Cyclic etching of SiO2 contact holes using heptafluoropropyl methyl ether having low global-warming potential

Abstract

SiO₂ contact holes having high aspect ratios were etched using a cyclic-etching process employing heptafluoropropyl methyl ether (HFE-347mcc3) which exhibits a low global-warming potential (GWP ≈530) compared with conventional perfluorocarbons (PFCs) and hydrofluorocarbons (HFCs). In this cyclic process, the etching steps involving HFE-347mcc3/O₂/Ar plasmas were alternated with deposition steps involving HFE-347mcc3/Ar plasmas to precisely control the fluorocarbon passivation films on the sidewalls. The effects of the deposition- and etching-step durations on the contact-hole profiles were systematically investigated. Increasing the duration of the deposition step initially improved sidewall protection, thereby significantly reducing bowing and enhancing anisotropy; however, excessively long deposition-step durations caused narrowing. Similarly, the duration of the etching step was optimized to achieve sufficient fluorocarbon-film hardening to prevent bowing while avoiding excessive narrowing. The degree of exposure to fluorocarbon plasma was introduced as a critical parameter to optimize the anisotropy. Under optimized conditions, a nearly vertical and highly anisotropic SiO₂ contact hole having a diameter of 100 nm and an aspect ratio of 24 was successfully obtained with minimal bowing (1 nm). The results revealed that HFE-347mcc3 is a viable and environmentally sustainable alternative to high-GWP PFCs or HFCs and provide both environmental benefits and excellent etching performance.