Abstract:
An imaging array and method for fabricating the same are disclosed. The imaging array includes a semiconductor substrate having a plurality of VIS pixel sensors and a plurality of SWIR readout circuits fabricated therein. An insulating layer is deposited on the semiconductor substrate. The insulating array has wells overlying the SWIR pixel sensors. A plurality of SWIR photodiodes are deposited in the wells. Each SWIR photodiode is located in a corresponding one of the wells and is connected by an electrically conducting path with the SWIR readout circuit underlying the SWIR photodiode. An electrically conducting transparent electrode overlying the SWIR photodiodes is connected to each of the SWIR photodiodes.

Description:
BACKGROUND 
       [0001]    Silicon-based image sensors provide high sensitivity in the visible portion of the optical spectrum. Improvements in CMOS sensors have provided the basis for low light detection in the visible portion of the spectrum. However, under night viewing conditions, a significant portion of the energy is at wavelengths greater than 1100 nm, which is the longest wavelength that is detectable in a silicon imaging array. Hence, an imaging system that is to operate under nighttime conditions requires a different material system to detect the energy in the near infrared (NIR) portion of the spectrum. 
         [0002]    Material systems that can detect NIR photons are known to the art. For example, quantum dot (QD) materials convert NIR photons into electrons that can be collected and measured using CMOS circuitry. QD materials have bandgaps that can be tuned by adjusting the dot size. Such materials have been suggested for use in multi-junction solar cells to improve the efficiency of the cells. 
         [0003]    An image sensor that can operate both in the NIR and the visible would be advantageous for forming images both under daylight and nighttime viewing conditions. Since QD based photodetectors rely on different material systems from conventional CMOS imaging detectors, providing such a hybrid imaging sensor presents significant challenges. 
       SUMMARY 
       [0004]    The present invention includes an imaging array and a method for fabricating the same. The imaging array includes a semiconductor substrate having a plurality of visible (VIS) pixel sensors that are sensitive to light in the visual portion of the optical spectrum and a plurality of short wave infrared (SWIR) readout circuits fabricated therein that process light from SWIR photodiodes that are sensitive to light in the short wavelength portion of the infrared spectrum. An insulating layer is deposited on the semiconductor substrate. The insulating array has wells overlying the SWIR readout circuits. A plurality of SWIR photodiodes are deposited in the wells. Each SWIR photodiode is located in a corresponding one of the wells and is connected by an electrically conducting path with the SWIR readout circuit underlying the SWIR photodiode. An electrically conducting transparent electrode overlies the SWIR photodiodes and is connected to each of the SWIR photodiodes. 
         [0005]    In one aspect of the invention, the SWIR readout circuits include a capacitive transimpedance amplifier with a feedback capacitor having a capacitance that is controlled by a first bias signal, the first bias signal varying in potential during a readout of one of the SWIR photodiodes connected to the SWIR readout circuit. 
         [0006]    In another aspect, the imaging array includes a bias circuit that applies the first bias signal to the SWIR readout circuits and a second bias signal to the electrically conducting transparent electrode. 
         [0007]    In another aspect of the invention, a controller sets the first and second bias signals based on the output of the SWIR readout circuits in a previous image. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a top view of a portion of a hybrid image sensor according to one embodiment of the present invention. 
           [0009]      FIG. 2  is a cross-sectional view of a portion of hybrid image sensor (HBIS)  20  through line  2 - 2  shown in  FIG. 1 . 
           [0010]      FIG. 3  is a cross-sectional view through one embodiment of the SWIR photodiode according to the present invention. 
           [0011]      FIG. 4  is a schematic drawing of one embodiment of a SWIR readout circuit according to the present invention. 
           [0012]      FIG. 5  illustrates an embodiment of a VIS pixel that can be used in the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    Refer now to  FIG. 1 , which is a top view of a portion of a hybrid image sensor according to one embodiment of the present invention. HBIS  20  includes a two-dimensional array of pixels organized as a plurality of rows and columns. The VIS pixels include pixels that are sensitive to light in the visual range of the optical spectrum such as pixel  21  and SWIR pixels that are sensitive in the short wavelength region of the optical spectrum such as pixel  22 . It should be noted that there are many more rows and columns of pixels than those shown in  FIG. 1 . To simplify the drawing only a 4×4 sub-array is shown. 
         [0014]    For the purposes of the present discussion, a SWIR pixel sensor is defined to be a pixel sensor that converts light having wavelengths between 1.1 μm and 2 μm to an electrical signal that depends on the intensity of that light. A VIS pixel sensor is defined to be a pixel sensor that converts light having wavelengths between 400 nm and 900 nm to an electrical signal that depends on the intensity of that light. 
         [0015]    Refer now to  FIG. 2 , which is a cross-sectional view of a portion of HBIS  20  through line  2 - 2  shown in  FIG. 1 . HBIS  20  includes a CMOS wafer  30  in which the VIS pixel sensors are fabricated. Exemplary VIS pixel sensors are shown at  31  and  32 . 
         [0016]    Each SWIR pixel has two components, a SWIR photodiode that converts light to an electrical signal and a SWIR readout circuit that processes the signal from the SWIR photodiode. The SWIR readout circuits are also constructed in CMOS wafer  30 . The SWIR photodiodes are deposited on top of CMOS wafer  30  after a patterned layer of SiO 2  with conducting pads  35  has been deposited on CMOS wafer  30 . Exemplary SWIR photodiodes are shown at  33  and  34 . As will be explained in more detail below, each conducting pad provides access to the anode of a corresponding SWIR photodiode. The cathodes of the SWIR photodiodes are accessed by a transparent electrically conducting contact  36  that is preferably constructed from ITO. 
