Abstract:
A scalable fuse design for individual pixels of a focal plane array of photodiodes comprises a fuse disposed on the upper surface of each photodiode in the array, wherein the fuse is situated proximal to a side of each photodiode. The fuse of each photodiode is electrically coupled to the active region thereof via a first bus and is electrically coupled to an ROIC via a second bus.

Description:
STATEMENT OF RELATED CASES 
       [0001]    This case claims priority of U.S. Pat. Appl. Ser. No. 62/301,058, filed Feb. 29, 2016 and which is incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to Avalanche Photodetector Focal Plane Arrays. 
       BACKGROUND OF THE INVENTION 
       [0003]    The successful operation of avalanche photodetector (APD) focal plane arrays is largely dependent upon all pixels producing sufficiently low current under no illumination. The necessary complexity of the read-out integrated circuit (ROIC) chips, which are typically flip-chip bonded to APD photodetector arrays (PDAs), places a premium on ROIC pixel real estate relative to that of ROICs used with conventional PDA technologies. This hinders individual pixel addressability and, in fact, APD PDAs are biased in parallel. Consequently, when individual pixels exhibit abnormally low impedance relative to their neighbors, these “leaky” pixels effectively short-circuit the entire FPA, preventing operation of the device. 
         [0004]    The present state of materials processing technology is sufficient to produce working APD PDAs, but point defects leading to catastrophically leaky pixels remain frequent enough to limit device yield. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention provides a scalable fuse design for the individual pixels of an APD FPA. In the illustrative embodiment, the fuse is disposed on the PDA side, integrated on the upper surface of each individual pixel. The fuses are designed so that their resistance is low enough to enable normal APD FPA operation, but high enough to ensure that the fuse melts in any pixel having problematically low impedance. The melted fuse permanently opens the electrical path of the faulty pixel, thereby isolating it, enabling the rest of the array to function normally. 
         [0006]    The inventors recognized that, due to the fuse&#39;s length and extreme thinness (i.e., a few nanometers), it would be exceedingly problematic to fabricate a fuse having a uniform thickness on a PDA having a “bumpy” or otherwise non-uniform surface profile (e.g., depositing on a mesoscopically-rough surface, the presence of a mesa structure, etc.). 
         [0007]    As such, in accordance with embodiments of the invention, the PDA is fabricated to present an atomically flat surface for fuse formation, such as by epitaxial deposition. Furthermore, the inventors recognized that offsetting ROIC metal-bump placement on each pixel relative to the device active area enables the fuse to be positioned along one side of the pixel and electrically connected to the photodetector/ROIC with wide buses. This approach results in a fuse design that is scalable to square pixel pitches as small as 25 μm with commonly-available process technologies. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1A  depicts a top view of a conventional 4×5, backside illuminated, APD PDA. 
           [0009]      FIG. 1B  depicts, in greater detail, a single pixel from the APD PDA of FIG. la. 
           [0010]      FIG. 2A through 2B and 2D through 2F  depict a fabrication sequence for a photodiode-side integrated fuse for individual pixels in an APD PDA, in accordance with the illustrative embodiment of the present invention. 
           [0011]      FIG. 2C  depicts an embodiment of a layer structure for the integrated fuse. 
           [0012]      FIG. 3  depicts a thermal equivalent circuit model of the integrated fuse depicted in  FIGS. 2B and 2C . 
           [0013]      FIG. 4  depicts a truncated, schematic-level cross-sectional view of the fully integrated fuse. 
           [0014]      FIG. 5  depicts the fully integrated fuse shown in  FIG. 4  with materials and dimensions for an exemplary embodiment. 
           [0015]      FIGS. 6A and 6B  depict the scalable nature of fuses in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    The following terms are defined for use in this disclosure and the appended claims:
       A “photodiode focal plane array” comprises, among other elements, an array of photodiodes, an array of micro lenses for focusing photons onto the photodiodes in the area, a read-out integrated circuit (“ROIC”).   A “photodiode array” comprises an array of photodiodes.   A “pixel” is a basic unit of an array. In the context of a photodiode array, the term “pixel” references a single photodiode of the array. In the context of a photodiode focal plane array, the term “pixel” collectively references a single photodiode in the photodiode array and a pixel in the ROIC, at a minimum. In the illustrative embodiment, there is a one-to-one correspondence between pixels in the photodiode array and pixels in the ROIC.       
 
