Patent Publication Number: US-11050961-B2

Title: High scanning frequency CMOS-TDI image sensor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from CN Application No. CN201410470253.5, filed Sep. 15, 2014 and PCT Application No. PCT/CN2014/093752, filed Dec. 12, 2014, the contents of which are incorporated herein in the entirety by reference. 
     TECHNICAL FIELD 
     The present invention relates to the technical field of analog integrated circuit design and more particularly, relates to design of a CMOS-TDI image sensor capable of realizing higher scanning frequency, that is, a high scanning frequency CMOS-TDI image sensor. 
     BACKGROUND ART 
     An image sensor is able to transform optical signals acquired by a lens to electrical signals which are easy to be stored, transferred, and processed. In terms of an operation manner, image sensors may be classified into the area type and the linear type. The working principle of an area image sensor is as follows: photos of an object are taken according to the pixel matrix of a two dimensional area so as to obtain the two dimensional image information, whereas the working principle of a linear image sensor is: photos of the object are taken according to the pixel matrix of one dimension and by scanning the object to get the two dimensional image information. The working manner of a linear image sensor is demonstrated in  FIG. 1 . The linear image sensor works in a special manner and therefore it is widely applied in kinds of fields such as aerial photography, space imaging, machine vision, medical imaging, and so on. However, as the object keeps moving during pixel exposure of the linear image sensor, the exposure period of pixels is seriously limited by the moving speed of the linear image sensor corresponding to the object to which pictures are taken. In particular, in case of a high movement and low illumination environment (for example in space imaging), the SNR (signal to Noise Ratio) of the linear image sensor will become very low. To resolve the problem of low SNR, a time delay integration (TDI) has been proposed. It is able to increase the SNR and sensitivity of the linear image sensor and it uses a special scanning manner to expose the same object several times, thus realizing a higher SNR and sensitivity. As such, it is suitable for an environment of high movement and low illumination. The basic principles of TDI lie in the linear scanning with an area pixel matrix, thus realizing multiple exposuers of the same moving object by pixels of different lines, and accumulating of the exposure result of each time. This equivalently extends the exposure integration time of the object, thereby greatly improving the SNR and sensitivity. 
     In early stages, the TDI technique is implemented using a charge coupled device (CCD) image sensor which is also an ideal device for implementing the TDI technique. It can realize accumulation of signals without noise. Currently, the TDI technique mostly applies to CCD image sensors. A widely used CCD-TDI image sensor has the structure similar to a rectangular area CCD image sensor and performs linear scanning. Shown as  FIG. 2 , the operation of a CCD-TDI image sensor is as follows: a N-leveled CCD-TDI image sensor has n lines of pixels; the charges collected during a first exposure period by pixels of the first line of a row are not output directly, they are added to the charges collected during a second exposure period by a second pixel located at the same row. By the same token, the charges collected by the pixels of the last line (the n th  line) of the CCD-TDI image sensor are added into the charges collected during the n−1 th  line and are read out according to the output manner of a normal linear CCD device. For a CCD-TDI image sensor, the amplitude of output signals is the accumulation of integration charges of n pixels. It is equivalent to the charges of a pixel collected during n times of the exposure time. The output signal amplitude is increased by n times while the noise amplitude is increased only by √{square root over (n)} times and accordingly, SNR may be improved by √{square root over (n)} times. 
     However, as a CCD image sensor has the disadvantages such as large power consumption and low integration, applications in various fields of the CCD image sensor have been gradually replaced by the CMOS (Complementary Metal Oxide Semiconductor) image sensor. If the TDI function can be realized by the CMOS image sensor (i.e., CMOS-TDI image sensor), the cost of a TDI camera will be decreased dramatically and find its wide application. In the prior art, to realize the TDI function using a CMOS image sensor, analog signal accumulators are incorporated into the CMOS image sensor to work as a CMOS-TDI image sensor. That is, analog signals output by the pixels are in advance input into the analog signal accumulator to realize accumulation of the identically exposed signals, and the