Cathodic-arc evaporation is a relatively simple physical vapor deposition (PVD) technology that produces a large flux of highly ionized vapor, valuable for depositing hard, dense, and well adhered industrial coatings. Cathodic-arc evaporation also produces macroparticles (MPs) that create defects in the films, relegating this technology to applications that are mostly insensitive to these defects, such as cutting tool coatings. Many methods have been tried over the years to filter out MPs. Although more or less successful at reducing MPs, all of these filtered cathodic-arc (FCA) sources also reduce the coating rate and area to such an extent that they are mostly relegated to the laboratory or to applications needing only extremely thin films over small areas. FCA technology has also typically been complicated, bulky and expensive.
The present convention in FCA, the curvilinear FCA, borrows from fusion research to bend the ions through a bent tube using magnet coils wrapped around the tube. MPs, unlike the ions, are unaffected by the magnetic field and travel in straight lines, getting caught on the walls of the tube and are thereby prevented from reaching the part to be coated. The main problem, also present in fusion technology, is that the ions are imperfectly confined and mostly don’t make it through the filter, which explains the low deposition rates inherent to conventional FCA. Curvilinear filters are also complicated to operate and typically quite large, sticking some distance out of the side of the vacuum chamber. Low deposition rates over small areas (magnetic restoring can increase coating area, but adds even more complication), difficult operation, bulky size, and high cost have prevented widespread adoption of filtered-cathodic-arc (FCA) technology, despite the many advantages of purified ion deposition.
The inventor of the Radial Arc source has extensive experience designing and operating curvilinear as well as other types of FCA, including a highly successful rectilinear FCA that helped pioneer the diamond coatings business. Frustrated by the limitations of prior art FCA technology, but having gained an understanding of the fundamentals required for achieving efficient ion transport (and thereby high deposition rates) along with robust operation, the inventor realized a different approach would be required if the benefits of ion deposition were to become widely available.
What emerged after this rethinking and more than a decade of development, is the Radial Arc source. By redesigning the magnetic field and the cathode and anode geometry, unprecedented ion transport efficiency and MP elimination is achieved from a compact, symmetrical, uncomplicated, and robust device. The use of permanent magnets instead of magnet coils decreases complexity and size. The ion flux is also inherently distributed, eliminating the need for magnetic rastoring to achieve coating uniformity. The novelty of Radial Arc technology was demonstrated by the awarding of a strong patent (US 6,756,596).
The geometry of the Radial Arc source can be visualized as a the volume created by rotating a ninety degree bent-tube filter around one of its two ends (see above).The ions travel in a radial direction out from the cathode and are carried around curved trajectories through the large open area of the filter by strong magnetic and electric fields, then directed to the substrate in a uniform distribution. The unique geometry of the Radial Arc provides for strong magnetic fields in a compact design. The cathode is tightly coupled to the filter so that the bend of the magnetic field occurs close to the cathode. The high density of electrons near the cathode and strong magnetic field then guide ions through the bend in the magnetic field with minimal ion losses. Thus, the high electronic currents inherent to cathodic arc are fully exploited for efficient ion transport. This visualization also indicates how MPs are filtered out by eliminating line-of-sight between the cathode and the substrate. In fact, unlike curvilinear filters, the large and abrupt angle that MPs would have to navigate to escape the filter further decreases the likelihood that MPs will reach the substrate.