Again, process requirements will be the
determining factor. One can expect that a cleanroom will have to maintain
relative humidity in a range between 30 to 60 percent. The humidity controls
should have a tolerance of + 5 percent RH and critical applications a
tolerance of + 0.5 percent RH.
Whether it's a semiconductor facility or a
pharmaceutical facility, humidity control in the cleanroom environment is quite
important. In a semiconductor plant, when the humidity level fluctuates, a
multitude of problems can occur, particularly in the wafer fab area. Resist
characteristics change, bake-out times increase and generally the entire process
becomes more random. Humidity levels above 35 percent RH affect the ultrafine
geometry of the photomasks, making them very vulnerable to corrosion.
In pharmaceutical manufacturing, high
humidity, particularly during the summer, allows fine particles to absorb
moisture, leading to agglomeration and clogging of powder feeds to the
tableting press. Powder inconsistency caused by moisture absorption results in
crumbling tablets. Humidity control provides optimum reliability by minimizing
the effects of outside weather conditions on the cleanroom.
One approach incorporates desiccants. In a
desiccant system, the process airstream passes through a desiccant bed or wheel.
Desiccants absorb (or adsorb) moisture directly from the airstream and the
resulting dehumidified air is routed to the cleanroom.
When outside humidity is high and outside
temperatures are near those of the cleanroom control environment, such as during
the spring and fall, air conditioning can be an ineffective method of humidity
control. In order to remove the moisture necessary to maintain control levels,
the air conditioning method requires the cooling of incoming air to very low
temperatures, which causes frost buildup on air conditioning coils. The air then
must be reheated to the proper control level. The overall effect of air
conditioning is an increase in energy consumption and a decrease in efficiency
of the humidity control system, since the air conditioning system is overwhelmed
trying to maintain both temperature and moisture levels.
Regardless of seasonal fluctuations,
desiccant dehumidifiers do not experience frost buildup like air conditioners
because they absorb moisture at the entering temperature. This method allows the
air to be dried to extremely low levels — far below the control levels required
in the typical cleanroom. Desiccant dehumidifiers do not require a reheating
procedure. The overall effect of desiccant dehumidification is control at low
moisture levels without the load sensitivity or maintenance associated with air
conditioning.
The desiccant system cannot only be used
for new cleanroom designs, but can also be retrofitted into existing mechanical
systems.