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.

Although high-performance ceramic groups play a vital role in new application development for rolling bearings, only two ceramics, Si3N4 and ZrO2, prove adequate to be used today.

Hot isostatic-pressed Si3N4 is especially applicable; full ceramic bearings consist of rings and balls made of it. Advantageous properties include extreme over-rolling resistance, low density, very good corrosion resistance, and high hardness and stiffness, as well as good temperature and abrasion characteristics. These properties are the result of the mainly covalent bonding of the ceramic that has been built up. After the development of series production technology for manufacturing powder, Si3N4 is the most frequently used ceramic in rolling bearings today. Typical Si3N4 materials are alloyed with yttrium oxide (Y2O3) or magnesium oxide (MgO).

Because of the very low expansion coefficient of Si3N4, the fit calculation requires special attention. Information regarding temperatures and surrounding parts are necessary to determine the optimal fit. The usage of tolerance-compensating elements, such as tolerance rings, is often necessary to allow functioning under higher temperature ranges.

Zirconium oxide is used as an alternative ring material in full ceramic bearings. This newer development of oxide ceramic distinguishes itself through its steel-like material properties. For instance, the connection to metallic surrounding parts is simplified through an almost identical heat expansion coefficient as standard bearing steel.

The various material characteristics of Si3N4, ZrO2 and standard bearing steel 100 Cr6 are shown in the Table.

Hybrid bearing rings are manufactured of high-value bearing steel, while the rolling elements consist of Si3N4. Mainly high-nitrogen steels, such as X30 (Cronidur 30) and X40, are used for the production of the rings. These high-nitrogen steel materials demonstrate extreme over-rolling and high corrosion resistance. Hybrid bearings nearly match the performance of full ceramic bearings under media lubrication and dry-running conditions, as well as at high rotational speeds under greased lubrication.

Our bearing cages are made of poly-ether-ether-ketone (PEEK), a thermoplastic that is used in a variety of semiconductor applications. PEEK is light, has very good mechanical properties, a high operating temperature and good media resistance. For extreme temperatures (to -253°C), rather than PEEK, polychlorotrifluoroethylene (PCTFE) is used, which also provides better media resistance. At temperatures above 250°C, heat resistant steels are used as cage material.

The question regarding lifetime calculation of hybrid or full ceramic bearings is often asked. Unfortunately, lifetime calculation according to DIN ISO 281 cannot be applied for usage with special demands. This calculation assumes that the lubrication of rolling bearings is optimal and does not consider the characteristics of ceramics.

Instead, bearing design is based on the calculation of the Hertzian stresses and experience gained with the usage of ceramics in comparable applications. Special attention needs to be paid toward insufficient lubrication or dry-running operations, which puts additional strain on the material through tangential and shear stresses in the rolling contact.

The analysis of test-stand experiments and experience shows that there are different limit loads and speeds depending on the lubrication condition. The influence of changing operating conditions needs to be estimated as well. A statement regarding the lifetime of a bearing can, therefore, often only be obtained through testing. However, our experience showed that a hybrid or full ceramic bearing may have a factor of 2–100× longer lifetime than a standard bearing.