JP2001512836A - Method for determining very small capacitances and sensors envisaged thereby - Google Patents

Abstract

(57) [Abstract] A small-capacitance capacitor is charged multiple times and discharged into a significantly larger-capacity measurement capacitor, and the voltage or charge state of the measurement capacitor is directly measured. A small capacitance or capacitance change as a result of the sensor measurement can thus be determined with high accuracy. Thus, the fingerprint sensor provided with an array of metal surfaces (4) below the mounting surface for the fingertip and provided with a transistor as a switch (S), a measuring capacitor (1) and a comparator (2) is a movable part. Feasible without. A predetermined potential is applied to the metal surface. When the fingertip is placed, the metal surface is charged to different heights according to the structure of the skin. The state of charge of the measuring capacitors is detected separately from one another according to the type of reading of the matrix memory.

Description

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method for determining a very small capacitance, which is suitable, for example, for evaluating measurement results in a micromechanical sensor.

[0002] For example, a micromechanical pressure sensor (EP0714017 = US5631)
428, WO 96/16319) or an acceleration sensor (WO 95/1957).
In a sensor for capacitive measurement such as that described in U.S. Pat. There is a problem of seeking. The measured value of capacity is distorted by the amount of disturbance. A variable disturbance quantity can come, for example, from an electromagnetic alternating magnetic field around the measuring device or be caused by mechanical vibrations. The amount of such an obstacle is
It must be compensated in a suitable manner or at least detected so that the measurement result can be corrected accordingly.

In order to determine the size or amount of a small capacitance, a sensitive amplifier is used. In the measuring process, a high-frequency alternating voltage or pulse voltage causes a capacitance to generate a measuring signal. The impairment signal is captured and detected directly by the measuring amplifier or is suppressed or quantitatively compensated after A / D conversion. This method is very complicated and requires high accuracy.

The publication “A 390dpi Live Fingerprint Imag” by M. Tartagni, R. Guerrieri
er Based on Feedback Capacitive Sensing Scheme ”(1997 ISSCC slide suppl
ement, p. 154-155, 402), provides an overview of the various possible implementations of fingerprint sensors. Sensors of this type are suitable for capturing and detecting fingerprints, i.e. diagrams with grooves and ridges in the skin of the fingertip, in an electronic manner and, if necessary, for further evaluation for personal identification purposes. With the capacitively measuring sensor shown there, the problem arises that very small capacitances have to be measured as well. This measurement must be performed in a very short time when the fingertip is resting on the sensor and is sufficiently accurate and contrast-rich so that different fingerprint lines can be distinguished from each other. The result should be provided.

[0005] EP 0 435 398 B1 describes a fingerprint sensor in which a finger is placed on a mounting surface on which a finger cannot be deformed and the capacitance generated by the sensor for an arrangement consisting of a metal surface is used to determine a fingerprint. ing. To this end, an amplifier and a switch are connected to each of the metal surfaces. This type of circuit is susceptible to faults.

It is an object of the present invention to provide a method for determining a very small capacity. Another problem is to provide a sensor that measures capacitively, for example, a sensor for recording fingerprints, which sends out accurate measurement results even when the capacitance changes only slightly. The method is solved by the features of claim 1, the sensor by the features of claim 3 and the fingerprint sensor by the features of claim 5. Respective embodiments are evident from the dependent claims.

In the method according to the invention, the capacitance to be measured is determined by first applying a voltage to the capacitor in question or the element acting as a capacitor and thus charging the capacitor. The capacitor is discharged into a significantly larger capacitor provided for this purpose, which is referred to below as the measuring capacitor. This process of charging and discharging is repeated several times for the small capacity to be measured. Therefore,
The charge is collected intermittently, ie in batches, in the measuring capacitor and can then be evaluated. This means that the voltage on the measuring capacitor is measured after a predetermined number of charging steps (charges to be charged together) or to charge the measuring capacitor to a predetermined voltage. This is done by either counting the number of charging steps required. In the former case, the voltage obtained after a predetermined number of charging steps is proportional to the magnitude of the capacity to be determined. In the latter case, the number of charging steps is inversely proportional to the size or amount of capacity to be determined.

In a sensor in which the distance between two electrodes functioning as capacitor electrodes changes depending on a measured amount, the capacitance to be measured is inversely proportional to the distance between the electrodes. There, the number of charging processes of the measuring capacitor (collectively charged charges) is therefore directly proportional to the distance to be measured, which simplifies the evaluation of the measurement result. The method according to the invention is therefore particularly advantageous in sensors of this type which may be embodied, for example, as micromechanical sensors, in that it immediately emits the desired digital result value and can subsequently process it quantitatively. It has special advantages. The disturbing effects of the measurements mentioned at the outset can be suppressed or quantitatively compensated.

