The present invention generally relates to optical lithography and more particularly relates to the fabrication of transparent or semitransparent phase shifting masks used in the manufacture of semiconductor devices. More particularly, the present invention implements a method for modifying anisotropically etched features on conventional alternating aperture phase shift masks (“aaPSMs”) using an isotropic plasma quartz etch process, which involves three processing stages: (1) defining the opaque region (e.g., chrome) using a chlorine-based decoupled plasma process; (2) forming an alternating anisotropic phase shift feature to a specific predetermined depth through the use of a decoupled plasma source with a fluorine etchant; and (3) changing the plasma conditions by interrupting the bias power applied across the mask and etching strictly in the inductively coupled plasma mode. These three processing stages achieve an isotropic undercutting of opaque layers which define the aaPSM.
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