         [0020]      FIG. 1A  depicts a top view of a conventional, backside illuminated, avalanche photodetector (APD) focal plane array  100 . The exemplary array depicted in  FIG. 1A  comprises twenty pixels  102  in a 4×5 array.  FIG. 1B  depicts further detail of a single pixel  102  from array  100  of  FIG. 1A . 
         [0021]    As seen in these Figures, etched isolation trench  108  having an exemplary width of 6 microns separates each pixel  102 .  FIG. 1B  depicts pixel  102  and a half width (i.e., 3 microns) of etched isolation trench  108 . 
         [0022]    An exemplary width of each pixel  102  is 50 microns. It is notable that active region  106  is centered within the pixel; this is important for limiting leakage currents. In the illustrative embodiment, surface  104  of pixel  102  comprises InP. In typical bump-bonded devices, active region  106  is coated with a metal film, and a metal bump is affixed concentrically to the top of this stack to enable bump-bonded contact with a CMOS ROIC. 
         [0023]      FIGS. 2A through 2B and 2D through 2F  depict a method for fabricating a photodiode-side integrated fuse for individual pixels  102  in an APD PDA, in accordance with the present teachings. Beginning with the basic pixel structure depicted in  FIG. 1B , surface  104 , which in the illustrative embodiment is InP, is coated with a passivating/insulating layer  210  in  FIG. 2A . In some other embodiments, the avalanche photodiodes are based on other materials systems (i.e., materials other than InP). It is within the capabilities of those skilled in the art to adapt the present teachings to APD based on such other materials systems. In the illustrative embodiment, layer  210  comprises silicon nitride, SiN x . SiN x  may be deposited via a variety of processes, including plasma enhanced chemical vapor deposition (PECVD), inductively coupled plasma chemical vapor deposition (ICP CVD), reactive ion beam deposition (RIBD), or physical vapor deposition (PVD) (also known as “sputtering”). Opening  212  is patterned in layer  210  to keep active area  106  exposed. In some other embodiments, passivating/insulation layer  210  comprises SiO 2 , although SiN x  is generally preferred. 
         [0024]    As depicted in  FIG. 2B , fuse  214  is deposited along one edge of (each) pixel  102 . In the illustrative embodiment, the fuse is deposited on the passivating/insulating layer of SiN x . In some alternative embodiments, polyimide is spun on the SiN x  (or SiO 2 ). As depicted in  FIG. 2C , the fuse is embodied as stack  218  having multiple layers  220 ,  222 , and  224  of metals, which are deposited as thin films. In the illustrative embodiment, stack  218  comprises thin-film layer  220  of titanium, thin-film layer  222  of aluminum over the titanium, and thin-film layer  224  of nickel over the aluminum. In the illustrative embodiment, layer  220  is used as an adhesion promoter (to layer  210 ) and layer  224  is used to prevent oxidation of the aluminum (i.e., layer  222 ). In some other embodiments, layer  220  is tantalum and/or layer  224  is chromium. In the illustrative embodiment, layer  222 —aluminum—can be considered to be the primary fuse material. In some other embodiments, layer  222  is, without limitation, nickel, titanium, palladium, tin, platinum, germanium, or gold. Bus regions  216  extend from opposite ends of fuse  214  along opposed edges of pixel  102 . In the illustrative embodiment, the width W B  of buses  216  is 4 microns. 
         [0025]    It is important that an apron or border region  226  having a minimum width of about one micron as measured between the “outer” edge of fuse  214  or buses  216  and the nearest edge of trench  108  is present. The border region ensures that the fuse will be deposited on a sufficiently flat surface and therefore not overlap the trench. The minimum width of about 1 micron between potentially interacting features is based on current photolithography mask registration tolerances. In the illustrative embodiment, the width W A  of apron region  226  is 2 microns. With filet features, a 6-micron trench and 2-microns clearance on a 50-micron pitch pixel, the length L F  of fuse  214  is about 25 microns. 
