An in-line, in-process or in-situ and non-destructive metrology system, apparatus and method provides composition, quality and/or thickness measurement of a thin film or multi-layer thin film formed on a substrate in a thin film processing system. Particularly, the subject invention provides a spectroscopic ellipsometer performing spectroscopic ellipsometry while the wafer is in a thin film processing system. In one form, the spectroscopic ellipsometer is associated with a wet bench system portion of the thin film processing system. The spectroscopic ellipsometer obtains characteristic data regarding the formed thin film to calculate penetration depth (D.sub.p) for a thin film formed on the substrate. Particularly, the ellipsometer obtains an extinction coefficient (k) which is used to calculate penetration depth (D.sub.p). Penetration depth (D.sub.p), being a unique function of the extinction coefficient (k) provides the information for the composition, quality and/or thickness monitoring of the thin film.