Patent ID: 6018392
Filing Date: 2000-01-25
Classification: G01N,G03F

Abstract:
A method of inspecting masks having phase shifting elements, comprising the steps of:providing a mask having phase shifting mask elements formed on a transparent mask substrate at a number of die positions;providing a light source;providing a first objective lens and a second objective lens;holding said mask in a mask holder;providing a phase adjustment unit, wherein the phase shift of light passing through said phase adjustment unit can be varied;adjusting the phase of the light exiting said phase adjustment unit to provide a 180.degree. phase shift;directing light from said light source through said transparent substrate and one of said phase shifting mask elements at a first said die position to said first objective lens;directing light from said first objective lens through said phase adjustment unit;directing light from said light source through said transparent mask substrate and one of said phase shifting mask elements at a second said die position to said second objective lens;combining the light exiting said phase adjustment unit and the light exiting said second objective lens at a first detector;combining the light exiting said phase adjustment unit and the light exiting said second objective lens at a second detector; andobserving the intensity of said combined light at said first detector or the intensity of said combined light at said second detector.