accumulated analog signals are sent to the ADC to be output quantitatively. Furthermore, the prior art has also proposed to quantitatively output signals of the CMOS image sensor through the ADC at first and then, finish accumulation of identically exposed signals by a digital domain accumulator built in chip. These two kinds of techniques, either performing accumulation and then quantitative output or performing quantification and then accumulative output, require reading out of the exposure result of all pixels of the CMOS image sensor during a single exposure period. As a result, the readout speed certainly will limit the shortest exposure period, i.e., the largest scanning frequency. To eliminate this problem, the prior art has proposed integration of a buffer cell into the pixels to realize signal delivery between adjacent pixels. Similar to the CCD type TDI image sensor capable of realizing accumulation of signals in a pipelined manner, only the output of the last line of pixels during each exposure time needs to be read out quantitatively. Accordingly, the limitation of the scanning frequency caused by the readout speed is eliminated, thereby realizing a faster scanning frequency. This technique, however, during the pipelined accumulation of pixel output signals, will introduce a great deal of kT/C noise and offset voltage of the operational amplifier. In addition, the fill factor of pixels is decreased due to the integration of buffer cells into the pixels, hence limiting the sensitivity of the sensor. 
     SUMMARY OF THE INVENTION 
     The present invention is intended to overcome drawbacks of the prior art and to better realize the TDI function of CMOS image sensors, improve the scanning frequency of the CMOS-TDI image sensor, and extend the application range of the TDI technique. To these ends, the present invention proposes a technical solution of a high scanning frequency CMOS-TDI image sensor. It includes a photodiode, an operational amplifier, integration capacitors C 1  and C 2  of the same capacitance, an offset voltage removing capacitor C 3 , and plural switches S 1 -S 10 . The anode of the photodiode is connected to a zero voltage ground wire, while the cathode thereof is connected to one end of the switch S 9 . The other end of the switch S 9  is connected to a reference voltage V ref . A left electrode plate of the offset voltage removing capacitor C 3  is coupled to the cathode of the photodiode, whereas a right electrode thereof is coupled to a negative input end of the operational amplifier. The switch S 10  is connected between the negative input end and an output end of the operational amplifier in series. A positive input end of the operational amplifier is coupled to the reference voltage V ref . The output end of the operational amplifier also works as the output end of entire pixels. The left electrode plate of the integration capacitor C 1  is connected to one end of each switch S 1  and S 3 . The other end of the switch S 1  is connected to the reference voltage V ref , the other end of the switch S 3  is connected to the cathode of the photodiode. The right electrode plate of the integration capacitor C 1  is connected to one end of each switch S 2  and S 4 . The other end of the switch S 2  is connected to an input end of the pixel, and the other end of the switch S 4  is connected to an output end of the pixel. The left electrode plate of the integration capacitor C 2  is connected to one end of each switch S 5  and S 7 . The other end of the switch S 5  is connected to the reference voltage V ref , and the other end of the switch S 7  is connected to the cathode of the photodiode. The right electrode plate of the integration capacitor C 2  is connected to one end of each switch S 6  and S 8 . The other end of the switch S 6  is connected to an input end of the pixel, and the other end of the switch S 8  is connected to an output end of the pixel. The cascading manner of the above pixels is described as follows: an input end of pixel  1  is connected to the reference voltage V ref , an input end of pixel  2  is connected to an output end of pixel  1  and then cascading by the similar manner; an output end of the last pixel is connected to a column-parallel ADC through a readout switch Read. 
     The switches S 1  and S 2  are controlled by the clock clk 1 ′, the switches S 3  and S 4  are controlled by the clock clk 2 , the switches S 5  and S 6  are controlled by the clock clk 2 ′, the switches S 7  and S 8  are controlled by the clock clk 1 , while the switches S 9  and S 10  are controlled by the clock rst. When the clock Read is at a high level, the pixel output is valid. The following operation of pixels is done in a pipelined manner: sample the input signal, and then add the sampled signal to the photocurrent integration signal generated during an exposure period of the pixel, output the accumulation result and express the output V int  as: 
     
