Abstract
The long wavelength infrared (LWIR) refers to the wave length ranging from 8-13 m. This wavelength range is often used in thermopile-based infrared pyrometers, which are commonly used to non-contact temperature measurement of Si wafers. Emissivity is, in practical, the only adjustable parameter for infrared pyrometers. From Kirchhoff’s law of radiation, at thermal equilibrium, emissivity is equal to absorptivity; the sum of absorptivity, reflectivity and transmissivity is one. The reflectivity of silicon in LWIR is independent of wafer thickness (d), while transmissivity does depend on d and absorption coefficient. In the literature, much has been studied on the effect of doping concentration to emissivity. Less has been known about the relation between thickness and emissivity. Recently, ultrathin material like silicene and graphene have been discovered and triggered many possible applications. The measurement and control of temperature of these ultrathin materials are important for their processing in device fabrication. If the emissivity of thin films can be precisely determined, then we can measure the temperature of silicene and graphene by pyrometers. The traditional method to calibrate emissivity is to put a sample and a nearly-perfect blackbody in a temperature-controlled furnace for comparative measurement of their radiance. However, the thermal radiation from the background furnace cannot be fully eliminated for thin samples in this method. In this study, we have proposed a new approach to obtain the average emissivity of ultrathin silicon wafer in LWIR by employing a combination of techniques XPS, thermocouples and pyrometers. In our method, the effect of background thermal radiation can be ignored. The experiment results show that the emissivity of silicon wafers decreases with their thickness, in agreement with theoretical predictions. At 732 K, the average emissivity is 0.08±0.02 for Si wafers of 20 μm in thickness; 0.68±0.02 for 550 μm. However, at temperatures higher than ~1067 K, the emissivity changes little for wafers with thickness >~20 um. The measured average emissivity for 20-μm-thick wafers is 0.61±0.01 and 0.68±0.01 for 550 μm.