"CHIPS: NIST previews wafer calibration technique"

As submicron precision becomes the industry bellwether, the American Society for Precision Engineering has become the premier venue for announcing developments in interferometry. Last week, at ASPE's Summer Topical Meeting on Precision Interferometric Metrology in Middletown, Conn., breakthroughs in submicron measurements were announced for semiconductor wafers, flat-panel displays, photolithography, automotive systems and other challenging environments. Interferometry is the science of combining two or more waveforms to create a higher-resolution measurement. An interferometer makes use of the principle that when two waves of the same phase coincide they amplify each other, while two waves with opposite phases will cancel each other out. At the conference, the National Institute of Standards and Technology (NIST) demonstrated an infrared-laser technique that it claims more precisely measures the thickness of 300-mm silicon wafers. The technique uses color infrared interferometry to produce a spatial map that represents variations in wafer thickness as different colors. Green represents the ideal thickness; red, orange and yellow show areas that are overly thick; and turquoise and blue shows areas that are too thin. NIST plans to offer a calibration service based on its infrared laser interferometry for the master wafers that the industry uses to calibrate wafer thickness.
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