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Wafer Flatness Measurements
Non-contact Indars Optical
Interferometer (patent pending) measures the flatness of thin transparent windows,
wafers and substrates by interferometer comparison to an optical reference
flat.
Indars Optical Interferometer
The introduction of thin,
transparent round and rectangular wafers and substrates for optoelectronic and
semiconductor processing presents the problem of measuring their surface
topography - flatness.
Since these wafers and substrates may
be thin (typical thickness 0.5mm and thinner) and their diameter can be large
(up to 300mm), the flatness measurement procedure can not be accomplished using
traditional methods. Neither the Newton interferometer which requires horizontal contact with an
optical flat possibly causing scratches in both, nor the classic
Fizeau interferometer method which requires application of a coating on
one side, can give a true flatness measurement.
Flatness Measurement Theory
Due to gravitational force, when positioned horizontally, thin wafers bend, causing
considerable disturbance of the interferogram. The resulting fringe pattern, then,
will not reveal a freeform profile
of the wafer surface. To eliminate the influence of gravity on the surface profile,
the wafer flatness should be measured when the wafer is in a vertical position in
which most bending components of forces applied by gravity and
supporting mechanisms of the instrument are eliminated.
When measurements for thin wafers
and substrates are carried out on a classic Fizeau laser interferometer:
- A flatness distorting opaque coating must be applied to one side.
- On some Fizeau Interferometers, the wafers and substrates are measured in horizontal
position.
To address the issues inherent with these traditional methods, the new arrangement of
the interferometer has been designed. The new Indars Interferometer scheme
differs from others which are now in practice, by the vertical placement of the
sample under test. Also, the need for application of distorting opaque film is
eliminated.
The
Indars Interferometer enables both
elements, the part to be measured and the reference
optical flat,
to be kept parallel to each other at a distance on the order of coherence length of a light
wave.
Indars Interferometer for Transparent Wafer Flatness Measurements
Related thin transparent wafer flatness measurement information:
Please call or e-mail
for further details.
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