Anisotropic quasi-atomic layer etching of InGaZnO₄ using unbiased CH₄ and continuous/pulsed biased O₂ plasmas

Abstract

In this study, an anisotropic quasi-atomic layer etching (qALE) process for InGaZnO4 (IGZO) removal was developed and systematically investigated, consisting of alternating unbiased CH4 plasma adsorption and biased (continuous or pulsed) O2 plasma desorption steps. The etching behavior was examined by independently varying the CH4 plasma time, O2 plasma time, surface temperature, bias voltage, and pulse duty cycle. The total cyclic etch rate was analyzed in terms of its physical and chemical components, with the two etching contributions examined separately. The physical cyclic etch rate increased linearly with O2 plasma processing time, bias voltage, and pulse duty cycle, while the chemical cyclic etch rate exhibited a self-limiting behavior with increasing CH4 and O2 plasma processing times. The chemical cyclic etch rate rose from 0.07 nm/cycle at 0% duty cycle in pulsed biased O2 plasma to 0.13 nm/cycle at 10%, remained constant between 10% and 70%, and further increased to 0.20 nm/cycle at 100%. This defined a qALE process window between 10% and 70% duty cycle at 60 V bias voltage, likely due to the complete removal of the saturated hydrocarbonated IGZO layer under moderate ion bombardment. ALE synergy decreased with increasing O2 plasma time, higher bias voltage, and higher duty cycle, reflecting the enhanced physical etching contribution. The cation composition of the IGZO film surface was analyzed using x-ray photoelectron spectroscopy after the qALE process. The Ga fraction in amorphous IGZO increased following qALE, due to the stronger Ga–O bond relative to In–O and Zn–O, resulting in a lower Ga etch rate. However, under higher bias voltage in the O2 plasma step, the Ga enrichment was less pronounced, suggesting the formation and desorption of Ga-containing etch by-products at elevated ion bombardment. This developed anisotropic qALE process was effectively applied to top-gate trench patterning for IGZO channel transistor fabrication, enabling precise control of channel thickness and yielding an excellent etch profile with high ALE synergy.