WO1999008121A1

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

Info

Publication number: WO1999008121A1
Application number: PCT/DE1998/002078
Authority: WO
Inventors: Paul-Werner Von Basse; Josef Willer; Thomas Scheiter; Stephan Marksteiner
Original assignee: Siemens Aktiengesellschaft
Priority date: 1997-08-05
Filing date: 1998-07-23
Publication date: 1999-02-18
Also published as: EP1000362A1; MXPA00001276A; KR20010022575A; JP2001512836A; CN1266494A; BR9811837A

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

Method for determining very small capacities and sensor designed with it

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

With capacitively measuring sensors, such as. B. micromechanical pressure sensors (EP 0 714 017 = US 5,631,428, WO 96/16319) or acceleration sensors (WO 95/19572, EP 0 730 157), when determining the parameters for small integrated components such as bipolar and MOS transistors , MIM capacitors, etc., the problem arises of reliably determining very small capacitances with great accuracy. The measured capacitance values are falsified by disturbance variables. Fixed disturbances are e.g. B. parasitic capacitances. Changing disturbance variables can e.g. B. originate from alternating electromagnetic fields from the surroundings of the measuring arrangement or caused by mechanical vibrations. These disturbance variables must be compensated for in a suitable manner or at least recorded in order to be able to correct the measurement result accordingly.

Sensitive amplifiers are used to determine the size of small capacitors. During the measuring process, the capacitances are excited by high-frequency alternating voltages or pulse voltages in order to generate the measuring signals. Interference signals are either recorded and suppressed directly with the measuring amplifier or numerically compensated after an analog-digital conversion. These methods are very complex and require high accuracy.

In the publication by M. Tartagni, R. Guerrieri: "A 390dpi Live Fingerprint Imager Based on Feedback Capacitive Sensing Scheme ", 1997 ISSCC slide Supplement, pp. 154-155 and 402, provides an overview of various possible implementations of fingerprint sensors. Such sensors are suitable for electronically recording the fingerprint, ie a diagram with the furrows and ridges of the skin surface of the fingertip, and, if necessary, to further evaluate it for the purpose of personal identification. The capacitive measuring sensors indicated there also have the problem that very small capacitances have to be measured. The measurement must be carried out in a very short time, during which a fingertip on the Sensor rests, performed and should provide a sufficiently accurate and high-contrast result so that different finger lines can be distinguished from each other.

EP 0 457 398 B1 describes a fingerprint sensor in which the finger is placed on a non-deformable support surface and the capacities resulting from an arrangement of metal surfaces in the sensor are used to determine the fingerprint. To this

For this purpose, electronic amplifiers and switches are connected to each of the metal surfaces. Such circuits are sensitive to interference.

The object of the present invention is to provide a method for determining very small capacities. A further task consists in specifying a capacitively measuring sensor, in particular a sensor for taking fingerprints, which delivers precise measurement results even with small changes in the capacities.

These objects are achieved with the method with the features of claim 1 or with the sensor with the features of claim 3 or with the finder impression sensor with the features of claim 5. Respective configurations result from the dependent claims. In the method according to the invention, the capacitances to be measured are determined by first applying an electrical voltage to the capacitors in question or components functioning as capacitors and thus charging the capacitors. The capacitors are discharged onto a capacitor of a considerably larger capacity, hereinafter referred to as a measuring capacitor. This process of loading and unloading is repeated several times because of the small capacities to be measured. The charge is therefore collected in batches on the measuring capacitor and can then be evaluated. Alternatively, this is done by measuring the voltage across the measuring capacitor after a predetermined number of charges (charge bursts) or by counting the number of charges required to charge the measuring capacitor to a predetermined voltage. In the first case mentioned, the voltage reached after a predetermined number of charges is proportional to the size of the capacitance to be determined. In the second case, the number of charges is inversely proportional to the size of the capacity to be determined.

In the case of sensors in which the distance between two electrodes functioning as capacitor plates varies as a function of the measured variable, the capacitance to be measured is inversely proportional to the distance between the electrodes. There, the number of charges of the measuring capacitor (charge bursts) is therefore directly proportional to the distance to be measured, which simplifies the evaluation of the measurement result. The method according to the invention therefore has in particular in such sensors, the z. B. can be implemented as micromechanical sensors, the particular advantage that it immediately delivers a desired digital result value that can be further processed numerically. The disturbing effects of the measurement mentioned at the beginning are suppressed or can be compensated numerically.

The process acts as an integrator because of the accumulation of the charge. This has the property of high-frequency interference to suppress. The low-frequency interference can be recorded and calculated using numerical methods which are known per se.

