초록 ▼

A method and apparatus for inspecting photomasks having phase shifting elements which shift the phase of light but are otherwise transparent. A coherent light source is directed through a mask to be inspected, through an objective lens, through an 180.degree. phase shifting unit, and to an image div

A method and apparatus for inspecting photomasks having phase shifting elements which shift the phase of light but are otherwise transparent. A coherent light source is directed through a mask to be inspected, through an objective lens, through an 180.degree. phase shifting unit, and to an image divider. The coherent light source is also directed through a transparent reference substrate to the image divider. The mask to be inspected is formed on a transparent mask substrate having the same thickness and formed from the same material as the transparent reference substrate. The intensity of the light exiting the image divider is proportional to the square of the cosine of 1800 plus the phase angle between the light exiting the reference substrate and the light exiting the mask under test. The light exiting the image divider is directed to a CCD image sensor. An image computer compares the output of the CCD image sensor with an image formed from the image database and identifies defects in the mask under test.

대표청구항 ▼

[ What is claimed is:] [1.] 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;direc

[ What is claimed is:] [1.] 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.