What is Ellipsometry?
Ellipsometry is an optical measurement technique that determines the change in polarization state of light reflected from a surface. Because this polarization change depends on the optical properties and thickness of any film present on the surface, ellipsometry can determine film thickness, refractive index, extinction coefficient and optical bandgap with remarkable accuracy and without any physical contact with or damage to the sample.
Spectroscopic vs. Single-Wavelength Ellipsometry
Single-wavelength ellipsometry measures at one fixed wavelength and provides thickness for simple single-layer transparent films. Spectroscopic ellipsometry (SE) measures across a broad wavelength range (typically 190-2100 nm), acquiring a complete optical spectrum that allows simultaneous determination of thickness and optical constants for complex multilayer systems, graded films and absorbing materials. SE is the standard for demanding semiconductor and optical coating applications.
What Ellipsometry Can Measure
- Film thickness from less than 0.1 nm (monolayers) to several micrometers
- Refractive index n and extinction coefficient k as a function of wavelength
- Optical bandgap and Urbach energy of semiconductors
- Surface roughness via effective medium approximation modeling
- Layer-by-layer structure of multilayer optical coatings
- Anisotropy and birefringence of crystalline and oriented films
The Measurement Process
Ellipsometry measures the ratio of Fresnel reflection coefficients for p- and s-polarized light, expressed as the angles Psi and Delta. These measured values are then fitted to an optical model using dispersion relations (Cauchy, Sellmeier, Tauc-Lorentz) to extract film thickness and optical constants. The quality of optical model construction is critical: a physically motivated model with appropriate dispersion law gives accurate, meaningful parameters.
Applications of Spectroscopic Ellipsometry
- Semiconductor manufacturing – gate oxide thickness, implant damage, photoresist profiles
- Optical coatings – anti-reflection, high-reflectance and filter coating analysis
- Organic electronics – OLED and organic photovoltaic layer characterization
- Biosensors – protein adsorption layer thickness and surface functionalization
- Corrosion science – passive oxide film growth kinetics
- Polymer films – thickness uniformity and optical properties of coatings
Choosing an Ellipsometer
The HORIBA UVISEL 2 phase-modulated spectroscopic ellipsometer covers 190-2100 nm with exceptional accuracy and is available in manual, automated mapping and in-situ configurations. Phase modulation technology provides inherently high signal-to-noise ratio and immunity to light source intensity fluctuations. NewRoad distributes HORIBA ellipsometry systems with full optical modeling software support.
