Patent Publication Number: US-6902946-B2

Title: Simplified upper electrode contact structure for PIN diode active pixel sensor

Description:
This is a divisional of application Ser. No. 09/810,852, filed Mar. 16, 2001 now U.S. Pat. No. 6,649,993. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to PIN photo diode active pixel sensors. In particular, the present invention relates to an elevated PIN diode sensor with a simplified upper electrode contact structure. 
     2. Description of the Background 
     Image sensors such as charged coupled devices (CCDs) and active pixel sensors are used in a wide range of applications such as digital cameras, camcorders, and night vision enhancement systems. In these applications, light detected at an array of such image sensors is converted to electrical signals that have amplitudes proportional to the intensity of the light. Thus, 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 CCDs. CCDs are relatively small and can provide a high-fill factor. However, CCDs are very difficult to integrate with digital and analog circuitry. Furthermore, CCD systems dissipate large amounts of power and suffer from image smearing problems. 
     Active pixel sensors are an alternative to CCD 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 (N-type layer)  14  of the PIN diodes to the substrate  10 , such as a silicon substrate. An I-layer (intrinsic layer)  16  is formed over the N-layer  14 . A P-layer (P-type 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 that 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. 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 is an electrical connection which is not reliable and which is expensive to produce. 
     It is desirable to have an active pixel sensor formed adjacent to a substrate in which a transparent conductor is reliably electrically connected to a pixel sensor bias voltage which originates on the substrate. 
     SUMMARY 
     An active pixel sensor is provided that includes a semiconductor substrate, an interconnection structure adjacent to the substrate, and a sensor interconnect structure adjacent to the interconnection structure. Photo sensors that contain individual pixel electrodes and an I-layer are formed over the sensor interconnect structure. A transparent conductor is deposited over both the photo sensors and an exposed conductive element in the interconnection structure. The conductive element passes through the interconnection structure to the substrate and allows a pixel sensor bias voltage that originates from circuitry within the substrate to be applied to the transparent conductor. A second conductive element in the interconnection layer is left exposed to allow connection to external packaging or other devices. 
     The substrate may contain active circuits to sense charge accumulation by the photo sensors due to the photo sensors receiving light. The photo sensors may include an additional P-layer formed between the I-layer and the transparent conductor, with the inner surface of the transparent conductor electrically connected to the P-layer, the I-layer, and the pixel interconnect layer. 
     In one embodiment, the semiconductor substrate contains a junction contact layer over which the interconnection structure has been removed. The transparent conductor is deposited over the photo sensors and the exposed junction contact layer in the substrate itself. This allows a pixel sensor bias voltage that originates from circuitry within the substrate to be applied directly to the transparent conductor. 
     In one embodiment, the active pixel sensor is formed by forming the interconnection structure adjacent the substrate and the sensor interconnect structure adjacent the interconnection structure. At least one pixel electrode is formed adjacent the sensor interconnect structure and an I-layer is deposited over the at least one pixel electrode and pixel interconnect layer. A portion of the I-layer and pixel interconnect layer is removed to expose the conductive element. A transparent conductor is deposited over the I-layer and conductive element. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  illustrates a cross-section of a prior art array of image sensors. 
         FIG. 2  is a cross-sectional view of an active pixel sensor in accordance with an embodiment of the invention. 
         FIG. 3  is a cross-sectional view of an active pixel sensor in accordance with another embodiment of the invention. 
         FIG. 4  is a cross-sectional view of a substrate with an interconnection structure and sensor interconnect structure formed over the substrate. 
         FIG. 5  is a cross-sectional view of pixel electrodes deposited on the sensor interconnect structure illustrated in FIG.  4 . 
         FIG. 6  is a cross-sectional view of an I-layer and P-layer deposited over the pixel electrodes illustrated in FIG.  5 . 
         FIG. 7  is a cross-sectional view showing the I-layer, P-layer and pixel interconnect layer of  FIG. 6  selectively etched to expose a conductive contact region in the interconnection structure. 
         FIG. 8  is a cross-sectional view of a transparent conductor deposited over the structure illustrated in FIG.  7 . 
