Focused ion beam milling for the fabrication of 160 nm channel length IGZO TFTs on flexible polymer substrates

Münzenrieder N, Shorubalko I, Petti L, Cantarella G, Shkodra B, Meister T, Ishida K, Carta C, Ellinger F & Tröster G

Flexible and Printed Electronics, 5(1), 015007 (6 pp.). 2020

https://doi.org/10.1088/2058-8585/ab639f

Abstract
The quest for short channel length transistors is an important challenge in the semiconductor industry. A similar trend is observed in the field of flexible electronics where sensor conditioning circuits and transceivers have to be realized on plastic foils. Here the use of a focused Ga+ ion beam (FIB) to structure the channel of a flexible InGaZnO-based thin-film transistor (TFT) is presented. The resulting flexible TFT exhibits a channel length of and an effective field effect mobility of 4 cm2 V−1 s−1. Furthermore, the optimized Ga+ beam milling does not damage the Al2O3 gate insulator underneath, leading to a gate leakage current of <200 pA. The extreme channel length demonstrates that focused ion beams can complement conventional fabrication approaches, overcoming current limitations imposed by flexible substrates. While the dimensions result in short channel effects and a drain conductance of limiting the DC applicability of the FIB TFT, the device also exhibits a high internal gain of 3.4 dB. Consequently, a transit frequency of ≈6 MHz and a maximum frequency of oscillation of ≈19 MHz is measured for supply voltages  ≼ 0.5 V. This shows that highly scaled flexible TFTs for analog circuits can be fabricated by ion beam milling.