Development of Three-Dimensional Atomic Force Microscope for Sidewall Structures Imaging with Controllable Scanning Density

In this paper, a three-dimensional atomic force microscope (3-D AFM) for imaging of the micro- and nanoscale sidewall structures is presented. The system mainly consists of two individual scanners, each with two degrees of freedom to drive the probe on YZ plane for the feedback control and the sample on XY plane for the raster scan. A slender optical fiber probe is used and can be laterally tilted with a rotary holder to access steep sidewalls with overhang or undercut features without any modification of the optical lever. Unlike the conventional AFM with the single-axis feedback control, the 3-D AFM is capable of tracking 3-D profiles of the sidewall structures with nonorthogonal scan due to the dual-axis feedback control with a vector probing scanning scheme. The motion vector angle of the probe is controllable, and varied according to the slope angle of the sample surface for accurate scan of the sidewall with a uniform image density. The 3-D AFM demonstrates great potential not only for characterization of 3-D surface topography, but also for critical dimension (CD) measurements of the sidewall structures. Three- dimensional imaging and CD measurement results of an AFM calibration grating, and a microcomb structure validate the capability and flexibility of the developed system.