The planar magnetron is a typical diode mode sputtering cathode consisting of a permanent magnet array behind it. This magnet array is organized in such a fashion to provide a magnetic field that is normal to the electric field in a closed path and to create a boundary ‘tunnel’ that traps electrons in the vicinity of the target surface. This electron trapping optimizes the efficiency of gas ion formation and limits the discharge plasma, thus enabling higher current at lower gas pressure. This, in turn, helps achieving a higher sputter deposition rate for physical vapor deposition (PVD) coatings.
Different Forms of Magnetrons
Although many different magnetron cathode/target shapes are available, the most common forms are rectangular and circular. Rectangular magnetrons are widely used in larger scale ‘in line’ systems, wherein the substrates are linearly passed the targets by means of a conveyor belt or carrier. Circular magnetrons are widely used in smaller scale ‘confocal’ batch systems or single wafer stations in cluster tools.
Even though it is possible to create more complex patterns, most cathodes, including almost all rectangular and circular ones, exhibit a simple concentric magnet pattern wherein the center is one pole and the perimeter is the opposite pole. For the rectangular magnetron, the center is typically a bar down the long axis surrounded by a rectangular ‘fence’ of the opposite polarity, with a gap in between. For the circular magnetron, the center would be a very small round magnet surrounded by an annular ring magnet of the opposite polarity, with a gap in between.
The gap is where the plasma will be, an elongated ‘race track’ in the rectangular magnetron or a circular ring in the circular magnetron. In larger cathodes, the magnets can be numerous individual segments instead of one solid piece. As it is utilized in PVD and material sputters off, these characteristic erosion patterns can be seen on the target face. The pole orientation of the individual magnets must be in such a fashion that the center has one pole and the perimeter has the opposite pole.
The magnetron operates with either magnetic alignment, i.e., the center can be south and the perimeter can be north, or vice versa. Nevertheless, in most sputter systems, multiple cathodes are closely placed with each other and the formation of north/south fields in between the targets is not desirable. These north/south fields must only be on the faces of the targets to create the desired magnetic tunnels there. Hence, it is desirable to ensure that all the cathodes in a system are arranged either as all north on their perimeters, or all south on their perimeters. This arrangement is also applicable to facilities having multiple sputter systems, thus enabling safer exchange of cathodes between the systems with no worry about magnet alignment.
Other Requirements
A majority of target materials are non magnetic and do not interfere with the magnetic field strength that is required. However, thinner targets or higher strength magnets or both will be needed if materials to be sputtered are magnetic such as iron or nickel in order to prevent the effect of the magnetic target material on the surface magnetic field. Electromagnets can also be used in place of permanent magnets in order to obtain some degree of programmable control over the magnetic field. However, it certainly increases the complexity and cost.
Sputter cathodes are usually cooled by water, typically involving a copper backing plate that is directly cooled by water, with the target either clamped or bonded to it. Moreover, in several cases, the magnets are placed inside the water jacket, in contact with the water, which also makes contact with the pole plate in such cathodes. Hence, appropriate measures must have to be taken to prevent corrosion of either the pole plate or the magnets. Corrosion of pole plates can be avoided by fabricating them from magnetic alloys of stainless steel. Moreover, by encapsulating the magnets in plastic or epoxy, they can be prevented from getting exposed to water.
Conclusion
Planar magnetrons and the clamp-on inset target magnetrons help achieving higher sputter deposition rates when compared to the simple diode configuration cathodes. They can sustain plasma at lower gas pressures, thus helping to achieve specific film properties as required. Many different cathodes and targets are available on the market, including several options for magnet strength and layout in order to improve specific aspects of the process if required. They have become a key element of magnetron PVD Sputter processing.
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This information has been sourced, reviewed and adapted from materials provided by Semicore Equipment.
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