Patent Publication Number: US-9425232-B2

Title: Very small pixel pitch focal plane array and method for manufacturing thereof

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 14/139,071, filed Dec. 23, 2013, which is a divisional of U.S. patent application Ser. No. 12/241,649, filed Sep. 30, 2008, now U.S. Pat. No. 8,634,005, the disclosures of which are hereby incorporated by reference in their entirety for all purposes. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to imaging devices. More particularly, the present invention relates to an imaging device having a focal plane array with a readout circuit and method of manufacturing thereof, where each pixel of the focal plane array has a very small pitch with dimensions corresponding to the cutoff wavelength of the photodetector of the respective pixel. 
     BACKGROUND OF THE INVENTION 
     The image resolution that can be achieved from conventional infrared focal plane arrays (IR FPA), even with the most favorable optics, are generally limited by the pixel pitch. In the most advanced conventional FPAs, the smallest pixel pitch dimensions are 12 microns for mid wavelength (MW) IR with a nominal cutoff wavelength of 5 microns. In the long wavelength (LW) IR bands with cutoff wavelengths of around 10 microns, the smallest pixel pitch observed by the applicant for conventional FPAs is 15 microns. 
     To enhance and further optimize image resolution, the pixel pitch of an FPA for an imaging device or photodetector needs to be comparable to the wavelength of radiation being detected. Among the primary limitations in reducing the pixel pitch of conventional imaging devices or photodetectors are the architecture of the FPA and associated readout circuit contacts and the fabrication technique for electrical interconnection of the FPA to the unit cells of the associated readout circuit. These limitations are especially compelling for the shorter wavelength IR bands with cutoff wavelengths of 2.5 microns or less. 
     Using high density vertically integrated photodiode (HDVIP®, a trademark of DRS Technologies, Inc.) architecture, a pixel pitch as small as 6 microns and via diameters as small as 2 microns are practically feasible. With these dimensions, fill factors of approximately 90% may be realized.  FIG. 1  shows the variation in fill factor versus pixel pitch for an FPA implementing pixels with HDVIP® architecture and having one of three different values of via diameters. As shown in  FIG. 1 , for smaller pixel pitches (e.g., less than 6 microns), the fill factor drops rapidly even for a via diameter of 2 microns and, thus, compromises the overall photodetector performance. 
     There is therefore a need for an FPA with associated readout circuit contact architectures and fabrication techniques that enables the realization of imaging devices with pixel pitches approaching the wavelength of radiation to be detected without compromising the fill factor of each photodetector. This is especially a stressing requirement for the short wavelength or SWIR spectral band with cutoff wavelengths ≦2.5 microns. In addition to image resolution, smaller pixel pitch IR FPAs enable reduced size of optics, reduced cooling requirements, which in turn leads to a smaller package, lower power consumption and reduced overall weight. 
     SUMMARY OF THE INVENTION 
     In accordance with systems and articles of manufacture consistent with the present invention, an imaging device having an improved focal plane architecture is provided. The imaging device comprises a semiconductor layer (such as an semiconductor infrared absorbing layer) and a photodetector having an implanted region formed in the semiconductor layer to define a p-n (or n-p) junction therein, and a pad formed or deposited over the implanted region. The pad has a malleable metal or metallic material, such as Indium. The imaging device further comprises a readout circuit having a contact plug. The contact plug has a base and a prong extending from the base and into the malleable metallic material of the pad. In one implementation, the prong is a first of a plurality of prongs extending from the base and into the malleable metallic material of the pad. The prongs have a structure effective to displace a portion of the malleable metallic material into a space between the first prong and a second of the prongs. 
     In addition, in accordance with methods consistent with the present invention, a method is provided for manufacturing an imaging device. The method comprises forming a contact pad having a malleable metallic material over a surface of a semiconductor substrate (e.g., such the pad is formed over the surface of a photodetector formed in the semiconductor substrate), and providing a readout circuit having a first side and a contact plug. The contact plug has a base affixed to the first side of the readout circuit and a plurality of prongs extending from the base away from the first side. The method further comprises moving the first side of the readout circuit towards the substrate surface so that the prongs of the contact plug are pressed into the pad and displace a portion of the pad into a space defined by and between a first and a second of the prongs. 
     In one implementation, the method further comprises: forming a first pair of stop elements over the semiconductor substrate surface so that the contact pad is disposed between the first pair of stop elements; and providing a second pair of stop elements on the first side of the readout circuit so that the base of each contact plug is disposed between the second pair of stop elements and in substantial alignment with the first pair of stop elements formed over the substrate surface. In this implementation, the first side of the readout circuit is moved towards the substrate surface in substantial axial alignment with the first and second pairs of stop elements until the first pair of stop elements contacts the second pair of stop elements. A first of the first pair of stop elements and a first of the second pair of stop elements have a combined thickness that is more than a length of each prong such that the prongs are inhibited from passing completely through the contact pad when the first pair of stop elements contacts the second pair of stop elements. 
