Patent ID: 8432151
Filing Date: 2013-04-30
Classification: G01B,G01R,H03B,H03L

Abstract:
1. A film-thickness measuring device having an oscillating circuit, including a piezoelectric element, and a frequency measurement circuit for measuring a frequency of an output signal of said oscillating circuit, in which a film forming material is deposited on said piezoelectric element, and a film thickness of said film forming material is measured based on a change of a natural frequency of said piezoelectric crystal by deposition of said film forming material, wherein said frequency measurement circuit comprises a phase comparator into which an output signal of said oscillating circuit is inputted as a reference signal, a loop filter into which the output signal of said phase comparator is inputted, an A/D converter for converting the output signal of said loop filter into a digital signal, a calculation circuit into which the digital signal outputted from said A/D convertor is inputted, and a DDS (direct digital synthesizer) for outputting a signal with a frequency according to a frequency setting signal outputted from said calculation circuit, the output signal of said DDS is arranged to be inputted into said phase comparator as a comparison signal which is compared with said reference signal, and a part of the output signal of said DDS is arranged to be injected into said oscillating circuit as an injection synchronization signal.