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Interferometer Helps Produce Reliable Touchpad Films

Photonics Spectra
May 1997
R. Winn Hardin

Acheson Colloids Co. has produced screen-printed conductive films used in touchpad keyboards such as those on computers, automated teller machines, cellular phones and microwave ovens. But when a giant supplier of electrical devices challenged Acheson with a higher standard of batch-to-batch repeatability -- and a whole new set of stringent specifications -- the company decided it had to look beyond conventional measuring techniques.Besides selecting the right conductive materials for a specific application, Acheson needed to keep close tabs on the thickness of the film. As the film changes in thickness, so does its electrical resistance. "Each customer has its own needs," said Tom Adcock from Acheson's Ontario, Calif., office.
In the past, conductive-material manufacturers used mechanical measurement devices, such as stylus profilometers. They would place the printed and cured film on a stage, or anvil, and lower a probe attached to a traveling spindle until it made contact with the film. These kinds of measurements could detect film "heights" of 0.35 mil with resolutions of 0.05 mil.
Acheson's new prospect needed thickness measurements with a variance of less than 0.01 mil. After searching for new techniques, company officials settled on a system made by Tucson, Ariz.-based WYKO Corp. that uses vertical scanning interference microscopy.
The WYKO NT-2000 noncontact surface profiler first splits a beam of white light from a tungsten halogen lamp. After one beam bounces off the sample surface, it recombines with the second, reference beam, creating an interference pattern. Because light travels in a wave pattern, combining two waves either amplifies portions of a wave as two crests or reduces a wave by combining a crest with a trough. The resulting light creates a pattern of fringes called an interference pattern.
Because the fringes are brightest at a specific focal length, WYKO's system can make specific height measurements based on the intensity of the interference pattern. By moving the optics up and down above the sample surface, and knowing the optimum focal length, the interferometer and electronics judge at which point the fringes are brightest and determine the thickness of a film laid across the sampling stage to within 0.001 mil (from 1 nm to 2 mm with a resolution of <1 A Ra, according to WYKO).
"By accurately knowing the thickness of the film, we could consistently predict the conductivity of the material, allowing us to target a reduction in our product variability," said Adcock.

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