Abstract:
A capacitive image sensor and a method for running the capacitive image sensor are disclosed. The capacitive image sensor includes a number of capacitive sensing elements, forming an array, each capacitive sensing element for transforming a distance between a portion of a surface of an approaching finger and a top surface thereof into an output voltage, wherein a value of the output voltage is changed by a driving signal exerted on the finger; an A/D converter, for converting the output voltage into a number and outputting the number; and a signal source, for providing the driving signal to the finger.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a pixel sensing element. More particularly, the present invention relates to a capacitive image sensor using the pixel sensing element, and a method for running the pixel sensing elements. 
       BACKGROUND OF THE INVENTION 
       [0002]    There are many human physiological characteristics which can be used to provide personnel identification for security purposes, such as fingerprint, retina, iris, DNA, or even face features. For all the devices that are capable of distinguishing some physiological characteristic of one person from others&#39;, a fingerprint reader has the lowest cost and complexity, while the identification results are generally pretty good. In addition, the size of data required to store the minutiae of one fingerprint is small (ranging from 120 bytes to 2K bytes). This makes fingerprint identification devices widely accepted in many applications. 
         [0003]    There are also many types of sensing techniques for capturing fingerprint. The popular ones are optical type and capacitive type. Optical fingerprint sensing modules utilize reflected light intensity from the surface of a finger to tell where the ridges and valleys are on the contact portion of the finger. The advantage of the optical technique is reliability and low cost. However, due to the size of the embedded optical lens, the form factor of an optical fingerprint sensing module cannot be kept small. It is difficult for the optical type sensor to be embedded in portable devices. The capacitive type fingerprint identification modules, on the other hand, are made out of silicon chips and can be made very compact. In some cases, when a fingerprint image can be fetched by slide scanning, the fingerprint sensor can be even thin and slim, too. The small form factor of capacitive type fingerprint identification module makes it suitable for portable applications such as access control badges, bank cards, cellular phones, tablet computers, USB dongles, etc. 
         [0004]    Capacitive fingerprint sensor is based on the principle that the capacitance of a two parallel conductive plates is inversely proportional to the distance between them. A capacitive fingerprint sensor consists of an array of sensing units. Each sensing unit contains a sensing plate. By using the sensing plate as one plate of the two-plated capacitor and a dermal tissue as another plate, ridges and valleys of a fingerprint can be located by measuring the different capacitances. There are many prior arts related to the capacitive type fingerprint identification module. Most of them have been applied to manufacture fingerprint sensors. However, there are also many problems pending for solutions. One of them is the accuracy of the sensing elements. 
         [0005]    Due to the high density nature, the popular capacitive fingerprint sensors are mainly manufactured with semiconductor processes. The precision of the sensing elements is affected by many factors inherited in the process technology, such as density of chemical impurities, alignment of photo masks, equipment control, etc., whose uncertainty or variation will be reflected in the different behavior between devices, or even a fixed pattern noise seen in the captured fingerprint images of the same device. To achieve best performance of personal identification, it is desirable to improve the quality of the capture fingerprint image by reducing the noise pattern. A common practice to eliminate fixed pattern noise is to calibrate the device before use. The calibration data can be calculated and stored as part of the manufacturing process, or right before the device is being used. However in either case, a certain amount of memory storage space must be set aside for the calibration data, and this storage space will increase the system cost. Therefore, an innovative pixel sensing element, a capacitive fingerprint sensor made by the pixel sensing elements and a method for running the pixel sensing element are desirable. 
       SUMMARY OF THE INVENTION 
       [0006]    This paragraph extracts and compiles some features of the present invention; other features will be disclosed in the follow-up paragraphs. It is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims. 
         [0007]    According to an aspect of the present invention, a capacitive image sensor is disclosed. The capacitive image sensor includes: a number of capacitive sensing elements, forming an array, each capacitive sensing element for transforming a distance between a portion of a surface of an approaching finger and a top surface thereof into an output voltage, wherein a value of the output voltage is changed by a driving signal exerted on the finger; an A/D converter, for converting the output voltage into a number and outputting the number; and a signal source, for providing the driving signal to the finger. The driving signal is a signal with voltage transition or transitions, formed by alternate positive waveform and negative waveform. The internal electric potential at each part of the capacitive sensing elements is initialized to a known constant value during a reset stage. The capacitive sensing elements receive the driving signal and convert it to an output voltage during a sensing stage. The A/D converter performs conversion during a measuring stage. A difference between two numbers converted from the output voltages occurred in each capacitive sensing element under one positive waveform and one negative waveform, respectively, is a noise-reduced value representing a pixel for the portion surface of the finger. Sequentially collect the noise-reduced values under the corresponding positive waveform and negative waveform of each pixel. Map the noise-reduced values to corresponding locations of capacitive sensing elements to obtain a noise-reduced image of the finger. 
         [0008]    Preferably, shapes of the positive waveform and the negative waveform may be symmetrical. The positive waveform or the negative waveform may be a step function. The pixel image value may be a numeric value that corresponds to the gray level of the pixel. 
         [0009]    In a first embodiment, the capacitive sensing element may further includes: a metal plate; a voltage follower, wherein an input end of the voltage follower is connected to the metal plate, and an output end of the voltage follower is connected to the A/D converter; a comparative capacitor, wherein one electrode of the comparative capacitor is electrically connected to the metal plate and the other electrode thereof is electrically connected to a ground end; a constant voltage source, for providing a constant bias voltage; and a constant bias voltage switch, connected to the constant voltage source and the metal plate, for switching supply of the constant bias voltage. The constant bias voltage switch is turned on during the reset stage and is turned off during the sensing stage and the measuring stage. 
         [0010]    According to the present invention, a parasitic capacitance having a value of C p  is formed between the metal plate and the ground end. The comparative capacitor has a value of C m . A value of a reference capacitance, C r , is obtained by C r =C m +C p . When the finger is approaching the capacitive sensing element, a signal capacitance is formed by the finger and the signal source. A finger capacitance is formed by the finger and the metal plate. The output voltage, V out , is obtained by 
         [0000]    
       
