Optimal strategy for controlling linewidth on spherical focal surface arrays

We present a method for patterning concave spherical substrates based on io n beam proximity lithography. The approach overcomes both the distortion an d radial dose variation that are inherent to projecting a flat mask pattern onto a sphere. We use a self-complementary mask (SCM) concept where an arr ay of discrete ion beamlets is scanned across the substrate to expose the c ircuit pattern. This scanning is implemented by varying the inclination of the incident ion beam with respect to the mask. Pincushion distortion is co rrected by applying a global transformation to the centers of the SCM openi ngs while a local transformation, applied within each opening, ensures that the beamlets stitch together properly on the substrate and that the ion do se is uniform. We show that the linewidth variation for 1 mum features is l ess than +/-80 nm over a 7.2 mm deep, spherical bowl with a 1.5 cm radius o f curvature. (C) 2000 American Vacuum Society. [S0734-211X(00)09606-2].