         FIG. 9  is a cross-sectional view showing the I-layer, P-layer and pixel interconnect layer of  FIG. 6  selectively etched to expose both the conductive contact region and bond pad in the interconnection structure. 
         FIG. 10  is a cross-sectional view of a transparent conductor that has been deposited over the structure illustrated in FIG.  9  and selectively etched from bond pad  65 . 
         FIG. 11  is a cross-sectional view showing the I-layer, P-layer, and pixel interconnect layer of  FIG. 6  selectively etched to expose both a conductive contact region and bond pad in the interconnection structure. 
         FIG. 12  is a cross-sectional view of a transparent conductor deposited over the structure illustrated in FIG.  11  and selectively etched from one of the bond pads. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  shows one embodiment of an active pixel sensor  200 . In sensor  200 , the transparent conductor  50  directly contacts conductive element  56 ,  57  in an interconnection structure  42  to electrically connect the transparent conductor  50  to a pixel sensor bias voltage that originates in the substrate  40 . 
     In the structure of sensor  200 , the interconnection structure  42  is formed adjacent to the substrate  40 . A sensor interconnect structure  43  is formed adjacent to the interconnection structure  42 . Each pixel sensor of an array of pixel sensors includes an individual pixel electrode  44  and an inner metal section  45 . Pixel electrodes  44  and the inner metal section  45  are formed adjacent to the sensor interconnect structure  43 . Each individual pixel electrode  44  is electrically connected to the substrate  40  through individual conductive vias  52 ,  54  in the sensor interconnect structure  43 . An I-layer (intrinsic layer)  46  is formed adjacent to the pixel electrodes  44 . A P-layer (P-type layer)  48  is formed adjacent to the I-layer  46 . 
     A transparent conductor  50 , which may be any conductive contact layer, is formed adjacent to the P-layer  48 . The transparent conductor  50  is electrically connected to the substrate  40  through direct contact with a conductive element  56 ,  57  in the interconnection structure  42 . In the sensor  200  illustrated in  FIG. 2 , the conductive element is shown as a conductive contact region  56 , e.g., a bond pad, and metal plug  57 . However any conductive element, such as a metal line, that passes through the interconnection structure  42  may be used to form the electrical connection to the substrate  40 . 
     Sensor structure  200  generally includes additional conductive elements, such as bond pad  65 , that do not contact conductor  50  and allow connection of the sensor to packaging or other external device. 
     The pixel sensors are photo sensors that conduct charge upon receiving light. The substrate  40  generally includes sense circuitry and signal processing circuitry. The sense circuitry senses how much charge the pixel sensors have collected during a “shutter” period. The amount of charge conducted represents the intensity of light received by the pixel sensors during the shutter period. Generally, the substrate circuitry can be CMOS (complementary metal oxide silicon), BiCMOS or Bipolar and can be any compound semiconductor such as, e.g., GaAs or InP. The substrate can include various types of substrate technology including charged coupled devices. 
     Typically, the interconnection structure  42  is a standard CMOS interconnection structure. The structure and methods of forming this interconnection structure are well known in the field of electronic integrated circuit fabrication. The interconnection structure  42  can be a subtractive metal structure, or a single or dual damascene structure. 
     The sensor interconnect structure  43  is typically formed from silicon oxide or a silicon nitride with metal filled vias. The sensor interconnect structure  43  provides reliability and structural advantages to the pixel sensor structure. The pixel interconnect structure allows for the formation of thin pixel electrodes  44  because the pixel electrodes  44  are formed over silicon rather than a metal pad located on the interconnection structure  42 . The pixel interconnect structure  43  electrically connects the pixel electrodes  44  to the interconnection structure  42 . 
     The conductive vias  52 ,  54  pass through the sensor interconnect structure  43  and electrically connect the pixel electrodes  44  to the substrate  40 . The sensor interconnect structure  43  allows this interconnection circuitry to be tightly packed because the vias  52 ,  54  are located directly underneath the pixel electrodes, which conserves lateral space. Additionally, the sensor interconnect structure  43  allows the formation of vias  52 ,  54  having a minimal diameter. Typically, conductive vias  52 ,  54  having a minimal diameter are formed from tungsten using a CVD process. 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. However, the temperatures required to form tungsten vias with a CVD process are greater than many of the materials (amorphous silicon for example) used to form the pixel electrodes can withstand. By forming the sensor interconnect structure  43  over the substrate  40 , and the pixel electrodes  44  over the sensor interconnect structure  43 , the vias  52 ,  54  can be formed before the pixel electrodes  44 , and thus, the pixel electrodes  44  are not subjected to the high temperatures required for the formation of vias  52 ,  54 . Other materials that may be used to form the conductive vias  52 ,  54  include copper, aluminum, or any other electrically conductive material. 
