Research On TOF-SIMS Primary Ion Optical Measurement And Control System And Parameters Adjustment Technology

Time-of-flight secondary ion mass spectrometry(TOF-SIMS)is a powerful method of surface analysis,which has the characteristics of high resolution,simple sample processing and almost non-destructive,and has become one of the most promising surface analysis technologies today.The primary ion optical system is one of the important components of the TOF-SIMS instrument,which directly determines the analytical performance of the instrument.The structure of the system is precise and complex,and many parameters need to be adjusted,including multiple sets of ion beam deflection plate voltages and focusing lens voltages,which are used to control the flight trajectory of ion beam to make the intensity and diameter of ion beam meet the experimental demand.The unreasonable setting of primary ion optical system parameters will directly lead to large errors in the experimental results or even fail to meet the analysis requirements.On the experimental platform of "isotope geology special TOF-SIMS" instrument developed by Beijing Ion Probe Center,this paper studies the TOF-SIMS primary ion optical measurement and control system and parameter regulation technology.The main work is as follows:(1)Design of primary ion optical measurement and control systemAiming at the problem that the primary ion optical system of TOF-SIMS instrument has many components and the control mode is not uniform,the overall design scheme of the primary ion optical measurement and control system is put forward,and all the measurement and control objects are controlled by the upper computer.The measurement and control system mainly includes the ion beam control part and the ion beam detection part.The direction of ion beam flight is controlled by controlling the high voltage power supply of multiple deflection plate groups and lens groups;The beam intensity is detected by the picoammeter and the diameter of beam spot is measured by knife edge method,so as to detect the ion beam strength and the beam spot size.(2)Hardware circuit development of the primary ion optical measurement and control systemIn order to control the high-voltage power supply of the deflection plate group to output high voltage and collect the feedback voltage of the power supply module,a multi-channel DA/AD module is developed based on the single-chip microcomputer and CPLD dual-chip control structure.The module performs data transmission with the host computer through the optical fiber interface.The DA output 0-5V is used as the control signal of the power module to control the deflection board power supply to output a ±300V voltage;after the high voltage output by the deflection board power supply is attenuated,the feedback voltage is collected by the ADC to detect the output voltage of the deflection board.After many tests,DA accuracy is better than ±0.1%,linearity R2>0.9999,AD accuracy is better than ±0.15%,reaching the design index,providing preconditions for the software development of the measurement and control system.(3)Development of primary ion optical measurement and control system softwareAiming at ion beam control and ion beam detection functions,according to the different control objects,it is divided into: deflection board power control module,lens power control module,step motor control module and leather ammeter control module.The software of TOF-SIMS primary ion optical measurement and control system is developed based on the Lab VIEW programming environment and the producer consumer model.Through the actual on-line test,the host computer can not only control the output voltage of each high voltage power supply module and collect the voltage and current value returned by the high-voltage module in real time,but also control the leather ammeter in real time and display the number on the software interface.By using the edge method,the diameter of beam spot is calculated by the data collected by the leather ammeter and the stepping motor,which lays foundations to the study of the automatic regulation technology for the parameters of subsequent instruments.(4)Research on control parameters adjustment technologyAiming at the complex problem of adjusting parameters of TOF-SIMS primary ion optical system,an automatic parameter adjustment method and an auxiliary parameter adjustment method based on neural network model prediction are proposed,Besides,the automatic adjustment method is integrated into the measurement and control software.Compared with the manual step-by-step debugging,the proposed automatic adjustment method reduses the adjustment time from a few hours to less than15 minutes,and it increases beam strength from 15 p A to about 25 p A,so it is obvious that the beam strength is enhanced;The mean square error(MSE),root mean square error(RMSE)and average absolute error(MAE)of the four-layer neural network model test set are 6.714,2.591,1.341.The prediction accuracy can reach 94.7%,indicating that the ion beam can be predicted accurately;By studying the typical variation curve of beam intensity and parameter voltage,the influence degree of each parameter on beam current is analyzed,which provides a guiding basis for the selection of adjustment interval and adjustment step size during instrument debugging.