         
           
             
               V 
               out 
             
             = 
             
               
                 V 
                 bias 
               
               + 
               
                 
                   
                     C 
                     f 
                   
                   
                     
                       C 
                       f 
                     
                     + 
                     
                       C 
                       r 
                     
                   
                 
                  
                 
                   
                     V 
                     
                       i 
                        
                       
                           
                       
                        
                       n 
                     
                   
                   . 
                 
               
             
           
         
       
     
         [0000]    V bias  is the value of the constant bias voltage. V in  is a voltage change of the driving signal. C f  is a value of the finger capacitance. 
         [0011]    Preferably, the comparative capacitor is a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) capacitor, a PIP (Polysilicon-Insulator-Polysilicon) capacitor or a MIM (Metal-Insulator-Metal) capacitor. 
         [0012]    In a second embodiment, the capacitive sensing element may further includes: a metal plate; a voltage follower, wherein an input end of the voltage follower is connected to the metal plate, and an output end of the voltage follower is connected to the A/D converter; a constant voltage source, for providing a constant bias voltage; and a constant bias voltage switch, connected to the constant voltage source and the metal plate, for switching supply of the constant bias voltage. The constant bias voltage switch is turned on during the reset stage and is turned off during the sensing stage and the measuring stage. 
         [0013]    According to the present invention, a parasitic capacitance having a value of C p  is formed between the metal plate and a ground end. A reference capacitor, C r , equals to C p . When the finger is approaching the capacitive sensing element, a signal capacitance is formed by the finger and the signal source. A finger capacitance is formed by the finger and the metal plate. The output voltage, V out , is obtained by 
         [0000]    
       
         
           
             
               V 
               out 
             
             = 
             
               
                 V 
                 bias 
               
               + 
               
                 
                   
                     C 
                     f 
                   
                   
                     
                       C 
                       f 
                     
                     + 
                     
                       C 
                       r 
                     
                   
                 
                  
                 
                   
                     V 
                     in 
                   
                   . 
                 
               
             
           
         
       
     
         [0000]    V bias  is the value of the constant bias voltage. V in  is a voltage change of the driving signal. C f  is a value of the finger capacitance. 
         [0014]    In a third embodiment, the capacitive sensing element may further includes: a metal plate; a voltage follower, wherein an input end of the voltage follower is connected to the metal plate, and an output end of the voltage follower is connected to the A/D converter; a working voltage source, for providing a working voltage; a working voltage switch, connected to the working voltage source with a first end of the working voltage switch, for switching supply of the working voltage; a comparative capacitor, wherein one electrode of the comparative capacitor is electrically connected to a second end of the working voltage switch and the other electrode is electrically connected to a ground end; a charge sharing switch, electrically connected to the metal plate and the second end of the working voltage switch, to balance electric charges in both ends when turned on; and a ground switch, for releasing electric charges accumulated in the capacitive sensing element to the ground end when turned-on, and for accumulating electric charges in the capacitive sensing element when turned-off. The working voltage switch is turned on during the reset stage and is turned off during the sensing stage and the measuring stage. The charge sharing switch is turned off during the reset stage and is turned on during the sensing stage and the measuring stage. The ground switch is turned on during the reset stage and is turned off during the sensing stage and the measuring stage. 
         [0015]    According to the present invention, a parasitic capacitance having a value of C p  is formed between the metal plate and the ground end. The comparative capacitor has a value of C m . When the finger is approaching the capacitive sensing element, a signal capacitance is formed by the finger and the signal source. A finger capacitance is formed by the finger and the metal plate. The output voltage, V out , is obtained by 
         [0000]    
       
         
           
             
               V 
               out 
             
             = 
             
               
                 
                   
                     C 
                     m 
                   
                   
                     
                       C 
                       m 
                     
                     + 
                     
                       C 
                       p 
                     
                     + 
                     
                       C 
                       f 
                     
                   
                 
                  
                 
                   V 
                   dd 
                 
               
               + 
               
                 
                   
                     C 
                     f 
                   
                   
                     
                       C 
                       m 
                     
                     + 
                     
                       C 
                       p 
                     
                     + 
                     
                       C 
                       f 
                     
                   
                 
                  
                 
                   
                     V 
                     in 
                   
                   . 
                 
