Polysilicon-germanium gate patterning studies in a high density plasma helicon source

Abstract

High density plasma etching processes using halogen based chemistries have been studied for 0.2 μm polysilicon-germanium gate patterning. Bilayer gate stacks consisting of 80 nm polycrystalline Si on 120 nm polycrystalline Si1−xGex (x was varied between 0.55 and 1) were grown on 4.5 nm SiO2 covered 200 mm diameter p-type silicon wafers. The bilayer gates were masked with oxide patterns. The wafers were etched in a low pressure, high density plasma helicon source. Various mixtures, based on Cl2, HBr, and O2 gases, have been used to investigate the etching of the Si/SiGe bilayer gates. The gas mixture and the plasma operating conditions have been optimized to obtain anisotropic etching profiles for features down to 0.2 μm, and to minimize the gate oxide consumption. Real time process control was achieved using HeNe ellipsometry in blanket areas, allowing the SiGe/oxide transition to be easily detected. A two step etching process using a Cl2/O2–He mixture was developed. The first step uses a high energy ion bombardment in order to obtain a high etch rate, and the second step uses a lower ion energy to achieve high SiGe/oxide selectivity. The second step is started 40 nm before reaching the SiGe/SiO2 interface in order to reduce gate oxide consumption and structural defects formation at the edges of the gate.