Fast Flexible Plastic Substrate ZnO Circuits

Abstract

We report thin-film transistors (TFTs) and circuits fabricated on flexible plastic substrates using ZnO thin films deposited by plasma-enhanced atomic layer deposition at 200 • C. Crossover test structures fabricated on flexible substrates have a good yield, and ZnO TFTs have a field-effect mobility of 20 cm 2 /V · s. The 15-stage ring oscillators are operated at > 2 MHz with a supply voltage of V DD = 18 V, corresponding to a propagation delay of < 20 ns/stage. To the best of our knowledge, these are the fastest oxide-semiconductor circuits on flexible substrates reported to date. Index Terms-Flexible circuits, plasma-enhanced atomic layer deposition (PEALD), thin-film transistors (TFTs).

Metal oxide semiconductor thin-film transistors for flexible electronics

“…47, the lowest stage delay has been reported for ZnO ring oscillators, which yield a 16 ns delay at 18 V supply voltage. 328 Already at 2 V supply voltage, the delay of the same ring oscillators increases to %300 ns. 328 The smallest ring oscillator is composed by 3 stages of IGZO TFTs on a metal foil and oscillates at f o ¼ 360 kHz (stage delay of 926 ns) with a supply voltage of 15 V. 314 Increasing the supply voltage to 50 V raises the oscillation frequency to 1.14 MHz and results in a stage delay of 291 ns.…”

“…328 Already at 2 V supply voltage, the delay of the same ring oscillators increases to %300 ns. 328 The smallest ring oscillator is composed by 3 stages of IGZO TFTs on a metal foil and oscillates at f o ¼ 360 kHz (stage delay of 926 ns) with a supply voltage of 15 V. 314 Increasing the supply voltage to 50 V raises the oscillation frequency to 1.14 MHz and results in a stage delay of 291 ns. 314 A larger IGZO ring oscillator (5 stages) oscillating at 182 kHz at 20 V (stage delay of 550 ns) and at f o ¼ 572 kHz at 30 V (stage delay of 350 ns) has been reported by Hsieh and Wu on PI foil.…”

“…328 Solutionprocessed metal oxide semiconducting materials are only rarely used for circuits, and in any case only for unipolar inverters or ring oscillators. 119,193,200,220,221,231 As gate dielectrics Al 2 O 3 or SiO 2 are mostly utilized, whereas source and drain electrodes are typically made of different metals like Au, Ti, Cr, and Cu, which can even be treated with special techniques (e.g., hydrogen plasma treatments) to reduce the contact resistance.…”

“…159 Similarly, vacuum-processed ZnO NOT gates on PI foils with gains of 1.5 V/V at supply voltages of only 9 V have also been demonstrated. 328 Additionally, also NOT gates with resistive pull-up loads employing solution-processed metal oxide semiconductors have been reported, like ion-gel gated ZnO NR NOT gates on paper yielding gains of 2 V/V at supply voltages of 1.3 V, 231 and aerosol-jet printed ZnO NOT gates on PI with gains up to 8 V/V (V DD ¼ 2 V). 193 to roll up to a radius of 25 lm after fabrication and release.…”

“…164 The same publication also presented a two clock shift register with 10 TFTs and 1 Capacitor per stage, which is suitable for display applications. 164 Further increasing the number of stages, Zhao, Mourey, and Jackson showed a ZnO 15-stage ring oscillator with 16 ns delay at 18 V and 300 ns delay at 2 V. 328 The realization of a flexible 19-stage IGZO ring oscillator with a stage delay of 19 ns at 20 V has only been possible due to the low TFT contact resistance achieved between source/drain and IGZO. 316 Even more TFTs have been utilized for an AMOLED line driver based on IGZO capable of 45 frames/s at 11 V on PEN foil, which has also been integrated with an optical display (64 Â 160 pixels) and a 2T1C pixel driver circuit.…”