Tips For Designing Clean-Room Equipment

As computer and electronics industries pack more circuits on semiconductor wafers, eliminating particles in the clean rooms where they are processed becomes more critical with each generation. In many wafer-processing environments, as well as in other clean rooms, mechanicalpositioning stages are large contributors to contamination. Sup-pliers are required to follow special design criteria that ensures a positioning stage's cleanliness according to certain standards.

Clean rooms are classified as Class 1, 10, 100, 1000, 10000, and 100000 according to standard FED STD 209E. A Class 1 clean room, for example, is defined by the number of allowable particles per cubic foot of air, in this case, no more than 35 particles of 0.1 m in diameter. Soon, a new ISO standard will likely displace the current FED STD 209E. The table, Clean-room standards, compares the ISO classification to the FED STD. The other two tables out-line the number of particles allowed in each size in a clean room.

ACTIVE GENERATION
Particles are generated actively and passively. Common active generators include positioning stages and other components with sliding surfaces, such as the seals on linear-bearing blocks, ball screws, and other metallic strip seals. Carbon black used in these seals is a source of airborne contamination, but seals made of virgin Teflon or unfilled urethanes resist abrasion and produce fewer particles.

Many manufacturers offer seals that have a small clearance between rail, shaft, and seal lips. This eliminates particle generation from sliding seal lips, yet retains lubricant critical for long-term operation.

In most clean rooms, however, contamination from other components is so well controlled that there is little danger of contaminating bearings with foreign particles, thus all bearings and ball screws can be run without seals. On the other hand, external electrical cables and air hoses rubbing over device housings are not so trouble-free. Their sliding motion generates particles. When external cable carriers can't be eliminated, specify Teflon cable casings and urethane or polyethylene air hoses or lines. All contacting surfaces should be as smooth as possible and devoid of sharp edges. Many cable-carrier manufacturers offer special carriers intended to minimize this sliding friction.

Other particle generators which are not so obvious include oil vapors and droplets dispersed in the air from lubricating oils used in bearing and drivetrain devices. Light oils and greases applied to precision bearings and ball screws produce airborne droplets from the motion of the races and the churning action of the balls. Bearings and ball screws for these clean rooms are typically packed with vacuum-grade, low-vapor pressure, lowmigration grease. Aroshell 5 and Kritox, for example, are inexpensive, general-purpose greases that meet these criteria.

Such greases used in ball bearings and ball screws work several ways. The most common is, of course, that the lubrication reduces friction to increase bearing life. Less friction also drops the wear rate and the amount of bearing material rubbed off during operation. Furthermore, grease holds ablated bearing material rather than dispersing it.

Because positioning-stage seals on ball screws and bearings typically hold the contamination inside the stage, steps must be taken to ensure that the sealing system itself does not contaminate the environment. Bellows, for instance, are a common method of isolating bearings and ball screws. However, bellows tend to be impractical because they generate large particles as they expand and contract. But worse than that, pressure differentials within the positioning stage cause bellows to blow air out, along with any contamination that is present.

Seals made from metals or belts are not susceptible to the pressure differentials during stage travel, but both typically slough off particles from sliding friction. Belt-sealing systems have substantially lower friction than metal-sealing systems which reduces the number of the particles generated for similar move profiles. In addition, belt-sealed positioning stages are evacuated more easily to remove the actively generated particles.

PASSIVE GENERATION
Passive debris comes from an exposed parent surface shedding its own particles or the particles of a foreign material trapped on that surface. Stainless steel, rather than plated or oxide-coated steel that shed material, is often used to help prevent such contamination. For example, black-oxide coated screws or plated screws should be replaced with stainless steel, and aluminum should be anodized to rid the surface of oils and other contaminants used during manufacturing. Both stainless steel and anodized aluminum resist corrosive elements and solutions that form contamination-producing oxides.

Surfaces of clean-room devices should be smooth, contain as few joints and cracks as possible, and never coated with textured paint. Surface irregularities trap particles that are released later because they can't be wiped off, and sharp edges snag strands of cleaning cloths.

PARTICLE REMOVAL
Most modern clean rooms contain filtration systems that introduce laminar airflow from elevated work surfaces to grated floors or to exhaust ducts at wall bases. FED STD 209E specifies maintaining the flow at 90 ft/min through a given cross section of the room. The ceiling-to-floor flow affects equipment designs two ways. First, don't position components over critical work areas such as water-processing areas, and hard-disk assembly areas that produce particles or turbulence. For example, control cabinets should be lower than or equal to the height of process or assembly surfaces. Furthermore, mechanical stages should not be moved over critical surfaces. Active components should be gripped from the side and kept out of the path of laminar airflow. Moreover, grippers should be shaped to let the air flow around them smoothly.

Ceiling to floor laminar flow also affects designs in a another way. Devices that are known to produce numerous particles should be located as close to the floor or exhaust ducts as possible, so the normal airflow can easily carry the particles away.

Applying a negative pressure or a vacuum to contamination-prone areas is another effective way to remove particles. Consider the airflow route when drawing a vacuum on a device. For example, louvers should be covered and the covers gasketed. Air inlets should be placed so flow moves through particle-producing areas and by devices that require cooling. The exhaust should be piped out through the house vacuum system, special filter system, or near the room exhaust ducts. High-pressure, low-flow purge systems are typically unacceptable because the high pressure tends to produce air turbulence that can disperse particles outside the influence of the vacuum. But low-pressure, high-flow evacuation minimizes air turbulence and maximizes particle exhaust.