A non-contact temperature measurement technique based on diffraction-analysis monitoring of the thermal expansion of materials is discussed. Due to the need for noncontact temperature measurements during semiconductor processing, silicon was chosen for this demonstration. The diffraction method requires a grating of suitable spatial frequency etched on the surface of the silicon wafer. The diffraction angle from the grating depends on the grating period which varies with temperature. Two symmetrically disposed incident beams are used to provide a differential measurement which is relatively independent of sample tilt. A computer system is used to monitor the diffraction order movement, from the order separation a relative temperature change can be calculated in near real-time. Temperature sensitivity for the diffraction technique is inversely dependent on the grating length (number of lines) and independent of the grating width. A sensitivity of 0.75°C is demonstrated for a 3-mm wide grating over a 20-700°C temperature range.