Process monitoring of hemispherical-grained silicon thin films for dynamic random access memory applications

Abstract

Spectroscopic ellipsometry is employed to correlate the effective thickness of hemispherical-grained silicon (HSG-Si) films on underlying Si capacitor plates to final device capacitance. The fabrication of HSG-Si was developed to increase the storage plate surface area, and hence the capacitance, of dynamic random access memory devices. In this article, a novel combination of modeling techniques is employed to calculate the effective thickness of the HSG-Si film, together with the thickness and degree of crystallinity of the underlying Si capacitor plate. Seven HSG-Si films were fabricated using the “seed and anneal” process. By utilizing a range of seeding temperatures, the morphology and surface area enhancement of the films were deliberately varied. Using broadband spectroscopic ellipsometry, combined with a harmonic oscillator model for the HSG-Si film, a correlation of >95% is demonstrated between the effective thickness of the as-deposited HSG-Si films and the completed device capacitance enhancement. Using a Bruggeman effective medium approximation to model the underlying Si capacitor plate, the crystallinity of that layer was determined to be in the range 78.9%–82.7% for six out of the seven wafers. One misprocessed wafer with a markedly more amorphous capacitor plate was detected. All spectral analyses were performed using a single combination of models for all seven HSG-Si/Si/SiO2/crystalline Si film stacks.