Atomic layer etching of InGaZnO thin films via plasma hydrocarbonation and oxygen radical reaction

Abstract

In this study, an atomic layer etching (ALE) process for InGaZnO4 (IGZO) was developed and systematically investigated, consisting of a plasma hydrocarbonation step with CH4 plasma and an O radical reaction step with O2 plasma. The etching process was examined by independently varying the CH4 plasma processing time, O2 plasma processing time, and surface temperature. The IGZO film surface was analyzed using time-of-flight secondary ion mass spectrometry and x-ray photoelectron spectroscopy (XPS) following the ALE process. A self-limiting cyclic etch rate was observed with increasing CH4 plasma processing time, corresponding to the saturation of hydrocarbon penetration into the IGZO layer after 30 s of CH4 exposure. A similar self-limiting trend was observed with increasing O2 plasma processing time, consistent with changes in OH emission intensity measured by optical emission spectroscopy, suggesting that embedded hydrogen atoms within the IGZO layer play a role in the etching process. The Ga fraction in the chemical composition remained stable as the duration of O radical reaction step increased from 30 s to 60 s but decreased at 150 s. The peak shifts toward higher binding energies in XPS spectra of In, Ga, and Zn after ALE likely corresponded to the formation of metal–hydrocarbon etch by-products, with or without oxygen. A potential etch mechanism involving two distinct etch phases was proposed based on these findings. Hydrocarbon and hydrogen radicals were introduced into the IGZO layer during the plasma hydrocarbonation step, with hydrogen atoms penetrating more deeply. During the O radical reaction step, O radicals promote the removal of hydrocarbonated IGZO by forming volatile etch by-products, M(CxHyOz) (where M represents In, Ga, or Zn), or by eliminating excess hydrocarbons, which enhances the desorption of M(CxHy) by-products. Once the embedded carbon is completely removed, hydrogen from deeper regions of the IGZO layer diffuses to the surface, facilitating the formation of a different etch by-product, MHx. The influence of surface temperature on the ALE process was also investigated. The etch rate remained almost constant at 0.40 nm cycle−1 as the temperature increased from 20 °C to 80 °C, while dramatically increased to 0.86 nm s−1 at 120 °C. This increase in the etch rate correlated with a reduction in the Ga fraction at 120 °C, likely due to the enhanced hydrogen diffusion and increased reactivity, which promote the formation of MHx etch by-product.