Film thickness detection system

摘要

A film flaw detection system has capacitive sensing members for sensing capacitance changes in correspondence with irregular film variations. The sensing members take two forms. One form senses variation in film thickness by using the film as a dielectric between two plates of the capacitor. Changes in film thickness result in changes in the dielectric and hence in the capacitance. The other form consists of feelers which ride along the surface of the film and which move transversely to the film movement in response to flaws in the film. Each feeler forms a plate of a capacitance, and movement of the feeler changes the distance between this plate and an associated fixed plate which results in a capacitance change. The detecting circuit comprises a phase detection system for measuring phase changes in a high frequency signal. These phase changes correspond to capacitance changes caused by flaws in the film and sensed by either of the two forms of sensing means.