# Cryogenic‐Assisted Hydrogen Fluoride Surface Reactions Enabling Reversibly Ultra‐High Selectivity of Atomic Layer Etching Between SiO2 and SiN

## TL;DR

A new method for ultra-selective etching of SiO2 and SiN is developed using hydrogen fluoride and ethanol under cryogenic conditions.

## Contribution

A cryogenic-assisted ALE process is introduced that enables reversible ultra-high selectivity between SiO2 and SiN.

## Key findings

- Ethanol addition at low temperatures enhances HF surface reactions on SiO2, increasing etch efficiency.
- SiN etch amount per cycle decreases to zero at lower temperatures due to a stable modification layer.
- Reversible etching selectivity between SiO2 and SiN is achieved with a selectivity ratio approaching infinity.

## Abstract

Atomic‐level precision processes are increasingly essential for advanced semiconductor devices with highly‐complicated small features. Plasma‐enhanced atomic layer etching (ALE) is considered as a promising technique to meet requirements of material diversity and highly selective processing. Here, the ALE processes of SiO2 and SiN are demonstrated through manipulating hydrogen‐fluoride (HF) reactions for surface modification, followed by argon ion bombardment for material removal. By varying substrate temperature (T
s) and introducing ethanol (C2H5OH) gas, which provides hydroxyl groups for cryogenic‐assisted synergistic reactions, the surface HF reactions and the properties of the surface modification layer can be significantly influenced. The etch amount per cycle (EPC) of SiN ALE decreases to zero with decreasing T
s, regardless of C2H5OH addition, due to increased stability of the (NH4)2SiF6 modification layer. No ALE synergy is observed for SiO2 when C2H5OH is not added during the HF dose step, irrespective of T
s. Conversely, the addition of C2H5OH at cryogenic temperatures enables the synergistic interactions between HF molecules and hydroxyl groups, enhancing the co‐adsorption of HF/C2H5OH and lowering activation energy for the fluorination reaction of the SiO2 that leads to the increased EPC. Consequently, the reversible etching selectivity between ALE SiO2 and SiN, reaching up to infinity, is achieved.

Plasma‐enhanced atomic layer etching (ALE) of SiO2 and SiN is demonstrated using HF dosing for forming a modification layer and material removal by Ar ions. By controlling temperature and adding ethanol for cryogenic‐assisted reactions, surface modification and etch selectivity are tuned, achieving reversible selectivity between SiO2 and SiN up to infinity.