JP3044660B1 - Sensor circuit for surface shape recognition - Google Patents

Abstract

Abstract: PROBLEM TO BE SOLVED: To improve detection accuracy of a sensor circuit for surface shape recognition. SOLUTION: A capacitance sensor element 11 whose capacitance changes according to the surface shape of a finger, such as unevenness of a fingerprint, a signal generation circuit 15 for generating a voltage signal according to the capacitance of the capacitance sensor element, a capacitance sensor element and a signal generation circuit N1 which is a connection point with
The signal generation circuit of the surface shape recognition sensor circuit including the detection circuit 20 for detecting the voltage signal of
s, and the first of the capacitive elements
Is connected to the node N1, the voltage of the second terminal of the capacitor is set to the first potential, and the second terminal is changed to the second potential after the node N1 is charged. The charge at the node N1 is extracted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface shape recognition sensor circuit for detecting a surface shape having minute irregularities such as a human fingerprint or an animal nose pattern.

[0002]

2. Description of the Related Art As a sensor for recognizing a surface shape having fine irregularities, a sensor specifically targeting fingerprint detection has been reported. As a technology for detecting a fingerprint pattern, a capacitance detection type sensor using an LSI manufacturing technology has been proposed. This is, for example, the 'ISSCC DIG
EST OF TECHNICAL PAPERS'F
EBRUARY 1998 pp. 284-285. As shown in FIG. 9, the capacitance detection type sensor is configured as a sensor array 2 in which each sense unit 1 is two-dimensionally arranged on an LSI chip, and is in contact with an electrode of each sense unit 1 via an insulating film. It detects the capacitance formed between the finger 3 and the skin of the finger 3 and senses the concavo-convex pattern of the fingerprint. Since the value of the capacitance formed by the unevenness of the fingerprint is different, the unevenness of the fingerprint can be sensed by detecting this minute difference in capacitance.

FIG. 7 is a block diagram showing a basic configuration of a conventional sensor circuit using this principle. That is, the sensor circuit for surface shape recognition includes a capacitance sensor element 11 having a capacitance value Cf formed between the electrode of the sense unit 1 and the skin of the finger touched via the insulating film. A signal generating circuit 13 including a switch SW2 for generating a voltage signal corresponding to the capacitance value Cf and a current source 12,
Detection circuit 2 for detecting a voltage signal by signal generation circuit 13
0 and a switch SW1 for supplying the external potential Vp to a node N1 which is a connection point between the electrode of the capacitance sensor element 11 and the signal generation circuit 13. Note that Cp in the figure indicates a parasitic capacitance. One set of the sense unit 1 is configured by the capacitance sensor element 11, the signal generation circuit 13, the detection circuit 20, and the like.

Here, the value Cf of the capacitance sensor element 11 is determined by the distance between the sensor electrode of the capacitance sensor element 11 (ie, the electrode of the sensor element 11 connected to the node N1 in FIG. 7) and the skin of the finger. Therefore, the value Cf of the capacitance sensor element 11 differs depending on the unevenness of the fingerprint. Therefore, when the finger 3 touches the position corresponding to the node N2 in FIG. 7, a voltage signal corresponding to the unevenness of the fingerprint is output to the node N1. This voltage signal is detected by the detection circuit 20 as a signal reflecting the unevenness of the fingerprint, and as a result, a fingerprint pattern is detected.

FIG. 8 is a timing chart for explaining the operation of the surface shape recognition sensor circuit shown in FIG. First, the control signal P for controlling the opening and closing of the switch SW1 is at a low level (FIG. 8A). The control signal S for controlling the opening and closing of the switch SW2 is also Lo.
It is at the w level (FIG. 8B). Therefore, the switches SW1 and SW2 are open. At this time, the node N1 has a potential equal to or lower than the external potential Vp (FIG. 8C).

In such a state, when the control signal P changes from the Low level to the High level at the time of FIG. 8A, the switch SW1 is closed and becomes conductive, and as a result, the potential of the node N1 becomes external. It is precharged to the potential Vp (FIG. 8C).

