Abstract:
A global shutter pixel cell includes a serially connected anti-blooming (AB) transistor, storage gate (SG) transistor and transfer (TX) transistor. The serially connected transistors are coupled between a voltage supply and a floating diffusion (FD) region. A terminal of a photodiode (PD) is connected between respective terminals of the AB and the SG transistors; and a terminal of a storage node (SN) diode is connected between respective terminals of the SG and the TX transistors. A portion of the PD region is extended under the SN region, so that the PD region shields the SN region from stray photons. Furthermore, a metallic layer, disposed above the SN region, is extended downwardly toward the SN region, so that the metallic layer shields the SN region from stray photons. Moreover, a top surface of the metallic layer is coated with an anti-reflective layer.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of U.S. Provisional Patent Application Ser. No. 61/479,059, filed Apr. 26, 2011, which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates, in general, to solid state imaging devices. More specifically, the present invention relates to imaging devices having a global shutter capability. 
     BACKGROUND OF THE INVENTION 
     Digital cameras sample the original light that bounces off a subject to create a digital image. A digital camera may use a shutter in combination with a sensor to determine an exposure time and acquire the proper amount of light to make a good image. The proper amount of light may come from accumulating ambient light over time until the proper amount of light is obtained. Alternatively, the addition of artificial light from a flash speeds up the amount of time, until the proper amount of light is obtained to make a good image. 
     In a typical mechanical shutter, the timing between scan lines of a sensor and the actuation of the shutter is such that the shutter is closed until the sensor (such as a complementary metal-oxide semiconductor (CMOS) sensor, or a charge coupled device (CCD) sensor) is ready to be exposed to the light in the picture frame. When the mechanical shutter opens, nearly 100% of the light is allowed to pass and be sensed by the light sensitive pixels in the sensor. When the mechanical shutter gives the command to close and the shutter closes, the light no longer reaches the sensor. 
     Some digital cameras have a mechanical shutter to control the exposure time of the CMOS sensor. Unfortunately, the inclusion of a mechanical shutter adds significant expense to the cost of a camera. Further, a mechanical shutter due to size and dimensions adds physical bulkiness to the camera. 
     Some cameras may have sensors implementing a global shutter timing mechanism for the light sensitive pixels. These sensors have electronic components added to each pixel location in the sensor. Typically, in a sensor implementing a global shutter timing mechanism, all the pixels begin and end their integration phase at the same time, and the intensities of all the pixels are simultaneously transferred to a light insensitive storage area at the same time. Sensors employing a global shutter timing mechanism, therefore, typically have little to no potential for motion artifacts. 
     A typical single chip CMOS image sensor  199  is illustrated by the block diagram of  FIG. 1 . Pixel array  190  includes a plurality of pixels arranged in a predetermined number of columns and rows. 
     Generally, the rows of pixels in array  190  are read out one by one. Accordingly, pixels in a row of array  190  are all selected for readout at the same time by a row select line, and each pixel in a selected row provides a signal representative of received light to a readout line in its column. In array  190 , each column also has a select line, and the pixels of each column are selectively read out, in response to the column select lines. 
     The row lines in pixel array  190  are selectively activated by a row driver  191 , in response to row address decoder  192 . The column select lines are selectively activated by a column driver  193 , in response to column address decoder  197 . The pixel array is operated by the timing and control circuit  195 , which controls address decoders  192 ,  197  for selecting the appropriate row and column lines for pixel signal readout. 
     The signals on the column readout lines, typically, include a pixel reset signal (Vrst) and a pixel image signal (Vsig) for each pixel. Both signals are read into a sample and hold circuit (S/H)  196 , in response to column driver  193 . A differential signal (Vrst−Vsig) is produced by differential amplifier (AMP)  194  for each pixel, and each pixel&#39;s differential signal is amplified and digitized by analog-to-digital-converter (ADC)  198 . The ADC  198  supplies the digitized pixel signals to an image processor  189 , which can perform appropriate image processing before providing digital signals defining the image. 
     An electronic shutter for image sensors has been developed to serve in place of a mechanical shutter. The electronic shutter controls the amount of photo-generated charge accumulated by a pixel cell by controlling the integration time of the pixel cell. This feature is especially useful when imaging moving objects, or when the image sensor itself is moving and a shortened integration time is necessary for quality images. 
