Research On Differential Parallel Confocal High Precision And Fast 3D Surface Topography Measurement Method

High precision and fast three-dimensional(3D)surface topography measurement methods play an important role in the fields of micro electro mechanics,optics,micro-electronics.As a popular three-dimensional surface topography measurement method at submicron scales,confocal imaging method has the advantages of axial chromatography,high contrast of imaging,non-contact,no requirement of special preparation of samples.Hence,confocal imaging method has become an important branch in the modern measurement science at micro-nano scales.Recently,with the advancement of micro-nano machining technologies,there exist an unmet demand that the required accuracy in both horizontal and axial directions is increasing daily while the measurement range is also constantly enlarging.However,the existing confocal measurement methods have the following limitations:(1)The measurement position is at the imaging focus where the optical signal is the strongest,but the changing rate of the axial response curve,i.e.,change of the light intensity with the axial position,is the minimum at the focus,which leads to the low accuracy of axial measurement;(2)As laser scanning confocal imaging method requires to conduct point by point scanning in order to imaging one axial layer,then layer by layer,the speed of 3D measurement is ratjer slow,especially not suitable for the samples with large surface area to be measured.Therefore,the issue of how to improve both the measurement accuracy and the measurement efficiency is a fundament problem for confocal 3D surface topography measurement,and it is also an important scientific problem to be solved in the modern precision measurement industry.The objective of this project is to solve the problem of how to jointly acquire high precision and high efficiency measurement in confocal imaging,and to provide the key technology and theoretical basis for the high-precision and rapid 3D surface profiling method for micro-nano structures,including step samples and samples with different reflectivity.The main research contents of this project are as follows:Firstly,to deal with the low efficiency theoretically limitated by traditional differential confocal measurement methods,a novel mechanism of differential parallel confocal(DPC)imaging microscopy is proposed and studied.Its advantagous characteristics including both high axial precision,high efficiency,and wide transverse measurement range are expounded in detail.The effects of different imaging parameters and different objective conditions on measurement curve of the DPC is discussed,the change rule of the DPC measurement characteristics is revealed.Then,to deal with the problem that the traditional differential confocal transverse resolution is still theoretically limited by the diffraction limit while conventional structure illumination microscopy(SIM)super-resolution imaging can't improve axial resolution,this project proposed a novel conjugate SIM super-resolution imaging method by designing a special structured illumination pattern for the 3D superresolution imaging.The designed illumination pattern satisfes both the superresolution requirement and the confocal principle.The principle,the design illumination pattern,and the image reconstruction algorithm for the conjugate SIM are described in detail.The experimental results show that with the proposed conjugate SIM superresolution method,the DPC achieves a transverse resolution of 173nm(NA=0.95),which is 58% higher than without SIM.Thirdly,an experimental measurement system based on the proposed DPC mechanism is built.Eperiments designed to verify the measurement principle of DPC were carried out.Measurement parameters that affecting the performance of DPC are also analyzed and optimized.3D surface topography measurement experiments with step samples verify that DPC has advantages of a high accuracy,large measurement range,and high efficiency.Experimental results using NA=0.75 objective with a standard step sample of a nominal height of 500 nm,the speed of the proposed DPC is about two orders of magnitude faster than a commercial white light interferremeter,while they have a similar measuring accuracy.Finally,to deal with the problem of spatial disturbance of both illumination and reflectivity,and to achieve high-precision three-dimensional measurement in full field of view,the project conducted error source tracing,error analysis,error separation,error compensation,and correction.A new methof for the construction of differential signal by first log axial response signals and then subtracting is proposed to eliminate errors caused by multiplicative noise such as uneven illumination and surface reflectivity and ensure the high-precision three-dimensional topography measurement through the full field of view.Experiments were designed and conducted to prove the effectiveess of the proposed error correction method.In conclusion,the proposed DPC,which simultaneously achieves high axial precision via the construction of differential confocal signal,and a super horizontal resolution via 2D isotropic conjugate SIM,and a high efficiency via paralleling imaging.The proposed DPC enriches the theory of confocal imaging.Due to its advantageous characteristics,it is anticipated that DPC has a great potential for 3D surface topography measurement.The research results can be applied to 3D surface topography reconstruction and measurement in MEMS,semiconductor manufacturing,precision optics,and similar other fields.