Improved Performance of Zinc Oxide Thin Film Transistor Pressure Sensors and a Demonstration of a Commercial Chip Compatibility with the New Force Sensing Technology

DOI: 10.1002/admt.201700279 rubrene-based transistors with microstructured gate electrodes,[2] microelectromechanical systems with strain gauges,[3] and other methods.[4,5] With the introduction of force sensing into mobile handheld devices, it becomes critical to develop force sensing solutions that are scalable, thin, light, and cost-effective. Force sensing functionality has been pursued using various methods that include strain sensors,[6] light emitters and detectors,[7] and surface acoustic wave sensors among others.[8–11] A powerful approach in pressure sensor development is the incorporation of both pressure sensing and switching functionalities into a single array element.[12] This allows the use of miniature sensor elements with high sensitivity and scalability to large areas. Zinc oxide (ZnO) is semiconducting, suitable for transistor fabrication in a vertically integrated process, transparent, and has a high piezoelectric coefficient that confers excellent pressure sensitivity,[13,14] positioning it as a forefront candidate for integration into in-cell touchscreen technologies.[15] A large portion of the emerging display technologies use materials based on ZnO, such as indium gallium zinc oxide (IGZO).[16,17] In addition, amenability to low temperature processing allows the ZnO pressure sensor arrays to be seamlessly integrated into pressure display technologies. When pressure is applied on top of a sputtered ZnO film, the net dipole moment in the c-direction is distorted, leading to accumulation or depletion of free charge carriers at the surface of the device.[18] The pressure-induced free charge carriers lead to a change in the drain current, with the magnitude of change being proportional to the applied pressure. In a field-effect transistor (FET) configuration, small pressures applied to the gate can lead to large drain current changes, especially when the transconductance of the ZnO FET is high. Vishniakou et al. reported the fabrication of 8×8 ZnO thin film transistor (TFT) arrays on rigid glass, with good pressure sensing characteristics.[12] However, their Ion/Ioff ratio was lower than 103, and the piezoelectric properties of the films were not explored in detail. For good transistor performance, the unintentionally doped ZnO films, usually n-type due to O2 vacancies and impurity donors, need to be compensated to achieve low leakage currents. Further, the film grains in the ZnO layers need to be A zinc oxide thin film transistor is developed and optimized that simultaneously functions as a transistor and a force sensor, thus allowing for scalable integration of sensors into arrays without the need for additional addressing elements. Through systematic material deposition, microscopy, and piezoelectric characterization, it is determined that an O2 rich deposition condition improves the transistor performance and pressure sensing characteristics. With these optimizations, a sensitivity of 4 nA kPa−1 and a latency of below 1 ms are achieved, exceeding the criteria for successful commercialization of arrayed pressure sensors. The functionality of 16 × 16 pressure sensor arrays on thin bendable glass substrates for integrated low weight and flexible touchscreen displays is fabricated and demonstrated and read-out electronics to interface with the arrays and to record their response in real-time are developed. Finally, the application of these sensors for mobile displays via their operation with an existing commercial touch integrated circuit controller is demonstrated.