Ellipsometry is an optical technique for measuring the thickness and optical properties of thin films, or layers, of material. The measurable properties are the refractive index, and the level of light absorption, called the absorption coefficient.

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 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. 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.

Our ellipsometer is a Rudolph Research AutoEL Ellipsometer. It is controlled by a microprocessor and can automatically measure the polarization variation of a laser beam reflected from the sample surface, whereby calculating various properties of the film on the sample, such as film thickness and refractive index. The intermediate values and final output are transmitted to and displayed on a digital computer. The facile operating process makes the ellipsometer very efficient in measuring thin film thickness down to less than 100 nm.