Comparison of Spectroscopic Ellipsometry and Spectral Reflectance: Thickness Measurement Guide

Both Spectroscopic Ellipsometry (SE) and Spectral Reflectance (SR) analyze reflected light to determine the thickness and refractive index of dielectrics, semiconductors, and thin metal films. The primary difference between the ellipsometry and reflectometry measurement techniques is that SE uses light reflected off the film at a low incident angle, while SR uses light that is reflected perpendicular (normal) to the film.

Spectroscopic Ellipsometry

This difference in spectroscopic ellipsometry's incident-light angle leads to differences in the cost, complexity, and capabilities of the ellipsometry and reflectometry measurement techniques. Since an ellipsometer’s light is incident at an angle, it must analyze the polarization of the reflected light, as well as its intensity, which gives the spectroscopic ellipsometer better capabilities for measuring very thin and complicated film stacks. Analyzing the polarization, however, also means that expensive, precision-moving optical components are required.

Spectral Reflectance

By using light that is perpendicular to the film, SR can ignore polarization effects (since most films are rotationally symmetric). Thus, SR can be realized without any moving components, which results in much simpler and lower-cost instruments. Spectral reflectance systems can also easily include transmittance analysis for even greater power.

Referring to the Spectral Reflectance / Spectroscopic Ellipsometry table below, spectral reflectance is usually the preferred thickness measurement technique for films greater than 10um, while spectroscopic ellipsometry is generally preferred for films thinner than 10nm thick. Between these two thicknesses, there are many applications where both spectral reflectance and spectroscopic ellipsometry techniques can be used. When this is the case, a spectroscopic reflectometer is often chosen due to its speed, simplicity, and low cost.