Abstract:
The present invention describes in detail the construction and operation of a CMOS Active Pixel Image Sensor that consists of pixels formed by only two transistors. The sensor can be fabricated with very small pixels sizes, which have only two metal contacts in them, have in-pixel offset non-uniformity correction, and bootstrapped reset lines. These features are achieved by employing the transistor body effect as the main photo-generated charge sensing means. The bootstrapped reset lines allow the sensor to operate at low bias voltages. Additional embodiments of the invention include: single line for addressing the pixels, column-clamping circuits to prevent the forward biasing of pixel&#39;s p-n junctions and trench isolation to minimize the pixel size

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to solid-state image sensors and specifically to Active Pixel CMOS image sensors that have only two transistors in each pixel.  
         BACKGROUND OF THE INVENTION  
         [0002]    State of the art CMOS image sensors have typically three or more transistors per pixel and are addressed and reset with two or more row lines. The pixels are also supplied with a power supply line and the output is obtained through a column sense line.  
           [0003]    A typical circuit schematic diagram  101  of a standard CMOS pixel is shown in FIG. 1. As light impinges on the pixel photodiode  102 , the diode is discharged and the resulting voltage of node  111  is supplied to the gate of the sensing transistor  103 . When a particular row “Yn” is ready to be addressed, the addressing signal is applied to line  108 , which turns the transistor  104  on. This connects the signal appearing on the source of the transistor  103  through the interconnect  107  to column line  112 . The current source load  113  loads the column sense line “Xm”. After the sensing is completed, the reset line  109  is pulsed, which resets the node  111  through the reset transistor  105  to the original level.  
           [0004]    The whole cycle of sensing and resetting is periodically repeated when the sensor is used in a movie mode or is terminated just after one cycle when the sensor is used in a still picture mode. The drain of the reset transistor  105  is connected together with the drain of the sense transistor  103 , and further through the interconnect  106  to the Vdd bus line  110 . Separate lines for the reset and Vdd are also possible. The voltage waveform that appears on the node  111  is shown in the graph  201  in FIG. 2. When the reset pulse  204  is applied to the pixel, the node voltage is reset to level  205  that corresponds to dark signal. The light impinging on the photodiode  102  gradually discharges the node causing the node voltage to follow an approximately linear line  202  assuming that the light intensity is constant. When an address pulse  203  is applied to the pixel, the source voltage is sensed and delivered to column processing circuits. The new reset pulse  204  then restores the pixel original dark level  205 . More information about Active Pixel Image (APS) Sensor design and operation can be found in the paper: “CMOS Image Sensor: Electronic Camera On a Chip” Eric Fossum published in the IEDM Technical Digest 1995 pp. 17-25. Additional information is also available in the U.S. Pat. No. 5,471,575 to Fossum at al.  
           [0005]    APS image sensors have advantages in low power consumption, high speed, require only low bias voltages, and can be produced with low cost. However, the complexity of the pixel design leads to a sacrifice of some light collecting area, which ultimately results in lower QE. Another disadvantage is in pixel-offset non-uniformities that are caused by the threshold variations of pixel transistors. The resulting pixel signal has to be processed to remove these non-uniformities. Pixels in each column can share the column processing circuitry, however, these circuits typically require at least two large capacitors for the offset subtraction and data storage, which may consume a significant portion of the chip area. Another problem resulting from the complex pixel circuitry is the need for several contacts between the transistor gates, source and drain regions, and metal busing. The pixel circuit  101 , shown in FIG. 1, requires at least three contacts. The contacts are also consuming pixel area, obstruct light, and prevent pixel size reduction beyond a certain limit. It is therefore desirable to reduce the pixel complexity and more importantly to reduce the number of in-pixel contacts. It is also desirable to simplify the pixel-offset non-uniformity processing circuits and minimize the number of large signal storage capacitors in each column.  
