Thin Film Inspection with Millimeter-Wave Reflectometer

Abstract Inspection of dielectric thin films (25.4–508 μm) placed directly above a conducting plate is performed with a W-band millimeter-wave network analyzer. The signal launched by a small antenna placed near the sample surface is picked up by a separate receiver. The phase of the received signal can be related to the thickness and permittivity of the thin films under inspection. While the transmitter and receiver are at larger distances from the dielectric surface, the system operates as a conventional reflectometer. But when they both are close to the surface, guided waves will be excited and even may become dominant over the direct reflection. The significance of the guided-wave contribution is controlled by the locations of the guided-wave poles in the complex k ρ -plane. In this paper we employ a modal approach that previously was developed for geophysical remote sensing to study the effects of the guided waves, and to outline the limits of the conventional reflectometric method for evaluating thin films.