Amorphous IGZO TFTs and Circuits on Highly Flexible and Dimensionally Stable Kovar (Ni-Fe alloy) Metal Foils

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 20-50μm. 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.