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Fourier-Spectrometer for Thickness Measurement Disclosure Number: IPCOM000106625D
Original Publication Date: 1993-Dec-01
Included in the Prior Art Database: 2005-Mar-21
Document File: 4 page(s) / 120K

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Korth, H: AUTHOR


A Self-Aligning Fourier-Spectrometer for Thickness Measurement

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Fourier-Spectrometer for Thickness Measurement

      A Self-Aligning Fourier-Spectrometer for Thickness Measurement

      The Fourier-Spectrometer, as described, is implemented as a
Michelson type interferometer with two retro-reflectors.  The optical
path difference within the interferometer is scanned, e.g., by
mechanical motion of one of the reflectors.  An integrated pulsed
laser and a multiplexing circuitry allow to monitor the path
difference and the spectrometer signal simultaneously with one
photodetector.  Thickness measurements of transparent plates (Si
wafers) can be done with miniaturized sensors using this measurement
principle.  The integration of the device into a hand-held unit
appears feasible.

      For routine measurements (e.g., of silicon wafers in the
thickness range from 10 microns to 1000 microns) a simple and cost
efficient device is required.  A miniaturized sensor would be
desirable.  This in turn means a minimized part count and, if
possible, no alignments.

      The critical alignment of the interferometer can be eliminated
if retro-reflecting elements are used.  Retro-reflectors can be
tilted and translated (moderately) without affecting the
characteristics of the reflected beam.

      The path difference within the interferometer can be modulated
if the reflectors are moved along their optical axis.  This may be
implemented by a voice-coil drive that oscillates one of the
reflectors.  If both reflectors are mounted on a rotary stage, the
effective path difference variation can be doubled.

      To monitor the path difference the interferometer itself can be
used.  The collimated and nearly monochromatic beam of a laser
(-diode) can be coupled into the optical path.  It will produce a
periodic modulation on the photosensor (diode) as a function of the
path difference.  To increase the coherence length in order to match
the maximum path difference, a narrow-band filter (Perrot Fabry
plate) may be provided.

      Spectrum sensing and monitoring can be done simultaneously.  If
the laser is pulsed rapidly (more than 4 pulses per laser-wavelength
path difference) a high-pass filter allows to separate the laser
signal from the sensor spectrum.  For evaluation, the number of laser
modulations between two spectrum 'pulses' gives the optical path
difference with respect to the laser wavelength.  The plate thickness
can be calculated dividing the path difference by two and by the
refractive index of the plate.  A dispersion of the refractive index
over the evaluated spectrum requires the calculation of an
'effective' refractive index.

      Optical Setup Fig. 1 shows a setup for transparent plate
measurement: The plate is illuminated normally with a (low aperture)
beam of 'white' light, e.g., from a halogenide lamp.  To measure
Silicon that is not transparent in the visible spectrum, a wavelength