Ellipsometry

Ellipsometry

Ellipsometry is an optical measurement technique that is based upon the fact that linearly polarized light, incident at an oblique angle to a surface, becomes elliptically polarized when it is reflected, transmitted, or scattered. Ellipsometry measures the change in amplitude and the phase difference and relates it to material and surface properties using theoretical models. The ellipsometry arrangement will comprise a light source, polarization generator, test sample, polarization detector, analyzer and data processing system. There are three different types of ellipsometry, namely reflection, transmission and scattering ellipsometry. The determination of a wide range of material and surface properties such as refractive index, film thickness, film uniformity, surface roughness and surface oxides can be done by Ellipsometry.

Ellipsometry is often used to measure the thickness of thin films on top of a substrate, from sub-nanometer to a few micron thicknesses. The wavelength of the incident light needs to be adjusted appropriately to minimize light absorption by the material. Some organic films may strongly absorb light at UV and IR wavelengths but not at mid-visible wavelengths. However, in metals, all wavelengths are absorbed, so the maximum layer for film thickness by Ellipsometry is typically about 100 nm. Mie-Scattering Ellipsometry is used on particles whose size are larger than incident wavelength, to measure size and refractive index of particles using ellipsometric parameters of elastically scattered light. In the semiconductor industry, the thickness of a thin layer of SiO₂ on a silicon wafer can be measured at the accuracy of angstrom units using Ellipsometry.

Common Uses of Ellipsometry

• Determination of Thickness and refractive index of thin films
• Calculation of pseudo-dielectric function of a substrate
• Determination of surface roughness or uniformity
• Study of solid-liquid or liquid-liquid interfaces

Advantages of Ellipsometry

• Extremely sensitive to very thin films
• Non-invasive, non-destructive, non-contact
• Determines several film properties simultaneously
• Rapid and reliable
• Measured parameters are independent of beam intensity.
• Simple and versatile, and can be performed in a wide range of experimental environments

Limitations of Ellipsometry

• If the surface is too rough, it scatters the probe beam away from the detector, which prevents spectroscopic ellipsometry measurements.
• Films thicker than several tens of microns experience interference oscillations

Industrial Applications of Ellipsometry

• Semiconductors
•  Dielectrics
•  Polymers,
• Organic films
• Metals
• superconductors
• Biological coatings
• Composites