Phase-shifting lithographic masks with improved resolution

Abstract

In order to print an isolated feature having a width W in a photoresist layer using an optical radiation imaging system whose lateral magnification is equal to m, a phase-shifting mask is used having a corresponding isolated feature. In one embodiment this corresponding isolated feature has a central square portion having a refractive index n.sub.1 and a thickness t.sub.1, and a peripheral (rim) square-ring phase-shifting portion having a width B, as well as a refractive index n.sub.2 and a thickness t.sub.2, such that the phase-shift .phi.=2.pi.(n.sub.2 t.sub.2 -n.sub.1 t.sub.1)/.lambda. of the central portion relative to the peripheral portion is equal to .pi. radian or odd multiple integral thereof. The width C of the central portion advantageously is selected such that mC/W is equal to at least 1.2, and preferably greater than 1.5, rather than unity. In another embodiment, the isolated feature has an opaque central portion, also of width C, and a peripheral square-ring phase-shifting portion having a width B and having a phase-shift of .pi. radian relative to a surrounding transparent wide area ('bright field')--in which case, advantageously the value of m(C+2B)/W is equal to less than 1/(1.2), and preferably less than 1/(1.5).