Sensing element arrangement for a fingerprint sensor
View Patent ↗For each pixel in an array of pixels in a fingerprint sensor, a fingerprint capacitor is defined by the fingerprint-bearing skin of a user's finger proximate to the top exposed surface of a sensing plate of the pixel. First and second plates are embedded in dielectric material beneath the sensing plate, and define therewith first and second capacitors of a sensing element. The capacitance of the fingerprint capacitor is coupled by the sensing element to an amplifier. During a sensing operation, the amplifier generates a pixel output signal that is a function of the variable capacitance of the fingerprint capacitor, which varies according to the presence of a fingerprint ridge or valley appearing directly above the sensing plate when the user's finger is in contact with the fingerprint sensor.
1. In a fingerprint sensor having a plurality of pixels for sensing a fingerprint of a user's finger, each pixel having a sensing element coupled to an amplifier for generating an output corresponding to a fingerprint characteristic appearing at the pixel location, the sensing element comprising:
a body of dielectric material;
a surface plate of conductive material retained by the dielectric body and having an upper surface defining a sensing surface for contact with the fingerprint-bearing skin of a user's finger;
first and second coplanar electrodes embedded in the dielectric body at a first level below the surface plate; and
a third electrode embedded in the dielectric body at a second level below the surface plate, the third plate being connected by a conductor to the surface plate;
wherein a first sensor is formed by the third electrode and the first electrode, a second sensor is formed by the third electrode and the second electrode, and a fingerprint sensor is formed between the surface plate and the skin of the user's finger thereover during a sensing operation.
2. The sensing element of claim 1 , wherein the first level is above the second level.
3. The sensing element of claim 2 , wherein the thickness of the portion of the dielectric body separating the surface plate from the first and second electrodes is at least ten times greater than the thickness of the portion of the dielectric body separating the first and second electrodes from the third electrode therebelow.
4. The sensing element of claim 1 wherein the first level is below the second level.
5. The sensing element of claim 4 , wherein the thickness of the portion of the dielectric body separating the surface plate from the third electrode is at least ten times greater than the thickness of the portion of the dielectric body separating the third electrode from the first and second electrodes therebelow.
6. The sensing element of claim 1 , wherein the dielectric body includes a thin layer separating the third electrode from the first and second electrodes, the thin dielectric layer having a thickness of less than one micron.
7. The sensing element of claim 6 , wherein the thickness of the thin dielectric layer is from 0.2 to 0.3 micron.
8. The fingerprint element of claim 1 , wherein said first, second and third electrodes comprise capacitor plates, and said first and second sensors comprise first and second capacitors.