The method is particularly suitable in the case of capacitively measuring micromechanical sensors, such as, for. B. to be used in pressure sensors or acceleration sensors. In the case of such pressure sensors, the deformation of an electrically conductive membrane (for example made of at least partially electrically conductive doped polysilicon) over a cavity, which is dependent on the external pressure, is determined in that the changed capacitance of this membrane relative to one present on the opposite side of the cavity Counterelectrode (e.g. an electrically conductive doped area in semiconductor material) is determined. Since small capacitances and changes in capacitance are involved here, the method according to the invention offers a possibility of improving the measuring accuracy of such sensors beyond the accuracy that was previously achievable.

The method according to the invention is particularly suitable for realizing a fingerprint sensor with a particularly simple construction, in which a good functionality is nevertheless achieved. A capacitive measuring method is also used in this fingerprint sensor according to the invention. The measurement is carried out without mechanical deformation of a sensor part by means of an arrangement (array) of metal surfaces which each form a plate of a capacitor. These metal surfaces are covered with a dielectric passivation layer, on the top of which a finger is placed. The furrows and ridges of the skin surface together with the metal surfaces form different capacities. The skin surface can be considered to be sufficiently electrically conductive and is practically at ground potential. If the metal surfaces are placed on a certain potential, they charge to different degrees. The charges are fed separately to a capacitor of larger capacitance as a measuring capacitor. The metal surfaces are loaded several times and the loading fertilize are collected on the measuring capacitor. In this way it is possible to add so many charges that an accurate measurement of the capacitance between the metal surface and the skin is possible via the amount of charge or the voltage reached.

In order to be able to record a fingerprint, the state of charge of the metal surfaces is checked e.g. B. determined in the manner in which a matrix memory is read out. In the case of a matrix-like arrangement of the metal surfaces, the metal surfaces are charged to the same potential in columns at the same time. This corresponds to the precharging of a memory array made of transistors via the word lines. The discharged metal surfaces are discharged line by line onto a measuring capacitor for each line.

The following is a more detailed description of the method according to the invention and of an exemplary embodiment of a sensor designed with it in the form of a fingerprint sensor with reference to FIGS. 1 to 6.

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

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

Figure 5 shows a schematic of a section of the sensor arrangement in supervision. Figure 6 shows a schematic of a section of the

Cross section of sensor arrangement.

The method according to the invention can be carried out with a circuit which comprises the components which are shown in FIG. 1. The charge stored in each charging process on the capacitors to be measured is passed to the respectively assigned measuring capacitor 1 by A charging voltage is switched off via switches and the capacitors, whose capacitances are to be measured, are each conductively connected to a plate of the measuring capacitor 1. The voltage drop across this capacitor that occurs when the measuring capacitor is charged is indicated by the arrow shown. A comparator 2 is provided for comparing the voltage applied to the measuring capacitor 2 with a reference voltage U _ref , so that the reaching of this reference voltage can be determined on the measuring capacitor.

FIG. 2 shows an alternative circuit in which the comparator is designed to determine after how many charges a certain reference voltage has been applied to the capacitor. The output signal of the comparator 2 is fed to the input of a counter 3, which is controlled by a clock pulse T _ch . With this counting device, the number of charges is counted, which is required until the capacitor 1 has reached the reference voltage U _ref . The number of charges is then inversely proportional to the capacitance of the capacitor to be measured.

The required switches are preferably formed by transistors. Via these switches, the capacitors, the capacities of which are to be determined simultaneously in an application cycle of the method, are simultaneously adjusted to a specific potential of z. B. charged 5 volts. The switches are opened to switch off the voltage source. At the same time, other switches are closed, via which the capacitors are connected to the respectively associated measuring capacitors. The measuring capacitors can optionally be set to a bias voltage of z. B. typically 1 volt. After the charge on the measuring capacitors has been discharged, the existing switches are actuated again so that the capacitors to be measured are recharged. Switching again leaves the charge on the measuring capacitors drain off, so that the charges are collected there in this way.

If the switches used in the circuit are implemented as transistors, it is expedient to also use transistors for the measuring capacitors. FIG. 3 shows such a component that can be used as a capacitor in cross section. A MOSFET with an insulation layer 7 _{t of} a gate electrode 8 over a channel region 11 and with regions for source and drain 10 diffused into the semiconductor material is provided with a contact such that the regions 10 for source and drain are electrically conductively connected to one another, ie short-circuited are. This contact 9 is shown in Figure 3. When the component is in operation, an inversion layer forms in the channel region 11. The insulation layer 7 forms a dielectric of a capacitor, which is formed on the one hand by the gate electrode 8 and on the other hand by the contact 9 and the inversion layer.

