WO1999008121A1 - Method for determining very low capacities and sensor designed therefor - Google Patents

Method for determining very low capacities and sensor designed therefor Download PDF

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Publication number
WO1999008121A1
WO1999008121A1 PCT/DE1998/002078 DE9802078W WO9908121A1 WO 1999008121 A1 WO1999008121 A1 WO 1999008121A1 DE 9802078 W DE9802078 W DE 9802078W WO 9908121 A1 WO9908121 A1 WO 9908121A1
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WO
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Patent type
Prior art keywords
capacitor
voltage
ƒkondensator
kapazit
number
Prior art date
Application number
PCT/DE1998/002078
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German (de)
French (fr)
Inventor
Paul-Werner Von Basse
Josef Willer
Thomas Scheiter
Stephan Marksteiner
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Siemens Aktiengesellschaft
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    • GPHYSICS
    • G01MEASURING; TESTING
    • G01RMEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R27/00Arrangements for measuring resistance, reactance, impedance, or electric characteristics derived therefrom
    • G01R27/02Measuring real or complex resistance, reactance, impedance, or other two-pole characteristics derived therefrom, e.g. time constant
    • G01R27/26Measuring inductance or capacitance; Measuring quality factor, e.g. by using the resonance method; Measuring loss factor; Measuring dielectric constants ; Measuring impedance or related variables
    • G01R27/2605Measuring capacitance
    • GPHYSICS
    • G06COMPUTING; CALCULATING; COUNTING
    • G06KRECOGNITION OF DATA; PRESENTATION OF DATA; RECORD CARRIERS; HANDLING RECORD CARRIERS
    • G06K9/00Methods or arrangements for reading or recognising printed or written characters or for recognising patterns, e.g. fingerprints
    • G06K9/00006Acquiring or recognising fingerprints or palmprints
    • G06K9/00013Image acquisition
    • G06K9/0002Image acquisition by non-optical methods, e.g. by ultrasonic or capacitive sensing

Abstract

A low capacity capacitor is charged and discharged several times onto a measuring capacitor with a substantially higher capacity and the voltage or charge thereof is measured directly, thereby enabling a low capacity or variation in capacity to be determined with a high degree of precision as the result of measurement by a sensor. A fingerprint sensor with an array of metal surfaces (4) underneath a contact area for a finger tip can be made without any moveable parts, whereby transistors are provided as switches (S), in addition to measuring capacitors (1) and comparators (2). A given electric potential is applied to the metal surfaces. When a finger tip is placed on the metal surfaces they become charged to varying degrees according to structure of the surface of the skin. The charges are carried to the measuring capacitors assigned thereto. The charge states of the measuring capacitors are determined separately according to the way in which the matrix memory is read.

Description

description

A process for the very small capacities, thus conceived sensor determination

The present invention relates to a method for determining very small capacitances which is particularly suitable for evaluation of the measurement result in micromechanical sensors.

In capacitively measuring sensors such. As micromechanical pressure sensors (EP 0714017 = US 5,631,428, WO 96/16319) or acceleration sensors (WO 95/19572, EP 0730157), when determining the parameters for small integrated Bauelemente as bipolar and MOS transistors , MIM capacitors and so arises the problem of determining very small capacitances reliably with great accuracy. The measured values ​​of the capacitances to be corrupted by interference. Solid disturbances are z. B. parasitic capacitances. Changeability Liehe disturbance variables z. B. electromagnetic alternating fields from the environment of the measuring arrangement come from or are caused by mechanical vibrations. This interference must be compensated for in an appropriate manner, or at least detected to correct the measurement result accordingly.

To determine the size of small capacities sensitive amplifiers are used. During the measuring capacities are excited by high-frequency AC voltage or pulse voltages to generate the measurement signals. Interference signals are detected either directly to the sense amplifier and suppresses or numerically compensated by an analog-to-digital conversion. These processes are very complicated and require a high degree of accuracy.

