Abstract:
An image plane includes a plurality of pixels. Each pixel comprises a photodiode and two transistors, and each pixel is connected by a signal bus to a respective storage node located off the image plane. Each storage node comprises two capacitors and associated switches. One of the transistors applies a reset pulse to the pixel, and the other transistor connects the pixel to a given conductor of the signal bus, which is then connected to the storage node. The pixel transistors can be operated simultaneously, and the sensed values can subsequently be transferred from the storage nodes sequentially.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a solid state imaging device which can be operated to provide an improved shutter function.  
         BACKGROUND OF THE INVENTION  
         [0002]    There are various basic CMOS pixel structures. One common type, with 3 transistors per pixel, is described in U.S. Pat. No. 4,407,010 (referred to as the CMOS 3T pixel), and is illustrated in FIG. 1 of the accompanying drawings. This is an efficient structure, wherein transistor M 1  amplifies an output from the photodiode while positioned within the pixel. Transistor M 2  resets the voltage on the pixel, and transistor M 3  is a multiplex transistor. Transistor M 3  enables many pixels in a column to be wired together, and only one pixel is enabled at a time. The device Iload is typically a sense amplifier that provides a load for the source follower transistor M 1 , and also measures the output voltage.  
           [0003]    The typical voltage on a photodiode is shown in FIG. 2. At point  1 , the pixel is reset by turning on transistor M 2  which sets the voltage on the reverse-biased diode to a preset voltage (VRT). After this point, light falling onto the pixel will create photo-generated electrons which will be attracted to the photodiode. This will cause the diode to be discharged. The amount of discharge is proportional to both the amount of light and also the amount of time. After a period of time (integration period Tint) the voltage on the pixel is measured. If the time Tint is kept constant, the swing will be proportional solely to the amount of light falling on the pixel.  
           [0004]    Typically, as shown in FIG. 3, the pixels are arranged into a 2-dimensional grid of rows and columns. There is one Iload/sense amplifier per column. The amplifier measures the output voltage of the pixel. Several pixels and usually all the pixels in a column share a single sense amplifier. Because of this structure, all the elements in a row are read out simultaneously into the sense amplifiers and the rows are addressed sequentially.  
           [0005]    As the rows are read out sequentially, they must also be reset sequentially. This keeps the integration time Tint constant for the whole sensor, and the brightness of the image constant over the image plane. This operation is called “rolling blade shutter” and is analogous to how a physical shutter in a 35 mm SLR camera works. In the CMOS 3T sensor, the integration time is variable. This is achieved by varying the time between the reset and readout pulse. This is also similar to how 35 mm SLR cameras work. The shutter blades move over the film at a constant rate, but a gap between the blades is adjusted to adjust the effective shutter speed.  
           [0006]    Another common type of CMOS pixel has 4 transistors. There are various types of implementation, one of which is shown in FIG. 4. The advantage of this design is that it has two storage capacitances per pixel. Cpd is formed by the parasitic capacitance of the photodiode. The storage node Csn is formed partly by the stray capacitance of M 1 , M 2  but also by creating a storage device inside the pixel. One advantage of a 4T pixel is sensitivity: V=Q/C. By reducing the value of Csn, the output voltage for a given photocharge is increased.  
           [0007]    The 4T pixel has another advantage, which is its ability to form an electronic shutter. Although arrays of either 3T or 4T pixels can be reset simultaneously, the sequential readout mechanism of the 3T pixel prevents simultaneous readout. The 4T pixel does not suffer from this problem since it has a storage element incorporated inside each pixel (Csn in FIG. 4). This permits the entire array to be sensed simultaneously, i.e., a photo-generated charge is transferred from each pixel&#39;s Cpd to the pixel&#39;s Csn simultaneously.  
