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 patt ern of holes in the mask be controlled. We have used the finite elemen t method provided by the ANSYS software package to calculate the two d imensional 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 p roduced by various simple geometries. For example, consider an asymmet ric 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 dia meter, between the two halves. The boundary between the two halves (al ong the diameter) is bowed by 182 nm. A method for reducing the distor tion is to cut a pair perforation rings along the perimeter. The perfo ration rings will have the effect that the central part of the membran e will have the stress relieved and it will be held in effect by ''spr ings'' at a constant, low stress level. We have calculated that the bo w of the diameter can be reduced to 18 nm by a suitable geometry of th e perforations. The perforation ring is an effective means of reducing distortion to well below an acceptable level. The uniform shrinkage o f the pattern due to stress relief is simply a small change in magnifi cation which is automatically corrected by the beam lock in the ion op tical column.