing directly underneath the pixels within the top level of metallization on the host CMOS wafer. In this case, a smooth, reflective surface and a large fill factor of these landing pads would also be beneficial. 
     After both host  420  and donor wafers ( FIG. 5 ) are planarized and prepped, they are aligned and bonded together (as shown, for example, in  FIG. 9 ) with the substrate removed, as described vide supra. The CMOS host wafer  420  may generally comprise a Si substrate  490 , n-type pixel interconnects  460 , multiple levels of metal interconnects  480 , alignment keys  430 , I/O pads  440 , p-type contacts  450  and/or the like. The host  420  and donor wafers ( FIG. 5 ) are bonded through interface  405  in metal interconnect layer  470 . After substrate removal (see  FIG. 10 ), the top surface of the bonded wafer composite is the original buried oxide layer  310  (BOX) of the SOI donor wafer ( FIG. 5 ) which acted as an etch stop in the substrate removal process. Alignment of all subsequent process steps may be made relative to the keys within the transferred layer that are now exposed from the back side. 
     Thereafter, as generally illustrated in  FIG. 11 , relatively high aspect ratio via holes  500 ,  510  are etched within each pixel from the composite structure&#39;s top surface, through: the BOX layer  310  at the surface; the Si semiconductor layer  320 ,  350 ; the bottom reflector  400 ; and the SiO 2  layers  470  on either side of the wafer bonded interface  405 , stopping on the landing pads  460  of the CMOS wafer  420 . Care must be taken with this etch step to ensure both optimal performance and reliability. To simultaneously meet the requirements of high pixel fill factor and thick optical absorption layer for high conversion efficiency, the aspect ratio of this etch must be suitably high, particularly as the pixel pitch is reduced. On the other hand, because of the differing material types that must be penetrated, it may be challenging to achieve sufficiently smooth sidewalls to allow high yield of the sidewall ohmic contacts and metal interconnection from the contact to the landing pad during subsequent processing. As shown in  FIG. 7 , this etch step is also used to define a via  500  within the p+ implanted regions  390  where sidewall contacts and interconnects will also be made. 
     As illustrated in  FIG. 12 , after the vias  500 ,  510  are cleared, metallization  505 ,  515  is deposited covering the sidewalls and bottom of the etched via  500 ,  510  forming ohmic contacts to the n +  material  380  (or p +  material  390 ) along the sidewall and interconnecting this contact to the via landing pad  450 . Optionally, the metallized vias  500 ,  510  may then be plugged with a plated metal  507 ,  517  (e.g., Cu and/or the like) then the surface re-planarized. 
     If a top side partial reflector  520  is to be included to for a resonant cavity around the absorptive Si layer, it may be deposited at this point in conjunction with a passivation layer  530  as generally depicted, for example, in  FIG. 13 . Typically, such a reflector might comprise a multi-layer dielectric DBR stack. 
     As a final fabrication step, as illustrated in  FIG. 14 , access vias  540  to the large area I/O pads  440  around the perimeter of the chip may be opened. This step has non-critical etch requirements and may easily be accomplished with standard etch processes. 
     To clarify the configuration of these elements,  FIG. 15  shows the development of an exemplary device, in accordance with representative aspects of the present invention, in plan view with the locations of the key elements clearly identified. The lower half  600  of  FIG. 15  is a region of the pixels array showing all or portions of eight (8) pixels, while the upper half  650  of the Figure show a peripheral region immediately adjacent to the array.  FIG. 15  also shows: the n-type implanted columns  380 ; the associated depletion region  550  surrounding them, within the intrinsic or p − -blanket epi layer  350 ; the common grounding strip  560  of the parent CMOS wafer  420 ; and the common cathode interconnect via and metal plugs  560 . Finally,  FIG. 15  also shows the access vias  540  to the I/O pads  440  for probing or wire bond interconnection off-chip. The displayed size of this I/O pad  440  is only a fraction of the actual size of the actual I/O pad for convenience of illustration. 
