Abstract:
Aspects of the invention can provide a fingerprint sensor with high sensing precision. The fingerprint sensor according to the invention can include capacitance detection circuits that output detection signals, which each correspond to a capacitance formed between a subject surface and the fingerprint sensor, to signal transmitting paths, and an amplification circuit that amplifies the detection signals outputted to the signal transmitting paths. The individual signal transmitting paths can be respectively connected to at least two capacitance detection circuits and the fingerprint sensor further includes a resetting means that resets the potential of the signal transmitting paths before the detection signals are outputted from the capacitance detection circuits to the signal transmitting paths.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of Invention  
         [0002]     Aspects of the invention relate to a capacitance detecting technology for reading, as variations in capacitance, the surface form of a subject, such as a fingerprint, that has minute convexes and concaves.  
         [0003]     2. Description of Related Art  
         [0004]     Japanese Unexamined Patent Publication Nos. H11-118415, 2000-346608, 2001-56204, and 2001-133213 disclose a related technology where the surface of a sensor electrode formed on a single-crystal silicon substrate is covered with a dielectric film, the potential of a fingertip is set as a reference potential, and variations in the capacitance formed between a fingertip and sensor electrodes due to the convexes and concaves of the fingerprint are used to identify the fingerprint.  
       SUMMARY OF THE INVENTION  
       [0005]     However, since related capacitance-type fingerprint sensors are formed on single-crystal silicon substrates, there has been the problem of the sensors breaking when firmly pressed with the fingertip of the user. Since a fingerprint sensor needs to be constructed in a size approximately equal to the area (around 20 mm by 20 mm) of a fingertip, there has also been the problem of high cost when forming sensors from single-crystal silicon substrates. In addition, in a construction where capacitance detection circuits that are combinations of sensor electrodes and transistors are disposed in a matrix, if the data lines that are the signal transmitting paths are respectively connected to a plurality of capacitance detection circuits, when the information of the capacitance detection circuits inside the matrix is read, the output information of the capacitance detection circuits in the same column but on previous rows remains and the initial potential of the data lines is not stable, so that there has been the problem that stable sensing has not been possible. In other words, when a plurality of circuits are connected to the same data line and successive data is read, the detection signals outputted from the later capacitance detection circuits will be affected by the detection signals outputted from the earlier capacitance detection circuits.  
         [0006]     An aspect of the invention can provide a capacitance detection apparatus, a fingerprint sensor, and a biometric authentication apparatus that solve the above problems and make highly precise and stable sensing possible. Further the invention to provide a method that can carry out sensing of capacitance with high precision.  
         [0007]     A capacitance detection apparatus, a fingerprint sensor, and a biometric authentication apparatus according to aspects of the invention can read convex/concave information on a subject surface using detection signals corresponding to a capacitance formed between the apparatus and the subject surface. A detection unit in which a plurality of capacitance detection circuits, which output the detection signals to signal transmitting paths, can be disposed, and an amplifying circuit that amplifies the detection signals outputted to the signal transmitting paths can be included. In particular, the respective signal transmitting paths are each connected to at least two of the capacitance detection circuits, and a resetting device for resetting a potential of the signal transmitting paths before the detection signals are outputted from the capacitance detection circuits to the signal transmitting paths is also included.  
         [0008]     The above capacitance detection apparatus can preferably also include a reset period setting device for setting a ratio between a reset period during which the resetting device resets the potential of the signal transmitting paths and a sensing period during which the capacitance detection circuits output the detection signals.  
         [0009]     Also, for the capacitance detection apparatus described above, a sensing period during which the capacitance detection circuits output the detection signals should preferably be set as a longer period than a reset period during which the resetting device resets the potential of the signal transmitting paths.  
         [0010]     Here, the expression biometric authentication apparatus refers to an apparatus equipped with a function for authentication individuals using fingerprint information as biometric information, and includes a variety of card media such as IC cards, cash cards, credit cards, and ID cards, and also all kinds of security systems, such as a user authentication apparatus for online trading, a room entry/exit controlling apparatus, and an authentication apparatus for a computer terminal apparatus.  
