Thermal atomic layer etching of SnO2 by fluorination and ligand-exchange/conversion reactions using sequential hydrogen fluoride and Al(CH3)3 exposures

Thermal atomic layer etching (ALE) of SnO2 was performed using a sequence of self-limiting fluorination and subsequent ligand-exchange/conversion reactions. The trimethylaluminum (TMA) can convert the SnO2 surface to an Al2O3 layer. Hydrogen fluoride (HF) then fluorinates the Al2O3 surface layer to form metal fluoride. Subsequently, TMA undergoes a ligand-exchange reaction and removes the metal fluoride by forming volatile products before again converting SnO2 to Al2O3. The initial SnO2 layers were prepared via atomic layer deposition by employing tetrakis(dimethylamino) tin and H2O2. The thermal SnO2 ALE was then studied using various techniques, including quartz crystal microbalance (QCM), x-ray reflectivity (XRR), quadrupole mass spectrometry (QMS), and atomic force microscopy (AFM) measurements. In situ QCM experiments monitored SnO2 ALE at temperatures from 225 to 300 °C. A linear reduction in SnO2 mass was observed as the number of HF and TMA cycles increased. QCM measurements confirmed that both HF and TMA reaction steps reached saturation with respect to reactant exposures, indicating self-limiting behavior. Higher etching temperatures led to higher SnO2 removal rates. The QCM analysis measured mass change per cycle (MCPC) values that varied from −32.6, −44.2, −100.2, and−123.5 ng/(cm2 cycle) at 225, 250, 275, and 300 °C, respectively. These MCPCs correspond to SnO2 etch rates of 0.47, 0.64, 1.44, and 1.78 Å/cycle for 225, 250, 275, and 300 °C, respectively. XRR measurements confirmed the linear removal of the SnO2 film thickness and the etching rates. QMS analysis also revealed the volatile etching products during the sequential HF and TMA exposures on SnO2 at 300 °C. These QMS investigations monitored Sn(CH3)3+ ion intensities during TMA exposures. The Sn(CH3)3+ ion intensity was consistent with Sn(CH3)4 as the main Sn etch product. In addition, AlxFy(CH3)z dimer and trimer species were identified as the ligand-exchange products. QMS studies also revealed that Al(CH3)3 exposures on initial SnO2 substrates prior to fluorination released Sn(CH3)4 products. These Sn(CH3)4 products are expected if Al(CH3)3 can convert SnO2 to Al2O3. These results indicate that Al(CH3)3 can both convert the SnO2 surface to an Al2O3 layer and remove the fluorinated Al2O3 layer by ligand-exchange reactions. The conversion and ligand-exchange reactions both produce Sn(CH3)4. Atomic force microscopy measurements also indicated that multiple thermal ALE cycles did not significantly change the roughness of the SnO2 surface.