Holographic nanometer alignment system for a half-micron wafer stepper

An excellent reticle to wafer overlay accuracy (0.064µm/3σ) was achieved using an holographic nanometer alignment system for a wafer stepper. This alignment system performs as an ideal interferometer using two conjugate beams diffracted from a wafer alignment grating. The remarkable characteristic of the alignment system is the high Signal/Noise ratio which is independent of the surface roughness such as caused by Al deposition. As a result, overlay accuracy was maintained within 0.07µm/3σ on various substrate conditions. Alignment offset caused by the alignment mark deformation was proportional to the asymmetricity of the alignment mark and that is consistent with the calculated results.