Laser induced stress wave thermometry applied to silicon wafer processing

Abstract

The research presented in this work employs laser-based generation and detection of ultrasound in anisotropic plate structures. Ultrasound generation is achieved in the thermoelastic region by a Q-switched Nd:YAG laser. Acoustic detection is performed using a fiber-optic-based interferometric technique using light from a CW Argon Ion laser. Both methods are non-contact, and non-invasive. The modeling and experimentation performed are directed at characterization of the thermo-mechanical properties of single crystal silicon wafers, specifically with respect to ultrasound propagation in the waveguide. The objective is to identify and quantify characteristics of interrogating ultrasonic waves which can be used as a temperature diagnostic during rapid thermal processing (RTP) of silicon wafers. Wafers of 0.76mm thickness and 200 mm diameter are used in the study at temperatures ranging from 23 to 400°C. The waveguide modes generated are identified with the aid of the orthotropic dispersion relations developed, and variations in the a₀ mode group velocity are ultimately used to serve as the temperature diagnostic.