         [0011]     An exemplary driving method for a capacitance detection apparatus according to the invention drives a capacitance detection apparatus including a signal outputting element that outputs a detection signal corresponding to a capacitance formed between the capacitance detection apparatus and a subject surface, a signal transmitting path for supplying the detection signal to an amplification circuit, a selection transistor for controlling a connecting and a disconnecting of the signal transmitting path and the signal outputting element, and a power supply line for supplying a predetermined voltage to the signal outputting element. In particular, the driving method can include a disconnecting step of placing the selection transistor in a closed state to electrically disconnect the signal transmitting path and the signal outputting element, a resetting step of resetting the signal transmitting path to a predetermined potential, a connecting step of placing, after the signal transmitting path has been reset, the selection transistor in an open state to electrically connect the signal transmitting path and the signal outputting element, and a sensing step of supplying a detection signal from the signal outputting element to the amplification circuit via the signal transmitting path and amplifying the detection signal.  
         [0012]     In the exemplary driving method described above, it should preferably be possible to change a ratio between a period during which the resetting step is executed and a period during which the sensing step is executed. Also in the exemplary driving method described above, a period during which the sensing step is executed can be changed should preferably be set longer than a period during which the resetting step is executed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     The invention will be described with reference to the accompanying drawings, wherein like numerals reference like elements, and wherein:  
         [0014]      FIG. 1  is an exemplary block diagram of a capacitance-type fingerprint sensor according to an embodiment of the present invention;  
         [0015]      FIG. 2  is an exemplary circuit diagram of a capacitance detection circuit provided in the above fingerprint sensor;  
         [0016]      FIG. 3  is an exemplary circuit diagram of a capacitance detection circuit according to a modification of  FIG. 2 ;  
         [0017]      FIG. 4  is an exemplary circuit diagram of an amplification circuit provided in the above fingerprint sensor;  
         [0018]      FIG. 5  is a timing chart for various signals in the above fingerprint sensor;  
         [0019]      FIG. 6  is a timing chart for various signals according to a modification of  FIG. 5 ;  
         [0020]      FIG. 7  is a cross-sectional view of the above capacitance detection circuit;  
         [0021]      FIG. 8  shows an application equipped with the above fingerprint sensor; and  
         [0022]      FIG. 9  is a flowchart showing an exemplary authentication procedure for a smart card that uses the above fingerprint sensor.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0023]     A first exemplary embodiment of the invention will now be described with reference to the drawings.  
         [0024]      FIG. 1  is an exemplary block diagram showing a capacitance-type fingerprint sensor  1  that is a capacitance detection apparatus according to the invention. As shown in  FIG. 1 , the fingerprint sensor  1  can include a data line driver  10  for selecting data lines  37  that are signal transmitting paths, a scan line driver  20  for selecting scan lines  36 , an active matrix unit  30  that functions as a fingerprint detection unit, and an amplification circuit  40  for amplifying detection signals. The data line driver  10  can include a shift register  11  that decides the timing at which the data lines  37  are successively selected and a reset period selecting circuit  14  that sets a reset period. The reset period selecting circuit  14  includes analog switches  12 , which connect and disconnect the respective data lines  37  and a global data line  38 , and logical AND circuits  15 , which take a logical AND between a column select signal of the shift register  11  and an enable signal ENB.  
         [0025]     The scan line driver  20  can include a shift register  21  that decides the timing at which the data lines  37  are successively selected. In the active matrix unit  30 , capacitance detection circuits  31  are arranged in a matrix (of M rows and N columns), with M scan lines  36  and M power supply lines  39  being laid out in the row direction and N data lines  37  and N data select lines  50  being laid out in the column direction. The data lines  37  are collectively connected to the single global data line  38  that is connected to the amplification circuit  40 . The global data line  38  is also connected to a data line resetting transistor  13  for supplying a reset voltage V SS  to the data lines  37 .  
         [0026]     In the above-described construction, when the M scan lines  36  are activated one line at a time, at any given time the N data lines  37  arranged on the active scan line  36  are driven in a point-sequential manner by being sequentially selected by the analog switch  12  and connected to the amplification circuit  40 .  
         [0027]     Here, the data line resetting transistor  13 , the reset period selecting circuit  14 , a driver (not shown) that outputs an ENB (enable) signal that defines the reset period, and a power supply circuit (also not shown) that supplies the reset voltage V SS  to the data lines  37  operate in concert to realize a resetting device. The reset period selecting circuit  14 , the driver (not shown) that outputs the ENB (enable) signal that defines the reset period also operate in concert to form a reset period setting device. The resetting device and reset period setting device are not necessarily realized by a plurality of hardware components, and may be realized by single hardware components.  
