Lateral metrology using scanning probe microscopes, 2D pitch standards and image processing

To use SPM as a metrological tool it is necessary to have traceable reference standards and image processing tools that can calculate correction parameters and produce accurate measurements. We describe new reliable methods for accurate characterization of SPM by imaging traceable 2D pitch standards and automated image processing. The scanning processes of the microscopes are characterized by correction parameters for lateral non-linearity, linear correction factors for the x- and y-axes and an x-y coupling factor. To obtain high accuracy we have improved a previously reported algorithm for calculation of the unit cell by subpixel Fourier analysis. We characterize the linearity by a new method where the positions of individual features associated with the unit cell are detected by a correlation technique and compared with predicted positions based on the calculated unit cell. The differences between the actual and predicted locations reflect the non-linearity. We are able to measure the non-linearity at sub-pixel level, i.e. smaller than 0.2% of the scan range for a 512 x 512 pixel image. Evaluation of the linearity has given us a tool that can confirm the unit cell measurements based on Fourier analysis to within 0.3% of the repeat distances. When analyzing a series of images acquired over a time frame of 6 months, we find a reproducibility of better than 0.7% for the calibration parameters. We demonstrate that we are able to measure angular distortions smaller than 0.2 degrees and that corrections of small residual non-linearities have a positive influence on the accuracy of the pitch measurements. The developed software is made freely available.