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Contactless Substrate Temperature Measurement by Laser Interferometry

IP.com Disclosure Number: IPCOM000039070D
Original Publication Date: 1987-Apr-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 2 page(s) / 53K

Publishing Venue

IBM

Related People

Brady, MJ: AUTHOR [+4]

Abstract

Substrate temperatures during molecular beam epitaxial (MBE) growth range from room temperature to over 700oC. Presently, temperature of the growth layers (for example, of AlGaAs on GaAs substrates) is monitored by one of two techniques: optical pyrometry and thermocouples. At temperatures below 500oC, pyrometry may not be practical, and at higher temperatures, the transmittance of the pyrometer viewport varies with time due to material deposition, introducing systematic measurement error. When monitoring with a thermocouple, it is necessary to make physical contact to the substrate, which abrades under rotation during deposition of growth layers. This abrasion contributes to propagation of dislocation defects through the substrate into the epitaxial layers.

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Contactless Substrate Temperature Measurement by Laser Interferometry

Substrate temperatures during molecular beam epitaxial (MBE) growth range from room temperature to over 700oC. Presently, temperature of the growth layers (for example, of AlGaAs on GaAs substrates) is monitored by one of two techniques: optical pyrometry and thermocouples. At temperatures below 500oC, pyrometry may not be practical, and at higher temperatures, the transmittance of the pyrometer viewport varies with time due to material deposition, introducing systematic measurement error. When monitoring with a thermocouple, it is necessary to make physical contact to the substrate, which abrades under rotation during deposition of growth layers. This abrasion contributes to propagation of dislocation defects through the substrate into the epitaxial layers.

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These problems can be avoided by the use of laser interferometric techniques and the intrinsic property of thermal expansion. The linear coefficient of thermal expansion of gallium arsenide is on the order of 6xl06 per degree C. For a nominal 2-inch diameter GaAs wafer, and a temperature change of 600oC, the expansion (WL) is on the order of l00 microns, assuming isotropic expansion. This expansion/contraction in one arm of an interferometer provides easily measured optical interference fringes which are not significantly affected by long term changes in MBE viewport transmittance. Fig. l depicts, schematically, how this interferometric technique is used to provide remote, contactless temperature sensing over a wide temperature range. A cleaved facet l0 (edge) of the substrate crystal l2 (GaAs) acts as a reflecting mirror in one arm of the interferometer. A second mirror Ml provides the reference beam. A laser beam, e.g., He-Ne, o = 0.63 microns, is passed through a beam splitter l4...