A dark field diffraction based overlay metrology device illuminates an overlay target that has at least three pads for an axis, the three pads having different programmed offsets. The overlay target may be illuminated using two obliquely incident beams of light from opposite azimuth angles or using normally incident light. Two dark field images of the overlay target are collected using ±1st diffraction orders to produce at least six independent signals. For example, the +1st diffraction order may be collected from one obliquely incident beam of light and the −1st diffraction order may be collected from the other obliquely incident beam of light. Alternatively, the ±1st diffraction orders may be separately detected from the normally incident light to produce the two dark field images of the overlay target. The six independent signals from the overlay target are used to determine an overlay error for the sample along the axis.