               
             
           
         
       
     
         [0000]    V dd  is the value of the working voltage. V in  is a voltage change of the driving signal. C f  is a value of the finger capacitance. 
         [0016]    Preferably, the comparative capacitor may be a MOSFET capacitor, a PIP capacitor or a MIM capacitor. 
         [0017]    A method for operating the capacitive image sensor includes the steps of: resetting the capacitive sensing elements to be capable of sensing in a first reset stage; exerting the driving signal having a first waveform to the capacitive sensing elements via the finger in a first sensing stage; reading numbers converted from the output voltages from every capacitive sensing elements in a first measuring stage; resetting the capacitive sensing elements to be capable of sensing in a second reset stage; exerting the driving signal having a second waveform to the capacitive sensing elements via the finger in a second sensing stage; reading numbers converted from the output voltages from every capacitive sensing elements in a second measuring stage; subtracting the numbers obtained from the same capacitive sensing element under different measuring stages to have the noise-reduced values; sequentially collecting the noise-reduced values under the corresponding positive waveform and negative waveform of each pixel; and mapping the noise-reduced values to corresponding locations of capacitive sensing elements. If the first waveform is a positive waveform, then the second waveform is a negative waveform; if the first waveform is a negative waveform, then the second waveform is a positive waveform. 
         [0018]    A method for operating the capacitive image sensor in the first embodiment includes the steps of: turning on the constant bias voltage switch in a first reset stage; turning off the constant bias voltage switch and exerting the driving signal having a first waveform to the capacitive sensing element via the finger in a first sensing stage; sending a first output voltage to the A/D converter in a first measuring stage; turning on the constant bias voltage switch in a second reset stage; turning off the constant bias voltage switch and exerting the driving signal having a second waveform to the capacitive sensing element via the finger in a second sensing stage; and sending a second output voltage to the A/D converter in a second measuring stage. If the first waveform is a positive waveform, then the second waveform is a negative waveform; if the first waveform is a negative waveform, then the second waveform is a positive waveform. 
         [0019]    A method for operating the capacitive image sensor in the second embodiment includes the steps of: turning on the constant bias voltage switch in a first reset stage; turning off the constant bias voltage switch and exerting the driving signal having a first waveform to the capacitive sensing element via the finger in a first sensing stage; sending a first output voltage to the A/D converter in the a first measuring stage; turning on the constant bias voltage switch in a second reset stage; turning off the constant bias voltage switch and exerting the driving signal having a second waveform to the capacitive sensing element via the finger in a second sensing stage; and sending a second output voltage to the A/D converter in a second measuring stage. If the first waveform is a positive waveform, then the second waveform is a negative waveform; if the first waveform is a negative waveform, then the second waveform is a positive waveform. 
         [0020]    A method for operating the capacitive image sensor in the third embodiment includes the steps of: turning on the working voltage switch and the ground switch, and turning off the charge sharing switch in a first reset stage; turning off the working voltage switch and the ground switch first, then turning on the charge sharing switch and exerting the driving signal having a first waveform to the capacitive sensing element via the finger in a first sensing stage; sending a first output voltage to the A/D converter in a first measuring stage; turning on the working voltage switch and the ground switch, and turning off the charge sharing switch in a second reset stage; turning off the working voltage switch and the ground switch first, then turning on the charge sharing switch and exerting the driving signal having a second waveform to the capacitive sensing element via the finger in a second sensing stage; and sending a second output voltage to the A/D converter in a second measuring stage. If the first waveform is a positive waveform, then the second waveform is a negative waveform; if the first waveform is a negative waveform, then the second waveform is a positive waveform. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a schematic diagram of a capacitive image sensor according to the present invention. 
           [0022]      FIG. 2  shows different aspects of the driving signal. 
           [0023]      FIG. 3  is a flow chart illustrating a procedure to operate the capacitive image sensor. 
           [0024]      FIG. 4  shows an equivalent circuit of a design of the capacitive sensing element of a first embodiment. 
           [0025]      FIG. 5  is a flow chart illustrating a procedure to operate the capacitive sensing element in the first embodiment. 
           [0026]      FIG. 6  shows the linear superposition of two voltages in the first embodiment. 
           [0027]      FIG. 7  shows an equivalent circuit of a design of the capacitive sensing element of a second embodiment. 
           [0028]      FIG. 8  is a flow chart illustrating a procedure to operate the capacitive sensing element in the second embodiment. 
           [0029]      FIG. 9  shows the linear superposition of two voltages in the second embodiment. 
           [0030]      FIG. 10  shows an equivalent circuit of a design of the capacitive sensing element of a third embodiment. 
           [0031]      FIG. 11  is a flow chart illustrating a procedure to operate the capacitive sensing element in the third embodiment. 
           [0032]      FIG. 12  shows an equivalent circuit of a first term in the design of the capacitive sensing element in the third embodiment. 
           [0033]      FIG. 13  shows an equivalent circuit of a second term in the design of the capacitive sensing element in the third embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0034]    The present invention will now be described more specifically with reference to the following embodiments. 
         [0035]    Please see  FIG. 1 .  FIG. 1  shows a schematic diagram of a capacitive image sensor  10  according to the present invention. The capacitive image sensor  10  is used to depict ridges and valleys of a surface of a finger  200 , further converting the results into a noise-reduced image of the fingerprint. There are many electronic components to implement the capacitive image sensor  10 . The main components include a number of capacitive sensing elements  100 , an A/D converter  160 , a signal source  170  and a controller  180 . Below are the descriptions for the functions of each main component. 
         [0036]    The capacitive sensing elements  100  form an array. Each capacitive sensing element  100  can be used to transform a distance between a portion of a surface of the approaching finger  200  and a top surface of itself into an output voltage. Details about how to generate the output voltage will be illustrated later. A value of the output voltage may change according to the distance therebetween and can be enhanced by a driving signal exerted on the finger  200 . The driving signal can be used to reduce noises when sensing the finger  200 . The A/D converter  160  converts the output voltage into a number and outputs the number. The number is a digitized value. The signal source  170  can provide said driving signal to the finger  200 . In practice, the signal source  170  may include a metal frame or metal strips (not shown) around the capacitive sensing elements  100 . The top surface area of the signal source  170  should be large enough for a finger to contact, and the impedance therebetween is so small that it can be ignored. In a simple way, the signal source  170  may also be a metal bar which can be touched by the finger  200  when the capacitive sensing elements  100  are sensing. The controller  180  may control the states of each capacitive sensing element  100  and cooperate with the signal source  170  to get the noise-reduced image. 
