Research On Key Techniques Of Microstructure Topography Measurement Based On Digital Holography

The surface topography characteristics of micro-nano structure devices are important parameters for evaluating their quality and performance,which are closely related to the reliability,service life,and mechanical properties of the devices.With the continuous improvement of the pattern density,depth-to-width ratio and integration degree of MEMS chips,it is a great challenge to measure the surface topography of the micro-nano scale.As having the advantages of non-invasive,full-field imaging and real-time quantitative phase measurement,digital holography is widely used in living cell detection and microstructures testing.This doctoral dissertation focuses on the key techniques in digital holography,such as numerical reconstruction,phase restoration,aberration compensation and dual-wavelength measurement.A phase restoration method with an adaptive reliability mask for phase unwrapping and a numerical phase aberration compensation method are proposed,and four digital holographic experimental devices are established,aiming to explore and realize the digital holography techniques with high resolution,low noise,aberration-free and large measurement range.The main work and innovations of this dissertation are summarized as follows:1.The current research status of microstructure topography measurement methods is compared and analyzed.The five main research directions in the field of digital holography are summarized,and the key problems that need to be solved in the measurement of microstructure topography by digital holography are proposed.2.The recording and reconstruction principles of three common digital holography methods are analyzed in detail,the generating mechanism of phase aberration and the spectrum characteristics of off-axis digital holography are theoretically studied.Aiming at the problem of large differences in the spectrum distributions corresponding to different holographic configurations and samples,a spatial filtering algorithm based on region recognition is used to adaptively generate the filtering window to accurately extract the targe spectrum.Experiments have proved the effectiveness of the algorithm.3.To improve the phase measurement accuracy of digital holography,a complete phase restoration should include phase filtering for noise reduction,phase unwrapping for discontinuity elimination and phase compensating for aberration removal.Various phase filtering methods are compared,and the simulation results show that windowed Fourier filtering(WFF)has the best noise reduction performance.Three classical phase unwrapping algorithms are analyzed in detail.Aiming at the phase anomaly caused by coherent noise and fringe dislocations in the wrapped phase maps,a phase restoration method with adaptive reliability mask for weighted least-squares phase unwrapping is proposed to realize accurate quantitative phase measurement in digital holographic microscopy(DHM).Simulation and experimental results demonstrate that the proposed method can restore a more accurate and reliable sample profile.4.To solve the phase aberration problem in digital holographic system,the advantages and limitations of double exposure method are analyzed experimentally,and the principles and implementation schemes of telecentric configuration,Zernike polynomial fitting and double fitting are explained in detail.For the dense microstructures with few and discontinuous background regions,a numerical phase aberration compensation method of DHM with curve fitting preprocessing and automatic background segmentation is proposed,which realizes the accurate measurement of 3D topography of the microstructure.Simulation and experimental results demonstrated that the proposed method is able to accurately compensate the phase aberration for the complicated dense microstructure.5.An experimental device of lensless Fourier transform digital holography(LFTDH)is established to measure the reflective microstructure.When the hologram recording distance is 13 cm,the actual lateral resolution of the reconstruction intensity image and phase map obtained from LFTDH are 17.54?m and 31.25?m,respectively.And obvious defects on the electrode surface of MEMS chip can be observed from LFTDH.A telecentric DHM experimental device is built to optically eliminate the quadratic phase aberration caused by microscope objective.Experimental results show that the actual lateral resolutions of the reconstructed intensity image and phase map obtained by the telecentric DHM are 3.91?m and 6.20?m,respectively,and verify the versatility of the proposed phase restoration algorithm.6.In order to expand the longitudinal measurement range of digital holography,an experimental device of single-shot dual-wavelength digital holography based on interference multiplexing is established.The wavelengths of the two lasers used are633 nm and 671 nm respectively,and the synthesized wavelength is 11.18?m.Aiming at the problem of aberration superposition and noise expansion in dual-wavelength holography,a dual-wavelength phase restoration method is proposed,which includes numerical aberration compensation of the wrapped phase maps,Curvelet transform filtering and immune algorithm.Finally,the accuracy of the proposed method is demonstrated by measuring a standard step sample with a nominal height of 1.99?m.