After the precharge is completed, the control signal P changes to Low level at the time of FIG. 8A, and at the same time, the control signal S changes to High level as shown in FIG. 8B. As a result, the switch SW1 is turned off,
The switch SW2 becomes conductive, and the electric charge charged to the node N1 by the current source 12 is extracted. As a result, the potential of the node N1 decreases (FIG. 8C). Here, assuming that the High level period of the control signal S is Δt, the potential drop ΔV of the node N1 after Δt has elapsed is ΔV = IΔt / (Cf + Cp) (1). Here, I is the current of the current source 12.

Here, the current I, the period Δt, and the parasitic capacitance C
Since p is constant, the potential drop ΔV is equal to the capacitance Cf
Is determined by Since each sense unit 1 is determined by the distance between the electrode of the capacitance sensor element 11 and the skin of the finger, the value of the capacitance Cf differs depending on the unevenness of the fingerprint. From this, the magnitude of the reduced potential ΔV changes reflecting the unevenness of the fingerprint.

That is, assuming that the value of the capacitive sensor element 11 corresponding to the concave part of the fingerprint is Cfv and the value of the capacitive sensor element 11 corresponding to the convex part of the fingerprint is Cfr, the difference ΔVi between the voltage signal of the concave part and the convex part of the fingerprint As shown in FIG. 8C, ΔVi = IΔt / (Cfv + Cp) −IΔt / (Cfr + Cp) (2) Since such a signal ΔVi is given to the detection circuit 20 as an input signal, the detection circuit 20 can identify the fingerprint unevenness and output a signal reflecting the fingerprint unevenness.

[0010]

In the conventional sensor circuit shown in FIGS. 7 and 8, the electric current at the node N1 is extracted by the current source 12. FIG. Since such a current source 12 is provided for each sense unit, it is necessary to uniformly control the amount of current of each current source in each sense unit and the time for extracting charges. However, in practice, it is difficult to uniformly manufacture all the current sources, and therefore, there has been a problem that the detection accuracy of the entire sensor circuit including each sense unit is deteriorated. Therefore, an object of the present invention is to improve the detection accuracy of the sensor circuit for surface shape recognition.

[0011]

SUMMARY OF THE INVENTION In order to solve the above problems, the present invention provides a capacitance sensor element whose capacitance changes according to the surface shape of a contacted object, and a voltage corresponding to the capacitance of the capacitance sensor element. A signal generation circuit that generates a signal, and a detection circuit that is connected to a node indicating a connection point between the capacitive sensor element and the signal generation circuit and detects a voltage signal at the node. A sensor circuit for recognizing a surface shape that detects and outputs a voltage signal of the node after the signal generation, wherein the signal generating circuit includes a capacitor having first and second terminals and a first terminal connected to the node. A capacitor having a value Cs, the voltage of the second terminal of the capacitor is set to the first potential, and the second terminal is changed to the second potential after the node is charged. From the capacitive element It characterized by that so as to generate pressure signal. In this case, the first potential is set at one of the power supply potential and the ground potential of the sensor circuit, and the second potential is set at the other of the power supply potential and the ground potential. Further, the capacitance element is formed of a semiconductor element. Further, the semiconductor element is constituted by a MOS transistor.
The gate terminal of the S transistor becomes the first terminal,
The source terminal and the drain terminal of the S transistor are the second terminals. When the parasitic capacitance value generated at the first terminal of the capacitance element is Cp, and the maximum and minimum capacitance values of the capacitance sensor element are Cfv and Cfr, respectively, the capacitance value Cs of the capacitance element is ｛(Cfv + Cp) (Cfr + Cp )｝ ^1/2
Is set to Further, the capacitance value Cs of the capacitive element
Is set to be equal to or less than the maximum value of the capacitance value of the capacitance sensor element. The capacitance Cs of the capacitance element is 10 fF
To 250 fF.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings. FIG. 1 is a block diagram of a sensor circuit for surface shape recognition according to the present invention. In FIG. 1, a sensor circuit for surface shape recognition includes a capacitance sensor element 11 having a value Cf formed between an electrode of a sense unit 1 and a skin of a finger touched via an insulating film; A signal generating circuit 15 including a driving capacitive element 14 having a capacitance value Cs for generating a voltage signal corresponding to the capacitance value Cf of the above, a detecting circuit 20 for detecting a voltage signal by the signal generating circuit 15,
The external potential Vp is applied to the capacitance sensor element 11 and the signal generation circuit 15.
Switch S for supplying to node N1 which is a connection point with
W1. Note that Cp in the figure indicates a parasitic capacitance.
One set of the sense unit 1 is configured by the capacitance sensor element 11, the signal generation circuit 15, the detection circuit 20, and the like.