In the publication by M. Tartagni, R. Guerrieri: "A 390dpi Live Fingerprint Imager Based on feedback Capacitive Sensing Scheme", ISSCC 1997 slide Supplement, pp 154-155 and 402, an overview of various possible realizations of fingerprint sensors is given. Such sensors are capable of printing the fingerprints electronically, that is, a diagram with the grooves and ridges of the skin surface of the fingertip to be detected, and optionally further evaluate persons for the purpose of identification. At the indicated there capacitive measuring sensors also is the problem that very small capacity must be measured. The measurement must be in a very short time during which rests a fingertip on the sensor, are carried out and is intended to provide a sufficiently accurate and high-contrast result, so that different finger lines can be distinguished from each other.

In EP 0457398 Bl, a fingerprint sensor is described in which the finger is placed on a non-deformable bearing area and which can be used compared with an arrangement of metal surfaces in the sensor resulting capacity to determine the fingerprint. For this

Purpose are connected electronic amplifiers and switches to each of the metal surfaces. Such circuits are sensitive to interference.

Object of the present invention is to provide a method for determining very small capacitances. A further object is to provide a capacitively measuring sensor, particularly a sensor for recording fingerprints, indicate that provides accurate measurements even with small changes of capacity.

These objects are solved with the method having the features of claim 1 and with the sensor having the features of claim 3 or the Finder Fingerprint sensor having the features of claim. 5 Respective embodiments are evident from the dependent claims. In the inventive method, the capacitances to be measured are determined by first applying a voltage to the respective capacitors or capacitors functioning components is applied, and so the capacitors are charged. The capacitors are discharged to a dedicated capacitor of much larger capacity, hereinafter referred to as the measuring capacitor. This process of charging and discharging is repeated several times because of the small capacitances to be measured. therefore on the measuring capacitor, the charge is collected in batches and can then be evaluated. This is done alternatively by the voltage at the measuring capacitor after a predetermined number of charges (charge bursts) is measured, or by the number of discharges is counted which is required lent to charge the sensing capacitor to a predetermined voltage. In the first mentioned case the voltage reached after a predetermined number of charges is proportional to the size of the capacity to be determined. In the second-mentioned case is the number of charges inversely proportional to the size of the capacity to be determined.

In sensors, in which the distance between two electrodes functioning as capacitor plates varies as a function of the measured variable, the measured capacitance is inversely proportional to the distance between the electrodes. Therefore, there the number of charges of the measurement capacitor (charge relapses) is directly proportional to the distance to be measured, which simplifies the evaluation of the measurement result. Therefore, the method according to the invention has in particular at such sensors, the z. B. can be implemented as a micromechanical sensors, the particular advantage that it immediately provides a desired result digital value that can be processed numerically. The aforementioned interference effects of the measurement can be suppressed or can be compensated for numerically.

The process acts as an integrator for the Aufsammeins the charge. This has to suppress high-frequency interference the property. The low-frequency interference can be detected and be eliminated with numerical methods, which are known per se.

The process is particularly suitable for capacitively measuring micromechanical sensors such. B. to be used in pressure sensors or acceleration sensors. In such pressure sensors is dependent on the external pressure deformation of an electrically conductive membrane (z. B. from at least partly electrically conductively doped polysilicon) over a cavity determined by the changed capacitance of this membrane to an existing on the opposite side of the cavity counter electrode (z. B. an electrically conductively doped region in semiconductor material) is determined. Since small capacities and capacity changes volved in-here, the inventive method offers a way to improve the accuracy of such sensors on the previously achievable accuracy also.

The inventive method is particularly suitable for realizing a fingerprint sensor with a particularly simple design, which still good operability is achieved. In this inventive fingerprint sensor is a capacitive measurement method is also applied. The measurement is effected without any mechanical deformation of the sensor member by means of an arrangement (Array) of metal surfaces, each forming one plate of a capacitor. These metal surfaces are covered with a dielectric passivation layer on top of which a finger is placed. The grooves and ridges of the skin surface form different capacities, together with the metal surfaces. The skin surface may be sufficiently be considered highly electrically conductive and is virtually at ground potential. If the metal surfaces down to a certain potential, invite to varying degrees. The charges are supplied separately to a capacitor of larger capacity than the measuring capacitor. The metal surfaces are loaded multiple times, and fertilize the laser are collected respectively on the measuring capacitor. In this way, it is possible to add as many charges that an accurate measurement of the capacitance between the metal surface and the skin over the amount of charge or the voltage reached is possible.

