Research On Microscopic Interferometry For Three-dimensional Topography Of Step-type Microstructures

Step-type microstructure components,with micro-and nano-scale,have been widely used in the fields of micro-optics,micro-mechanical,microelectronics,and biomedicine.Their topography can significantly influence the response sensitivity of the system and physical or biocompatible performance.At present,there is an urgent need for a non-destructive measurement technology for the three-dimensional(3D)topography or parameter of the steptype microstructure components,which can be utilized to optimize the manufacturing process and evaluate the quality of elements.Low coherence microscopic interferometry enables nondestructive 3D topography measurement for step-type microstructures.It has the advantages of high-precision,non-contact,non-phase blur,and so on.However,the step-type microstructure with the height near the wavelength,a period near the diffraction limit of the system,or a high aspect ratio,together with the low coherence microscopic interferometry system,produce complex multiple modulations on the detection light.It further distorts the zero optical path difference position of coherence signals,induces the strange signals containing several envelopes,and generates signals with a low signal-to-noise ratio,so that the coherence peak localization and phase calculation include errors.Finally,the 3D topography measurement algorithms,utilized by the current commercial equipment,fail and need to be continuously improved or proposed.Based on the theoretical research for coherence signals,this work establishes simulation models to study the modulation mechanism of the structures and the system on the detection light.Analyzing the characteristics of coherence signals helps to propose effective signal demodulation algorithms and establish an efficient coherent scanning strategy.This paper dedicates to achieving high-precision measurement for 3D topography of step-type microstructures.The main research work and innovation are as follows:Step-type microstructures and low coherence microscopic interferometry systems generate common modulations on the detection light.Theoretical models of the coherence signals are studied concerning three parameters,namely,the height near the wavelength,the period near the diffraction limit,and the high aspect ratio.For the step-type microstructures with the height near the wavelength,the modulation law of the step-type microstructures on the incident light is analyzed based on Fourier harmonic analysis,linear system transfer function theory,and scalar diffraction theory.Besides,the research studies the response bandwidth of the system receiving spectrum information and establishes the model of modulated coherence signals.The influence of system and structure parameters on the coherence signals is further studied to explain the batwing effect caused by diffraction.For step-type microstructures with a period near the diffraction limit of the system or high aspect ratio,the coherence signals model is established based on vector diffraction theory.This work discusses the characteristics of doubleenvelope or multi-envelope anomalous coherence signals caused by complex diffraction effects at the step edge and lower surface.The establishment of different simulation models provides a theoretical basis for analyzing the intensity,spectral characteristics,and variation laws of coherence signals,concerning these three typical structures.Meanwhile,the models lay a foundation for proposing a coherence signal demodulation algorithm and completing highprecision 3D topography measurements.For the step-type microstructures with a height close to the working wavelength,the 3D topography measurement result,calculated by the conventional algorithms,appears to batwing effects at step edges,so that the coherence peak location misleads the phase order calculation.Thus,this work proposes a batwing effect suppression algorithm based on the differential analysis of neighborhood pixels.The correction of phase order error and random mutation of topography is completed by locating the position of abnormal shape jump,analyzing coherent information,and averaging the shape of neighboring pixels.The standard step structure with a height of 459.8±3 nm is measured by the white light microscopic interference system developed in the group.The average value of the 10 repeated measurement results and the furtherly calculated uncertainty is 459.1±3.24 nm(k=2).For the step-type microstructures with a period close to the diffraction limit of the system,anomalous coherence signals with double envelopes appear at the edges of the step.The positions of the two envelopes correspond to the upper and lower surfaces of the structure.However,conventional algorithms cannot discriminate the effective envelope.This work proposes a binarization algorithm to locate all sampling points in the field of view by analyzing the intensity and contrast information extracted from the coherence signals.And a new type of constructed Morlet wavelet family function is to perform wavelet transformation and identify abnormal coherence signals.Combined with the binarization results,the effective envelope corresponding to the actual surface,where the sampling point locates,is automatically extracted and demodulated.The periodic step-type micropillar array structure in the metasurface is measured by the self-developed white light microscopic interference system.The diameters of7 types of micro-pillars distribute between 590 and 1350 nm,the periods in the orthogonal direction are 1740 nm and 3020 nm,and the height is 1850 nm.The structural features are close to the lateral resolution of the system of 501.9 nm.Compared with the scanning electron microscope results with higher accuracy,the height detection error of micropillars is within31.9 nm.For the 3D topography measurement problem of microgroove structures with a high aspect ratio,the Mirau-type white-light microscopic interference system induces a limited amount of probe light passing through the structure.The received coherence signals are weak.However,when using the self-developed Linnik-type near-infrared microscopic interferometry system,there are problems such as long coherent scanning time,signals with a low signal-to-noise ratio,and multi-envelope aliasing coherence signals on the step edge and lower surface.An automatic coherent scanning strategy based on auto-focusing establishes to avoid the acquisition of redundant interferograms and shorten the detection time.Coherence signals preprocessing algorithm based on complete ensemble empirical mode decomposition with adaptive noise,Fourier transform,and window filtering is proposed to eliminate low-frequency drift and highfrequency noise in the signal.Combining the intensity and envelope information extracted from the coherence signals completes the binarization process and the location of all sampling points in the field of view.According to the auto-focusing results,effective coherence signals are extracted by window filtering for demodulation.The system measures two standard groove samples.The depth and the line width are 101.77 m,10.97 m,as well as 200.99 m,30.00m,respectively.The averages of 10 repeatable depth detection results are 101.09 m and202.03 m,respectively.Uncertainty is furtherly calculated,and the result of the latter is202.03±1.96 m(k=2).