         [0037]    Here, the driving signal is a signal with voltage transition or transitions. It is formed by alternate positive waveform and negative waveform. Voltage of the positive waveform is increasing with time while voltage of the negative waveform is decreasing with time. Shapes of the positive waveform and the negative waveform should be symmetrical. Please see  FIG. 2 .  FIG. 2  shows different aspects of the driving signal with different positive waveforms and negative waveforms. The driving signal shown in the upper side of  FIG. 2  has a smoothly changing shape. Positive waveforms and negative waveforms are marked separately. However, the driving signal may be exerted in a sharply changing manner as shown in the lower side of  FIG. 2 . Preferably, the positive waveform or the negative waveform is a step function. Internal electric potential at each part of the capacitive sensing elements  100  is initialized by applying a constant voltage, which provides the bias voltage of the sensing elements, to a known constant value during a reset stage. The capacitive sensing elements  100  receive the driving signal and convert it to an output voltage during a sensing stage. The A/D converter  160  performs conversion during a measuring stage. The reset stage, sensing stage and measuring stage are the states of the capacitive sensing elements  100  and will be explained in details along with the description of the capacitive sensing elements  100  later. A difference between two numbers converted from the output voltages occurred in each capacitive sensing element  100  under one positive waveform and one negative waveform, respectively, is a noise-reduced value. The noise-reduced value represents a pixel for the portion of the surface of the finger  200 . Namely, the portion of the surface of the finger  200  is the portion just above the capacitive sensing element  100  which is sensing said portion of the surface of the finger  200 . A noise-reduced image of the finger  200  (fingerprint) can be obtained by: sequentially collecting the noise-reduced values under the corresponding positive waveform and negative waveform of each pixel, and mapping the noise-reduced values to corresponding locations of capacitive sensing elements  100 . 
         [0038]    Preferably, the pixel image value is a numeric value that corresponds to the gray level of the pixel. Thus, any portion of the finger  200  can be presented by a specific grayscale, representing the distance between the capacitive sensing element  100  and the portion of the finger  200  above it. A fingerprint image can be obtained. 
         [0039]    The procedure to operate the capacitive image sensor  10  is illustrated by the flow chart in  FIG. 3 . A first step is to reset the capacitive image elements  100  to be capable of sensing in a first reset stage (S 01 ). Here, “capable of sensing” means the capacitive image elements  100  are ready for sensing and can also be called ready state. The ready state may be different for different implementations (or embodiments) of the present invention. Then, exert the driving signal which has a first waveform to the capacitive sensing elements  100  via the finger  200  in a first sensing stage (S 02 ). The first waveform, for example, is a positive waveform. A third step is reading numbers converted from the output voltages from every capacitive sensing elements  100  in a first measuring stage (S 03 ). Step S 03  may be carried out by the controller  180  or other specific element designed for this job. Next, reset the capacitive sensing elements  100  to be capable of sensing in a second reset stage (S 04 ). Again, the capacitive sensing elements  100  are in the status of capable of sensing for the following steps. Exert the driving signal which a second waveform to the capacitive sensing elements  100  via the finger  200  in a second sensing stage (S 05 ). The second waveform is a negative waveform. Similarly, read numbers converted from the output voltages from every capacitive sensing elements  100  in a second measuring stage (S 06 ). After two cycles of collecting numbers from said capacitive sensing elements  100 , subtract the numbers obtained from the same capacitive sensing element  100  under different measuring stages, namely, from the first and second measuring stages to have the noise-reduced values (S 07 ). Then, sequentially collect the noise-reduced values under the corresponding positive waveform and negative waveform of each pixel (S 08 ). Finally, map the noise-reduced values to corresponding locations of capacitive sensing elements  100  to get the noise-reduced image of the finger  200  (fingerprint). It should be noticed that if the first waveform is a positive waveform, then the second waveform is a negative waveform. Otherwise, if the first waveform is a negative waveform, then the second waveform is a positive waveform. Again, the positive waveform and the negative waveform should be symmetrical. 
         [0040]    In a first embodiment, an equivalent circuit of a design of the capacitive sensing element  100  is shown in  FIG. 4 . The capacitive sensing element  100  is basically composed of a metal plate  101 , a voltage follower  102 , a comparative capacitor  103 , a constant voltage source  104  and a constant bias voltage switch  105 . There are many parasitic capacitances naturally formed the metal plate  101  and other metal layers in the sensor (not shown), a net effect of all the parasitic capacitances can be considered as one single equivalent capacitor, having a value of C p , formed between the metal plate  101  and the ground end  108 . The parasitic capacitance inevitably exists in the capacitive sensing elements  100 . However, its capacitance, C p , can be well controlled under proper design such that the parasitic capacitance in every capacitive sensing element  100  has the same value. An equivalent parasitic capacitor  106  is used to denote the parasitic capacitance. When the finger  200  is approaching the capacitive sensing element  100 , a signal impedance is formed between the finger  200  and the signal source (not shown), and a finger capacitance is formed by the finger  200  and the metal plate  101 . Because the overlapping area between the signal source and the finger is much larger than that of a single metal plate  101  of the capacitive sensing element  100 , the signal impedance is so small that it is negligible. An equivalent finger capacitor  107  is used to denote the finger capacitance. A voltage change of the driving signal (value of V in  and produced by the change of waveforms) can be exerted to the capacitive sensing element  100  via the finger  200 . An input end  102   a  of the voltage follower  102  is connected to the metal plate  101  while an output end  102   b  of the voltage follower  102  is connected to the A/D converter  160 . The voltage follower  102  gives effective isolation for the output end  102   b  from the input end  102   a , connected to the metal plate  101 , to avoid drawing power form the input end  102   a , and is better designed as close to an ideal voltage follower as possible. The comparative capacitor  103  is implemented by circuit elements such as MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) capacitor. It can also be a PIP (Polysilicon-Insulator-Polysilicon) capacitor or a MIM (Metal-Insulator-Metal) capacitor. One electrode of the comparative capacitor  103  is electrically connected to the metal plate  101  and the other electrode of the comparative capacitor  103  is electrically connected to the ground end  108 . The comparative capacitor  103  has a value of C m . The constant voltage source  104  is used to provide a constant bias voltage. The constant bias voltage switch  105  is connected to the constant voltage source  104  and the metal plate  101 , for switching supply of the constant bias voltage. The constant bias voltage switch  105  is turned on during the reset stage. At this stage, the constant bias voltage is applied to the metal plate  101 , the comparative capacitor  103 , and the parasitic capacitor  106  to establish proper operating conditions for the capacitive sensing element  100 . The constant bias voltage switch  105  is turned off during the sensing stage and the measuring stage. 
         [0041]    The procedure to operate the capacitive sensing element  100  in this embodiment is illustrated by the flow chart in  FIG. 5 . First, turn on the constant bias voltage switch  105  in a first reset stage (S 11 ). The constant voltage source  104  provides the bias voltage for the circuit in the capacitive sensing element  100 . The capacitive sensing element  100  can start sensing operation. Then, turn off the constant bias voltage switch  105  and exert the driving signal which has a first waveform to the capacitive sensing element  100  via the finger  200  in a first sensing stage (S 12 ). It is clear that the sensing stage is the time when the driving signal is applied for the capacitive sensing element  100  to get the output voltage (sensing stages used in the description of the present invention mean the same condition). Next, send a first output voltage to the A/D converter  160  in a first measuring stage (S 13 ). During the measuring stage, the output voltage is sent to the A/D converter  160  and the A/D converter  160  converts all received output voltages from those capacitive sensing elements  100  in the measuring stage to respective numbers (measuring stages used in the description of the present invention mean the same condition). The following steps are turning on the constant bias voltage switch  105  in a second reset stage (S 14 ), turning off the constant bias voltage switch  105  and exerting the driving signal which has a second waveform to the capacitive sensing element  100  via the finger  200  in a second sensing stage (S 15 ), and sending a second output voltage to the A/D converter  160  in a second measuring stage (S 16 ). Basically, step S 14  to S 16  repeat the actions from step S 11  to S 13 . The only difference between the two cycles is different waveforms are applied. Similarly, if the first waveform is a positive waveform, then the second waveform is a negative waveform. Otherwise, if the first waveform is a negative waveform, then the second waveform is a positive waveform. The two consequent waveforms must be symmetrical in shape. 
         [0042]    The output voltage, V out , at the input end  102   a  of the voltage follower  102  can be obtained by 
         [0000]    
       