     In accordance with systems and articles of manufacture consistent with the present invention, another imaging device having an improved focal plane architecture and effective to provide two color detection is provided. The imaging device comprises a first semiconductor layer having a first surface and a second surface; and a first photodetector having a first implanted region formed in the first semiconductor layer and a pad formed over the first implanted region. The pad has a malleable metallic material. The imaging device also comprises a readout circuit disposed over the first surface of the first semiconductor layer. The readout circuit has a plurality of contact plugs facing the first surface of the first semiconductor layer. A first of the contact plugs has a first base and a first prong extending from the first base and into the malleable metallic material of the pad of the first photodetector. The imaging device further comprises a second semiconductor layer disposed below the second surface of the first semiconductor and a second photodetector having a second implanted region formed in the second semiconductor layer. In addition, the imaging device has a metalized via extending through the first semiconductor layer through an insulated via and the second semiconductor layer so that the metalized via electrically only connects the second implanted region of the second photodetector to a second of the contact plugs of the readout circuit, enabling the imaging device to detect two wavelength bands or two portions of a band (e.g., two colors of the visible band or infrared band). 
     Other systems, methods, features, and advantages of the present invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an implementation of the present invention and, together with the description, serve to explain the advantages and principles of the invention. In the drawings: 
         FIG. 1  is a graph depicting the variation in the fill factor percentage of a conventional HDVIP® photodetector of a pixel as a function of the pixel&#39;s pitch for three different “via” diameters for various pixel pitch on a focal plane array (FPA); 
         FIGS. 2A-2B  show a flow chart depicting a process for manufacturing an imaging device in which a photodetector array and a ROIC are aligned and interconnected in accordance with the present invention; 
         FIGS. 3A to 3F and 3H to 3J  are cross sectional views of an exemplary photodetector array and an exemplary ROIC of an imaging device manufactured in accordance with the process depicted in  FIG. 2 , where the photodetector array and the ROIC are illustrated at various steps of the manufacturing process; 
         FIG. 3G  is a top level view of the exemplary photodetector array corresponding to the cross-sectional view in  FIG. 3F  and before the ROIC having one or more contact plugs is applied in accordance with the present invention to the photodetector array as depicted in  FIGS. 3H-3J ; and 
         FIG. 4  is a cross sectional view of another imaging device manufactured in accordance with the present invention, in which the imaging device has an exemplary ROIC, a first exemplary photodetector array manufactured in accordance with the process depicted in  FIG. 2 , and a second exemplary photodetector array that collectively form a two color focal plane array for the imaging device. 
     
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
     Reference will now be made in detail to an implementation in accordance with methods, systems, and products consistent with the present invention as illustrated in the accompanying drawings. 
     Methods consistent with the present invention provide a process  200  depicted in  FIGS. 2A-2B  for manufacturing a focal plane array (FPA) of an imaging device, where an array of photodetectors with dimensions comparable to the cutoff wavelength of the respective photodetector is aligned with and interconnected to a unit cell of a readout integrated circuit (ROIC or readout circuit) to enable the realization of very small pixel pitches.  FIGS. 3A to 3F and 3H to 3J  are cross sectional views of an exemplary photodetector array  302  and an exemplary ROIC  304  of an imaging device  300  (as completed in  FIG. 3J ), where the photodetector array  302  and the ROIC  304  are illustrated at various steps of the manufacturing process  200 .  FIG. 3G  is a top level view of the exemplary photodetector array  302  corresponding to the cross-sectional view in  FIG. 3F  and before the ROIC  304  having one or more contact plugs  350   a - 350   c  is applied in accordance with the present invention to the photodetector array  302  as depicted in  FIGS. 3H-3J . Each contact plug  350   a - 350   c  is associated with and reflects a respective unit cell of the ROIC  304 . 
     As shown in  FIG. 2A  and  FIG. 3A , a passivation layer  310  is initially formed over a semiconductor substrate or layer  312  having a first conductivity type (step  202 ). In a pre-processing step, the semiconductor layer  312  may initially be formed over or deposited on a substrate (not shown in figures) comprising cadmium zinc telluride (e.g., when the semiconductor layer  312  comprises mercury-cadmium telluride), indium phosphide (e.g., when the semiconductor layer  312  comprises indium gallium arsenide), or other material suitable for forming a semiconductor layer. The substrate upon which the semiconductor layer  312  is formed may be removed using any known semiconductor device manufacturing technique, which is not described to avoid obscuring the present invention. 
     The passivation layer  310  may comprise cadmium telluride (CdTe), cadmium zinc telluride, cadmium telluride selenium, zinc sulfide, or any other suitable passivation material. The passivation layer  310  has a thickness in a range of 30 nm to 250 nm. 
