Improved gauge capability for ion implant monitors using temperature compensation for resistivity measurements

Abstract

Recent gauge capability studies for ion implant monitors have shown that the ambient temperature variation in the fab is a large contributor to the lack of reproducibility in the sheet resistance measurement. The temperature variation in many fabs can be as large as 4–8°C. For films with a temperature coefficient of resistance (TCR) of 0.8%/°C this causes a 3.2 to 6.4% variation in sheet resistance. TCR values are well documented for National Institute of Science and Technology (NIST) Standard Reference Material (SRM) and accurate temperature measurement allows for easy correction of measured sheet resistance to the nominal temperature of 23°C, at which the standards are certified. Unfortunately, the TCR values of most implant films have not been documented so it has not been possible to correct sheet resistance readings for variations in measurement temperature. An automated apparatus has been developed to measure the TCR of implanted layers in the temperature range of 15 to 35°C. The variation of sheet resistance with temperature can be considered linear over this range and these TCRs can be used to correct subsequent R s measurements to a nominal reference temperature (usually 23°C). With ever tightening process capability and gauge capability requirements the use of temperature correction will often mean the difference between a process or gauge that meets requirements and one that does not. The TCRs are also important in understanding how temperature variations will effect device performance where resistance is important. Examples of sheet resistance versus temperature and TCRs are presented for some typical ion implants. Results of a gauge study repeated on the same set of wafers reported on at IIT'90, using this apparatus are also reported.