Determination of SiGe Optical Properties for Accurate Ellipsometry Measurement

For mobility enhancement through local stress SiGe films are very attractive candidates for the 28nm CMOS technology node. In a production environment the in-line measurements of the SiGe thickness and Ge content become key parameters. Although techniques such as XRD and SIMS are traditionally adopted as reference technique for Ge thickness and concentration there is a clear need for rapid and non-destructive inline measurements such as spectroscopic ellipsometry. One of the key requirements for ellipsometry measurement is a very accurate determination of the optical properties of SiGe versus Ge%. As it stands, the experimental determination of the n & k dispersions curves is inherently affected by experimental errors that can spoil the correctness of the measurements. The SiGe dispersion curves are indeed usually determined independently for each percentage of Ge. This kind of approach can lead to discrepancies between the curves for different Ge percentage. We present in this article an original technique that recalculate the experimental values by ensuring that for each wavelength the relation between n and k versus Ge% is regular and free of scattered points (Figure 1). The resulting set of n & k functions, as defined in the bidimentional domain of the wavelength and the Ge%, are smooth and free of irregularities. This is clearly better adapted for a non-linear search of the optimal fit solution of ellipsometric data. In this paper we show that an improvement in SiGe thickness and Ge% measurement can be achieved by the implementation of this technique both for monitor and production wafers. 45 5. 3 nm wavelength 455.3 nm Wavelength (nm) x (Ge%) FIGURE 1. SiGe refractive index versus wavelength for different Ge%. In the insert n vs Ge% (λ=455.3 nm). REFERENCES1. Safa O. Kasap and Peter Capper, Springer Handbook of Electronic and Photonic Materials, Springer, 2006, pp. 448-449.