Low Drift Scanning Method For SPM Based On Correlation-Steered Strategy

Scanning probe microscopes (SPM) form images of sample surfaces in the atom-ic scale and achieve nano-scale measurement, which have been widely used in many fundamental fields such as precision machinery, material science, nanotechnology and bio-chemical technology. However, SPM images are often found to be distorted by thermal drift, severely affect measurement based on image. Currently, the drift cor-rection methods had been confined in tracking a certain area or image correction after images are acquired, which makes it difficult for SPMs to obtain low drift image. So in this dissertation, a correlation-steered scanning method was proposed for SPMs to calculate and compensate the three-dimensional drift in real time.This dissertation was based on the proposed correlation-steered scanning method, studied its working principle theoretically. Simulation and analysis was performed to verify this method using MATLAB software. Hybrid programming method using Lab-VIEW and MATLAB was employed to implement this method in the experimental plat-form based on a commercial AFM, and a final image obtained using this method was compared and analyzed. The main research contents and innovations of this dissertation are as follows:(1) This dissertation studied the proposed correlation-steered scanning method the-oretically. Sample image is scanned band by band with overlapping scanning area and reversed scanning direction between adjacent bands. For lateral drift, each band is artifi-cially divided into several blocks, digital image correlation technique is applied between block image and the corresponding overlapping area of previous band, the drift is calcu-lated and compensated to steer the scanning of the subsequent blocks. When each band is scanned completely, the blocks’drifts of this band are then used to recover the effect of lateral drift. The vertical drift can be considered as linear and can be eliminated by subtracting fitted background reference plane image.(2) Simulation analysis was performed using MATLAB to verify the proposed correlation-steered scanning method in overcoming the lateral drifts. A series of offset which change with the scanning time is added into the original coordinates to simulate the drift effect during the scanning process when acquiring the coordinates of pixel-s. The simulation results showed that the correlation calculation can derive the exact drifts which are set artificially in the scanning process, the difference between the cal-culated and preset value is less than0.5pixel. The final image after the drift corrected was quite the same as the original image, which proves the feasibility of the correlation-steered scanning method. The simulation also analyzed the impact of overlapping ratio and drift speed on the final recovered image, larger overlapping ratio is needed when the drift speed is faster, however, this will reduce the scanning area.(3) Experimental platform based on a commercial AFM was built and a hybrid pro-gramming method using Lab VIEW and MATLAB was employed to control the AFM. Lab VIEW program together with the data acquisition card PCI-6024E are linked to con-trol the two-dimensional planar movement with two output analog channels and acquire the height value in the vertical direction with one input channel. The MATLAB code which is built in the Lab VIEW program is responsible for assembling the height data into final image. The program method realized the basic control of the AFM, and laid the foundation to fulfill the proposed correlation-steered scanning method to be applied in the equipment.(4) This essay studied the hysteresis characterization of piezoelectric actuator un-der voltage drive and missed scanning brought by the drift compensation. A sinusoidal model was built for the piezoelectric tube and image process was applied to eliminate the large part of hysteresis. The small residual hysteresis was derived together with drift during scanning by correlation calculation. Virtual adjustment strategy was proposed to address the missed scanning problem, a band with larger area was scanned directly without drift calculation and compensation. The data of each block is picked up in the scanned band image according to the drift of previous block, and then the calculated drift is compensated to steer next block’s data. The lateral drift is accumulated in each band and is compensated when the scan of band is finished. This strategy can prevent the image of each band from the creep effect, and can also guarantee all the area is s-canned in each band, and thus to get an intact and distortion-free image after the drifts are corrected.(5) Under the control of Lab VIEW and MATLAB, image was obtained using the proposed correlation-steered scanning method in the AFM.15bands of a homemade Blu-Ray disk sample were scanned with160x1000pixels in each band, and an image with720x1000pixels was combined. A polynomial together with a first-order sinu-soidal function were applied to represent and fit the trends of total drifts and residual hysteresis separately so as to get the drifts of the first band, then finally, an image with the drifts of the first band corrected was obtained. Compared with the image scanned using the traditional raster scanning method, the final image manifested little effect from thermal drift and hysteresis, the measurement error based on the image was proved to be less than1.5%.Based on the above researches, the proposed and realized correlation-steered scan-ning method is proved to be a pretty good scanning method for SPMs to overcome the three-dimensional drift. Due to the real-time drift calculation and compensation, an im- age with large area in the nano-scale can be obtained. This method has both theoretical and practical value for extending the application of SPMs.