Infrared-laser interferometric thermometry: A nonintrusive technique for measuring semiconductor wafer temperatures

Abstract

We report an optical, interferometric technique for measuring the temperature of semiconductor substrates during heating or cooling, which is applicable in vacuum. The technique circumvents many of the problems associated with thermocouple or pyrometer measurements. A low-power infrared (IR) laser (e.g., λ=1.15-μm He–Ne laser) having an energy below the band gap is directed at a wafer that is polished on both sides, where either reflected or transmitted laser light is detected by a photodiode. Interference results between reflections off the front and back surfaces of the wafer. As the temperature of the wafer is either increased or decreased, the temperature dependence of the refractive index, along with a smaller contribution from thermal expansion, causes the optical path within the wafer to change by λ/2n (i.e., a full interference cycle) for every ∼3 K for a typical Si, GaAs, or InP wafer thickness of 500 μm. Consequently, temperature changes of ±0.2 K are easily detected. This technique, has been used between room temperature and 600 °C on GaAs substrates in a low-pressure metal-organic chemical vapor deposition (MOCVD) system, and in an ultrahigh vacuum thermal desorption experiment. This method can be used well below room temperature, as well as at temperatures above 650 °C with the optimum choice in laser wavelength. Application of this method to other processes such as molecular beam epitaxy (MBE), reactive ion etching, and rapid thermal processing should be straightforward. We also describe a refinement of the method for measuring the sign, as well as magnitude of temperature changes for typical, slightly tapered wafers during heating or cooling cycles. In this case the reflected laser beam contains a series of parallel lines that move toward the thinner end of the region probed by the laser beam as the temperature increases. Sensing the direction that these spatial interference fringes move can be used to determine whether the sample is heating or cooling.