Research On Autofocus And Fast Vertical Scanning Method Of White Light Interferometr

White light micro-interferometer is an important instrument for high-precision and non-destructive measurement of micro-topography.When detecting the surface microscopic three-dimensional topography,it is generally necessary to collect multiple white light interference patterns on the surface of the tested part,and use a special algorithm to obtain the surface microscopic three-dimensional topography data.In order to measure accurately,the surface of the tested part must be positioned at the localized surface of the white light interference fringe with the highest signal-to-noise ratio,that is,the micro-interference system is required to quickly and accurately focus on the tested part;on the other hand,if the surface is highly fluctuating up to dozens or hundreds of microns,like step or high aspect ratio groove,it is necessary to collect hundreds of interferograms to recover the surface microscopic three-dimensional topography.Therefore,it is necessary to study the autofocus and fast vertical scanning methods of white light interferometry to improve the measurement speed and accuracy of surface microscopic three-dimensional topography.Starting from the principle of white light interferometry autofocus,this paper analyzes the construction principle of autofocus evaluation function,compares the performance of five typical evaluation functions through simulation,and selects the most suitable evaluation function.After explaining the influence of the interference peak in the evaluation function value curve on the autofocus,a strategy to eliminate the interference peak is proposed,and simulation and experimental verification are carried out.Based on the completion of automatic focusing,this paper analyzes four typical detection methods for finding coherent regions on the upper and lower surfaces of the tested object,and proposes an improved frame difference method based on the characteristics of white light interference fringes to realize fast vertical scanning of white light interferometry.Based on the calculated length of the coherent region,the method uses the frame difference method to accurately locate the boundary of the coherent region after directly jumping half the length of the coherent region,which greatly reduces the search times of the frame difference method and greatly speeds up the acquisition time of vertical scanning.With 90.5 nm and 7.805μm step plates,autofocus experiments were carried out on the compound search method based on spectral distribution characteristics proposed in this paper.Experiments show that the focus error of this method is withinλ₀/8,the average focus error is less than 20 nm,and the repeatability fluctuation is less than0.15%;the 7.805μm step plate is used to conduct a fast vertical scanning experiment on the improved frame difference method based on the white light interference fringe characteristics proposed in this paper.The experiment shows that the average measurement error of the method in this paper is 0.104%,the measurement repeatability fluctuation is 0.091%,the average acquisition time for vertical scanning is 45.4 s,which is 42.2 s faster than the traditional vertical scanning method,and the acquisition rate is increased by 48.2%;with the high aspect ratio（7:1）trench on silicon,the fast vertical scanning experiment is carried out on the improved frame difference method based on the white light interference fringe characteristics proposed in this paper.The experiment shows that the method proposed in this paper is also suitable for near-infrared micro-interferometer.Compared with the traditional vertical scanning time 517.6 s,the average acquisition time of the vertical scan of the method in this paper is only 232.1 s,which is 285.5 s faster,and improves the acquisition rate by 55.2%.