Fingerprint detection has long been used as a method of identification. In the time that fingerprints have been examined and compared, no two areas of friction ridge skin on any two fingers or palms have been found to have the same friction ridge characteristics. As such, fingerprinting has been accepted as a primary method of identifying an individual.
Although fingerprinting was initially used for forensic purposes, computerized identification, verification and authentication systems that utilize the digital analysis of fingerprints have been developed more recently to protect personal property such as laptops, computers and cellular phones, prevent credit card and calling card fraud, limit access to secure computers, information and areas, and ensure security for network based financial transactions.
The technology for the actual scanning or sensing of fingerprints has also advanced over time. For example, ink roller methods were replaced with optical and mechanical sensing technologies. Such technologies generally used scanners or cameras to capture a fingerprint image and then digitize the image for subsequent processing. One problem with such technologies is that the lighting conditions available at the time of image capture can affect the quality of the resulting fingerprint image.
More recently, capacitive fingerprint sensors have been introduced. Such sensors are typically silicon semiconductor-based devices that have an array of capacitor electrodes covered by a protective coating at the surface of the sensor (i.e., the platen). The sensor detects varying capacitor charges that correspond to die distance between the ridges and valleys of a, fingerprint and the electrodes when a finger is pressed against the platen. These measurements are then converted into a digital image of the fingerprint. An early capacitive fingerprint sensor is disclosed in U.S. Pat. No. 4,353,056 to Tsikos, which is incorporated herein by reference in its entirety.
One issue with capacitive fingerprint sensors is that placing a linger that holds a sufficient level of electrostatic charge onto the sensor has the potential to significantly damage sensitive electronic components in the sensor, other electronic components coupled to the sensor (including circuitry in any authentication computer systems coupled to the sensor) or otherwise interfere or affect the operation of the sensor. This effect is known as electrostatic discharge (“ESD”). Conventionally, sensor manufacturers have sought to control ESD by placing electrodes or an internal metal grid underneath the sensor to dissipate electrostatic charge received from a finger. Exemplary patents incorporating such methods include U.S. Pat. No. 6,628,812 to Setlak et al. and U.S. Pat. No. 6/737,329 to Lepert et al, each of which is incorporated herein by reference in its entirety. However, such methods do not reduce the electrostatic charge received at the surface of sensors. As such, despite the potential of such electrodes or metal grids to distribute or dissipate current once the finger contacts the surface of the sensor, substantial ESD effect or damage may still occur to any sensitive components in the sensor that tire not in adequate proximity to the electrodes or grid. In particular, sensors placed in environments that have the potential to produce ESD finger events that result in high voltages (e.g., greater than 12 kV) may not be adequately protected by such conventional solutions. What is therefore needed is a structure or architecture where ESD is dissipated prior to contact by the finger with the platen.