Digital beam control for fast differential voltage contrast

A new method for voltage contrast enhancement by image subtraction is presented. It is based on a modification of a commercial column automation system that digitizes beam positions and intensity signals in a scanning electron microscope (SEM). The new modulus performs via hardware all the operations of bias control and signal differentiation at each image point, resulting in a minimum acquisition time of 4 μs per point, that is less than 0.3 seconds to obtain a 256 × 256 pixels image with 8 bit gray resolution. The digital handling of intensity data, gives high precision differential images and the control of beam position prevents spatial differentiation effects, enabling noise reduction by time integration of intensity data in each point.