           [0006]    It is the purpose of this invention to teach how to overcome the above-described limitations and how to achieve, small pixel size, fewer transistors per pixel, and fewer in-pixel contacts, and to teach how to compensate pixel offset non-uniformities by storing the corrective signal in the pixel itself. The prior art does not show how to design an Active Pixel CMOS image sensor with a small pixel size that has only two transistors per pixel and only two in-pixel contacts. The prior art does not teach how to compensate for pixel offset non-uniformities by storing the error-compensating signal in the pixel itself. Finally the prior art does not teach how to improve the image sensor performance at low biasing voltages by bootstrapping the reset gate signal.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention provides an Active Pixel CMOS image sensor that has only two transistors in each pixel, in-pixel offset non-uniformity correction, and bootstrapped reset lines. The reduced number of active pixel elements leads to higher Quantum Efficiency (QE) and smaller pixel size. The unique pixel row addressing and reset technique that uses only a single line further improves the sensor&#39;s QE. Separate column lines for the pixel output and reset reference allow for efficient compensation of pixel-offset non-uniformities. The pixel-offset non-uniformities are individually detected after each photo-generated signal has been read out. Special array column or array external processing circuits compute the corresponding pixel non-uniformity error-compensating signal and supply it back through the reset line to each pixel. As a result the photo-generated signal is always referenced to an absolute reference level free of any pixel-offset error. This type of non-uniformity correction thus reduces the number of column capacitors, normally needed for the standard non-uniformity correction and data storage, since the offset compensation is stored in the pixel itself. This technique significantly reduces the chip size and eventually the cost of the sensor.  
           [0008]    These and other objects are achieved by using the body effect of p-type transistor as the main photo-signal sensing means. The n-well region containing the p-type transistor is reset to a reference voltage by a second n-type pixel transistor and then left floating. Impinging light creates electron-hole pairs and collected electrons are changing potential of the n-well. The potential changes modulate the threshold of the p-type transistor and these changes then represent the pixel output signal. The reset reference for each pixel is adjustable individually to compensate for the intrinsic p-type transistor threshold variations. This feature, when activated, effectively cancels the pixel-offset non-uniformities.  
           [0009]    Another embodiment of the invention includes incorporation of trench isolation into each pixel. The trench isolation allows reducing the transient regions between the p-type and the n-type doping regions within the pixel and between the pixels of the array. This in turn leads to reduction of pixel size.  
           [0010]    Yet additional embodiment of the invention includes the stacked double poly reset line that generates bootstrapped reset signal for the n-well reset transistor. This allows resetting the n-well to a higher voltage level, which increases the pixel output voltage swing and thus the overall sensor Dynamic Range. This feature is important when low bias voltages are required to operate the sensor.  
           [0011]    The main advantage of the above-described pixel structure is that it contains only two transistors and consequently requires only two contacts to the array bus lines. The n-type transistor resets the n-well without the need for in-pixel metal interconnects and thus the pixel does not need additional contacts. This leads to an aggressive pixel size reduction without any significant sacrifice in the sensor sensitivity and QE. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0012]    The invention will be described in detail in the following description of preferred embodiments with reference to the following figures wherein:  
         [0013]    [0013]FIG. 1 shows a simplified circuit diagram of the prior art active pixel that consists of three transistors and a photodiode;  
         [0014]    [0014]FIG. 2 shows a timing diagram for the reset and address pulses that are applied to the prior art pixel whose circuit diagram is given in FIG. 1, and includes a graph of the signal waveform that appears on the photodiode during the course of the pixel operation;  
         [0015]    [0015]FIG. 3 a  shows a simplified circuit diagram for the pixel of the invention that contains only two transistors, one addressing line, and two column lines;  
         [0016]    [0016]FIG. 3 b  shows a simplified circuit diagram for the pixel of the invention that contains only two transistors, address line, independent reset line, and two column lines;  
         [0017]    [0017]FIG. 4 shows a timing diagram for the pixel of the invention that indicates relative positions of the sense and reset pulses, and separate graphs of voltage waveforms appearing on the n-well and on the pixel output during the pixel operating cycle are also given;  
