Patent ID: 6134014
Filing Date: 2000-10-17
Classification: G03F

Abstract:
A method of inspecting masks having phase shifting elements, comprising:providing a coherent light source;providing an objective lens;providing a mask holder which can be moved in a predetermined sequence in a single plane;providing a transparent reference substrate;directing light from said coherent light source through said transparent reference substrate;placing a mask having phase shifting mask elements formed on a transparent mask substrate, wherein said mask comprises a number of mask segments and said transparent mask substrate is formed of the same material and has the same thickness as said transparent reference substrate, in said mask holder;providing an image database for said mask;moving said mask holder so that light from said coherent light source is directed through each of said mask segments to said objective lens in a predetermined sequence;directing light from said objective lens through a phase adjustment unit, wherein the phase of the light exiting said phase adjustment unit is shifted in phase by 180.degree. relative to the phase of the light entering said phase adjustment unit;providing an image divider having a first input, a second input, and an output wherein one half of the light entering said first input of said image divider and one half of the light entering said second input of said image divider exit said output of said image divider;directing the light exiting said phase adjustment unit to said first input of said image divider;directing the light exiting said transparent reference substrate to said second input of said image divider;providing an image acquisition unit having an input and an output;directing the light exiting said output of said image divider to said input of said image acquisition unit; andcomparing said output of said image acquisition unit and said image database of said mask, thereby identifying defects in said mask.