Abstract:
A complementary metal-oxide semiconductor (CMOS) image sensor system and method thereof produce a control signal to make a input terminal repeatedly switch between high potential and low potential, thereby modulating image signals at a specific frequency to prevent image quality from being affected by direct current (DC) voltage variations. The mechanism thus helps improving the image quality.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to TAIWAN Patent Application Ser. No. 102127073, filed Jul. 29, 2013, the entireties of which is incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    The invention relates to an image sensor system and, in particular, to a CMOS image sensor system that utilizes CMOS as an active pixel image sensor. The invention also relates to the method thereof. 
       BACKGROUND ART 
     Description of Related Art 
       [0003]    In recent years, rapid developments and popularity of semiconductor technology have enabled complementary metal-oxide semiconductor (CMOS) image sensors to compete with charge coupled device (CCD) sensors. Particularly in the low-end market, the CMOS image sensors have a lower cost because they do not require a special manufacturing process. Therefore, the CMOS image sensors have become the mainstream in the low-end market. 
         [0004]    Generally speaking, the quality of images produced by a CMOS image sensor is not as good as that by a CCD image sensor. Nevertheless, the CMOS has a lower cost and is power-effective, posing a large attraction for portable devices. It is thus the primary issue for vendors to improve the image quality of CMOS. 
         [0005]    In view of this, some propose to improve the manufacturing process and packaging method. For example, the integrated technology of backside illumination (BSI) and through-silicon via (TSV) has been proposed to improve the image quality. However, this method cannot be applied to the CMOS image sensor made using conventional process and packaging. On the other hand, new manufacturing processes and structures result in lower yield and higher cost. Therefore, the above-mentioned solution cannot effectively address the problem of bad image quality for CMOS. 
         [0006]    In summary, the prior art always has the problem of bad image quality for CMOS image sensors. It is imperative to provide a better technique to solve the problem. 
       SUMMARY 
       [0007]    The invention discloses a CMOS image sensor system and the method thereof. 
         [0008]    The disclosed system includes: a controlling unit, a sensor array, and a signal processing module. The controlling unit generates a control signal to make a input terminal repeatedly switch between high and low potentials. The sensor array includes at least one active pixel sensor unit. After each of the active pixel sensor units detects light and produces charges, direct current (DC) and alternating current (AC) signals are generated according to the high/low potential on the input terminal. The DC and AC signals are output to a column output. The signal processing module is electrically connected with the sensor array to filter out the DC current, receiving only the AC signal from the column output, to avoid DC voltage variations and noise interference. The signal processing module further processes the AC signal to generate an image signal. 
         [0009]    The disclosed method includes the steps of: generating a controlling signal to make a input terminal repeatedly switch between high and low potentials; after detecting light and generating charges, generating DC and AC signals according to the high/low potential on the input terminal, and outputting the DC and AC signals to the column output; filtering out the DC signal from the column output and receiving only the AC signal to avoid DC voltage variations and noise interference, and processing the AC signal to generate an image signal. 
         [0010]    The disclosed system and method differ from the prior art in that the controlling signal is produced to switch the input terminal between the high and low potentials, thereby modulating the image signal at a specific frequency to prevent DC voltage variations and noise interference from affecting the image quality. 
         [0011]    Through the above-mentioned technique, the invention can improve the image quality. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The invention will become more fully understood from the detailed description given herein below illustration only, and thus is not limitative of the present invention, and wherein: 
           [0013]      FIG. 1  is a block diagram of the disclosed CMOS image sensor system; 
           [0014]      FIGS. 2A and 2B  are flowcharts of the disclosed CMOS image sensor method; 
           [0015]      FIGS. 3A and 3B  are schematic views of the disclosed signal processing module; and 
           [0016]      FIG. 4  shows waveforms of signals at various terminals of the invention. 
           [0017]      FIG. 5  shows the waveforms of signals at various terminals of a conventional CMOS image sensor. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    The present invention will be apparent from the following detailed description, which proceeds with reference to the accompanying drawings, wherein the same references relate to the same elements. 
         [0019]    Before describing the disclosed CMOS image sensor and the method thereof in detail, we first explain the structure of the invention. The invention differs from the prior art in that in the active pixel sensor unit, the input terminal repeatedly switches between high and low potentials to obtain DC and AC signals. The DC signal is then filtered out, leaving the AC signal to be detected and used for image formation. In comparison, the prior art can only obtain and use the DC signal. Thus, the invention can prevent DC signal shifts due to differences in electronic properties, thereby avoiding bad image quality. The invention can be applied to a conventional sensor array and achieve the goal of image quality improvement. Besides, semiconductors have higher flicker noises on low-frequency and DC signals. Tuning the image signal at a specific frequency can reduce this interference, further improving the image quality. The specific frequency can be adjusted according to the intensity of light detected by the sensor array. As the light intensity increases, the frequency can be higher. The duty cycle of the specific frequency also can be adjusted according to the intensity of light detected by the sensor array. As the light intensity increases, the duty cycle can be longer. In addition to the above-mentioned method, one can also change the frequency range to adapt to different light intensities. As the light intensity increases, the frequency range is also enlarged. 
