Thermal atomic layer etching of HfO2 using HF for fluorination and TiCl4 for ligand-exchange

Abstract

Thermal atomic layer etching (ALE) can be accomplished using sequential fluorination and ligand-exchange reactions. HF has been a typical fluorination reactant. Various metal reactants have been used for ligand-exchange, such as Sn(acac)2, Al(CH3)3, AlCl(CH3)2, and SiCl4. This study explored TiCl4 as a new metal chloride reactant for ligand-exchange. Thermal HfO2 ALE using HF and TiCl4 as the reactants was studied using in situ quartz crystal microbalance (QCM) measurements from 200 to 300 °C. The HfO2 films were etched linearly versus the number of HF and TiCl4 reaction cycles. The sequential HF and TiCl4 reactions were also self-limiting versus reactant exposure. The QCM studies observed a mass change per cycle (MCPC) of −10.2 ng/(cm2 cycle) at 200 °C and −56.4 ng/(cm2 cycle) at 300 °C. These MCPCs correspond to HfO2 etch rates of 0.11 Å/cycle at 200 °C and 0.59 Å/cycle at 300 °C. To explore the selectivity of thermal ALE using HF and TiCl4 as the reactants, spectroscopic ellipsometry (SE) measurements were also employed to survey the etching of various materials. The SE results revealed that HfO2 and ZrO2 were etched by HF and TiCl4. In contrast, Al2O3, SiO2, Si3N4, and TiN were not etched by HF and TiCl4. The etching selectivity can be explained by the reaction thermochemistry and the stability and volatility of the possible etch products. Al2O3 can also serve as an etch stop for HfO2 ALE.