Development And Experimental Study Of In-situ AFM Control System In High Vacuum Environment

Designing an in-situ Atomic Force Microscope(in-situ AFM)system that works in conjunction with a Scanning Electron Microscope(SEM)will have important significance for the quantitative characterization of sample morphology information and the characterization of three-dimensional micro-nano structural profile information.Its unique advantages are mainly reflected in:?SEM can provide nano-level fast navigation for the in-situ AFM needle tip;?Realize in-situ complementary imaging of SEM and AFM to avoid sample transfer measurement process;?SEM can monitor the in-situ AFM needle tip in real time to avoid the in-situ AFM needle tip being contaminated or damaged,causing in-situ AFM imaging distortion;?SEM can provide real-time visual feedback for in-situ AFM and improve the operation speed and effectiveness of in-situ AFM.This article summarizes the existing research of in-situ AFM from home and abroad,based on which,aiming at the problems faced by the development of in-situ AFM control system under high vacuum environment of SEM cavity,has indepth study from such four aspects,namely,in-situ AFM imaging mechanism and its distortion model,in-situ AFM imaging distortion correction method,in-situ AFM control system design based on distortion correction,and experimental study of in-situ AFM control system.The specific research contents are as follows:(1)In-situ AFM imaging mechanism and its distortion model.This paper designs the overall structure of the in-situ AFM control system based on the analysis of the in-situ AFM imaging mechanism and the identification of in-situ AFM control system,and the cause of in-situ AFM imaging distortion caused by SEM is studied.The first reason for distortion is that the in-situ AFM self-induced piezoresistive micro-cantilever(PRC)force sensor readings caused by the SEM focused electron beam radiation continue to drift,and the in-situ AFM force sensor reading drift model is proposed accordingly.The second reason for distortion:the high vacuum environment of the SEM cavity makes it difficult to dissipate heat,the temperature increase in the cavity will reduce the measurement accuracy and sensitivity of the in-situ AFM semiconductor strain gauge(SCSG)displacement sensor,resulting in the in-situ AFM piezoelectric scanner cannot be accurately moved.Based on this,an in-situ AFM displacement sensor reading drift model is proposed.(2)In-situ AFM imaging distortion correction method.Aiming at the drift model of in-situ AFM force sensor reading,an active drift suppression method for in-situ AFM self-induction PRC force sensor is proposed and experimented.The experimental results show that the drift rate of the in-situ AFM force sensor reading drops from about 13 nm/min to about 1 nm/min.Aiming at the in-situ AFM displacement sensor reading drift model,the traditional hardware compensation method is first improved,and then combined with the software compensation method to correct the hardware-compensated displacement output value of the integrated temperature compensation method and experimented.The experimental results show that the improved hardware compensation method increases the usable measurement range of the displacement sensor by about 40%,and the integrated temperature compensation method improves the measurement accuracy of the displacement sensor by about 220%.(3)In-situ AFM control system design based on distortion correction.On the basis of eliminating the influence of SEM on in-situ AFM imaging,a design of in-situ AFM control system based on distortion correction is proposed.In the hardware design,the system selects STM32F429IGT6 to control the 18-bit DAC to output analog signals.After low-voltage and high-voltage amplification,it controls the movement of the X/Y/Z piezoelectric scanner;The system uses a 24-bit ADC to collect the feedback signals from the inductive PRC force sensor and the SCSG displacement sensor.During the automatic needle insertion,the linear piezoelectric positioning platform performs coarse adjustment process and the Z-direction piezoelectric scanner performs fine adjustment process.STM32F429IGT6 completes the data exchange with the in-situ AFM host computer and SEM through USB2.0 protocol and Ethernet TCP/IP protocol respectively;In the software design,the system introduces the embedded operating system uCLinux,adopts multi-thread control and maps the system hardware into character devices and network devices,which simplifies the development of device drivers and improves the real-time nature of the system.(4)Experimental study of in-situ AFM control system.In this paper,the in-situ AFM system modules are first tested to ensure the stable operation of each module;then the standard AFM calibration chip SiC/0.75 is used as a sample to test the imaging accuracy of in-situ AFM.The experimental results show that the imaging accuracy of the in-situ AFM system designed in this paper is about 0.7706nm.In the in-situ AFM system experiment with SEM-guided scanning speed dynamic changes;Firstly,SEM secondary electron mode is used to image the sample;Secondly,the characteristic metric map of the sample is generated;Then,through the mapping function,the sample characteristic metric map is converted into the in-situ AFM scanning speed map,and smoothed;Finally,downloaded to the in-situ AFM main controller,in-situ AFM imaging dynamically adjusts the scanning speed of each point on the sample according to the scanning speed map.The standard AFM calibration chip VGRP-UM is used as a sample to perform low-speed scanning mode,SEM guided in-situ AFM scanning mode with dynamically changing scanning speed and high-speed scanning mode three sets of comparative experiments.The experimental results show that under the condition that the imaging accuracy is almost the same as the low-speed scanning mode,SEM guided in-situ AFM scanning mode with dynamically changing scanning speed saves about 56%of the total scanning time;In the case where the total scan time is the same as the high-speed scanning mode,the imaging accuracy of SEM guided in-situ AFM scanning mode with dynamically changing scanning speed is improved by about 43%.