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Phase Modulated Ellipsometry

Most ellipsometers operated today use a mechanically rotated polarising element to analyse the polarisation of the light beam that is reflected by the sample. Many instruments are still based on the nulling technique that was already used in the first ellipsometers at the beginning of the century (although today's null ellipsometers are somewhat more sophisticated). In this technique, the polarising elements are rotated until the signal at the detector is extinguished (nulled). The light sources used in these instruments were often fixed to a single wavelength. In recent years, the advent of computer control and multichannel detectors has made it possible to develop fast spectroscopic ellipsometers that can scan an entire range of wavelengths simultaneously. These instruments still use a rotating polariser (or analyser), but employ Fourier analysis of the detector signal to determine the ellipsometric parameters faster and more accurately. Still, these instruments suffer from low sensitivity under certain conditions, and have a poor signal-to-noise ratio, especially when the sample is not highly reflective.

In 1968, Jasperson and Schnatterley introduced a different approach to the analysis of the state of polarization, called the phase modulation technique. They used a strained piece of amorphous silica to modulate the state of polarisation of the light. The silica becomes birefringent when strained, with the amount of birefringence (the phase retardation of a light beam passing through the optical element) proportional to the strain. The strain is applied by piezoelectric transducers at the resonance frequency of 50 kHz. The resulting detector signal has a large unmodulated component, with two superimposed modulated signals at 50 and 100 kHz (and higher harmonics). The ellipsometric parameters can be directly deduced from these modulated signals, which can be easily separated by phase sensitive detection through lock-in amplifiers. The technique allows a fast response time and has a superiour signal-to-noise ratio due to the use of lock-in detection and a high signal throughput. However, because of the high frequency of the modulation, multichannel detection that is needed for imaging and fast spectroscopic measurements is extremely difficult with this design.

However, the phase modulation technique does not rely on the use of a photoelastic modulator: the same phase modulation can be achieved by rotating a quarter wave plate. While the slower rotation increases the instrument's response time, it allows the use of slower detectors such as CCD arrays for imaging or spectroscopic multichannel measurements.

Phase modulated ellipsometers have virtually no insensitive regions, low signal noise, and are less influenced by small changes in the angle of incidence. Therefore, in applications such as dynamic studies, measurements on liquid surfaces, and ellipsometric imaging, phase modulated ellipsometry has significant advantages over conventional techniques.