     The inner metal section  45  typically includes a thin conductive material. The inner metal section  45  may be formed, for example, from a degenerately doped semiconductor layer, aluminum, titanium, titanium nitride, copper or tungsten. The inner metal section  45  should be thin (approximately 500 angstroms) and smooth. The inner metal section  45  should be smooth enough so that any surface roughness is substantially less than the thickness of the pixel electrode  44  formed over the inner metal section  45 . To satisfy the smoothness requirement, polishing of the inner metal section  45  may be required. 
     The inner metal section  45  can be optional. However, the inner metal section  45  has a lower resistance than the materials used to form the pixel electrodes  44 . Therefore, the inner metal section  45  provides better current collection. 
     The pixel electrodes  44  are generally formed from a doped semiconductor. The doped semiconductor can be, for example, an N-layer (N-type layer) of amorphous silicon, which may be doped with, for example, phosphorous. Alternatively, the pixel electrodes  44  can be implemented with a conductive nitride, e.g., titanium nitride. The pixel electrode must be thick enough and doped heavily enough so that the pixel electrodes  44  do not fully deplete when biased during operation. 
     Although an N-layer of amorphous silicon is typically used when the active pixel sensors have a PIN diode configuration, the active pixel sensors can include an NIP sensor configuration. In this case, the pixel electrodes  44  are formed from a P-layer, and the P-layer  48  of  FIG. 2  is replaced with an N-layer. 
     The sensor  200  includes an I-layer  46  that is typically formed from a hydrogenated amorphous silicon. I-layer  46  is electrically connected to the transparent conductor  50 . The I-layer includes a resistive path between the electrodes  44  and the transparent conductor  50 . The resistance of the resistive path between the end electrode (the electrode  44  electrically connected to the conductive via  54 ) and the transparent conductor  50  is directly dependent on the distance  47 . Increasing the resistance minimizes leakage current-that flows through the resistive path. Therefore, the end electrode should be located so that a distance  47  between the edge of the end electrode and the transparent conductor  50  is maximized. 
     The P-layer  48  is generally formed from amorphous silicon. Typically, the P-layer  48  is doped with Boron. The P-layer  48  thickness must generally be controlled to ensure that the P-layer  48  does not absorb too much short wavelength (blue) light. 
     Another embodiment of sensor  200  does not include a P-layer  48 . The P-layer can be eliminated with proper selection of the composition of the material within the transparent conductor  50 , and proper selection of the doping levels of the pixel electrodes  44 . For this embodiment, the transparent conductor  50  provides a conductive connection between a top surface of the I-layer  46  of the pixel sensors and the interconnection structure  42 , rather than just between an edge surface of the I-layer  46  and the interconnection structure  42 . 
     As previously described, the pixel electrodes  44 , the I-layer  46 , and the P-layer  48  are generally formed from amorphous silicon. However, the pixel electrodes  44 , the I-layer  46 , and the P-layer  48  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 transparent conductor  50  provides a conductive connection between the P-layer  48  and the I-layer  46  of the pixel sensors, and the interconnection structure  42 . Transparent conductor  50  is typically transparent to light in the visible wavelength range, but, as sensor  200  may be constructed to detect various wavelengths of electromagnetic radiation, e.g., x-rays, transparent conductor  50  need only be transparent to the relevant wavelengths, and may be any conductive contact layer. 
     Light that is received by the pixel sensors must pass through the transparent conductor  50 . Both the selection of the type of material to be used within the transparent conductor  50 , and the determination of the desired thickness of the transparent conductor  50 , are based upon minimizing the reflection of light received by the pixel sensor. Minimization of the reflection of light received by the pixel sensor helps to optimize the amount of light detected by the pixel sensor. Transparent conductor  50  is typically formed from an indium tin oxide, but may also be formed from titanium nitride, thin silicide, or certain types of transition metal nitrides or oxides. 