     The optically active layer may be transferred to the CMOS wafer via any wafer bonding technique now known or otherwise hereafter described in the art. This result can be achieved in many ways, but a representative and exemplary approach may be one in which the two components to be bonded are coated with SiO 2  and subsequently planarized. The surfaces are then chemically treated and thereafter mated together to form a covalent-based bond at the SiO 2  coordination interface. The bonding process generally takes only several seconds and may be performed at room temperature. The substrate originally containing the optical active layer is then removed by chemical and/or mechanical means (e.g., CMP). In order to achieve suitably thin transferred layers, it may be helpful to the substrate removal process if this wafer further comprises a suitably adapted etch stop layer. 
     There are many possible modes of implementing the desired vertically integrated structure. Some representative variables in the process may include the partitioning of the photodiode fabrication process into pre-bond and post-bond sections. At one extreme, a “blank” optical active layer having substantially no pre-processing may be bonded with the final diode fabrication and interconnection to the CMOS wafer performed at some point after the bonding. This may be attractive from the standpoint that critical alignment of the blank layer to the CMOS wafer would generally not be required. A drawback of this approach is that is highly undesirable to have to do high temperature processing to fabricate the photosensors after the bonding process as this may have an adverse impact on the already processed CMOS devices below. The preferred approach may be more toward the other extreme in which the photodiodes are substantially completely fabricated prior to bonding leaving only via etching, ohmic contact formation, inter-active layer interconnection and passivation processing for post-bond processing. 
     Two representative and exemplary modes are disclosed in which bonding may be performed. The first is wafer-to-wafer bonding in which two (or more) wafers (preferably of matching diameter for greatest efficiency) are mated and bonded in a substantially unitary step. If the photodiodes (along with alignment keys) are pre-processed in the active layer, a critical alignment must generally be done before the wafers are mated. Subsequent processing is then best aligned to the optically active layer with any misalignment tolerance generally accommodated in oversized interconnect metallization pads in the CMOS wafer. If the optically active layer is blank, alignment may be registered to keys in the CMOS wafer (if made visible with a coarsely aligned clearing etch step, for example). In this bonding mode, the surface area of the optical active layer is generally not efficiently utilized since, typically, the active area does not match the entire area of the CMOS chip. 
     The second representative bonding mode is die-to-wafer. Here, the wafer containing the optical active layer is typically diced prior to bonding and the chips are individually aligned to corresponding sites on the CMOS wafer. If the optically active layer was initially blank, then these die-to-wafer bonds generally do not require any critical alignment and substantially all of the post-bond processing may be easily aligned to the exposed alignment keys in the CMOS wafer. Contrariwise, if the optically active layer contains processed photodiodes, then each die generally needs to be critically aligned to each site on the CMOS wafer. Subsequent processing typically must then be aligned at each individual site, or if aligned simultaneously, to the CMOS wafer, where any error in the die-to-wafer alignment step is generally absorbed in the optical active layer. 
     Regardless of the bonding method employed, the end result is a monocrystalline semiconductor layer that has been transferred to the surface of the CMOS wafer within which the array of photodiodes reside. The primary advantage of having this optically active layer comprised of single crystal material (as opposed to amorphous or poly-crystalline material) is the potential for higher performance and flexibility. In principle, the single crystal material generally demonstrates much lower dark currents due inter alia to the absence of a high density of grain boundaries and other mid-gap defects as compared with non-crystalline material. In fact, there is no fundamental reason why photosensors fabricated in such materials cannot approach the performance of high quality CCDs. A secondary benefit is the potential for additional levels of electronic integration within this layer that would otherwise be unavailable in the conventional art. 
