Ion-assisted deposition in unbalanced-magnetron sputtering systems

Abstract

In conventional balanced-magnetron (CBM) sputtering, the amount of ion bombardment of the growing film, as measured by the substrate bias current density, is limited to less than 1 mA cm −2. Fast secondary electrons escaping from the target surface quickly find a path to ground and undergo few ionizing collisions with gas atoms near the substrate. If a CBM cathode is unbalanced by making one set of the magnets, usually the outer ones, stronger than the other, the expanding magnetic field lines confine the fast secondary electrons. A dense secondary plasma can be formed in the region of the substrate, and ions from this plasma can then be used for bombardment of the growing film. Such bombardment, particularly for hard coatings, is necessary to form a fully dense, well-adhered coating. With unbalanced-magnetron (UBM) sputtering, it is now possible to achieve substrate bias current densities in excess of 5 mA cm −2. Although high current densities are achieved, the uniformity of the current density varies with position relative to the target. It is highest directly in front of the target, and it decreases towards the edge of the target. In multiple-cathode systems, the magnetic fields must be linked to form a trap for the electrons, and an even number of cathodes must be used in order to close the trap. UBM sputtering can be scaled up, and today industrial systems are taking advantage of this technology.