Optimized measurement of strained Si thickness and SiGe virtual substrate composition by spectroscopic ellipsometry

Optimized parameters for measuring strained Si wafers by spectroscopic ellipsometry (SE) are presented. It is shown that SE sensitivity to the main wafer characteristics of interest–native SiO2 thickness, strained Si thickness, and composition of the underlying fully relaxed SiGe virtual substrate–is optimized for an incident angle of ∼78° and an energy (wavelength) range of ∼2.5–3.6 eV (∼350–500 nm). A series of 20 strained Si samples with thickness ranging from 10 to 26 nm were grown on either Si0.81Ge0.19 or Si0.85Ge0.15 virtual substrates and measured by SE using the optimized parameters. To enable accurate data fits the complex refractive index of strained Si was experimentally obtained and is presented, and justification is provided for ignoring the expected birefringence. Excellent agreement is demonstrated between strained Si thickness measurements made by SE and grazing incidence X-ray reflectivity. Good agreement was also obtained between the underlying SiGe virtual substrate composition measured by SE and that measured by highly accurate {224} reciprocal space X-ray diffraction maps. These results establish the suitability of SE as a measurement technique for strained Si wafers.