     Typically, a pixel cell having an electronic shutter includes a shutter transistor and a storage device, which is typically a pn-junction capacitor. The storage device stores a voltage representative of the charge generated by a photo-conversion device in the pixel cell. The shutter transistor controls when, and for how long, charge is transferred to the storage device. This controls the integration time of the pixel cell. 
     There are two typical modes of operation for an electronic shutter, namely, rolling shutter and global shutter. When an electronic shutter operates as a rolling shutter, each row of pixels in the array integrates photo-generated charge, one at a time, and each row is read out one at a time. When an electronic shutter operates as a global shutter, all pixels in the array integrate photo-generated charge simultaneously, and each row is read out one at a time. 
     Global shuttering provides advantages over row shuttering. Essentially, global shuttering is able to provide a “snap shot” of the imaged object. Consequently, global shuttering offers increased accuracy of an imaged object, with uniform exposure time and uniform image content. 
     On the other hand, because the pixel cells of the pixel array are read out row by row, pixel cells in a row, that are read out last, must store photo-generated charge in their respective storage devices longer, than pixel cells in rows that are read out earlier. These storage devices may lose charge over time, and the longer these storage devices must store photo-generated charge, there is more charge that is lost. Therefore, charge loss is especially problematic for pixel cells in the last read row. When charge is lost by a pixel cell, the resultant image may have a poor quality, or may be distorted. 
     Additionally, a common problem associated with active pixel imager cells, when operated in global shutter modes, is that storage devices, or storage nodes (SNs), accumulate parasitic charges while holding charge transferred from each photodiode (PD). This lowers the global shutter pixel efficiency and results in appearance of vertical shading; a moving object may also have smears and shadings. Accordingly, what is needed is a pixel cell with an electrical shutter having improved charge transfer efficiency, minimal charge loss, and reduced accumulation of parasitic charges. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The invention may be understood from the following detailed description when read in connection with the accompanying figures: 
         FIG. 1  is a block diagram of a conventional image sensor. 
         FIG. 2  is a global shutter pixel cell forming part of the image sensor shown in  FIG. 1 , in which the global shutter pixel cell is configured in accordance with an embodiment of the present invention. 
         FIG. 3  is a cross-sectional view of a conventional global shutter pixel cell forming part of an image sensor in a semiconductor chip. 
         FIG. 4  is a cross-sectional view of a global shutter pixel cell, configured in accordance with an embodiment of the present invention. 
         FIG. 5  is a timing diagram of the operation of the global shutter pixel cell shown in  FIG. 2 , in accordance with an embodiment of the present invention. 
         FIG. 6  is a plot of storage node parasitic quantum efficiency versus wavelength. 
         FIG. 7  is a plot of the ratio of pixel quantum efficiency to parasitic storage node quantum efficiency versus wavelength. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As will be explained, the present invention provides pixel cells with one or any combination of the following features for increasing global shutter pixel efficiency:
         a. a photodiode (PD) implanted deeper and under an angle covering space under a storage node (SN);   b. the bottom portion of the PD is expanded sideways;   c. a high voltage potential to the PD is applied during pixel storage time;   d. a high dose p-type implant is included in the p-substrate, immediately below the SN, in order to lower the probability of parasitic charges diffusing to the SN;   e. a smaller distance is provided between a metal shield and the SN to limit the amount of light directly flowing into the SN area;   f. metal lines are covered with black material, such as titanium nitride (TiN), in order to limit reflections to the SN area; and   g. less light transparent dielectric, such as hydrogen infused silicon nitride (Si3N4), used everywhere outside of PD area.       

     Referring first to  FIG. 2 , there is shown a global shutter pixel, generally designated as  200 . The global shutter pixel, it will be appreciated, is one of multiple global shutter pixels arranged in rows and columns of an imager array, such as pixel array  190  shown in  FIG. 1 . 
     Each pixel  200  includes a photosensor, for example, a pinned photodiode, or a photogate for accumulating photo-generated charges in a specified portion of the pixel&#39;s substrate. As shown, pixel  200  includes a photosensor, which is implemented as a pinned photodiode (PD), designated as  207 . Pixel  200  also includes storage gate (SG) transistor  202 , storage node (SN) region  209 , anti-blooming (AB) transistor  201 , transfer gate (TX) transistor  203 , floating diffusion (FD) region  205 , reset (RST) transistor  204 , source follower (SF) transistor  206  and row select (SEL) transistor  210 . 