To capture a fingerprint, the charge is state of the metal surfaces such. As determined in the manner as a matrix memory is read out. In the case of ma trixförmigen arrangement of metal surfaces, the metal surfaces are charged in columns simultaneously on the same potential. This corresponds to the precharging a memory array of transistors via the word lines. The unloading of the loaded metal surfaces line by line on each one existing for each line measuring capacitor.

There follows a more detailed description of the method and an embodiment of a sensor so designed in the form of a fingerprint sensor with reference to Figures 1 to. 6

Figures 1, 2 show diagrams of circuit parts, which are suitable for the novel process. Figure 3 shows a transistor device that can be used as a capacitor in the circuit.

Figure 4 shows the sensor arrangement in the scheme in supervision.

Figure 5 shows a diagram of a section of the sensor array in plan view. Figure 6 shows a diagram of a section of the

Sensor arrangement in cross section.

The inventive method can be performed with a circuit which includes the components that are shown in Figure 1. The information stored in each case a charging process on the to be measured, capacitor charging is conducted on the respectively associated measurement capacitor 1, by a charge voltage is switched off via switches and capacitors whose capacitances are to be measured, are conductively connected in each case with a plate of the measuring capacitor. 1 Which adjusting at charged measuring capacitor voltage drop across this capacitor is indicated by the marked arrow. A comparator 2 is adapted to compare the voltage across the measuring capacitor 2 voltage with a reference voltage U ref, so that the attainment of this reference voltage can be detected on the measuring capacitor.

2 shows an alternative circuit is illustrated, wherein the comparator is adapted to determine after how many charges applied a certain reference voltage to the capacitor. The output signal of the comparator 2 is supplied to the input of a counting device 3, which is controlled by a clock pulse T ch. With this counting, the number of discharges is counted which is required until the capacitor 1 ref is the reference voltage U ER has sufficient. The number of charges is then inversely proportional to the capacity of the measured capacitor.

The necessary switches are preferably formed by transistors. These switches the condensers are reindeer, whose capacity is to be determined in an application cycle of the process at the same time, simultaneously to a certain potential of z. B. charged 5 volts. The switches are opened to turn off the power source. Simultaneously, other switches are closed, through which the capacitors are connected to the respective associated measuring capacitors. The measuring capacitors may, if necessary, via a suitable additional circuit at the beginning of the measurement cycle to a bias voltage of, for. B. typical 1 volt be summoned. After the charge is abgeflos- sen to the measuring capacitors, the existing switches are operated again, so that the capacitors to be measured to be recharged. Switching again makes the charge on the measuring capacitors drain, so that the charges are collected there in this way.

When the switches used in the circuit are implemented as transistors, it is convenient to use for the measurement capacitors transistors. Figure 3 shows such a capacitor usable component in cross section. A MOSFET with an insulating layer 7 t a gate electrode 8 over a channel region 11 and diffused into the semiconductor material regions for source and drain 10 is provided with a contact that the regions 10 are electrically conductively connected to the source and drain each other, ie short-circuited are. This contact 9 is drawn in FIG. 3 In operation of the device, an inversion layer forms in the channel region. 11 The insulating layer 7 forms a dielectric of a capacitor of the inversion layer is formed on the one hand by the gate electrode 8 and on the other hand by the contact 9 as well.

In a realized using the method according to the invention fingerprint sensor in the preferred embodiment, a patterned metallization layer is present, which has a grid-like arranged metal surfaces. These areas are for. B. arranged in a double grid-like grid according to Figure 4, in which the individual metal surfaces 4 are shown schematically in plan view. A lateral portion 5 serves to accommodate the required electronics circuit. On the patterned metallization is a dielectric passivation layer on the upper side a support surface for the finger tip is. The thickness of the passivation layer determines the size of the capacitances formed by the individual metal surfaces 4, together with the skin of the fingertip. The metal surfaces can,. B. be a metallization on a semiconductor chip in which the control electronics or other components are integrated. The circuit diagram shown in Figure 1 for explaining the measuring method is used in accordance with the fingerprint sensor. A scheme for the interconnection of the arrangement of metal surfaces 4 is shown in FIG. 5 An intended for collecting the charges measurement capacitor 1 is provided for each row of metal surfaces. 4 The capacities of these measuring capacitors are connected between a hundred times and thousand times greater than that of capacity is obtained when one of the metal surfaces (the electrically conducting skin surface z. B.) is complemented with a capacitor having disposed in the plane of the support surface electrical conductor. 5 shows the arrangement of drawn metal surface the associated switches Si and S 2 are shown to each segment in the illustrated, which are preferably constituted by transistors. Via charging lines L and the closed switch S all in a column (or row) are charged to the arrangement existing metal surfaces to an intended electric voltage by way of word lines in a memory matrix, the switches remain open S2. According to the different existing capacities, the metal surfaces invite to varying degrees. After the switches S x have been opened and the switches have been S 2 is closed, the charges flow via discharge lines R line by line from the metal surfaces (or column-wise) to a respective row (or column) associated with the measuring capacitor 1, and will be collected there.

