| As modern equipments keep updating,new industries are demanding higher and higher processing fineness for their products,and precise and rapid inspection of components is becoming more and more important.Optical interferometry is gaining attention because of its high sensitivity and high accuracy,but traditional interferometric instruments are difficult to achieve high resolution,large field of view,real-time calculation,and probe miniaturization at the same time,and thus are not suitable for some industrial scenarios where continuous precise machining and in-situ real-time inspection are desired.For example,the contradiction between the resolution,field of view and real-time of the system of white light interferometer is irreconcilable,and the probe is so large that not suitable for online real-time measurement.The reason is that it uses a two-dimensional surface array camera to capture the shape of a three-dimensional object,which needs to be scanned in the axial direction and wait for all the three-dimensional images to be acquired before it can start calculating the height.To realize a measurement system with high resolution,large field of view,good real-time performance,and miniaturized probes at the same time,this paper proposes a spectral interferometry system based on fiber optical path to measure the three-dimensional shape of samples.The main work of this paper is as follows:(1)A spectral interferometry system is proposed,which is based on fiber optical path to measure the three-dimensional shape of samples.The system uses a fiber optical path instead of a free space optical path,and the probe can be separated from the reference arm and detector to achieve a miniaturized probe;a fiber optic spectrometer is used to collect the interferometric spectrum and the relative height of a measured point can be calculated;a scanning spot scanning module based on a galvanometer-scanner is used to achieve 3D topographic measurement with a typical field of view with resolution of about 7 μm beam radius in a large field of view of 18mm×18mm.The sample scanning and topography calculation can be carried out simultaneously with good real-time performance due to the light spot scanning.(2)The Energy Centrobaric Correction Method(ECCM)is designed as an algorithm for topography measurement applications,which improves the calculation speed and reduces the measurement error.In response to the problem of slow calculation speed and high error of the widely used Phase Slope Method(PSM),this paper designed the ECCM,which adopts simple primitive operations,and discards phase calculation,and does not need to iterate to obtain the slope of the phase,thus the calculation speed is fast.Simulation shows that the calculation time of the ECCM is 84% lower than that of the PSM,and the calculation error is96% lower,and the bandwidth requirement of the digital filter for the ECCM is lower,which can avoid the interference of pseudo-peaks and better meet the practical application requirements.The experiments also show that the ECCM has lower error.(3)The influence of the local reflectivity change of the sample on the topography measurement results is studied,which provides a direction for data analysis scheme improvement.Since the point-scan spectral interferometry system is not yet widely used as a new inspection tool and its reliability has yet to be verified,this paper has measured and calibrated the topography of a variety of samples with different characteristics.The experiments show that the local reflectance variation of the sample and the radius of the scanning beam can jointly affect the lateral topography results of the measured samples,for instance the width.But the measurement results of the system,especially the depth measurement results,are basically consistent with commercial instruments such as the white light interferometer and the confocal microscope and are reliable.The typical relative error is less than 1%.This conclusion is a guideline for the topography analysis of general samples and small holes with large aspect ratios.(4)A 300 μm pinhole placed above the sample was designed to simulate micro-deep holes with different aspect ratios,and it was demonstrated that the spectral interferometry system can correctly measure the topography of micro-deep holes.Compared with non-interferometric measurement methods,the spectral interferometric system has high sensitivity and large range,which is especially suitable for measuring micro-deep hole samples.However,few studies have analyzed the effect of deep micro-apertures on the measurement of the bottom shape of the hole.In this chapter,we propose to simulate micro-apertures with different depth-to-diameter ratios using a 300 μm pinhole placed above the sample,and the experiments show that the spectral interferometry system can correctly measure the bottom shape of micro-apertures.This system is successfully used to measure the press-fit deep micro-hole samples in multilayer circuit boards with an aspect ratio of about 20 and a hole diameter of about 250 μm,which are difficult to measure by non-interferometric means.(5)Designing to extend the focal depth of the spectral interferometry system using a diffraction-free beam,it is demonstrated that the Airy beam has a longer focal depth than the corresponding Gaussian beam,and the measured topography is basically correct.To extend the focal depth of the spectral interferometry system,a Gaussian beam,a Bessel beam,and an Airy beam with a half-height and full width of about 10 μm are designed and implemented through theoretical simulation and experiment.The three beams were used as the scanning beam of the spectral interferometry system,and the same shallow-aperture samples were scanned and measured.The measured topography showed that the depth of the hole was about 3 μm when the Gaussian beam was in focus,but the topography of the small hole changed significantly after 1 mm out of focus;the measured results of the Bessel beam did not change significantly in the out-of-focus range of 6mm,but the measured depth of the sample was 0.5 μm,which changed significantly from the actual one,probably due to the influence of the sidelobe;the measured morphology of the Airy beam was basically unchanged in the out-of-focus range of 0～2 mm The experiments demonstrate that the Airy beam has a longer depth of focus than the corresponding Gaussian beam,and can acquire the depth of the holes correctly.In summary,this paper designs and implements a spectral interferometry system for microstructure topography measurement on product surfaces;verifies the reliability of this new device;and analyzes the measurement characteristics of the system.And the designed system can be used to measure micro-deep hole samples that are difficult to detect with conventional equipment and has good real-time performance.This paper also investigates the use of diffraction-free beam to achieve the extended depth of focus of the spectral interferometry system,and experiments show that the generated Airy beam can extend the depth of focus and can acquire the depth of the holes correctly within a defocus of 2 mm. |
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