         [0020]    Please refer to  FIG. 1 , which is a block diagram of the disclosed CMOS image sensor system. The disclosed system includes: a controlling unit  10 , a sensor array  11 , and a signal processing module  20 . The controlling unit  10  generates a control signal to make a input terminal  114  repeatedly switch between high and low potentials. In practice, the repeated switches mean that the potential is cyclically switched between high and low. The number of switches is such that the output image signal is sufficiently stable. Suppose the image signal can be stabilized after five switches between high and low potentials. The control signal generated by the controlling unit  10  controls the input terminal  114  to repeatedly switch between the high and low potentials for at least five times. We will describe the waveforms as the input terminal  114  switches between the high and low potentials with accompany figures. 
         [0021]    The sensor array  11  includes at least one active pixel sensor unit  110   a - 110   n.  After each of the active pixel sensor units  110   a - 110   n  detects light and produces charges, DC and AC signals are generated according to the high/low potential on the input terminal  114 . The DC and AC signals are output to a column output  116 . In an embodiment of the invention, each of the active pixel sensor units  110   a - 110   n  includes a photodiode  111 , transistor  112   a - 112   c,  a capacitor  113 , a input terminal  114 , a row select  115 , and a column output  116 . The photodiode  111  and the capacitor  113  are connected in parallel. The photodiode  111  can generate electric charges according to the light incident thereon. The row select  115  controls the transistor  112   c  for the charges generated by the photodiode  111  to be output to the column output  116 . In practice, each of the active pixel sensor units  110   a - 110   n  can consist of three transistor active pixel sensors (3 T APS&#39;s) or four transistor active pixel sensors (4 T APS&#39;s). The invention does not put any restriction on the active pixel sensor units  110   a - 110   n.  Any element that can perform pixel sensing should be considered as part of the invention. 
         [0022]    The signal processing module  20  is electrically connected with the sensor array  11  to filter out the DC current, receiving only the AC signal from the column output  116 , to avoid DC voltage variations and noise interference. A high-pass filter is used to allow the passage of the AC signal from the column output  116 . Afterwards, an amplifier amplifies the AC signal that passes through the high-pass filter. A rectifier or demodulator then performs half or full wave rectification or signal demodulation. The rectifier or demodulator converts the AC signal into a signal with a higher frequency and a DC signal. Finally, a low-pass filter smoothes the AC signal after the half or full wave rectification or demodulation, and outputs the final signal. Detailed features of the signal processing module  20  will be described with reference to accompanying figures later. 
         [0023]    Please refer to  FIGS. 2A and 2B , which are flowcharts of the disclosed CMOS image sensor method. The disclosed method includes the steps of: generating a control signal to make the input terminal  114  repeatedly switch between high and low potentials (step  210 ); after detecting light and generating charges, generating DC and AC signals according to the high/low potential on the input terminal  114 , and outputting the DC and AC signals to the column output  116  (step  220 ); filtering out the DC signal from the column output  116  and receiving only the AC signal from the column output  116  to avoid DC voltage variations and noise interference, and processing the AC signal to generate an image signal (step  230 ). Through the above-mentioned steps, the control signal is generated for the input terminal  114  to repeatedly switch between high and low potentials, so that the image signal is modulated at a specific frequency, thereby preventing DC voltage variations and noise interference from deteriorating the image quality. In practice, the signal processing in step  230  is accomplished through the steps of: allowing the AC signal of the column output to pass (step  231 ); amplifying the passed AC signal (step  232 ); performing half or full wave rectification or signal demodulation on the amplified AC signal (step  233 ), smoothing the AC signal after half or full wave rectification or demodulation and outputting the signal (step  234 ). 
