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Stability of alignment marks for x‐ray masks

IBM’s x‐ray lithography program uses optical alignment to position the mask relative to the wafer. Optical alignment places the constraint of high optical transparency for the B‐doped Si and B–N membranes presently used in the program. This problem may be solved by using alignment marks located outside the x‐ray exposure area, with the substrate’s transparency in the alignment region increased by (a) thinning of the B–Si, (b) eliminating the B–Si and supporting the alignment marks on polyimide, or (c) removing the supporting background completely. In any of these solutions the stability of the alignment marks becomes a separate issue from that of the stability of the mask itself. Processes developed to fabricate alignment marks by all three schemes are described, and measurements performed to ascertain the stability of the marks are reported.