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Wavelength Modulated Interferometric Thermometry for Measurement of Non-monotonic Temperature Change

IP.com Disclosure Number: IPCOM000121989D
Original Publication Date: 1991-Oct-01
Included in the Prior Art Database: 2005-Apr-04
Document File: 4 page(s) / 218K

Publishing Venue

IBM

Related People

Holber, WM: AUTHOR [+4]

Abstract

Disclosed are several related schemes for interferometric thermometry which utilize the wavelength modulation capabilities of distributed feedback diode lasers. Because these schemes permit measurement of both the magnitude and direction of temperature change, it becomes possible to use this appealing, non-contact thermometry technique for absolute determinations of arbitrarily varying temperatures, opening up important applications, such as temperature control (where the temperature can be expected to oscillate around a set point).

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Wavelength Modulated Interferometric Thermometry for Measurement
of Non-monotonic Temperature Change

      Disclosed are several related schemes for interferometric
thermometry which utilize the wavelength modulation capabilities of
distributed feedback diode lasers.  Because these schemes permit
measurement of both the magnitude and direction of temperature
change, it becomes possible to use this appealing, non-contact
thermometry technique for absolute determinations of arbitrarily
varying temperatures, opening up important applications, such as
temperature control (where the temperature can be expected to
oscillate around a set point).

      Substrate temperature is widely recognized as an important
processing parameter in the fabrication of a wide variety of thin
film materials and devices, particularly in the microelectronics
industry.  This fact has prompted a renewed interest in the
application of laser interferometric thermometry (LIT) techniques for
temperature measure ments.  A variant of an old thermometry method
[1,2], this non-contact optical technique utilizes laser
interferometry to determine temperature changes from the thermal
expansion and refractive index changes of a transparent substrate
whose front and back faces are polished and approximately parallel.
Recent applications include temperature measurements on optically
absorbing semiconducting materials, such as Si and GaAs, using IR
lasers at 1.15 mm [3], 1.5 mm [4], and 3.39 mm [4].  Temperature
changes are determined by counting fringes (i.e., oscillations in
reflectance); for a laser wavelength of 1.52 mm and a silicon sample
of Z640 mm in thickness, one fringe corresponds to Z6.2~C.

      In the earliest refraction thermometers [1,2], which were based
on a wedge-shaped sensor design, the sign of the temperature change
was determined from the direction of fringe motion across a fiducial
mark.  This approach has also been proposed in [3], with pre-existing
substrate thickness non-uniformities replacing the wedge.  For flat
samples of uniform thickness, however, the previous laser
interferometers [3 - 6] are not sensitive to the direction of
temperature change, and, as a consequence, temperature changes must
be monotonic for unambiguous counting of the fringes.

      One fixed wavelength interferometer design [7] gets around the
direction problem by simultaneously monitoring a second point on the
sample with a different local thickness corresponding to a Z1/4
fringe difference in phase.  In this arrangement, a turning point in
temperature would always be detected as a kink in at least one of the
reflectance traces, since both traces could never simultaneously be
at a reflectance minimum or maximum.  However, this is awkward, since
it would typically require special sample preparation and alignment
for certainty as to the sign of the initial phase difference.

      In contrast to interferometric schemes based on lasers with a
fixed emissi...