This method acts as an integrator for charge storage. This has the property of suppressing high frequency interference. Low-frequency disturbances can be detected and calculated by numerical methods known per se.

The method is particularly suitable for use in capacitively measuring micromechanical sensors such as, for example, pressure sensors or acceleration sensors. In a pressure sensor of this type, an external pressure-dependent deformation of a conductive diaphragm (for example, at least partially conductively doped polysilicon) is applied to the opposite sides of the cavity via the cavity. The diaphragm is determined in such a way that the capacitance of the diaphragm changed with respect to the counter electrode (conductively doped region in the semiconductor material) present on the side. Because of the inclusion of small capacitances and capacitance variations, the method of the present invention offers the ability to improve the measurement accuracy of this type of sensor far beyond what has been previously achievable. .

The method according to the invention is particularly suitable for realizing a fingerprint sensor with a particularly simple configuration but with satisfactory functionality. In the fingerprint sensor of the present invention, a capacitive measuring method is similarly applied. The measurement is performed without mechanical deformation of the sensor part using an arrangement of metal surfaces, each forming an electrode of a capacitor. These metal surfaces are covered with a dielectric inactive layer, on which a finger is placed. Grooves and bumps in the skin form different volumes with the metal surface. The skin can be considered sufficiently high conductivity and is actually at earth potential. When the metal surfaces are placed at a given potential, they are charged with different magnitudes. The charge is separately supplied to a relatively large capacitor as a measuring electrode. The metal surface is charged several times and the charge is collected in each case on the measuring capacitor. In this way, it is possible to accumulate such a number of 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 detect a fingerprint, the state of charge of the metal surface is, for example,
For example, it is detected in such a way that the matrix memory is read. When the metal surfaces are arranged in a matrix, the metal surfaces are simultaneously charged to the same potential for each column. This corresponds to bias charging the memory array of transistors via the word lines. The discharge of the charged metal surface takes place row by row in the measuring capacitors present for each row.

An embodiment of the method according to the invention and the sensor envisaged thereby, in the form of a fingerprint sensor, will now be described in detail with reference to FIGS.

1 and 2 schematically show circuit parts suitable for the method of the present invention, FIG. 3 shows a transistor element that can be used in the circuit as a capacitor, and FIG. 4 shows a sensor device in a plan view. FIG. 5 schematically shows a part of the sensor device in a plan view, and FIG. 6 schematically shows a part of the sensor device in a cross section.

The method of the present invention can be performed by a circuit that includes the components illustrated in FIG. The charge stored in the capacitor to be measured in each charging process is reduced by switching off the charging voltage via a switch and electrically connecting the capacitor whose capacitance is to be measured to the electrode of the measuring capacitor 1 in each case. Is transferred to the measuring capacitor 1 assigned to. The voltage drop across the charged measuring capacitor that occurs in this capacitor is represented by the arrow shown. The comparator 2 is provided for comparing the voltage generated in the measuring capacitor 1 with the reference voltage Uref, and in that case, it can detect that the measuring capacitor has reached this reference voltage.

FIG. 2 shows an alternative circuit, in which a comparator is provided for detecting how long after charging the capacitor a predetermined reference voltage is generated. The output signal of the comparator 2 is supplied via a clock pulse Tch to the input of a controlled counting device 3. This counting device counts the number of times of charging required until the capacitor 1 reaches the reference voltage Uref. In that case, the number of times of charging is inversely proportional to the capacitance of the capacitor to be measured.

The necessary switches are advantageously formed by transistors. Via this switch, the capacitor whose capacitance is to be determined simultaneously in the application cycle of the method is simultaneously charged to a predetermined potential, for example 5V. The switch is opened to shut off the voltage source. At the same time, further switches are closed, via which the capacitors are connected to the respective measuring capacitors. The measuring capacitor can be precharged if necessary at the beginning of the measuring cycle, for example to a bias voltage of typically 1 V, via suitable additional circuitry. After the charge has flowed through the measuring capacitor, the existing switch is operated again, so that the capacitor to be measured is charged again. The new switching causes a charge to flow through the measuring capacitor, and the charge is thus collected there.

