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).
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.
ellipsometric data is displayed as images of the
two fundamental ellipsometric quantities Re(r)
and Im(r), with r = rp/rs. These parameters are
a much more direct representation of the ellipsometry
signal than the ellipsometric angles
which makes results more accessible to interpretation.
However, the data can always be converted to the
traditional notation using
and if necessary.
configuration has an intensity throughput ~Rs
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.