Amorphous IGZO TFTs and Circuits on Highly Flexible and Dimensionally Stable Kovar (Ni-Fe alloy) Metal Foils
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Amorphous IGZO TFTs and Circuits on Highly Flexible and Dimensionally Stable Kovar (Ni-Fe alloy) Metal Foils Shahrukh A. Khan, Xiaoxiao Ma, Nack Bong Choi and Miltiadis Hatalis Department of Electrical Engineering, Lehigh University, Bethlehem, PA 18015 ABSTRACT We have fabricated high performance amorphous IGZO TFTs and integrated circuits on flexible kovar (Ni-Fe 42 alloy) foils. Excellent dimensional stability on kovar foils is obtained by a pre-anneal process at 800°C that limits the thermal run-out to within 100ppm. After substrate annealing, Ni-Fe 42 alloy retains high yield strength and good flexibility with the re-crystallized structure containing large isotropic grains between 2050μm. Amorphous IGZO TFTs and circuits with a staggered, bottom-gate architecture are fabricated and tested. Non-flexed TFTs have field effect mobility of 12 cm2/V.s, threshold voltage around 2 V and sub-threshold swing of 0.6 V/decade and ON/OFF current ratio exceeding 107. Under prolonged uniaxial tensile strain upto 0.8%, TFTs exhibited minimal change in performance which augers well for use of Ni-Fe foil as flexible substrates. To demonstrate the viability of oxide-based device integration, n-type pseudo logic ring oscillator circuits are also evaluated. Sub 300 ns propagation delay is confirmed at a rail-rail supply voltage of 40 V. The results suggest that device integration on such a highly flexible substrate is amenable to roll-to-roll processing of future electronics. INTRODUCTION Electronics on flexible substrates is a field of rapidly growing interest for displaybased applications and for integration of large area systems in general. Inexpensive and light-weight flexible electronics would be more rugged and portable than the ones on more conventional rigid-substrates. Although plastic substrates are considered as possible flexible electronics vehicle, they still pose significant engineering challenges due to their chemical and mechanical instability [1]. Thin metal foils on the other hand offer greater dimensional stability and thus very attractive for the fabrication of flexible AMOLED type of displays [2]. Low temperature process ability of IGZO TFTs is compatible with plastic substrates but the greater dimensional stability afforded by thin metal foils allows implementation of circuit designs with smaller feature sizes. Kovar (Ni-Fe 42 alloy) is such a metal foil with low and nominally constant coefficient of thermal expansion over a wide range of temperature and excellent mechanical and chemical robustness. For successful integration of high performance devices on Ni-Fe 42 foils, substrate stabilization is of paramount interest. Correlation between macrostructure of the substrate undergoing thermal processes during fabrication and its dimensional stability thus needs detailed attention. Furthermore, mechanical bending of TFT/foil substrate structures show that TFTs and photodiodes [3] are surprisingly rugged when made on thin foils. In this regard, Ni-Fe 42 foils can have added advantage over choice metal substrates l
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