Research On Cofocal Simultaneous Phase-Shifting Interference Imaging Microscopy

With the development of microelectronics, microsystem and microoptics technologies, it is in urgent need of the precision measurement of three-dimensional micrometer or sub-micrometer scale microstructures. Among all three-dimensional technologies of microstructure measurement, confocal scanning microscopy has attracted much attention due to its unique optical sectioning capability and high lateral resolution performance. However, there are still problems when applying confocal scanning microscopy to the three-dimensional measurement of microstructures, which include: first, there is a contradiction between the high lateral resolution and long working distance in conventional confocal microscopy; second, the influence introduced by changing of specimen reflectivities, is difficult to overcome in the current confocal and differential confocal measurements using the linear range scanning method, thus restricting its application in the measurement of structures with mixed materials and elements with their surface reflectivities changing dramatically; finally, the imaging theory of confocal microscopy is not completed yet, lacking in a detailed analysis of the widely used infinity confocal imaging system. The three problems above have becoming the main technical bottlenecks of confocal scanning microscopy, restricting its practical application in the three-dimensional precision measurement of microstructures.The subject"research on confocal simultaneous phase-shifting interference imaging microscopy"including the theoretical modeling, performance analysis and experimental study, aims to build up a theoretical model for the infinity confocal imaging system, as well as provide a three-dimensional confocal imaging measurement of surface topography with high resolution, long working distance and wide applicability. The research of the dissertation improves the imaging theory of practical confocal microscopy, lays a theoretical foundation for the application of infinity confocal and interference confocal systems, and provides a technical basis for the practical use of confocal scanning microscopy in the wide fields of microelectronics, microsystem and microoptics, etc.The major innovations in this dissertation are as follows: 1. Confocal simultaneous phase-shifting interference microscopy has been proposed. First, the main cycle of the interference phase curve is located using the bipolar property of differential confocal measurement, and then the defocused quantity is solved using the phase within the main cycle. The restriction of NA of the objective lens on the axial resolution is broke through when combining the simultaneous phase-shifting interference with differential confocal microscopy; meanwhile, the working distance is significantly increased and the influence of reflectivity changing of specimen surfaces on measurement results is overcame.2. Theoretical model for the infinity confocal imaging system has been established. The imaging process of conventional confocal system and confocal interference system is described based on the scalar diffraction theory and Fourier optics theory; It is pointed out that the lateral displacement of the point detector has a much more significant and sensitive influence on the confocal interference phase-shift that the confocal axial intensity response; when using the single-mode fiber as the confocal point detector, the lateral displacement of fiber head leads to the position of the peak point shifting axially with its intensity profile changing slightly; the theoretical lateral resolution of the confocal interference imaging system is 1.12 as high as that of conventional bright-field optical microscope.Finally, experimental verifications of the researches related with the subject have been implemented. First, experimental verifications of proposed confocal simultaneous phase-shifting interference microscopy were conducted. The experimental results show: 1) the axial resolution of the confocal simultaneous phase-shifting interference microscope reaches the order of 1nm under various NA conditions of the objective lens; 2) under the same experimental conditions of NA=0.25 for the objective lens, NA=0.1 for the collector lens andλ=632.8nm for the light wavelength, meanwhile, its working distance is extended as long as 10mm; 3) the practical surface measurement with its relative reflectivity difference of 80% can still be implemented using the confocal simultaneous phase-shifting interference microscopy. Next, experimental verification of the established imaging model of infinity confocal system was conducted. The experimental results show: 1) when using finite sized detector to receive confocal signals, with NA of the objective lens decreasing and the focal length f of the collector lens shortening, the confocal axial response will be broadened and thus the resolution is lowered; 2) with the extent of the optical source increasing, the confocal axial response will be broadened, the system resolution is lowered, but the optical sectioning capability is still observed; 3) with the conjugate distance d0 increasing from 20cm to 100cm, FWHM value of the axial intensity response changes less than 7%, and thus the conjugate distance has no significant influence on the confocal axial response; 4) when the lateral displacement of the confocal point detector using single-mode fiber increases to 5μm, the peak intensity of the axial response is lowered to be half of that with no lateral displacement, the position of the peak intensity point shifts about 2.5μm axially, but the normalized axial intensity curves remain almost the same; 5) the lateral displacement of the point detector has a much more significant influence on the confocal interference phase than the confocal axial intensity response.