Patent ID: 8030632

Claim:
A method of performing optical metrology with multiple beams directed to a structure in a workpiece, the method comprising: generating a first illumination beam having a first plurality of wavelengths using a first light source; generating a second illumination beam having a second plurality of wavelengths using a second light source; projecting the first and second illumination beams onto an illumination secondary mirror; reflecting the first and second illumination beams onto an illumination primary mirror, the reflected first and second illumination beams projected onto a structure on the workpiece at a first angle of incidence and a second angle of incidence respectively, the reflected first and second illumination beams generating a first and second detection beams respectively; projecting the first and second detection beams onto a detection primary mirror, generating an output first and second detection beams; and reflecting the output first and second detection beams onto a detection secondary mirror; wherein an optimum first angle of incidence is derived based on the first light source and a calculated numerical aperture of the first illumination beam and an optimum second angle of incidence is derived based on the second light source and a calculated numerical aperture of the second illumination beam; wherein the illumination primary mirror and illumination secondary mirror are positioned so as to make the first angle of incidence substantially equal or close to the optimum first angle of incidence and the second angle of incidence substantially equal or close to the optimum second angle of incidence; wherein the detection secondary mirror is positioned relative to the detection primary mirror so as to reflect the first detection beam through a first detection aperture and the second detection beam through a second detection aperture; and wherein the illumination secondary mirror is a single piece mirror and the detection secondary mirror is a single piece mirror, the illumination secondary mirror located proximate to the detection secondary mirror.