Because of growing process demands, standard bearings are more rapidly reaching their limits of employability. With the proper choice of bearing technology, lubricant and fit, failures can be minimized and lifetimes extended.
Andreas Nobis, Christine Kessler and Johannes Kreuser, Cerobear GmbH, Herzogenrath, Germany -- Semiconductor International, 9/1/2008.
The vacuum and semiconductor industries place very high demands on mechanical components. In the case of bearings, it is typically not possible to use the same general principles in vacuum environments that hold true under normal atmospheric conditions. For these applications, the choice of the right material, lubricant and fit is critical.
Full ceramic bearings consist of rings and balls made of ceramic. In contrast, the rings of a hybrid bearing are made of a high-value bearing steel (Fig. 1). Ceramic and hybrid bearings have a variety of advantages over standard bearings. For instance, test-stand examinations of a standard steel vs. hybrid bearings (Fig. 2) show that under the same load conditions, the steel bearing shows a 2–3× higher frictional force than the hybrid bearing. There is also a noticeable strong fluctuation of the friction coefficient of the steel bearing, which suggests high abrasion.
1. Hybrid bearings consist of high-nitrogen steel rings with silicon nitride ceramic (Si3N4) balls. The high stiffness of the bearings plays a vital role in exact positioning.
2. Friction force of steel-angular contact ball bearings 7306 in dry-run mode (2.6 kN pre-load) vs. the friction force of hybrid-angular contact ball bearings 7306 in dry-run mode (2.6 kN pre-load.
Seizing is one of the main failure modes in standard steel bearings because of poor lubrication conditions. This phenomena is practically not possible in full ceramic or hybrid bearings, allowing use with poor lubrication or even without lubricants. Dry-running bearings also lead to a drastic reduction of friction and particle emission. The high Young's modulus of silicon nitride ceramic (Si3N4) allows a high stiffness of the bearing arrangement, which plays a vital role in exact positioning in the semiconductor industry. The extreme corrosion resistance makes the application of aggressive media as a lubricant possible, which then makes extensive sealing against the environment unnecessary. Furthermore, the applicability in strong magnetic fields is achieved by the non-magnetic characteristics of Si3N4 and zirconium oxide (ZrO2). Many coating plants also take advantage of ceramic's characteristic of electrical isolation in rolling bearings.
Dry running is advantageous in many circumstances. For example, rolling bearings have been used in handling systems in vacuum application and under high thermal stress.
In turbomolecular pumps, safety bearings for magnetic bearings are often exposed to extreme shock loads and accelerations. Lifetime improvements up to 40× may be possible. These bearings need to be able to reach a rotational speed of ~40,000 revolutions per minute (rpm) within 100 msec. For this application, hybrid bearings provide two advantages: light ceramic balls can be accelerated faster than steel balls and the friction loss in the rolling contact is typically 20–30% lower.
The bearing-related downtime of excimer lasers (KrF and ArF) used in microlithography could be reduced to ~1/32 of current levels by applying full ceramic bearings. In this application, the bearings directly run in contact with aggressive process gases. Ideal adapted materials in the bearing could even lead to a greater number of shoots with the same gas filling.
The corrosion resistance of Si3N4 full ceramic bearings is critical in wafer cleaning applications. The cleaning medium is also used as lubricant for the bearings, as the use of grease is forbidden.