         
           
             
               V 
               out 
             
             = 
             
               
                 V 
                 bias 
               
               + 
               
                 
                   
                     C 
                     f 
                   
                   
                     
                       C 
                       f 
                     
                     + 
                     
                       C 
                       r 
                     
                   
                 
                  
                 
                   
                     V 
                     in 
                   
                   . 
                 
               
             
           
         
       
     
         [0000]    V bias  is the value of the constant bias voltage, V in  is a voltage change of the driving signal, and C f  is a value of the finger capacitance (equivalent finger capacitor  107 ). A value of a reference capacitance, C r , is obtained by C r =C m +C p . Derivation of the above formula is illustrated below. The circuit in  FIG. 4  is a linear circuit. Therefore, the output voltage can be written as a linear combination of two terms: the first term represents the part affected by the constant voltage V bias , and the second term represents the part affected by the voltage change V in . On the left of  FIG. 6  is a simplified circuit of the first term, where the reference capacitor  103   a  (a combination of the comparative capacitor  103  and the equivalent parasitic capacitor  106 ) and the finger capacitor  107  has been charged to the constant voltage, V bias . A first voltage output, V out   _   A , can be obtained by V out   _   A =V bias . On the right of  FIG. 6  is a simplified circuit of the second term when V in  is applied. A second voltage output, V out   _   B , can be obtained by 
         [0000]    
       
         
           
             
               V 
               
                 out 
                  
                 _ 
                  
                 B 
               
             
             = 
             
               
                 
                   
                     C 
                     f 
                   
                   
                     
                       C 
                       f 
                     
                     + 
                     
                       C 
                       m 
                     
                     + 
                     
                       C 
                       p 
                     
                   
                 
                  
                 
                   V 
                   m 
                 
               
               = 
               
                 
                   
                     C 
                     f 
                   
                   
                     
                       C 
                       f 
                     
                     + 
                     
                       C 
                       r 
                     
                   
                 
                  
                 
                   
                     V 
                     in 
                   
                   . 
                 
