Multi-pass spectroscopic ellipsometry

• We present multi-pass spectroscopic ellipsometry (MPSE), a multi-pass approach to ellipsometry.
• Different detectors, samples, angles of incidence and number of passes were tested.
• N passes improve polarization ratio sensitivity to the power of N.
• N reflections improve phase shift sensitivity by a factor of N.
• MPSE can significantly improve thickness measurements in thin films.

Spectroscopic ellipsometry is an established technique, particularly useful for thickness measurements of thin films. It measures polarization rotation after a single reflection of a beam of light on the measured substrate at a given incidence angle. In this paper, we report the development of multi-pass spectroscopic ellipsometry where the light beam reflects multiple times on the sample. We have investigated both theoretically and experimentally the effect of sample reflectivity, number of reflections (passes), angles of incidence and detector dynamic range on ellipsometric observables tanΨ and cosΔ. The multiple pass approach provides increased sensitivity to small changes in Ψ and Δ, opening the way for single measurement determination of optical thickness T, refractive index n and absorption coefficient k of thin films, a significant improvement over the existing techniques. Based on our results, we discuss the strengths, the weaknesses and possible applications of this technique.