Non-invasive process temperature monitoring using laser-acoustic techniques

A method of temperature measurement is studied that is suitable for in situ monitoring of semiconductor wafer temperature, based on the temperature dependence of acoustic waves. The change in the dispersion relations of the plate modes through the wafer as a function of temperature is exploited to provide a viable temperature-monitoring scheme with advantages over both thermocouples and pyrometers. Temperature based on the velocity dependence of acoustic waves through a thin layer of ambient directly above the silicon wafer is expected to be important in better controlling processes where the mass transport mechanisms and the effects of stagnant layers play a significant part in the process. The information about the temperature of the layer of ambient can also be used to obtain the wafer bulk temperature. Because of the high temperature sensitivity of the acoustic wave velocity in gases, this system can be used as a practical in situ temperature measurement scheme with sensitivities better than ±1°C