For years, the historical Kaufman ion source enabled vacuum process engineers and scientists to extract the advantages of ion beam processing of materials. The adoption of the Kaufman source in material processes resulted in many commonly understood benefits such as the densification of thin films, deposition of smooth multilayer film stacks, the activation of polymer surfaces, and the plasma etch of hard chemically inert materials from patterned wafers.
The Kaufman ion source is a plasma generation device, which on one end of the plasma volume boundary resides a multi-aperture grid set called ion optics. The typical ion optics construction consists of two grids, one called the screen grid and the other called the accelerator grid. The ions generated in the plasma discharge chamber are extracted from the plasma and accelerated through the apertures of the grids. The accelerated ions from each aperture will overlap to form a broad ion beam. Several centimeters downstream of the grids, an electron source injects low energy electrons into the beam to create a charge neutralized beam. Besides the ion optics, the typical construction of a Kaufman source consists of a filament cathode, metal anode, and a permanent magnet assembly whose magnetic field exists between the cathode and anode.
In simple terms, the operation of a Kaufman source can be described as follows. The filament cathode thermionically emits electrons. The electrons are attracted to the positively biased anode however their path is not straight but rather the electrons drift in a cycloidial path due to a confining magnetic field. As the electrons travel, they will impact a neutral atom (molecule) from a working gas fed into discharge chamber, resulting in ionization. The ionization cascade establishes a high density plasma in the discharge chamber. The plasma is elevated to a positive potential with respect to ground by the biased anode. At the ion optics, the plasma is contained within the discharge region chamber by the multi-aperture screen grid. However, when a negative bias is applied to the accelerator grid, an electric field is established which extracts ions from the plasma discharge. The extracted ions flow through the grid apertures on their way to form an ion beam.
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