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Interferometry
has been a technique in existence for more than one hundred years and consists
of viewing the optical path difference between a sample beam and a reference
beam; the beams undergo constructive and destructive interference and this
results in a pattern of bright and dark fringes. In the case of an interference
microscope, the objective lens is coupled with a beam splitter so that some of
the light is reflected from a reference mirror at 90 degrees (Michelson type) or
co-linear with the light path (Mirau type) see figure. |
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Illumination
from a white light
beam passes through a filter, then through a microscope objective lens to the
sample surface. The light reflecting back from the surface recombines with the
reference beam and interference fringes are formed. The pattern of these fringes
is captured on a CCD camera array. If the sample is a perfectly flat mirror but
tilted, and the illumination is by monochromatic light then the resulting
interference pattern will be a series of fringes. The distance between
the maximum of the dark or light fringes is proportional to the wavelength of
light used and the tilt of the mirror. Each band in the interference pattern
represents a contour height difference of half the wavelength of light used in
the measurement. The contour bands are purely sinusoidal and the phase of the
interference fringe pattern can be measured to very high accuracy.
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| Two
techniques of interferometry are described, Phase Shift Interferometry (PSI) and Vertical Scanning
Interferometry (VSI). |
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| Phase Shift Interferometers have been developed to
measure the surface height of very smooth and continuous surfaces with
sub-nanometer resolution. The sample, which must be in focus, is scanned
vertically in a few steps, which are a very precise fraction of the
wavelength. The profiling algorithms produce a phase map of the surface,
which is converted to the corresponding height map by means of a suitable
unwrapping procedure. PSI profiling provides sub-nanometer vertical
resolution for all NA. Very low magnifications (2.5X) can be employed to
measure large fields of view with the same height resolution. However, the
measurement range is limited by the coherence length to a few micrometers.
PSI algorithms enable to profile shape features on the nanometer scale, as
well as to assess texture parameters of super smooth surfaces on the
sub-nanometer scale. |
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| White-light vertical scanning interferometers have been
developed to measure the surface height of smooth to moderately rough
surfaces. Maximum fringe contrast occurs at the best focus position for
each point on the surface of the sample. The sample is scanned vertically
in steps so that every point on the surface passes through the focus. The
height of the surface at each pixel location is found by detecting the
peak of the narrow fringe envelopes. VSI profiling provides nanometer
vertical resolution for all NA. The VSI algorithms enable to use all the
available magnifications to profile shape features with the same height
resolution. The measurement range is intrinsically unlimited although in
practice it is limited. Scan speeds and data acquisition rates can be very
fast, although this leads to a significant loss of vertical resolution. |
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