Capacitive finger detection for fingerprint sensor

• US 6,501,284 B1
• Filed: 08/28/2000
• Issued: 12/31/2002
• Est. Priority Date: 08/28/2000
• Status: Expired due to Term

• Current Assignee: Apple Inc.
• Original Assignee: STMicroelectronics Incorporated (STMicroelectronics NV)
• Inventors: Gozzini, Giovanni
• Primary Examiner(s): Le, N.
• Assistant Examiner(s): Deb, Anjan K.
• Application Number:
  US09/648,968
• Time in Patent Office: 855 Days
• Field of Search: 324/681, 324/686, 324/661, 324/662, 324/658, 324/663, 324/667, 324/668, 324/672, 324/688
• US Class Current: 324/681
• CPC Class Codes: G06V 40/1306 non-optical, e.g. ultrasonic or capacitive sensing
• CPC Class Codes: G06V 40/40 Spoof detection, e.g. liveness detection

Abstract

Within a capacitive fingerprint detection device, finger detection is provided by a capacitive grid overlying the fingerprint sensor electrodes to measure the absolute capacitance of the finger placed on the sensor surface. The capacitive measurement is converted to a representative frequency, which is then compared to a reference frequency or frequency range to determine whether the measured capacitance matches the expected bio-characteristics of living skin tissue. The finger detection thus provides anti-spoofing protection for the fingerprint detection device.

Claims

1. A method of finger detection, comprising:

measuring a capacitance between a capacitive grid within a fingerprint detection device and a finger placed on a surface of the fingerprint detection device, wherein the capacitive grid is independent from capacitive sensor electrodes employed within the fingerprint detection device to detect fingerprint features;

converting the measured capacitance to a corresponding, representative frequency; and

comparing the frequency representing the measured capacitance to a frequency range corresponding to living skin tissue to determine whether a finger has been placed on the surface of the fingerprint detection device.

2. The method of claim 1, wherein the step of measuring a capacitance between a capacitive grid within a fingerprint detection device and a finger placed on a surface of the fingerprint detection device further comprises:

measuring the capacitance at a capacitive grid replacing selected capacitive sensor electrodes or over the capacitive sensor electrodes.

3. The method of claim 1, wherein the step of measuring a capacitance between a capacitive grid within a fingerprint detection device and a finger placed on a surface of the fingerprint detection device further comprises:

measuring the capacitance at a capacitive grid on the surface of the fingerprint detection device.

4. The method of claim 1, wherein the step of comparing the frequency representing the measured capacitance to a frequency range corresponding to living skin tissue to determine whether a finger has been placed on the surface of the fingerprint detection device further comprises:

comparing the frequency representing the measured capacitance to a frequency range corresponding to capacitive bio-characteristics of living skin tissue.

5. The method of claim 1, wherein the step of comparing the frequency representing the measured capacitance to a frequency range corresponding to living skin tissue to determine whether a finger has been placed on the surface of the fingerprint detection device further comprises:

comparing the frequency representing the measured capacitance to a threshold frequency; and

determining that a finger has been placed on the surface of the fingerprint detection device if the frequency representing the measured capacitance exceeds the threshold frequency.

6. The method of claim 1, wherein the step of comparing the frequency representing the measured capacitance to a frequency range corresponding to living skin tissue to determine whether a finger has been placed on the surface of the fingerprint detection device further comprises:

comparing the frequency representing the measured capacitance to a predefined frequency range; and

determining that a finger has been placed on the surface of the fingerprint detection device if the frequency representing the measured capacitance falls within the predefined frequency range.

7. The method of claim 1, further comprising:

generating a signal indicating whether a finger has been placed on the surface of the fingerprint detection device.

8. Anti-spoofing protection for a fingerprint detection device, comprising:

a capacitive grid independent from capacitive sensor electrodes employed within the fingerprint detection device to detect fingerprint features;

a capacitance-to-frequency converter detecting a capacitance between the capacitive grid and a finger on a surface of the fingerprint detection device and generating a corresponding, representative frequency; and

a frequency comparator comparing the frequency representing the capacitance to a frequency range corresponding to capacitive bio-characteristics of living skin tissue to determine whether a finger has been placed on the surface of the fingerprint detection device.

9. The anti-spoofing protection of claim 8, wherein the capacitive grid replaces selected capacitive sensor electrodes.

10. The anti-spoofing protection of claim 8, wherein the capacitive grid is either between the capacitive sensor electrodes and the surface of the fingerprint detection device or on the surface of the fingerprint detection device.

11. The anti-spoofing protection of claim 8, further comprising:

an oscillator generating at least one reference voltage employed by the frequency comparator to determine whether a finger has been placed on the surface of the fingerprint detection device.

12. The anti-spoofing protection of claim 11, wherein the oscillator generates a threshold frequency defining a minimum frequency corresponding to converted capacitive bio-characteristics of living skin tissue.

13. The anti-spoofing protection of claim 11, wherein the oscillator generates a pair of frequencies defining a frequency range corresponding to converted capacitive bio-characteristics of living skin tissue.

14. The anti-spoofing protection of claim 8, wherein the frequency comparator generates a signal indicating whether a finger has been placed on the surface of the fingerprint detection device.

15. A fingerprint detection device, comprising:

capacitive sensor electrodes employed to detect fingerprint features;

a capacitive grid independent of end between the capacitive sensor electrodes and a surface of the fingerprint detection device on which a finger is placed for detection of a fingerprint;

a capacitance-to-frequency converter converting a capacitance between the capacitive grid and a finger on the surface of the fingerprint detection device into a corresponding, representative frequency; and

a frequency comparator comparing the frequency representing the capacitance to a frequency range corresponding to capacitive bio-characteristics of living skin tissue to determine whether a finger has been placed on the surface of the fingerprint detection device.

16. The fingerprint detection device of claim 15, wherein the capacitive grid is either between selected capacitive sensor electrodes or between the capacitive sensor electrodes and the surface of the fingerprint detection device.

17. The fingerprint detection device of claim 15, wherein the capacitive grid is on the surface of the fingerprint detection device.

18. The fingerprint detection device of claim 15, further comprising:

an oscillator generating at least one reference voltage employed by the frequency comparator to determine whether a finger has been placed on the surface of the fingerprint detection device.

19. The fingerprint detection device of claim 18, wherein the oscillator generates a threshold frequency defining a minimum frequency corresponding to converted capacitive bio-characteristics of living skin tissue.

20. The fingerprint detection device of claim 18, wherein the oscillator generates a pair of frequencies defining a frequency range corresponding to converted capacitive bio-characteristics of living skin tissue.

21. The fingerprint detection device of claim 15, wherein the frequency comparator generates a signal indicating whether a finger has been placed on the surface of the fingerprint detection device.