Cryogenic etching characteristics of SiO2, SiN, and a-C films with HF/fluorocarbon-containing plasmas

In this study, the etching characteristics of SiO2, SiN, and amorphous carbon films processed in CF4/H2 and hydrogen fluoride (HF)/CF4 plasmas were investigated. In CF4/H2 plasmas, where HF is generated within the plasma, SiO2 cryogenic etching was primarily governed by ion-enhanced surface reactions driven by co-adsorbed HF/H2O species. However, under conditions where fluorocarbon deposition becomes dominant—such as high H2 concentration in CF4/H2 plasmas or low ion-energy regimes—the deposited fluorocarbon layer also plays a critical role in controlling the etching reactions. The SiN etch rate decreased with increasing H2 concentration in CF4/H2 plasma and lower temperature due to reduced CFx ions and F radical densities and the increased thermal stability of the ammonium fluorosilicate layer formed on the surface. For amorphous carbon films in CF4/H2 plasma, the etch rate decreased with increasing H2 concentration and transitioned from etching to net deposition once a critical H2 concentration was exceeded as a result of hydrogen-induced fluorine scavenging. In HF/CF4 plasmas, where HF is externally supplied, the SiO2 etch rate was insensitive to the CF4 addition when its concentration was below 20%, indicating a usable process window for fine-tuning selectivity without sacrificing throughput. In contrast, the addition of CF4 increased the SiN etch rate due to the reduced ammonium fluorosilicate formation and/or enhanced its removal, presumably facilitated by HF dilution and CFx species. For amorphous carbon films, the etch rate exhibited a minimum with a small amount of CF4 addition, suggesting that fluorocarbon radical deposition characteristics are a key factor governing etch suppression. Notably, adding a small amount of fluorocarbon to cryogenic HF plasma enhanced the selectivity of both SiO2 and SiN over amorphous carbon by approximately 2.5 times, respectively, without compromising etching throughput.