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Ellipsometry measures thin film thickness and optical properties using an ellipsometer that shines a laser onto a material and captures the reflection. The analysis depends on Snell’s law and the material must meet certain physical properties. Ellipsometers are used in semiconductor manufacturing and are non-destructive, making them increasingly popular in biological sciences.
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 take these measurements.
Ellipsometers work by shining a well-defined light source onto a material and capturing the reflection. Modern ellipsometers use lasers, typically helium-neon lasers, as the source. The ellipsometer’s beam first passes through a polarizer so that only light directed in a known direction can pass. It then passes through a device called a compensator, which elliptically polarizes the light beam. The remaining light is then bounced off the material under test.
The analysis depends on Snell’s law; when a ray of light strikes a material, some will immediately reflect and some will go through to the opposite 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 variation is used to calculate the refractive index and absorption coefficient.
For an ellipsometer to function properly, the material under test must meet certain physical properties. The sample must be composed of a small number of well-defined layers. The layers must be optically homogeneous, have an identical molecular structure in all directions, and reflect significant amounts of light. If any of these requirements are violated, standard procedures will not work.
Ellipsometers are extremely sensitive devices, capable of measuring layers as thin as an atom. They are used extensively in semiconductor manufacturing where successive layers of material are chemically grown on top of each other.
Ellipsometry is non-destructive; a material measured by an ellipsometer is not adversely affected by the process. Because of this feature, the use of ellipsometers in the biological sciences is increasing. Biological materials are much less uniform than manufactured materials and generally lack the physical characteristics necessary for traditional ellipsometry. New techniques, such as using multiple ellipsometers set at different angles, have been developed for working with such materials.