     In one implementation, the semiconductor substrate or layer  312  comprises an infrared sensitive material, such as mercury cadmium telluride (HgCdTe), mercury zinc telluride (HgZnTe), mercury cadmium zinc telluride (HgCdZnTe), cadmium telluride (CdTe), cadmium zinc telluride (CdZnTe), indium gallium arsenide (InGaAs), or indium antimonide (InSb), doped with a material, such as Arsenic (As) or Copper (Cu), to form a p-type semiconductor layer. Alternatively, the semiconductor layer  312  may comprise silicon, germanium, gallium arsenide (GaAs), indium antimonide (InSb), other III V or II VI compound semiconductors, or the like, suitable for forming a photodetector. In an alternative implementation, the semiconductor substrate or layer  312  may be doped with another material so that the semiconductor layer  312  has a second conductivity type (e.g., n-type) different from the first conductivity type (e.g., p-type). For example, the semiconductor substrate or layer  312  may be doped with Boron (B) or other n-type material layer. Accordingly, each photodetector (as reflected by implanted regions  314   a - 314   c  in  FIG. 3B ) fabricated in the photodetector array  302  in accordance with the present invention may have a p-on-n or a n-on-p architecture and corresponding junction without departing from the scope of the present invention. 
     In addition, as shown in  FIGS. 3A-3F and 3H-3J , the starting detector material may have another passivation layer  316  formed on a back-side  318  of the semiconductor layer  312  to form a double sided passivated semiconductor layer  302 , allowing for front-side illumination of the photodetector array  302 . In an alternative implementation, the manufacturing steps discussed here may be applied so that the photodetectors of the array  302  are formed on the back-side  318  of the semiconductor layer  302  with the passivation layer  310  remaining intact over the front-side of the semiconductor layer  302 , allowing for back-side illumination of the photodetector array  302 . 
     Continuing with  FIG. 2A , the passivation layer  310  is patterned using photolithography and etched using a dry or a wet etching technique, to form openings such as  320   a ,  320   b  and  320   c  in the passivation layer  310  (step  204 ) that expose the substrate  312 . One of ordinary skill in the art would appreciate that other patterning and etching techniques may be employed without departing from the present invention. The openings  320   a ,  320   b  and  320   c  may each have a width (w) within the range of 0.3 μm to 1 μm (and preferably less than 0.8 μm), allowing each photodetector (as represented by the implanted regions  314   a - 314   c ) to be formed to a pixel pitch that is approximately equal to a cutoff wavelength of the radiation to be detected by the respective photodetector. 
     Next, one or more implanted regions  314   a ,  314   b  and  314   c  are formed in the substrate, via the one or more openings, where each implanted region has a second conductivity type that is different than the first conductivity type of the substrate  312  (step  206 ). For example, when the semiconductor layer  312  is comprised of HgCdTe doped with Arsenic (As) to have a p-type conductivity, Boron (B+) may be implanted, via each opening  320   a ,  320   b  and  320   c , into the substrate  312  to form implanted regions  314   a ,  314   b  or  314   c  having an n-type conductivity. Each implanted region  314   a ,  314   b  or  314   c  having the second conductivity type forms a junction with the substrate  312  having the first conductivity type to effectively form a n-on-p architecture associated with a respective photodetector. Alternatively, each photodetector may be implemented to have a p-on-n junction architecture when the semiconductor layer  312  is doped with Indium, for example, to have n-type conductivity. Note, in an alternative implementation, following the patterning, the implants may be formed through the passivation layer before the formation of the contact openings  320   a ,  320   b  and  320   c  via etching. 
     Each implanted region  314   a ,  314   b  and  314   c  has the approximate width (w) of the opening  320   a ,  320   b  or  320   c  through which the respective implanted region was formed. Thus, the photodetector represented by the respective implanted region  314   a ,  314   b  or  314   c  is formed to a pixel pitch that is approximately equal to a cutoff wavelength of the radiation to be detected by the respective photodetector. For example, the pixel pitch (e.g., d 1 , as shown in  FIG. 3H ) of the photodetector represented by implanted region  314   a  may be approximately 2.5 microns or less, corresponding to the cutoff wavelength of the SWIR spectral band. 
     After forming the implanted regions  314   a ,  314   b  and  314   c , a base contact  322   a ,  322   b  or  322   c  is formed over each implanted region  314   a ,  314   b  and  314   c  in each contact opening  320   a ,  320   b  and  320   c  (step  208 ). Each base contact  322   a ,  322   b  or  322   c  is comprised of a metal or metal alloy and is formed to a thickness substantially equal to the thickness of the initial passivation layer  310 . In one implementation, each base contact  322   a ,  322   b  and  322   c  has two layers  324   a  and  326   a ,  324   b  and  326   b  or  324   c  and  324   c . The first layer  324   a ,  324   b  or  324   c  is comprised of a first type of material that substantially bonds or adheres to the material comprising the implanted region  314   a ,  314   b  or  314   c  (e.g., Boron doped HgCdTe). The second layer  326   a ,  326   b  or  326   c  is comprised of a second type of material that substantially bonds or adheres to the first layer material and to the malleable metallic material comprising each pad (e.g., each pad  332   a ,  332   b  or  332   c  in  FIG. 3F  comprises Indium). In one implementation, the first layer  324   a ,  324   b  or  324   c  of each base contact comprises Nickel (Ni) deposited over a respective implanted region  314   a ,  314   b  or  314   c  to have a thickness of approximately 100 Angstroms or within a range of 50 Angstroms to 150 Angstroms. In this implementation, the second layer  326   a ,  326   b  or  326   c  of each base contact comprises Titanium (Ti) deposited over a respective first layer  324   a ,  324   b  or  324   c  to have a thickness of approximately 100 Angstroms or within a range of 50 Angstroms to 1000 Angstroms. The thickness of the Titanium is equal to or greater than the thickness of the Nickel in the base contact. In one implementation, the thickness of the Nickel is sufficient to enable the Titanium portion of the base contact to adhere to a respective pad  332   a ,  332   b  or  332   c  comprised of Indium. In this implementation, the second layer  326   a ,  326   b  or  326   c  of Titanium adheres better than Nickel to the Indium used to form the contact pad. 
