Using a 120 kV STEM equipped with a backscattered electron detector and operated as a conventional SEM. voids in metal lines can be detected through 1 μm of passivation. By applying current to passivated thin metal lines while in the microscope, voids can be observed while electromigration is in progress. Voids move significant distances during electromigration. On at least some occasions. failure of the line is not the result of a void growing until the width of the line is reached. On these occasions, when the size of the void approaches the width of the line. the void breaks up into smaller voids.