Basic operating principles of the Beaglehole Ellipsometer


The Picometer Ellipsometer uses the configuration P-BM-S-A (Polariser-Birefringence Modulator-Sample-Analyser).

biref_modulator Instead of mechanically rotating a polariser to analyze the ellipticity of the reflected light beam, the incident light is modulated by the Birefringence Modulator. This element is operated in longitudinal resonant vibration at a frequency of 50 kHz, but has no mass motion. Because there are no moving parts, there are no beam deflections which plague rotating element Instruments. The Birefringence Modulator is a balanced 3 component device. A gauge crystal provides feedback on the oscillation amplitude which leads to a high long term stability that is independent of environmental parameters such as temperature. The exact reproducibility of the modulation amplitude eliminates the need for frequent recalibrations or elaborate cross-checking of the phase shift. This is especially important for spectroscopic measurements where the modulation amplitude is tracked across the spectrum. With a modulation frequency of 50 kHz the instrument can work with response times as short as 1 ms. The high frequency modulation moves the signal away from 1/f noise region. Phase lock detection further reduces the noise, leaving photon shot noise as the only limiting factor to the instrument sensitivity. Further details on the modulator method are contained in a Beaglehole Instruments Technical note.

To eliminate signal offsets, the detected ellipsometric parameters can be subjected to a low frequency analyser modulation which removes spurious optical signals.

All components are interfaced to the host computer through a microcontroller that acts as an independent instrument controller and data logger. The microcontroller ensures true real-time operation and minimizes the risk of data loss by keeping a copy of the last measured data until it has been saved by the host computer. It also enables analysis to be completed on previously acquired data while a measurement is completed
Due to the modular design of the Picometer Ellipsometer, the default P-BM-S-A configuration can be easily configured for many other optical measurements such as measurements of reflectivity, transmission, birefringence, dichroism etc. When operated at normal incidence the Picometer Ellipsometer becomes a fast and highly sensitive instrument for studying reflectance anisotropy. This allows the study of small surface effects by suppressing the bulk ellipsometry signal.

The two lock-in signals are directly related to Re(r) and Im(r), where r is the fundamental quantity of ellipsometry, being the ratio of the amplitude reflectivities r = rp/rs. Traditionally this ratio is often written

r = tanY eiD
so that
Re(r) = tanY cosD    and     Im(r) = tanY sinD

The parameter r is a much more direct representation of the ellipsometry signal than the ellipsometric angles Y and D, which makes results more accessible to interpretation. However, since the Picometer measures both Re(r) and Im(r) simultaneously, the data can always be converted to the traditional notation using Y and D. Beaglehole Instruments Technical Notes are available to elaborate on the theory of reflection and ellipsometric parameters.

For applications looking at surface effects the greatest sensitivity is obtained at the Brewster angle of incidence where Re(r) = 0. Unlike a null ellipsometer where the intensity is zero, the Picometer Ellipsometer at the optimum angle has a large throughput intensity, and thus can operate under optimum conditions. Further, stray phase shifts introduced by glass windows and sample cells have little effect on Im(r) when D = 90°. The incidence and reflection arms are driven by stepping motors that can be synchronized in a sample feedback loop to maintain the ellipsometer at the Brewster angle in dynamic situations.

A more through description of the operating principles of the instrument is contained in the Manual and a Beaglehole Instruments Technical note