         [0028]      FIG. 2  is an exemplary circuit diagram for a capacitance detection circuit  31  that converts convex/concave information on the fingerprint of the subject to an electric signal. The capacitance detection circuit  31  can include a selecting transistor  32  and a data line selecting transistor  35  for selecting the present capacitance detection circuit  31 , a capacitance  33  that is formed between the fingertip of the subject and a sensor electrode, a signal amplifying transistor  34  that outputs a detection signal that carries the convex/concave information of the fingerprint based on minute variations in the capacitance  33 , the scan lines  36  that transmit signals for opening/closing control of the selecting transistor  32 , the data select line  50  that transmits a signal for opening/closing control of the data line selecting transistor  35 , the data line  37  for transmitting the detection signal, the power line  39  for supplying a voltage V DD  to the signal amplifying transistor  34 , and a reference capacitance Cs of a fixed capacitance.  
         [0029]     If the detected capacitance for the capacitance  33  is expressed as Cd, the detected capacitance Cd is determined in accordance with the distance between the convexes and concaves of the subject&#39;s fingerprint and the sensor electrode (see  FIG. 7 ).  
         [0030]     In the above-described construction, when the scan line  36  is activated in accordance with the successive selection of lines by the scan line driver  20 , the selecting transistor  32  is placed in the open state. As a result, a detection current determined by the gate potential of the signal amplifying transistor  34  flows in the data line  37 . The gate potential of the signal amplifying transistor  34  is determined by the respective ratios between the parasitic capacitance Ct (not shown) of the signal amplifying transistor  34  itself, the reference capacitance Cs, and the detected capacitance Cd.  
         [0031]     For example, when the fingertip of the subject has approached the surface of the capacitance detection apparatus, if a convex part of the fingerprint becomes adjacent to the sensor electrode, the detected capacitance Cd of the capacitance  33  becomes sufficiently larger than the parasitic capacitance Ct and the reference capacitance Cs, so that the gate potential of the signal amplifying transistor  34  approaches ground potential. As a result, the signal amplifying transistor  34  is placed in a substantially off state with only an extremely small current flowing between the source and drain of the signal amplifying transistor  34 .  
         [0032]     On the other hand, if a concave part of the fingerprint becomes adjacent to the sensor electrode, the detected capacitance Cd becomes sufficiently smaller than the parasitic capacitance Ct and the reference capacitance Cs, so that the gate potential of the signal amplifying transistor  34  approaches a high potential V DD . As a result, the signal amplifying transistor  34  is placed in a substantially on state with a larger current than the extremely small current mentioned above flowing between the source and drain of the signal amplifying transistor  34 .  
         [0033]     Here, since the source terminal of the signal amplifying transistor  34  is connected to the high-potential power line  39 , the direction of the current flowing through the signal amplifying transistor  34  is oriented from the capacitance detection circuit  31  to the data line  37 .  
         [0034]      FIG. 3  shows another example of a capacitance detection circuit  31 . Parts that functionally correspond to components of the capacitance detection circuit  31  shown in  FIG. 2  have been given the same reference numerals. In the capacitance detection circuit  31  shown in  FIG. 2 , the high potential-side terminal of the reference capacitance Cs is connected to the scan line  36 , but in the example in  FIG. 3 , the potential-side terminal of the reference capacitance Cs is connected to the data select line  50 . In the examples in both  FIG. 2  and  FIG. 3 , when the capacitance detection circuit  31  is selected, a selection signal is supplied to the scan line  36  and the data select line  50 , so that both capacitance detection circuits  31  can achieve the same function.  
         [0035]     In the exemplary embodiment, as a preliminary stage before the detection signal is outputted on the data line  37 , the data line  37  is reset to a predetermined voltage V SS  to stabilize the potential of the data line  37 . The data line selecting transistor  35  carries out control to electrically connect and disconnect the data line  37  and the signal amplifying transistor  34 , and is controlled so as to be placed in a closed state when the data line  37  has been reset. Opening and closing control of the data line selecting transistor  35  is controlled by the data select line  50 .  
         [0036]      FIG. 4  is an exemplary circuit diagram of the amplification circuit  40  that amplifies the detection signals of the capacitance detection circuits  31 . A voltage signal LD OUT  outputted from a data line  37  to the global data line  38  is inputted into an input terminal of the amplification circuit  40 . The amplification circuit  40  compares the input voltage LD OUT  with a reference voltage VR and outputs a digital value as V OUT . When the input voltage LD OUT  is smaller than the reference voltage VR, the output voltage V OUT  becomes a low-level potential (V SS ). Conversely, when the input voltage LD OUT  is larger than the reference voltage VR, the output voltage V OUT  becomes a high level potential (V DD ). By doing so, it is possible to obtain fingerprint information composed of binary data.  
