Mechanically strained a-Si:H AMOLED driver circuits
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Mechanically strained a-Si:H AMOLED driver circuits P. Servati, S. Tao, E. Horne, D. Striakhilev, K. Sakariya, and A. Nathan Electrical and Computer Engineering Department, University of Waterloo Waterloo, Ontario, Canada N2L 3G1 ABSTRACT This paper examines the variations in performance of amorphous silicon (a-Si:H) thin-film transistor (TFT) pixel driver circuits for active-matrix organic light-emitting diode (AMOLED) displays, which are subject to compressive or tensile mechanical strain. The external strain is induced by bending of the TFT substrate, and is measured by the observed changes in resistance of in-situ strain gauges. Mechanical strain impacts the performance of the circuit in terms of its drive current, which may be attributed to mobility and Fermi energy shifts in the individual TFTs. The effect of strain-induced shifts in the TFTs as a function of strain orientation (longitudinal or transverse) with respect to direction of current flow is also examined. Our measurements show that the variation in the drain current of a longitudinally oriented TFT can be as much as ~ 1.5% for strains as high as 10-3. Proper layout and circuit design can suppress the effect of strain-induced shifts, and should be taken into consideration when designing stable TFT driver circuits for mechanically flexible AMOLED displays. INTRODUCTION Successful fabrication of TFTs on compliant substrates [1,2] is bringing the idea of mechanically flexible AMOLED displays closer to reality. Here, development of processes for roll-to-roll production of TFT backplanes is especially attractive because of the potentially lower production cost [3]. In addition, compared to the conventional (rigid) glass substrate, flexible polymeric substrates have reduced fragility and weight [3,4]. Most transparent plastic substrates have a glass transition temperature ~ 200oC and significant thermal expansion coefficient mismatches with the TFT layers. Therefore, the TFT processing temperature has to be sufficiently reduced without compromising the properties of the thin-film materials and TFT performance. Low temperature (< 150oC) processes have been developed for fabrication of TFTs on substrates such as polyethylene terephthalate (PET) [5] and polyimide substrates [6]. High performance 120ºC TFTs with switching ratio of 107, threshold voltage (VT) of 2 V, subthreshold slope of 0.5 V/dec, and field effect mobility (µeff) of 1.0 cm2/Vs have been reported [1,2] and lower temperature processes are also under investigation [7]. Aside from the challenges for realization of TFTs on compliant substrates, the capability to bend the thin-film sandwich structure of an AMOLED display to a desired minimum radius of curvature (ρmin) without cracking the layers is a vital requirement. Here, in addition to the substrate, the thin-film sandwich structure includes the buffer, TFT, interconnect, planarization, OLED, and encapsulation layers. The stress in these layers is dependent on the thickness and stiffness of the different layers. For instance, the use of
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