We present a method for patterning concave spherical substrates based on ion beam proximity lithography. The approach overcomes both the distortion and 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 array of discrete ion beamlets is scanned across the substrate to expose the circuit pattern. This scanning is implemented by varying the inclination of the incident ion beam with respect to the mask. Pincushion distortion is corrected by applying a global transformation to the centers of the SCM openings while a local transformation, applied within each opening, ensures that the beamlets stitch together properly on the substrate and that the ion dose is uniform. We show that the linewidth variation for 1 mum features is less than +/-80 nm over a 7.2 mm deep, spherical bowl with a 1.5 cm radius of curvature.