By means of a downstream comparator 2 which compares the voltage across the capacitor 1 voltage with a reference voltage U ref, the voltage measured at the measuring capacitor. If only a rough measurement is to be performed, it may be sufficient if it is determined by the comparator when a predetermined voltage on the measuring capacitor 1 is exceeded. then you get pressure, a diagram of the fingerprint that is point-black or white. When various gray shades are desired, a terminal of the measuring capacitor 1 at different inputs of comparators •

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Claims

Patentanspr├╝che
1. A method for measuring Kapazitäten, in which a) to a capacitor, a predetermined voltage is applied and the capacitor is charged by the Kapazität is to be determined, b) the capacitor is disconnected from the charging electric voltage , c) is connected to the capacitor with a Meßkondensator such daß an existing charge on the capacitor to this Meßkondensator abfließt, d) steps a to c are repeated until an intended number of charges of Me ßkondensators has occurred or is achieved at the Meßkondensator to a designated voltage, and e) determining the voltage at the Meßkondensator or the number of charges and from the measured Kapazität is determined.
2. The method of claim 1, wherein the Kapazitäten a plurality of capacitors is determined by the steps of a plurality of capacitors and bfür be ausgeführt together and the steps c to e at the same time, these capacitors für are ausgeführt using each of a separate Meßkondensators.
3. Sensor capacitive Meßvorrichtung where
- this Meßvorrichtung umfaßt at least one capacitor of variable Kapazität, - with this capacitor a Meßkondensator is present,
- means are present with which applied to the capacitor, an electric voltage, and this voltage can be separated from the condenser,
- means are present with which an existing on the capacitor electric charge on the Meßkondensator can be übertragen, and - means are provided with which a voltage or a charge state of the Meßkondensators or a number of times of charges of Me ßkondensators can be erfaßt, and in which an evaluation device is provided which equips it overall,
- repeated charge the capacitor,
each übertragen to an existing on the capacitor charge on the Meßkondensator and -
- dezustand from the voltage at the Meßkondensator or from the laser or of the number of charges of the Meßkondensators Kapazität the capacitor of the Meßvorrichtung or abhängende thereof to be determined to Größe determine.
4. Sensor according to claim 3, wherein the Meßkondensatoren provided by field effect transistors whose für source and drain regions (10) are short-circuited with each other are formed.
5. fingerprint sensor
- structured with a grid of Metallflächen (4) metallization,
- with a number of Meßkondensatoren (1),
- with switches (S), the über an intended electric potential may be respectively applied to a designated number of Metallflächen and über the Metallflächen each of these can be electrically conductively connected to a Meßkondensator,
- approximately level with a passivation layer on the metallization,
- with a Auflagefläche (6) für a fingertip on this passivation layer and
- a circuit for determining the voltages or the Ladezustände the Meßkondensatoren.
6. The fingerprint sensor of claim 5, wherein said circuit umfaßt each case a comparator (2) which is dafür provided to compare the voltage at the respective Meßkondensator with a reference voltage.
7. The fingerprint sensor of claim 6, wherein a Zähleinrichtung (3) is present, which is provided dafür, until reaching the reference voltage agree to a number of loading completion Ladevorgänge to each Meßkondensator.
8. The fingerprint sensor of claim 5, wherein said circuit umfaßt each plurality of comparators which are dafür provided to compare the voltage at the respective Meßkondensator with a scale of reference voltages.
9 are fingerprint sensor according to one of Ansprüche 5 to 8, wherein the Meßkondensatoren provided by field effect transistors whose für source and drain regions (10) are short-circuited with each other, is formed.
10. The fingerprint sensor of any one of Ansprüche 5 to 9, wherein the Kapazitäten the Meßkondensatoren between a hundred times and größer thousand times than that Kapazität which results when one of the Metallflächen is ergänzt having disposed in the plane of Auflagefläche electrical conductor to a condenser.
PCT/DE1998/002078 1997-08-05 1998-07-23 Method for determining very low capacities and sensor designed therefor WO1999008121A1 (en)

