An optical shadowgraph microscope for a semiconductor wafer bump height measurement

We describe an optical micro shadowgraph for measuring the height of a micro solderball on a semiconductor wafer. The proposed optical system consists of a white-light microscopic inclination illumination and a telecentric objective. The solderball shadow image resulting from a slant illumination onto the protruded bump/solderball on the wafer is clearly captured. In terms of geometrical optics, a formula is derived to resolve the bump height from the length of the resulting shadow on the wafer surface. Experimental results demonstrate that the bump height measurement can be readily implemented.