Patent Publication Number: US-2016227142-A1

Title: Fingerprint sensor and sensing method thereof

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This Application claims priority of China Patent Application No. 201510053091.X, filed on Feb. 2, 2015, the entirety of which is incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a fingerprint sensor, and more particularly, to a fingerprint sensor capable of removing common mode noise. 
     2. Description of the Related Art 
     In recent years, biological identification technology has become increasingly mature, and different biological features can be used for identifying users. Since the recognition rate and accuracy of fingerprint identification technology are better than those of other biological feature identification technologies, fingerprint identification and verification are used extensively in various areas. 
     Fingerprint identification and verification technology detects a user&#39;s fingerprint image, captures fingerprint data from the fingerprint image, and saves the fingerprint data as a template. Thereafter, the user presses or slides the finger on or over the fingerprint sensor such that a fingerprint is captured and compared with a template. If the two match, then the user&#39;s identity is verified. 
     BRIEF SUMMARY OF THE INVENTION 
     A fingerprint sensor and a sensing method thereof are provided. An embodiment of a fingerprint sensor is provided for sensing fingerprint information of a finger. The fingerprint sensor includes a sensing array, a readout module and a processor. The sensing array includes a plurality of sensing units. Each of the sensing units includes a sensing electrode. The readout module provides a sensing output according to a first sensing voltage from the sensing array and an average voltage of a plurality of second sensing voltages from the sensing array. The processor generates the fingerprint information of the finger according to the sensing output. The first sensing voltage is provided by the sensing electrode of a first sensing unit of the sensing units, and each of the second sensing voltage is provided by the sensing electrode of a second sensing unit of the sensing units. The second sensing units are neighboring to the first sensing unit. 
     Furthermore, an embodiment of a sensing method for a fingerprint sensor, wherein the fingerprint sensor comprises a sensing array having a plurality of sensing units disposed in a plurality of row lines and a plurality of column lines. A first sensing voltage of a first sensing unit of the sensing units is read. A plurality of second sensing voltages of a plurality of second sensing units of the sensing units are read. A sensing output is generated according to the first sensing voltage and an average voltage of the second sensing voltages, by a differential amplifier. Fingerprint information of a finger is generated according to the sensing output. 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1  shows a fingerprint sensor according to an embodiment of the invention; 
         FIG. 2  shows a schematic diagram illustrating that the fingerprint sensor of  FIG. 1  is used to obtain the fingerprint of the user; 
         FIG. 3  shows a sectional schematic illustrating the finger of the user contacting the fingerprint sensor of  FIG. 1 ; 
         FIG. 4  shows a fingerprint sensor according to another embodiment of the invention; 
         FIG. 5  shows a sensing array for illustrating the relation among a target sensing unit and a plurality of reference sensing units according to an embodiment of the invention; 
         FIG. 6  shows a sensing array for illustrating the relation among a target sensing unit and a plurality of reference sensing units according to another embodiment of the invention; 
         FIG. 7  shows a sensing array for illustrating the relation among a target sensing unit and a plurality of reference sensing units according to another embodiment of the invention; and 
         FIG. 8  shows a sensing method for a fingerprint sensor according to an embodiment of the invention, wherein the fingerprint sensor comprises a sensing array having a plurality of sensing units, a readout module and a processor. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
     When a user presses or slides his or her finger on or over a fingerprint sensor, the fingerprint sensor will sense the ridges and the valleys of the fingerprint, and generate different capacitance values corresponding to the ridges and valleys. Next, voltage values corresponding to the capacitance values are obtained by using a charge-sharing technique, and the voltage value is converted into a digital code. The digital code is provided to a processor for subsequent operation and fingerprint identification. 