Priority Applications (2)

Application Number Priority Date Filing Date Title
DE19733865.8 1997-08-05
DE19733865 1997-08-05

Applications Claiming Priority (5)

Application Number Priority Date Filing Date Title
MXPA00001276A MXPA00001276A (en) 1997-08-05 1998-07-23 Method for determining very low capacities and sensor designed therefor.
JP2000506535A JP2001512836A (en) 1997-08-05 1998-07-23 The methods and sensor envisioned by this for obtaining a very small volume
KR20007001166A KR20010022575A (en) 1997-08-05 1998-07-23 Method for determining very low capacities and sensor designed therefor
EP19980945041 EP1000362A1 (en) 1997-08-05 1998-07-23 Method for determining very low capacities and sensor designed therefor
BR9811837A BR9811837A (en) 1997-08-05 1998-07-23 Process for the determination of very small capacity sensor and so designed

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WO1999008121A1 true true WO1999008121A1 (en) 1999-02-18

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JP (1) JP2001512836A (en)
KR (1) KR20010022575A (en)
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WO (1) WO1999008121A1 (en)

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JP4511064B2 (en) * 2001-03-05 2010-07-28 三菱電機株式会社 Unevenness detecting sensor
DE10333154A1 (en) 2003-07-22 2005-02-24 Vega Grieshaber Kg Method and circuit arrangement for evaluating a measurement capacitance
JP4036798B2 (en) * 2003-07-29 2008-01-23 アルプス電気株式会社 Capacitance detection circuit and a detection method and a fingerprint sensor
US7075316B2 (en) 2003-10-02 2006-07-11 Alps Electric Co., Ltd. Capacitance detector circuit, capacitance detection method, and fingerprint sensor using the same
CN101526989B (en) 2008-03-06 2011-02-09 奇景光电股份有限公司 Capacitive fingerprint sensor and panel thereof
FR2938344B1 (en) 2008-11-07 2010-11-19 Continental Automotive France Device for measuring a variation in the capacitance of a capacitive variable structure
JP5874505B2 (en) * 2012-04-10 2016-03-02 オムロン株式会社 Vibration energy detector, vibration energy detection system
CN103679163B (en) * 2012-09-18 2017-03-22 成都方程式电子有限公司 New capacitive fingerprint image capture system
CN105631432A (en) * 2016-01-04 2016-06-01 京东方科技集团股份有限公司 Fingerprint detection circuit and driving method and display device

Citations (3)

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Publication number Priority date Publication date Assignee Title
US4039940A (en) * 1976-07-30 1977-08-02 General Electric Company Capacitance sensor
EP0509915A1 (en) * 1991-04-18 1992-10-21 Marelli Autronica Process and appliance for measuring a capacitor
WO1997017668A1 (en) * 1995-11-06 1997-05-15 Dew Engineering & Development Ltd. Contact imaging device

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4039940A (en) * 1976-07-30 1977-08-02 General Electric Company Capacitance sensor
EP0509915A1 (en) * 1991-04-18 1992-10-21 Marelli Autronica Process and appliance for measuring a capacitor
WO1997017668A1 (en) * 1995-11-06 1997-05-15 Dew Engineering & Development Ltd. Contact imaging device

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
TARTAGNI M ET AL: "FP 12.3: A 390DPI LIVE FINGERPRINT IMAGER BASED ON FEEDBACK CAPACITIVE SENSING SCHEME", IEEE INTERNATIONAL SOLID STATE CIRCUITS CONFERENCE, vol. 40, February 1997 (1997-02-01), pages 200/201, 456, XP000753070 *

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CN1266494A (en) 2000-09-13 application
JP2001512836A (en) 2001-08-28 application
EP1000362A1 (en) 2000-05-17 application
KR20010022575A (en) 2001-03-26 application

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