     The PD  207  is connected to SN region (diode)  209  by SG transistor  202 , when the SG transistor is activated by a global storage control signal, SG control. The SN region is connected to FD region  205  via TX transistor  203 , during the time when the TX transistor is activated by a transfer gate control signal, TX control. 
     The RST transistor  204  is connected between FD region  205  and a pixel supply voltage, VaaPix. A reset control signal, RST control, is used to activate RST transistor  204 , which resets FD region  205  to the pixel supply voltage level of VaaPix. Additional activation of the TX and RST transistors resets SN region  209  to the pixel supply level of VaaPix. The SF transistor  206  has its gate connected to FD region  205  and its source and drain terminals connected between supply voltage VaaPix and SEL transistor  210 . 
     The SF transistor  206  converts the charge stored at the FD region into an electrical voltage output signal. The SEL transistor  210  is controlled by a row select signal, SEL control, for selectively connecting the SF transistor and its output voltage signal to a column line of the pixel array. The AB transistor  201  connects PD diode  207  to the supply voltage VaaPix during the time that the AB control signal is applied to the AB transistor. 
     A common problem associated with active pixel imager cells, such as pixel cell  200  of  FIG. 1 , is that when it is operated in a global shutter mode, SN region  209  accumulates parasitic charges while holding charge transferred from PD diode  207 . The accumulation of charge lowers the global shutter pixel efficiency and results in vertical shading, as well as object smearing. The parasitic charges may be accumulated by storage node  209  and its associated capacitance  208 . These parasitic charges are dominated by photon induced parasitic current, as well as charges generated by photons directly within the SN region. 
     Referring next to  FIG. 3 , there is shown a cross-sectional view of pixel cell  300 , which may be included on a semiconductor chip as part of an integrated circuit. It will be understood that any pixel cell described herein, in connection with examples of the present invention, may be included on a semiconductor chip. For simplicity, only pixel cell  300  is depicted as part of the semiconductor chip. 
     As shown in  FIG. 3 , pixel cell  300  is a global shutter pixel with highlighted parasitic current sources that lower global shutter efficiency. For clarity, gate contacts, many dielectric layers and many metal layers have been omitted. Included, however, are dielectric layers  301 ,  302  and  303  containing metal layers  307  and  308 , as indicated. Also included is P-well region  304 , which is embedded in epitaxial layer  305 , the latter disposed above P-substrate  306 . 
     Various elements of pixel cell  200  ( FIG. 2 ) are shown in a cross-sectional view in  FIG. 3 . Viewing  FIG. 3  from left to right, there are the VaaPix node, the anti-bloom gate AB, the storage gate SG, the transfer gate TX, and the reset gate RST. The pinned photodiode region PD is disposed below and between the AB gate and the SG gate. The SN region is located below and under the SG gate; and the FD region is located below and between the transfer gate TX and the reset gate RST. 
     As may clearly be seen in  FIG. 3 , photons may pass around, or via metal shield gaps, reflecting from metal layers, and may hit the SN region directly to generate parasitic charges. A majority of the parasitic charges are generated below or on the sides of the SN region. These parasitic charges may diffuse easily into the SN region, thus resulting in lower shutter efficiency and image degradation. 
     In order to increase global shutter pixel efficiency, the present invention implements the following modifications (these modifications may be a single modification or may be a combination of the listed modifications): 
     (1) Implant the PD region deeper and under an angle covering the space under the SN region; expand sideways the bottom portion of the PD region so that it is under the SN region; and apply a high voltage potential to the PD region during pixel storage time. 
     (2) Implant a high dose p-type isolation region in the p-substrate, immediately below the SN region to lower the probability of parasitic charges diffusing to the SN region. 
     (3) Provide a smaller distance between the metal shield and the SN region to limit the amount of light that directly goes into the SN region; and cover the metal lines with black material, such as titanium nitride (TiN) in order to limit reflections going to the SN region. 
