Filtered vacuum arc deposition of semi-conductor thin films

The cathode spot vacuum produces a jet of highly ionized plasma plus a spray of liquid droplets, both consisting of cathode material. The droplets are filtered from the plasma by passing the plasma through a curved, magnetized duct. A radial magnetic field may be applied to the face of the cathode to rotate and distribute the cathode spots in order to obtain even erosion and avoid local overheating. The choice of axial magnetic field strength in the vicinity of the cathode is a compromise between a relatively high field desired to collimate a large fraction of the plasma flux, and the need to collect a substantial fraction of the plasma at the anode in order to reduce arc voltage and insure arc stability. The transmission of the filter duct increases with magnetic field strength until a saturation value. Entrainment of the droplets in the plasma jet can decrease the effectiveness of the filter at high plasma flux. Semiconducting thin films of amorphous silicon were prepared using cathodes of heavily B-doped Si. Arcs of 35 A current produced a deposition rate of 10 A/s. The electrical conductivity of the films was similar to conventional a-Si:H films deposited by conventional Silane based CVD at high temperatures, but had a higher room temperature conductivity, despite the absence of hydrogen to terminate dangling bonds. Transparent conducting films on Sn-O were deposited at rates of up to 100 Å/s using 160 A arcs on a Sn cathode while injecting O₂ gas in the vicinity of the substrate. Adjustment of the O content is critical for optimizing conductivity, and complicated by pumping effects of the arc. Optimal conductivity was achieved at an oxygen pressure of 6 mTorr. Conductivities equal to the best reported to date were achieved by subjecting the room temperature deposited films to a 30 s rapid thermal annealing at 350 C. Both the as deposited and annealed films are amorphous. The deposition rates achieved by the filtered vacuum arc technique for these semiconductor films are an order of magnitude greater than achieved with conventional methods, while the conductivities are equivalent or better. The results contravene accepted assumptions that hydrogenation is necessary for high conductivity a-Si, and that high conductivity Sn-O requires a crystalline microstructure. It is suggested that the unusual results achieved are a result of higher density coatings caused by the high energy of the depositing ions.