How to specify linear motion components for vacuum applications

Vacuum applications are common in the manufacturing of semiconductors, liquid crystal and plasma displays, fiber optics, and aerospace components, where linear motion systems are used for the positioning, handling, and alignment of critical equipment. Although vacuum applications are often likened to cleanroom applications, in the category of “special environments,” there is a critical difference between the two. The standards that define a cleanroom (ISO 14644-1 and the now-revoked Federal Standard 209E) refer to the presence and release of solid particles into the environment, while vacuum environments are affected by the release of gasses and vapors into the environment, which can hinder the ability to create or maintain the required vacuum level.

Technically, any environment with a pressure less than atmospheric pressure (101,325 Pa or 760 Torr) is considered a vacuum, but vacuum levels are often broken down into these five categories:

Units of measure for vacuum include inches of mercury (in. Hg), millimeters of mercury (mm Hg), atmosphere (atm), and micron (μ, where 1 μ = 0.001 Torr). The most common units for specifying vacuum levels in industrial applications are:

Torr, which is used primarily in the U.S. (1 Torr = 1 mm Hg)
Pascal, which is the SI unit (1 Pa = 1 N/m²)
millibar (1 mbar = 0.001 bar; 1 mbar = 100 Pa)

Outgassing and bake-out

The primary concern in vacuum environments is outgassing — the desorption, or release, of gasses or vapors into the vacuum chamber, which makes it more difficult to achieve low-pressure conditions. All surfaces outgas to some extent under vacuum, but the most significant contributors to outgassing are plastics, elastomers, and glues; porous metals and ceramics; greases; and humans, via fingerprints, hair, and skin cells.

The rate of outgassing is calculated as:

Outgassing Rate = Pressure * Volume per unit Area * Time

Typical units are:

Torr * liter per square centimeter per second
Pascal * cubic meter per square meter per second
millibar * liter per square centimeter per second

Outgassing can be reduced through a process known as a bake-out, in which the materials or equipment to be used in the vacuum are heated to a high temperature — typically 200°C, although higher temperatures are sometimes required — for anywhere from several hours to several days. The bake-out process forces the release of trapped gasses and vapors, ensuring there is little or no outgassing when the product is introduced to the vacuum environment. Performing a bake-out before using a product in a vacuum chamber is standard practice for most levels of vacuum, but bake-out is mandatory for ultrahigh vacuum (UHV) environments.

Requirements for linear motion components in vacuum applications

When choosing linear motion components for vacuum applications, there are two primary goals: use materials with minimal outgassing, and ensure those materials can withstand the required bake-out process. Fortunately, stainless steel, which is a standard material (or a widely available option) for many components, is one of the preferred materials for vacuum environments, because of its low outgassing properties, wide temperature range, and corrosion resistance.

Most types of recirculating bearing linear guides and screws, including cam roller guides, can be made entirely from stainless steel. However, for components that use recirculating balls, replacing the standard bearing steel balls with stainless steel versions causes a 30 to 40 percent reduction in load capacity. For vacuum applications that require high load capacity, the load-bearing balls can be made of ceramic, which is non-porous, non-corrosive, lightweight, and can withstand extreme temperatures, including temperature fluctuations. And even rack and pinion assemblies can be made with stainless steel racks and bronze pinions, suitable for vacuum environments.

For components or systems where stainless steel is not an option, electroless nickel plating — which can be applied to steel or aluminum — is an acceptable surface treatment for vacuum environments. (Anodizing, which is frequently used on aluminum, is not recommended in vacuum applications because it can cause the surface to retain water vapor and become a significant source of outgassing.)

Plain linear guides and lead screws can also be good choices for vacuum applications since plain bearings and lead screw nuts can be made from a wide range of materials. For example, although most polymers are sources of outgassing, some materials, such as PEEK, PPS, and PTFE, are relatively low offenders and are suitable for most vacuum environments. (However, be sure to check the material’s allowable temperature range if the component will be subject to bake-out.)

Lubrication is also a significant source of outgassing from linear motion components. Of course, in the ideal world, no lubrication would be present in the vacuum environment, but a completely lubricant-free system is rarely achievable. (Remember that screws use radial bearings for end support, and other components in the vacuum chamber, such as motor bearings, also require lubrication.)

One solution to the lubrication issue is to use plain bearings or lead screw nuts made of self-lubricating, vacuum-compatible polymers or composites, such as PEEK infused with PTFE. And for recirculating linear guides and ball screws, ceramic balls eliminate the need for lubrication.

Solid lubricants, such as molybdenum disulfide or tungsten disulfide, are recommended for some vacuum environments, but these materials aren’t suitable for semiconductor and LCD manufacturing applications due to particle generation. Another solid lubricant option is to apply a thin layer of silver (recommended specifically by some ball screw manufacturers), which provides lubricating properties and is suitable for semiconductor manufacturing and UHV environments. And when there’s no avoiding the use of grease, many vacuum-compatible options are available that can be used in rolling bearing linear guides and ball screws.

This video from HepcoMotion shows an assembly using cam roller type guides with a rack & pinion drive to transport semiconductor wafers in a 10^-9 mbar (10^-7 Pa) vacuum chamber.

Another important design feature for linear motion components in vacuum applications is the elimination of gas pockets, which are most often caused by blind screw holes. Blind holes should be replaced with through holes when possible. Where blind holes are necessary, vented fasteners should be used. Vented screws, for example, have a hole drilled through the middle and a groove on the faster head, to prevent the formation of an air pocket under the screw and to vent the cavity under the screw head. This applies to fasteners used for assembly of the components as well as to fasteners used for mounting.