1. A mask comprising: a substrate; a first attenuating layer disposed on the substrate, having a first material and a first thickness; a second attenuating layer having a second material and disposed on the first attenuating layer; and a third attenuating layer interposed between the first attenuating layer and the substrate, wherein the first, second, and third attenuating layers define: a first feature having a first opening extending through the first, second, and third attenuating layers, wherein the first feature is a main feature; and a second feature having a second opening extending through the second attenuating layer and exposing the first attenuating layer, wherein the second feature is a first assistant feature proximate to the main feature and has a first phase shift relative to the main feature, the first phase shift being in a range such that the main feature and the second feature are out of phase; and a third feature having a third opening extending through the second and first attenuating layers and exposing the third attenuating layer, wherein the third feature is a second assistant feature proximate to the main feature and has a second phase shift relative to the main feature, the second phase shift being different from the first phase shift.