Factors affecting the precision and accuracy of surface temperature measurement using light-pipe radiation thermometers (LPRTs)
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The accurate measurement of temperature in Rapid Thermal Processing (RTP), a key technique that processes single silicon wafers at a lower cost and in a shorter period of time, is of vital importance for improving the productivity of high quality devices. In order to meet the requirement of the International Technology Roadmap for SemiConductor 2004 (ITRS-2004), which is an uncertainty of + 1.5 o C at 1,000 o C, light-pipe radiation thermometers (LPRTs) are the sensor of choice to monitor the wafer temperature during the processing. To achieve this goal of uncertainty, a unique test bed, which is an axisymmetric design chamber with a three-zone ceramic heater, was developed by the University of Texas and used to calibrate the LPRT system by comparing its reading with the temperature reading obtained from an instrumented wafer. However, a difference of 10 o C to 30 o C between these two readings was found. This dissertation focuses on finding the error sources with three different types of light-pipes: fused silica, fused quartz, and sapphire. The thermal environment effect is the first factor to be determined. The diffuse reflectivity caused by the surface imperfections of the LPRT is determined in this research. Three different surface roughness values of fused silica light-pipe created by different type of sandpapers were performed, and their results were compared with previous Monte Carlo simulation results. Furthermore, different types of light-pipe can be affected differently. To explain which light-pipe material can be most influenced by the thermal environment effect based on its spectral properties, the sensitive wavelength range of our photo-detector was measured. Another study is the effect of the separation distance between the light-pipe tip and the measured object on the object surface made by the light-pipe probe. To determine which type of object and light-pipe materials are causing the separation distance effect, ceramic and molybdenum, which were painted with flat black ultra-high-temperature paint, were used as the measured targets. Moreover, the experimental results were compared to a computer model using the finite-difference method in order to predict the temperature depression as the space between the tip of the light-pipe and the target increases. To obtain higher accuracy in the computer simulation, the spectral properties of each material were measured by using an infrared spectroradiometer. To understand the directional range over which the LPRT can detect the radiation signal, the acceptance angles of each light-pipe materials were also measured.