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Advances in stencil mask technology: control of distortion in ion-projection lithography masks

We study the problems of (1) pattern-dependent, (2) thermal, and (3) bonding distortion in stencil masks for ion projection lithography. (1) Pattern-dependent distortion was found to consist mainly of a correctable magnification error in representative circularly symmetric masks whose amplitude is determined by the size and average density of the pattern. After magnification correction, distortion decreases rapidly with increasing modulation frequency, falling to values below 5 nm for spatial frequencies greater than 0.23 cycles/mm. (2) We show that the use of a cold cylinder to enhance radiation cooling can limit temperatures to within 1.5 K of 300 K over the surface of 120 mm membrane. This minimizes pattern-dependent, conductive heat flow and reduces distortion to below 10 nm. (3) An aluminum metallic bonding technology is described for attaching silicon support rings to membranes with a radial bonding stress below 1 MPa. We conclude that the most serious potential distortion problems for silicon stencil masks should have practical solutions.