Tech Briefs Magazine - May 2024 - Sensor-16

TECHNOLOGY LEADERS Sensors
* With a measuring angle of 4°, ideally
suited for flat surfaces
* Resolution up to 30 picometer
interferoMETER Measuring Principle
The measuring principle of an interferometer
is based on the wave nature
of light. This leads to the fact that
overlapping waves can either intensify
or cancel each other, depending on
whether wave crest meets wave crest
Figure 3. White light interferometers offer picometer-level resolutions, ideal in the semiconductor industry.
When loading wafers, white light interferometers are used to measure the horizontal tilt of wafers. The interferometers
provide absolute distance values at a subnanometer resolution. The measurement ensures the greatest
possible positional accuracy when picking up and removing wafers. (Image: Micro-Epsilon)
portfolio. They too, are available as
multi-peak systems, which opens up
numerous new potential applications.
Interferometers are used when high-resolution
distance and thickness measurements
are required.
Multi-peak versions enable high-precision
detection of transparent layers.
Depending on the controller model,
this is achieved via calculations based on
the respective distance values and taking
the refractive indices into account. The
thickness measuring system, however,
does not perform distance measurements
- it measures the thickness of
the individual layers and their combinations.
For industrial use, the interferometers
are equipped with protection class
IP65 sensor housings.
Thanks to absolute distance measurement,
the scanning of steps with white
light interferometers takes place with
high signal stability and precision. When
measuring on moving objects, the height
differences of heels, steps, and depressions
can thus be detected without additional
referencing.
It is also suitable for precise thickness.
The large thickness measuring
range allows the measurement of thin
16
layers, flat glass, and films. Since the
white light interferometer works in the
near infrared range, thickness measurement
of anti-reflective coated glass is
also possible.
Finally, it is suitable for distance measurements
with picometer resolution.
Thanks to the absolute measurement,
sampling is performed without signal
loss. When measuring on moving objects,
this enables the differences in the
height of heels, steps, and depressions
to be reliably detected. The measuring
system offers sub-nanometer resolution
with a large offset distance in relation to
the measuring range.
In addition to excellent linearity,
the white light interferometers offer a
resolution up to < 30 picometers. Configuration
is possible via an integrated
web interface.
Characteristics of Interferometer
Measurement
* Absolute distance measurement with
nanometer accuracy
* Precise thickness measurements independent
of the distance from the sensor
* Multi-peak distance measurement and
multi-layer thickness measurement
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or wave crest meets wave trough. If a
light beam is split so that it takes different
paths and the two partial beams
subsequently overlap again, interference
occurs that depends on the difference
between the two paths. If the
length of one of the two paths changes
by half a wavelength of the light used,
this leads to a complete change from
positive interference (amplification)
to negative interference (extinction).
This enables accuracy in the nanometer
or even sub-nanometer range. In
order to measure distances using this
method, for example, one of the two
partial beams is reflected at the measuring
object and then superimposed
on the reference beam. If the distance
from the target changes, this change in
distance can be detected very sensitively
by the interference.
When measuring the thickness of films
or glasses, it takes advantage of the fact
that both the front and back of the measuring
object reflect. Thickness changes
then also cause the interference signal
to change, so for that application, a separate
reference beam is not necessary.
A special feature of thickness measurement
is that since the two interfering
partial beams originate from the upper
and lower surfaces, the measurement result
is independent of the distance from
the measuring object.
In Conclusion
Confocal chromatic sensors and interferometers
using white light can perform
numerous extremely precise measurements
even under continuous manufacturing
scenarios.
This article was written by Martin
Dumberger, Managing Director, MicroEpsilon
America. For more information,
contact Mr. Dumberger at martin.
dumberger@micro-epsilon.com
Sensor Technology, May 2024

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Tech Briefs Magazine - May 2024

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