Author: JUERGEN BRODERS ; Type of thesis: Foreign Institute Thesis
Abstract: A cylindrical postmagnetron has been designed for sputtering Niobium films into 1,5 Ghz monocell Copper cavities of the TESLA-type.
For sputtering 1.5 GHz monocells we built a cylindrical post-magnetron . The idea under such a design was the search of high sputtering rates and high thickness uniformity of the coating sampled along the cavity profile. Higher sputtering rates respect to classical cylindrical magnetron are possible when sputtering from the whole target surface. The advantage is twofold: the fraction of impurities trapped in the film linearly decreases versus deposition rate, moreover the deposition takes shorter time.
On the other side for the classical cylindrical magnetron, when the discharge is switch on, plasma do not live simultaneously on the whole target surface, instead it is strictly confined onto the target surface portion where ionizing electrons are trapped by magnetic field lines. A large portion of target surface, the one just outside of the magnetic trap, reaches high temperatures as well, but there is not sputtering from it, unless the electromagnet is moved toward this region.
On the basis of our experience on cylindrical magnetrons of such a design, we have observed that films untouched by plasma have systematically worse purity than films immersed in the discharge. This problem becomes not important for postmagnetrons,since the whole target surface is sputtered simultaneously. When designing a post-magnetron two choiceswere possible: a) the cathode must follow the profile of the cavity, keeping the magnetic field lines constantly parallel to the target. End losses being avoided by an electrostatic mirror outside of the cavity. In this case the shape of the target is rather complicated since to get into the narrow bore of the cutoff,it must be collapsible. b) the cathode is a straight tube and
the magnetic field follows the shape of the cavity.