Signal Process Algorithms for White-Light Interferometer of Surface Morphology Measurement

• Main Authors: Cheng Yu Pai, 白丞祐
• Other Authors: J. W. Liaw
• Format: Others
• Published: 2010
• Online Access: http://ndltd.ncl.edu.tw/handle/42635917481175039976

碩士 === 長庚大學 === 機械工程學系 === 98 === Three algorithms were used to process the interference signals of white-light interferometer in order to obtain the surface profile of a sample- the absolute value method (AVM), the envelope method (EM), and the 2nd derivative method (2nd DM). Via a low-pass filter, a reference signal of the interference signal is obtained, and then a preprocessed signal which is the difference of the interference signal and its reference one is used for the process of these algorithms. After that, the surface morphology of sample is constructed. In our experiment, a step of 75 nm was set for a PZT motor, the scanning span in z-axis was 10.5 m, and a 50-fold Mirau interferometer was used. Two waveforms were observed in the interference signal, when the observing point is very close to the interface of two different-height planes; one is the interference signal of the reflected light from the higher plane interfering with the reference light in Mirau interferometer, and the other is that from the lower plane. A notch profile with a width of 250 nm and a depth of 550 nm was used for measurement. The EM can preciously estimate the width and depth, because the stronger envelope of the lower plane is identified, rather than the smaller one of the higher plane. In addition, a polynomial fitting was used to find the peak of this envelope. In contrast, the depth of the notch was under-estimated by the software of a commercial system.