     As shown in  FIG. 2A  and  FIG. 3D , a self-limiting photo-resist layer  328  is then deposited or formed over each base contact  322   a ,  322   b  or  322   c  and remaining initial passivation layer  310  to have a first thickness within a range of 0.5 μm to 1 μm (step  210 ). As shown in  FIG. 3E , a pad opening  330   a ,  330   b  or  330   c  is formed (e.g., via patterning and etching) in the photo-resist layer  328  over each base contact  322   a ,  322   b  and  322   c  (step  212 ). Each opening  330   a ,  330   b  and  330   c  may extend from the top of the photo-resist layer  328  to the base contact  322   a ,  322   b  and  322   c , which may be exposed via the etching process. 
     Next, a pad  332   a ,  332   b  or  332   c  comprised of a malleable metallic material or alloy, such as Indium or suitable Indium alloy, is formed in each pad opening  330   a ,  330   b  and  330   c  (step  214 ) as shown in  FIG. 3F . In one implementation, each pad  332   a ,  332   b  and  332   c  is formed to have the first thickness of the photo-resist layer  328 . 
     As depicted in  FIG. 3F , the photo-resist layer  328  is then patterned and etched to form at least a first pair of stop elements (e.g.,  334   a  and  334   d ) on the initial passivation layer  310  so that each pad  332   a ,  332   b  and  332   c  is disposed between the first pair of stop elements (e.g.,  334   a  and  334   d ) and each of the first pair of stop elements has a second thickness that is different than the first thickness of the pad (step  216 ). As discussed in further detail below, at least the first pair of stop elements (e.g.,  334   a  and  334   d ) on the photodetector array  302  are adapted to contact at least a second pair of stop elements (e.g.,  362   a  and  362   d  in  FIG. 3H ) on the readout circuit  304  so as to inhibit further movement of the readout circuit  304  towards the photodetector array  302 , preventing damage to the base contacts  322   a ,  322   b  and  322   c  and underlying photodetector implanted regions  314   a ,  314   b  and  314   c . In the implementation shown in  FIGS. 3F and 3G , the first pair of stop elements may be two of a plurality of stop elements  334   a - 3341  formed from the photo-resist layer  328  on the initial passivation layer  310  of the photodetector array  302  in accordance with the present invention. In one implementation, each pad  332   a ,  332   b ,  332   c ,  332   d ,  332   e  and  332   f  may be disposed between a respective pair of stop elements. For example, as shown in  FIG. 3G , a pad  332   a  may be disposed between a pair of stop elements  334   a  and  334   b  located near opposing sides  336  and  338  of the pad  332   a  or between a pair of stop elements  334   a  and  334   f  located near opposing corners  334   a  and  334   f  so as to contact a corresponding pair of stop elements on the readout circuit  304  so as to inhibit further movement of the readout circuit  304  towards the respective pad  332   a  of the photodetector array  302 , preventing damage to the base contact  322   a  and photodetector implanted region  314   a  underlying the pad  332   a . To provide further reliability in connecting the ROIC  304  to the photodetector array  302  as further described herein, each pad  332   a - 332   f  may be disposed between a respective four stop elements located about the pad  332 - 333   f . For example, as shown in  FIG. 3G , each pad (e.g., pad  332   a ) may be disposed between a respective four stop elements (e.g.,  334   a ,  334   b ,  334   e  and  334   f ) located near corners (e.g.,  340 ,  342 ,  344  and  346 ) of the respective pad. 
     In addition, although the pads  332   a - 332   f  are shown in  FIG. 3G  as having a square shape, each pad  332   a - 332   f  may have a polygon, circular or other shape without departing from the spirit of the present invention. Furthermore, although the stop elements  334   a - 3341  are depicted as square posts, the stop elements  334   a - 3341  may also have a polygon, circular or other shape and be formed in a strip or line without departing from the spirit of the present invention. Moreover, in an alternative implementation, each pad  332   a - 332   f  may be surrounded on at least three sides by a single stop element (e.g.,  334   a ) formed in a strip or line. 