The value Cf of the capacitance sensor element 11 is determined by the distance between the sensor electrode of the capacitance sensor element 11 (that is, the electrode of the sensor element 11 connected to the node N1 in FIG. 1) and the skin of the finger. Therefore, the value Cf of the capacitance sensor element 11 differs depending on the unevenness of the fingerprint. Therefore, when the finger touches the position corresponding to the node N2 in FIG. 1, a voltage signal corresponding to the unevenness of the fingerprint is output to the node N1. This voltage signal is detected by the detection circuit 20 as a signal reflecting the unevenness of the fingerprint, and as a result, a fingerprint pattern is detected.

A signal generating circuit 1 in the sensor circuit shown in FIG.
Reference numeral 5 indicates that the electric charge at the node N1 is extracted by using the charge and discharge of the driving capacitance element 14, and the amount of electric charge to be extracted is the capacitance Cs of the driving capacitance element 14 and the driving voltage Vs
Is controlled by Here, the driving voltage Vs shown in FIG.
Through the switch SW3, for example, the power supply voltage level (V
DD) or ground level (GND) to control the amount of charge to be extracted. The control based on charging / discharging of the capacitance value Cs of the driving capacitance element 14 has higher accuracy than the conventional control based on the current value of the current source 12 shown in FIG. 7, and the control of the driving voltage Vs of the driving capacitance element 14 is as follows. The control can be performed easily and with high accuracy as compared with the conventional time control shown in FIG.

Next, referring to FIG. 2, the signal generation circuit 15 of FIG.
Will be described. Here, a case where the surface of the finger is detected as the surface shape will be described. FIG.
At the time point (a), the potential of the control signal P is set to High level, the switch SW1 is closed, and the external potential Vp is precharged to the node N1. Thereafter, at the time point of FIG. 2A, the potential of the control signal P is set to the Low level, and the switch SW1 is opened. At the same time, as shown in FIG.
The drive voltage Vs of the drive capacitance element 14 in 5 is reduced by ΔVs to extract the electric charge at the node N1 to generate an input signal to the detection circuit 20 side.

Here, the precharge potential is Vp, the value of the capacitance sensor element 11 is Cf, the parasitic capacitance value is Cp, the capacitance value of the driving capacitance element 14 is Cs, and the variation of the driving voltage Vs of the driving capacitance element 14 is ΔVs. Then, the change amount ΔV of the input signal applied to the detection circuit 20 side is as shown in Expression (3). ΔV = ΔVs / {1+ (Cf + Cp) / Cs} (3)

The dynamic range ΔVi of the signal change amount is as follows. ΔVi = ΔVMAX−ΔVMIN = ΔVs / ｛1+ (Cfv + Cp) / Cs｝ −ΔVs / ｛1+ (Cfr + Cp) / Cs｝ (4) In the expression (4), Cfv is a concave portion of a fingerprint of the capacitance Cf of the capacitive sensor element 11. , And Cfr indicates a capacitance value of the capacitance Cf of the capacitance sensor element 11 which corresponds to the convex portion of the fingerprint.

The driving capacitance element 14 can also be realized by a semiconductor element. In particular, as shown in FIG. 3B, a MOS transistor can be used as the driving capacitance in FIG. 3A. FIG. 3B shows an example of an NMOS transistor. However, when the polarity of the power supply voltage of the sensor circuit is different, the PMO
An S transistor may be used. Further, M
By connecting the gate terminal of the OS transistor to the capacitance sensor element 11 and controlling the voltages of the source terminal and the drain terminal, the parasitic capacitance of the source terminal and the drain terminal of the MOS transistor is not connected to the capacitance sensor element 11. The drive capacitance element 14 can be realized.

