Study On White Light Scanning Interferometry: Measurement Method And System

The development of ultra-precision machining technology has put forward higher requirements for the measurement technology, which has given birth to the ultra-precision measurement methods. As an important measuring technique, white light scanning interferometry can realize fast and non-contact measurements, and it is now widely used in the field of ultra-precision testing. So far, the research interest on this technique is to make improvements on the accuracy and efficiency, as well as widening its application areas. Focusing on the white light scanning interferometry, this dissertation studied the related measurement principle and presented a nano-measuring machine (NMM) based white light interferometric profiler. The mainly achievements in this dissertation are listed as follows:1. The state-of-the-art development situation and trends about white light scanning interferometry were described in detail. To discuss the different types of white light correlogram demodulation algorithms, this paper classified them into three main groups, spatial domain methods, frequency domain methods, and white light phase shifting interferometry. Based on the theory deduction and the experiments analysis, the characteristics of each algorithm were summarized.2. With a nano-measuring machine (NMM) employed as the positioning platform, a white light microscopic interferometry profiler was designed and constructed. In addition to the routine vertical scan, the system is also capable of performing the tilt scan, speed-variable scan, and film measurement. Taking advantages from the excellent large scale positioning ability, the measurement range of the proposed system is enlarged to 25 mm×25 mm×5 mm. By laser interferometer real-time calibration, the scan step length error is minimized.3. The Carrémethod based white light phase shifting interferometry was described. Compared with the spatial domain methods, the presented algorithm can improve measurement accuracy without losing the processing efficiency. Combined with the centre of gravity method, the phase extraction is located within the zero-order fringe. Hence, the phase unwrapping is no longer needed, and the height error due to visibility variation is also minimized.4. A kind of speed-variable scan technique specialized for large step height measurement was presented. This technique has two realizations: the pre-configuration mode and the auto-focusing mode. During the scanning, the image collection is limited within the coherence area, and in other positions the NMM speeds up the motion. As a result, the measurement efficiency for the large size step height is significantly improved.5. The two-angle configuration based white light tilt scanning interferometry was presented. This new technique provides a solution to expand the horizontal measurement range of white light scanning interferometry, without the need of stitching. By means of this new technique, a millimeter size surface can be evaluated within one single round scan. Additionally, because of the two-angle configuration, the experiment adjusting process is quite simple.6. An image segmentation based film measurement technology was presented. In the signal analysis, the image segmentation is utilized to set the threshold dynamically. Therefore, the proposed algorithm can derive the film topography of the upper and lower surfaces, the thickness, the roughness and homogeneity simultaneously. Through the correlogram identification process, the application of the new method was expanded to evaluate the structures, which are partially coated by a film.