Methods and apparatus for testing lithographic masks using electron beam scanning

Claim 1

A method for testing a mask to be used in a lithographic process wherein radiation is directed onto said mask, said mask including a patterned layer of material on a substrate in which photoconductivity and photoemissivity cannot be easily induced, including the steps of: coating said patterned layer with a thin layer of a low work function material in which photoemission can readily be induced, said coating being substantially transparent to an electron beam and to said radiation, scanning said patterned layer with an electron beam to test for defects in said patterned layer, said electron beam causing a buildup of electrons on the surface of said coated mask, and directing ultraviolet light onto said coated mask in an area overlapping the area of said mask being hit by said electron beam to cause photoemission of electrons from said thin coating layer in an amount to produce a positive charge thereon sufficient to balance the negative charge induced by the incidence of electrons from said electron beam.

Claim 2

A method for testing a mask to be used in a lithographic process wherein radiation is directed onto said mask, said mask including a patterned layer of material on a substrate in which photoconductivity and photoemissivity cannot be easily induced, including the steps of: coating said patterned layer with a thin layer of a low work function material in which photoemission can readily be induced, said coating being substantially transparent to an electron beam and to said radiation, scanning said patterned layer with an electron beam to test for defects in said patterned layer, said electron beam causing a buildup of electrons on the surface of said coated mask, and directing ultraviolet light onto said coated mask in an area overlapping the area of said mask being hit by said electron beam to cause photoemission of electrons from said thin coating layer in an amount to produce a positive charge thereon sufficient to balance the negative charge induced by the incidence of electrons from said electron beam.

Claim 3

A method for testing lithographic masks to be used in lithographic processes wherein radiation is directed onto said mask, including the steps of: placing said masks in a vacuum environment in which a scanning electron beam will be directed to said mask for detection of said faults therein, said mask including a substrate having a patterned layer thereon, said patterned layer including first regions which are transparent to said radiation and second regions which are opaque to said radiation, said substrate being comprised of a material in which photoconductivity and photoemission are only very weakly induced by ultraviolet excitation having an energy spectrum less than about 6eV, coating said mask with a thin layer of a material having a low work function in which photoemission can readily be induced, said coating being substantially transparent to said radiation, testing said mask by impinging a high resolution scanning electron beam thereon over an area termed a scan field, said testing producing a net negative charge due to the electrons incident on said mask, producing a net positive charge of an amount sufficient to balance the said negative charge in order to discharge said mask, said net positive charge being produced by the application of ultraviolet energy having an energy spectrum of energies less than 6eV, where said ultra- violet radiation is applied to said thin coating layer over an area which overlaps said scan field of said electron beam.

Claim 4

A method for testing lithographic masks to be used in lithographic processes wherein radiation is directed onto said mask, including the steps of: placing said masks in a vacuum environment in which a scanning electron beam will be directed to said mask for detection of said faults therein, said mask including a substrate having a patterned layer thereon, said patterned layer including first regions which are transparent to said radiation and second regions which are opaque to said radiation, said substrate being comprised of a material in which photoconductivity and photoemission are only very weakly induced by ultraviolet excitation having an energy spectrum less than about 6eV, coating said mask with a thin layer of a material having a low work function in which photoemission can readily be induced, said coating being substantially transparent to said radiation, testing said mask by impinging a high resolution scanning electron beam thereon over an area termed a scan field, said testing producing a net negative charge due to the electrons incident on said mask, producing a net positive charge of an amount sufficient to balance the said negative charge in order to discharge said mask, said net positive charge being produced by the application of ultraviolet energy having an energy spectrum of energies less than 6eV, where said ultra- violet radiation is applied to said thin coating layer over an area which overlaps said scan field of said electron beam.

Claim 7

A method as claimed in any one of claims 1 to 4 in which said mask is an optical or x-ray mask.

Claim 8

A method as claimed in any one of claims 1 to 5, in which the thickness of said thin coating layer is less than approximately 1000A.

Claim 9

A method as claimed in any one of claims 1 to 6, in which said thin coating layer has a work function less than approximately 3eV.

Claim 11

A method as claimed in any one of claims 1 to 8, in which said ultra-violet radiation includes a spectrum of energies less than approximately 6eV.

Claim 12

A method as claimed in any one of claims 1 to 9, in which said thin coating layer comprises CsTe, CsI, or one of the alkali metal halides.

Claim 16

An apparatus for testing lithography masks, said masks being used in lithographic processes wherein radiation is directed onto said masks for exposure of layers behind said masks, comprising in combination: a mask comprising a patterned layer located on a substrate, said patterned layer including first regions which are substantially transparent to said radiation and second regions which are substantially opaque to said radiation, said substrate being comprised of a material which does not strongly support induced photoconductivity and photoemission at excitations less than about 6eV, a thin coating layer formed on said patterned layer comprised of a material which exhibits a strong photoemissive effect at an excitation less than about 6eV, said coating layer being sufficiently thin that it is transparent to said radiation, a vacuum chamber into which said mask is placed for testing, an electron beam source for directing a high energy scanning beam of electrons into said vacuum chamber and onto said coated mask for testing said mask, said electron beam striking said mask over an area defined as a scan field, the intensity of said electron beam being sufficient that negative charge will occur on said coated mask of an amount sufficient to alter the trajectory of said electron beam to thereby cause errors in the testing of said masks, means for shining ultraviolet light of an energy less than about 6eV on said mask in an area overlapping said scanned field for producing photoemission from said thin coating layer of an amount to produce a net positive charge which balances the net negative charge produced by said electron beam to thereby sufficiently discharge the surface of said coated mask so that the trajectory of the scanning electron beam is not adversely affected.

Claim 17

An apparatus for testing lithography masks, said masks being used in lithographic processes wherein radiation is directed onto said masks for exposure of layers behind said masks, comprising in combination: a mask comprising a patterned layer located on a substrate, said patterned layer including first regions which are substantially transparent to said radiation and second regions which are substantially opaque to said radiation, said substrate being comprised of a material which does not strongly support induced photoconductivity and photoemission at excitations less than about 6eV, a thin coating layer formed on said patterned layer comprised of a material which exhibits a strong photoemissive effect at an excitation less than about 6eV, said coating layer being sufficiently thin that it is transparent to said radiation, a vacuum chamber into which said mask is placed for testing, an electron beam source for directing a high energy scanning beam of electrons into said vacuum chamber and onto said coated mask for testing said mask, said electron beam striking said mask over an area defined as a scan field, the intensity of said electron beam being sufficient that negative charge will occur on said coated mask of an amount sufficient to alter the trajectory of said electron beam to thereby cause errors in the testing of said masks, means for shining ultraviolet light of an energy less than about 6eV on said mask in an area overlapping said scanned field for producing photoemission from said thin coating layer of an amount to produce a net positive charge which balances the net negative charge produced by said electron beam to thereby sufficiently discharge the surface of said coated mask so that the trajectory of the scanning electron beam is not adversely affected.