Closed-Loop Autofocus Scheme for Scanning Electron Microscope

In this paper, we present a full scale autofocus approach for scanning electron microscope (SEM). The optimal focus (in-focus) position of the microscope is achieved by maximizing the image sharpness using a vision-based closed-loop control scheme. An iterative optimization algorithm has been designed using the sharpness score derived from image gradient information. The proposed method has been implemented and validated using a tungsten gun SEM at various experimental conditions like varying raster scan speed, magnification at real-time. We demonstrate that the proposed autofocus technique is accurate, robust and fast.