Abstract: An improved method for minimizing interferences from random noise and correlated fluctuations which obscure electrical signals converted from optical emissions. In particular, an improved method for the removal of interferences from optical emission signals during endpoint determination in dry etching processes for the fabrication of microelectronic devices which derives information in the presence of random noise, correlated fluctuations and periodic modulations of the plasma by maximizing the signal to random noise ratio and minimizing the obscuring effects of correlated fluctuation.

Abstract: The device of this invention employs a short focal length anamorphic lens to collect light from a source with asymmetric angular dispersion of the flux. Focal lengths are chosen to provide an approximate line fuzzy focus with a f/number equal to or smaller than one of the associated detector. Fiber optic cable systems are used to deliver the condensed light and the diameter of the optical fibers is chosen to give a numerical aperture greater than that of the detector. The number of optical fibers in the cable should be large enough to over-fill the limiting area of the etendue of the detector. In a preferrred embodiment the fiberoptic cable is bifurcated and fibers from the separate channels are interleaved.