Spectroscopic Ellipsometer

Overview

Ellipsometry is an optical technique for measuring the thickness and optical constants (refractive index and extinction coefficient) of thin films from the polarization state of reflected light. When linearly polarized light reflects from a film, the amplitude and phase of the reflected s- and p-polarized components change differently, so the reflected light becomes elliptically polarized. By measuring this ellipticity with high precision, using rotating analyzer and polarizer optics, the system deduces the film's thickness and complex refractive index. Spectroscopic ellipsometry extends this across 250–1700 nm, building a refractive-index spectrum and high-precision thickness profile in a single measurement.

The Light Source Assembly emits broadband UV-visible light, coupled through a fiber to the Polarizer Arm Assembly. The Polarizer Arm Assembly rotates a linear polarizer to set the incident polarization state. Light reflects from the sample on the Sample Stage Assembly, where the incident angle is fixed (typically 70°) or motorized through a goniometer. The Analyzer Arm Assembly spins an analyzer to dissect the reflected light's polarization, and the Spectrometer Module measures intensity as a function of wavelength. The Control Electronics synchronizes polarizer and analyzer rotation with spectrometer readout, and the Data Processor Unit iteratively fits the spectrum to a multi-layer film model, returning thickness and refractive index.

How it works

Ellipsometry quantifies the complex reflection coefficients rₚ and ᵣₛ for p- and s-polarized light. The key parameter is tan(ψ) = |rₚ|/|rₛ| and Δ = phase(rₚ) − phase(rₛ). These parameters are measured by rotating the Polarizer Arm Assembly through a sequence of azimuths and recording the reflected light intensity at each angle as the Analyzer Arm Assembly also rotates. Data fitting uses the Fresnel equations for stratified media: for a known substrate (e.g., silicon) and unknown overlying film, software assumes a film model (thickness t, refractive index n), computes its predicted ψ and Δ, and adjusts (t, n) to match the measured values.

Spectroscopic operation repeats this measurement at many wavelengths, typically 400 points across 250–1700 nm. The Spectrometer Module uses a diffraction grating and CCD array; at each wavelength, the reflection coefficient depends on optical constants and film thickness in a way that is highly sensitive to both. By fitting the entire spectrum jointly, the Data Processor Unit uses the wavelength dependence of the refractive index (Kramers-Kronig consistency) and the multiple-reflection interference in the film to constrain the solution, often yielding sub-angstrom accuracy on thickness and precise spectral n(λ) profiles.

The Sample Stage Assembly can be motorized to step the incident angle (45°–75°) or azimuth, allowing the measurement of anisotropic films or mapping spatial variation across a substrate.

Accuracy is limited by light source stability, alignment (calibrated with the Calibration Assembly), and model assumptions. Real samples with roughness, inhomogeneity, or unknown chemistry can introduce systematic errors, so expert judgment is required in model selection.

Applications

Ellipsometry is the standard metrology for semiconductor thin films: gate oxides, dielectric spacers, photoresist thickness, and copper or aluminum interconnects are all verified by ellipsometry. In optical coatings, it measures anti-reflective coatings, high-index and low-index layer thicknesses in multilayer stacks, and coating uniformity. In materials science, it characterizes native oxides on metals, corrosion films, and polymer adsorption layers. In biology, it measures lipid bilayer thickness and protein layer thickness on biosensing chips. In optoelectronics manufacturing, ellipsometry monitors quantum-well layer thickness in epitaxial structures and verifies doping profiles in GRIN lenses.