Highly Sensitive Active-Matrix Driven Self-Capacitive Fingerprint Sensor based on Oxide Thin Film Transistor

Abstract

The fingerprint recognition has been widely used for biometrics in mobile devices. Existing fingerprint sensors have already been commercialized in the field of mobile devices using primarily Si-based technologies. Recently, mutual-capacitive fingerprint sensors have been developed to lower production costs and expand the range of application using thin-film technologies. However, since the mutual-capacitive method detects the change of mutual capacitance, it has high ratio of parasitic capacitance to ridge-to-valley capacitance, resulting in low sensitivity, compared to the self-capacitive method. In order to demonstrate the self-capacitive fingerprint sensor, a switching device such as a transistor should be integrated in each pixel, which reduces a complexity of electrode configuration and sensing circuits. The oxide thin-film transistor (TFT) can be a good candidate as a switching device for the self-capacitive fingerprint sensor. In this work, we report a systematic approach for self-capacitive fingerprint sensor integrating Al-InSnZnO TFTs with field-effect mobility higher than 30 cm 2 /Vs, which enable isolation between pixels, by employing industry-friendly process methods. The fingerprint sensors are designed to reduce parasitic resistance and capacitance in terms of the entire system. The excellent uniformity and low leakage current (<10 −12 ) of the oxide TFTs allow successful capture of a fingerprint image.

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“…It operates on the basis of the parallel-layer model, where the electrode and bottom (or user's finger) act as the two layers of the capacitor. As capacitance is added with each "touch", the capacitance on the electrode of the self-capacitive system increases [26]. Contrarily, any intended or unintended capacitance between two "charge-holding objects" can be termed mutual capacitance [27].…”

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“…Considering a parallel-layer model, the electrode forms one layer of a capacitor, with the other layer being either ground or the user’s finger. A touch causes the electrode capacitance to increase, as the human body ‘adds’ capacitance to that of the system [ 15 ]. Mutual capacitance is the intentional or unintentional capacitance between two ‘charge holding objects’ [ 16 ].…”

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