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 = 90°.
Instead of mechanically rotating a polariser to
analyze the ellipticity of the reflected lightbeam,
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. To eliminate signal offsets, both
Re(r) and Im(r) can be subjected to a low frequency
analyser modulation which removes spurious optical
signals. 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.