32-2: High-Speed Capacitive Touch Sensor with Pseudo-CMOS Buffer using a-IGZO TFTs on Plastic

Abstract

We propose a capacitive touch sensor with a-IGZO TFTs on plastic using pseudo-CMOS inverter as output buffer. Benefit from the pseudo-CMOS structure, the sensor achieves both high-speed and large output swing between touched/untouched state. This result demonstrates a novel approach to large area flexible touch sensing applications. Author KeywordsOxide thin-film transistor (TFT), capacitive touch, pseudo-CMOS. BackgroundAdvantages of a-IGZO TFT with high mobility, low temperature process, and scalability [1] resulting in a broadened possibility of large area flat panel displays. And combination of oxide TFTs and flexible substrate [2] has become a major candidate for future flexible electronics. The presented AMOLED pixel circuits [3], gate drivers [4], and other integrated circuits [5] show the potential of using a-IGZO TFTs to achieve panels with more integrated functions. Recently, we proposed high-speed circuits using pseudo-CMOS structure and bulk-accumulation a-IGZO TFTs [6], which can be a good approach to high performance integrated circuits on panel.On the other hand, even though capacitive touch sensing has become one of the most important and popular technology for human-computer interaction, its large area application still suffers from technical obstacles in passive electrode matrix like environment noise, high material resistance, etc. Although some concept of capacitive sensing with TFTs in pixel has been proposed [7], the output voltage difference between touched / untouched state (ǻV) still remains small, or the system is with relatively slow response speed.In this work, we propose a capacitive touch sensor using a-IGZO on flexible substrate. By using the pseudo-CMOS buffer, the circuit shows potential of working under high refresh rate up to 500 kHz with large output ǻV, which makes it robust to be applied to large area touch sensing applications. Experiments TFT FabricationAmorphous IGZO TFTs using back channel etch (BCE) and dual-gate (DG) structure have been fabricated. The proposed flexible substrate is a spin-coated polyimide thin film on a detachable glass [8]. Fig. 1(a) shows the cross sectional view of the TFT structure. Details of the TFT process flow are described elsewhere [9]. Fig. 1 (b) and (c) shows the transfer and output curve of the fabricated TFT, respectively. The TFT has a turn on voltage close to 0V and mobility up to 12 cm 2 /Vs. In order to analyze the speed of the fabricated DG TFTs on plastic, a 7-stage pseudo-CMOS ring oscillator has also been fabricated, as shown in Fig. 2(a). When given a supply voltage of 20V, the ring oscillator exhibits a high oscillating speed of 2.9MHz, as shown in Fig. 2(b), corresponding to a propagation delay of 25ns per stage.This result further proves the potential of DG TFTs and pseudo-CMOS structure to be applied to high-speed circuit designs. Touch Sensor DesignThe circuit diagram of the proposed capacitive touch sensor is shown in Fig. 3(a). The circuit consists of two parts. First part is targeting to transfer the variation of ...

P‐24: Integrated Gate Driver for 2700‐ppi 8K 120Hz Displays using a‐IGZO TFTs

“…And, the flexible substrate can be applied to oxide TFT backplane seamlessly [2]. The presented AMOLED pixel circuits [3], gate drivers [4][5], sensors [6][7][8] and other integrated circuits [9][10][11][12] show the potential of using a-IGZO TFTs to achieve panels with more integrated functions. On the other hand, mobile and AR/VR applications rise the demand for displays with higher pixel density.…”

63‐3: Capacitive Touch Sensor using a‐IGZO TFTs for Flexible AMOLED

“…However, capacitive touch sensing with passive electrode matrix suffers from driving noise and material resistance, etc. In order to overcome those shortcomings, we recently proposed a high-speed capacitive touch sensing circuit using pseudo-CMOS structure and bulk-accumulation a-IGZO TFTs [6][7]. Although this circuit has achieved high speed and robust output for touch sensing, it is relatively complicated and requires complex driving clocks, thus limiting the possibility of it to be integrated with high-resolution displays.…”