Real-time determination of the direction of wafer temperature change by spatially resolved infrared laser interferometric thermometry

Interferometric thermometry is a noncontact method of directly measuring the temperature of a semiconductor wafer. Periodic modulation in the intensity of reflected laser light is detected as the temperature-dependent change in the optical pathlength causes transitions from constructive to destructive interference between the front and back surfaces of the wafer. In its simplest implementation, only a temperature change is detected; heating and cooling are indistinguishable. In this article, a simple method for determining the direction of temperature change is described and demonstrated, based on sensing phase differences in periodic signal intensity detected from two regions of slightly different thickness. A statistical analysis reveals that this method will have an extremely high probability of correctly determining the direction of temperature change in almost all instances.