Particle Generation Overview

Particles are generated actively and passively. Common active generators are positioning stages and other components with sliding surfaces. Sliding motion generates particles. Lubricants can also be a source of particle generation in the form of vapors and droplets dispersed during the rapid motion of balls in precision bearings and ball screws. Low pressure, low migration, perfluorinated grease reduces friction and thereby the wear rate of ball and race surfaces. As a viscous fluid, it traps and holds in suspension the particles created from the bearing surfaces.

In systems using cable carriers, particle generation occurs as a result of cables and air lines rubbing against each other, against the cable carrier, and from the carrier itself.

Stage protection devices are also a source of particles. Bellows sealing assemblies generate large particles as they expand and contract. Pressure differentials within the stage cause bellows to blow air out, along with any contamination that may be present.

Passive contamination is the result of a surface shedding particles, the shedding of a trapped foreign material, or outgassing. The best cleanroom surfaces are ones that do not require coatings, such as stainless steels and Teflon.

Cleanrooms contain filtration systems that produce laminar-air flow from elevated work surfaces to grated floors to exhaust ducts at wall bases. US FED STD 209E specifies maintaining the flow at 90ft/min through a given cross section of the room.