       
         
           
             
               
                 
                   
                     V 
                     int 
                   
                   = 
                   
                     
                       V 
                       ref 
                     
                     + 
                     
                       V 
                       0 
                     
                     + 
                     
                       
                         
                           ∫ 
                           0 
                           
                             T 
                             int 
                           
                         
                         ⁢ 
                         
                           
                             i 
                             ph 
                           
                           ⁢ 
                           dt 
                         
                       
                       
                         C 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         1 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     Wherein V 0  is the signal collected during the previous exposure period of the pixel, and i ph  is the photocurrent value of the photodiode. 
     The operating procedure of the pixel is described as follows: 
     In the initializing status, clk 1 =clk 2 =0, clk 1 ′=clk 2 ′=rst=1, the input and output voltage of all pixels are V ref  by this time. After that, all the pixels are subject to a first exposure period, and here, clk 1 =clk 1 ′=1, clk 2 =clk 2 ′=rst=0, the integration capacitor C 1  of each pixel begins to collect the output signal of its previous pixel and at the same time, the integration capacitor C 2  begins to integrate photocurrent of the photodiode. When the first exposure period ends, the signal stored in the integration capacitor C 2  of x th  pixel is V int(1,x) , and the signal stored in the integration capacitor C 1  thereof is V int(1,x−1) . And then, all the pixels are subject to a resetting status and here, clk 1 =clk 1 ′=clk 2 =clk 2 ′=0, rst=1, and the integration capacitors C 1  and C 2  of each pixel are in a floating status by this time and signals stored in these capacitors are kept unchanged. Resetting action is done to the photodiode. After that, all the pixels are subject to a second exposure period and here, clk 1 =clk 1 ′=rst=0, clk 2 =clk 2 ′=1, the integration capacitor C 2  of each pixel begins to collect the output signal of its previous pixel and at the same time, the integration capacitor C 1  begins to integrate photocurrent of the photodiode. When the second exposure period ends, the signal stored in the integration capacitor C 1  of x th  pixel is V int(1,x−1) +V int(2,x) , and the signal stored in the integration capacitor C 2  thereof is V int(1,x−2) +V int(2,x−1) . By the similar manner, after exposure of N times, the output of the Nt h  pixel can be expressed as:
 
 V   int_tot   =V   int(1,1)   +V   int(2,2)   +V   int(3,3)   + . . . +V   int(N,N)   (2)
 
