Abstract:
An array of active pixel sensors includes a substrate. An interconnect structure is formed adjacent to the substrate. The interconnect structure includes a plurality of conductive vias. A plurality of photo sensors are formed adjacent to the interconnect structure. Each photo sensor includes a pixel electrode. Each pixel electrode is electrically connected to the substrate through a corresponding conductive yet. A I-layer is formed over each of the pixel electrodes. The array of active pixel sensors further includes a conductive mesh formed adjacent to the photo sensors. An inner surface of the conductive mesh is electrically and physically connected to the photo sensors, and electrically connected to the substrate through a conductive via. The conductive mesh providing light shielding between photo sensors thereby reducing cross-talk between the photo sensors. The conductive mesh includes apertures that align with at least one of the pixel electrodes of the photo sensors.

Description:
FIELD OF INVENTION 
     This invention relates generally to photo diode active pixel sensors. In particular it relates to a conductive mesh bias connection for an array of elevated active pixel sensors which provides light shielding between the active pixel sensors. 
     BACKGROUND 
     An array of image sensors or light sensitive sensors detect the intensity of light received by the image sensors. The image sensors typically generate electronic signals that have amplitudes that are proportionate to the intensity of the light received by the image sensors. The image sensors can convert an optical image into a set of electronic signals. The electronic signals may represent intensities of colors of light received by the image sensors. The electronic signals can be conditioned and sampled to allow image processing. 
     Integration of the image sensors with signal processing circuitry is becoming more important because integration enables miniaturization and simplification of imaging systems. Integration of image sensors along with analog and digital signal processing circuitry allows electronic imaging systems to be low cost, compact and require low power consumption. 
     Historically, image sensors have predominantly been charged coupled devices (CCDs). CCDs are relatively small and can provide a high-fill factor. However, CCDs are very difficult to integrate with digital and analog circuitry. Further, CCDs dissipate large amounts of power and suffer from image smearing problems. 
     An alternative to CCD sensors are active pixel sensors. Active pixel sensors can be fabricated using standard CMOS processes. Therefore, active pixel sensors can easily be integrated with digital and analog signal processing circuitry. Further, CMOS circuits dissipate small amounts of power. 
     FIG. 1 shows a cross-section of a prior art array of image sensors. This array of image sensors includes PIN diode sensors located over a substrate  10 . An interconnection structure  12  electrically connects an N-layer  14  of the PIN diodes to the substrate  10 . An I-layer  16  is formed over the N-layer  14 . A P-layer  18  is formed over the I-layer  16 . The P-layer  18 , the I-layer  16  and the N-layer  14  form the array of PIN diode sensors. A first conductive via  20  electrically connects a first diode sensor to the substrate  10 , and a second conductive via  22  electrically connects a second diode sensor to the substrate  10 . A transparent conductive layer  24  is located over the array of diode sensors. A conductive lead  26  is connected to the transparent conductive layer  24 . The conductive lead  26  is connected to a bias voltage which allows biasing of the P-layer  18  of the array of PIN diode sensors to a selected voltage potential. 
     A limitation of the image sensor structure of FIG. I is the lack of shielding between the image sensors. Light received by a given sensor is also received by a neighboring sensor because there is no shielding of light between neighboring sensors. Light received by a given image sensor will also effect neighboring image sensors because current can flow through the N-layer  14  between neighboring image sensors. Charge can flow between the image sensors especially when the light intensity of the received light varies greatly between neighboring image sensors. The P-layer  18 , the I-layer  16  and the N-layer  14  are shared by neighboring image sensors. A trench  28  is formed to provide some isolation between the image sensors by increasing the resistance between the N-layers sections of neighboring image sensors. 
     Another limitation of the image sensor structure of FIG. 1 is the electrical connection between the conductive lead  26  and the transparent conductive layer  24 . The transparent conductive layer  24  must be electrically conductive to allow biasing of the PIN diodes, and must be transparent to allow the PIN diodes to receive light. Generally, it is very difficult to bond to the types of materials that must be used to form the transparent conductive layer  24 . Therefore, the conductive lead  26  must be attached to the transparent conductive layer  24  with the aid of some type of clamp or support structure. The result being an electrical connection which is not reliable and which is expensive to produce. 
     It is desirable to have an array of active pixel sensors formed adjacent to a substrate in which light received by an active pixel sensors of the array is shielded from the other active pixel sensors of the array. That is, it is desirable that isolation exist between the active pixel sensors which reduces the effect that light received by an active pixel sensor of the array has on the other active pixel sensors of the array. It is also desirable that the active pixel sensor array include a conductive layer that provides a bias voltage to the array of active pixel sensors, and that is reliably electrically connected to a pixel sensor bias voltage which originates on the substrate. 
