A method of forming a dual damascene structure on a substrate having a dielectric layer already formed thereon. A phase detector drives an oscillator in the forward path of the phase locked loop. Near one mirror are positioned current blockers which surround part of the cavity and prevent current from reaching regions of the cavity which are near edges of that mirror. A non-return check valve is located between the plenum and the source. In some embodiments, the first and second drift regions are stacked vertically, with the first drift region being shallower than the second drift region.