         [0037]      FIG. 5  is a timing chart for various signals outputted from the data line driver  10 . In this chart, SP is a start pulse, CLK is a clock signal, X( 1 ), X( 2 ), . . . X(N) are output signals of the respective columns of the shift register  11 , ENB is an enable signal for opening/closing control of the data line resetting transistor  13 , XSEL( 1 ), XSEL( 2 ), . . . , XSEL(N) are select signals outputted on the data select lines  50 , LD OUT  is a voltage signal outputted from the data lines  37  to the global data line  38 , and V OUT  is an output signal of the amplification circuit  40 . Out of a period during which one capacitance detection circuit  31  is selected by the successive selection of the shift register  11  inside the data line driver  10 , a former half is set as a reset period A and the latter half is set as a sensing period B.  
         [0038]     During the reset period A, the enable signal ENB is inactive (a logic level L) and the data line resetting transistor  13  is placed in an open state. When an output signal X(k) with a logic level H is outputted from the shift register  11 , the k th  analog switch  12  of the reset period selecting circuit  14  is placed in a connecting state so that the reset potential V SS  is supplied to the k th  data line  37 . By doing so, the potential of the data line  37  in question is reset. It should be noted that at this time, the logic level of the enable signal ENB is L, so that the output of the logical AND circuit  15  of the reset period selecting circuit  14  is the logic level L and an L level signal is outputted to the data select line  50 . This results in all of the data line selecting transistors  35  being placed in the closed state, and in the flow of current from the capacitance detection circuits  31  being suppressed.  
         [0039]     Next, there is a transition to the sensing period B, and when the enable signal ENB becomes active (logic level H), the data line resetting transistor  13  is placed in the closed state, so that the reset voltage V SS  and the data lines  37  are disconnected (resetting complete). On the other hand, an H-level output signal X(k) is supplied to the k th  logical AND circuit  14   b  of the reset period selecting circuit  14  from the k th  column of the shift register  11  and an H-level enable signal is supplied, so that an H-level signal is outputted on the data select line  50  that is the output line of the k th  logical AND circuit  14   b . When this happens, the data line selecting transistors  35  aligned in the k th  column are placed in the open state. By doing so, the signal amplifying transistor  34  is connected to the data line selecting transistor  35  and via the selecting transistor  32  to the data line  37 . In addition, in the amplification circuit  40  shown in  FIG. 4 , the enable signal ENB is activated so that the voltage outputted from a capacitance detection circuit  31  via the data line  37  is amplified by the amplification circuit  40 .  
         [0040]     Also, as described above, according to the exemplary embodiment, by carrying out a resetting operation for the potential of the data lines  37  in the preliminary stage of sensing, the potential of the data lines  37  is stabilized and the operation margin is widened. In addition, since the potential of the data lines  37  is stabilized, the fingerprint information can be detected at higher speed.  
         [0041]     It should be noted that in the exemplary timing chart shown in  FIG. 5 , the ratio between the reset period A and the sensing period B have a 1:1 relationship, but the invention is not limited to this, and as shown in  FIG. 6 , by changing the duty ratio of the enable signal, the ratio between the reset period A and the sensing period B (i.e., the duty ratio) may be adjusted. By shortening the reset period A and stabilizing the potential of the data lines  37  at a predetermined potential in a short time, it can be possible to maintain a sufficiently long sensing period. By doing so, highly precise sensing becomes possible. The power supply voltage for resetting is not limited to V SS  described above, and it is possible to use any freely chosen voltage.  
         [0042]      FIG. 7  is a cross-sectional construction diagram of a capacitance detection circuit  31  centered on the sensor electrode. As shown in  FIG. 7 , a signal amplifying transistor  34  that outputs a detection signal that carries the convex/concave information of a fingerprint and a sensor electrode (detection electrode)  71 , between the fingertip F of the subject and which the capacitance  33  is formed, are formed in the capacitance detection circuit  31 . The signal amplifying transistor  34  is a MOS transistor including a gate electrode  70 , a gate insulating film  68 , a polycrystalline silicon layer  63 , and a source/drain electrode  69 . The capacitance  33  is a variable capacitance that varies in accordance with the convex/concave pattern of a fingerprint. The potential of the fingertip F is set at a reference potential. The sensor electrode  71  is connected to the gate electrode  70 , and is constructed so that variations in the detected capacitance Cd due to the convexes and concaves of the fingerprint are transmitted to the signal amplifying transistor  34  and variations in the capacitance can be sensed due to the amplifying action of a drain current flowing in a channel.  