         [0017]    The area to be occupied by the SWIR photodiodes is bounded by additional SiO 2  deposits to create wells in which the SWIR photodiodes are deposited. A transparent conducting layer such as ITO is deposited over SWIR pixels and the SiO 2  regions  37  to provide one of the two contacts needed to power the SWIR photodiodes, the remaining contact being transparent electrically conducting contact  36 . 
         [0018]    The readout circuitry for the SWIR pixels is divided into two components. The first component shown at  38  is constructed in CMOS wafer  30 . There is one such SWIR readout circuit per SWIR photodiode. The other component is shared by all of the SWIR photodiodes that are connected to transparent electrically conducting contact  36 . 
         [0019]    Refer now to  FIG. 3 , which is a cross-sectional view through one embodiment of the SWIR photodiode according to the present invention. SWIR photodiode  40  is constructed by successive depositions into a corresponding well whose sides are determined by the SiO 2  regions shown at  37  in  FIG. 2 . Initially, an Au bond pad  41  is deposited over the corresponding Al pad shown in  FIG. 2 . An electron blocking layer (EBL)  42  is then deposited on Au bond pad  41 , followed by a layer  43  of quantum dot material. In one embodiment, PbS quantum dots are used for quantum dot layer  43 . Finally, a hole blocking layer (HBL)  44  is deposited over quantum dot layer  43 . 
         [0020]    Refer now to  FIG. 4 , which is a schematic drawing of one embodiment of a SWIR readout circuit according to the present invention. Readout circuit  51  is connected to the anode of SWIR photodiode  52 , which is biased via driver circuit  53 . Readout circuit  51  includes a capacitive transimpedance amplifier (CTIA)  61  that has a gain that is set by voltage controlled capacitor  56 . Prior to making a measurement, voltage controlled capacitor  56  is shorted via switch  57 . The signal from SWIR photodiode  52  is typically prone to noise and dark current problems. By making a number of measurements at different bias levels these problems can be substantially reduced. Bias circuit  54  allows the bias of the SWIR photodiode to be set to a different voltage from the control signal to the voltage controlled capacitor. 
         [0021]    The SWIR photodiodes have a parasitic capacitance associated with the diodes. This parasitic capacitance alters the gain of CTIA  61  and alters the overall noise. The output bias voltage from bias circuit  54  is determined by a logic circuit included in controller  91  that analyzes the digitized output from the CTIA and adjusts the bias voltage accordingly. For example, if the original output from CTIA is too low, the logic block will control the bias circuit  54  to generate a higher bias voltage; if the CTIA output is too high, the bias voltage will be reduced. 
         [0022]    In one aspect of the invention, the bias voltage will be applied globally to all the SWIR pixels. The control logic in controller  91  makes decisions based on the averaged output from all the CTIAs. The bias voltage is adjusted from frame to frame. If controller  91  determines that the output from the CTIAs are low in the previous frame, the bias voltage is changed before the next frame integration starts. 
         [0023]    In one embodiment, the SWIR pixel sensors are arranged in a rectangular array of rows and columns of SWIR pixel sensors. Each SWIR pixel sensor in a given column is connected to a column readout line  59  by a gate  62  that is controlled by a word line that selects all of the SWIR pixel sensors in a given row. The output from the SWIR pixel sensor currently connected to each column readout line is processed by a column readout circuit  58  that optionally includes an analog-to-digital converter that provides a digital value that measures the intensity of light detected by the SWIR pixel sensors in that column. 
         [0024]    Refer now to  FIG. 5 , which illustrates an embodiment of a VIS pixel that can be used in the present invention. The VIS pixel sensors are organized as a plurality of columns and rows of pixel sensors. During the readout operations, VIS pixel sensors in a selected row are readout in parallel. Each of the VIS pixel sensors in the selected row is connected to a corresponding readout line. In the example shown here, VIS pixel sensor  71  is connected to readout line  82  by applying a word select signal on line  78 . At any given time, at most one pixel sensor is connected to readout line  82 . The output from column readout circuit  58  is processed by controller  91  to provide the image from the VIS pixels. Controller  91  also operates the word select lines  78  and generates the Reset and Tx signals. 
         [0025]    VIS pixel sensor  71  typically includes a photodiode  72  that accumulates charge during the image exposure. At the end of the image exposure, floating diffusion node  73  is reset to Vr by closing gate  74  using a signal on a Reset line. The voltage on floating diffusion node  73  after the reset operation is measured via column readout circuit  81  and stored therein. Source follower  76  provides the power amplification needed to drive readout line  82  and the circuitry connected thereto. 
         [0026]    After this voltage measurement, gate  75  is placed in a conducting state in response to a signal on Tx. The positive potential on floating diffusion node  73  causes all of the charge on photodiode  72  to be transferred to floating diffusion node  73 . The transferred charge results in a decrease in the potential of floating diffusion node  73  relative to the reset potential. The potential on floating diffusion node  73  is then measured again by column readout circuit  81 . The difference in voltage between the two readings is then used to determine the charge that was generated by photodiode  72  during the exposure. 
         [0027]    In the above-described embodiments, the SWIR pixel sensors are connected to different column readout lines. However, embodiments in which both types of sensors in a given column are connected to the same column readout line can be constructed. In such common column readout line embodiments, the column readout circuitry must be able to switch between two modes, one for the VIS pixel signals and one for SWIR pixel sensors. 
         [0028]    The above-described embodiments of the present invention have been provided to illustrate various aspects of the invention. However, it is to be understood that different aspects of the present invention that are shown in different specific embodiments can be combined to provide other embodiments of the present invention. In addition, various modifications to the present invention will become apparent from the foregoing description and accompanying drawings. Accordingly, the present invention is to be limited solely by the scope of the following claims.