         [0026]    As shown in  FIG. 2D , buses  228 A and  228 B, which each comprise one or more layers of electrically conductive metal(s), such as titanium, platinum, and gold, extend from respective ends of fuse  214 , being deposited over buses  216 . Within buses  228 A and  228 B, the gold is primarily used to transport current. The titanium and platinum are used as diffusion barriers to prevent gold/indium from diffusing into the underlying InP of the APD. In an alternative embodiment, silver replaces gold as the primary current-transport layer and a tungsten/titanium alloy is used as the diffusion barrier. Alternatively, the upper layer of tungsten/titanium alloy is replaced by layers of titanium (40 nm), nickel (200 nm), and gold (100 nm). Bus  228 A connects active region  106  (see, e.g.,  FIG. 2A ) to fuse  214 . Bus  228 A terminates in circular portion  230  that overlies active region  106 . In the illustrative embodiment, the diameter D O  of circular portion  230  is 8 microns. Bus  228 B extends to form conductive base or pad  230  for an offset metal bump for eventual connection to read-out circuitry (not depicted). In the illustrative embodiment, diameter D P  of pad  232  is 18 microns. With the aforementioned dimensions, gap G between the circular portion  230  and pad  232  is 2 microns. 
         [0027]    Per  FIG. 2E , passivation/insulation layer  234  is deposited on top of all features except for pad  232 . Passivation/insulation layer  234  is used to provide electrical and thermal insulation. For example, layer  234  further insulates the active-area bus (i.e.,  228 A/ 230 ) from direct contact with the ROIC-side bus (i.e.,  228 B/ 232 ). In the illustrative embodiment, layer  234  comprises SiNX. In other embodiments, other materials can suitably be used, such as and without limitation, polyimide and SiO 2 . 
         [0028]    Finally, per  FIG. 2F , bump  236 , which is comprises an electrically conductive material, such as indium, is affixed to the top of pad  232 , enabling electrical connection between the photodiode  102  and the eventual bump-bonded CMOS circuitry (i.e., the ROIC). Other materials known to those skilled in the art may suitably be used in place of indium for bump  236 . For example and without limitation, bump  236  can comprise: SnPb37, InAg, AuSn80/20, SnAg3.5, or Cu/CuSn pillars. 
         [0029]    Specific layer thicknesses are dependent on desired fuse properties, which in turn are highly dependent on the choice of fuse material and in-depth knowledge of the underlying APD technology. The inventors have learned, from their own implementation of APD cameras, that with the inclusion of a safety margin, fuses will need to carry up to 1 mA current without adverse reaction for normal operation. However, current ROIC implementations begin to malfunction at the array level beyond 15 mA of applied current. Based on results from finite-element analysis, the inventors determined that thin-film fuses can effectively be treated as thermally insulted from their surroundings when integrated on substrates with substantially lower thermal conductivities. Highly electrically- and thermally-conductive fuse materials surrounded by material of a lower electrical and thermal conductivity will allow for a build-up of heat in the fuse before dissipation, enabling the temperature at the center of the fuse to be calculated using a two-element equivalent thermal circuit.  FIG. 3  depicts equivalent circuit  340  for the illustrative fuse implementation. 
         [0030]    Because the fuse can be treated as though it is surrounded by a perfect thermal insulator for the duration of its joule heating, the maximum temperature T max  will be reached in its center. The thermally conductive path length to thermal “ground” is therefore equal to L/2, where L is the total length of the fuse. 
         [0031]    In this implementation, the thermal equivalent circuit is governed by: 
         [0000]        PR   Θ =( T   max   −T   RT )   [1]
 
