In our last month’s Blog I talked a bit about the proper operating pressures for plasma depositions under various working conditions but I left out one rather important aspect for all vacuum process equipment. What I didn’t talk about is ground potential and how to properly ground the vacuum system.
Almost all vacuum deposition systems utilize some form of high energy electrical power supply to either produce heat for thermal transfer, to generate a plasma, or electrons, or ions or some aspect of energy transfer to perform a material transformation from bulk material to thin film form. This energy transformation can be performed via. many different configurations but in all cases it requires a live electrical potential being properly controlled within the vacuum chamber. This potential must be controlled both for safety reasons as well as for operational performance and process control reasons. In all cases, care must be taken to insure that a proper grounding network is established for both the vacuum system and all the associated power supplies. This is particularly important if an rf generator and tuning network is part of the vacuum equipment being installed. It is impossible to null out the reflected power without a proper ground.
Ideally this can be accomplished by preparing a proper grounding spike. If possible, a one inch (1″) diameter copper rod measuring six-eight feet (6′ – 8′) in length should be driven directly into the ground somewhere in close proximity to the vacuum system during the initial equipment installation. The copper spike should be driven into the earth ground perpendicular to the floor with around six to twelve inches (6″-12″) of the spike left protruding above the floor level. It will probably be necessary to drill a hole through the concrete floor prior to installing the stake so be certain as to the proper location of the system and the spike prior to commencing. If the system is being installed in a clean room simply install the grounding spike in the equipment chase adjacent to the room.
Once the ground spike is properly prepared, clamp one inch (1″) wide braded copper straps onto the extended portion of the rod and then run them to the vacuum system frame assembly, the vacuum system chamber and the power supply frame as well as to any other operational equipment that may come in contact to high voltage exposure. If the vacuum chamber has multiple components, such as a door or removable top plate for example, make certain to properly ground each component separately.
If it is impossible to provide a proper ground spike directly into the earth, as in the case of the system not being located on the ground floor, substructures impede the possibility, physical locality prevents it, etc. then the next best thing is to attach the same braided ground straps described above that are running from the various equipment connections directly to the copper water pipe coming from the supply line of the building. This is not ideal but it is the best alternative to a true ground spike.