1. A mask comprising: a substrate; a first attenuating layer disposed on the substrate, having a first material and a first thickness corresponding to a phase shift; and a second attenuating layer having a second material and disposed on the first attenuating layer, wherein the first and second attenuating layers define: a first feature having a first opening extending through the first and second attenuating layers; and a second feature having a second opening extending through the second attenuating layer and exposing the first attenuating layer, wherein one of the first and second features is a main feature and the other one is an assistant feature proximate to the main feature.