         [0032]    where: P is heat flow;
       R Θ is the thermal resistance; and   T RT  is the temperature of the fuse at its end points.
 
With the length of the thermally conductive path to thermal ground equal to L/2, the equation governing the temperature difference between the center of the fuse and its end points is thus:
       
 
         [0000]    
       
         
           
             
               
                 
                   
                     
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         [0035]    where: K is thermal conductivity; and
       A is the cross-sectional area.       
 
         [0037]    In the case of joule heating, P=I 2 R, where: P is power, I is current, and R is the total electrical resistance of the fuse. Using R=ρL/A, where ρ is electrical resistivity, and recognizing that the melting point, thermal conductivity, and electrical resistivity are determined by the choice of fuse material, the necessary length-to-area ratio of the fuse can be expressed as a function of prescribed electrical current and fuse material: 
         [0000]    
       
         
           
             
               
                 
                   
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         [0038]    where: ΔT=(T melt −T RT )
       T melt  is the melting point of the fuse material.       
 
         [0040]    Ideally, a thin-film fuse is relatively straightforward to fabricate (i.e., on a flat surface), with a lower melting point, higher thermal conductivity, and lower electrical resistivity than its surroundings. Using aluminum as an example, having a melting point of 993 K, a thermal conductivity of 205 W/m·K, and an electrical resistivity of 2.82×10 −8  Ω·m, and with a desired current in the range of about 1 to about 15 mA, a desired L/A ratio is in the range of about 3.2×10 9  to about 2.1×10 8  m −1 . Using a practical minimum for area dimensions, a fuse thickness of five nanometers (nm) and a fuse width of 0.25 microns, a minimum range of necessary fuse lengths in the range of about 0.3 to about 4 microns is obtained. Allowing for some margin on thickness and/or width, the length requirement increases, emphasizing a need to design a layout that maximizes available length for the fuse. 
         [0041]      FIGS. 4 and 5  depict an exemplary integrated fuse  214  in cross-section (see also,  FIG. 2C ), with  FIG. 5  depicting exemplary materials and layer thicknesses. 
         [0042]    In the illustrative embodiment, fuse comprises aluminum, deposited to a thickness of about 10 nm. A very thin layer of titanium is used as an adhesive layer between passivation/insulation layer  210  (e.g., silicon nitride, etc.) and the aluminum. The titanium must be thick enough to enable adhesion between the underlying substrate and the fuse material. In the illustrative embodiment, a thickness of 2 nm was sufficient. In other embodiments in which a different substrate is used, a slightly thicker layer might be required to produce a flat, cohesive adhesion film. A thickness in the range of about 2 to 10 nm is expected to be sufficient for most substrates. However, one skilled in the art can readily verify the thickness requirement via simple experimentation. A very thin layer of nickel is deposited on the aluminum to protect the fuse against aluminum oxidation. The capping layer of nickel must be thick enough to prevent oxygen transport to the fuse. A layer of nickel having a thickness of 2 nm was determined by experimentation to be sufficient for this purpose. If a different material is used for as the capping layer, a different thickness might be required to prevent oxygen transport. In such situations, those skilled in the art will be able to determine the required thickness via simple experimentation. 
         [0043]    The bus metals (titanium, platinum, and gold in the illustrative embodiment) of circular portion  230  of bus  228 A provide electrical connection to the APD (at the left in  FIGS. 4 and 5 ) and the bus metals of pad  232  of bus  2288  provides electrical connection to the ROIC (at the right in  FIGS. 4 and 5 ). In the illustrative embodiment, buses  228 A and  2288  comprise a layer of titanium having a thickness of 30 nm, a layer of platinum having a thickness of 40 nm on top of the titanium, and a layer of gold having a thickness of 100 nm on top of the platinum. The aforementioned layer thicknesses are for the illustrative embodiment. For all such layers, there is a minimum thickness that must be exceeded in order to ensure that the layer, as deposited, contains no pinholes that reach to the underlying layer. That minimum is about 10 nm. There is no particular maximum thickness; the maximum is bounded by the specifics of other depositions in the overall process. An upper bound for the thickness of the titanium and platinum layers is about 100 nm for each layer. An upper bound for the thickness of the gold layer is about 1 micron. 
         [0044]    In the illustrative embodiment, passivation/insulation layer  210  comprises silicon nitride having a thickness of 150 nm and passivation/insulation layer  234  comprises silicon nitride having a thickness of about 170 nm. Once again, the aforementioned layer thicknesses are for the illustrative embodiment. As previously discussed, there is a minimum thickness that must be exceeded in order to ensure that the layer, as deposited, contains no pinholes that reach to the underlying layer. And the maximum is bounded by the specifics of other depositions in the overall process. A range for the thickness of these layers is typically between about 10 nm and about 1 micron. In the illustrative embodiment, lower layer  210  must be thinner than bus  228 A (i.e., the stack of Ti+Pt+Au), which is 170 nm. Hence, a thickness of 150 nm was selected for layer  210 ). And upper layer  234  is ideally about the same thickness as bus  228 B. Hence, a thickness of 170 nm was selected for layer  234 . 
         [0045]      FIGS. 6A and 6B  depict how, with the aforementioned reasonable fabrication limits on trace deposition, the design is readily scalable to a PDA with 25 micron pitch. The resistance of the fuse can be kept within the design range at shorter fuse lengths by reducing its width. For example, for a PDA having a plurality of pixels with a 50 micron pitch, the fuse length L F  is about 25 microns and fuse width W F  is 2 microns. For a PDA having a plurality of pixels with a 25 micron pitch, the fuse length L F  is about 6.5 microns and fuse width W F  is 0.5 microns. It is expected that for most applications, fuse length L F  will be in the range of about 1 micron to 30 microns. 
         [0046]    The following guidelines are provided for the scalable design:
       a minimum 1 micron clearance between fuse and pixel isolation trench;   a minimum 2 micron clearance between the two Ti/Pt/Au circular regions;   SiN x  covers the fuse and the central Ti/Pt/Au deposition to ensure electrical isolation from bump  236  (providing connection to the ROIC);   a minimum 1 micron trench width;   a minimum bus width-to-fuse width ratio of 2; and   a minimum filet radius of curvature equal to bus width.
 
Acceptable margins between critical features can be maintained down to a pixel pitch of about 25 microns.
       
 
         [0053]    It is to be understood that the disclosure teaches just one example of the illustrative embodiment and that many variations of the invention can easily be devised by those skilled in the art after reading this disclosure and that the scope of the present invention is to be determined by the following claims.