Methods of forming half-tone phase-shift masks include the steps of forming a series of layers on a face of a transparent substrate such as quartz. These layers include a phase-shift layer of MoSiON, a layer of opaque material (e.g., chrome) for blocking light on the phase-shift layer and a photoresist layer on the layer of opaque material. The photoresist layer is then patterned to define a mask having openings therein which expose the layer of opaque material. The layer of opaque material is then patterned using a wet etchant, to expose portions of the phase-shift layer. The patterned photoresist layer is then stripped and a cleaning step is then performed to remove residual defects and marks from the patterned layer of opaque material. The patterned layer of opaque material is then used as a mask during the step of anisotropically dry etching the phase-shift layer using a gas containing CF.sub.4 and O.sub.2, but not CHF.sub.3. The use of a gas containing CF.sub.4 and O.sub.2 inhibits parasitic sputtering of chrome from the patterned layer of opaque material onto the exposed portions of the face of the transparent substrate, during the dry etching step. In contrast, the use of a gas containing CHF.sub.3 and O.sub.2 during dry etching of the phase-shift layer may cause the formation of parasitic defects containing chrome on the face of the transparent substrate. These parasitic defects typically cause a reduction in yield when the phase-shift mask is used in the formation of integrated circuits.