A prototype RTP system has been developed which allows for in-situ emissivity and temperature measurements. The wafer emissivity is measured by using an optical detector at a wavelength of 2.4 μm and by modulation of the lamp power. This method permits accurate temperature determination in the range from 400 to 1200°C, independent of wafer backside roughness, backside layers, and transmit tance. The feasibility of the temperature measurement technique is demonstrated by using wafers with built-in thermocouples and highly As-doped wafers with different backside roughnesses or layers. The emissivity variations during processing can also be used to study thin film reactions in-situ. This is demonstrated for Co silicidation using probing wavelengths varying from 0.6 to 3.2 μm.