1.  Field of the Invention
Embodiments of the present invention generally relate to a method and apparatus for plasma etching photomasks and, more specifically, to a method and apparatus with improved control of distribution of plasma species. 
2.  Description of the Related Art
The fabrication of microelectronics or integrated circuit devices typically involves a complicated process sequence requiring hundreds of individual steps performed on semiconductive, dielectric and conductive substrates.  Examples of these process steps include oxidation, diffusion, ion implantation, thin film deposition, cleaning, etching and lithography.  Using lithography and etching (often referred to as pattern transfer steps), a desired pattern is first transferred to a photosensitive material layer, e.g., a photoresist, and then to the underlying material layer during subsequent etching.  In the lithographic step, a blanket photoresist layer is exposed to a radiation source through a reticle or photomask containing a pattern so that an image of the pattern is formed in the photoresist.  By developing the photoresist in a suitable chemical solution, portions of the photoresist are removed, thus resulting in a patterned photoresist layer.  With this photoresist pattern acting as a mask, the underlying material layer is exposed to a reactive environment, e.g., using wet or dry etching, which results in the pattern being transferred to the underlying material layer. 
The pattern on a photomask, which is typically formed in a metal-containing layer supported on a glass or quartz substrate, is also generated by etching through a photoresist pattern.  In this case, however, the photoresist pattern is created by a direct write technique, e.g., with an electron beam or other suitable radiation beam, as opposed to exposing the photoresist through a reticle.  With the patterned photoresist as a mask, the pattern can be transferred to the underlying metal-containing layer using plasma etching.  An example of a commercially available photomask etch equipment suitable for use in advanced device fabrication is the Tetra™ Photomask Etch System, available from Applied Materials, Inc., of Santa Clara, Calif. The terms “mask”, “photomask” or “reticle” will be used interchangeably to denote generally a substrate containing a pattern. 
With ever-decreasing device dimensions, the design and fabrication of photomasks for advanced technology becomes increasingly complex, and control of critical dimensions and process uniformity becomes increasingly more important.  Therefore, there is an ongoing need for improved process monitor and control in photomask fabrication.