Portland, Ore. 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.
Techniques covered at the meeting included unconventional sources and wavelengths, such as deep-ultraviolet, infrared and terahertz radiation; roughness and texture analysis; interference microscopy; thin films on patterned surfaces using dissimilar materials; free-form optics and aspheres; and the use of traditional tools to assess form, figure and waviness.
Researchers today use monochromatic light sources (lasers) to build interferometers with submicron resolution. 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.
The calibration service will help the semiconductor industry meet the quality control goals of the International Technology Roadmap for Semiconductors out to 2010, NIST researchers Ulf Griesmann, Bob Polvani and Quandou Wang said in a presentation. NIST's instrument will accurately measure differences in thickness across 300-mm wafers with a repeatability of 5 nanometers, they said.
Dubbed the IR3 (for Improved Infrared Interferometer), the device uses intersecting waves of light to create interference patterns, like all interferometers; but instead of using red lasers it uses longer-wavelength, infrared lasers. The researchers said the longer wavelength lets the laser harmlessly penetrate the wafers to illuminate the top and bottom simultaneously. The resulting three-dimensional spatial map can reveal variations in wafer thickness in a single pass, whereas conventional interferometry requires the wafer to be spun so that measurements can be made from different orientations.
One limitation of the technique, according to NIST, is that the index-of-refraction of the wafer must be known in order to calibrate for the amount by which the lasers are bent when passing through it. Today that limitation results in measurements that are relative to a wafer's average thickness, but the researchers are confident that they can overcome this limitation to make absolute-thickness measurements by the time the calibration service goes online circa 2010.
In another presentation, QED Technologies Inc. (Rochester, N.Y.) discussed techniques for enhancing measurement accuracy with subaperture-stitching interferometry (SSI) a stitching algorithm that self-calibrates corrections for errors of motion, geometric distortion and reference waves. According to QED Technologies, SSI expands the useful range of interferometry for calculating surface figure, pixel scaling and radius of curvature by automatically controlling positioning and nulling while compensating for errors.
Interferometer pioneer and Lawrence Livermore National Lab researcher Gary Sommargren, who died in March, was honored in a retrospective session. Among Sommargren's most important contributions was the development of the "absolute interferometer," as opposed to instruments that measure relative to a reference. The work pointed the way to today's instruments, with subnanometer accuracy. Sommargren was winner of an ASPE Lifetime Achievement Award in 2003.
ASPE also hosted its traditional "open forum" evening, wherein a free-flowing discussion was stimulated by short ad hoc reports on the latest, most provocative results and technical directions in the field of interferometry.