The Planar Magnetron is a classic “diode” mode sputtering cathode with the key addition of a permanent magnet array behind the cathode. This magnet array is arranged so that the magnetic field on the face of the target is normal to the electric field in a closed path and forms a boundary “tunnel” which traps electrons near the surface of the target. This improves the efficiency of gas ionization and constrains the discharge plasma, allowing higher current at lower gas pressure and achieves a higher sputter deposition rate for PVD (Physical Vapor Deposition) coatings.
Several different Magnetron cathode/target shapes have been used, but the most common are circular and rectangular. Rectangular Magnetrons are most often found in larger scale “in line” systems where substrates scan linearly past the targets on some form of conveyor belt or carrier. Circular Magnetrons are more commonly found in smaller scale “confocal” batch systems or single wafer stations in cluster tools.
Although more complex patterns can be done, most cathodes – including virtually all circular and rectangular ones – have a simple concentric magnet pattern with the center being one pole and the perimeter the opposite. For the circular Magnetron, this would be a relatively small round magnet in the center, and an annular ring magnet of the opposite polarity around the outside with a gap in between.
For the rectangular Magnetron the center one is normally a bar down the long axis (but less than the full length) with a rectangular “fence” of the opposite polarity all the way around it with a gap in between. The gap is where the plasma will be, a circular ring in the circular Magnetron or an elongated “race track” in the rectangular. Note that, especially in larger cathodes, the magnets can be several individual segments rather than one solid piece.
As it is used in PVD and material sputters off, you will be able to see these characteristic erosion patterns on the target face. In fact, in the event of any magnet problems such as missing, misaligned, or upside down, the erosion path will be abnormal and this can be a good diagnostic indication of such problems within your Magnetron cathode.
The pole orientation of the individual magnets must be such that one pole is formed at the center, and the opposite pole at the perimeter. There are a couple of ways to do this. The most common is to install the magnets’ north/south poles perpendicular to the plane of the target, one pole toward the target and the other end – the “free” end / opposite pole – magnetically bridged to the other magnets by a pole plate made of magnetic (normally ferrous) material.
The complete magnetic circuit is thus an open north pole of one magnet (or a chain of individual magnets if not one piece) with its opposite south pole coupled by the magnetic material to the north pole of another, whose south pole is then open. These two magnetically opposite open ends face toward the target and the resulting magnetic field arches above the surface of the target to form the electron trapping, plasma concentrating tunnel
Note that the Magnetron works with either magnetic alignment – the center can be north and the perimeter can be south, or vice versa. However, in most sputter systems, there are multiple cathodes in fairly close proximity to each other, and you do not want stray north / south fields formed in between the targets.
Those N/S fields should only be on the targets’ faces, forming the desired magnetic tunnels there. For this reason, it is entirely desirable to make sure all the cathodes in one system are aligned the same way, either all north on their perimeters, or all south on their perimeters. And for facilities with multiple sputter systems, it is likewise desirable to make them all the same so that cathodes can safely be exchanged between the systems without worrying about magnet alignment.
There are additional considerations and options regarding the magnets. Most target materials are non magnetic and thus do not interfere with the required magnetic field strength. But if you are sputtering magnetic materials such as iron or nickel, you will need either higher strength magnets, or thinner targets, or both in order to avoid having the surface magnetic field effectively shorted out by the magnetic target material.
Beyond that, the magnet details, such as magnetic strength and gap dimensions, can be designed to improve target material utilization, or to improve uniformity along the principal axis of a rectangular target. It is even possible to use electromagnets instead of permanent magnets, which can afford some degree of programmable control of the magnetic field, but does, of course, increase complexity and cost.
Sputter cathodes are normally water cooled, generally involving a copper backing plate that is directly water cooled, with the target either bonded or clamped to it. And in many cases the magnets are located inside the water jacket, in contact with the water, which likely also contacts the pole plate in such cathodes.
In such cases, it therefore becomes necessary to take appropriate steps to avoid corrosion of either the magnets or the pole plate. Pole plates can be fabricated from magnetic alloys of stainless steel, for instance, to avoid corrosion, and magnets which, depending on their composition, could otherwise be affected by exposure to water can be encapsulated in plastic or epoxy.
Planar Magnetrons, and their close cousins the clamp-on inset target Magnetrons, allow the user to achieve higher sputter deposition rates than simple diode configuration cathodes. They are also able to sustain plasma at lower gas pressures, which can open up that parameter for additional process adjustment to achieve specific film properties as may be desired.
A wide variety of such cathodes and targets are commercially available, including various options for magnet strength and layout to enhance particular aspects of the process when needed. They have become a mainstay of Magnetron PVD Sputter processing.
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