Thermal Atomic Layer Etching of ZnO by a “Conversion-Etch” Mechanism Using Sequential Exposures of Hydrogen Fluoride and Trimethylaluminum

The atomic layer etching (ALE) of ZnO thin films was demonstrated using sequential, self-limiting thermal reactions with hydrogen fluoride (HF) and trimethylaluminum (TMA) as the reactants. The initial polycrystalline ZnO films were grown by atomic layer deposition (ALD) using diethylzinc (DEZ) and H 2 O at 150 °C. The thermal ZnO ALE process was then studied between 205 and 295 °C using various techniques. In situ quartz crystal microbalance (QCM) measurements monitored ZnO ALE at 265 °C. The ZnO etching was linear versus number of ALE cycles. The HF exposure caused a mass gain of Δ M HF = +53 ng/cm 2 from fluorination. The subsequent TMA exposure caused a large mass loss of Δ M TMA = −172 ng/cm 2 . This mass loss was much larger than expected from metal fluoride removal resulting from ligand-exchange transmetalation. The large mass loss suggested that there is a “conversion-etch” mechanism where TMA also converts the ZnO surface to an Al 2 O 3 surface layer. This conversion reaction is believed to occur according to 3ZnO + 2Al(CH 3 ) 3 → Al 2 O 3 + 3Zn(CH 3 ) 2 . The ALE reaction would then proceed by Al 2 O 3 + 6HF + 4Al(CH 3 ) 3 → 6AlF(CH 3 ) 2 + 3H 2 O. In this etching process, the Al 2 O 3 layer is first fluorinated by HF to produce an AlF 3 surface layer. The AlF 3 surface layer is then removed by ligand-exchange transmetalation with TMA to yield volatile AlF(CH 3 ) 2 reaction products. After the AlF 3 removal, TMA then reacts with additional ZnO to regenerate the Al 2 O 3 surface layer. The average mass change per cycle (MCPC) during ZnO ALE was −119 ng/cm 2 at 265 °C. This MCPC corresponds to an etch rate of 2.11 Å/cycle using a ZnO film density of 5.62 g/cm 3 . The QCM measurements showed that the sequential HF and TMA reactions were self-limiting versus reactant exposure. Ex situ spectroscopic ellipsometry (SE) determined that the etch rates were temperature dependent, and the etch rates leveled off at higher temperatures. The etch rates ranged from 0.01 Å/cycle at 205 °C to 2.19 Å/cycle at 295 °C. Atomic force microscopy (AFM) measurements observed that the ZnO ALD films were smoothed by ZnO ALE. The conversion-etch mechanism may occur during the ALE of other metal compounds. This conversion-etch mechanism reaction may expand the number of materials that can be etched by thermal ALE methods.