Tip to tail processing in the larger multi pallet systems
|3.5.2017||Posted by Dominic under Nezařazené|
Although sputtering target is possible to use other types, most of the cathodes in such systems are rectangular. As a rule, the long axis of the rectangular cathode is across the chamber and the short axis is along the direction of pallet travel. And, although it is possible to configure cathodes for intentionally non uniform coating, the great majority of users want their substrates to be uniformly coated. In an in line system as we are discussing, the uniformity in the direction of pallet travel is dependent on the stability of the cathode power and chamber pressure/gas mixture, along with the stability of the transport speed, and finally the start/stop positions in front of and behind the deposition zone.
For a single pallet, or for the first and last pallet in a tip to tail continuous run, the start position (as well as the stop position) must be far enough out from directly under the target to avoid accruing unplanned deposition during any pre-sputter stabilization period, prior to starting the scan. Any starts, stops, or reversals of scan direction should take place outside the actual deposition zone and the scan should be steady and uninterrupted through the deposition zone. Scans can be single pass in either direction, or can be back and forth to build up thicker coatings.
Three and four rotary sputtering target are quite common, and the chamber length can be increased to accommodate additional sources as required. With enough power supplies, multiple targets can be simultaneously used in a single pass. With different target materials on the cathodes, multiple layers can thus be deposited in a single pass, or with duplicate targets, thicker coatings can be achieved in a single pass.
Uniformity in the other axis, perpendicular to the pallet scan direction, is determined by the performance of the cathode, including, especially for reactive sputtering, possible gas distribution issues. With Magnetrons, the placement and strength of the magnets can affect both target utilization and inherent uniformity, and there is normally a trade off between those two aspects. Along the center of the target's length, Vacuum coating equipment and utilization are normally quite good, but at the ends, where the "race track" erosion path turns around, the deposition rate and resulting film thickness will drop off unless magnets are adjusted to compensate, but if that is done the erosion channel gets deeper there and that reduces target utilization (the percentage of the total target mass that can be sputtered off before the deepest erosion point breaks through to the backing plate).
Tip to tail processing in the PVD coating pallet systems is also quite beneficial for target material utilization in terms of getting more on your substrates and less on shields and other chamber parts. In a single pallet system, the lead pallet is the only pallet, and as it is leaving the deposition zone, it must continue to scan until the trailing edge – the tail – is all the way out, with the target still burning the whole time, which effectively wastes some of the target material.