         [0043]     To manufacture the capacitance detection circuit  31  shown in  FIG. 7 , a base insulating film  62 , of silicon oxide or the like, is laminated on the insulating substrate  61  and a film of amorphous silicon is formed thereon and crystallized to form the polycrystalline silicon layer  63 . Next, the gate insulating film  68  and the gate electrode  70  are formed on the polycrystalline silicon layer  63 , a dopant is introduced and dispersed in the polycrystalline silicon layer  63  to form the source and drain regions by self-alignment. Next, after a first interlayer insulating film  64  has been formed, contact holes are opened to form the source/drain electrode  69 . In addition, the second interlayer insulating films  65 ,  66  are laminated, a contact hole is opened, and a sensor electrode  71  is formed. Finally, the entire surface is covered with a passivation film  67 . Here, although the second interlayer insulating films  65 ,  66  have a two-layer construction so that the lower second interlayer insulating film  65  is kept flat and a desired thickness is obtained for the upper second interlayer insulating film  66 , a single-layer construction may be used.  
         [0044]     It should be noted that the formation of a semiconductor element, such as a transistor, on the insulating substrate  61  is not limited to the fabrication method described above, and for example a semiconductor element such as a transistor may be formed on the insulating substrate  61  by an exfoliation transfer technique disclosed by Japanese Unexamined Patent Publication No. H11-312811 or in S. Utsunomiya et. al. “Society for Information Display” p. 916 (2000). By using an exfoliation transfer technique, a low-cost substrate with appropriate strength, such as a plastic substrate or glass substrate, can be used as the insulating substrate  61 , so that the mechanical strength of the capacitance-type fingerprint sensor  1  can be increased.  
         [0045]     Next, an example application of the capacitance-type fingerprint sensor  1  will be described.  FIG. 8  is an exemplary block diagram of a smart card  81  which can include the capacitance-type fingerprint sensor  1  described above, an IC chip  82 , on which a CPU, a memory element, and the like are mounted, and a display apparatus  83 , such as a liquid crystal display. Fingerprint information on the card owner is registered in the IC chip  82  as biometric information.  
         [0046]      FIG. 9  shows an authentication procedure for the smart card  81 . By having the user touch the fingerprint sensor  1  with his or her fingertip, fingerprint information is inputted into the smart card  81  (step S 1 ) and the fingerprint information is compared with fingerprint information registered in advance (step S 2 ). Here, when the fingerprint matches (“YES” in step S 2 ), a personal identification number (PIN) is issued (step S 3 ). Next, a PIN is inputted by the card owner (step S 4 ). It is then checked whether the PIN issued in step S 3  matches the PIN inputted in step S 4  (step S 5 ), and when the numbers match (“YES” in step S 5 ), usage of the card is permitted (step S 6 ).  
         [0047]     In this way, by authentication the user using fingerprint information in addition to the security number, it is possible to provide a high-security smart card. A smart card equipped with a biometric authentication function can be used as a cash card, a credit card, an identification card, and the like. The fingerprint sensor according to the exemplary embodiment can be applied to all kinds of biometric authentication apparatus for authentication individuals. For example, the embodiment can be applied to a system in which a fingerprint sensor according to the embodiment is attached to a door as a security system for controlling entry/exit for a room, fingerprint information on an entrant inputted by the fingerprint sensor is compared with fingerprint information registered in advance, entry into the room is permitted when such information matches, entry is prohibited when the information does not match, and a security firm or the like is informed as necessary. For online trading that is conducted via an open network such as the Internet, the fingerprint sensor according to the present embodiment can be effectively applied to a biometric authentication apparatus for verifying individuals. In addition, the sensor has a wide range of applications, such as in a user authentication apparatus of a computer terminal apparatus or in a control apparatus for users of a copier.  
         [0048]     It should be noted that although an example of a fingerprint sensor has been described as an exemplary embodiment of a capacitance detection apparatus of the invention, the invention is not limited to this, and can be applied to all kinds of reader apparatuses that read a minute convex/concave pattern of a subject as minute variations in capacitance. As one example, the invention can be applied to authentication of noseprints and the like of animals.