     Turning to  FIG. 2B , a readout circuit  304  is formed or provided that has a first side  348  and a contact plug  350   a ,  350   b  or  350   c  for each pad  332   a ,  332   b  and  332   c  (step  218 ). Each contact plug  350   a ,  350   b  or  350   c  has a base  352   a ,  352   b  and  352   c  affixed to the first side  348  of the readout circuit  304  and one or a plurality of prongs  354   a - 354   c ,  356   a - 356   c  or  358   a - 358   c , extending from the base  352   a ,  352   b  or  352   c  away from the first side  348  of the ROIC  304 . Each contact plug  350   a ,  350   b  or  350   c  may comprise a material (such as Tungsten) that may be pressed into and substantially bond or adhere to the malleable metallic material or metal alloy (e.g., Indium or alloy thereof) comprising a respective pad  332   a - 332   f , either with or without an annealing processing step. Each prong of a respective contact plug  350   a ,  350   b  or  350   c  may have a polygon, circular or other shape. Each prong  354   a - 354   c ,  356   a - 356   c  or  358   a - 358   c  may extend a length (L) from the top side  348  of the ROIC  304  that is within a range of 0.25 microns to 0.5 microns. In addition, each prong  354   a - 354   c ,  356   a - 356   c  or  358   a - 358   c  may have a diameter or width (w) is within a range of 0.25 microns to 0.5 microns. Each set of adjacent prongs (e.g.,  354   a  and  354   b ) define a space  360  therebetween. In one implementation, the combined width of the prongs and spaces  360  between adjacent prongs for a contact plug (e.g.,  350   a ) does not exceed the width (d) of the pad (e.g.,  332   a ) to which the contact plug is to be connected as described below or the pixel pitch (d 1 ) of the photodetector associated with the pad (e.g.,  332   a ). 
     As shown in  FIG. 2B , a second pair of stop elements (e.g., stop elements  362   a  and  362   d  in  FIG. 3H ) are provided or formed on the first side  348  of the readout circuit  304  so that the base  352   a ,  352   b  or  352   c  of each contact plug  350   a ,  350   b  and  350   c  is disposed between the second pair of stop elements (e.g., stop elements  262   a  and  262   d ) and in substantial axial alignment with the first pair of stop elements (e.g.,  334   a  and  334   d ) of the photodetector array  302  (step  220 ). Each of the stop elements of each contact plug  350   a ,  350   b  and  350   c  has a third thickness such that the combined thickness of a stop element (e.g.,  362   a ) of a contact plug (e.g.,  350   a ) and a corresponding stop element (e.g.,  334   a ) of the photodetector array  302  is more than the length (L) of each prong (e.g.,  354   a - 354   c ) of the respective contact plug (e.g.,  350   a ) such that the prongs (e.g.,  354   a - 354   c ) are inhibited from passing completely through the associated pad (e.g.,  332   a ). 
     In the implementation shown in  FIG. 3H , the second pair of stop elements of the readout circuit  304  may be two of a plurality of stop elements  362   a - 362   d  provided or formed on the first side  348  of the readout circuit  304 . In one implementation, each contact plug  350   a ,  350   b  and  350   c  may be disposed between a respective pair of stop elements  362   a - 362   d . For example, as shown in  FIG. 3H , a contact plug  350   a  may be disposed between a pair of stop elements  362   a  and  362   b  located near opposing sides (as reflected by prongs  354   a  and  354   c ) of the contact plug  350   a , or between a pair of stop elements located near opposing ends or corners (not shown in figures) of the contact plug  350   a . In the implementation of the photodetector array  302  shown in  FIG. 3G , the readout circuit  304  may have a stop element  362   a - 362   l  (elements  362   e - 362   l  not shown in the figures) for each stop element  334   a - 334   l  of the photodetector array  302 . In this implementation, each of the stop elements  362   a - 362   l  of the readout circuit  304  are substantially aligned with and adapted to contact a corresponding stop element (e.g., element  334   a - 334   l ) of the photodetector array  302  so as to inhibit further movement of the readout circuit  304  towards the pads  332   a - 332   f  of the photodetector array  302 , preventing damage to the base contact and photodetector implanted region underlying each pad  332   a - 332   f.    
     Continuing with  FIG. 2B , the first side  348  of the readout circuit  304  is moved towards the semiconductor substrate or layer  312  of the photodetector array  302  in substantial axial alignment with the stop elements  334   a - 334   d  of the photodetector array and the stop elements  362   a - 362   d  of the readout circuit  304  such that the prongs  354   a - 354   c ,  356   a - 356   c  or  358   a - 358   c  of each contact plug  350   a ,  350   b  and  350   c  are pressed into each pad  332   a ,  332   b  and  332   c  and displace a portion (e.g.,  364   a ,  364   b ,  364   c ,  364   d ,  364   e  or  364   f ) of the respective pad into the space  360  between adjacent prongs (step  222 ). 