     Wherein, from V int(1,1)  to V int(N,N) , each represents the exposure result of the same object during a respective transit time from 1 to N by a respective pixel from 1 to N. Accordingly, the output of the N th  pixel is the result of N-leveled integration accumulation. This result is quantitatively output by a subsequent column-parallel ADC during a high level period of the clock Read, thus completing the entire reading out process. During each exposure period, the output of the N th  pixel is the result of N times integration accumulation. 
     The layout is denoted below. 
     A square with a central distance P is a photosensitive region of the photodiode, and a laterally adjacent square with the same size is the location where the operational amplifier, switches and capacitor layout are disposed and it is called as the circuitry region. Every two laterally adjacent squares constitute a layout of a pixel. Pixels of the odd column are not laterally adjacent to those of the even column. The pixels of the even column entirely locate below the pixels of the odd column. The photosensitive region of pixels of the even column is aligned with the circuitry region of the pixels of the odd column. Except for the first column, the photosensitive region of pixels of the odd column is aligned with the circuitry region of the pixels of the even column. Consequently, along a direction perpendicular to the scanning direction, that is, the length direction of the sensor array, a photosensitive region of which the fill factor is almost 100% is disposed at every distance of P. 
     The present invention has the following features and good effects. 
     The pixel structure of the CMOS-TDI image sensor is able to realize the transfer of the output signal of previous pixels to next pixels at the exposure time, and obtained the pipelined accumulation of exposure result to the same object by the pixels of the same column. In each exposure period, the output of the last line pixels is required to be read out, thus decreasing the limitation of the shortest exposure time imposed by the readout speed, and improving the largest scanning frequency of the sensor. The offset isolation technique is applied to the pixels to remove offset voltage. In addition, only one sampling operation is introduced in the course of receiving forwarded pixel output signals, thus reducing the introduced thermal noise. The layout suitable for this pixel structure may significantly improve the fill factor of the pixel photosensitive region without decreasing the equivalent central distance of the pixels. The instant invention may better realize the TDI function, improve the scanning frequency of the CMOS-TDI image sensor, and expand the application range of the TDI technique. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  shows a schematic operation manner of a prior art linear image sensor; 
         FIG. 2  shows a schematic operation manner of a prior art CCD-TDI image sensor; 
         FIG. 3  shows a pixel circuit diagram employed by a high scanning frequency CMOS-TDI image sensor of the present invention; 
         FIG. 4  indicates a schematic pixel timing sequence control of the present invention; 
         FIG. 5  shows a circuit diagram of pixels of a single column of the present invention; 
         FIG. 6  shows a schematic layout view of the pixel array of the invention; and 
         FIG. 7  shows an embodiment schematic view of a single pixel layout of the current invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The pixel structure employed by a CMOS-TDI image sensor of the present invention is illustrated in  FIG. 3 . It mainly includes a photodiode, an operational amplifier, integration capacitors C 1  and C 2  (of the same capacitance value), an offset voltage removing capacitor C 3 , and plural switches S 1 -S 10 . These components of the pixel have the connection relationship as denoted below. The anode of the photodiode is connected to a ground wire (0V), while the cathode thereof is connected to one end of the switch S 9 , the other end of the switch S 9  is connected to a reference voltage V ref . A left electrode plate of the offset voltage removing capacitor C 3  is coupled to the cathode of the photodiode, whereas a right electrode thereof is coupled to a negative input end of the operational amplifier. The switch S 10  is connected between the negative input end and an output end of the operational amplifier in series. A positive input end of the operational amplifier is coupled to the reference voltage V ref . The output end of the operational amplifier also works as the output end of entire pixels. The left electrode plate of the integration capacitor C 1  is connected to one end of each switch S 1  and S 3 , the other end of switch S 1  is connected to the reference voltage V ref, and the other end of the switch S 3  is connected to the cathode of the photodiode. The right electrode plate of the integration capacitor C 1  is connected to one end of each switch S 2  and S 4 . The other end of the switch S 2  is connected to an input end of the pixel, the other end of the switch S 4  is connected to an output end of the pixel. The left electrode plate of the integration capacitor C 2  is connected to one end of each switch S 5  and S 7 . The other end of the switch S 5  is connected to the reference voltage V ref , the other end of the switch S 7  is connected to the cathode of the photodiode. The right electrode plate of the integration capacitor C 2  is connected to one end of each switch S 6  and S 8 . The other end of the switch S 6  is connected to an input end of the pixel, the other end of the switch S 8  is connected to an output end of the pixel. The controlling timing sequence of the switches S 1 -S 10  is shown in  FIG. 4 . Wherein, T L  is transit time, T int  is pixel exposure time, the high level indicates turning on of the switches, while the low level indicates turning off thereof. Also, the switches S 1  and S 2  are controlled by the clock clk 1 ′, the switches S 3  and S 4  are controlled by the clock clk 2 , the switches S 5  and S 6  are controlled by the clock clk 2 ′, the switches S 7  and S 8  are controlled by the clock clk 1 , while the switches S 9  and S 10  are controlled by the clock rst. When the clock Read is at the high level, the pixel output is valid. Under the control of the timing sequence of  FIG. 4 , the pixel structure of  FIG. 3  may perform the following actions in a pipelined manner: sample the input signal, and then add the sampled signal to the photocurrent integration signal generated during an exposure period of the pixel, output the accumulation result and express the output V int  as: 
     
