UC San Diego

We have developed zinc oxide thin film transistors with a unique device architecture that exhibit robust and high-performance pressure sensing while maintaining transparency for next generation pressure-sensitive touchscreen displays. We systematically studied and optimized the material growth and device fabrication for improved device performance. We then fabricated top and bottom gate TFTs using both top and bottom contact geometries and identified the optimal device architecture to attain high pressure sensitivity while maintaining excellent transistor performance. Simultaneous operation of a single device as a switch and a pressure sensor allows simple integration of sensors into arrays without the addition of external switching elements. The all-solid-state sensors are capable of measuring steady-state pressure and transient pressure variations. The sensing mechanism stems from the piezoelectric characteristics of RF sputtered ZnO. When zinc oxide is used as the channel material in thin film transistors, its piezoelectric property results in a shift of the transistor threshold voltage upon pressure application. This shift causes a modulation of the drain current flowing through the transistor at a steady bias. A linear dependence of the current change with the applied pressure is observed. Our first-generation TFTs together with a readout circuit built on a breadboard allow discrete pressure measurements from a single sensor at a frequency of 2 kHz. We demonstrated the operation of a transparent 8x8 pressure sensor array fabricated on glass, and the integrated system of pressure sensors and actuators for the recording and reproduction of touch. We further advanced our pressure sensors by studying the effects of sputtering gas, substrate temperature, and seeding layer on the ZnO film properties. The second-generation devices achieved an on-off ratio of $10^5$ and allowed us to successfully demonstrate a larger, 16x16 array. We designed a complete system for real-time pressure signal acquisition and display, including read-out electronics, mechanical integration, electrical connections, and display software for signal visualization. We measured the latency of the pressure sensors to be less than 1 ms and the recovery time to be less than 20 ms. The new pressure-sensing technology enables the development of force-sensitive touchscreens that will make possible new mobile applications with a richer user-machine interface.