Development of Laser Scanning Lateral Differential Confocal Microscopic System
Objective With the rapid development of ultra-precision manufacturing,various precision components with micro and nanostructures are widely employed in fields such as engineering materials,biomedicine,optical imaging,and semiconductor manufacturing.Meanwhile,they are developing toward high-performance manufacturing with increasing scale,increasingly fine structures,and increasingly complex structural shapes.Testing techniques and processing techniques complement each other to truly reflect the processing quality of ultra-precision manufacturing devices and study device functions,which is an essential part of the development of ultra-precision manufacturing technology.A laser scanning lateral differential confocal microscope is developed in our study to tackle the challenge of high-speed and high-precision measurement of complex surface morphology of microstructured samples.Methods Firstly,the emitted laser enters the galvanometer beam scanning module after passing through the splitter prism.The laser emitted from the galvanometer beam scanning module focuses on its focal plane by the scan lens,and the rear focal plane of the scan lens coincides with the rear focal plane of the tube lens.After being collimated by the tube lens,it enters the measurement objective.Finally,the measured objective converges the laser beam on the surface of the sample.By changing the control signal,the laser exits from the two-dimensional galvanometer in different directions,causing the parallel light direction to enter the measurement objective to change accordingly.This changes the lateral position of the light spot that converges on the surface of the sample,forming the measurement beam.Even if the sample is not moved,the laser spot and the measured sample can still be changed by controlling the reflection angle of the galvanometer scanning module.Additionally,the relative position is adjusted to achieve beam scanning.The light beam reflected by the tested sample returns to the original path and passes through the measurement objective,tube lens,scan lens,and galvanometer beam scanning module in sequence.After being reflected by the splitter prism,it forms a detection beam via the collection lens.The system inserts a D-shaped aperture into the detection optical path.The surface height change of the tested sample will cause the detection spot to move laterally.After being modulated by the D-shaped aperture,two point-detectors consisting of a double pinhole and a two quadrant detector set at the focal plane of the collection lens collect the light intensity signal to obtain the front and back focal axis response signals.Differential subtraction is adopted to finally obtain the laser scanning lateral differential confocal axial response signal.Meanwhile,the high linearity near the zero-crossing point of the axial response signal is adopted to achieve high-speed measurement without axial scanning.Combined with the two-dimensional galvanometer for high-speed beam lateral scanning,high-speed and high-resolution three-dimensional imaging is realized.Results and discussions Experiments show that the axial measurement resolution of the system reaches 1 nm(Fig.10),and the lateral measurement resolution is 400 nm(Fig.11).Compared with the standard etching sample calibrated by atomic force microscopy,the measurement results are basically consistent(Fig.12).For micro and nanostructured devices such as semiconductor wafers and others with heights at the micrometer level,the laser scanning lateral differential confocal microscopy improves the measurement efficiency by about three times compared to the Olympus OLS4000 laser scanning confocal microscope(Fig.13).Additionally,when the surface structure height profile of micro/nanostructured devices is lower,the laser scanning lateral differential confocal microscopy will have a more significant effect on measurement efficiency improvement.Conclusions In our study,a laser scanning lateral differential confocal microscopy system is constructed.The system inserts a D-shaped aperture into the collection path of the confocal system,which causes changes in the surface height of the measured sample to make the collection spot move laterally on the focal plane for lateral differential confocal detection.By utilizing the characteristic of the large slope and good linearity of the axial response curve,polynomial fitting is performed on the linear region of the response curve,and the height of the measured sample is directly measured from the light intensity response.By adopting a two-dimensional galvanometer as the lateral high-speed beam scanning component,this system is different from ordinary differential confocal systems and has the ability of beam scanning,ultimately achieving high-speed and high-precision three-dimensional microscopic imaging of the surface morphology of microstructure samples.Finally,by taking semiconductor wafer samples in practical engineering as an example and conducting comparative tests on existing commercial laser scanning confocal microscopes,it is proved that this system has the ability to measure and analyze the surface three-dimensional morphology of microstructured samples in ultra-precision machining and detection.Therefore,it is a new and effective engineering high-speed high-resolution microscopy system.
optical instrumentthree-dimensional microscopic imaginglateral differential confocal microscopygalvanometer scanninghigh-speed and high-resolution
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