     The first side  348  of the readout circuit  304  continues to be moved towards the photodetector array  302  in substantial axial alignment with the stop elements  334   a - 334   d  of the photodetector array and the stop elements  362   a - 362   d  of the readout circuit  304  until each stop element of the photodetector array  302  (or the first pair of stop elements  334   a  and  334   d ) contacts a corresponding stop element of the readout circuit  304  (or the second pair of stop elements  362   a  and  362   d ) as shown in  FIG. 3I . As previously noted, the combined thickness of each stop element of the photodetector array  302  (e.g., each of the first pair of stop elements  344   a  and  334   d ) and the corresponding stop element of the readout circuit  304  (e.g., corresponding one of the second pair of stop elements  362   a  and  362   d ) is more than the length (L) of each prong  354   a - 354   c ,  356   a - 356   c  and  358   a - 358   c  such that the prongs are inhibited from passing completely through the respective pad  332   a ,  332   b  or  332   c  (step  224 ). 
     In an alternative implementation, step  220  is skipped and stop elements  362   a ,  362   b ,  362   c  and  362   d  are not formed that the first side  348  of the readout circuit  304 . In this implementation, in step  222 , the first side  348  of the readout circuit  304  is moved towards the semiconductor substrate or layer  312  of the photodetector array  302  such that the prongs  354   a - 354   c ,  356   a - 356   c  or  358   a - 358   c  of each contact plug  350   a ,  350   b  and  350   c  are pressed into a respective one of the pads  332   a ,  332   b  and  332   c  and displace a portion (e.g.,  364   a ,  364   b ,  364   c ,  364   d ,  364   e  or  364   f ) of the respective pad into the space  360  between adjacent prongs. In addition, in this implementation in step  224 , the first side  348  of the readout circuit  304  continues to be moved towards the photodetector array  302  until each stop element of the photodetector array  302  (or the first pair of stop elements  334   a  and  334   d ) contacts the first side  348  of the readout circuit  304 . In this implementation, the thickness of each stop element of the photodetector array  302  (e.g., each of the first pair of stop elements  344   a  and  334   d ) is more than the length (L) of each prong  354   a - 354   c ,  356   a - 356   c  and  358   a - 358   c  such that the prongs are inhibited from passing completely through the respective pad  332   a ,  332   b  or  332   c.    
     In one implementation, to facilitate the hybridization of each contact plug  350   a - 350   c  of the readout circuit  304  to a corresponding pad  344   a - 344   c  of the photodetector array  302 , the photodetector array  302  may be warmed to a predetermined temperature that is below the melting point of the material used to form each pad  332   a - 332   c . For example, when each pad  332   a - 332   c  is comprised of Indium, the photodetector array  302  may be warmed to a predetermined temperature that is equal to or less than 80° C. 
     Before or while the photodetector array  302  is being warmed, epoxy  366  (shown in  FIG. 3J ) may be injected or wicked into a cavity  368   a - 368   f  (shown in  FIG. 3I ), a portion of which is defined by two or more of the following: a pad  332   a ,  332   b  or  332   c ; a stop element  334   a - 334   n  of the photodetector array  302  (e.g., a first  334   a  of the first pair of stop elements  334   a  and  334   d ); a corresponding stop element  362   a - 362   n  of the readout circuit  304  (e.g., a first  362   a  of the second pair of stop elements  362   a  and  362   d ) in contact with the stop element (e.g.,  334   a ) of the photodetector array  302 ; the initial passivation layer  310  and the first side  348  of the readout circuit  304  (step  226 ). Note the predetermined temperature may be above the melting point of the material used to form the photo-resist layer  328  from which the stop elements  344   a - 344   d  of the photodetector array  302  are formed. In this implementation, the stop elements  344   a - 334   d  and/or the epoxy  366  flows to surround and encapsulate the perimeter of each pad  332   a ,  332   b  or  332   c , preventing excess malleable metallic material (e.g., Indium) of one pad (e.g., pad  332   a ) from shorting to an adjacent pad (e.g.,  332   b  or  332   d  in  FIG. 3G ). Once the photodetector array  302  is cooled back to room temperature, the wicking process is ended as the epoxy  366  and/or stop elements  344   a - 334   d  made from photo-resist material solidify. The combination of the photodetector array  302  and the respective unit cells of the ROIC  304  form the FPA of the imaging device  300 , and may be mounted on a chip carrier. 
     Turning to  FIG. 4 , another imaging device  400  manufactured consistent with the present invention is shown. The imaging device  400  incorporates the ROIC  304  (i.e., ROIC  404  in  FIG. 4 ) and the photodetector array  302  (i.e., photodetector array  402  in  FIG. 4 ) of the imaging device  300 . The photodetector array  402  and ROIC  404  are each manufactured and connected to each other in accordance with the process depicted in  FIG. 2  as previously discussed, except as noted below. As shown in  FIG. 4 , the imaging device  400  also includes a second photodetector array  406  formed below the first photodetector  402 . As further described herein, the ROIC  404 , the first photodetector array  402  and the second photodetector array  406  collectively form a two color focal plane array of the imaging device  400 , in which each unit cell (as shown in  FIG. 4 ) of the focal plane array has a smaller pitch (e.g., D equal to or less than 15 μm) than other conventional two color imaging devices. 
