Miniaturized confocal imaging system for large-volume high-speed 3-D measurement

Abstract

Recently, the fast 3-D measurement of large-area and micro-surface is demanded from various high-tech industries, for example, flat panel display (back light unit), bump inspection in the semiconductor manufacturing process, plasma display, and MEMS etc. The measurement field mentioned above is “Meso-measurement”, which is a middle area between micro- and macro- and has several centimeters measurement area and $1-10 \mum$ resolution. In addition, large depth scanning range is needed in dental CAD/CAM industry. Miniaturizing an equipment is an additional issue because of a space-restricted inspection environment such as mouth cavity of a human. Large-area 3-D measurement methods, such as Moire topography, white-light scanning interferometry, confocal microscopy using micro-lens array and slit beam scanning topography etc., have been developed so far, but still have many problems in terms of imaging speed, resolution, depth scanning range and size of the system. In this thesis, miniaturized confocal imaging system for high-speed large-volume 3-D measurement was developed. To increase the 2-D scan speed, the structure of direct-view confocal microscopy (DVCM) was adopted. A pinhole array was used to generate a multi-source and multi-beam array in DVCM. In addition, electrically tunable lens (ETL) was implemented to the DVCM system to enhance the speed of axial scanning. The proposed system, therefore, has not any mechanical moving part so that stable and fast 3-D image acqui-sition is available. The 3-D imaging theory of the DVCM was derived and some simulations are performed to investigate the influence of pinhole array characteristic (such as pinhole size, pinhole pitch) on the performance of the axial discrimination strength. In addition, the signal intensity, i.e. the ratio of the area of the pinholes to its total area in the pinhole array, was considered according to the pinhole size and the pinhole separation. As a result, optimal specification of the pinhole array was determined and it is applied to the design of the system. The design of a miniaturized DVCM with an ETL was performed. The design goal of the system was specified with a field of view of $10 mm \times 10 mm$, a depth scanning range of 5 mm and a resolution of $20 \mum$ in both lateral and axial direction. Then the design procedures were fulfilled by dividing a system into three parts

the source generation part, the relay optics part and the detection part. The source generation part consists of a LED, a condenser lens and a pinhole array, which generates a multi-source array. The relay optics part is composed of a tube lens, an ETL set and an objective lens, which satisfies the desired system magnifications. The optimal design of a tube lens and an objective lens were performed to minimize the optical aberrations and the system’s size. The detection part consists of an imaging lens and a CMOS camera, which transfers the im-age formed in the pinhole array to the camera. At last, its 3D cad design was completed. Implementation and performance tests of the proposed system were executed. At first, the system was calibrated to find a relation between the applied current into the ETL and the focal shift of the system. It has shown a good linear relation between the applied current and the following z-position over a range of 9 mm. After the calibration, we evaluated an axial response of the planar mirror and it was well matched to the theo-retical value. The FOV and the both lateral and axial resolution were tested and these were satisfied desired value. At last, the fast three-dimensional imaging capability was demonstrated by imaging certified step height specimens and the dental plaster within 1 second.