Wavelength-specific pyrometry as a temperature measurement tool

A wavelength-specific 9.4±0.3-μm pyrometer with a 0.41 constant emissivity was used to measure the temperature of 2 to 6 Ω-cm, n-type silicon coated with 1.3 μm of thermal oxide from 270°C to 600°C with an accuracy of ±1%. At lower temperatures, the emissivity monotonically decreases to 0.36 at 200°C with a slope that is proportional to the thermally activated free-carrier absorption of the silicon. This dependency introduces. at any temperature, a temperature uncertainty that is proportional to the emissivity change divided by the constant emissivity. By comparison, the same accuracy inherent in constant emissivity measurements is limited to 430°C when sensed with a more typical 11±3-μm pyrometer. Furthermore, the emissivity change at 200°C is three times larger, resulting in an equally large temperature uncertainty. A four-phase optical model with the constraint of substrate opacity is used to approximate the constant emissivity, as a function of the spectral bandwidth of the pyrometer. The greatest discrepancy between calculation and measurement is 0.14 emissivity or 24°C at 430°C