     Consistent with the photodetector array  302  and the manufacturing process depicted in  FIG. 2 , the photodetector array  402  of the imaging device  400  includes a first semiconductor layer  312  having a first surface  317  (which may be a front-side surface) and a second surface  318  (which may be a back-side surface) upon which a respective passivation layer  310  or  316  is formed. The first semiconductor layer  312  has a first conductivity type (e.g., p-type) and include an infrared sensitive material, such as HgCdTe, HgZnTe, HgCdZnTe, CdTe, CdZnTe, InGaAs or InSb. A first implanted region  314   a  is formed in the first semiconductor layer  312  to form a p-on-n or a n-on-p architecture for a first photodetector of the first photodetector array  402 . 
     A base contact  322   a  is formed over the implanted region  314   a . The base contact  322   a  is comprised of a metal or metal alloy and is formed to a thickness substantially equal to the thickness of the initial passivation layer  310 . As previously described, the base contact  322   a  may have two layers  324   a  and  326   a . In this implementation, the first layer  324   a  is comprised of a first type of material (e.g., Nickel) that substantially bonds or adheres to the material comprising the implanted region  314   a  (e.g., Boron doped HgCdTe). The second layer  326   a  is comprised of a second type of material (e.g., Titanium) that substantially bonds or adheres to the first layer material and to the malleable metallic material comprising the pad  332   a , which is formed on the base contact  332   a  for the first photodetector (as reflected by the implant  314   a ) of the photodetector array  402 . In one implementation, the malleable metallic material comprising the pad  332   a  is Indium or a suitable Indium alloy. 
     The ROIC  404  is disposed over the first surface  317  of the first semiconductor layer  312  and the passivation layer  310  formed thereon. The ROIC  404  has a plurality of contact plugs (e.g.,  350   a  and  450   a  in  FIG. 4 ) facing the first surface  317  of the first semiconductor layer  312 . A first  350   a  of the contact plugs  350   a  and  450   a  has a first base  352   a  and one or more prongs  354   a - 354   c  extending from the first base  350   a  and into the malleable metallic material of the pad  332   a  of the first photodetector. A second  450   a  of the contact plugs is disposed adjacent to the first contact plug  350   a . The second contact plug  450   a  includes a second base  452   a  and may include one or more prongs  454   a  and  454   b  extending from the second base  452   a.    
     The second photodetector array  406  includes a second semiconductor layer  412  disposed below the second surface  318  of the first semiconductor layer  312 . A respective passivation layer  410  and  416  may be formed on a front-side surface  417  and back-side surface  418  of the second semiconductor layer  412  in the same manner as described for the first semiconductor layer  312 . 
     A second implanted region  414  is formed in the second semiconductor layer  412  adjacent to and below (but not directly beneath) the first implant region  314   a  of the first semiconductor layer  312 . The second semiconductor layer  412  has a conductivity type (e.g., p-type) that is different from the conductivity type (e.g., n-type) of the second implanted region  414  to form the p-on-n or n-on-p architecture for the second photodetector in the second semiconductor layer  412 . 
     In one implementation as shown in  FIG. 4 , two or more layers  419   a - 419   b  of filler material and/or epoxy may be disposed between and used to attach the lower passivation layer  316  formed on the second surface  318  of the first semiconductor layer  312  and the upper passivation layer  410  formed on the front-side  417  or upper surface of the second semiconductor layer  412 . 
     As shown in  FIG. 4 , the imaging device  400  includes a metalized via  420  extending through the first photodetector array  402  (and the first semiconductor layer  312  thereof) and through the second photodetector array  406  (and the second semiconductor layer  412  thereof) so that the metalized via  420  electrically connects the second implanted region  414  formed in the second semiconductor layer  412  to the second base  452   a  or prong  454   a  or  454   b  of the second contact plug  450   a  of the ROIC  404 . Thus, each unit cell of the ROIC  404  has two contact plugs  350   a  and  450   b , each of which is connected to a respective photodetector (as reflected by implants  314   a  and  414 ) formed in one of the two semiconductor layers  312  and  412 , enabling the imaging device  400  to detect two different wavelengths or colors in a predetermined band. 
     The metalized via  420  may be formed using known via boring techniques. In one implementation, the second implant region  414  in the second semiconductor layer  412  (as well as a third implant region  422  in the first semiconductor layer  312 ) is formed during the via boring process. In one process for forming the metalized via  420 , the first photodetector array  402  and the ROIC  404  are first formed and connected together in accordance with the manufacturing process depicted in  FIG. 2 . A first bore hole (having side walls  426  defined by the third implant region  422  in  FIG. 4 ) is then formed through the first semiconductor layer  312  (and the passivation layers  310  and  316  sandwiching the first semiconductor layer  312 ) in perpendicular alignment with the second contact plug  450   a  of the ROIC  404 . An insulation film  424  is then deposited on the side walls  426  of the first bore hole using known deposition techniques to prevent contact between the metalized via  420  and the first semiconductor layer  312  or the third implant region  422  therein. In one implementation, the insulation film  424  may be deposited so that the insulation film  424  extends through the first semiconductor layer  312  to the base  452   a  of the second contact plug  450   a . If necessary, the first bore hole may be re-bored at a smaller diameter in order to remove excess insulation or insulation blocking access to the base  452   a  of the second contact plug  450   a . A first portion  428  of the metalized via  420  may then be deposited over the insulation film  424  in the first bore hole so that the metalized via  420  is electrically connected to the second contact plug  450   a  but not the first semiconductor layer  312  or the third implant region  422  that may be formed therein during the boring process. A filler material  430 , such as epoxy, may be deposited over the first portion  428  of the metalized via  420  to fill any excess area in the first bore hole. A second portion  432  of the metalized via  420  may be deposited as a layer on top of the filler material  430  to provide a base contact extension in proximity to the second or back-side surface  318  of the first semiconductor layer  312 . 