     Unlike the Thin Film on ASIC (TFA) approach of fabricating integrated photosensing arrays, representative and exemplary embodiments of the present invention provide a CMOS-based imaging army having photosensing elements fabricated in a vertically integrated optically active layer comprising monocrystalline material(s) that is/are subsequently transferred to the CMOS wafer surface. The thickness of the optically active layer is generally sufficiently thin to allow for a high density of electrical interconnects to the CMOS wafer (e.g., about one per pixel; on the order of about 1 connection per 10–250 square microns, and more preferably about 1 connection per 10–100 square microns). The interconnections Themselves may be manufactured using standard, wafer level, CMOS via and metalization technology. 
     Skilled artisans will appreciate that another benefit of a transferred optical active layer is the ability to utilize materials that are generally non-native to the Si substrate containing the CMOS. Such non-native semiconductor materials may have widely varying optical absorption characteristics, allowing the option to choose a material that is the most optimally suited for a particular application. These materials may include the compound semiconductors such as GaAs, InP, and GaN and related materials, as well as Ge, SiGe, SiC, etc. These representative materials generally span the entire range of UV, visible, and NIR wavelengths. The absorption of Si is relatively weak across the visible range while many of the other materials are an order of magnitude stronger. This fact implies that optically active layers of non-Si materials could be much thinner, for the same absorption level, making the fabrication of the photosensors substantially easier. 
     Image sensing outside of the visible band may be applied to many applications. In the near UV, some examples may include: forensics; dermatology for detection of skin cancers; “solar blind” imaging for applications in which solar illumination (sunlight) is perceived as a noise source; and/or the like. NIR imaging may also be used for “night vision” (using either natural or artificial illumination), alcohol-based flame detection, biological analysis, etc. 
     In other applications, multi-spectral sensing and comparative imaging may be of great value. Here, information may be extracted from the differences (or similarities) in images taken within different ranges of the spectrum (e.g., UV, visible, NIR). With the vertical integration methodology described herein, one sensor may be included within the CMOS layer (for example, a visible sensor) and another sensor (for example, UV or NIR) could be implemented within the vertically integrated active layer. 
     What has been described thus far is the implementation of only photosensing devices within the vertically integrated active layer. Clearly, more functionality could be implemented within this layer in the form of, for example, CMOS electronics; which circuitry could be used inter alia for parallel processing of captured images. 
     With the bonding approaches described, vertical integration of active layers need not be limited to a single layer. As long as the wafer surface can be sufficiently planarized to allow substantially void-free bonding, the process could be repeated indefinitely; in principle, to provide a vertical stack of active layers all or some of which are interconnected and each providing some distinct or otherwise additional functionality. 
     In the foregoing specification, the invention has been described with reference to specific exemplary embodiments; however, it will be appreciated that various modifications and changes may be made without departing from the scope of the present invention as set forth in the claims below. The specification and figures are to be regarded in an illustrative manner, rather than a restrictive one and all such modifications are intended to be included within the scope of the present invention. Accordingly, the scope of the invention should be determined by the claims appended hereto and their legal equivalents rather than by merely the examples described above. For example, the steps recited in any method or process claims may be executed in any order and are not limited to the specific order presented in the claims. Additionally, the components and/or elements recited in any apparatus claims may be assembled or otherwise operationally configured in a variety of permutations to produce substantially the same result as the present invention and are accordingly not limited to the specific configuration recited in the claims. 
     Benefits, other advantages and solutions to problems have been described above with regard to particular embodiments; however, any benefit, advantage, solution to problems or any element that may cause any particular benefit, advantage or solution to occur or to become more pronounced are not to be construed as critical, required or essential features or components of any or all the claims. 
     As used herein, the terms “comprises”, “comprising”, or any variation thereof, are intended to reference a non-exclusive inclusion, such that a process, method, article, composition or apparatus that comprises a list of elements does not include only those elements recited, but may also include other elements not expressly listed or inherent to such process, method, article, composition or apparatus. Other combinations and/or modifications of the above-described structures, arrangements, applications, proportions, elements, materials or components used in the practice of the present invention, in addition to those not specifically recited, may be varied or otherwise particularly adapted by those skilled in the art to specific environments, manufacturing specifications, design parameters or other operating requirements without departing from the general principles of the same.