Minimizing outgassing from cleanroom equipment

Cleanrooms require minimization of friction-related particle generation. Now let’s consider another source of contamination — outgassing — and how to minimize it.

Especially in cleanroom applications involving the manufacture of LEDs, optics, or glass, or the processing of silicon wafers, outgassing can damage end product.

Outgassing is the desorption of vapors or gasses, either from within or from the surface of a material. ISO standard 14644-8 addresses outgassing in cleanrooms and specifically refers to airborne molecular contamination by chemicals and non-particulate substances in the gaseous or vapor state that may degrade product, process, or equipment.

These molecular vapors are smaller than particles and pass right through HEPA and ULPA filters so are difficult to control once released.

Minimize outgassing through material selection

Automation components for minimizing outgassing feature smooth materials that make it more difficult for airborne molecular contaminants to adhere to their surfaces. Stainless steel and anodized aluminum are preferred materials, although some epoxy paints can sufficiently prevent outgassing-type issues.

Thankfully, most components such as linear guides and drives, motors, and gearboxes can be made of stainless steel. On linear motion components, stainless-steel endcaps should replace plastic whenever that’s possible. Likewise, standard steel fasteners should be replaced with stainless-steel hardware.

Steel can maintain hardness, durability, or load-carrying capacity, and nickel plating can reduce the steel’s tendency to outgas. One caveat: Some nickel-plating formulas include a polytetrafluoroethylene or PTFE or Teflon topcoat. PTFE is a solid lubricant with low shear strength and low friction coefficient against metal. As a coating, it can be incompatible with cleanroom applications. Where PTFE acts within a matrix (for example, in the fiber-reinforced thermoplastic of a motion component) the material may be suitable.

Nickel-plated steel and epoxy-coated aluminum are suitable, but stainless steel or anodized aluminum (with the least amount of plastic possible) are preferred. Smooth surfaces (with no textured paints) also benefit these settings.

For example, some suppliers of engineered-plastic components sell some mechanical components with proprietary solid-lubricant chemistry embedded as microscopic units in fiber-reinforced polymer matrices. Safety and performance data sheets imply fluorinated polymers like PTFE that can spread a thin film onto any counterface during motion to permanently lower friction.

Other embeddable solid lubricants include graphite and molybdenum disulfide (MoS₂) that have high dry-running PV performance though these aren’t cleanroom compatible.

Reduce outgassing from lubricants

Lubricants are major outgassing offenders, readily releasing water and oil vapor into the atmosphere. But most motion components need lubrication. Any absolutely necessary lubrication must be suitable for cleanroom settings. Cleanroom lubricants have base oils distilled to remove volatile molecules that easily evaporate. Hydrocarbons, esters, silicones, and polyalphaolefins are a few used in cleanrooms. However, PTFEs often deliver the least outgassing with high performance. Cleanroom-rated lubricants are acceptable, but any chance to eliminate lubrication should be taken.

Various cleanroom-rated lubricants are suitable for bearings, motors, gearboxes, and other motion components. Consider how screws and motors output shafts have rotary bearings for end support. Here, plain bearings of polymers or composites such as PEEK infused with PTFE have been used — as well as ceramic balls that eliminate the need for lubrication.

Solid molybdenum disulfide or tungsten disulfide lubricants are suitable for some vacuum environments but not for certain semiconductor and LCD manufacturing applications due to particle generation. Another solid lubricant is silver applied in a thin layer — specifically recommended by some ballscrew makers for its suitability in semiconductor manufacturing and UHV environments. Where grease is absolutely required, many vacuum-compatible options are available for linear guides and drives especially.

Many cleanroom-approved lubricants are special formulations having no (or less than typical) solid-particle aluminum, silica, and PTFE additives.

Lubrication reduces friction and therefore particle generation — plus traps generated particles and prevents them from scattering into the environment.

Seals and covers can prevent rotating components from slinging grease into and onto critical work areas.

So let’s detail other ways to make linear-motion systems cleanroom compatible.

Separating load-bearing elements: Particle-count tests confirm that bearing-ball spacers or cages (that separate the balls to prevent collisions during their recirculation) can reduce particle generation in profile-rail guides and ballscrews.

Cleanroom-friendly materials are a must: Preferred materials for cleanroom environments are stainless steel and PVC although aluminum and carbon steel dominate in linear motion components. In fact, there are ways to make aluminum and standard carbon steel cleanroom-compliant.

Anodizing aluminum gives it good corrosion resistance. Carbon-steel parts can be coated with black chrome or nickel to prevent oxidizing.

It’s quite common for miniature guides and miniature screws come in stainless-steel versions. So, these are good choices for cleanroom applications with shorter stroke lengths and lighter loads. Many miniature components also come with low-friction seals and low preload as standard options. Plus, just due to size they generate fewer particles than their full-size counterparts.

Not to be overlooked, fasteners are often coated with a black-oxide finish — a coating that sheds particles even when these components are static. So, for cleanroom applications, specify stainless steel hardware wherever possible.

Some linear actuators with toothed belt drives (for flexible positioning) can work even in ISO-2 cleanrooms.

Miniature linear guides and ballscrews are often available in stainless steel versions, making them well-suited for cleanroom applications.

There are yet other ways to make a linear system cleanroom friendly. Mounting orientation can be chosen to minimize the release of particles into the atmosphere — and that’s more critical than system dynamics.

If it’s possible, the best mounting keeps moving components below the work area. Laminar cleanroom airflow evacuates particles so they can’t contaminate critical work areas above. Right-side-up orientations trap particles inside a motion assembly than other mounting orientations.