       
         
           
             
               
                 
                   
                     V 
                     int 
                   
                   = 
                   
                     
                       V 
                       ref 
                     
                     + 
                     
                       V 
                       0 
                     
                     + 
                     
                       
                         
                           ∫ 
                           0 
                           
                             T 
                             int 
                           
                         
                         ⁢ 
                         
                           
                             i 
                             ph 
                           
                           ⁢ 
                           dt 
                         
                       
                       
                         C 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         1 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
     Wherein V 0  is the signal collected during the previous exposure period of the pixel, and i ph  is the photocurrent value of the photodiode. The offset voltage removing capacitor C 3  of the pixel has the ability of isolating integration capacitors C 1  and C 2  from the operational amplifier offset voltage. As a result, the integration result output by the pixel contains no offset voltage of the operational amplifier. Moreover, only one input signal sampling operation is introduced in the course of signal accumulation by the pixel. That is, sampling thermal noise is introduced only once, thus reducing the thermal noise level during the accumulated readout. 
     TDI signal accumulation function is achieved by cascading these pixels. A single column of pixels after cascading N pixels is illustrated in  FIG. 5 . Here, an input end of pixel  1  is connected to the reference voltage V ref , an input end of pixel  2  is connected to an output end of pixel  1  and then cascading by the similar manner; and finally, an output end of the last pixel is connected to a column-parallel ADC through a readout switch Read. The operating procedure of the pixel is described as followed. In the initializing status, (clk 1 =clk 2 =0, clk 1 ′=clk 2 ′=rst=1), the input and output voltage of all pixels are V ref  by this time, a resetting action is finished for integration capacitors C 1  and C 2  of all pixels, and a resetting action is also applied to the photodiode. After that, all the pixels are subject to a first exposure period(clk 1 =clk 1 ′=1, clk 2 =clk 2 ′=rst=0), the integration capacitor C 1  of each pixel begins to collect the output signal of its previous pixel and at the same time, the integration capacitor C 2  begins to integrate photocurrent of the photodiode. When the first exposure period ends, the signal stored in the integration capacitor C 2  of x th  pixel is V int(1,x) , and the signal stored in the integration capacitor C 1  thereof is V int(1,x−1) . Next, all the pixels are subject to a resetting status (clk 1 =clk 1 ′=clk 2 =clk 2 ′=0, rst=1), the integration capacitors C 1  and C 2  of each pixel are in a floating status by this time and signals stored in these capacitors are kept unchanged. Resetting action is applied to the photodiode. After that, all the pixels are subject to a second exposure period(clk 1 =clk 1 ′=rst=0, clk 2 =clk 2 ′=1), the integration capacitor C 2  of each pixel begins to collect the output signal of its previous pixel and at the same time, the integration capacitor C 1  begins to integrate photocurrent of the photodiode. When the second exposure period ends, the signal stored in the integration capacitor C 1  of x th  pixel is V int(1,x−1) +V int(2,x) , and the signal stored in the integration capacitor C 2  thereof is V int(1,x−2) +V int(2,x−1) . By the similar manner, after exposure of N times, the output of the N th  pixel may be expressed as:
 
 V   int_tot   =V   int(1,1)   +V   int(2,2)   +V   int(3,3)   + . . . +V   int(N,N)   (formula 2)
 