         [0018]    [0018]FIG. 5 a  shows a plan view of the pixel layout for the pixel of the invention with two metal bus line contacts to the pixel&#39;s active elements;  
         [0019]    [0019]FIG. 5 b  shows the plan view of the pixel layout for the pixel of the invention that is using trench isolation to reduce the pixel size;  
         [0020]    [0020]FIG. 6 a  shows a simplified cross section for the pixel of the invention. The cross section is taken through the line  5 - 5  indicated in FIG. 5 a;    
         [0021]    [0021]FIG. 6 b  shows a simplified cross section, taken through  6 - 6  of FIG. 5 b,  for the pixel of the invention that is using trench isolation;  
         [0022]    [0022]FIG. 7 shows a simplified schematic diagram for the bootstrapped driver;  
         [0023]    [0023]FIG. 8 shows the block diagram for the section of a complete image sensor array that incorporates the pixels of the invention with the pixel offset correction circuits are shown block diagram form;  
         [0024]    [0024]FIG. 9 shows a timing chart of the image sensor array shown in FIG. 8, and a graph of the column signal waveform is shown with the details that indicate where the pixel-offset signal is being sensed; and  
         [0025]    FIG. 10  shows the circuit diagram for the column switches that allow sensing the reset error signal and computing the reset level correction externally to the array, and the reset level bias correction that is supplied back to the corresponding column reset busses. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0026]    Simplified circuit diagrams  301  and  312  for two versions of the CMOS pixel of the invention are shown in FIG. 3 a  and FIG. 3 b . The pixel consists of one p-type transistor  302  that is formed in a standard CMOS n-type well  304  and one n-type transistor  303 . The drain of a p-type transistor is connected to p-type substrate by extending the p+ drain region over the n-well edge. This feature eliminates the need for a metal interconnect. The source of transistor  302  is connected to column signal bus line  308  by the interconnect  307 . The column signal bus line is shared by all the pixels in the same column and is loaded by a suitable current source  310 . N-type transistor  303  is used for the reset, and when turned on, it resets the n-well  304  to a potential that is present on the bus line  306  at that moment. Each pixel in the same column can thus be reset to a different reset level since the pixels are reset at different times. The interconnect  305  connects the drain of transistor  303  to column reset bus line  306 . The n+ source of transistor  303  is connected to the n-well  304  similarly as that of the p-type transistor, which is by extending the n+ diffusion over the edge of the p-region or p-well. Gates of both transistors  302  and  303  are connected to the same address line  309  even thou this is not always necessary since separate lines for the address transistor  309  and the reset transistors  311  can be provided as shown in FIG. 3 b.  Since the transistors are of a different conductivity type, it is possible to distinguish the reset from addressing in case of the common line just by applying different polarity pulses. The reset is accomplished by a positive pulse excursion while the addressing is accomplished by a negative pulse excursion. For more clarity, the signal waveforms appearing on the n-well  304  and on the source node of transistor  302  are shown in the graph  401  in FIG. 4. The n-well  304  is reset to a potential level  408  by the reset pulse  405 , and photo-generated carriers causes the level to drop along the line  402  providing that photon flux is constant during this interval.  
         [0027]    The n-well potential change is affecting the threshold voltage of transistor  302  through the body effect phenomenon and the potential of transistor  302  source is changed accordingly starting from the reset level  407  along the line  403  as indicated by the dotted curve if it were turned on. However, the transistor is turned off, since its gate is not addressed when biased to an intermediate V m  level  404 . When address pulse  406  is applied, the transistor turns on and the potential level  409  is supplied to the column sense line as the pixel output.  
         [0028]    An example of one possible pixel layout  501  is shown in FIG. 5 a.  Regions  506  represent address lines that are typically formed from a single polysilicon layer. As will be explained later, the lines can also be formed as stacked structures of two polysilicon layers separated by a suitable dielectric. This is not visible in the drawing.  
         [0029]    Region  502  indicates the n-well. The P-type substrate or p-well is the region  503 , which is everything outside the n-well  502 . Region  507  defines a p-type transistor with its drain overlapping the n-well  502  to facilitate the drain to substrate connection as explained previously. Similarly, region  508  defines a n-type transistor with its source overlapping the n-well  502  to make the required connection. Metal bus lines  504 , for reset, and  505 , for signal, are slightly offset, but this is only for the purpose of drawing clarity. In practice they may be located on top of each other and elsewhere in the pixel. Transistor contacts to corresponding bus lines are indicated in the drawing by dots  509  and  510 .  
