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. A sensing element for a capacitive sensor, comprising:
a body of dielectric material;
a surface plate disposed on the dielectric body, the surface plate having an upper surface defining a portion of a sensing surface of the capacitive sensor;
first and second plates separated from each other and from the surface plate by portions of the dielectric body;
a third plate separated from the surface plate, the first plate and the second plate by portions of the dielectric body; and
a conductor connecting the third plate to the surface plate;
wherein the first plate defines a first capacitor with the third plate, and wherein the second plate defines a second capacitor with the third plate, the first and second capacitors being connected in series in a circuit within the capacitive sensor for communicating a signal therein that is a function of a variable capacitance sensed at the upper surface of the surface plate.
2. The sensing element of claim 1 wherein the first, second and third plates are each embedded in the dielectric body a like distance beneath the surface plate, the third plate lying between and adjacent to the first and second plates, the first capacitor being defined by respective facing edges of the first and third plates, the second capacitor being defined by respective facing edges of the second and third plates.
3. The sensing element of claim 2 wherein the first and second plates have fingers extending toward the third plate, and the third plate has fingers extending toward the first and second plates, the fingers of the first plate lying in an interdigitated relationship to adjacent fingers of the third plate and forming the facing edges that define the first capacitor, and the fingers of the second plate lying in an interdigitated relationship to adjacent fingers of the third plate and forming the facing edges that define the second capacitor.
4. The sensing element of claim 1 wherein the first and second plates are embedded in the dielectric body beneath the surface plate and separated therefrom by a first thickness of dielectric material, and the third plate is disposed beneath the first and second plates and separated therefrom by a second thickness of dielectric material, the first thickness being greater than ten microns and the second thickness being less than one micron.
5. The sensing element of claim 1 wherein the third plate is embedded in the dielectric body beneath the surface plate and separated therefrom by a first thickness of dielectric material, and the first and second plates are disposed beneath the third plate and separated therefrom by a second thickness of dielectric material, the first thickness being greater than ten microns and the second thickness being less than one micron.
6. The sensing element of claim 1 wherein the first, second and third plates consist essentially of aluminum embedded in the dielectric body.
7. The sensing element of claim 1 wherein the surface plate consists essentially of titanium oxide.