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Browse Prior Art Database

Thickness Measurements using IR Tunable Laser Source

IP.com Disclosure Number: IPCOM000108845D
Original Publication Date: 1992-Jun-01
Included in the Prior Art Database: 2005-Mar-23
Document File: 4 page(s) / 148K

Publishing Venue

IBM

Related People

Holber, WM: AUTHOR [+4]

Abstract

Disclosed here is an interferometric method of determining substrate thickness, such as Si and GaAs wafer, which utilizes a tunable fiber laser. Thickness is determined from analysis of the oscillations in a trace of reflectance vs. wavelength. The wafer thickness L is related to the wavelength spacing of one complete period of the oscillation (or adjacent fringes) wg by L = (g2/2n)/wg, where g is the laser wavelength and n is the wafer's refractive index.

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Thickness Measurements using IR Tunable Laser Source

       Disclosed here is an interferometric method of
determining substrate thickness, such as Si and GaAs wafer, which
utilizes a tunable fiber laser.  Thickness is determined from
analysis of the oscillations in a trace of reflectance vs.
wavelength.  The wafer thickness L is related to the wavelength
spacing of one complete period of the oscillation (or adjacent
fringes) wg by L = (g2/2n)/wg, where g is the laser wavelength and n
is the wafer's refractive index.

      The disclosed method is non-contact, and expected to be
accurate, potentially inexpensive, and compact.  It is primarily
intended for substrates with smooth, parallel faces whose temperature
would subsequently be monitored with an interferometric thermometry
technique (1-3) (which requires knowledge of the sample thickness).

      The principle upon which this measurement technique is based is
not new.  Previously, substrate thicknesses have been measured in
infrared (IR) spectrophotometers, and thin (500 o - 10 mm) film
thicknesses have been measured by the fringe spacing in the visible
spectrum with Film Thickness Analyzers (FTAs) (4).  For accurate
measurements, the wavelength resolution Ww must be much smaller than
the fringe spacing wg.  Because of the wavelength resolution, FTAs
are typically limited to thin films which have relatively widely
spaced fringes.  Since the fringe spacing is larger at longer
wavelengths, thicker samples can be measured in the IR. However,
typical IR spectrophotometers are bulky and expensive.  All versions
of this technique are limited to materials which are transparent in
the wavelength region probed.

      A tunable fiber laser at a nominal wavelength of 1.5 mm can be
utilized for interferometric thickness measurements. For a silicon
wafer of 600 mm thickness, the fringe spacing wg would be about 5 o .
The laser described below has a spectral width Wg of about 0.15 o
(i.e., Wg<<wg) and a maximum continuously tuning range of about 20 nm
(i.e., about 40 fringes). The tuning speed is limited by the speed of
the wavelength selective element that provides the feedback for the
fiber laser, typically longer than the laser build-up time which is
estimated to be N50 ns.  In our experimental setup, a
piezoelectrically driven Fabry-Perot fiber filter is used as the
wavelength selective element.  The wavelength tuning could be
achieved by numerous means, e.g., by changing the optical index of an
electro-optic material, such as liquid crystal in a Fabry-Perot
filter (5) or tuning the applied RF frequencies of the
acousto-optical filter (6).  An accuracy of about 1% should be
expected for a 3-fringe scan (i.e., .15 o over 15 o).  However, the
reflected signal must be normalized by the laser intensity the laser
intensity varies significantly over the tuning range.  The technology
for fabricating this laser is mature; for example, the Er3+ doped
fibers will be used as o...