     Wherein, from V int(1,1)  to V int(N,N) , each represents the exposure result of the same object during a respective transit time from 1 to N by a respective pixel from 1 to N. Accordingly, the output of the N th  pixel is the result of N-leveled integration accumulation. This result is quantitatively output by a subsequent column-parallel ADC during a high level period of the clock Read, thus completing the entire reading out process. During each exposure period, the output of the N th  pixel is the result of N times integration accumulation. Therefore, the ADC only needs to perform readout once thus reducing the limitation on the scanning frequency imposed by the readout speed. A readout time of a microsecond level can lead to a scanning frequency of hundreds of KHz. 
     Operational amplifiers and capacitors are integrated into the pixel structure of the invention, and these circuits certainly reduce the fill factor of the photosensitive region of the photodiode. To overcome this problem, the present invention suggests a layout as shown in  FIG. 6  and suitable for this pixel structure. Here, each square has a central distance of P. the hatched squares are photosensitive regions of the photodiode, while the laterally adjacent white squares with the same size are locations where the operational amplifier and capacitor layout are disposed (are called as circuitry regions). Accordingly, each hatched square and a laterally adjacent white square constitute a layout of a pixel. Pixels of the odd column are not laterally adjacent to those of the even column. The pixels of the even column entirely locate below the pixels of the odd column. The photosensitive region of pixels of the even column is aligned with the circuitry region of the pixels of the odd column. Except for the first column, the photosensitive region of pixels of the odd column is aligned with the circuitry region of the pixels of the even column. Consequently, along a direction perpendicular to the scanning direction (that is, the length direction of the sensor array), a photosensitive region of which the fill factor is almost 100% is disposed at every distance of P. By using such layout, the influence of the circuit portion of the pixel on the fill factor of the photosensitive regions is decreased without decreasing an equivalent pixel central distance, and almost 100% of the fill factor can be formed. Only a constant time difference N×TL exists between the output of the odd and even columns of pixels. This difference may be removed by simply processing output digital signals and this will not burden post circuits. 
     To further illustrate the objects, technical solution and advantages of the invention, a detailed description of the embodiments of the invention will be provided. In one embodiment, the length of the sensor is 1024 pixels, the levels of the TDI are 50, the scanning frequency is 100 KHz, the central distance of pixel photosensitive region is 15 μm, and a Cyclic ADC with a resolution of 10 bit and conversion rate of 100 KHz is used as a column-parallel on-chip ADC. The layout of an individual pixel is shown in  FIG. 7 . The photodiode has the size of 15 μm×15 μm, capacitors C 1 , C 2  and C 3 , switches and operational amplifier are all placed into a 15 μm×15 μm sized layout. Here, the size of the capacitors C 1 , C 2  is both 7.5 μm×7.5 μm, the capacitor C 3  has the size of 2.5 μm×15 μm, the rest area is for placing the switches and operational amplifier. If the capacitors are realized by a MIM capacitor with a unit area capacitance of 2 fF/μm 2 , then the capacitance value of the capacitors C 1 , C 2  will be about 112.5 fF, while that of the capacitor C 3  will be about 75 fF. Moreover, MIM is generally made of an upper layer of metal and as a result, transistors and the lower layer of metal wiring may be positioned below the capacitors. As such, part of switches and operational amplifiers may be disposed into the space below the capacitors to make full use of the space. The length of the sensor pixel array is 15 μm×1024=15360 μm, height is 15 μm×50×2=1500 μm, and equivalent photosensitive region central distance is 15 μm. For a scanning frequency of 100 KHz, the transit time T L  of the sensor is 10 μs, including 9 μs of T int  and 1 μs of pixel resetting time. The conversion rate of in-chip column-parallel ADC is 100 KHz which meets the requirement of readout speed. During the period of sampling the previous pixel output by each pixel, the root mean square value of the introduced thermal noise is √{square root over (kT/C 1 )}≈192μV. Accordingly, after accumulation of TDI of 50 levels, the root mean square value of the introduced total thermal noise is √{square root over ( 50  kT/C 1 )}≈1.36 mV. The reference voltage V ref  of the pixel may be 1V. If the maximum voltage output by the pixel is 2.6V, the maximum signal amplitude output by the pixels after accumulation of 50 levels is 1.6V. If only thermal noise caused during the readout process is considered, the maximum SNR of the output signals is 20 log(1.6V/1.36 mV)=61.4 dB, satisfying the SNR requirement of ADC of 10-bit resolution.