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 demonstratedusing sequential exposures of Al(CH3)(3) (trimethyl-aluminum(TMA)) and HF (hydrogen fluoride). ZnS is one of the first sulfidematerials to be etched using thermal ALE techniques. In situ spectroscopicellipsometry (SE) studies were performed on ZnS films grown at 100 & DEG;C using atomic layer deposition (ALD) techniques. These studiesrevealed that the etch rate during ZnS ALE increased with temperaturefrom 1.4 & ANGS;/cycle at 225 & DEG;C to 2.1 & ANGS;/cycle at 300 & DEG;C.ZnS ALE was also self-limiting at longer TMA and HF exposures. A possiblemechanism for ZnS ALE is fluorination and ligand exchange where ZnSis fluorinated by HF and then ZnF2 undergoes ligand exchangewith Al(CH3)(3) to yield Zn(CH3)(2). Because Al(CH3)(3) may also have theability to convert ZnS to Al2S3, a second possiblemechanism for ZnS ALE is ligand exchange/conversion by TMA togetherwith fluorination by HF. To verify the conversion mechanism, in situquadruple mass spectrometry (QMS) studies revealed that Al(CH3)(3) exposures on initial ZnS substrates releasedZn(CH3)(2) products, as expected for a conversionreaction. In addition, no H2S products were observed byQMS analysis during HF exposure on the initial ZnS substrates. However,after Al(CH3)(3) exposures on ZnS, QMS measurementsmonitored H2S from HF exposures, as expected if Al(CH3)(3) converts ZnS to Al2S3.These QMS results provide direct evidence for the conversion of ZnSto Al2S3 during ZnS ALE. Time-dependent QMSresults also revealed that the conversion/ligand-exchange and fluorinationreactions were self-limiting. In addition, QMS analysis observedAl( x )F( y )(CH3)( z ) dimers and trimers as ligand-exchangeproducts during the Al(CH3)(3) exposures. Becausethe Al2S3 conversion layer thickness is dependenton Al(CH3)(3) exposures, larger Al(CH3)(3) pressures over equivalent times led to higher ZnS etchrates. In contrast, larger HF pressures over equivalent times hada small effect on the ZnS etch rate because HF is able to fluorinateonly the converted Al2S3 layer thickness. TheZnS etch rate was slightly dependent on the ZnS ALD growth temperature.The ZnS etch rate at 300 & DEG;C was 1.4, 1.0, and 0.8 & ANGS;/cyclefor ZnS ALD films grown at 100, 200, and 300 & DEG;C, respectively.The lower ZnS etch rates for the ZnS ALD films grown at higher temperatureswere attributed to the larger density and higher sulfur content ofZnS ALD films grown at higher temperatures. The ZnS ALD films witha 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 7.4to 5.9 & ANGS; after 15 ALE cycles. However, after 30 ALE cycles, theRMS roughness gradually increased with the ALE cycles.