Temperature Measurement of Semitransparent Silicon Wafers Based Upon Absorption Edge Wavelength Shift

The temperature dependence of silicon wafer transmittance is well understood, and is caused by various absorption mechanisms over a wide spectral range. As the wavelength increases, the photon energy decreases until it becomes lower than the minimum energy gap in the silicon band structure. At this point, which is often referred to as the absorption edge wavelength, there is a rapid drop in absorption. The absorption edge shifts to a longer wavelength with increasing temperature, because the bandgap narrows with increasing temperature. Experiments were carried out with varying wavelength (900 nm to 1700 nm), polarization (p- and s-polarized), and direction (from normal to 80°), using specimens with different resistivities (0.01 Ω · cm to 2000 Ω · cm). A characteristic curve relating the absorption edge wavelength and temperature was obtained for all of the silicon wafers, despite their differing resistivity. This method enables in situ temperature measurements of silicon wafers from room temperature to 900 K, using wavelengths to which the wafer is semitransparent. In this article, an experimental apparatus and measurement results are described in detail, and several remaining problems are discussed.