     Next, a second bore hole (having side walls  434  defined by the second implant region  414 ) is formed through the second semiconductor layer  412  (and the passivation layers  410  and  416  sandwiching the second semiconductor layer  412 ) in perpendicular alignment with base contact extension (e.g., the second portion  432  of the metalized via  420 ) and the second contact plug  450   a  of the ROIC  404 . A third portion  436  of the metalized via  420  may then be deposited on the side walls  434  of the second bore hole so that the metalized via  420  electrically connects the second implant region  414  to the base contact extension  432  and, thus, to the second contact plug  450   a . Thus, the ROIC  404  is structured to read the first photodetector defined by the first implant region  314   a  in the first semiconductor layer  312  and to read the second photodetector defined by the second implant region  414  in the second semiconductor layer  412 , where the second implant region  414  is disposed adjacent to and below (but not directly beneath) the first implant region  314   a  of the first photodetector in the first semiconductor layer  312 . In this implementation, the first photodetector as defined by the first implant region  314   a  is effective to detect a first wavelength associated with a first portion of a predetermined band (e.g., the visible band or infrared band) that passes through the second semiconductor layer and into the first semiconductor layer  312 . The second photodetector defined by the second implant region in the second semiconductor layer is effective to detect a second wavelength associated with a second portion of the predetermined band. Wavelengths  460  associated with the first portion of the predetermined band and detected by the first photodetector in the first semiconductor layer  312  are longer than wavelengths  470  associated with the second portion of the predetermined band and detected by the second photodetector in the second semiconductor layer  412 . 
     In accordance with the present invention, the imaging device  400  further comprises a first pair of stop elements (e.g.,  334   a  and  334   b ) each of which is disposed over the first surface  317  of the first semiconductor layer  312  such that the pad  332   a  of the first photodetector is disposed between the first pair of stop elements  334   a  and  334   b . The ROIC  404  has a second pair of stop elements (e.g.,  362   a  and  362   b ) disposed on the first side  348  of the ROIC  404 . The base  352   a  of the first contact plug  350   a  is affixed to the first side  348  of the ROIC  404  such that the prong  354   a  of the first contact plug  350   a  extends away from the first side  348  of the ROIC  404  and between the second pair of stop elements  362   a  and  362   b . When the ROIC  404  is moved towards the first surface  317  of the first semiconductor layer  312 , the first pair of stop elements  334   a  and  334   b  formed over the semiconductor layer  312  (and formed on the passivation layer  310  in one implementation) contact the second pair of stop elements  362   a  and  362   b  of the ROIC  404  such that each prong  354   a - 354   c  of the first contact plug  322   a  is inhibited from passing completely through the pad  332   a  of the first photodetector. 
     As shown in  FIG. 4 , the imaging device  400  further comprises a third pair of stop elements  480   a  and  480   b  each of which is disposed over the first surface  317  of the first semiconductor layer  312  such that the metalized via  420  is disposed between the third pair of stop elements  480   a  and  480   b . The ROIC  404  has a fourth pair of stop elements  490   a  and  490   b  disposed on the first side  484  of the ROIC  404 . The third pair of stop elements  480   a  and  480   b  is disposed relative to and contacting the fourth pair of stop elements  490   a  and  490   b  such that each prong  454   a  and  454   b  of the second contact plug  450   a  is inhibited from contacting the first surface  318  of the first semiconductor layer  312 . 
     As previously discussed, when the first side  348  of the ROIC  404  is moved towards the first semiconductor layer  312  of the first photodetector array  402  in substantial axial alignment with the stop elements  334   a - 334   b  and  480   a - 480   b  of the first photodetector array and the stop elements  362   a - 362   b  and  490   a - 490   b  of the ROIC  404 , the prongs  354   a - 354   c  of the first contact plug  350   a  is pressed into the pad  332   a  and displaces a portion of the pad  332   a  into the space between adjacent prongs (e.g.,  354   a  and  354   b  or  354   b  and  354   c ). 
     While various embodiments of the present invention have been described, it will be apparent to those of skill in the art that many more embodiments and implementations are possible that are within the scope of this invention. Accordingly, the present invention is not to be restricted except in light of the attached claims and their equivalents.