Plasma etching of HfO₂ at elevated temperatures in chlorine-based chemistry

International audiencePlasma etching of HfO2 at an elevated temperature is investigated in chlorine-based plasmas. Thermodynamic studies are performed in order to determine the most appropriate plasma chemistry. The theoretical calculations show that chlorocarbon gas chemistries (such as CCl4 or Cl2–CO) can result in the chemical etching of HfO2 in the 425–625K temperature range by forming volatile effluents such as HfCl4 and CO2. The etching of HfO2 is first studied on blanket wafers in a high density Cl2–CO plasma under low ion energy bombardment conditions (no bias power). Etch rates are presented and discussed with respect to the plasma parameters. The evolution of the etch rate as function of temperature follows an Arrhenius law indicating that the etching comes from chemical reactions. The etch rate of HfO2 is about 110Å/min at a temperature of 525K with a selectivity towards SiO2 of 15. x-ray photoelectron spectroscopy analyses (XPS) reveal that neither carbon nor chlorine is detected on the HfO2 surface, whereas a chlorine-rich carbon layer is formed on top of the SiO2 surface leading to the selectivity between HfO2 and SiO2. A drift of the HfO2 etch process is observed according to the chamber walls conditioning due to chlorine-rich carbon coatings formed on the chamber walls in a Cl2–CO plasma. To get a very reproducible HfO2 etch process, the best conditioning strategy consists in cleaning the chamber walls with an O2 plasma between each wafer. The etching of HfO2 is also performed on patterned wafers using a conventional polysilicon gate. The first result show a slight HfO2 foot at the bottom of the gate and the presence of hafnium oxide-based residues in the active areas