| Summary: | 碩士 === 國立中正大學 === 機械工程所 === 95 === Three-dimensional micro-feature inspection is a critical issue of semiconductor and optoelectronic industry. The most important characteristics of optical metrology using the interferometer technique are the “non-contact” and “high-resolution”. “High-speed” is another interested characteristic often demanded by the industrial applications. Holography is a well-proven method to quickly catch the 3D information of a sample by one-shot measurement and provide high resolution of measurement. However, conventional holography is not a real-time measurement method because the postprocessing of the holographic film (called the hologram) is a wet processing method and it takes time to develop the hologram. The reconstruction the 3D information from the analog hologram is also a tedious and time-consuming task that involves complex procedure of optical setup and mechanical alignement. Digital holography (DH) solves the problem of the conventional holography by exploiting the advantage of modern 2D image sensors and computational capability of micro-computers. Digital holography stores the interference information of the sample in the catch digital 2D image (called the digital hologram). A computational algorithm is then applied to numerically reconstruct the 3D information of the sample from the digital hologram. With high speed image sensor and fast computational algorithm, it is possible now to measure the 3D surface in video rate.
This paper is focused on the measurement of micro-three-dimensional object using the digital holographic microscope (called the DHM). We start from the discussion of the theory of near field diffraction. Based on the near field diffraction, the so-called “Huygens-fresnel diffraction” can be derived and formulated in both analog and numerical forms. The numerical reconstruction of the digital hologram is implemented in a computer program written based on the MATLAB software. We evaluated the correctness of our written computer program using different digital image inputs. The digital images include the amplitude only images and the virtual 3D images that include the amplitude and phase information. We also evaluate the reconstruction errors due the effects of reconstruction distance and the apodization. We also extended the DH method to the digital holographic microscope by adding the function of high numerical aperture microscope objective lens (MO lens) to the previous DH method. The DHM is purposed for improving the lateral resolution of the DH method that is limited by the total size of the CCD image sensor. However, the high NA lens also adds phase aberration on the digital hologram. We demonstrate that phase aberration induced by the high NA lens can be easily corrected by adding a digital phase mask in the numerical reconstruction process.
In conclusion, we have demonstrated that the DHM can be a fast, accurate and non-contact method to measure the 3D profile of micro-structures. Although the measurement speed of our DHM system can just spend 1/15 second to take one digital hologram, it spends about 3 second to numerically reconstruct the 3D profile. The physical resolution of the digital hologram is 13.1μm that is determined by the CCD pixel size and reconstruction distance. By applying the 80x MO lens to magnify the micro-features of the real sample, the resolution can be increased as 0.17um. However, the real resolution will be limited by the point spread function of the MO lens that is typically greater than 0.5mm.
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