PATTERNING OF HAFNIUM ALUMINIUM OXIDE/POLY-SI GATE STACK FOR ADVANCED CMOS APPLICATIONS

Abstract

The patterning of hafnium aluminium oxide/PolySi (HfAlO/polySi) gate stack was investigated. To etch polySi with high etch rate, high RF power of 400W and Cl-based plasma(s) were incorporated into the polySi recipe. HBr-based plasma together with He-O₂ gases showed best selectivity for etching HfAlO films compared to Cl-based plasma. AFM scans showed that Root Mean Square (RMS) roughness of HfAlO film etched by H-2 recipe was quite low and lower than that of etched-HfO₂ films by Pearton's group [24]. The final gate stack, consisting of 80Ǻ HfAlO and 2000 Ǻ polySi film, was dry etched using photoresist mask. It showed relatively a good fidelity transfer with straight sidewalls and good resist integrity during HBr plasma etch as no facet formation was observed. Etch artifacts such as microtrenching, facet formation and undercutting, were not observed on the pattern. However, polymer film was found on the pattern from cross section SEM. The presence of the polymer film was in good agreement with Auger Electron Spectroscopy (AES) spectra, which was able to detect the significant carbon concentration on three different regions, namely region near the sidewalls of the gate stack, region between the pattern and region on the pattern. This polymer film was shown to aid in the formation of straight sidewalls as this passivation layer is etch resistant in HBr plasma. This film was found by cross section SEM to be 20 nm thick. Sidewall bowing was minimal, hence, no microtrenching was observed. Further work needs to be performed for polymer film removal employing NH₄OH and H₂O₂ chemical solution. A relatively straight sidewall profiles developed during the HBr etching explain the nonoccurrence of microtrenches. The absence of microtrenching is probably associated with the absence or the minimum sidewall bowing near the bottom of the gate stack. The microtrenching is probably associated with two mechanisms: ion scattering from tapered sidewalls and the focusing of directional ions by bowed sidewalls onto the feature bottom.