A measurement device for measuring a thickness of a film layer over a substrate utilizes a microwave source and a resonant cavity having an open side. A microwave signal is introduced at a first end of the resonant cavity with the open side against a surface measurement sample having a film layer over a substrate, and an output signal detector senses the output power of the signal at a far end of the resonant cavity. A processor uses a difference in the resulting resonant frequency of the cavity from that using a substrate without the film layer to determine the thickness of the film layer.