LASER INTERFEROMETRIC TECHNIQUES FOR TEMPERATURE MEASUREMENT

LASER APPLICATION FOR NONCONTACT TEMPERATURE MEASUREMENT OF A SEMICONDUCTOR WAFER IS BEING DEVELOPED. THE TECHNIQUE CAN BE APPLIED FOR EITHER CHEMICAL VAPOR DEPOSITION (CVD) OR BY RAPID THERMAL PROCESSING (RTP) SYSTEMS. THE METHOD IS BASED ON THERMAL EXPANSION OF A SEMICONDUCTOR WAFER, AND USES SPECKLE PATTERNS GENERATED FROM THE BACK (ROUGH SURFACE) OF THE WAFER. SPECKLE PATTERNS ARE GENERATED BY COHERENT ILLUMINATION BY TWO SYMMETRICALLY INCIDENT LASER BEAMS. SPECKLE PATTERNS ARE RECORDED AT VARIOUS TEMPERATURES AND SUBTRACTED FROM AN INITIAL FRAME STORED AT A KNOWN TEMPERATURE. SPECKLE CORRELATION FRINGES PRODUCED BY THIS ELECTRONIC SUBTRACTION PROCESS (ESPI) REPRESENT CONTOURS OF EQUAL DISPLACEMENT. FOR 6.0 MM WIDE ILLUMINATION REGION, A TEMPERATURE SENSITIVITY OF APPROXIMATELY 30-DEGREE/FRINGE IS OBSERVED. SPECKLE DECORRELATION TAKES PLACE AS THE NUMBER OF FRINGES APPROACHES TYPICAL SPECKLE SIZE. SIGNAL-TO-NOISE RATIO OF THE SPECKLE FRINGES IS IMPROVED BY INCORPORATING SINGLE-STEP PHASE SHIFT IN THE DATA ACQUISITION AND PROCESSING SYSTEM. TEMPERATURE MEASUREMENTS UP TO 400 DEGREES ARE DEMONSTRATED WITH A SINGLE REFERENCE STORED FRAME. FAST FOURIER TRANSFORM (FFT) OF SPECKLE FRINGE PATTERNS IS APPLIED FOR CALCULATION OF SPECKLE FRINGE PERIOD TO EXTRACT TEMPERATURE INFORMATION. A DETAILED INVESTIGATION OF THESE TECHNIQUES IS PROPOSED FOR SPECKLE TEMPERATURE MEASUREMENTS UP TO 1000 DEGREES.