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NONINTRUSIVE THERMOMETRY for TRANSPARENT THIN FILMS by LASER INTERFEROMETRIC MEASURMEMENT of THERMAL EXPANSION (Limotex) USING SINGLE or DUAL BEAMS

IP.com Disclosure Number: IPCOM000039473D
Original Publication Date: 1987-Jun-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 4 page(s) / 37K

Publishing Venue

IBM

Related People

Appleby-Hougham, G: AUTHOR [+4]

Abstract

This system measures the temperature of transparent thin films using laser interferometry to measure minute changes in film thickness resulting from thermal expansion. It can be used as either a direct (non- contact) or indirect (contact) temperature sensor. In the former case, the temperature of the thin film (a 5-50 mm layer, for example) is of primary interest. In the latter case, a thin film decal can be applied to the surface of an object whose temperature is to be measured. Alternatively, the thin film can be packaged as a capsule at the end of a protected optical fiber and then placed in contact with the object. (Image Omitted) film's back surface.

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NONINTRUSIVE THERMOMETRY for TRANSPARENT THIN FILMS by LASER INTERFEROMETRIC MEASURMEMENT of THERMAL EXPANSION (Limotex) USING SINGLE or DUAL BEAMS

This system measures the temperature of transparent thin films using laser interferometry to measure minute changes in film thickness resulting from thermal expansion. It can be used as either a direct (non- contact) or indirect (contact) temperature sensor. In the former case, the temperature of the thin film (a 5-50 mm layer, for example) is of primary interest. In the latter case, a thin film decal can be applied to the surface of an object whose temperature is to be measured. Alternatively, the thin film can be packaged as a capsule at the end of a protected optical fiber and then placed in contact with the object.

(Image Omitted)

film's back surface. Intensity maxima occur when the optical path length difference 2L between the two reflected beams is an integral multiple of laser wavelength in the film, /n, where n is the refractive index of the film. Conventional temperature sensing systems using thermocouples or thermistors are difficult to use with accuracy in the presence of RF of other electrically hostile environments due to
(1) electrical noise, (2) heating field distortions by the probe, (3) perturbation of the temperature of the materials and (4) spurious heating of wires and probes themselves from the RF fields. Infrared and optical pyrometry techniques also have limitations, having poor sensitivities except at high temperature, and being limited to materials of predictable emissivities. Recently, fluoroptic thermometry has gained a following, but even that technique can be somewhat perturbing for thin film measurements because of the probe's physical size and thermal mass. The present system has absolutely none of these difficulties when used as a direct temperature sensor. We first used the LIMOTEX (Laser Interferometric Measurement of Thermal Expansion) technique to obtain a, the "effective" TEC for 30 mm films of polyimide on silicon wafers by measuring WL for a known WT and L. Temperature was measured by #22 Chromel-Alumel thermocouples cemented to both sides of the wafer with thermally-conducting grease. Fig. 2 shows the development of interference fringes as the substrate temperature is cycled between 25 and 250OEC. for = 6328 ~ and L = 31 mm. We found a (in the dimension perpendicular to the substrate) to be 83 + 3 x 10-6/OEC and

_ independent of temperature. This is about 3 times the reported linear CTE. Some of this discrepancy can be attributed to the fact that the film can only expand in one direction since it is bonded to a relatively non-expanding substrate. However, the "effective" TEC, a, is in fact the sum of the real TEC, a and (1/n)(dn/dT), where dn/dT is the derivative of the refractive index with respect to temperature.

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NONINTRUSIVE THERMOMETRY FOR TRANSPARENT THIN FILMS BY LASER
INTERFEROMETRIC MEASURMEMENT OF THERMAL EXPANSION (LIMOTEX)...