               
             
           
         
       
     
         [0000]    Since V out  is the linear superposition of V out   _   A  and V out   _   B , 
         [0000]    
       
         
           
             
               V 
               out 
             
             = 
             
               
                 
                   V 
                   
                     out 
                      
                     _ 
                      
                     A 
                   
                 
                 + 
                 
                   V 
                   
                     out 
                      
                     _ 
                      
                     B 
                   
                 
               
               = 
               
                 
                   V 
                   bias 
                 
                 + 
                 
                   
                     
                       C 
                       f 
                     
                     
                       
                         C 
                         f 
                       
                       + 
                       
                         C 
                         r 
                       
                     
                   
                    
                   
                     
                       V 
                       in 
                     
                     . 
                   
                 
               
             
           
         
       
     
         [0000]    Since all parameters but C f  in the equation above are known, V out  is a function of C f  representing the distance between the metal plate  101  in a capacitive sensing element  100  and the portion of the surface of the finger  200  above it. By collecting all outputs from every capacitive sensing elements  100  and converting the outputs to numbers as grayscale values, a fingerprint image can be obtained. 
         [0043]    In a second embodiment, the capacitive sensing element has different design but the same operation procedure. The capacitive image sensor  10  can be configured with this different capacitive sensing element while its operation procedure doesn&#39;t change. Below is an illustration about the second kind of capacitive sensing element. 
         [0044]    In this embodiment, an equivalent circuit of a design of the capacitive sensing element  100  is shown in  FIG. 7 . Structure of the capacitive sensing element  100  is changed and basically composed of a metal plate  111 , a voltage follower  112 , a constant voltage source  114  and a constant bias voltage switch  115 . Obviously, there is no comparative capacitor used in this design. There are many parasitic capacitances naturally formed the metal plate  111  and other metal layers in the sensor (not shown), a net effect of all the parasitic capacitances can be considered as one single equivalent capacitor, having a value of C p , formed between the metal plate  111  and the ground end  118 . Similarly, C p  can be well controlled under proper design such that the parasitic capacitance in every capacitive sensing element  100  has the same value, and is used as a reference capacitance, i.e. C r =C p . An equivalent parasitic capacitor  116  is used to denote the parasitic capacitance. When the finger  200  is approaching the capacitive sensing element  100 , a signal impedance is formed between the finger  200  and the signal source (not shown), and a finger capacitance is formed by the finger  200  and the metal plate  111 . The signal impedance is so small that it is negligible. An equivalent finger capacitor  117  is used to denote the finger capacitance. A voltage change of the driving signal (value of V in  and produced by change of waveforms) can be exerted to the capacitive sensing element  100  via the finger  200 . An input end  112   a  of the voltage follower  112  is connected to the metal plate  111  while an output end  112   b  of the voltage follower  112  is connected to the A/D converter  160 . Functions of the voltage follower  112  are the same as that of the voltage follower  102  in the previous embodiment. The constant voltage source  114  is used to provide a constant bias voltage. The constant bias voltage switch  115  is connected to the constant voltage source  114  and the metal plate  111 , for switching supply of the constant bias voltage. The constant bias voltage switch  115  is turned on in the reset stage. At this stage, the constant bias voltage is applied to the metal plate  111  and the parasitic capacitor  116  to establish proper operating conditions for the capacitive sensing element  100 . The constant bias voltage switch  115  is turned off during the sensing stage and the measuring stage. 
         [0045]    The procedure to operate the capacitive sensing element  100  in this embodiment is illustrated by the flow chart in  FIG. 8 . First, turn on the constant bias voltage switch  115  in a first reset stage (S 21 ). Like the previous embodiment, the constant voltage source  114  provides the bias voltage for the circuit in the capacitive sensing element  100 . The capacitive sensing element  100  can start sensing operation. Then, turn off the constant bias voltage switch  115  and exert the driving signal which has a first waveform to the capacitive sensing element  100  via the finger  200  in a first sensing stage (S 22 ). Next, send a first output voltage to the A/D converter  160  in a first measuring stage (S 23 ). The following steps are turning on the constant bias voltage switch  115  in a second reset stage (S 24 ), turning off the constant bias voltage switch  115  and exerting the driving signal which has a second waveform to the capacitive sensing element  100  via the finger  200  in a second sensing stage (S 25 ), and sending a second output voltage to the A/D converter  160  in a second measuring stage (S 26 ). Step S 24  to S 26  repeat the actions in step S 21  to S 23 . The only difference between the two cycles is different waveforms are applied. Similarly, if the first waveform is a positive waveform, then the second waveform is a negative waveform. Otherwise, if the first waveform is a negative waveform, then the second waveform is a positive waveform. The two consequent waveforms must be symmetrical in shape. 
         [0046]    The output voltage, V out , at the input end  112   a  of the voltage follower  112  can be obtained by 
         [0000]    
       
         
           
             
               V 
               out 
             
             = 
             
               
                 V 
                 bias 
               
               + 
               
                 
                   
                     C 
                     f 
                   
                   
                     
                       C 
                       f 
                     
                     + 
                     
                       C 
                       r 
                     
                   
                 
                  
                 
                   
                     V 
                     in 
                   
                   . 
                 