         [0030]    For more clarity a pixel cross section  601  is shown in FIG. 6 a.  The cross section is taken through the line  5 - 5  as indicated in FIG. 5 a.  Structures  602  through  610  correspond directly to regions  502  through  510  in FIG. 5 a.  Region  611  is the p+ type substrate on which the p-type bulk material is epitaxially grown. Other substrate materials and material combinations are possible as can be easily surmised by those skilled in the art. Transistor&#39;s active regions with thin gate oxide  613  and inactive field regions  612  with thick oxide are also standard in the industry. Other doped regions, to prevent parasitic channel formations, transistor punch-through, and other unwanted phenomena, are not indicated in the drawing since they are not essential for this invention and are well known to those skilled in the art of CMOS technology. One important detail, however, is the region  614  under the p-type transistor gate  614 . This region includes a combination of suitable p-type and n-type doping layers that optimize the p-type transistor threshold and at the same time maximize its body effect. Particular doping levels and energies are easily obtained by simulations and experiments and are unique to each device design. It is therefore not practical to specify them here in any more detail.  
         [0031]    Another example of possible pixel layout is shown in FIG. 5 b . In this embodiment the trench isolation regions  551  are used to minimize the transition regions between the p-well  553  and n-well  552  doping. The trench also reduces the n-well capacitance, which increases the pixel conversion factor. The p-channel transistor  557  and the n-channel transistor  558 , with the common gate  556 , correspond to the same structures in FIG. 5 a.  The metal buss lines have been omitted in this figure, but the contact holes  559  and  560  are indicated. The simplified device cross-section taken through the line  6 - 6 , in FIG. 5 b,  is shown on the drawing in FIG. 6 b.  The structures  602  through  614  in FIG. 6 a  correspond directly to structures  652  through  664  in FIG. 6 b.    
         [0032]    When CMOS image sensors are required to operate at low bias voltages, the threshold voltage of the reset transistor reduces the voltage swing of node  111  (FIG. 1) or  304  (FIG. 3 a ). The node is typically reset to a level that is approximately a threshold voltage below V dd .  
         [0033]    A simplified schematic diagram  701 , shown in FIG. 7, illustrates a bootstrapping technique that can minimize this problem and is particularly suitable for the pixel of the invention. During the pixel integration time, switch  707  is on and the gate bus of pixel transistors  703  is kept at an intermediate potential level Vm. For the reset, switch  707  is turned off, and reset pulse  710  is applied to the input of CMOS driver  706 . The output from driver  706  then drives the line  702 . Line  702  is located directly on top of the line  703 , and is separated from it by a suitable dielectric. This arrangement forms a strong capacitive coupling between these lines. The coupling capacitance  705  is typically much larger than all the pixel gate capacitances  704  added together, which leads only to a small pulse amplitude attenuation. When the reset pulse is applied, the full reset pulse amplitude, starting from the level V m , appears on the gate bus  703 . No pixel dynamic range is sacrificed and nodes  111  (FIG. 1) or  304  (FIG. 3) can thus be fully reset to V dd  or at least close to this level. The pixel addressing is accomplished simply by turning the switch  707  momentarily off and the switch  708  momentarily on.  
         [0034]    For completeness a simplified block diagram  801  of a CMOS image sensor that includes the pixels of the invention is given in FIG. 8. FIG. 8 shows only a 2×2 sub array of pixels  804  and block diagrams of essential peripheral circuits. Current sources  802  located at the top of the array edge are biasing the column signal lines  806 . Vdd is supplied to the current sources through a common Vdd line  803 . The current sources can be switched off when not in use to save power, but this feature is not indicated in the diagram. Column reset lines  805  together with the column signal lines  806  interface directly with the column signal processing circuit modules  815 . The circuit modules are also connected to the reset reference voltage line  827 ; the strobe clock line  825 , and the error-enable clock line  826 . The processed column signal, that is stored on the column circuit block capacitors  820 , is supplied via interconnects  816  to horizontal switches  817 .  