     (4) Use a dielectric material outside of the PD area that is less transparent to light than silicon oxide (SiO2), such as hydrogen infused silicon nitride (Si3N4), to cover the space between the metal layers that may be dictated by layout constraints. 
     Referring next to  FIG. 4 , there is shown an example of an embodiment of the present invention.  FIG. 4  includes dielectric layers  401 ,  402  and  403 , as well as P-well region  404 , epitaxial layer  405  and p-substrate layer  406 , which are similar to elements  301 - 306  shown in  FIG. 3 . Differences between the two figures, however, are highlighted below. These differences contribute to obtaining a global shutter pixel cell having lower parasitic storage node (SN) charges. 
     As shown in  FIG. 4 , the PD region is extended downwardly into the epitaxial layer  405 . In addition, the PD region is widened, so that it extends under the SN region. In extending the PD region sideways and under the SN region, an angled region, designated as  407 , is formed. By extending the PD region toward and below the SN region, the amount of bulk photocurrent that may flow to the SN region is significantly limited. Instead of flowing to the SN region, the photocurrent flows to the PD region. 
     In addition, by applying a positive potential to the PD region during storage time (as described with respect to  FIG. 5 ), parasitic current flows away from the SN region. This may be achieved, for example, by connecting the cathode of PD  207  to VaaPix, upon activation of AB transistor  201 . 
     Furthermore, by adding a high dose P-isolation implant directly below the SN region, as shown in  FIG. 4 , the probability of parasitic charges diffusing to the SN region from below is further reduced. 
     Further still, as shown in  FIG. 4 , providing a smaller distance between the metal shield and the SN region allows less light to pass directly to the SN region. Also shown as a further possible implementation, the present invention may add anti-reflective black material, such as titanium nitride (TiN), to limit the amount of reflected light flowing to the SN region. Additionally, a less transparent dielectric layer (for example, hydrogen infused silicon nitride (Si3N4), located immediately above the SN region limits the amount of light that may also generate parasitic current. 
     In an embodiment of the present invention,  FIG. 5  illustrates signal timing for the global shutter pixel. Operation of the pixel begins with a global reset phase, when the PD, FD and SN structures are connected to VaaPix via the AB, SG, TX and RST gates upon applying corresponding AB, SG, TX and RST control signals. 
     Upon completion of the global reset phase, an acquisition phase begins, when the PD region collects photon generated charges. The acquisition phase is finished with the global SG control signal when it transfers the PD charges to the SN region (within each pixel) for storage. Before this PD to SN charge transfer, all SN regions are discharged completely by connecting the SN regions to the VaaPix terminal via the TX and RST transistors. 
     Upon completion of the charge transfer from PD to SN, the AB transistor is connected to VaaPix to allow parasitic generated current to flow away from the pixel, thus increasing shutter efficiency. Additionally, the AB transistor, when connected to the VaaPix terminal, prevents any pixel blooming effect. 
     During the readout phase, charge stored in the SN region of each pixel in the array is read out by completing a full correlated double sampling (CDS). This is achieved by first connecting the FD region to the VaaPix terminal via the RST transistor, and then performing the FD reset voltage level readout; and, subsequently, connecting the SN region to the FD region via the TX transistor and reading the FD signal level. Potential difference between the FD reset and the signal levels is directly proportional to the SN charge and represents the level of charge accumulated by the PD region during the acquisition phase. 
     It will be appreciated that, as shown in  FIGS. 2 and 5 , the SN region is set to VaaPix during the global reset phase, when the AB transistor and the SG transistor are activated. In addition, the SN region is reset during the acquisition phase, when the TX transistor and the RST transistor are activated. 
     The aforementioned modifications to the global shutter pixel decreases parasitic storage node quantum efficiency during the storage time, due to the PD and SN changes described above, and consequently, improves global shutter efficiency multiple times. It will be appreciated, however, that the aforementioned changes need not be all implemented into the global shutter pixel. Any one change or a combination of changes also result in increased global shutter efficiency. 
     Turning lastly to  FIGS. 6 and 7 , improvements in global shutter efficiency are shown by comparing a conventional PD region with an extended PD region. The extended PD region has a decreased parasitic storage node quantum efficiency during the storage time ( FIG. 6 ) and increased global shutter efficiency ( FIG. 7 ). 
     Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.