     A protective layer may be formed over the transparent conductor  50 . The protective layer provides mechanical protection, electrical insulation, and can provide some anti-reflective characteristics, and is typically formed from a thin dielectric film such as SiO 2 , which may be, for example, 5,000 angstroms thick. 
     Another embodiment includes Schottky diode sensors. Schottky diode sensors include several different configurations. A first Schottky diode configuration includes the electrodes  44  being formed from a conductive metal. This configuration also includes the I-layer  46  and the P-layer  48 . A second Schottky diode configuration includes the electrodes  44  being formed from a conductive metal and the P-layer  48  being replaced with a transparent conductor or a transparent silicide. A third Schottky diode configuration includes the electrodes  44  being formed from an N-layer, and the P-layer being replaced with a transparent conductor. The transparent conductor of the third configuration must exhibit a proper work function. Conductive metals that may be used for the Schottky configurations include, but are not limited to, chrome, platinum, aluminum and titanium. 
       FIG. 3  illustrates another embodiment for an active pixel sensor. Sensor  300  is similar to sensor  200  except that instead of using conductive element  56 ,  57  through interconnection structure  42  to electrically connect transparent conductor  50  to substrate  40 , transparent conductor  50  directly contacts the substrate  40 . The semiconductor substrate  40  has, for example, a junction contact layer  58  made from, e.g., titanium silicide, that is electrically connected to a pixel sensor voltage that originates in circuitry within the substrate  40 . The physical contact between the transparent conductor  50  and the junction contact layer  58  electrically connects the transparent conductor  50  to the pixel sensor voltage. 
       FIGS. 4-8  illustrate a process sequence that can be used to fabricate an active pixel sensor  200  as illustrated in FIG.  2 . 
       FIG. 4  shows a substrate  40  with a standard interconnection structure  42  and a sensor interconnect structure  43  formed over the substrate  40 . The methods of forming structures  42 ,  43  are well known in the field of electronic integrated circuit fabrication. The interconnection structure  42  can be a subtractive metal structure, or a single or dual damascene structure, and typically includes metal plugs  57 , conductive contact regions  56 , or other conductive element such as metal lines, and bond pads  65 . The conductive vias  52 ,  54  are typically formed using a chemical vapor deposition (CVD) process of tungsten or other metal. 
       FIG. 5  shows pixel electrodes  44  and inner metal sections  45  deposited on the sensor interconnect structure  43 . An inner metal layer and a pixel electrode layer are first deposited on the sensor interconnect structure  43 . The pixel electrode layer and an inner metal layer are then etched according to a predetermined pattern to form the pixel electrodes  44  and the inner metal layers  45 . An individual pixel electrode  44  and inner metal section  45  are formed for each pixel sensor. 
     The pixel electrodes  44  are typically deposited using plasma enhanced chemical vapor deposition (PECVD). The PECVD is performed with a phosphorous containing gas, for example, PH 3 . A silicon containing gas, such as Si 2 H 6  or SiH 4 , is included when forming amorphous silicon pixel electrodes  44 . 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  46  and a P-layer  48  deposited over the plurality of pixel electrodes  44 . The I-layer  46  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 so that hydrogen is retained within the film. The P-layer  48  can also be deposited using PECVD. The PECVD is performed with a Boron containing gas, for example B 2 H 6 . A silicon containing gas is included when forming an amorphous silicon P-layer  48 . 
       FIG. 7  shows the P-layer  48  and the I-layer  46  having been etched to remove these layers from the portion of the interconnect structure  43  which lies over the conductive contact region  56  and bond pad  65 . The sensor interconnect structure  43  is then selectively etched from the region over conductive contact region  56  to expose conductive contact regions  56 . As discussed above, the conductive contact region  56  is electrically connected to a reference voltage on the substrate  40  that is used to bias the array of pixel sensors. 
