A method includes exposing a first photoresist layer through a refractive mask to form a first pattern of above-threshold exposure spots in the first layer and exposing a second photoresist layer through the same mask to form a second pattern of above-threshold exposure spots in the second layer. Coordination numbers of exposure spots are larger in the first pattern than in the second pattern, nearest-neighbor pairs of the exposure spots have larger spacings in the first pattern than in the second pattern or largest ones of the exposure spots have larger diameters in the first pattern than in the second pattern.