       FIG. 1  shows a fingerprint sensor  100  according to an embodiment of the invention. The fingerprint sensor  100  comprises a sensing array  110 , a readout module  120 , a processor  130  and an insulating surface  140 . The sensing array  110  is formed by a plurality of sensing units  115  arranged in a two-dimensional manner, wherein the insulating surface  140  overlays all of the sensing units  115  of the sensing array  110 . The readout module  120  obtains a target sensing voltage V sen  and a plurality of reference sensing voltages V ref1 -V refn  from the sensing array  110 , wherein the target sensing voltage V sen  is provided by a target sensing unit  115 S which is going to sense in the sensing array  110 , and the reference sensing voltages V ref1 -V refn  are respectively provided by a plurality of sensing units  115 R neighboring to the target sensing unit  115 S in the sensing array  110 . The readout module  120  comprises an average unit  122  and a differential amplifier  124 . The average unit  122  is used to average the reference sensing voltages V ref1 -V refn , to provide a reference average voltage V avg . Next, the differential amplifier  124  provides a sensing output D sen  corresponding to the target sensing unit  115 S to the processor  130  according to the target sensing voltage V sen  and the reference average voltage V avg . After obtaining the sensing output D sen  of the target sensing unit  115 S, the processor  130  determines whether the user&#39;s finger is in contact with the insulating surface  140 , and further obtains fingerprint information of the finger, so as to determine that the sensing output D sen  corresponds to a fingerprint ridge or a fingerprint valley of the finger. Thus, according to the sensing outputs D sen  of all sensing units  115 , the processor  130  obtains the binary or gray-level fingerprint data for subsequent processes, for example, a fingerprint identification operation is performed by a fingerprint identification algorithm. 
       FIG. 2  shows a schematic diagram illustrating that the fingerprint sensor  100  of  FIG. 1  is used to obtain the fingerprint of the user. In  FIG. 2 , when the finger  210  contacts the fingerprint sensor  100 , the fingerprint ridges  220  on the surface of the finger  210  will contact and press the sensing units  115  via the insulating surface  140 . Thus, the fingerprint sensor  100  obtains a capacitance curve  230  corresponding to the fingerprint ridges  220 , and identifies the shape of the fingerprint ridges  220  according to the shape of the capacitance curve  230 , so as to obtain a fingerprint pattern  240 . Next, the other circuits or devices can perform subsequent processes according to the fingerprint pattern  240 . 
       FIG. 3  shows a sectional schematic illustrating the finger of the user contacting the fingerprint sensor  100  of  FIG. 1 . In  FIG. 3 , the insulating surface  140  is disposed on the semiconductor substrate  310 . In general, the insulating surface  140  is a protective dielectric layer formed by performing the integrated circuit manufacturing process. The thickness of the insulating surface  140  is d 1 , wherein an equivalent capacitor C 1  of the insulating surface  140  is determined by the thickness d 1 . Label  320  represents a fingerprint ridge of the finger, wherein the fingerprint ridge  320  of the finger will directly contact the insulating surface  140 . Moreover, Label  330  represents a fingerprint valley of the finger, wherein a distance between the fingerprint valley  330  of finger and the insulating surface  140  is d 2 , and a capacitor C 2  between the fingerprint valley  330  and insulating surface  140  is determined by the distance d 2 . As described above, the sensing array  110  is formed by a plurality of sensing units  115 . Each sensing unit  115  comprises the electrodes E 1  and E 2 , wherein the electrodes E 1  and E 2  are formed by different metal layers within the semiconductor substrate  310 . The electrode E 1  is formed by a top metal layer and is disposed below the insulating surface  140 , and the thickness of an insulation layer between the insulating surface  140  and the electrode E 1  is d 3 , wherein an equivalent capacitor C top  on the insulation layer is determined according to the thickness d 3 . Therefore, when the fingerprint ridge  320  contacts the insulating surface  140 , a sensing capacitor C sen  between the fingerprint ridge  320  and the electrode E 1  is formed by the capacitor C top  and the capacitor C 1  connected in series. Furthermore, comparing with the sensing capacitor C sen  of the fingerprint ridge  320 , a sensing capacitor C sen  between the fingerprint valley  330  and the electrode E 1  is formed by the capacitor C top , the capacitor C 1  and the capacitor C 2  connected in series. Thus, when the finger contacts the insulating surface  140 , the fingerprint ridge  320  and the fingerprint valley  330  will cause different capacitances, wherein the sensing capacitor C sen  corresponding to the fingerprint valley  330  is smaller than the sensing capacitor C sen  corresponding to the fingerprint ridge  320 . Therefore, the readout module  120  of  FIG. 1  can obtain the sensing voltage V sen  corresponding to the sensing capacitor C sen  via the electrode E 1  of the sensing unit. Moreover, the electrode E 2  is disposed below the electrode E 1 , wherein the electrode E 2  is coupled to a ground GND. 