     SUMMARY OF THE INVENTION 
     The present invention is an array of elevated active pixel sensors formed adjacent to a substrate that includes a conductive mesh that is reliably electrically connected to the pixel sensors and a pixel sensor bias voltage which is located on the substrate. The conductive mesh provides shielding of light received by each pixel sensor from the other pixel sensors. That is, the conductive mesh provides isolation between the active pixel sensors which reduces the effect that light received by an active pixel sensor of the array has on the other active pixel sensors of the array. The conductive mesh provides a bias voltage to the array of active pixel sensors. The substrate can be a CMOS substrate which includes image processing circuitry. 
     A first embodiment of this invention includes an array of active pixel sensors. The array of active pixel sensors includes a substrate. An interconnect structure is formed adjacent to the substrate. The interconnect structure includes a plurality of conductive vias. A plurality of photo sensors are formed adjacent to the interconnect structure. Each photo sensor includes a pixel electrode. Each pixel electrode is electrically connected to the substrate through a corresponding conductive via. An I-layer is formed over each of the pixel electrodes. The array of active pixel sensors further includes a conductive mesh formed adjacent to the photo sensors. An inner surface of the conductive mesh is electrically and physically connected to the photo sensors, and electrically connected to the substrate through a conductive via. 
     A second embodiment of this invention is similar to the first embodiment. The second embodiment includes the conductive mesh providing light shielding between photo sensors thereby reducing cross-talk between the photo sensors. 
     A third embodiment of this invention is similar to the second embodiment. The third embodiment includes apertures of the conductive mesh aligning with at least one of the pixel electrodes of the photo sensors. 
     A fourth embodiment of this invention is similar to the first embodiment, but further includes a P-layer formed between the I-layer and the conductive mesh. The inner surface of the conductive mesh is electrically connected to the P-layer and the interconnect structure. 
     Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a cross-section of a prior art array of image sensors. 
     FIG. 2 shows an embodiment of the invention. 
     FIG. 3 shows a top view of an embodiment of the invention. 
     FIG. 4 shows another embodiment of the invention. 
     FIG. 5 shows pixel electrodes deposited on the pixel interconnect structure. 
     FIG. 6 shows an I-layer and a P-layer deposited over a plurality of pixel electrodes. 
     FIG. 7 shows the P-layer and the I-layer selectively etched to allow access to a conductive via. 
     FIG. 8 shows a conductive layer deposited over the P-layer which provides an electrical connection between the P-layer and the interconnection structure. 
     FIG. 9 shows the transparent conductive layer having been etched according to a predetermined pattern. 
    
    
     DETAILED DESCRIPTION 
     As shown in the drawings for purposes of illustration, the invention is embodied in an array of elevated active pixel sensors formed adjacent to a substrate that includes a conductive mesh that is reliably electrically connected to the pixel sensors and a pixel sensor bias voltage which is located on the substrate. The conductive mesh provides shielding of light received each pixel sensor from the other pixel sensors. That is, the conductive mesh provides isolation between the active pixel sensors which reduces the effect that light received by an active pixel sensor of the array has on the other active pixel sensors of the array. The conductive mesh provides a bias voltage to the array of active pixel sensors. 
     FIG. 2 shows a first embodiment of the invention. This embodiment includes a substrate  200 . The substrate  200  includes electronic circuitry. An interconnection structure  210  is formed adjacent to the substrate  200 . Pixel electrodes  222 ,  224 ,  226  are formed adjacent to the interconnection structure  210 . Each pixel sensor of an array of pixel sensors includes an individual pixel electrode  222 ,  224 ,  226 . An I-layer  220  is formed adjacent to the pixel electrodes  222 ,  224 ,  226 . A P-layer  230  is formed adjacent to the I-layer  220 . A conductive mesh  240  is formed adjacent to the P-layer  230 . A first pixel electrode  222  of a first pixel sensor is electrically connected to the substrate  200  through a first conductive via  214 . A second pixel electrode  224  of a second pixel sensor is electrically connected to the substrate  200  through a second conductive via  216 . A third pixel electrode  226  of a third pixel sensor is electrically connected to the substrate  200  through a third conductive via  218 . The conductive mesh  240  is electrically connected to the substrate  200  through a fourth conductive via  212 . 
     FIG. 3 shows a top-view of an embodiment of the invention. The top-view more clearly depicts the physical characteristics of the conductive mesh  240 . The conductive mesh  240  include apertures  310  which align with the pixel electrodes  222 ,  224 ,  226  of the pixel sensors. The conductive mesh  240  provides shielding of light between the pixel sensors. The apertures  310  within the conductive mesh  240  allow light to be received by pixel sensors which align with the apertures  310  while shielding the received light from other pixel sensors. 
