Fingerprint sensors

Abstract

A sensor for detecting fingerprints is provided having first and second substrates, a two-dimensional array of sensing elements formed on the first substrate, and a plurality of thin-film transistors or TFTs for controlling the sensing elements at pixel location along the array. Each of the sensing elements detects one of electrical signals (e.g., capacitance, resistance, or impedance), temperature, or light via one of the first or second substrates representative of one or more fingerprints. The top of the second substrate or the bottom of the first substrate may provide a platen upon which one or more fingers can be disposed. The sensor may be utilized in a fingerprint scanner having one or more processors driving sensing elements or reading from sensing elements analog signals representative of one or more fingerprints, and generating an image representative of the one or more fingerprints from the analog signals.

Description

[0001]This application claims the benefit of priority to U.S. Provisional Patent Application No. 61 / 486,766, filed May 17, 2011, which is herein incorporated by reference.FIELD OF THE INVENTION[0002]The present invention relates to sensors, methods, and systems for detecting one or more fingerprints, and particularly a sensor for detecting one or more fingerprints utilizing thin-film transistors (TFT). The present invention is particularly useful in providing a fingerprint sensor having sensing element(s) at each pixel location of a two-dimension array where sensing element(s) are coupled to one a TFT at the pixel location for use in a fingerprint scanner to provide a two-dimension fingerprint image of one or more fingers. At each pixel location, a single sensing element of desired type or characteristic (e.g., light, electrical, or thermal sensing) may be coupled to a TFT enabling driving and readout of the sensing element, or multiple sensing elements of the same or different type...

AI Technical Summary

Benefits of technology

[0005]Accordingly, it is a feature of the present invention is to provide a fingerprint sensor having arrays of sensing elements controlled by TFTs for detecting one of more fingerprints, and thereby provide a TFT fingerprint sensor, which avoids free-space optics of conventional fingerprint scanners.

[0011]Preferably, the arrays of TFT and coupled sensor(s) thereto are in 2D orthogonal grids. As the sensor of the present invention avoids the need for free-space optics described earlier, the magnification of the fingerprint(s) detected when light sensing elements are present is typically 1:1, but may be a slightly higher magnification in one axis due to the presence of a tilted platen as in the case of a prism element. For systems that have 1:1 magnification, the maximum pixel size desired for AFIS (Automatic Fingerprint Identification Systems) corresponds to 50.8×50.8 μm which translates to 500 points-per-inch (ppi). For a system that incorporates a 45 deg prism, the maximum pixel size is 50.8*sin(45)=35.9 μm.

[0014]TFTs, since they use amorphous silicon rather than crystalline silicon, can be printed on flexible substrates. Flexible substrates can be advantageous since they can allow more compact fingerprint scanners wherein folds or rolls up when not in use. Alternately or in addition to, these flexible substrates can be fabricated in a “sling-like” configuration suitable for capture of rolled print equivalents. Flexible TFT-based arrays can be made very thin in order to accommodate “smart ID card” applications.

[0015]A surface of the second substrate facing the first substrate may be the top of the substrate in the sandwich light assembly of the first and second substrates and the array disposed there between. Alternatively, the surface of the first substrate opposite the array formed thereupon may be the top of the substrate, in this case the sensor may be considered “up-side-down”, in which case a metal (like steel or aluminum) plate or substrate may be provided as part of the assembly. Thus, the array may be printed where the second substrate is considered the bottom of the sensor. The TFT is printed on top of the first substrate and the light to be collected comes from above the sensor. However, if the TFT needs to be encapsulated with a gel or epoxy, trapped air bubbles will hinder light coming from the object and reach the TFT sensor. By printing the sensor “upside-down” one avoids this potential issue. In other words, the sensor is made such that light is expected to be collected from the bottom on of first substrate. This way, if any encapsulating is required, trapped air bubbles will only affect the illumination light (and not even this if LEDs or OLEDs are present at each pixel in the array. This and can thus increase optical resolution of a scanner utilizing the sensor. The metal plate may encapsulate the “upside down” sensor, and strengthen the sensor as well as to act as a heat sink for temperature stabilization.

Problems solved by technology

Because of the presence of free-space optics, a relatively large distance must be between the finger and the sensor, resulting in a fingerprint scanner that is larger and bulkier than would be desired for certain applications (in particular mobile, hand-held applications).

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