       FIG. 4  shows a fingerprint sensor  400  according to another embodiment of the invention. The fingerprint sensor  400  comprises a sensing array  410 , a differential amplifier  420  and a processor  430 . The sensing array  410  comprises a plurality of sensing units  415 , a plurality of switches  413 , a plurality of switches  417 _ 0 - 417 _n and a plurality of switches  419 _ 0 - 419 _n. In the embodiment, each sensing unit  415  is coupled to the corresponding row line via the corresponding switch  413 , wherein each switch  413  is controlled by the column line corresponding to the sensing unit. Furthermore, each row line is coupled to a non-inverting input terminal (positive input terminal) of the differential amplifier  420  via the corresponding switch  417 , and is coupled to an inverting input terminal of the differential amplifier  420  via the corresponding switch  419 . For example, the switch  417 _ 0  is coupled between the row line R 0  and the non-inverting input terminal of the differential amplifier  420 , and the switch  419 _ 0  is coupled between the row line R 0  and the inverting input terminal of the differential amplifier  420 . In  FIG. 4 , assuming that the target sensing unit  415 S which is going to sense is located in an intersection of the row line R 1  and the column line C 2  in the sensing array  410 , the processor  430  will provide a control signal Ctrl to the sensing array  410 , so as to turn on the switch  413  corresponding to the target sensing unit  415 S, and to turn off the other switches  413  in the row line R 1 . Furthermore, the processor  430  provides a control signal SW to the sensing array  410 , so as to turn on the switch  417 _ 1  and turn off the switch  419 _ 1 . Thus, the target sensing voltage V sen  from the sensing electrode of the target sensing unit  415 S is transmitted to the non-inverting input terminal of the differential amplifier  420 . Simultaneously, the processor  430  provides the control signal Ctrl to turn on all of the switches  413  in the row line R 0  and the row line R 2 , and provides the control signal SW to turn on the switches  419 _ 0  and  419 _ 2  and turn off the switches  417 _ 0  and  417 _ 2 . Moreover, the processor  430  provides the control signal Ctrl to turn off all of the switches  413  in the row lines R 3 -Rn, and provides the control signal SW to turn off the switches  417 _ 3 - 417 - n  and the switches  419 _ 3 - 419 - n . Thus, the reference sensing voltages V ref  of all of the reference sensing units  415 R that are in the row line R 0  and the row line R 2  and are adjacent to the target sensing unit  415 S, are transmitted to the inverting input terminal of the differential amplifier  420 . Since the sensing electrodes of all of the reference sensing units  415 R are coupled to the inverting input terminal of the differential amplifier  420 , the sensing capacitors C sen  of all of the reference sensing units  415 R provide an equivalent average sensing voltage V avg  to the inverting input terminal of the differential amplifier  420  according to a voltage divided result. Thus, the differential amplifier  420  can remove a common mode noise between the average sensing voltage V avg  and the target sensing voltage V sen , i.e. the interference signals received by each sensing electrode can be removed. Therefore, the obtained sensing output D sen  comprises the signal sensed by the sensing electrode of the target sensing unit  415 S, and does not comprise the unwanted noise components. 