In the case of a fingerprint sensor implemented using the method according to the invention, in the preferred exemplary embodiment there is a structured metallization layer which has metal surfaces arranged in a grid. These areas are e.g. B. arranged in a double lattice-like grid according to Figure 4, in which the individual metal surfaces 4 are shown schematically in supervision. A lateral area 5 serves to accommodate the necessary circuit electronics. There is a dielectric passivation layer on the structured metallization level, on the top of which there is a contact surface for the fingertip. 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 e.g. B. be a metallization level on a semiconductor chip in which the control electronics or other components are integrated. The circuit diagram shown in FIG. 1 to explain the measuring method is used accordingly in the fingerprint sensor. A diagram for the interconnection of the arrangement of metal surfaces 4 is shown in FIG. 5. A measuring capacitor 1 provided for collecting the charges is provided for each row of metal surfaces 4. The capacitances of these measuring capacitors are between 100 times and 100 times greater than the capacitance that results when one of the metal surfaces is supplemented to form a capacitor with an electrical conductor arranged in the plane of the contact surface (for example the electrically conductive skin surface). In FIG. 5, the associated switches Si and S ₂ , which are preferably formed by transistors, are shown for each metal surface shown in the section of the arrangement shown. Via charging lines L and the closed switches S, all metal surfaces present in a column (or row) of the arrangement are charged to an intended electrical voltage in the manner of word lines in a matrix memory, the switches S ₂ remaining open. Depending on the different capacities available, the metal surfaces charge to different extents. After the switches S _{x have been} opened and the switches S _{2 have been} closed, the charges flow through discharge lines R from the metal surfaces row by row (or column by column) to the measuring capacitor 1 assigned to a respective row (or column) and become collected there.

The voltage across the measuring capacitor is measured by means of a downstream comparator 2, which compares the voltage across the capacitor 1 with a reference voltage U _ref . If only a rough measurement is to be carried out, it may be sufficient if the comparator is used to determine when a certain voltage on the measuring capacitor 1 is exceeded. A diagram of the fingerprint is then obtained, which is black or white at points. If different shades of gray are desired, the measuring capacitor 1 can be connected to different inputs of comparators •

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Claims

claims

1. Method for measuring capacitance, in which a) a predetermined electrical voltage is applied to a capacitor whose capacitance is to be determined and the capacitor is thereby charged, b) the capacitor is separated from the charging electrical voltage, c) the Capacitor is connected to a measuring capacitor in such a way that a charge present on the capacitor flows to this measuring capacitor, d) steps a to c are repeated until a predetermined number of charges of the measuring capacitor has taken place or until a predetermined electrical voltage at the measuring capacitor is reached is and e) the voltage across the measuring capacitor or the number of charges is determined and the capacitance to be measured is determined therefrom.

2. The method according to claim 1, wherein the capacitances of a number of capacitors is determined by carrying out steps a and b together for a plurality of capacitors and carrying out steps c to e simultaneously for these capacitors, using a separate measuring capacitor.

3. Sensor with capacitive measuring device, in which

this measuring device comprises at least one capacitor of variable capacitance, a measuring capacitor is provided for this capacitor,

Means are available with which an electrical voltage can be applied to the capacitor and this voltage can be separated from the capacitor,

- Means are available with which an existing electrical charge on the capacitor can be transferred to the measuring capacitor, and Means are available with which a voltage or a state of charge of the measuring capacitor or a number of charges of the measuring capacitor can be detected, and with which there is an evaluation device which enables it,

- repeatedly charging the capacitor,

- To transfer a charge present on the capacitor to the measuring capacitor and

- to determine the capacitance of the capacitor of the measuring device or a variable to be determined that depends on the voltage across the measuring capacitor or on its charge state or on the number of charges of the measuring capacitor.

4. Sensor according to claim 3, in which the measuring capacitors are formed by field effect transistors, the regions (10) of which are provided for source and drain are short-circuited to one another.

5. Fingerprint sensor

with a metallization level structured into a grid of metal surfaces (4),

- With a number of measuring capacitors (1),

with switches (S), by means of which a proposed electrical potential can be applied to a specified number of metal surfaces and via which each of these metal surfaces can be connected in an electrically conductive manner to a measuring capacitor,

- with a passivation layer on this metallization level,

- With a contact surface (6) for a fingertip on this passivation layer and

- With a circuit for determining the voltages or the state of charge of the measuring capacitors.

6. Fingerprint sensor according to claim 5, wherein the circuit each comprises a comparator (2) which is provided for comparing the voltage at the relevant measuring capacitor with a reference voltage.

7. Fingerprint sensor according to claim 6, in which a counting device (3) is provided which is intended to determine a number of charging processes for each measuring capacitor until the reference voltage is reached.

8. Fingerprint sensor according to claim 5, wherein the circuit comprises a plurality of comparators, which are intended to compare the voltage across the measuring capacitor in question with a scale of reference voltages.

9. Fingerprint sensor according to one of claims 5 to 8, in which the measuring capacitors are formed by field effect transistors whose regions (10) intended for source and drain are short-circuited to one another.

10. Fingerprint sensor according to one of claims 5 to 9, in which the capacitances of the measuring capacitors between lOO times and lOOO times is greater than the capacitance which results when one of the metal surfaces is supplemented with a capacitor arranged in the plane of the bearing surface to form a capacitor .