Methods and apparatus for inspecting workpieces by spatial filtering subtraction

Claim 3

The method according to claim 1 including the further step of, prior to said displaying step, passing said beam through a second lens to reimage the Fraunhoffer diffraction pattern upon said detecting means.

Claim 4

The method according to claim 1 wherein said workpiece is an integrated circuit photomask.

Claim 5

The method according to claim 1 wherein said workpiece is a thin-film photomask.

Claim 6

The method according to claim 1 wherein said optical grating is an absorption grating.

Claim 7

The method according to claim 1 wherein said optical grating is a phase grating.

Claim 8

The method according to claim 1 wherein said optical grating is asymmetrically positioned laterally, with respect to said optical axis, by one-quarter period.

Claim 9

The method according to claim 1 wherein said optical grating is asymmetrically positioned laterally, with respect to said optical axis, by one-half period.

Claim 10

The method according to claim 1 wherein the element-to-element spacing along each row of the array and the width of each aperture is b, and the aperture-to-aperture spacing is 2b.

Claim 11

The method according to claim 9 wherein said array includes N rows and M columns, where N > or = 1 and M > or = 4, said method comprising the further steps of: respectively aligning said apertures with the first and third columns in the array; comparing, at said detecting means, said first and third columns to detect nonperiodic errors therein; and then laterally displacing said workpiece, relative to the optical axis, to successively compare the second column with the fourth column, the (M-2)th column with the Mth column.

Claim 12

The method according to claim 1 wherein the element-to-element spacing along each row of the array is b, the width of each aperture is q, and the aperture-to-aperture spacing is 2q, where q is an integral multiple of b or sub-multiple of b which is < or = 1/2 b.

Claim 13

The method according to claim 11 wherein said array includes N rows and M columns, and the method includes the further steps of: aligning said apertures with the first and the (1 + J)th columns in the array, where J > or = 1; comparing, at said detection means, said first and said (1 + J)th columns to detect nonperiodic errors therein; and then laterally displacing said workpiece, relative to the optical axis, by (M - J - 1) times the width of one column, to successively compare all remaining columns or fractions of columns in the array one with the other, up to and including said Mth column.

Claim 18

Apparatus for detecting nonperiodic errors in a workpiece containing a matrix-like array of normally identical elements, which comprises: a source of a beam of coherent, collimated radiation, said beam illuminating the workpiece to be inspected; a mask, positioned to intercept said beam after passage through said workpiece, said mask having at least two apertures therein, said apertures being aligned with the columns of the array, the widths of the apertures and the aperture-to-aperture spacing being functions of the element-to-element spacing along the rows of the array; a lens, positioned to intercept the beam after passage through said mask, for forming Fraunhoffer diffraction patterns of portions of said workpiece in the focal plane of the lens; an optical grating, positioned in said focal plane, to form a central image, and at least two side images, of each of the apertures in the mask, the rulings of said grating being parallel to a major axis of the apertures in the mask and asymmetric with respect to the optical axis, and the grating periodicity being a function of the element-to-element spacing along the rows of the array; and means, positioned to intercept the beam after it has been diffracted by said grating, for displaying the central and side images of each of said apertures, the centermost side image of each aperture overlapping and cancelling, except for nonperiodic errors which are not cancelled and, hence, are detected.

Claim 19

Apparatus for detecting nonperiodic errors in a workpiece containing a matrix-like array of normally identical elements, which comprises: a source of a beam of coherent, collimated radiation, said beam illuminating the workpiece to be inspected; a mask, positioned to intercept said beam after passage through said workpiece, said mask having at least two apertures therein, said apertures being aligned with the columns of the array, the widths of the apertures and the aperture-to-aperture spacing being functions of the element-to-element spacing along the rows of the array; a lens, positioned to intercept the beam after passage through said mask, for forming Fraunhoffer diffraction patterns of portions of said workpiece in the focal plane of the lens; an optical grating, positioned in said focal plane, to form a central image, and at least two side images, of each of the apertures in the mask, the rulings of said grating being parallel to a major axis of the apertures in the mask and asymmetric with respect to the optical axis, and the grating periodicity being a function of the element-to-element spacing along the rows of the array; and means, positioned to intercept the beam after it has been diffracted by said grating, for displaying the central and side images of each of said apertures, the centermost side image of each aperture overlapping and cancelling, except for nonperiodic errors which are not cancelled and, hence, are detected.

Claim 20

Apparatus according to claim 15 wherein said displaying means comprises: a transforming lens, positioned to intercept said beam after passage through said diffraction grating, to reconstruct said Fraunhoffer Diffraction patterns; and a screen, positioned to receive said reconstructed patterns, said centermost side images overlapping and cancelling on the surface of said screen.

Claim 21

Apparatus according to claim 15 wherein said optical grating is an absorption grating.

Claim 22

Apparatus according to claim 15 wherein said optical grating is a phase grating.

Claim 23

Apparatus according to claim 15 wherein said optical grating is asymmetrically positioned laterally, with respect to the optical axis, by one-half period.

Claim 24

Apparatus according to claim 15 wherein said optical grating is asymmetrically positioned laterally, with respect to the optical axis, by one-quarter period.

Claim 25

Apparatus according to claim 15 wherein, if the element-to-element spacing along the rows of the array is b, the width of each aperture is b and the aperture-to-aperture spacing in the mask is 2b.

Claim 26

Apparatus according to claim 15 wherein, if the element-to-element spacing along the rows of the array is b, the width of each aperture is q and the aperture-to-aperture spacing in the mask is 2q, where q is an integral multiple of b or sub-multiple of b which is < or = 1/2 b.

Claim 27

Apparatus for detecting nonperiodic errors in a workpiece including a marix-like array of normally identical elements, which comprises: a source of a beam of coherent, convergent radiation, said beam illuminating the workpiece to be inspected, and forming Fraunhoffer diffraction patterns of portions of the workpiece at the focal point of the converging beam; a mask, positioned to intercept the convergent beam after passage through said workpiece, said mask having at least two apertures therein, parallel to the columns of the array, the widths of the apertures and the aperture-to-aperture spacing being a function of the element-to-element spacing along the rows of the array; an optical grating, positioned at said focal point, to form a central image, and at least two side images, of each aperture in the mask, the rulings of said grating being parallel with a major axis of the apertures of the mask and asymmetric with respect to the optical axis; and means, positioned to intercept said beam after it has been diffracted by said grating, for displaying the center and side images of said apertures, the centermost side image of each of said apertures overlapping and cancelling, except for nonperiodic errors which do not cancel and, hence, are detected.

Claim 28

Apparatus for detecting nonperiodic errors in a workpiece including a marix-like array of normally identical elements, which comprises: a source of a beam of coherent, convergent radiation, said beam illuminating the workpiece to be inspected, and forming Fraunhoffer diffraction patterns of portions of the workpiece at the focal point of the converging beam; a mask, positioned to intercept the convergent beam after passage through said workpiece, said mask having at least two apertures therein, parallel to the columns of the array, the widths of the apertures and the aperture-to-aperture spacing being a function of the element-to-element spacing along the rows of the array; an optical grating, positioned at said focal point, to form a central image, and at least two side images, of each aperture in the mask, the rulings of said grating being parallel with a major axis of the apertures of the mask and asymmetric with respect to the optical axis; and means, positioned to intercept said beam after it has been diffracted by said grating, for displaying the center and side images of said apertures, the centermost side image of each of said apertures overlapping and cancelling, except for nonperiodic errors which do not cancel and, hence, are detected.