       FIG. 5  shows a sensing array  500  for illustrating the relation among a target sensing unit  515 S and a plurality of reference sensing units  515 R according to an embodiment of the invention. In the embodiment, the target sensing unit  515 S is arranged in a specific column line (e.g. C m ) of the sensing array  500 , and the reference sensing units  515 R are arranged in two neighboring column lines (e.g. the neighboring lines C m−1  and C m+1 ) adjacent to the specific column line. Since the reference sensing units  515 R are adjacent to the target sensing unit  515 S, the reference sensing units  515 R and the target sensing unit  515 S have similar sensing outputs for the same interference noise. Thus, the processor can obtain the actual sensing result according to the difference between the target sensing voltage V sen  of the target sensing unit  515 S and the reference average voltage V avg  of the reference sensing units  515 R. It should be noted that the number of the reference sensing units  515 R adjacent to the target sensing unit  515 S and the number of the column lines where the reference sensing units  515 R are disposed, are determined according to the actual application. 
       FIG. 6  shows a sensing array  600  for illustrating the relation among a target sensing unit  615 S and a plurality of reference sensing units  615 R according to another embodiment of the invention. In the embodiment, the target sensing unit  615 S is arranged in the specific row line (e.g. R n ) of the sensing array  600 , and the reference sensing units  615 R are arranged in four neighboring row lines (e.g. R n−2 , R n−1 , R n+1  and R n+2 ) adjacent to the specific row line. As described above, since the reference sensing units  615 R are adjacent to the target sensing unit  615 S, the reference sensing units  615 R and the target sensing unit  615 S have similar sensing outputs for the same interference noise. Thus, the processor can obtain the actual sensing result according to the difference between the target sensing voltage V sen  of the target sensing unit  615 S and the reference average voltage V avg  of the reference sensing units  615 R. It should be noted that the number of the reference sensing units  615 R adjacent to the target sensing unit  615 S and the number of the row lines where the reference sensing units  615 R are disposed, are determined according to the actual application. 
       FIG. 7  shows a sensing array  700  for illustrating the relation among a target sensing unit  715 S and a plurality of reference sensing units  715 R according to another embodiment of the invention. In the embodiment, the target sensing unit  715 S is arranged in the intersection of a specific row line and a specific column line (e.g. R n  and C m ) of the sensing array  700 , and the reference sensing units  715 R are arranged in a reference area  720  formed by a plurality of row lines and a plurality of column lines, wherein the target sensing unit  715 S is arranged in the center of the reference area  720 . In the embodiment, the reference area  720  is a rectangular area formed by the row line R n−2  to the row line R n+2  and the column line C m−2  to the column line C m+2 . Specifically, in the reference area  720 , the reference sensing units  715 R are disposed around the target sensing unit  715 S and surrounding the target sensing unit  715 S. As described above, the location and range of the reference area  720  can be determined according to the actual application. 
       FIG. 8  shows a sensing method for a fingerprint sensor according to an embodiment of the invention, wherein the fingerprint sensor comprises a sensing array having a plurality of sensing units, a readout module and a processor. First, in step S 810 , the readout module obtains the target sensing voltage V sen  of the target sensing unit in the sensing array. Next, in step S 820 , the readout module obtains the reference sensing voltages V ref  of a plurality of reference sensing units, wherein the reference sensing units are neighboring to the target sensing unit, i.e. located in a neighboring area surrounding the target sensing unit. Next, in step S 830 , the readout module obtains the reference average voltage V avg  of all the reference sensing voltages V ref . Next, in step S 840 , the readout module obtains the sensing output D sen  according to the reference average voltage V avg  and the target sensing voltage V sen . For example, the differential amplifier is used to remove the common mode noise between the target sensing voltage V sen  and the reference average voltage V avg , so as to obtain the sensing signal without interference. Next, in step S 850 , the processor obtains the fingerprint information of the finger according to the sensing output D sen . 
     While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.