When the switches used in the circuit are realized as transistors, it is advantageous to use transistors also for the measuring capacitors. FIG. 3 shows a cross section of such a device which can be used as a capacitor. The MOSFET having the isolation layer 7, the gate electrode 8 above the channel region 11, and the region 10 for the source and the drain, which is diffused in the semiconductor material, has the region 10 for the source and the drain electrically connected to each other. That is, the contact forming means is provided so as to be short-circuited. This contact 9 is shown in FIG. During operation of the device, an inversion layer is formed in the channel region 11.
The isolation layer 7 forms the dielectric of the 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 a fingerprint sensor implemented using the method of the present invention, in an advantageous embodiment:
There is a structured metallization layer, which has a number of metal surfaces arranged in a grid. These planes are arranged, for example, in a double grid type grid as shown in FIG. FIG. 4 schematically shows the individual metal surfaces 4 in plan view. The lateral area 5 is used to accommodate the necessary circuit electronics. Above the structured metal surface is a dielectric passivation layer, above which is the resting surface for the fingertip. The thickness of the inert layer, along with the skin of the fingertip,
Determines the magnitude of the capacitance formed by The metal surface may be, for example, a metallized surface on a semiconductor chip on which control electronics or other components are integrated.

The circuit schematic shown in FIG. 1 for explaining the measuring method is used accordingly in a fingerprint sensor. A schematic for the interconnection of a device comprising a metal surface 4 is shown in FIG. A measuring capacitor 1 provided for collecting charge is present for each row of the metal surface 4. The capacitance of these measuring capacitors is
An electrical conductor (eg, a conductive skin surface) that is located at the level of the mounting surface
Together with the capacitance produced when forming a capacitor, which is 100 times
It is between 0 times. Figure 5 is, for each of the metal surface shown in the illustrated portion of the device, switches S ₁ and S ₂ belong is shown. These switches are preferably formed by transistors. Via a switch S ₁ which is the charging line L and closed, depending on the type of the word lines in the matrix memory, the voltage of all of the metal surfaces that are present on the column of the apparatus (or row) is set It is charged, whereby the switch S ₂ is maintained at the open state. Depending on the differently present capacities, the metal surface is charged with different magnitudes. After the switch S ₁ is now ready to be opened and the switch S ₂ is closed, the charge is assigned to each row (or column) of a metal surface via the discharge line R for each row (or Retsugoto) And collected there.

The voltage at the measuring capacitor is measured using a comparator 2 connected downstream, which compares the voltage developed at the capacitor 1 with a reference voltage Uref. If only a rough measurement is to be performed, it may be sufficient for the comparator to detect when the measuring capacitor 1 exceeds a predetermined voltage. In that case, a fingerprint diagram is obtained which is black or white at each point. When different gray shades are desired, the connection terminals of the measuring capacitor 1 can be guided to various inputs of the comparator. In this case, the comparator converts the voltage to A / D
Depending on the type of converter, different reference voltages of different magnitudes can be compared and in this way the different states of charge of the individual capacitors can be determined stepwise.

The number of charging steps to reach a predetermined voltage at the measuring electrode (total charge flowing into the measuring capacitor in each operation) is determined by the distance to be measured between the metal surface and the main surface Is directly proportional to Therefore, when the measurement method is applied to a fingerprint sensor using the device of FIG. 2, a digital result of the measurement is immediately obtained, which can be processed immediately and quantitatively. Interference effects that can occur during the measurement can be suppressed or compensated quantitatively. In the exemplary embodiment according to FIG. 1, a counter is also provided, which is used to detect a certain duration, after which time the continuous charging process is interrupted and the charge present at the individual measuring electrodes 1 Or the voltage can be measured.

The entire circuit in the fingerprint sensor can be realized by microelectronic elements, for example, transistors as described above. For direct control of the metal surface and the measuring capacitor, so that the metal surface arranged in a grid shape used for the measurement can be provided as close as possible to the surface on which the finger rests Unnecessary electronic parts are arranged on the sides of this grid-shaped arrangement. In the schematic plan view of FIG. 4, this example is shown as a square,
The arrangement of the metal surface 4 is shown. On its side, the electronic components are arranged in an adjacent area 5. Therefore, the electronic circuits used for evaluation of the measurement results or for further processing can be integrated on a single chip together with the fingerprint sensor. This results in shorter measurement times and reduced disturbance effects. Typical dimensions for the metal surface 4 are approximately 50 μm × 50 μm to about 100 μm × 100 μm. The entire device on which the fingerprint sensor is mounted is approximately 13 mm x 13 mm to 15 mm
It has a size of × 15 mm.

FIG. 6 schematically shows a cross section of the fingerprint sensor. The metal surface 4 is below the inert layer, and the upper surface 6 of the inert layer is used as a mounting surface for a fingertip. The metal surface 4 is commonly charged to a predetermined potential via a suitable switch (not shown in this simplified diagram for clarity). Discharge and metal surfaces, therefore switches are provided for the collection of individual charges of the measurement capacitor 1 is provided in S (corresponding to the switch S ₂ in FIG. 5) is a diagram in the form of a circuit diagram 6 is shown schematically in the lower part. The evaluation of the charge collected in the measuring capacitor 1 is performed in the comparator 2. A number of comparators are illustratively shown in the device section. The remaining circuit components are connected to the output of the comparator.

