Omnidirectional 3-D AFM Integrated With a Rotary Stage for High-Precision Sidewall Structure

Atomic force microscopy (AFM) is widely used to examine the 3-D structure in a few nanometer resolutions for numerous fields. However, the scan performance of 3-D profiles for sidewall structures may be significantly influenced by inherent AFM probe shape. To address the above issue, a novel 3-D AFM system integrated with a rotary sample stage has been introduced, providing the sample with the freedom to rotate and enabling omnidirectional scanning. First, based on optical imaging, an optimized calibration method is proposed to determine the center of the rotary stage, which is then aligned with the probe tip. Next, a geometrical assessment of the admissible sidewall orientations (ASOs) for the Akiyama probe is also presented. After the initial AFM scan, the orientations of the sample sidewalls with respect to the scan coordinate system can be derived using the proposed local sidewall orientation analysis (LSOA). Generally speaking, the sidewall scan has better quality if its orientation is transverse to the tip’s scan direction and if the sidewall is on the “rising” edge (rather than the “falling” edge), rendering that a single AFM scan is unlikely to secure good quality of all sidewalls with versatile orientations. Therefore, based on the results of ASO and LSOA, we propose a sample rotation planning algorithm (SRPA) to determine the minimal number of sample rotations via the rotary stage and their corresponding rotation angles to seek high scan quality of all sidewalls provided that multiple AFM scans proceed subsequently after each sample rotation. Next, an effective fusion protocol based on the iterative-closest-point (ICP) method is also presented to accurately merge the 3-D point clouds of the scan topography from the multiple AFM scans with respect to different sample rotations. To verify the effectiveness of the proposed omnidirectional 3-D AFM, extensive experiments with both square island samples and cylinder samples are conducted to demonstrate the high-precision sidewall scanning, regardless of the limitation of the AFM scan due to the adverse probe geometry. To this end, various quantitative metrics are employed to evaluate the scanning quality of the proposed work, showing the high promise of the research in this article.