               
             
           
         
       
     
         [0000]    Based on the same definitions, V bias  is the value of the constant bias voltage, V in  is a voltage change of the driving signal and C f  is a value of the finger capacitance (equivalent finger capacitor  117 ). However, a value of a reference capacitance, C r , equals to C p  since there is no comparative capacitor in this embodiment. Derivation of the above formula is illustrated below. The circuit in  FIG. 7  is a linear circuit. Therefore, the output voltage can be written as a linear combination of two terms: the first term represents the part affected by the constant voltage V bias , and the second term represents the part affected by the voltage change V in . On the left of  FIG. 9  is a simplified circuit of the first term, where the reference capacitor  116  (the parasitic capacitor  116  is the reference capacitor in this embodiment) and the finger capacitor  117  has been charged to the constant voltage, V bias . A first voltage output, V out   _   A , can be obtained by V out   _   A =V bias . On the right of  FIG. 9  is a simplified circuit of the second term when V in  is applied. A second voltage output, V out   _   B , can be obtained by 
         [0000]    
       
         
           
             
               V 
               
                 out 
                  
                 _ 
                  
                 B 
               
             
             = 
             
               
                 
                   
                     C 
                     f 
                   
                   
                     
                       C 
                       f 
                     
                     + 
                     
                       C 
                       p 
                     
                   
                 
                  
                 
                   V 
                   m 
                 
               
               = 
               
                 
                   
                     C 
                     f 
                   
                   
                     
                       C 
                       f 
                     
                     + 
                     
                       C 
                       r 
                     
                   
                 
                  
                 
                   
                     V 
                     in 
                   
                   . 
                 
               
             
           
         
       
     
         [0000]    Since V out  is the linear superposition of 
         [0000]    
       
         
           
             
               
                 V 
                 
                   
                     out 
                      
                     _ 
                      
                     A 
                   
                    
                   
                       
                   
                 
               
                
               and 
                
               
                   
               
                
               
                 V 
                 
                   out 
                    
                   _ 
                    
                   B 
                 
               
             
             , 
             
               
                 V 
                 out 
               
               = 
               
                 
                   
                     V 
                     
                       out 
                        
                       _ 
                        
                       A 
                     
                   
                   + 
                   
                     V 
                     
                       out 
                        
                       _ 
                        
                       B 
                     
                   
                 
                 = 
                 
                   
                     V 
                     bias 
                   
                   + 
                   
                     
                       
                         C 
                         f 
                       
                       
                         
                           C 
                           f 
                         
                         + 
                         
                           C 
                           r 
                         
                       
                     
                      
                     
                       
                         V 
                         in 
                       
                       . 
                     
                   
                 
               
             
           
         
       
     
         [0000]    Since all parameters but C f  in the equation above are known, V out  is a function of C f  representing the distance between the metal plate  111  in a capacitive sensing element  100  and the portion of the surface of the finger above it. By collecting all outputs from every capacitive sensing elements  100  and converting the outputs to numbers as grayscale values, a fingerprint image can be obtained. 
         [0047]    In a third embodiment, the capacitive sensing element has different design and different operation procedure. However, the capacitive image sensor  10  can be configured with this different capacitive sensing element while its operation procedure doesn&#39;t change. Below is an illustration about the third kind of capacitive sensing element. 
         [0048]    Please see  FIG. 10 .  FIG. 10  shows an equivalent circuit of a design of the capacitive sensing element  100 . The capacitive sensing element  100  mainly has a metal plate  121 , a voltage follower  122 , a comparative capacitor  123 , a working voltage source  124 , a working voltage switch  125 , a charge sharing switch  129  and a ground switch  130 . Functions of the metal plate  121  and the voltage follower  122  are the same as their counterparts in the previous embodiments. An input end  122   a  of the voltage follower  122  is connected to the charge sharing switch  129  while an output end  122   b  of the voltage follower  122  is connected to the A/D converter  160 . The comparative capacitor  123  is implemented by circuit elements such as MOSFET capacitor. It can also be a PIP capacitor or a MIM capacitor. One electrode of the comparative capacitor  123  is electrically connected to a second end  125   b  of the working voltage switch  125  and the other electrode of the comparative capacitor  123  is electrically connected to a ground end  128 . The comparative capacitor  123  has a capacitance of C m . The working voltage source  124  can provide a working voltage for the circuit in the capacitive sensing element  100  to operate. The working voltage switch  125  is connected to the working voltage source  124  with a first end  125   a . It is used for switching supply of the working voltage. The charge sharing switch  129  is electrically connected to the metal plate  121  and the second end  125   b  of the working voltage switch  125 . It switches to balance electric charges in both ends when turned on. The ground switch  130  is used to release electric charges accumulated in the capacitive sensing element  100  to the ground end  128  when it is turned-on. The ground switch  130  can let the capacitive sensing element  100  accumulate electric charges when it is turned-off. The working voltage switch  125  and the ground switch  130  are turned on in the reset stage and are turned off in the sensing stage and the measuring stage. The charge sharing switch  129  is turned off in the reset stage and is turned on in the sensing stage and the measuring stage. There are many parasitic capacitances naturally formed the metal plate  121  and other metal layers in the sensor (not shown), a net effect of all the parasitic capacitances can be considered as one single equivalent capacitor, having a value of C p , formed between the metal plate  121  and the ground end  128 . It can be presented by an equivalent parasitic capacitor  126  to the ground end  128 . When the finger  200  is approaching the capacitive sensing element  100 , a signal impedance is formed between the finger  200  and the signal source (not shown), and a finger capacitance is formed by the finger  200  and the metal plate  121 . The signal impedance is so small that it is negligible, and the finger capacitance can be presented by an equivalent finger capacitor  127 . 
         [0049]    The procedure to operate the capacitive sensing element  100  in this embodiment is illustrated by the flow chart in  FIG. 11 . First, turn on the working voltage switch  125  and the ground switch  130 , and turn off the charge sharing switch  129  in a first reset stage (S 31 ). Unlike the previous embodiments, the operation is that the working voltage is applied to the comparative capacitor  123  rather than that the constant bias voltage is applied to both the reference capacitor and the finger capacitor. Once entering the sensing stage, the charges accumulated in the comparative capacitor  123  will change. Details about how charge redistribution affects output voltage will be illustrated later. Then, turn off the working voltage switch  125  and the ground switch  130  first, then turn on the charge sharing switch  129  and exert the driving signal which has a first waveform to the capacitive sensing element  100  via the finger  200  in a first sensing stage (S 32 ). Next, send a first output voltage to the A/D converter  160  in a first measuring stage (S 33 ). The following steps are turning on the working voltage switch  125  and the ground switch  130 , and turning off the charge sharing switch  129  in a second reset stage (S 34 ), turning off the working voltage switch  125  and the ground switch  130  first, then turning on the charge sharing switch  129  and exerting the driving signal which has a second waveform to the capacitive sensing element  100  via the finger  200  in a second sensing stage (S 35 ), and sending a second output voltage to the A/D converter  160  in a second measuring stage (S 36 ). Step S 34  to S 36  repeat the actions in step S 31  to S 33 . The only difference between the two cycles is different waveforms are applied. Similarly, if the first waveform is a positive waveform, then the second waveform is a negative waveform. Otherwise, if the first waveform is a negative waveform, then the second waveform is a positive waveform. The two consequent waveforms must be symmetrical in shape. 
         [0050]    The output voltage, V out , at the input end  122   a  can be obtained by 
         [0000]    
       
