Electron beam system with reduced charge buildup

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 6 eV,
• 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 6 eV, where said ultraviolet radiation is applied to said thin coating layer over an area which overlaps said scan field of said electron beam.

An apparatus for testing for lithography masks, said mask being used in lithographic processes wherein radiation is directed onto said masks for exposure of layers behind said mask, 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 6 eV,
• 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 6 eV, 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 mask,
• means for shining ultraviolet light of an energy less than about 6 eV 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.