Apparatus for aligning semiconductor wafer using mixed light with different wavelengths

An alignment system for a semiconductor wafer, comprising:

• a first light source for generating a first laser beam having a first wavelength;
• a second light source for generating a second laser beam having a second wavelength;
• a plurality of beam splitters for combining the first and second laser beams and transmitting the combined laser beam from the first and second light sources;
• a spatial filter for transmitting the combined laser beam from the beam splitters therethrough;
• first condensing means for condensing the combined laser beam from the spatial filter unto a wafer mounted on a wafer stage, the wafer having alignment marks thereon, the combined laser beam impinging on the alignment marks on the wafer at an incident angle of less than 90.degree. and being diffracted by the alignment marks to generate a diffracted laser beam;
• second condensing means for condensing the diffracted laser beam from the alignment marks;
• diffraction means for changing the direction of the diffracted laser beam from the second condensing means;
• a light collecting device for collecting the diffracted laser beam from the diffraction means and converting it into an electrical signal;
• and
• a control unit for controlling the wafer stage to control alignment of the wafer thereon in accordance with the electrical signal from the light collecting unit.