A new technique is presented to measure the angular distribution of plasma ions bombarding the substrate surface with a planar retarding field analyzer. By varying the effective aspect ratio of the analyzer’s aperture, ions with different angular spread that are allowed through the device for detection are controlled. The analytical theory developed to define the ion current as a function of incident ion angle, ion energy, aperture geometry, and aspect ratio is shown. The method used to vary the effective aspect ratio of the aperture is also discussed. The mathematical theory is derived and the numerical solution discussed. Ion energy distributions, as a function of ion angle, with resolution as low as 3° can be measured.

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