Thermoelectric sensor for fingerprint thermal imaging
The present invention provides a fingerprint sensing mechanism using a two-dimensional thermoelectric sensor array to capture the thermal image related to the ridges and valleys on the finger, wherein its fabricating method is totally compatible with integrated circuits processing. Using the body temperature of a human being as the stimulation source for biometrics, a temperature difference is produced from a ridge of a fingerprint contacting the thermoelectric sensor and the temperature gradient is converted into an electrical signal. A plurality of thermoelectric sensors arranged in a two-dimensional array forms a fingerprint sensor so as to obtain the electrical signal output of the ridge profile of the fingerprint.
1. A fingerprint sensor comprising:
a plurality of thermoelectric sensors arranged in a two-dimensional array including a plurality of signal processing circuits on a single chip, said chip having a hot-junction region and a cold-junction region wherein said cold-junction region surrounds said hot-junction region, whereby a ridge of a fingerprint produces a temperature gradient between said hot-junction region and said cold-junction region when said ridge of said fingerprint closes or contacts said thermoelectric sensor, said thermoelectric sensor further converting said temperature gradient into an electrical voltage signal;
a silicon substrate;
a field oxide layer or a trench isolation layer on said silicon substrate acting as a thermal-isolation structure;
at least a thermocouple connected in series to form a thermopile, wherein said hot-junction region of said thermopile is located at a central portion of said field oxide layer and said cold-junction region is located on a thin oxide layer which is surrounding said field oxide layer; and
a heat pipe structure comprising at least an interconnection layer and at least a via hole metal, wherein said heat pipe structure is located between said central portion of said field oxide layer and a passivation layer which is on a surface of said substrate.
2. The fingerprint sensor chip according to claim 1 , further comprising a thermo-electrical cooler arranged under said fingerprint sensor chip to stabilize ambient temperature of said thermoelectric sensor being higher or lower than a temperature of a finger.
3. The fingerprint sensor chip according to claim 1 , wherein said thermocouple includes a first thermocouple material and a second thermocouple material, and wherein said first thermocouple material and said second thermocouple material are respectively made of by selected from the combination of a N type silicon conductor and a P type silicon conductor, and a combination of a silicon conductor and a metal conductor.
4. The fingerprint sensor chip according to claim 1 , further comprising a heating resistor arranged over said thermal-isolation structure to heat said hot-junction region of said thermocouple to keep a temperature of said hot-junction region constant and higher than said temperature of said finger.
5. The fingerprint sensor chip according to claim 4 , wherein said heating resistor is a silicon conductor.
6. A CMOS thermoelectric sensor comprising at least a polysilicon layer and at least two interconnection layers, said thermoelectric sensor comprising following elements:
a silicon substrate;
a thermo-isolation structure defined on said silicon substrate, wherein said thermo-isolation structure is defined by utilizing a field oxide layer or a trench isolation layer;
a thin oxide layer formed surrounding said thermo-isolation structure, wherein said thin oxide layer is used as a gate oxide layer;
at least a thermocouple composing of a first thermocouple material and a second thermocouple material, wherein a hot-junction region of said thermopile is located at a central portion of said field oxide layer and a cold-junction region is located on a thin oxide layer which is surrounding said field oxide layer;
at least a via hole metal to connect said first thermocouple material and said second thermocouple material; and
a heat pipe structure comprising at least an interconnection layer and at least a via hole metal plug, wherein said heat pipe structure is located between said central portion of said field oxide layer and a passivation layer which is on a surface of said substrate.
7. The thermoelectric sensor according to claim 6 , wherein said first thermocouple material comprises polysilicon material and said second thermocouple material is an interconnection layer.
8. The thermoelectric sensor according to claim 6 , wherein said via hole metal comprises tungsten.