Date of Award
Doctor of Philosophy (PhD)
Chemistry & Biochemistry
Steven M. George
Carl A. Koval
David M. Walba
Atomic control of thin film growth and removal is essential for semiconductor processing. Atomic layer deposition (ALD) is a thin film deposition technique that is based on self-limiting surface reactions that deposit thin conformal films with atomic scale precision. The ALD of metal fluorides has been limited by the difficulty of handling the hydrogen fluoride (HF) precursor that is necessary for metal fluoride film growth. The use of HF-pyridine as an HF reservoir has allowed the development of metal fluoride ALD processes such as AlF3, LiF, lithium ion conducting (AlF3)(LiF)x alloy, ZrF4, HfF4, MnF2, MgF2, and ZnF2. AlF3 ALD was studied using trimethylaluminum (TMA) and HF at temperatures from 150-300°C by in situ techniques such as quartz crystal microbalance (QCM), quadrupole mass spectrometry (QMS) and Fourier transform infrared (FTIR) spectroscopy. Ex situ characterization of the films was conducted using X-ray reflectivity (XRR) and spectroscopic ellipsometry (SE), grazing incidence X-ray diffraction (GIXRD), X-ray photoelectron spectroscopy (XPS), and Rutherford backscattering spectrometry (RBS). These high quality metal fluoride ALD films will be useful for many applications such as optical coatings and Lewis acid catalysts.
”Reverse ALD”, i.e. atomic layer etching (ALE), is a film removal technique that is able to remove thin films with atomic level control. Current ALE processes based on halogen adsorption and ion-enhancement are inherently directional processes that lead to an anisotropic removal of material. New thermal approaches for Al2O3 and HfO2 ALE were demonstrated using Sn(acac)2 and HF as the reactants. In situ and ex situ studies showed that thermal ALE of Al2O3 and HfO2 is possible at temperatures from 150-250°C. In the proposed reaction mechanism, the HF reactant fluorinates the metal oxide surfaces to form the corresponding metal fluoride and H2O as reaction products. The Sn(acac)2 reactant induces a ligand exchange reaction to produce volatile SnF(acac) and Al(acac)3 or Hf(acac)4. This etching approach based on fluorination and ligand exchange is general and can be applied to other metal oxides, as well as metal nitrides, metal phosphides, metal arsenides and elemental metals. Thermally driven ALE processes will provide an important tool for isotropically etching materials at the atomic scale.
Lee, Younghee, "Atomic Layer Etching of Metal Oxides and Atomic Layer Deposition of Metal Fluorides" (2015). Chemistry & Biochemistry Graduate Theses & Dissertations. 152.