Apparatus and method of inspecting phase shift masks using comparison of a mask die image to the mask image database
First Claim
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°
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; and
comparing said output of said image acquisition unit and said image database of said mask, thereby identifying defects in said mask.
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Accused Products
Abstract
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° 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.
48 Citations
16 Claims
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1. A method of inspecting masks having phase shifting elements, comprising:
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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°
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; and comparing said output of said image acquisition unit and said image database of said mask, thereby identifying defects in said mask. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9)
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10. An apparatus for inspecting masks having phase shifting elements, comprising:
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a coherent light source; a mask holder which can be moved in a predetermined sequence in a single plane; 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, wherein said image divider is placed so that said mask holder is between said coherent light source and said image divider; an objective lens placed between said image divider and said mask holder; a transparent reference substrate placed between said coherent light source and said second input of said image divider so that light passing through said transparent reference substrate enters said second input of said image divider without passing through said mask holder or said objective lens; a phase adjustment unit placed between said objective lens and said first input of said image divider, wherein the light exiting said objective lens enters said first input of said image divider, after passing through said phase adjustment unit, with a 180°
phase shift relative to the light entering said second input of said image divider;an image acquisition unit, having an input and an output, positioned so that the output of said image divider enters said input of said image acquisition unit; and means for comparing said output of said image acquisition unit to the image database of a mask placed in said mask holder, thereby identifying defects in the mask placed in said mask holder. - View Dependent Claims (11, 12, 13, 14, 15, 16)
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Specification