         
           
             
               V 
               out 
             
             = 
             
               
                 
                   
                     C 
                     m 
                   
                   
                     
                       C 
                       m 
                     
                     + 
                     
                       C 
                       p 
                     
                     + 
                     
                       C 
                       f 
                     
                   
                 
                  
                 
                   V 
                   dd 
                 
               
               + 
               
                 
                   
                     C 
                     f 
                   
                   
                     
                       C 
                       m 
                     
                     + 
                     
                       C 
                       p 
                     
                     + 
                     
                       C 
                       f 
                     
                   
                 
                  
                 
                   
                     V 
                     in 
                   
                   . 
                 
               
             
           
         
       
     
         [0000]    V dd  is the value of the working voltage, V in  is a voltage change of the driving signal and C f  is a value of the finger capacitance. Derivation of the above formula is illustrated below. The circuit in  FIG. 10  is a linear circuit. Therefore, the output voltage can be written as a linear combination of two terms: the first term represents the part affected by the working voltage V dd , and the second term represents the part affected by the voltage change V in . The first term, V out   _   A , can also be called the “sharing term”. In the reset stage, the comparative capacitor  123  is charged to the working voltage, V dd . In the measuring stage, an equivalent circuit of the first term (sharing term) is shown in  FIG. 12 . In this stage, the charges accumulated in the comparative capacitor  123  before the charge sharing switch is turned on are redistributed. In other words, the charges are shared with the finger capacitor  127  and the parasitic capacitor  126  in the measuring stage (the charge sharing switch is turned on). When reaching stable equilibrium, the stable voltage, 
         [0000]    
       
         
           
             
               V 
               
                 out 
                  
                 _ 
                  
                 A 
               
             
             = 
             
               
                 
                   C 
                   m 
                 
                 
                   
                     C 
                     m 
                   
                   + 
                   
                     C 
                     p 
                   
                   + 
                   
                     C 
                     f 
                   
                 
               
                
               
                 
                   V 
                   dd 
                 
                 . 
               
             
           
         
       
     
         [0000]    A simplified equivalent circuit giving the relationship between the second term, V out   _   B , and the driving signal, V in , is shown in  FIG. 13 , which represents the effect of the voltage change of the driving signal. The second voltage output (driving signal term), V out   _   B , can be obtained by 
         [0000]    
       
         
           
             
               V 
               
                 out 
                  
                 _ 
                  
                 B 
               
             
             = 
             
               
                 
                   C 
                   f 
                 
                 
                   
                     C 
                     m 
                   
                   + 
                   
                     C 
                     p 
                   
                   + 
                   
                     C 
                     f 
                   
                 
               
                
               
                 
                   V 
                   in 
                 
                 . 
               
             
           
         
       
     
         [0000]    Since V out  is the linear superposition of V out   _   A  and V out   _   B , 
         [0000]    
       
         
           
             
               V 
               out 
             
             = 
             
               
                 
                   V 
                   
                     out 
                      
                     _ 
                      
                     A 
                   
                 
                 + 
                 
                   V 
                   
                     out 
                      
                     _ 
                      
                     B 
                   
                 
               
               = 
               
                 
                   
                     
                       C 
                       m 
                     
                     
                       
                         C 
                         m 
                       
                       + 
                       
                         C 
                         p 
                       
                       + 
                       
                         C 
                         f 
                       
                     
                   
                    
                   
                     V 
                     dd 
                   
                 
                 + 
                 
                   
                     
                       C 
                       f 
                     
                     
                       
                         C 
                         m 
                       
                       + 
                       
                         C 
                         p 
                       
                       + 
                       
                         C 
                         f 
                       
                     
                   
                    
                   
                     
                       V 
                       in 
                     
                     . 
                   
                 
               
             
           
         
       
     
         [0000]    Since all parameters but C f  in the equation above are known, V out  is a function of C f  representing the distance between the metal plate  121  in a capacitive sensing element  100  and the portion of the surface of the finger above it. By collecting all outputs from every capacitive sensing elements  100  and converting the outputs to numbers as grayscale values, a fingerprint image can be obtained. 
         [0051]    While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not be limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims, which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.