A system, apparatus, and methods for interfacing a scan engine for scanning optical code symbols and a decoder. The compositions and associated methods of the invention result in similar removal rates of both the tungsten and the dielectric material. A constraint space C\u2032 is then derived from the state space S\u2032, where C\u2032 is the logical complement of S\u2032. Then, one of the gas flows from the first excitation part is biased toward the \u2212X direction by a first biasing member arranged in the first gas flow passage, and the other gas flow from the second excitation part is biased toward the +X direction by a second biasing member arranged in the second gas flow passage.