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In contrast to conventional ellipsometers based
on the nulling technique, the Picometer Ellipsometer
uses phase modulation to achieve a higher sensitivity
and lower noise under almost any measurement condition.
Phase modulation makes the instrument insensitive
to small variations in the measurement conditions
which are inevitably introduced by glass windows,
sample cells, or variations in the sample surface
itself such as waves on a liquid.
The
ellipsometer uses a Photoelastic Birefringence
Modulator to modulate the polarisation of the
incident light beam at a high, well defined frequency.
Lock-in detection of the modulated detector signal
eliminates most sources of noise to achieve the
high sensitivity. The sensitivity is usually shot
noise limited, and typically is equivalent to
an ellipticity of about 10-5
, or to a polarisation
angle of 10-3
degrees at a time constant of 1
s. This is equivalent to a detection limit of
less than 10-12m
of oxide on the silicon
surface (about 1/100 of a monolayer).
Instead of a rotating polariser, a Photoelastic
Birefringence Modulator is used to analyze the
ellipticity of the reflected beam with a response
time of up to 1 ms. Beaglehole Instruments'
unique modulator features a three element design
that makes it independent of temperature variations.
The
response time can be as short as 1 ms. The ellipsometer
is then fast enough to follow most dynamic situations
in real time. In its standard configuration, the
Picometer Ellipsometer operates at a single fixed
wavelength. It can be modified with components
from our Modular System at any time - for example,
the instrument can be fitted with a monochromator
and white light source for spectroscopic measurements,
or can be converted into an Imaging Ellipsometer.
The Picometer Ellipsometer is more than just an
ellipsometer: it is a base system that can be
configured for a variety of optical measurements.
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