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Introduction to Ellipsometry

Ellipsometry is an optical technique that uses polarised light to probe the dielectric properties of a sample. The most common application of ellipsometry is the analysis of very thin films. Through the analysis of the state of polarisation of the light that is reflected from the sample, ellipsometry can yield information about layers that are thinner than the wavelength of the light itself, down to a single atomic layer or less. Depending on what is already known about the sample, the technique can probe a range of properties including the layer thickness, morphology, or chemical composition.

The name "ellipsometry" stems from the fact that the most general state of polarization is elliptic. The technique has been known for almost a century, and today has many standard applications. It is mainly used in semiconductor research and fabrication to determine properties of layer stacks of thin films and the interfaces between the layers. However, ellipsometry is also becoming more interesting to researchers in other disciplines such as biology and medicine. These areas pose new challenges to the technique, such as measurements on unstable liquid surfaces and microscopic imaging.

There are many different ways of determining the polarisation of a beam of light. In the first ellipsometers, the operator observed the light beam that was reflected off the sample through an eyepiece. The polarisers and retarders were rotated by hand until the effect of the polarisation was inverted and no light would pass through the instrument. This is called the nulling technique. Modern nulling ellipsometers use computers to rotate the elements and to automatically calculate the ellipsometry signal very quickly. However, the nulling technique is not ideal for automated instruments because it is based on measuring a zero signal. This was an advantage in the early ellipsometers because the human eye is very sensitivy to small changes in the signal around the 'null'. However, modern light detectors exhibit significantly higher noise at low intensities. A technique that is more suited to modern-day instrumentation is Phase Modulated Ellipsometry, where the polarisation of the light is modulated at a high light throughput. Phase modulated ellipsometry is described in more detail on the next page.