The dynamic range ΔVi shown in equation (4)
Is the input signal to the detection circuit 20, so the larger is better. Here, the parasitic capacitance value Cp = 50fF, Cfv = 10f
FIG. 4 shows the relationship between the dynamic range ΔVi and the value Cs of the driving capacitive element 14 when F, Cfr = 100 fF, and ΔVs = 2.7 V.

In FIG. 4, the dynamic range ΔVi
There is a value Cs of the driving capacitance element 14 at which the maximum value is obtained. In this case, the value Cs of the driving capacitance element 14 is about 95 fF. From the above equation (4), the dynamic range ΔVi is maximized when the value Cs of the driving capacitive element 14 is Cs = {(Cfv + Cp) (Cfr + Cp)} ^1/2 (5). Value C of drive capacitive element 14
s may be selected.

The size of the sensor electrode is limited by the fingerprint pattern and the sensitivity of the sensor circuit.
It becomes about 00 um square. Cfr at this time is 20 fF or more.
350fF, Cfv is 5fF or less, Cp is 10fF ~ 1
It is about 70 fF. Since Cfv is small, it may be fixed at 5 fF, and the optimum value Cs of the drive capacitance element 14 when Cfr and Cp are changed is as shown in FIG.
From the above, the value Cs of the driving capacitive element 14 may be set to be equal to or smaller than the capacitance Cfv of the concave portion of the fingerprint.

Further, in actual production, Cp = Cfv /
3, it may be considered that Cfv = Cfv / 20. In this case, the optimal value of the value Cs of the driving capacitance element 14 is about 0.7 Cfv. FIG. 6 shows this relationship. Therefore, the value Cs of the driving capacitive element 14 is optimally about 0.7 times Cfv,
The range is from 10 fF to 250 from the above Cfv range.
about fF. Thus, in the signal generation circuit 15,
Since the electric charge at the node N1 is extracted by using the charging and discharging of the driving capacitance element 14 without extracting the electric charge by the current source, the current source is incorporated in a large number of sense units, and the current amount and the electric charge are extracted. It is not necessary to precisely control the time for extracting the sensor, and the sensing accuracy of the sensor circuit can be improved with a simple configuration.

[0023]

As described above, according to the present invention, a capacitance sensor element whose capacitance changes according to the surface shape of a contacted object, a signal generation circuit for generating a voltage signal corresponding to the capacitance of the capacitance sensor element A surface shape recognition sensor circuit including a detection circuit connected to a node which is a connection between a capacitance sensor element and a signal generation circuit, the detection circuit detecting a voltage signal, the signal generation circuit having first and second terminals A first terminal is formed of a capacitor having a capacitance value Cs connected to the node, and the voltage of the second terminal of the capacitor is set to the first terminal.
And a voltage signal is generated from the capacitive element by changing the second terminal to the second potential after the node is charged with electric charge, so that the current source is connected to a large number of sense units. It is possible to avoid controlling the amount of current of each current source and the time for extracting the electric charge with high accuracy, thereby improving the detection accuracy of the sensor circuit. At this time, the first potential applied to the second terminal of the capacitor is set to one of the power supply potential and the ground potential, and the second potential applied to the second terminal is set to the power supply potential and the ground potential. Since the potential is set to either of the potentials, the potential of the second terminal can be controlled with a simple configuration. Further, since the capacitor is formed of a semiconductor element, the gate terminal of the MOS transistor as the semiconductor element is a first terminal, and the source terminal and the drain terminal of the transistor are the second terminal. Thus, an accurate capacitor can be manufactured. Further, the parasitic capacitance value generated at the first terminal of the capacitive element is represented by C
p, when the maximum and minimum capacitance values of the capacitance sensor element are Cfv and Cfr, respectively, the capacitance value Cs of the capacitance element is
Since {(Cfv + Cp) (Cfr + Cp)} is set to ^1/2 , when detecting the unevenness of the fingerprint as the surface shape, the concave and convex portions of the fingerprint can be accurately identified. Also,
Since the capacitance value Cs of the capacitance element is set to be equal to or less than the maximum value of the capacitance of the capacitance sensor element, the surface shape of the object having fine irregularities can be detected with high accuracy. In addition, since the capacitance value Cs of the capacitance element is in the range of 10 fF to 250 fF, a high-precision capacitance element can be easily manufactured.

