Role of CF4 Addition in Gas-Phase Variations in HF Plasma for Cryogenic Etching: Insights from Plasma Simulation and Experimental Correlation

The fabrication of semiconductor devices with three-dimensional architectures imposes unprecedented demands on advanced plasma dry etching processes. These include the simultaneous requirements of high throughput, high material selectivity, and precise profile control. In conventional reactive ion etching (RIE), fluorocarbon plasma provides both accelerated ion species and reactive neutrals that etch the feature front, while the CFx radicals promote polymerization that protects sidewalls and enhance selectivity to the amorphous carbon layer (ACL) mask. In this work, we present computational results on the role of CF4 addition to hydrogen fluoride (HF) plasma for next-generation RIE, specifically cryogenic etching. Simulations were performed by varying the CF4 concentration in the HF plasma to evaluate its influence on ion densities, neutral species concentration, and electron density. The results show that the densities of CFx (x = 1–3) ions and radicals increase significantly with CF4 addition (up to 20%), while the overall plasma density and the excited HF species remain nearly unchanged. The results of plasma density and atomic fluorine density are consistent with the experimental observations of the HF/CF4 plasma using an absorption probe and the actimetry method. It was verified that the gas-phase reaction model proposed in this study can accurately reproduce the plasma characteristics of the HF/CF4 system. The coupling of HF-based etchants with CFx radicals enables polymerization that preserves SiO2 etching throughput while significantly enhancing etch selectivity against the ACL mask from 1.86 to 5.07, with only a small fraction (~10%) of fluorocarbon gas added. The plasma simulation provides new insights into enhancing the etching performance of HF-based cryogenic plasma etching by controlling the CF2 radicals and HF reactants through the addition of fluorocarbon gases.