How hot is it? Semiconductor manufacturers need to know the answer to this question, especially for their rapid thermal processing systems. To reach the quality and device-performance requirements set for next-generation integrated circuits, chipmakers say they need a threefold improvement in their ability to measure and control temperature during single-wafer processing.
A new National Institute of Standards and Technology (NIST)-patented calibration wafer promises to deliver the desired level of accuracy. The wafer, instrumented with thin-film thermocouples, can link temperature measurements to the international temperature scale, making it easier to replicate processing conditions in different rapid thermal process chambers and at different facilities.
During trial runs, chipmakers use test wafers to relate wafer temperatures to rapid thermal process chamber temperatures, which are recorded by a light pipe radiation thermometer. Inserted through the bottom of the chamber, the thermometer tracks temperature during actual production. With current methods, the light pipe radiation thermometers can be calibrated with an accuracy of ~9 to ~11°F (5 to 6°C). The 1999 Technology Road-map for Semiconductors set the goal of reducing measurement uncertainty to ~4°F (2°C).
To develop a more sensitive sensor, NIST substituted on-wafer, thin-film thermocouples for the wire ones currently being used to measure temperature at different points on the surface. The thin-film approach eliminates the large junctions where pairs of thermocouple wires meet, avoiding the measurement-complicating heat transfer that occurs at these points. In addition, the NIST team developed methods for calibrating thin-film thermocouples on the international temperature scale, achieving an uncertainty of about 0.3°C. In turn, light pipe radiation thermometers can be calibrated on the scale to within ~4°F (2°C).
For more information about the calibration wafer, contact NIST's Kenneth G. Kreider at (301) 975-2619.