| AVS 65th International Symposium & Exhibition | |
| Thin Films Division | Tuesday Sessions |
| Session TF+PS-TuA |
| Session: | Atomic Layer Processing: Chemistry & Surface Reactions for Atomic Layer Processing |
| Presenter: | Andrew Cavanagh, University of Colorado at Boulder |
| Authors: | A.S. Cavanagh, University of Colorado at Boulder J.W. Clancey, University of Colorado at Boulder S. Sharma, University of Colorado at Boulder S.M. George, University of Colorado at Boulder |
| Correspondent: | Click to Email |
Thermal atomic layer etching (ALE) of Al2O3 can be accomplished using sequential, self-limiting fluorination and ligand-exchange surface reactions with hydrofluoric acid (HF) and trimethyl aluminum (TMA, Al(CH3)3) as the precursors. Fluorination by HF converts the surface of Al2O3 to AlF3. Ligand-exchange reactions then occur between TMA and the AlF3 surface. The first ligand-exchange reaction is believed to be: AlF3(s) + Al(CH3)3(g) → AlCH3F2(s) + Al(CH3)2F(g) where “s” indicates a surface species and “g” indicates a gas phase species. Additional ligand-exchange reactions can then react AlF2CH3(s) to AlF(CH3)2(g). Recent quadrupole mass spectrometry (QMS) studies have observed that the main etch products during Al2O3 ALE are the dimers AlF(CH3)2-AlF(CH3)2 and AlF(CH3)2-Al(CH3)3. These dimers may be formed from the monomer AlF(CH3)2 etch product pairing with itself or with the Al(CH3)3 metal precursor.
To understand these dimer etch products, density functional theory (DFT) calculations were performed on all possible dimers that could be produced from the four possible monomer species (Al(CH3)3, Al(CH3)2F, AlCH3F, AlF3). Each dimer consisted of a pair of bridging ligands between the two Al metal centers and four terminal ligands. The bridging ligands could be (F, F), (F, CH3) or (CH3, CH3). The (F, F) bridges resulted in the most stable dimers while the (CH3, CH3) bridges resulted in the least stable dimers. In agreement with the QMS results, these DFT calculations predict that the AlF(CH3)2-AlF(CH3)2 dimer with a (F,F) bridge and four terminal methyl groups is the most viable etch product.
Additional DFT computational studies have also been performed for ligand-exchange reactions on fluorinated surfaces of Al2O3, ZrO2 and Ga2O3 with various metal precursors including Al(CH3)3, Al(CH3)2Cl, SiCl4, GeCl4, SnCl4, and TiCl4. These calculations model the ligand-exchange surface reactions during Al2O3, ZrO2 and Ga2O3 ALE. For all systems studied to date, the calculations indicate that dimer species are the preferred etch products. Future QMS experiments will observe etch products and compare with the DFT computational studies for a more complete understanding of thermal ALE.