     The conductive mesh  240  is physically and electrically connected to the P-layer  230 . The conductive mesh  240  is also electrically connected to the substrate  200  through the fourth conductive via. The conductive mesh  240  is driven to a bias voltage potential by electronic circuitry within the substrate  200 . The bias voltage potential biases the pixel sensors. 
     The pixel sensors conduct charge when the pixel sensors receive light. The substrate  200  generally includes sense circuitry and signal processing circuitry. The sense circuitry senses how much charge the pixel sensors have conducted. The amount of charge conducted represents the intensity of light received by the pixel sensors. Generally, the substrate  200  can be CMOS (complementary metal oxide silicon), BiCMOS or Bipolar. The substrate  200  can include various types of substrate technology including charged coupled devices. 
     Typically, the interconnection structure  210  is a standard CMOS interconnection structure. The structure and methods of forming this interconnection structure  210  are well known in the field of electronic integrated circuit fabrication. The interconnection structure  210  can be a subtractive metal structure, or a single or dual damascene structure. 
     The conductive vias  214 ,  216 ,  218  pass through the pixel interconnect structure  210  and electrically connect the pixel electrodes  222 ,  224 ,  226  to the substrate  200 . The fourth conductive via  212  passes through the pixel interconnect structure  210  and provides a reliable electrical connection between the conductive mesh  240  and the substrate  200 . Typically, the conductive vias  212 ,  214 ,  216 ,  218  are formed from tungsten. Tungsten is generally used during fabrication because tungsten can fill high aspect ratio holes. That is, tungsten can be used to form narrow and relatively long interconnections. Typically, the conductive vias  212 ,  214 ,  216 ,  218  are formed using a chemical vapor deposition (CVD) process. Other materials which can be used to form the conductive vias  212 ,  214 ,  216 ,  218  include copper, aluminum or any other electrically conductive material. 
     The pixel electrodes  222 ,  224 ,  226  are generally formed from a doped semiconductor. The doped semiconductor can be an N-layer of amorphous silicon. The pixel electrodes  222 ,  224 ,  226  must be thick enough, and doped heavily enough that the pixel electrodes  222 ,  224 ,  226  do not fully deplete when biased during operation. The pixel electrodes  222 ,  224 ,  226  are typically doped with phosphorous. 
     The pixel electrodes  222 ,  224 ,  226  are typically deposited using plasma etched chemical vapor deposition (PECVD). The PECVD is performed with a phosphorous containing gas. The phosphorous gas can be PH 3 . A silicon containing gas is included when forming amorphous silicon pixel electrodes. 
     An N-layer of amorphous silicon is typically used when forming the pixel electrodes  222   224 ,  226  of the PIN diode active pixel sensors. However, the diode active pixel sensors can include an NIP sensor configuration. In this case, the pixel electrodes  222 .  224 ,  226  arc formed from a P-layer, and the P-layer  230  of FIG. 2 is replaced with an N-layer. 
     The I-layer  220  is generally formed from a hydrogenated amorphous silicon. The I-layer  220  can be deposited using a PECVD or a reactive sputtering process. The PECVD process must include a silicon containing gas. The deposition should be at a low enough temperature that hydrogen is retained within the film. The I-layer  220  is approximately one micron thick. 
     The P-layer  230  is generally formed from amorphous silicon. Typically, the P-layer  230  is doped with Boron. The P-layer  230  can be deposited using PECVD. The PECVD is performed with a Boron containing gas. The Boron containing gas can be B 2 ,H 6 . A silicon containing gas is included when forming an amorphous silicon P-layer  230 . The P-layer  230  thickness must generally be controlled to ensure that the P-layer  230  does not absorb too much short wavelength (blue) light. 
     Another embodiment of the invention does not include a P-layer  230 . The P-layer  230  can be eliminated with proper selection of the composition of the material within the conductive mesh  240 , and proper selection of the doping levels of the pixel electrodes  222 ,  224 ,  226 . For this embodiment, the conductive mesh  240  provides a conductive connection between a top surface of the I-layer  220  of the pixel sensors and the interconnection structure  210 . 
     As previously described, the pixel electrodes  222 ,  224 ,  226 , the I-layer  220  and the P-layer  230  are generally formed from amorphous silicon. However, the pixel electrodes  222 ,  224 ,  226 , the I-layer  220  and the P-layer  230  can also be formed from amorphous carbon, amorphous silicon carbide, amorphous germanium, or amorphous silicon-germanium. It should be understood that this list is not exhaustive. 
