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Triple-Phase Overlay Detector

IP.com Disclosure Number: IPCOM000099491D
Original Publication Date: 1990-Jan-01
Included in the Prior Art Database: 2005-Mar-15
Document File: 4 page(s) / 144K

Publishing Venue

IBM

Related People

Korth, HE: AUTHOR

Abstract

White light interferometry is used to analyze the thickness of transparent films from the fringe pattern in the spectrum of the light reflected at the object surface. Spectral evaluation is unambiguous, being limited only by the resolution of the spectrometer and the capabilities of the evaluation algorithm. This approach is extremely sensitive (a 1 value of 0.02 nm was measured with an FTP (film thickness probe), because a multitude of spectral channels are evaluated such that single channel noise is averaged out. The accuracy of the approach is determined by spectrometer calibration and the quality of data evaluation.

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Triple-Phase Overlay Detector

       White light interferometry is used to analyze the
thickness of transparent films from the fringe pattern in the
spectrum of the light reflected at the object surface.  Spectral
evaluation is unambiguous, being limited only by the resolution of
the spectrometer and the capabilities of the evaluation algorithm.
This approach is extremely sensitive (a 1  value of 0.02 nm was
measured with an FTP (film thickness probe), because a multitude of
spectral channels are evaluated such that single channel noise is
averaged out.  The accuracy of the approach is determined by
spectrometer calibration and the quality of data evaluation.

      For using the sensitivity of white light interferometry for
surface topography measurements, the influence of surface tilt must
be neutralized.  This can be done by probing the test surface at
three equidistant and collinear locations.

      Three coherent light beams can be readily produced with a
sinusoidally modulated phase grating.  Similarly, the three beams can
be recombined.  With a phase shift of zero (or a multiple of 2f) the
light is directed into the zeroth order.  Alternatively, it is
diffracted into the      1st order.  Non-sinusoidal or amplitude
gratings can also be used if reduced efficiency is acceptable and the
higher diffraction orders are blocked out.

      The interference fringe pattern of three superimposed light
beams can be analyzed to determine the path differences between them.
 If the path difference between two of the beams is small compared to
the wavelength, then their amplitudes roughly add up arithmetically,
and a fringe pattern similar to a two-beam interferogram is obtained.

      Minimum path difference between side beams is readily obtained
by leveling the object or tuning the optical path. In this case, the
fringe density is proportional to the optical path difference between
the center beam and the averaged side beams.

      For analyzing very small path differences, a bias phase shift
may be introduced into the center beam.  This produces an
interferogram with a number of fringes that can be evaluated with
maximum accuracy.  The self-referencing properties of the periodic
pattern allow eliminating the influence of variations in fringe
amplitude and contrast.

      The bias method permits measuring etch grooves or
non-transparent lines having a height of a few nanometers with an
accuracy of 0.01.  Surface roughness or waviness can be analyzed with
similar precision.

      A negative bias allows analyzing very thick profiles by
shifting the measurement range to an appropriate offset.

      Fig. 1 shows an example of a set-up combining the functions of
the beam splitter separating incident and reflected light with the
grating for triple beam generation and recombination.  The grating
consists of metallic stripes on a transparent carrier.  It is located
at a plane that is imaged int...