Abstract
Substrate temperature is increasingly being recognized as an important processing parameter in the fabrication of a 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 measurements. A variant of an old thermometry method1,2, this noncontact optical technique uses 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 transparent dielectric materials by using a red HeNe laser at 633 nm3,4, as well as on optically absorbing semiconducting materials such as Si and GaAs by using IR lasers at 1.15 μm5, 1.5 μm6,7, and 3.39 μm.7 Here we use a compact distributed feedback (DFB) laser diode emitting at 1.55 μm for interferometric thermometry on silicon samples. In contrast to earlier interferometric schemes based on lasers with a fixed emission wavelength, the present work uses the distinctive injection-modulation feature of the laser diode to modulate the laser wavelength. Analysis of the resulting small amplitude oscillations in reflectance allows determination of the direction of temperature change, a previously inaccessible parameter.
© 1991 Optical Society of America
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