Abstract:
An image sensor pixel suitable for use in a back-side-illuminated or a front-side-illuminated sensor arrangement is provided. The image sensor pixel may be a small size pixel that includes a source follower implemented using a vertical junction field effect (JFET) transistor. The vertical JFET source follower may be integrated directly into the floating diffusion node, thereby eliminating excess metal routing and pixel area typically allocated for the source follower in conventional pixel configurations. Pixel area may instead be allocated for increasing the charge storage capacity of the photodiode or can be used to reduce pixel size while maintaining pixel performance. Using a vertical junction field effect transistor in this way simplifies pixel addressing operations and minimizes random telegraph signal (RTS) noise associated with small size metal-oxide-semiconductor (MOS) transistors.

Description:
[0001]    This application claims the benefit of provisional patent application No. 61/569,743, filed Dec. 12, 2011, which is hereby incorporated by reference herein in its entirety. 
     
    
     BACKGROUND 
       [0002]    This relates to solid-state image sensors and, more specifically, to image sensors with small size pixels that are either front-side illuminated or back-side-illuminated. The small pixel size helps to reduce the cost of image sensor arrays. Sensor performance, however, should not be compromised as the size of pixels is reduced. Conventional image sensors detect light by converting impinging photons into electrons that are integrated (collected) in sensor pixels. Upon completion of each integration cycle, the collected charge is converted into voltage signals, which are then supplied to corresponding output terminals associated with the image sensor. Typically, the charge to voltage conversion is performed directly within the pixels, and the resulting analog pixel voltage signals are transferred to the output terminals through various pixel addressing and scanning schemes. The analog signal can sometimes be converted on-chip to a digital equivalent before being conveyed off-chip. Each pixel includes a buffer amplifier commonly referred to as a source follower (SF), which is used to drive output sensing lines that are connected to the pixels via respective address transistors. 
         [0003]    After the charge-to-voltage conversion is complete and after the resulting signals are transferred out from the pixels, the pixels are reset before a subsequent integration cycle begins. In pixels having floating diffusions (FD) serving as the charge detection node, this reset operation is accomplished by momentarily turning on a reset transistor that connects the FD node to a fixed voltage reference for draining (or removing) any charge remaining at the FD node. 
         [0004]    Removing charge from the floating diffusion node using the reset transistor, however, generates kTC-reset noise as is well known in the art. The kTC noise must be removed using correlated double sampling (CDS) signal processing technique in order to achieve desired low noise performance. Image sensors that utilize CDS typically require three transistors (3T) or four transistors (4T) per pixel. An example of the 4T pixel circuit with a pinned photo-diode can be found in Lee (U.S. Pat. No. 5,625,210), incorporated herein as a reference. 
         [0005]      FIG. 1  is a simplified rendering of a cross-sectional side view of a conventional image sensor pixel  100 . As shown in  FIG. 1 , conventional image sensor pixel  100  includes a photodiode  107  configured to collect photon-generated carriers, charge transfer transistor gate  108 , N+ doped floating diffusion region  111 , reset transistor gate  109 , and source follower transistor gate  110 . The reset transistor and the source follower transistor share an N+ drain region  112  that is biased to a fixed positive power supply voltage Vdd. The source follower transistor has an N+ source region  113  that is connected to a column sensing line Vout through metal via  115  (i.e., an output line to which each pixel in a given column is connected). 
         [0006]    Note that floating diffusion region  111  is connected to source follower gate  110  via connection  116 . This connection supplies the signal collected at the floating diffusion region to the source follower transistor gate. Pixel  100  may include an address transistor interposed between the region  113  and sensing line Vout that is common to all pixels in a given column of image sensor pixels. For simplicity, the address transistor is not shown in  FIG. 1 . 
         [0007]    Pixel  100  is fabricated in an epitaxial substrate  101 . A P+ doped layer  102  is deposited on the back surface of the sensor if the sensor is a back-side-illuminated image sensor. Substrate  101  may also be deposited on a substantially thicker P+ substrate (relative to layer  102 ) for the front-side-illuminated image sensors. Epitaxial layer  101  is covered by an oxide layer  103  that provides electrical isolation for gates  108 ,  109 , and  110 . Oxide material  103  typically extends into and fills up shallow trench isolation (STI) regions  114 . An additional oxide layer  104  is deposited over the gates and serves as isolation for the metal wiring formed over pixel  100 . Additional oxide isolation layers and the metal wiring layers are typically deposited over the top of pixel  100  (not shown). 
