In-situ optical wafer temperature measurement

The need for increasingly tighter process control is eminently apparent as semiconductor device dimensions become smaller and wafers larger. Today "Thermal Budgets" are shrinking and ramp rates are increasing throughout wafer processing. Wafer temperature is perhaps the most universally critical process variable in front-end integrated circuits (IC) manufacturing. The use of pyrometry and optical lightpipes continues to gain widespread acceptance as the standard temperature control method in many processes. Lightpipes are used for controlling temperature in chemical vapor deposition (CVD), rapid thermal processing (RTP), epitaxial film growth (EPI) and physical vapor deposition (PVD). Optical thermometry offers numerous advantages over other forms of wafer temperature measurement. This paper presents the current strengths and limitations in optical wafer temperature measurement. Many factors continue to drive the measurement technology. As IC junctions become shallower, thermal budget concerns drive process temperatures down. Processing time and ramp rates continue to shorten in particular for implant anneals. Increasingly, process control requires complete thermal histories of wafers throughout IC manufacturing. These factors and new materials (copper and low-kappa dielectrics) push tool manufactures and pyrometer vendors toward lower temperatures while still requiring high sensitivity, and accuracy. The accuracy of most in-situ optical temperature measurement continues to be dominated by uncertainty in wafer emissivity. Factors that limit accuracy, e.g., from wafer to wafer and from tool to tool, and advances in the technology are discussed.