Capacitive fingerprint sensor

• US 20030190061A1
• Filed: 04/01/2003
• Published: 10/09/2003
• Est. Priority Date: 04/03/2002
• Status: Active Grant

• Current Assignee: Egis Technology Inc.
• Original Assignee: Lightuning Tech Incorporated
• Inventors: Chang, Ben, Chou, Bruce C.S., Cheng, Wallace Y.W.
• Granted Patent: US 7,099,497 B2
• Time in Patent Office: Days
• US Class Current: 382/124
• CPC Class Codes: G06V 40/1306 non-optical, e.g. ultrasonic or capacitive sensing

Abstract

A capacitive fingerprint sensor includes a plurality of capacitive sensing members arranged in a 2D array. A charge-sharing principle is utilized to read signals in the capacitive sensing member. Each capacitive sensing member includes an insulating surface layer, a sense electrode, a reference electrode, a reference capacitor, and a signal reading circuit. The sense electrode is below the insulating surface layer. When a finger contacts the insulating surface layer, a sense capacitor is formed therebetween. The reference capacitor is connected between the reference and sense electrodes. The signal reading circuit is connected to the sense and reference electrodes, repeatedly provides control signals to charge the reference capacitor and then enable the reference and sense capacitor to share charges. Finally, the sense voltage of the sense electrode may be read.

Claims

1. A capacitive fingerprint sensor, comprising:

a capacitive sensing members array including a plurality of capacitive sensing members;

an insulating surface layer covering over the sensing members array to serve as a contact surface for a finger; and

a basic peripheral circuit, comprising;

a row decoder arranged beside the sensing members array;

a plurality sets of control lines inserted, in parallel, into the sensing members array and connected to the row decoder;

a column multiplexer arranged beside the sensing members array and at a side perpendicular to the row decoder;

a plurality of signal lines, which is vertically inserted into the sensing members array to intersect the control lines and is connected to the column multiplexer, wherein;

each of the capacitive sensing members comprises a sensor structure and a signal reading circuit, the sensor structure includes a sense electrode, a reference electrode, and an insulating layer between the sense and reference electrodes, the signal reading circuit is arranged beside the sensor structure to connect the sensor structure to the corresponding control and signal lines, a sense capacitor is formed between the finger and the sense electrode, and a reference capacitor is formed between the sense and the reference electrodes; and

the basic peripheral circuit repeatedly inputs, through the control lines;

a set of the same first and second control voltages to the signal reading circuit for reset function to enable two terminals of the reference capacitor to have voltage difference and two terminals of the sense capacitor to have no voltage difference; and

another set of the same third and fourth control voltages and a pulse control voltage to the signal reading circuit so as to enable the sense capacitor and the reference capacitor to share charges and to read sense signals.

2. The fingerprint sensor according to claim 1, wherein the basic peripheral circuit further comprises:

an analog signal processing unit comprising a programmable gain amplifier and an analog-to-digital converter for amplifying and converting voltage signals output from the column multiplexer into digital gray-scale image data, respectively.

3. The fingerprint sensor according to claim 1, wherein the signal reading circuit comprises:

a reset switch for controlling charging of the reference capacitor and cleaning residual charges of the sense capacitor.

a shutter switch for controlling connection between the sense capacitor and the reference capacitor, wherein the sense capacitor and the reference capacitor share the charges at a moment when the shutter switch is turned on and then off; and

an NMOS source follower serving as a buffer for coupling balanced voltage after charge-sharing between the sense and reference capacitors to the signal lines.

4. The fingerprint sensor according to claim 1, wherein the peripheral circuit further comprises:

a comparator connected to the column multiplexer for comparing the voltage signals output from the column multiplexer with a reference voltage of the comparator, and then outputting binary image data of 0 and 1.

5. The fingerprint sensor according to claim 1, further comprising a mesh metal film arranged on the insulating surface layer and connected to ground to avoid ESD damage.

6. The fingerprint sensor according to claim 1, further comprising a mesh metal film arranged below the insulating surface layer and connected to ground, wherein a part of the insulating surface layer is removed to expose a part of the metal film for electrostatic discharge therethrough so as to avoid ESD damage.

7. The fingerprint sensor according to claim 1, further comprising a trigger switch for turning on a sensor power after the sensor is judged to be in contact with the finger.

8. The fingerprint sensor according to claim 7, wherein the trigger switch comprises:

a trigger electrode;

a trigger dielectric layer arranged on the trigger electrode; and

a trigger phase inverter, wherein when the finger contacts the trigger switch, a trigger capacitor is formed between the finger and the trigger electrode, the trigger capacitor and a storage capacitor below the trigger electrode share charges to decrease a voltage of the trigger electrode below a reverse voltage for inverting the trigger phase inverter, and then to turn on the sensor power of the capacitive fingerprint sensor.

9. The fingerprint sensor according to claim 8, further comprising a layer of barium titanate, strontium titanate, silicon carbide or tantalum oxide formed on the trigger dielectric layer.

10. The fingerprint sensor according to claim 1, further comprising a layer of barium titanate, strontium titanate, silicon carbide or tantalum oxide formed on the insulating surface layer.