Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R27/00—Arrangements for measuring resistance, reactance, impedance, or electric characteristics derived therefrom
  • G01R27/02—Measuring real or complex resistance, reactance, impedance, or other two-pole characteristics derived therefrom, e.g. time constant
  • G01R27/26—Measuring inductance or capacitance; Measuring quality factor, e.g. by using the resonance method; Measuring loss factor; Measuring dielectric constants ; Measuring impedance or related variables
  • G01R27/2605—Measuring capacitance
• • G—PHYSICS
  • G06—COMPUTING; CALCULATING OR COUNTING
  • G06V—IMAGE OR VIDEO RECOGNITION OR UNDERSTANDING
  • G06V40/00—Recognition of biometric, human-related or animal-related patterns in image or video data
  • G06V40/10—Human or animal bodies, e.g. vehicle occupants or pedestrians; Body parts, e.g. hands
  • G06V40/12—Fingerprints or palmprints
  • G06V40/13—Sensors therefor
  • G06V40/1306—Sensors therefor non-optical, e.g. ultrasonic or capacitive sensing

Definitions

• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• the voltage reached after a predetermined number of charges is proportional to the size of the capacitance to be determined.
• the number of charges is inversely proportional to the size of the capacity to be determined.
• the capacitance to be measured is inversely proportional to the distance between the electrodes.
• 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.
• the deformation of an electrically conductive membrane for example made of at least partially electrically conductive doped polysilicon
• Counterelectrode e.g. an electrically conductive doped area in semiconductor material
• 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.
• 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.
• 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.
• 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
• 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 .
• T ch the clock pulse
• 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.
• 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.
• 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.
• 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.
• 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.
• FIG. 1 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).
• 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.
• 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 •

Landscapes

• Engineering & Computer Science (AREA)
• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Human Computer Interaction (AREA)
• Multimedia (AREA)
• Theoretical Computer Science (AREA)
• Measurement Of Length, Angles, Or The Like Using Electric Or Magnetic Means (AREA)
• Image Input (AREA)
• Measuring Fluid Pressure (AREA)
• Measurement Of Resistance Or Impedance (AREA)

Priority Applications (5)

Applications Claiming Priority (2)

Publications (1)

Family

ID=7838065

Family Applications (1)

Country Status (7)

Cited By (2)

Families Citing this family (9)

Citations (3)

• 1998
  • 1998-07-23 KR KR1020007001166A patent/KR20010022575A/ko not_active Application Discontinuation
  • 1998-07-23 BR BR9811837-4A patent/BR9811837A/pt not_active Application Discontinuation
  • 1998-07-23 JP JP2000506535A patent/JP2001512836A/ja active Pending
  • 1998-07-23 EP EP98945041A patent/EP1000362A1/de not_active Withdrawn
  • 1998-07-23 WO PCT/DE1998/002078 patent/WO1999008121A1/de not_active Application Discontinuation
  • 1998-07-23 MX MXPA00001276A patent/MXPA00001276A/es unknown
  • 1998-07-23 CN CN98808036A patent/CN1266494A/zh active Pending

Patent Citations (3)

Non-Patent Citations (1)

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