         [0008]    Photodiode  107  includes a P+ layer  105  that is formed directly below layer  103  and that is connected to ground. This P+ doping layer reduces dark current by filling the silicon-silicon dioxide interface states with holes. Photon generated electrons are accumulated in N-type doped region  106 . The accumulated charge is transferred to N+ floating diffusion region  111  when transfer gate  108  is turned on. Prior to turning on charge transfer gate  108 , floating diffusion region  111  needs to be reset by pulsing signal that is supplied to gate  109  of the reset transistor. 
         [0009]    An additional bottom P-implant (BTP) layer  117  is extended from P+ layer  105  formed under STI region  114  to the region under reset transistor gate  109  and source follower transistor gate  110  (see,  FIG. 1 ). Layer  117  is connected to ground and serves to block photon generated electrons from entering regions  111 ,  112 , and  113 . 
         [0010]    As is apparent from  FIG. 1 , a large portion of the valuable pixel area is occupied by transistor gates  108 ,  109 , and  110 . Forming transistors side-by-side on the surface of substrate  101  using this arrangement may be disadvantageous. It may therefore be desirable to provide image sensors with reduced pixel area, where a smaller portion of the pixel area is occupied by transistors and a larger portion of the pixel area is occupied by the photodiode. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a simplified cross-sectional side view of a conventional image sensor pixel. 
           [0012]      FIG. 2  is a simplified schematic diagram of an illustrative image sensor pixel with a vertical junction gate source follower in accordance with an embodiment of the present invention. 
           [0013]      FIG. 3  is a simplified cross-sectional side view of an illustrative image sensor pixel of the type shown in  FIG. 2  in accordance with an embodiment of the present invention. 
           [0014]      FIG. 4  is an example of a timing diagram illustrating how an image sensor pixel of the type shown in  FIG. 2  may be operated in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]      FIG. 2  is a simplified circuit diagram showing one suitable circuit arrangement for an image sensor pixel such as pixel  200 . As shown in  FIG. 2 , pixel  200  may include a pinned photodiode  207 , a charge transfer transistor  208 , a reset transistor  209 , and a source follower transistor  300 . Charge generated from impinging photons may be collected using photodiode  207 . Reset transistor  209  may be used to reset a floating diffusion node  302  associated with pixel  200  (e.g., by temporarily pulsing reset gate control signal Rx). Following the reset operation, the collected charge may be transferred to floating diffusion node  302  via the gate of charge transfer transistor  208  (e.g., by temporarily pulsing charge transfer gate control signal Tx). Source follower transistor  300  may be used to buffer the signal resulting from the transferred charge that appears on the floating diffusion node and to drive the corresponding column output line Vout. 
         [0016]    Photodiode  207  may have a P-type doped region that is coupled to ground power supply terminal  312  (e.g., a power supply line on which ground power supply signal Vss is provided) and an N-type doped region that is coupled to floating diffusion node  302  via charge transfer transistor  208 . Reset transistor  209  may have a drain region that is coupled to adjustable power supply terminal  310  (e.g., an adjustable power supply terminal on which adjustable power supply voltage Vddx is provided) and a source region that is coupled to floating diffusion node  302 . 
         [0017]    In accordance with an embodiment of the present invention, source follower transistor  300  may be a junction field effect transistor (JFET), whereas transistor  208  and  209  may be n-channel metal-oxide-semiconductor (NMOS) transistors. Source follower transistor  300  may have a source that is coupled to output sensing line Vout, a drain that is coupled to ground  312 , and a gate that is coupled to floating diffusion region  302 . As shown in  FIG. 2 , address transistor  304  (e.g., a transistor that is selectively turned on using address signal Addr) may optionally be interposed in the output path between the source of transistor  300  and output line Vout. 
         [0018]      FIG. 3  shows a simplified cross-sectional side view of image sensor pixel  200 . As shown in  FIG. 3 , pixel  200  may be formed in an epitaxial substrate  201 . A P+ doped layer  202  may be deposited on the back side of substrate  201  if pixel  200  is being used in a back-side-illuminated arrangement. If pixel  200  is being used in a front-side-illuminated configuration, epitaxial substrate  201  may be deposited on a thicker P+ silicon carrier (i.e., a silicon carrier with a thickness that is substantially larger than that of layer  202 ). Layer  202  may serve to reduce dark current by filling the silicon interface states with holes and thus quenching dark current generation. The silicon to silicon dioxide interface on the front side of substrate  201  in a pinned photodiode region  207  is also lined with a P+ doped layer  205  to help reduce dark current generation. Layer  205  may extend into substrate  201  under shallow trench isolation region  250 . 