[Brief description of the drawings]

FIG. 1 is a block diagram of a sensor circuit for surface shape recognition according to the present invention.

FIG. 2 is a time chart showing an operation state of the sensor circuit of FIG.

FIG. 3 is a diagram illustrating an example of a driving capacitance element in a signal generation circuit included in the sensor circuit of FIG. 1;

FIG. 4 is a characteristic diagram of the sensor circuit of FIG. 1;

FIG. 5 is a characteristic diagram of the sensor circuit of FIG. 1;

FIG. 6 is a characteristic diagram of the sensor circuit of FIG. 1;

FIG. 7 is a block diagram of a conventional sensor circuit.

FIG. 8 is a time chart illustrating an operation state of the sensor circuit of FIG. 7;

FIG. 9 is a diagram illustrating a sensor array in which each sense unit is formed in a lattice shape.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Sense unit, 2 ... Sensor array, 3 ... Finger, 11
... Capacitive sensor element, 14 ... Drive capacitive element, 15 ... Signal generation circuit, 20 ... Detection circuit, SW1, SW3 ... Switch,
Cp: parasitic capacitance, N1, N2: node, Vp: external voltage.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-11-118415 (JP, A) JP-A-1-285801 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01B 7/00-7/34 102 G01D 5/00-5/252 G01D 5/39-5/62 G06T 1/00

Claims (7)

(57) [Claims] 1. A capacitance sensor element whose capacitance changes according to the surface shape of a contacted object, a signal generation circuit connected to the capacitance sensor element for generating a voltage signal according to the capacitance of the capacitance sensor element, A detection circuit connected to a node indicating a connection point between the capacitance sensor element and the signal generation circuit, the detection circuit detecting a voltage signal at the node; and the detection circuit detects the voltage signal at the node after the node is charged with electric charge. Wherein the signal generating circuit has first and second terminals, and the first terminal is connected to the node Cs.
The signal generation circuit sets the voltage of the second terminal of the capacitor to a first potential, and sets the second terminal to the second potential after the node is charged with electric charge. Wherein the voltage signal is generated from the capacitive element by changing the potential of the capacitor element to a potential of the surface shape. 2. The power supply potential and the ground potential according to claim 1, wherein the first potential is set to one of a power supply potential and a ground potential of a sensor circuit for surface shape recognition. A sensor circuit for recognizing a surface shape, which is set to one of the other. 3. The sensor circuit for recognizing a surface shape according to claim 1, wherein the capacitance element is formed of a semiconductor element. 4. The semiconductor device according to claim 3, wherein the semiconductor element comprises a MOS transistor.
A sensor circuit for surface shape recognition, wherein a gate terminal of an OS transistor serves as the first terminal, and a source terminal and a drain terminal of the MOS transistor serve as the second terminal. 5. The method according to claim 1, wherein a parasitic capacitance value generated at a first terminal of the capacitance element is C.
p, when the maximum and minimum capacitance values of the capacitance sensor element are Cfv and Cfr, respectively, the capacitance value C of the capacitance element
s is set to {(Cfv + Cp) (Cfr + Cp)} ^1/2 . 6. The surface according to claim 1, wherein the capacitance value Cs of the capacitance element is set to be equal to or less than a maximum value of the capacitance value of the capacitance sensor element. Sensor circuit for shape recognition. 7. The capacitance element according to claim 1, wherein a capacitance value Cs of the capacitance element is 10 fF to 250 fF.
A sensor circuit for recognizing a surface shape having a range of