Electrostatic discharge protection of a capacitive type fingerprint sensing array
View Patent ↗A planar fingerprint pattern detecting array includes a large number of individual skin-distance sensing cells that are arranged in a row/column configuration. Each sensing cell includes an amplifier having an ungrounded input mode and an ungrounded output node. Output-to-input negative feedback that is sensitive to the fingerprint pattern is provided for each amplifier by way of (1) a first capacitor plate that is placed vertically under the upper surface of a dielectric layer and is connected to the ungrounded amplifier input node, (2) a second capacitor plate that is placed vertically under the upper surface of the dielectric layer in close horizontal spatial relation to the first capacitor plate and is connected to the ungrounded output node, and (3) an ungrounded fingertip whose fingerprint pattern is to be detected, which ungrounded fingertip is placed on the upper surface of the dielectric layer in close vertical spatial relation with the first and second capacitor plates. Electrostatic discharge protection relative to electrostatic potential that may be carried by the ungrounded fingertip is provided by placing a number of grounded metal paths within the dielectric layer to spatially surround each of the first and second capacitor plates, this being done in a manner that does not disturb the ungrounded state of the fingertip.
1. A method of providing electrostatic discharge for a capacitive fingerprint pattern detecting array, comprising:
providing a fingerprint pattern detecting array having a number of individual skin-distance sensing cells that are arranged in a closely spaced physical configuration;
providing a dielectric layer for said array, said dielectric layer having an upper surface and a lower surface;
placing a fingertip having a fingerprint pattern on said upper surface;
providing each sensing cell with an amplifier having an ungrounded input node and an ungrounded output node;
providing an ungrounded feedback signal from the output node to the input node for each of said amplifiers, said signal feedback being modified according to a skin distance within said fingerprint pattern;
providing said feedback for each of said amplifiers by way of
(1) a first capacitor plate that is placed vertically under said upper surface or said dielectric layer and is connected to said ungrounded input node,
(2) a second capacitor plate that is placed vertically under said upper surface of said dielectric layer in close horizontal spatial relation to said first capacitor plate and is connected to said ungrounded output node, and
(3) said fingertip in vertical spatial relation with said first and second capacitor plates;
providing a metal path to at least partially spatially surround but not physically engage said first and second capacitor plates; and
connecting said metal path to ground and to thereby protect said input and output nodes from electrostatic potential that may be carried by said fingertip by providing an electrostatic discharge path for said electrostatic potential.
2. The method of claim 1 further comprising:
forming said first capacitor plate, said second capacitor plate, and said metal path within said dielectric layer at a location that is spaced from said upper surface of said dielectric layer.
3. The method of claim 1 , further comprising:
placing said first and second capacitor plates and said metal path on a common physical plane.
4. The method of claim 1 , further comprising:
providing said amplifier for each sensing cell as signal-inverting amplifier; and
providing said ungrounded feedback for each of said amplifiers as negative feedback signal.
5. The method of claim 1 , further comprising:
prior to said step of placing said ungrounded fingertip on said upper surface, momentarily shorting said input and output nodes of each amplifier of each sensing cell.
6. The method of claim 5 , further comprising:
providing said amplifier for each sensing cell as signal-inverting amplifier; and
providing said ungrounded feedback for each of said amplifiers as negative feedback.