           [0008]    The readout mechanism then proceeds in a row sequential fashion, similar to the mechanism used in the 3T pixels. As all the pixels in the array are reset and measured simultaneously, the array captures a snapshot of the light pattern falling on the sensor, unlike the rolling blade shutter of the 3T pixels. This technique is of great value for hand-held operation of the camera as the effect of camera shake is reduced as the total time for which the array is collecting light is minimized, as opposed to the time for which an individual pixel is collecting light.  
           [0009]    There are significant disadvantages with a 4T pixel. The extra circuitry (M 4 , Csn) occupies an area on the pixel and this reduces the amount of light reaching the photodiode. Transferring all the charge from Cpd to Csn is difficult to achieve. Special CMOS manufacturing techniques are often employed to change the structure of the photodiode Cpd or the transfer transistor M 4 . These manufacturing techniques are very costly since as they are non-standard and are also difficult to reliably achieve.  
           [0010]    There are also some linear arrays (see FIG. 5) with two rows of pixels which have separate electronics on both top and bottom. However, these structures are limited to a maximum of two rows. Other prior art in this area includes U.S. Pat. Nos. 4,835,617; 5,576,762; 5,134,489; 5,122,881; 5,471,515, and European Patent WO 98/08079.  
         SUMMARY OF THE INVENTION  
         [0011]    An object of the present invention is to provide a solid state image sensor which, like the 3T sensor, can be manufactured by standard techniques, but which also is capable of providing a true electronic shutter.  
           [0012]    The invention and preferred features thereof are defined in the appended Claims.  
           [0013]    Briefly stated, the invention is based upon locating the readout electronics off the image plane of the device. In preferred forms of the invention, this is facilitated by connecting each pixel to its associated readout electronics via a multi-conductor signal bus. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    Embodiments of the invention will now be described, by way of example only, referring to the drawings in which:  
         [0015]    FIGS.  1  to  5  illustrate the prior art as discussed above;  
         [0016]    [0016]FIG. 6 shows part of one column of an array structure embodying the invention;  
         [0017]    [0017]FIG. 7 is a timing diagram illustrating the operation of FIG. 6;  
         [0018]    [0018]FIG. 8 shows a typical system layout of a sensor incorporating the circuitry of FIG. 6;  
         [0019]    [0019]FIG. 9 shows one pixel and read-out circuitry of a modified version of FIG. 6;  
         [0020]    [0020]FIG. 10 is a timing diagram illustrating the operation of FIG. 9;  
         [0021]    [0021]FIG. 11 shows one pixel plus read-out circuitry of a further modification of FIG. 6;  
         [0022]    [0022]FIG. 12 is a timing diagram illustrating the operation of FIG. 11;  
         [0023]    [0023]FIG. 13 is a view similar to FIG. 8 but showing a modified system layout; and  
         [0024]    [0024]FIG. 14 shows a preferred readout arrangement for the circuit of FIG. 11. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]    A basic feature of the invention is to provide a storage node per pixel, and to avoid degrading the fill factor and hence light sensitivity, by locating the storage element away from the image plane. Referring to FIG. 6, this embodiment has only two transistors, M 1  and M 2  per pixel, thus improving the fill factor and sensitivity. The array is not multiplexed, and therefore there is no mulitplex transistor in the pixel equivalent to M 3  in FIG. 1. Instead, there is a connection to the signal bus  10  which runs through the column.  
         [0026]    The switches S 2 - 1 , S 2 - 2  will typically be implemented as MOSFET transistors. The current loads Iload are to ensure correct operation of sense transistor M 1 . FIG. 6 shows only two pixels, but in a practical array there are several pixels in a column.  
         [0027]    The operation of the array is as follows. At point  1  (see FIG. 7) the RST signal goes high, causing all the M 2  transistors (M 2 _ 1 , M 2 _ 2 , etc.) to conduct and the voltage Vpix on the photodiode to be reset to Vrt. At a time later point  2  (see FIG. 7), all the S 1  switches (S 1 _ 1 , S 1 _ 2 , etc.) are closed simultaneously and the output of the sense transistors (M 1 ) are stored on the sense capacitors (Csn_ 1 , Csn_ 2 ). Subsequently (not shown), the signals on the sense capacitors are readout sequentially by sequentially closing switches S 2  (S 2 _ 1 , S 2 _ 2 , etc.).  
