Capacitive fingerprint sensing device and method for capturing a fingerprint using the sensing device

1. A capacitive fingerprint sensing device for sensing a fingerprint pattern of a finger, said capacitive fingerprint sensing device comprising:

a protective dielectric top layer having an outer surface forming a sensing surface to be touched by said finger;

a two-dimensional array of electrically conductive sensing structures arranged underneath said top layer and configured for use in capturing a fingerprint image based on a capacitive coupling with the finger;

readout circuitry coupled to each of said electrically conductive sensing structures to receive a sensing signal, based on a capacitive coupling between the finger and a respective sensing structure, indicative of a distance between said finger and said sensing structure; and

an electroacoustic transducer arranged underneath said top layer and configured to generate an acoustic wave, and to transmit the acoustic wave through the protective dielectric top layer towards the sensing surface to induce an ultrasonic vibration potential in a ridge of finger placed in contact with the sensing surface.

2. The sensing device according to claim 1 , wherein the electroacoustic transducer is an ultrasonic transmitter configured to generate an ultrasonic wave.

3. The sensing device according to claim 1 , wherein the electroacoustic transducer is a planar electroacoustic transducer.

4. The sensing device according to claim 1 , wherein the electroacoustic transducer is configured such that the transmitted acoustic wave is a plane wave.

5. The sensing device according to claim 1 , wherein the top layer is configured to have an acoustic impedance matching an acoustic impedance of a finger.

6. The sensing device according to claim 1 , wherein the electroacoustic transducer is a piezoelectric transducer.

7. The sensing device according to claim 6 , wherein the electroacoustic transducer is a Piezoelectric Micromachined Ultrasonic Transducer, PMUT.

8. The sensing device according to claim 1 , wherein the electroacoustic transducer is a Capacitive Micromachined Ultrasonic Transducer, CMUT.

9. The sensing device according to any claim 1 , wherein the electroacoustic transducer is of the same size as the array of sensing structures.

10. The sensing device according to claim 1 , comprising a plurality of electroacoustic transducers, each electroacoustic transducer having an area corresponding to an area of a sub-array of the array of sensing structures.

11. The sensing device according to claim 1 , wherein the array of sensing structures is arranged between the transducer and the protective dielectric top layer.

12. The sensing device according to claim 11 , further comprising a delay layer arranged between the electroacoustic transducer and the array of sensing structures.

13. The sensing device according to claim 12 , wherein the delay layer comprises a plastic material or PMMA.

14. The sensing device according to claim 1 , further comprising a shielding layer arranged between the electroacoustic transducer and the array of sensing structures to electrically shield the array of sensing structures from the electroacoustic transducer.

15. The sensing device according to claim 14 , wherein the shielding layer comprises an electrically conductive structure connected to ground potential.

16. The sensing device according to claim 1 , wherein the electroacoustic transducer is arranged between the array of sensing structures and the protective top layer.

17. A method for controlling a capacitive fingerprint sensing device comprising:

a protective dielectric top layer having an outer surface forming a sensing surface to be touched by a finger;

a two-dimensional array of electrically conductive sensing structures arranged underneath said top layer;

readout circuitry coupled to each of said electrically conductive sensing structures to receive a sensing signal, based on a capacitive coupling between the finger and a respective sensing structure, indicative of a distance between said finger and said sensing structure; and

an electroacoustic transducer arranged underneath said top layer, the method comprising:

activating the electroacoustic transducer, generating an acoustic wave, and transmitting the acoustic wave through the protective dielectric top layer towards the sensing surface to induce an ultrasonic vibration potential in a ridge of a finger placed in contact with the sensing surface; and

capturing a main fingerprint image by reading out a capacitive coupling between the finger and the sensing structures by means of the readout circuitry.

18. The method according to claim 17 , further comprising:

before the step of activating the electroacoustic transducer, capturing an initial fingerprint image;

comparing the initial fingerprint image with the main fingerprint image; and

if the difference between the initial and the main fingerprint image is larger than a predetermined threshold, determining that the fingerprint image originates from an authentic finger.

19. The method according to claim 17 , further comprising:

before the step of activating the electroacoustic transducer, capturing an initial fingerprint image;

comparing the initial fingerprint image with the main fingerprint image; and

if the difference between the initial and the main fingerprint image is smaller than a predetermined threshold, determining that the fingerprint image originates from a fake finger.

20. A method for controlling a capacitive fingerprint sensing device comprising:

a protective dielectric top layer having an outer surface forming a sensing surface to be touched by a finger;

a two-dimensional array of electrically conductive sensing structures arranged underneath said top layer;

readout circuitry coupled to each of said electrically conductive sensing structures to receive a sensing signal, based on a capacitive coupling between the finger and a respective sensing structure, indicative of a distance between said finger and said sensing structure; and

an electroacoustic transducer arranged underneath said top layer, the method comprising:

activating the electroacoustic transducer, generating an acoustic wave, and transmitting the acoustic wave through the protective dielectric top layer towards the sensing surface to induce an ultrasonic vibration potential in a ridge of a finger placed in contact with the sensing surface;

deactivating the electroacoustic transducer; and

while the ultrasonic vibration potential in the finger is detectable, capturing a fingerprint image by reading out a capacitive coupling between the finger and the sensing structures by means of the readout circuitry.