Analysis of stencil mask distortion in ion projection lithography

Accurate feature placement on the wafer in ion projection lithography requires that the distortion due to stress relief produced by the pattern of holes in the mask be controlled. We have used the finite element method provided by the ANSYS software package to calculate the two dimensional mask distortion due to non-symmetrical patterns of openings in stencil masks and to analyze a method for controlling distortion. The ion projection lithography mask is a circular silicon membrane 120 mm in diameter 2.5 mu m thick rigidly held around the perimeter. The central 60 x 60 mm square area will contain the pattern to be printed, demagnified by 3x. We take the intrinsic stress of the membrane to be 10 MPa, the Young's modulus of the membrane to be E = 1.5 x 10(11) Pa and the Poisson ratio v = 0.17. We have explored the x-y distortion produced by various simple geometries. For example, consider an asymmetric grid of square holes which is 22% open on one half and 10% open on the other half. High distortion in this case will occur along the diameter, between the two halves. The boundary between the two halves (along the diameter) is bowed by 182 nm. A method for reducing the distortion is to cut a pair perforation rings along the perimeter. The perforation rings will have the effect that the central part of the membrane will have the stress relieved and it will be held in effect by ''springs'' at a constant, low stress level. We have calculated that the bow of the diameter can be reduced to 18 nm by a suitable geometry of the perforations. The perforation ring is an effective means of reducing distortion to well below an acceptable level. The uniform shrinkage of the pattern due to stress relief is simply a small change in magnification which is automatically corrected by the beam lock in the ion optical column.