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Zero-Order Reflecting Reference Optics Disclosure Number: IPCOM000099489D
Original Publication Date: 1990-Jan-01
Included in the Prior Art Database: 2005-Mar-15
Document File: 3 page(s) / 120K

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The basic set-up of an FTP (film thickness probe) is shown in Fig. 1. The layered object surface is illuminated with white light.

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Zero-Order Reflecting Reference Optics

       The basic set-up of an FTP (film thickness probe) is shown
in Fig. 1.  The layered object surface is illuminated with white

      In the intermediate image plane, the light is fed to a
fiber-optic cable and transmitted to the spectrophotometer integrated
on a personal computer attachment card.

      An identical set-up may be used to analyze various surface and
material parameters.  A symmetrical grating is shown in Fig. 2.  If
the grating constant is sufficiently small, the diffracted light is
not returned to the spectrometer.  This means that the grating
structure cannot be resolved by the optical system.  The observable
zeroth order reflected light is the superposition of two plane waves,
i.e., the zeroth order of gratings reflecting, respectively, on the
top and the bottom surface.  Thus, the observed spectrum is identical
with the interference pattern of two parallel plane waves separated
by twice the grating depth.  The density of the interference fringes
in the spectrum is proportional to the grating depth.

      The above considerations are not limited to grating structures.
 Any structure with two surface levels and unresolved lateral texture
behaves similarly.  The fringe contrast varies according to the ratio
of the top and the bottom area in the image field, whereas the fringe
pattern remains unchanged.

      A phase reconstruction algorithm as used in the FTP may be
employed for analyzing structures producing interference fringes in
the photometer spectrum in real-time.  For shallow structures
producing spectra without extrema, the reflectance spectrum may be
modelled with a variable amount of unmodulated light until an optimum
match with the measured data is obtained.  Alternatively, the maximum
and minimum reflectances may be calculated from the relative
extension of the structures measured on the basis of or extractable
from the design data.

      An inverted spectrum can be measured in a "dark field"
arrangement where the zeroth order light is blocked out. This may be
useful for analyzing objects with a low-area density.

      Spatial filtering techniques may be used to mask the diffracted
light of object structures of predefined shape. This allows
sequential measurements of various features on complex
photolithographically prepared structures.  It is also feasible to
construct an etch rate/etch endpoint monitor with selective
sensitivity for small features.

      Zero-order spectrometry is suitable for measuring a multitude
of conditions on semiconductor chips during processing, one example
being the verification of the surface topography of circuitry, i.e.,
the average location of several layers.

      The above-described set-up...