Metrology of lithographic features: balanced illumination eliminating diffractive bias

Optical microscope metrology of shallow relief patterns is shown to be capable of greater accuracy, to smaller dimensions, when phase features are imaged rather than intensity ones. This comes from the inherent symmetry in diffraction from pure phase discontinuities. Thus when intensity variations are suppressed, as by appropriate illumination or filtering, the diffractive bias can be eliminated. The approach is analyzed and demonstrated. When it can be used, greater accuracy is available than with traditional approaches. In ideal conditions the approach has the potential for absolute accuracy within 0.1lambda, down to dimensions of