       FIG. 8  shows the transparent conductor  50  deposited over the P-layer  48  and conductive contact region  56  to provide an electrical connection between the P-layer  48 , the I-layer  46 , and the substrate  40 . The transparent conductor  50  is generally deposited through a reactive sputtering process, which is well known in the art of integrated circuit fabrication. However, the transparent conductor  50  can also be grown by evaporation. If the transparent conductor  50  is formed from titanium nitride, then typically a CVD process or a sputtering process must be used to deposit the transparent conductor  50 . 
     The transparent conductor  50  and the sensor interconnect structure  43  are then etched according to a predetermined pattern to expose additional conductive elements such as bond pad  65 . This etching allows access to the additional conductive elements of the interconnection structure  42 , resulting in the structure of sensor  200  illustrated in FIG.  2 . 
     To form the sensor structure  300  illustrated in  FIG. 3 , a similar process sequence as that illustrated in  FIGS. 4-8  may be used with some modification. The formation of conductive element  56 ,  57  in interconnection structure  42  is unnecessary. Additionally, when etching through the sensor interconnect structure  43 , as illustrated in  FIG. 7 , the etch is continued to remove a portion of the interconnection structure  42  over the junction contact layer  58  (shown in FIG.  3 ), to expose the junction contact layer on the surface of substrate  40 . Transparent conductor  50  is then deposited and etched as described above in reference to FIG.  8 . 
       FIGS. 9 and 10  illustrate another process sequence that may be used to form the active pixel sensors. In this embodiment, after depositing the I-layer  46  and P-layer  48 , as was illustrated in  FIG. 5 , portions of the I-layer  46  and P-layer  48  which are not part of the photo sensor diode (e.g., not over pixel electrodes  44 ) are selectively etched from the structure. The interconnection structure  42  is then selectively etched to remove interconnection structure  42  that is over the conductive contact region  56  and bond pad  65 , as illustrated in FIG.  9 . 
     A layer of transparent conductor  50  is then deposited over the entire structure of  FIG. 9 , and then patterned to remove conductor  50  from everywhere except over the photo sensor diode and conductive contact region  56 , leaving sensor structure  400  as illustrated in FIG.  10 . Thus, in sensor  400 , the transparent conductor  50  remains only over the photo sensor and conductive element  50 , which may reduce the likelihood of shorting the active pixel sensor when making contact to bond pad  65 . 
     In the process sequence illustrated in  FIGS. 4-8  and  9 - 10 , two selective etches are needed to obtain the structures illustrated in FIG.  7  and  FIG. 9. A  portion of the I-layer  46  and P-layer  48  is first selectively etched from the region that is over the conductive contact region  56  and bond pad  65 . A portion of the sensor interconnect structure  43  is then selectively etched from the region over conductive contact region  56 , as shown in  FIG. 7 , or from both the region over conductive contact region  56  and bond pad  65 , as shown in FIG.  9 . This process sequence may be simplified, as illustrated in  FIGS. 11 and 12 . 
     After depositing the I-layer  46  and P-layer  48 , as was illustrated in  FIG. 6 , the portion of the I-layer  46 , P-layer  48 , and sensor interconnect structure  43  that is over both conductive contact region  56  and bond pad  65  can be entirely removed, as illustrated in FIG.  11 . These layers may be removed in a single etch, which is typically a wet or dry chemical etch as described above. 
     The transparent conductor  50  is then deposited over the P-layer  48  and the exposed conductive contact region  56  and bond pad  65 . The transparent conductor  50  is subsequently etched according to a predetermined pattern to expose to bond pad  65  of the interconnection structure  42 , resulting in active pixel sensor structure  500  of FIG.  12 . The sensor structure  500  can thus be formed with fewer processing steps than the sensor structure  200  of FIG.  2 . However, sensor interconnect structure  43  may contain a passivation layer, so removal of the entire layer as illustrated in  FIG. 11  may leave the sensor  500  unpassivated. 
     As stated previously, after deposition of transparent conductor  50 , a protective layer (not shown) may be formed over the transparent conductor  50  using conventional methods. The protective layer provides mechanical protection and electrical insulation, and can provide some anti-reflective characteristics. 
     While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspects. Therefore, the appended claims are to encompass within their scope all such changes and modifications as fall within the scope of this invention.