Film Thickness and Composition Metrology using Spectroscopic Ellipsometry By Arun R. Srivatsa, Ph.D
-----------------------------------------------------------------------------------------
Arun is a principal staff technologist at KLA-Tencor
Corporation where he has specialized in developing and
extending applications of Spectroscropic Ellipsometry (SE)
in the semiconductor industry.
“Traditional” process control for deposition, etch, litho and CMP processes is typically based on monitoring thickness and index of films. With the introduction of newer materials and processes, like SiON and high-k gate dielectrics and B-doped Silicon Germanium for source/drain, there is an emerging need to measure or infer composition for process control. Elemental analysis techniques like X-ray Photoelectron Spectroscopy (XPS) and Secondary Ion Mass Spectrometry (SIMS) suffer from limitations in thickness measurement capability, throughput, and product wafer measurement capability. Recent advances in SE have enabled use of this technology for film thickness and composition control for several processes. Composition is inferred by establishing correlations to optical dispersions (variation of n and k with wavelength). The primary advances enabling these measurements are: a) improved spectral fidelity allowing the extraction of additional information while minimizing metrology errors b) extension of the wavelength range to 150nm, since there is more sensitivity to changes in composition at Deep Ultra-Violet (DUV) wavelengths and c) advances in applications and algorithms capabilities. 
Figure 1. Tracking %SiO2 and %N in HfSiON films across DoE with SE;
site-site measurements show very good correlation to XPS
Some critical applications using SE for thickness and composition control are: HfSiON gate metrology and SiGe:B. Candidate high-k materials are largely Hf-based and include HfO2, HfSiOx and HfSiOxNy. Typically the high-k dielectric is between 20-40 Å thick and an interfacial layer (intentionally grown or formed) about 5 to 10 Å thick is present between the high-k film and Si. Process control schemes usually rely on thickness and composition monitoring of “bulk” high k dielectric, and electrical monitoring of the interface between the high-k dielectric and Si. Optically, the interlayer can be modeled as an SiO2 layer. The optical properties of high-k materials usually vary systematically with composition, with increased sensitivity at shorter wavelengths down to 150nm due to increased absorption. Combined with recent advances in hardware, algorithms and applications methodologies, this enables the use of SE to simultaneously monitor two compositional parameters to control the HfSiOxNy layer. Figure 1 shows results using a recently developed algorithmic model to simultaneously compute both %SiO2 and %N in the film across aDoE (Design of Experiment). As seen, there is very good correlation with the baseline across the wide range of compositions in the DoE. As with high-k, a systematic variation is seen in the optical properties of SiGe with increasing Ge concentration. The presence of boron (B) at high dopant concentrations has a secondary effect on the optical properties. To measure the Ge concentration in SiGe:B, a DoE was designed with relatively constant B concentration and variation in Ge. An SE solution was developed to measure both single layer SiGe:B and bi-layer Si-cap/SiGe:B/Si films with the same recipe. Across a four wafer DoE with varying Ge concentration, excellent correlation was achieved to the baseline (X-Ray Diffraction in this instance) for Ge concentration measurements with SE. The thickness of the SiGe:B layer was in excess of 1000 Å with a thin Si cap layer. The results also showed excellent tool-to-tool matching for this measurement in a production environment. SE has made significant inroads in composition metrology for critical front-end applications in both production and development fabs. To learn more about film thickness and composition measurements, go to:
www.kla-tencor.com/08JanuaryUS | 