     The conductive mesh  240  provides a conductive connection between the P-layer  230  and the interconnection structure  210 . Light must pass through the apertures  310  of the conductive mesh  240  and is received by the pixel sensors. The conductive mesh  240  is formed from an opaque conductive material. Materials that can be used to form the conductive mesh  240  include indium tin oxide, aluminum, tungsten or copper. It is to be understood that this list is not exhaustive. 
     The conductive mesh  240  can be formed by depositing the conductive mesh material  240  by a sputtering process, and then etching the conductive mesh material according to a mesh pattern to form the conductive mesh  240 . Deposition through sputtering and etching according to a pattern are well known in the art of integrated circuit fabrication. 
     FIG. 4 shows another embodiment of the invention. This embodiment includes I-layer sections  410  rather than a single I-layer  220 . That is, each pixel sensor includes a separate I-layer section  410  rather than a single I-layer  220  which is formed as a part of many pixel sensors. Like the pixel electrodes  212 ,  214 ,  216 , the I-layer sections can be etched according to a predetermined pattern. 
     FIGS. 5-9 show processing steps which can be used to fabricate the embodiments shown in FIG. 2, FIG.  3  and FIG.  4 . 
     FIG. 5 shows a substrate  200  with an interconnection structure  210  formed over the substrate  200 . Pixel electrodes  222 ,  224 ,  226  are formed over the interconnection structure  210 . The structure and methods of forming this interconnection structure  210  are well known in the field of electronic integrated circuit fabrication. The interconnection structure  210  can be a subtractive metal structure, or a single or dual damascene structure. 
     The interconnection structure  210  includes conductive vias  212 ,  214 ,  216 ,  218  that are generally formed from tungsten. Tungsten is generally used because during fabrication, tungsten can fill high aspect ratio holes. That is, tungsten can be used to form narrow and relatively long interconnections. Typically, the conductive vias  212 ,  214 ,  216 ,  218  are formed using a chemical vapor deposition (CVD) process. Other materials which can be used to form the conductive vias  212 ,  214 ,  216 ,  218  include copper, aluminum or any other electrically conductive material. 
     The pixel electrodes  222 ,  224 ,  226  are deposited on the interconnection structure  210 . A pixel electrode layer is first deposited over the interconnection structure  210 . Then the pixel electrode layer is etched according to a predetermined pattern forming the pixel electrodes  222 ,  224 ,  226 . An individual pixel electrode  222 ,  224 ,  226  is formed for each pixel sensor. The pixel electrodes  222 ,  224 ,  226  can be implemented with N-layer sections. Alternatively, the pixel electrodes  222 ,  224 ,  226  can be implemented with a conductive nitride, like, titanium nitride. 
     The pixel electrodes  222 ,  224 ,  226  arc typically deposited using PECVD. The PECVD is performed with a phosphorous containing gas. The phosphorous containing gas can be PH 3 . A silicon containing gas, such as Si 2 H 6  or SiH 4 , is included when forming amorphous silicon pixel electrodes. The predetermined pixel electrode pattern is formed through a wet or dry etch of the deposited pixel electrode material. 
     FIG. 6 shows an I-layer  220  and a P-layer  230  deposited over the plurality of pixel electrodes  222 ,  224 ,  226 . The I-layer  220  is generally deposited using a PECVD or reactive sputtering process. The PECVD must include a silicon containing gas. The deposition should be at a low enough temperature that hydrogen is retained within the film. The P-layer  230  can also be deposited using PECVD. The PECVD is performed with a Boron containing gas. The Boron containing gas can be B 2 H 6 . A silicon containing gas is included when forming an amorphous silicon P-layer  230 . 
     FIG. 7 shows the P-layer  230  and the I-layer  220  having been etched to provide access to the fourth conductive via  212 . The fourth conductive via  212  is electrically connected to a reference voltage on the substrate  200  which is used to bias the array of pixel sensors. 
     FIG. 8 shows a conductive layer  810  deposited over the P-layer  230  which provides an electrical connection between the P-layer  230  and the fourth conductive via  212 . The conductive layer  810  is formed from an opaque conductive material. Materials that can be used to form the conductive layer  810  include aluminum, tungsten or copper. It is to be understood that this list is not exhaustive. 
     The conductive layer  810  is generally deposited through reactive sputtering. However, the conductive layer  810  can also be grown by evaporation, or deposited through chemical vapor deposition or plasma vapor deposition. 
     FIG. 9 shows the conductive layer  810  having been etched according to a predetermined pattern forming the conductive mesh  240 . Etching of conductive materials is well known in the art of semiconductor processing. 
     Although specific embodiments of the invention have been described and illustrated, the invention is not to be limited to the specific forms or arrangements of parts so described and illustrated. The invention is limited only by the claims.