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Millimeter-wave measurement of subwavelength thickness with surface waves

Thickness measurements of cellulose acetate thin films (25 - 76 microns) placed directly above a conducting plate are made with W-band millimeter-waves (75 - 110 GHz). The surface wave excited by a transmitter near the sample surface is picked up by a separate receiver. The phase of the received signal was then related to the film thickness. The radiation from the transmitter is modeled as that produced by a vertical electric dipole over a two-layer medium. A contour integration is performed to derive the theoretical dependence of the phase shift on the dielectric thickness. A quadratic dependence was expected theoretically and observed experimentally. It was also found that achieving thickness resolution to several microns appears feasible. This technique displayed relatively high tolerance to variations in placement of the receiver and the dielectric samples.