Thermal Atomic Layer Etching of Silica and Alumina Thin Films Using Trimethylaluminum with Hydrogen Fluoride or Fluoroform

Rezwanur Rahman; Eric C Mattson; Joseph P Klesko; Aaron Dangerfield; Sandrine Rivillon-Amy; David C Smith; Dennis Hausmann; Yves J Chabal

doi:10.1021/acsami.8b10899

Thermal Atomic Layer Etching of Silica and Alumina Thin Films Using Trimethylaluminum with Hydrogen Fluoride or Fluoroform

ACS Appl Mater Interfaces. 2018 Sep 19;10(37):31784-31794. doi: 10.1021/acsami.8b10899. Epub 2018 Sep 4.

Authors

Rezwanur Rahman¹, Eric C Mattson¹, Joseph P Klesko¹, Aaron Dangerfield¹, Sandrine Rivillon-Amy², David C Smith³, Dennis Hausmann³, Yves J Chabal¹

Affiliations

¹ Department of Materials Science and Engineering , University of Texas at Dallas , Richardson , Texas 75080 , United States.
² Electronic Fluorocarbons, LLC , Hopkinton , Massachusetts 01748 , United States.
³ Lam Research Corporation , Tualatin , Oregon 97062 , United States.

PMID: 30179460
DOI: 10.1021/acsami.8b10899

Abstract

Thermal atomic layer etching (ALE) is an emerging technique that involves the sequential removal of monolayers of a film by alternating self-limiting reactions, some of which generate volatile products. Although traditional ALE processes rely on the use of plasma, several thermal ALE processes have recently been developed using hydrogen fluoride (HF) with precursors such as trimethylaluminum (TMA) or tin acetylacetonate. While HF is currently the most effective reagent for ALE, its potential hazards and corrosive nature have motivated searches for alternative chemicals. Herein, we investigate the feasibility of using fluoroform (CHF₃) with TMA for the thermal ALE of SiO₂ and Al₂O₃ surfaces and compare it to the established TMA/HF process. A fundamental mechanistic understanding is derived by combining in situ Fourier transform infrared spectroscopy, ex situ X-ray photoemission spectroscopy, ex situ low-energy ion scattering, and ex situ spectroscopic ellipsometry. Specifically, we determine the role of TMA, the dependence of the etch rate on precursor gas pressure, and the formation of a residual fluoride layer. Although CHF₃ reacts with TMA-treated oxide surfaces, etching is hindered by the concurrent deposition of a fluorine-containing layer, which makes it unfavorable for etching. Moreover, since fluorine contamination can be deleterious to device performance and its presence in thin films is an inherent problem for established ALE processes using HF, we present a novel method to remove the residual fluorine accumulated during the ALE process by exposure to water vapor. XPS analysis herein reveals that an Al₂O₃ film etched using TMA/HF at 325 °C contains 25.4 at. % fluorine in the surface region. In situ exposure of this film to water vapor at 325 °C results in ∼90% removal of the fluorine. This simple approach for fluorine removal can easily be applied to ALE-treated films to mitigate contamination and retain surface stoichiometry.

Keywords: FTIR; atomic layer etching; fluorine removal; fluoroform; thermal etching; trimethylaluminum.