         [0019]    Pinned photodiode  207  may be formed from P+ doped region  205  and N doped  206 . Impinging photons may generate charge (e.g., electrons) that is temporarily stored in N-type region  206 . Epitaxial substrate  201  may be covered by a dielectric layer such as silicon dioxide layer  203 . Layer  203  may be formed between gate conductors of pixel  200  (e.g., the gate conductors  208 ′ and  209 ′) and substrate  201 . 
         [0020]    N+ region  218  may serve as the drain for reset transistor  209 . N+ region  218  may be coupled to adjustable power supply voltage Vddx through conductive via  219 . Voltage bias Vddx may be used as a reference voltage level to which pixel  200  may be reset. Reset transistor  209  has a gate  209 ′ configured to receive signal Rx via corresponding control routing paths formed on top of inter-level oxide layer  204 . 
         [0021]    Similarly, charge transfer transistor  208  has a gate  208 ′ configured to receive signal Tx via corresponding control routing paths formed on top of layer  204 . 
         [0000]    The detailed routing of control paths on which signals Tx, Vout, Rx, and Vddx are provided is not shown in  FIG. 2  for simplicity. A dielectric stack that also includes alternating layers of conductive via layers and metal routing layers may be formed on top of pixel  200  over layer  204 . 
         [0022]    As shown in  FIG. 3 , pixel  200  may include N-type doped regions  210 ,  212 , and  213  that collectively serve as floating diffusion region  302  for pixel  200 . Floating diffusion region  302  formed in this way simultaneously serves as a source-drain for charge transfer transistor  208 , as a source for reset transistor  209 , and as a gate for source follower transistor  300  (e.g., transistor  300  may have a gate that is integrated into floating diffusion region  302 ). 
         [0023]    A P+ doped region  211  that serves as the source for SF transistor  300  may be formed in region  210 . P-type doped regions  214  and  215  that serve as the channel for transistor  300  may be formed in region  212  and  213 , respectively. Transistor  300  having a p-type channel may sometimes be referred to as a p-channel JFET. A P+ doped region  217  may be formed below and adjacent to regions  214  and  215  to serve as the drain for transistor  300 . Region  217  may be used as a photo-electron blocking layer for preventing pixel cross-talk and may sometimes be referred to as a bottom P-implant (BTP) layer. BTP layer  217  may be continuous with region  205  and may be coupled to ground  312  (see, e.g.,  FIG. 2 ). 
         [0024]    When charge from region  206  in photodiode  207  is transferred onto floating diffusion  302  (i.e., region  210 ,  212 , and  213 ), a voltage change may occur at the floating diffusion node  302 . This voltage change may cause change of the source voltage of the JFET transistor  300  when constant current (or holes) flow from region  211  to region  216 , as indicated by arrows  216 . 
         [0025]    In the example of  FIG. 3 , layers  212  and  213  may serve to define the channel length Lch for transistor  300  that extends vertically into substrate  201 . Transistor  300  formed vertically within the surface of substrate  201  may therefore sometimes be referred to as a vertical JFET. If Lch is too short, SF transistor  300  may exhibit reduced gain due to drain-induce barrier lowering (DIBL) effects. It may therefore be desirable to form multiple regions such as regions  212  and  213  between P+ regions  211  and  217  so that Lch is greater than a predetermined minimum length. For example, at least three N-type regions, at least four N-type regions, or at least five N-type regions each of which includes P-type doped channel regions may be formed between the source and drain regions of vertical JFET source follower  300 . Optimizing source follower channel length in this way does not introduce any undesired pixel area overhead. 
         [0026]    The arrangement as described in connection with  FIG. 3  in which pixel  200  uses P-type doped epitaxial substrate and n-type PD layer  206  to collect photo-generated electrons is merely illustrative and does not serve to limit the scope of the present invention. In some cases it may be advantageous to build pixels that accumulate holes instead of electrons. The same vertical JFET transistor concept described in connection with  FIGS. 2 and 3  can thus be also used for such pixels. The doping type of the substrate, the doping types of the source-drain regions, and the type of the carrier collection should not be construed as a limiting case for this invention. 
