Research On Analysis And Correction Of Cross-Talk Effect In AFM Measurement With Lateral Scan

With the rapid development of semiconductor integrated circuit (IC) manufacturing, the line width of chip has been shrinked into sub-100nm as the critical dimension of process technologies. The improvement in the accuracy of semiconductor manufacturing requires more accuracy for the measuring equipment. Atomic force microscopes (AFM), which can obtain three dimensional images of sample surface with nanometer level resolution, are increasingly being used as tools for nanometer dimensional measurement. The research on the scan ways and measurement error of AFM has theoretical significance and use value in the field of measurement of semiconductor lines by AFMs. The constant force mode is one of the AFM main operation modes, which has the advantage of high resolution and rapid measurement speed. There are two scan ways in this mode including parallel scan and lateral scan. However, the measurement results under both scan ways are seriously affected by cross-talk effect caused by tip-sample angle, friction, rotation of PSPD and deviation between tip axis and cantilever axis, leading to large measurement error. Based on the analysis of the affecting factors above, a new scan method of AFM is proposed in this paper for eliminating the factors.Based on the summary about the influence factors on AFM measurement results in the constant force mode, the influence of tip-sample angle and friction on the measurement results under parallel scan and lateral scan, including the influence of the measured topography and tip-sample force, are analyzed, respectively. The analysis indicates that the tip-sample angle will cause a blind scan region in parallel scan, while friction determining the size of the not-for-scan region in lateral scan. It also indicates that lateral scan is more suitable for topography measurement than parallel scan because there is no error obtained under lateral scan theoretically, however, the large force between tip and sample may damage the sample surface.Based on the study of the generation mechanism of cross-talk effect in AFM measurement, including geometrical cross-talk, mechanical cross-talk and tip cross-talk, the influence of the effects on the measurement results are analyzed, respectively. The analysis indicates that comparing to the other two cross-talk effects, the geometrical cross-talk has the largest influence on the measurement results. The analysis indicates that the torsion of the cantilever directly causes the light path cross-talk, therefore, the tip-sample angle and friction is the basic reason leading to this kind of cross-talk effect. The simulation results show that the cross- talk effects will also cause large measurement error under lateral scan.For eliminating the affecting factors mentioned above, a lateral scan method with constant force is proposed. In this method, the tip-sample force is controlled in a small value in order not to damage the sample surface, and the movement of the scanner and the placement at the end of cantilever are then both used to calculate the height signal at each scanning point. Cross-talk effects are corrected in this method. Firstly, the rotating angle of PSPD can be acquired to eliminate the geometrical cross-talk through an experimental method. Secondly, the real torsion and bending can be calculated from output of PSPD through an arithmetic approach, and light path cross-talk can be then corrected. Finally, the ratio of torsion and bending under vertical force can be obtained to compensate the mechanical cross-talk. Considering that lateral scan is ultilized in this method, the influence of tip-sample angle and friction can also be eliminated by using this method for measuring. For verifying the feasibility of this method, experiment is operated by scanning a standard silicon sample with trapezium form. By comparing with the under parallel scan and lateral scan, the experimental results demonstrate that the new scan method can eliminate the affecting factors effectively with accurate measuring results.At the end of this paper, the influence of convolution effect caused by the conic angle of tip, as well as influence the stiffness of the cantilever on the measurement results under the new scan method, is demonstrated by the other two experiments. Corresponding solution is proposed to restrain the affection of the two factors.