Abstract
I. SUMMARY AND MOTIVATION A unique requirement of flexible electronic systems is the need to simultaneously optimize their electrical and mechanical performance. Amorphous InGaZnO thin-film transistors (TFTs) fabricated on free-standing large-area plastic substrates address this issue by providing a carrier mobility >10 cm 2 /Vs, and bendability down to radii as small as 25 μm. At the same time, limitations such as a constrained minimum lateral feature size, the lack of appropriate p-type materials, or the influence of strain have to be considered when designing circuits. Here, models describing the scaling and bending behavior of flexible InGaZnO TFTs, together with the design of strain insensitive circuits operating at megahertz frequencies are presented. II. INTRODUCTION Flexible, unobtrusive and cheap sensor devices fabricated on imperceptible substrates and combined with everyday objects, are the next major milestone for smart assistance systems and healthcare products [1-3]. Besides sensors, flexible conditioning circuits and data transceivers are needed to enable such applications. In this context, amorphous InGaZnO (IGZO) TFTs have been identified as a promising technology to fabricate flexible electronics [4]. This is due to the good electrical properties of IGZO [5], and its ability to be deposited on a variety of deformable but temperature sensitive substrates [6, 7]. At the same time, the use of flexible substrates can lead to strain induced performance variations and imposes limitations on the fabrication process, which limits the circuit complexity. All this has to be considered when designing flexible circuits. III. RESULTS AND METHODOLOGY The TFTs shown here are fabricated on free-standing 50 μm thick polyimide using UV lithography. Fig. 1a shows the device structure. The substrate is first encapsulated with 50 nm SiN [PECVD]. A 35 nm thick Cr gate is then e-beam evaporated and wet etched. The gate is insulated by 25 nm Al2O3 [ALD] deposited at 150°C, which is the highest temperature used in the fabrication process. Next, 15 nm IGZO are RF sputtered in an Ar atmosphere. The Al2O3 and IGZO are independently structured by wet etching. The source and drain contacts (as well as interconnection lines for circuits) are made from 10 nm Ti, and 75 nm Au, e-beam evaporated and structured by lift-off. Finally, the devices are passivated by an additional 25 nm thick Al2O3 layer. Fig. 1b shows a fully processed substrate. Representative transfer and output characteristics of a single TFT are plotted in Figs. 1c and 1d. The shown TFT exhibits a threshold voltage (VTH) of 1 V, a field effect mobility (μFE) of 15.5 cm 2 /Vs, an on/off current ratio >10 8 , a subthreshold swing of 110 mV/dec., and a maximum transconductance of 570 μS. A. Mechanical strain Minimizing the impact of mechanical deformation is of primary importance when dealing with flexible circuits. Figs. 2a and 2b show a bending test and the parameter shifts caused by strain parallel to the channel. Tensile strain increases μFE and decreases VTH, while compressive strain has the opposite effect [8]. These shifts are reversible, but excessive stain beyond ≈0.7% tensile or ≈2.2% compressive (bending radii of 3.5 mm and 1.1 mm, respectively [9]) cause cracks and permanently destroys the devices. Although the tolerable tensile strain and minimum bending radius can be improved to 1.7% and 25 μm by using encapsulation layers [10], thin substrates [11], or ductile materials [12], circuits have to consider bending effects. Therefore a SPICE level 61 transistor model was modified to simulate the influence of strain [13]. In particular for digital circuits where the output voltage depends on the transconductance ratio between multiple TFTs Fig. 1: Flexible IGZO TFTs. a) Layer structure, b) Processed substrate and micrograph of single TFT, c) transfer and b) output characteristics.