         [0027]    For example, the doping type of each region in pixel  200  may be swapped so that pixel  200  uses an N-type doped epitaxial substrate and a P-type PD layer  206  to collect photo-generated holes instead of electrons. Vertical source follower transistor  300  may be an n-channel JFET, wherein the P+ doped BTP layer  217  is replaced by an N+ bottom-implant (BTN) layer. Floating diffusion region  302  may include P-type doped implants  210 ,  212 , and  213 , whereas region  211  becomes an N+ doped region while regions  214  and  215  becomes N-type regions. Likewise, region  218  for associated with reset transistor  209  may also become a P+ doped region. 
         [0028]    The biasing scheme for this type of pixel  200  would also be inverted. For example, charge transfer gate  208  and reset gate  209  may be turned on by respectively pulsing gate control signals Tx and Rx to a negative voltage level. Pixel  200  may instead by reset to an adjustable negative power supply level instead of Vddx. 
         [0029]    As discussed previously in connection with  FIG. 2 , an additional transistor for addressing pixel  200  need not be formed (e.g., an address transistor connecting the source terminal of transistor  300  to column sense line Vout need not be used). For example, the pixel reset bias level that is applied to region  218  may be modulated for pixel addressing purposes.  FIG. 4  shows one suitable addressing scheme for pixel  200 . During integration, Vddx and Rx are held high to drain any existing overflow charge from the overexposed photodiodes. This technique of draining overflow charge is sometimes referred to as blooming control. At the onset of a readout operation (at time t 1 ), reset signal Rx on non-selected lines is driven low, thereby keeping the floating diffusion nodes associated with non-selected pixels at nominal positive power supply voltage Vdd 1  during the entirety of the current readout operation. Doing so effectively ensures that the p-channel JFETs associated with the non-selected pixels are turned off. 
         [0030]    At time t 2 , Vddx may be lowered from Vdd 1  to a reduced positive power supply voltage level Vdd 2 . Bias level Vdd 1  may be equal to 3.3 V, whereas Vdd 2  may be equal to 2 V (as an example). Doing so will bring the reset level of the addressed floating diffusion regions lower, which turns on the p-channel JFETs in the selected pixels. At time t 3 , signal Rx associated with the selected line is driven low. When all the reset lines are deactivated, all the floating diffusion regions are now floating. 
         [0031]    At time t 4 , the source follower output is sampled to obtain a reference sampling level that is stored in a CDS reference storage node. At time t 5 , transfer signal Tx associated with the selected pixels is pulsed high to transferred photo-generated charge into the floating diffusion region. At time t 6 , the source follower output is sampled to obtain a signal sampling level. The CDS may then subtract the signal sampling level from the stored reference sampling level to obtain a corresponding output signal level. Subsequently, Vddx and Rx may be driven high so that the overflow charge from the photodiodes can again be drained to Vdd 1  in preparation for another integration or readout cycle. 
         [0032]    Having thus described the preferred embodiments of the novel pixel for the back side illuminated or for the front side illuminated image sensor arrays that have small pixel size, high well capacity, low dark current, and the vertical JFET transistors serving as source followers (which are intended to be illustrative and not limiting), it is noted that the principles of this invention and various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention. 
         [0033]    Various embodiments have been described illustrating a small size pixel design exhibiting improved storage well capacity and low dark current. The pixel may include a photodiode (e.g., a pinned photodiode), a charge transfer transistor (sometimes referred to as a charge transfer gate), a reset transistor (sometimes referred to as a reset gate), and a source follower transistor. The small size pixel may include a vertical junction field effect transistor (JFET) that serves as the source follower. 
         [0034]    The small size pixel may include a floating diffusion region that is shared among a source-drain of the charge transfer gate, a source-drain of the reset gate, and a gate of the vertical JFET source follower. The floating diffusion region may be formed using a plurality of vertically stacked N-type doping regions (as an example). A P+ doped region may be formed in one the of stacked regions to serve as a source for the vertical SF, whereas P-type doped regions may be formed in the remaining stacked regions to serve as a channel for the vertical JFET source follower. A bottom P+ implant layer may be formed below the stacked floating diffusion regions to serve as a drain for the vertical JFET source follower. 
         [0035]    The use of the vertical JFET source follower may provide reduced random telegraph signal (RTS) noise typically associated with small metal-oxide-semiconductor source follower transistors. If desired, an additional address transistor need not be used. A selected pixel may be reset to a reduced bias level to differentiate from unselected pixels along a column of pixels that may have the SF gates held at higher bias levels. 
         [0036]    The foregoing is merely illustrative of the principles of this invention which can be practiced in other embodiments.