         [0035]    The horizontal switches are consecutively turned on and off by the command signals received from the horizontal scan decoder  821  via the lines  823 . Sometimes it is advantageous to replace the horizontal scan decoder by a shift register. The input into the horizontal scanner is received through bus lines  824 . The sequential pixel output appears on the horizontal sense line  818  and is amplified and buffered by the output amplifier  819  that drives the chip output terminal  822 . The array vertical scanning is accomplished by decoder  810 , which supplies the necessary control signals to line drivers  809  and finally through them to pixel address lines  807 . The vertical scanner receives its input through bus  811 . An important element of the array peripheral circuits is the column-clamping switch  814 . The primary function of the switch is to keep the column signal lines from drifting to a high bias level, and start forward biasing the pixel&#39;s p-n junctions when there is no pixel addressed. A suitable clamping bias is supplied to all the switches via line  813 , and the switches are controlled by a command signal supplied via line  812 .  
         [0036]    A simplified timing diagram  901  that includes some of the array&#39;s essential control signals is shown in FIG. 9. The pixel addressing pulse  904  connects the pixel output form one row of pixels to the corresponding column sense lines. Prior to this moment, sense lines were clamped to the level  907 . The column sense lines always remain clamped to this level when no addressing pulses are applied. Addressing pulses cause the column sense lines to change their potential to a new level  908 . The level  908  is subsequently sampled by pulses  916  and stored on holding capacitors  820  (FIG. 8) in the column signal processing circuits for a later scan and horizontal readout. In the next step reset pulse  903  is applied to the pixels followed by a new same-address pulse  904 . The reset pulse  903  resets pixels to level  912 . Since all charge from the pixels in the addressed row was removed by the reset, only dark levels  909  now appear on the column sense lines. The column error-processing circuits activated by pulse  915  process these levels and generate the reset level correction signal  913 . The second reset pulse  905  then resets the pixels again to the new offset correcting reset levels. If the pixels were addressed again the column signal would be at the level  906 , which is a common reference for all the pixels in the array. The above-described process continues for the next row of pixels in the column, which generate different output levels  910  and also different dark levels  911 . The corresponding reset corrections  914  have now, for example, a different polarity since the level  911  was above the level  906 . This process thus sets all the pixels in the array to an identical dark reference, which removes all the unpleasant dark signal variations from the sensor image.  
         [0037]    Another embodiment of the column signal processing circuitry  1001  is shown on the circuit diagram in FIG. 10. When the image sensor pixel has a small size, there is not enough room available within the column pitch to accommodate all the required pixel non-uniformity correction circuitry. The circuit schematic diagram shown in FIG. 10 provides a solution to this problem. Both the signal and the reset error are scanned column by column and supplied to the processing bloc that is located near the image sensor array or entirely off chip. The error-processing block then supplies the correction signals back to the array. In this case the error processing circuit is only one for all the columns in the array and can be as complicated as needed or can even be using sophisticated software programmable DSP when off chip option is selected. As the individual columns are scanned, the corrections are loaded back to the proper columns of the array. A suitable wiring of switches  1022  to the horizontal scanner can easily accommodate the delay that might be encountered in the external error correcting circuits. The function of this system can be understood as follows: After the row of pixel is addressed the column signal is stored on the capacitors  1020  by turning the switches  1018  momentarily on. This is accomplished by applying a short pulse to the line  1019 . After that the selected row of pixels is reset to a reference voltage supplied to the column-reset lines  1005  by switches not shown in the drawing. The same row of pixel is addressed again, which causes the zero signals to appear on the columns  1006  where it becomes stored. The horizontal scanner  1027  then simultaneously turns the switches  1021  and  1017  momentarily on via the control lines  1023 , and supplies the signal and the error data to the sense lines  1026  and  1025 , and, at the same time, the externally computed correction to the reset level is supplied to the column reset line  1005  of the previous column. The correction is supplied via the bus line  1024  and the switches  1022 . If more delay is needed for computing the reset error, the wiring lines  1023  that control the reset column line switches  1022  can skip over several columns rather than only as indicated in FIG. 10.  
         [0038]    Having described preferred embodiments of novel semiconductor Active Pixel CMOS image sensor with Two-Transistor Pixel, In-Pixel Non-uniformity Offset Correction, and Bootstrapped Reset Lines, which are intended to be illustrative and not limiting, it is noted that modification and variations can be made by persons skilled in the art in light of the teachings. It is therefore to be understood that changes may be made in the particular embodiments of the invention disclosed which are within the scope and spirit of the invention as defined by the appended claims. Having thus described the invention with the details and particularity required by the patent laws, what is claimed and desired protected by Letters Patent is set forth in the appended claims.