An SEM image is acquired. The SEM image shows a metal line and a via hole disposed above the metal line. The via hole exposes a portion of the metal line vertically aligned with the via hole. A first portion and a second portion of the via hole are each vertically not aligned with the metal line and are disposed on opposite sides of the metal line. The acquired SEM image is processed to enhance a contrast between the first and second portions and their surrounding areas. A first dimension of the first portion and a second dimension of the second portion of the via hole are measured in a first direction. The first direction is different from a second direction along which the metal line extends. An overlay between the via hole and the metal line is determined based on the first dimension and the second dimension.