         [0028]    [0028]FIG. 8 shows a typical layout of a system with an image array  12  and sample capacitor area  14 . To simplify the drawing, a 6×6 pixel structure is shown but the array would typically be larger. The output from each pixel is wired (indicated by the X in FIG. 8) to a different conductor of the signal bus  10 . Each cell has a width A within the system.  
         [0029]    The embodiment of FIGS.  6  to  8  shows signal bus lines planar with the image plane, i.e., using the same conductor layer. One improvement (not shown) is to stack the conductors, that is, to use different conductive layers. This reduces the amount of metal covering the pixel and thus improves the amount of light collected by the pixel.  
         [0030]    The system described in FIG. 6 is area and cost efficient, but it suffers from a fixed pattern noise in the form of brightness variations on the picture. This is due to the varying amount of threshold voltage of transistors M 1  over the array. These variations are a normal part of the CMOS manufacturing process. A practical way of cancelling this offset is to measure, on a per-pixel basis, the reset voltage after the source follower.  
         [0031]    Referring to FIGS. 9 and 10, this is achieved by closing switch S 3  (FIG. 9) immediately after the end of the reset pulse ( 2  in FIG. 10). This signal is then stored on Cres, and switch S 3  is opened. For a period of time ( 3  in FIG. 10), the pixel collects light and the photo-charge discharges the photodiode. At the end of this period ( 4  in FIG. 10) the signal is sampled on Csn. During image readout ( 5  in FIG. 10), switches S 2  and S 4  are closed simultaneously and both the signal and reset values are output onto the output signal and reset value conductors. The threshold voltage can then be compensated by subtracting the reset value from the output signal.  
         [0032]    This technique is similar to that used in U.S. Pat. No. 5,122,881 but is modified to deal with the present situation where no multiplex transistor is present.  
         [0033]    Although the technique described previously (FIG. 9) cancels the offset, it degrades the rate at which the system can operate since it is not possible to perform image acquisition and readout simultaneously. This is because the reset signal ( 2  in FIG. 10) occurs at the start of an image acquisition, but is required during readout. A new acquisition is therefore not possible until readout has been completed.  
         [0034]    The solution to this problem is shown in FIG. 11. An extra capacitor per pixel is used to enable simultaneous image acquisition and readout.  
         [0035]    To understand the operation of the circuit in FIG. 11, reference is made to the timing diagram in FIG. 12. At point  1 , Vrst goes high causing all the M 2 s in the array to conduct for resetting the photodiodes in the array. As soon as this is complete, (point  2 ) S 2  goes high enabling CresA to sample the reset value of the pixel. The image array collects light until time 3 when the voltage corresponding to the pixel&#39;s exposure to light is collected. S 1  is closed and the voltage is stored on the pixel&#39;s Csn.  
         [0036]    At this time the system has collected a complete set of reset and image values and is ready to readout. Before this occurs, the next acquisition cycle starts. At point  4 , Vrst goes high causing all the M 2 s in the array to conduct for resetting the photodiodes in the array. As soon as this is complete, (point  5 ) S 4  goes high enabling CresB to sample the reset value of the pixel. As the image array collects light, the pixels&#39; capacitors are accessed sequentially. At point  6 , S 2  is closed to output the image value Vsn stored on Csn onto the output signal conductor. For this sequence of images, S 4  is closed to output the reset value Vres stored on CresA onto the reset value A conductor. The image array collects light until time 7 when the voltage corresponding to the pixel&#39;s exposure to light is collected. S 1  is closed and the voltage is stored on the pixel&#39;s Csn.  
