Design And Research Of A High-speed Imaging Atomic Force Microscope (AFM )System

Atomic force microscopy(AFM)is a useful tool in nanoscale measurement.However,a key hurdle to meet the needs of microstructure detection or nanofabrication is slow scanning speed,due to the limited bandwidth of AFM components.Especially,the bandwidth of the traditional scanner and control system are usually too low to achieve high-speed scanning.In this dissertation,a simple,compact,miniaturized and high integrated home-made AFM has been designed and developed.As an important part of the scanning system,a parallel kinematic piezoelectric actuator(PZT)driven xyz scanner is proposed,and detailed parametric design,model validation and prototype testing are performed,respectively.A series of actions confirmed that it can achieve high bandwidth and low coupling errors.Additionally,in order to improve the tracking performance of the scanner,an effective hybrid control strategy is applied.Meanwhile,field programmable analog array(FPAA)is used to implement the required controller algorithm,and the excellent and stable tracking capability of the xyz scanner is demonstrated.To operate the home-made AFM system quickly and concisely,an operating software is developed,which integrates the processing and imaging of all acquired data.Finally,scanning imaging operation is performed in the home-made AFM system,and some effective imaging results can be obtained.The following innovation work have been completed:A simple,compact,miniaturized and high integration scanning imaging system is designed.It mainly consists of an optical measuring system,a probe system and a scanner.To improve the optical measurement accuracy,theoretical analysis of the sensitivities of the optical beam deflection(OBD)sensor for the cantilever’s deflection and translation.An optical system design method that aims to reduce the measuring crosstalk error is proposed.Compare the lateral and parallel scanning methods,it’s found that the lateral force crosstalk in the z direction is more obvious in the parallel scanning method;the corresponding dynamic characteristics simulation also verified that the cantilever probe has poor scanning stability due to lateral forces under the parallel scanning.The above comparison shows that the lateral scanning method is more suitable for high-speed scanning.As a very important component of AFM system,scanner has a significant impact on the imaging and operation.To overcome the shortcomings of traditional scanners,a parallel kinematic piezoelectric actuator(PZT)xyz scanner is proposed.Aimed at high bandwidth and low coupling error design goals,parallel leaf flexure is used as a guide mechanism,detailed stiffness and parametric design are performed.Both the model validation and prototype testing confirmed the high resonance frequency and the effectively decoupling motion,theexperimental results indicated an effectively workspace about 8.66 × 8.4 × 10.6 μm3 with the maximum coupling of 1.5% between the three working axes,and the resonant frequency of x-,y-and z-axes is 8.8,9.1 and 33.4 k Hz,respectively.Most AFMs use digital signal processor(DSP)as a feedback controller,in such digital implementation,the signal needs to be sampled and afterwards quantized by analog to digital converter(ADC)before it is sent to the DSP processor,which will inevitably affect the process speed of control system.To break this limitation and achieve fast imaging,an all-analog controller was used in the home-made AFM system.Control is performed directly for analog signals and allows for a much higher control bandwidth than an ordinary commercial control system.Additionally,the inherent hysteretic and creep nonlinear characteristics seriously restrict piezoelectric actuator’s positioning accuracy,which has become a limitation of its applications.Therefore,based on the inverse Bouc-Wen model,a feedforward/feedback hybrid control strategy is proposed,and successively implemented on the analog controller,which can effectively improve the tracking performance of the xyz scanner.Finally,the excellent and stable tracking capability of the xyz scanner is demonstrated.Using the proposed calibration method,the laser sensitivity coefficient and piezoelectric ceramic expansion factor can be successively calibrated,respectively,to ensure the imaging accuracy of the home-made AFM system.Additionally,the force-distance curve can be described base on contact scanning model.Finally,in order to operate the home-made AFM system quickly and concisely,Lab VIEW software is used to design the operating interface,which integrates the processing and imaging of all acquired data.It can achieve accurately obtain the topology information of the sample surface in real-time.A simple,compact,miniaturized and high integration imaging system is developed,a series of scanning operation experiments were implemented.The z noise was tested and determined firstly.And comparison of two scanning methods,the experimental results demonstrate that the lateral scanning method is more suitable for high-speed imaging.Based on this method,and some optimistic results can be obtained by scanning of standard gratings with the scanning speed up to 250 Hertz,and confirmed full range of imaging capabilities of the proposed AFM system.Finally,application of the high-speed imaging system to scan polymer nanofibers,the imaging rate up to 10 frame/s.