Improving the cost-effectiveness of coating processes is a permanent challenge, even in the field of optical technologies. One aspect here is the uniformity of the coatings. Depending on the specifications for spectral properties, variations in coating thickness should be subject to relatively wide or only extremely narrow tolerances. Various approaches to a solution have been developed over recent decades. One of these is EOSS®, a sputtering system developed at the Fraunhofer IST.
The objective: uniform coatings and clean processes
One of the simplest ways of making uniform and homogeneous coatings is to move the workpiece to be coated with, for example, a rotary movement. Since demands to the cleanness of the coatings are extremely strict, solutions such as using planetary gear trains for movement are not be suitable. In the field of magnetron sputtering, especially in turntable configurations, subrotations are frequently realized by means of a separate drive for each substrate holder. This approach is however associated with a certain financial cost both to purchase and in operation. This is one of the reasons why systems without subrotation hold their ground in the market.
Coating installations without subrotation use masks which modify the deposition profile of the typical planar targets in such a way that uniform coatings are created. This concept involves a great deal of effort in operation. In addition, the formation of erosion furrows on the target surface changes the distribution of atomized material over the life of the target.
With the aim of achieving an optimum coating, the following are some of the measures taken in production:
- Production of less critical coatings with suitable batch planning at the beginning and end of the target's life cycle,
- production of critical coatings at the optimum point in time,
- replacement of masks,
- adjusting the magnetic fields of the targets, and
- use of cylindrical targets.
The EOSS® sputtering system
In the EOSS® sputtering system developed at the Fraunhofer IST the cylindrical targets already established in the flat glass industry are used for the first time in a production system for precision optical interference filters. The surface of these targets erodes evenly which means that the emission characteristic of the atomized material remains virtually unchanged over the lifetime of the target. The advantages are obvious: readjustments, batch plans and other measures are no longer necessary.
These advantages are confirmed by an analysis of the behavior of the cylindrical targets in the EOSS® system. It has been demonstrated that uniformities in the range of ±0.15% can be achieved with long-term stability. This means that the EOSS® concept has a cost-effectiveness which no other manufacturer can currently reach.