         [0024]    Please refer to  FIGS. 3A to 5  for an embodiment of the invention.  FIGS. 3A and 3B  are schematic views of the disclosed signal processing module. As mentioned before, the signal processing module  20  filters out the DC signal of the column output. In practice, the signal processing module  20  includes a high-pass filter  201 , an amplifier  202 , a rectifier  203   a,  and a low-pass filter  204 , as shown in  FIG. 3A . The high-pass filter  201  is electrically connected to the column output  116  of the sensor array  11  to allow the AC signal of the column output  116  to go through. That is, the DC signal is filtered out, and only the AC signal is allowed to pass. The amplifier  202  is electrically connected to the high-pass filter  201  to amplify the AC signal passing through the high-pass filter  201 . The rectifier  203   a  is electrically connected to the amplifier  202  to perform half or full wave rectification on the passed AC signal. The AC signal is then converted into a signal with a higher frequency and a DC signal. The low-pass filter  204  is electrically connected to the rectifier  203   a  to smooth the rectified AC signal and to output the final signal. Besides, the signal processing module  20  can use a demodulator  203   b  instead of the rectifier  203   a,  as shown in  FIG. 3B . The other components are unchanged. The demodulator  203   b  demodulates the passed AC signal. The low-pass filter  204  smoothes the demodulated AC signal and outputs the result. The purpose of the above-mentioned half or full wave rectification or signal demodulation is to increase the original AC signal to a higher frequency. The difference between the two methods is merely in the circuit. For the convenience of reference to the waveforms at various terminals, the terminal between the high-pass filter  201  and the amplifier  202  is denoted by N 3 , that between the rectifier  203   a  or the demodulator  203   b  and the amplifier  202  by N 4 , that between the low-pass filter  204  and the rectifier  203   a  or the demodulator  203   b  by N 5 , and the output terminal of the low-pass filter  204  by N 6 . 
         [0025]      FIG. 4  shows waveforms of signals at various terminals of the invention. Please refer to  FIGS. 1 and 3  at the same time. The control signal generated by the controlling unit  10  makes the waveform of the signal at the input terminal  114  switch rapidly between high and low potentials, as shown in  FIG. 4 . The high potential is denoted by Vdd, and the low potential by Vss. Take the active pixel sensor unit  110   a  of  FIG. 1  as an example. The input terminal  114  repeatedly switch between the high and low potentials, the input terminal  114  electrically connects to the gate of the transistor  112   a,  the drain of the transistor  112   a  electrically connects to the high potential Vref, and the source of the transistor  112   a  electrically connects to the photodiode  111 . Therefore, the waveform at terminal N 1  switches from the low potential Vss to the high potential Vref. As the switch time of the input terminal  114  between the high and low potentials is very short, the waveform at terminal N 1  is maintained at the high potential Vref during the switches of the input terminal  114 , as shown in  FIG. 4 . Besides, the gate of the transistor  112   b  electrically connects to terminal N 1 . The drain of the transistor  112   b  electrically connects to the high potential Vdd. The source of the transistor  112   b  electrically connects to the drain of the transistor  112   c.  The gate of the transistor  112   c  electrically connects to the row select  115 . The source of the transistor  112   c  electrically connects to the column output  116  (i.e., terminal N 2 ). Therefore, terminal N 2  has a triangle wave at the AC part, as shown in  FIG. 4 . This waveform is processed by the high-pass filter  201  to produce the waveform at terminal N 3  in  FIG. 4 . The amplifier  202  amplifies the waveform of terminal N 3  to generate the waveform at terminal N 4 . The waveform at terminal N 4  is processed with half or full wave rectification or signal demodulation by the rectifier  203   a  or the demodulator  203   b  to generate the waveform at terminal N 5 . Finally, the low-pass filter  204  smoothes the waveform at terminal N 5  and outputs the waveform at terminal N 6 . After the above-mentioned processing, the image signal is modulated to a specific frequency (e.g., high frequency) to avoid the problem of bad image quality due to DC voltage variations. Since the image signal is modulated to a higher frequency, the invention can further avoid the problem of noises of semiconductors at low frequencies and DC signals. It should be mentioned that the waveforms at terminal N 1  and terminal N 2  have different heights due to their different electronic properties. However, after the high-pass filter, the waveforms at terminal N 3  and terminal N 6  do not have any difference in height due to electronic properties. 
         [0026]    Finally, please refer to  FIG. 5  for the waveforms of signals at various terminals of a conventional CMOS image sensor. Different from the repeated switches between high and low potentials at the input terminal  114 , terminal N 1  is raised to the high potential V REF  as the input terminal switches from the low potential to the high potential in the prior art. When the row select still stays at the low potential Vss, no signal is output and the input terminal switches from the high potential to the low potential. This period of time until the row select switches from the low potential to the high potential is called the photodiode integration time. During this period, the potential of terminal N 1  drops with time until the row select switches from the low potential to the high potential for signal output. 
         [0027]    In summary, the disclosed system and method differ from the prior art in that the controlling signal is produced to switch the input terminal between the high and low potentials, thereby modulating the image signal at a specific frequency to prevent DC voltage variations and noise interference from affecting the image quality. Through the above-mentioned technique, the invention can improve the image quality. 
         [0028]    Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternative embodiments, will be apparent to persons skilled in the art. It is, therefore, contemplated that the appended claims will cover all modifications that fall within the true scope of the invention.