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 method^1,2, 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 nm^3,4, as well as on optically absorbing semiconducting materials such as Si and GaAs by using IR lasers at 1.15 μm⁵, 1.5 μm^6,7, 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