Condensation inside a chamber during rapid roughing: the gas expands almost adiabatically, its temperature drops, and water vapour condenses into droplets or ice crystals that can carry particles onto wafers and optics. It is easiest to see through a viewport on a humid day. Slower roughing and venting with dry nitrogen prevent it.
Saw fogging in the load lock window on the first pump-down after the humidity spiked, so we slowed the soft pump.
Also heard as
- pump-down fog
- condensation fog
Related terms
Soft pump
Slow, throttled roughing from atmosphere that avoids stirring up particles and condensing moisture.
Vapor pressure
Pressure at which a liquid or solid is in balance with its own vapour, which decides what can go inside a chamber.
Wet
Said of a chamber whose walls still hold adsorbed water, so it pumps down slowly and stalls high.
Rough out
To pump a vented chamber from atmosphere to crossover with the roughing pump, before the high vacuum valve opens.