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Imaging Ellipsometer: Configuration

imelli_3D picture Modular component setup for the Imaging Ellipsometer.

(a) The Imaging Ellipsometer uses a rotating retarder to modulate the polarisation of the incident and reflected beams.
(b) The illumination and imaging optics are chosen so that the polarising elements are placed in quasi-parallel light beams. The divergence angle of the imaging beam is exaggerated in this figure. Imaging Ellipsometer Setup

The ellipsometric configuration is essentially the same as that used in our high sensitivity Picometer Ellipsometer, but instead of the Birefringence Modulator, a rotating retarder is used. The configuration is P-R-S-A (Polariser, Retarder, Sample, Analyser). The analog lock-in amplifiers of the Picometer Ellipsometer are replaced by the computer algorithm that calculates the ellipsometry images. Images are recorded continuously with a sensitive high-resolution digital CCD camera over one or more revolutions of the retarder, and the ellipsometric images are calculated by a fast Pentium II computer. The ellipsometric data is displayed as images of the two fundamental ellipsometric quantities Re(r) and Im(r), with r = r_p/r_s. These parameters are a much more direct representation of the ellipsometry signal than the ellipsometric angles y and D, which makes results more accessible to interpretation. However, the data can always be converted to the traditional notation using y and D if necessary.
The configuration has an intensity throughput ~R_s I at the Brewster angle, in contrast to zero for the null ellipsometer. The photon noise is thus significantly smaller, resulting in a higher resolution and faster measurement time.

Optimal sensitivity is obtained at the Brewster angle where Re(r) is zero, and the phase shift between s and p waves is 90°. This makes the measurement insensitive to small extraneous phase shifts which are inevitably introduced by glass windows, sample cells, or small variations in the sample surface itself such as waves on a liquid.

The instrument uses a vertical table that allows use with reflection from the free surfaces of liquids. The two arms are motor driven and can be held at the Brewster angle to follow surface properties over extended periods. The configuration with operation at the Brewster angle is alignment-tolerant.

The sensitivity of Re(r) and Im(r) of 10^-3 corresponds to ~1/2Å of an oxide layer on silicon. With the phase-modulated Imaging Ellipsometer, this sensitivity is achieved without the use of specially selected polarisers or positioning devices which can impose severe limitations on an imaging system.

Modular System

All components of the Imaging Ellipsometer are compatible with our Modular System, which makes it easy to modify the instrument for a variety of optical measurements. For example, a filter wheel or a monochromator can be added for spectroscopic measurements. Other modifications can be made to customer specifications. The instrument can also be converted into a Picometer Ellipsometer with only a few additional components.

Our new Optical Table has been especially designed to meet the Imaging Ellipsometer’s specification of < 3µm spatial resolution at the highest magnification, even with additional components mounted on the arms.