What is important for the fingerprint sensor of the invention is that, in accordance with the described measuring method, rather than directly amplifying small charges, they accumulate them in relatively large capacitors, which makes them relatively simple. This means that the small charge is enhanced so that it can be detected by any conventional measuring method. A particular advantage of this concept is that the measuring process directly outputs a digital output signal, so that it can be processed in an efficient manner.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a circuit portion suitable for the method of the present invention.

FIG. 2 is a schematic diagram of a circuit portion suitable for the method of the present invention.

FIG. 3 is a schematic diagram of a transistor element that can be used in a circuit as a capacitor.

FIG. 4 is a schematic plan view of the sensor device.

FIG. 5 is a schematic plan view of a part of the sensor device.

FIG. 6 is a schematic cross-sectional view of a part of the sensor device.

[Procedural Amendment] Submission of translation of Article 34 Amendment of the Patent Cooperation Treaty

[Submission date] February 3, 2000 (200.2.3)

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] Claims

[Correction method] Change

[Correction contents]

[Claims]

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Thomas Scheider Oberhaching Fresserweg, Germany 13 (72) Inventor Stephan Marksteiner, Germany Munich Gustav-Heinemann-Ring 39 F-term (reference) 2F055 AA40 BB20 CC60 DD20 EE25 FF11 GG31 2G028 AA01 BB06 CG07 DH03 FK01 FK08 GL07

Claims (10)

[Claims] 1. a) applying a predetermined voltage to a capacitor whose capacitance is to be measured and charging it thereby; b) disconnecting said capacitor from said charged voltage; c) connecting the capacitor to the measuring capacitor so that the charge present in the capacitor flows out of the measuring capacitor; d) until a predetermined charging process is performed on the measuring capacitor; or Steps a) to c) until the voltage set on the measuring capacitor is reached
E) measuring the voltage or the number of charging steps in the measuring capacitor and determining therefrom the capacitance to be measured. 2. The steps a) and b) are performed in common for a plurality of capacitors and the steps c) to e) are performed simultaneously on the plurality of capacitors using separate measuring capacitors. The method of claim 1, wherein the performing measures the capacitance of a number of capacitors. 3. A sensor having a capacitive measuring device, said measuring device comprising at least one capacitor of variable capacitance, wherein one measuring capacitor is present for said capacitor, Means for applying a voltage to the capacitor and disconnecting the voltage from the capacitor are provided; means for transferring the charge present in the capacitor to the measuring capacitor are provided; and Means are provided for detecting the voltage or state of charge of the capacitor or the number of charges performed on the measuring capacitor, and an evaluation circuit is provided, by which the capacitor is repeatedly discharged, The charge present in the capacitor is transferred to the measuring capacitor each time,
From the voltage at the measuring capacitor or from its state of charge or the number of charging steps of the measuring capacitor depends on the capacity of the capacitor of the measuring device or on it,
A sensor that requires the quantity to be measured. 4. The sensor according to claim 3, wherein the measuring capacitor is formed by a field-effect transistor whose regions provided for the source and the drain are short-circuited to each other. 5. A metallization plane having a metal surface (4) structured in a grid-like manner, comprising a plurality of measuring capacitors (1), comprising a switch (S) and set via said switch. A potential can be applied in each case to a set number of metal surfaces and each of the metal surfaces can be connected to one measuring capacitor, comprising one inert layer in the metallization plane A fingerprint sensor comprising: a surface (6) for placing a fingertip on the inactive layer; and a circuit for measuring a voltage or a state of charge of the measurement electrode. 6. The fingerprint sensor according to claim 5, wherein each of the circuits has a comparator (2) for comparing the voltage at the measuring capacitor with a reference voltage. . 7. The fingerprint sensor according to claim 6, wherein a counting device is provided for measuring the number of charging steps performed on each measuring capacitor before reaching the reference voltage. 8. The fingerprint sensor according to claim 5, wherein each of the circuits includes a plurality of comparators, and the comparator is configured to compare a voltage at the measurement capacitor with a scale of a reference voltage. 9. The measuring capacitor according to claim 5, wherein the region provided for the source and the drain is formed by a field-effect transistor in which the region is short-circuited to each other. A fingerprint sensor as described. 10. The capacitance of the measuring capacitor is 100 to 1000 times greater than the capacitance that occurs when one of the metal surfaces forms a capacitor with a conductor arranged in the plane of the mounting surface. 10. The fingerprint sensor according to any one of items 1 to 9.