High-Vacuum Applications
Strangely enough, it is possible to use air bearings in a high vacuum without significantly increasing the supply pressure. A process chamber with a high vacuum has a pressure in the range of 10-3 to 10-7 mbar. Such a vacuum is required in EUV lithography, for example.
In an air bearing, the air flows laterally out of the bearing gap into the open environment. For an air bearing to be optimally used in a vacuum environment, exhaust air outside of the bearing gap must be extracted by suction as much as possible before it can flow into the vacuum area. For this reason, multiple suction grooves are built into the air bearing surface around the active bearing surface. Figure 1 shows an air bearing with three suction grooves. The grooves are connected to vacuum pumps for different pressure ranges. The area between the grooves and between the outermost groove and the environment are located on the same plane as the active air bearing surface. The gap towards the mating surface, as with the active bearing surface, is therefore only a few micrometres. It therefore forms a contactless gap seal that prevents the bearing air from escaping into the vacuum unhindered. Doing so, the exhaust air from the air bearing can be effectively withdrawn by suction without it flowing into the vacuum.
Figure 2 systematically shows the effect of the three-stage suction. The height of the gap between the bearing and mating surface is disproportionate compared to the width of the three exhaust air grooves. The pressure or number of air particles in the bearing gap decreases sharply towards the vacuum area.
As part of a joint research project with the IMMS in Ilmenau, the suitability of air bearings for high-vacuum applications was examined. Figure 3 shows the gap height d and the pressure curve at different points when the supply pressure is switched on and off. After switching on the supply pressure, the air gap increases to approx. 8 µm both in the active air bearing area and gap sealing area between the suction grooves and the vacuum area. The pressure measured in the high-vacuum chamber (red curve) changes subtly and remains below 1x10-6 mbar at all times.
This shows that air bearings are suitable for use in a high vacuum with appropriate suction grooves.
Figure 1: Round air bearing with three suction grooves. The active bearing area with laser nozzles is located in the middle.
Figure 2: Plan showing the functionality of the exhaust air grooves and the gap seal between the bearing and mating surface.
Figure 3:
Measurement of the pressure curve when switching on (55s) and switching off (128s) the supply pressure of the air bearing. Above: Air gap (measured in the three gap seals). Below: pressure in the chamber (red) and suction grooves. The supply pressure is 1 bar, the bearing load 50 N.