Research On Methods Of Improving AFM Measurement Precision Considering Tip Wear

The demand of military and civilian industries for microelectronic devices and optical parts has promoted the development of ultra-precision machining technologies.The manufacturing precision of parts' functional surfaces has gradually reached nanometer level,and higher requirements have been put forward for ultra-precision surface measurement tools.Atomic force microscopy uses a nano-probe contact surface to obtain micro-nano-scale three-dimensional topographic information of the surface,and there is no vacuum environment and sample conductivity requirements,so the application is increasingly widespread.However,the existence of artifacts in the current measurement results does not give a good performance to the instrument's high resolution performance.Although many scholars have conducted researches to improve the measurement accuracy,such as optimizing the scanning parameters,the influence of the probe tip has not been considered.Because the scanning parameters affect the control accuracy of the system,it also affects the wear of the tip,thereby reducing the lateral resolution of the AFM.However,if the sampling parameters(sampling interval)are not properly adjusted after the probe tip wears out,if the sampling interval is too small,high-frequency error information larger than the horizontal resolution will be introduced,which will increase the sampling time,introduce more noise,and reduce the measurement accuracy.Therefore,this article has carried out a method to improve the measurement accuracy considering probe wear.The main contents are as follows:(1)A simple method based on the step structure for evaluating the wear of the tip was proposed.The scanned image is actually the result of the convolution of the probe tip and the surface topography of the sample.The changes in the tip radius will inevitably cause changes in the scanning results of the step structure.According to this principle,the contact model of the tip radius and the standard step structure is established.The radius of the obtuse circle of the worn tip is evaluated by the new probe(the radius of the tip arc is known).(2)The criterion of evaluating the feedback control accuracy by using the amplitudeerror is proposed.The influence of the scanning parameters(proportional gain,integral gain,amplitude setpoint,scanning frequency,and free amplitude)on the feedback accuracy of the feedback system is studied.At the same time,based on the analysis of the influence of the scanning parameters on the extreme conditions of the probe tip wear(zero amplitude scanning of the probe tip),the scanning parameter setting principle of improving the measurement accuracy with both the tip wear and the feedback accuracy is proposed.(3)The optimal sampling interval selection method to ensure measurement accuracy was studied.The effects of sampling interval and wear tip on the measurement results were studied by surface 3D evaluation parameters and power spectral density evaluation indexes.Based on the study of sampling interval and the influence of probe tip wear,a reasonable sampling interval calculation model considering the wear of the tip was established.The model can effectively improve the measurement efficiency and accuracy.