Thermal Atomic Layer Etching of Zinc Sulfide Using Sequential Trimethylaluminum and Hydrogen Fluoride Exposures: Evidence for a Conversion Mechanism

Thermal atomic layer etching (ALE) of zinc sulfide (ZnS) was demonstrated using sequential exposures of Al(CH3)3 (trimethylaluminum (TMA)) and HF (hydrogen fluoride). ZnS is one of the first sulfide materials to be etched using thermal ALE techniques. In situ spectroscopic ellipsometry (SE) studies were performed on ZnS films grown at 100 °C using atomic layer deposition (ALD) techniques. These studies revealed that the etch rate during ZnS ALE increased with temperature from 1.4 Å/cycle at 225 °C to 2.1 Å/cycle at 300 °C. ZnS ALE was also self-limiting at longer TMA and HF exposures. A possible mechanism for ZnS ALE is fluorination and ligand exchange where ZnS is fluorinated by HF and then ZnF2 undergoes ligand exchange with Al(CH3)3 to yield Zn(CH3)2. Because Al(CH3)3 may also have the ability to convert ZnS to Al2S3, a second possible mechanism for ZnS ALE is ligand exchange/conversion by TMA together with fluorination by HF. To verify the conversion mechanism, in situ quadruple mass spectrometry (QMS) studies revealed that Al(CH3)3 exposures on initial ZnS substrates released Zn(CH3)2 products, as expected for a conversion reaction. In addition, no H2S products were observed by QMS analysis during HF exposure on the initial ZnS substrates. However, after Al(CH3)3 exposures on ZnS, QMS measurements monitored H2S from HF exposures, as expected if Al(CH3)3 converts ZnS to Al2S3. These QMS results provide direct evidence for the conversion of ZnS to Al2S3 during ZnS ALE. Time-dependent QMS results also revealed that the conversion/ligand-exchange and fluorination reactions were self-limiting. In addition, QMS analysis observed AlxFy(CH3)z dimers and trimers as ligand-exchange products during the Al(CH3)3 exposures. Because the Al2S3 conversion layer thickness is dependent on Al(CH3)3 exposures, larger Al(CH3)3 pressures over equivalent times led to higher ZnS etch rates. In contrast, larger HF pressures over equivalent times had a small effect on the ZnS etch rate because HF is able to fluorinate only the converted Al2S3 layer thickness. The ZnS etch rate was slightly dependent on the ZnS ALD growth temperature. The ZnS etch rate at 300 °C was 1.4, 1.0, and 0.8 Å/cycle for ZnS ALD films grown at 100, 200, and 300 °C, respectively. The lower ZnS etch rates for the ZnS ALD films grown at higher temperatures were attributed to the larger density and higher sulfur content of ZnS ALD films grown at higher temperatures. The ZnS ALD films with a larger density may be more difficult to convert from ZnS to Al2S3 during the Al(CH3)3 reaction. The RMS roughness of the ZnS films was slightly decreased from 7.4 to 5.9 Å after 15 ALE cycles. However, after 30 ALE cycles, the RMS roughness gradually increased with the ALE cycles.