         [0037]    At this time the system has collected another complete set of reset and image values and is ready to readout. Before this occurs, the next acquisition cycle starts. Point  8  is identical to point  1 , and point  9  is identical to point  2 . As the image array collects light, the pixels&#39; capacitors are accessed sequentially. At point  10 , S 2  is closed to output the image value Vsn stored on Csn onto the output signal conductor. For this sequence of images, S 6  is closed to output the reset value Vres stored on CresB onto the reset value B conductor.  
         [0038]    The system continues to operate using the sequence described above. The important feature to note in FIG. 12 is that Vsn is able to be output on each frame.  
         [0039]    In the layout shown in FIG. 8, the pitch of the sample capacitors is ⅙ th  the pitch of the pixels as there are 6 pixels vertically. For a larger array, a greater number of sample capacitors need to be fitted into the width of a pixel. This presents a practical limit to the architecture. The minimum width of sample capacitors is determined by the manufacturing technology used by the architecture. The maximum size of the pixel is determined by cost factors.  
         [0040]    An improved layout is shown in FIG. 13. This architecture has sample capacitors  14 A and  14 B at the top and bottom of the array  12 . There are now two signal buses  10 A and  10 B divided in the center, and the cell width B is equal to ⅓ of a pixel. There are two advantages. The fewer signal bus conductors running across each pixel requires less metal, and hence, there is less obstruction of the pixel (i.e, a higher fill-factor) and hence greater sensitivity from the pixel. As the array is divided into two parts, the sample capacitors are shared top and bottom, resulting in twice the width available.  
         [0041]    The following Table 1 illustrates the advantages.  
                                   TABLE 1                           Column   Pixel   Pixel   Image   Imaging       Layout   Width   Array   Size   Plane   Area                       2 μm   100 × 100   200 μm ×   200 mm ×   400 m 2                     200 μm   200 mm           2 μm   100 × 100   100 μm ×   100 mm ×   100 m 2                     100 μm   10 mm                  
 
         [0042]    As can be seen in the final column, the improved layout technique of FIG. 13 produces a four-fold increase in area, and hence a corresponding reduction in cost per unit area.  
         [0043]    Turning to FIG. 14, a preferred scheme for measuring and amplifying the two output signals will now be described. Associated with the switches S 2 , S 4 , S 6  and the conductors output signal  18 , reset value A  20 , and reset value B  22 , are unwanted stray capacitances. As the array size increases, the number of pixels, and therefore, the number of switches increases. The cumulation of all these switches can produce an unwanted capacitance roughly equal to that of the sampling capacitances. When the signals are read out (switches S 2 /S 4 /S 6  closed), part of the charge stored on the capacitors Csn/CresA/CresB is used to charge the stray capacitors. This problem is known as charge sharing. This can easily be 50% to 70% of the signal, and reduces the output swing to ½ or ¼ of the true signal.  
         [0044]    Using a differential, charge sensitive amplifier  16  as shown in FIG. 14, charge sharing is avoided. Before the signal is read out, the switches S 7 , S 8  are closed and the amplifier  16  is put into its common mode reset state. This discharges the capacitors Cf 1 , Cf 2  on the feedback of the operational amplifier  16  and forces the conductors  18 ,  20 ,  22  to the common mode voltage. Switches S 7 /S 8  are opened and S 2 , S 4  (or S 6 ) are then closed.  
         [0045]    The nature of the operational amplifier is to ensure that its input remains at the common mode voltage. By doing so there is no change in the voltage on the lines  18 ,  20  and  22  and so there can be no loss of charge. During the readout, the voltages on Csn, CresA, CresB are also set to the common mode voltage. The change in voltage from that which was measured off the array requires a current to flow. This comes from the output of the op-amp  16  via the feedback capacitors Cf 1 , Cf 2 . For correct operation (symmetrical operation) the capacitance of Cf 1 =Cf 2  and Csn=CresA=CresB. Hence: 
         Out1−Out2=(Vsignal−Vreset)xCsn/Cf1 
         [0046]    Modifications and improvements may be made to the foregoing within the scope of the invention.