Ellipsometry is an optical technique for measuring the thickness and optical properties of extremely thin films, or layers, of material. The measurable properties are the refractive index, or how much light is bent, and the level of light absorption, called the absorption coefficient. An ellipsometer is a device used to perform these measurements.
Ellipsometers work by shining a well-defined source of light on a material and capturing the reflection. Modern ellipsometers use lasers, typically Helium-Neon lasers, as the source. The ellipsometer beam first goes through a polarizer so that only light orientated in a known direction is allowed to pass. It then goes through a device called a compensator, which elliptically polarizes the light beam. The remaining light is then bounced off the material under study.
The analysis is dependent on Snell's Law; when a beam of light strikes a material, some will reflect immediately, and some will pass through to the far side of the material before reflecting. By measuring the difference between the two reflections, the thickness of the device can be determined. The reflected light also undergoes a change in polarization; this change is used to calculate the refractive index and absorption coefficient.
For an ellipsometer to work properly, the material being examined must satisfy certain physical properties. The sample must be composed of a small number of well-defined layers. The layers must be optically homogeneous, have identical molecular structure in all directions, and reflect significant amounts of light. If any of these requirements are violated, the standard procedures will not work.
Ellipsometers are extremely sensitive devices, able to measure layers as thin as one atom. They are widely used in semiconductor manufacturing where successive layers of material are chemically grown on top of each other.
Ellipsometry is non-destructive; a material being measured by an ellipsometer is not adversely affected by the process. Because of this feature, use of ellipsometers in the biological sciences is increasing. Biological materials are far less uniform than manufactured materials, and do not generally have the physical characteristics necessary for traditional ellipsometry. New techniques, such as using multiple ellipsometers arranged at different angles, have been developed to work with the such materials.