Temperature Measurement in Rapid Thermal Processing Using Acoustic Techniques (Wafers, Lamb Waves)
A key prerequisite for successful rapid thermal processing is the ability to monitor and control the wafer temperature accurately. Optical pyrometers, which are currently used on most rapid thermal processors, however, have significant limitations. Because the temperature measurement is based on the thermal radiation of the wafer, it is strongly affected by the emissivity of the wafer which is a function of a variety of factors such as film depositions, backside roughness, doping levels, and temperature. Heating lamp radiation also interferes with the measurements due to the finite overlap between the optical spectrums of the lamps and the detectors. The velocity of Lamb waves is a fairly strong function of temperature, and this property is exploited to obtain reliable temperature measurement of the wafer. The quartz pins that support the wafer are used to transmit and receive the Lamb waves, and the time of flight measurement is used to determine temperature. The sensor has been installed into a rapid thermal processor, and temperature measurements from 20 C to 1000 C with better than 5 C accuracy has been achieved. The velocity of the acoustic wave in the processing ambient has a stronger dependence on temperature. By measuring the phase delay between two lines directly adjacent the frontside of the wafer, the temperature of the ambient at the wafer-ambient interface from 20 C to 300 C with 5 C resolution has been measured. This measurement provides information on the thermal energy of the reactant species at the wafer surface. A variety of other acoustic techniques for temperature measurement are also presented. These include photoacoustic measurements of longitudinal waves, photoacoustic measurements of Lamb waves, photoacoustic measurements of ambient waves, pin to pin Lamb wave phase measurements, and resonance frequency measurements.
