Friday, March 27, 2009

Humidity Control in Clean Rooms

Many products manufactured and processed in a clean room
environment are moisture-sensitive. For this reason, clean
room specifications often include relative humidity (RH)
control. These control points range from 35-65%RH for yearround
operation. These RH levels generally are maintained in a
narrow band ±2 percent RH at temperatures below 70°F. The
effects of higher humidity levels in close tolerance
environments can be detrimental to product quality and
production schedules.
In semiconductor manufacturing, when the humidity level
fluctuates in a wafer fabrication area, a multitude of problems
can occur. Bake-out times typically increase, and the entire
process generally becomes harder to control. Humidity levels
above 35 percent RH make the components vulnerable to
corrosion. Additionally, as developer solvents are sprayed
onto the wafer surface, the solvents evaporate rapidly, cooling
the wafer enough to condense moisture from the air. This extra
water can change the developer characteristics and be
adsorbed onto the semiconductor layers. This can cause
swelling and further product quality problems, necessitating
additional process control.
In pharmaceutical manufacturing facilities, high humidity
causes fine powders to adsorb moisture, clogging the powder
feed to the tableting press. Powder inconsistency caused by
moisture adsorption results in crumbling tablets and clogged
tablet dies. Variations in humidity mean difficult adjustments in
bed temperature and spraying rates, resulting in heat damage
and moisture intrusion. Humidity in air ductwork creates moist
places for bacterial colonies to grow and cause process
Two common approaches to humidity control are air
conditioning and desiccants. Air conditioning lowers the
temperature of a surface exposed to the clean room airstream
below the dew point of that airstream. Excess water vapor
condenses and the resulting air is dehumidified. The air must
then be reheated to the proper control temperature and routed
to the clean room. Standard refrigeration equipment can
produce dew points of +40°F (4°C) on a reliable basis.
In a desiccant system, the process airstream passes through a
desiccant medium. The desiccant adsorbs moisture directly
from the airstream, and the resulting dehumidified air is routed
to the clean room. Desiccant dehumidifiers can produce dew
points below 0°F (-18°C), a fivefold reduction in the air
moisture beyond what can be achieved with standard HVACgrade
refrigeration systems.
Either the General Eastern MDR series relative humidity probe
or the MMR 31 series transmitter can be used to monitor the
relative humidity and temperature of clean room environments.
Housed in the tip of the MDR or MMR is a silicon-based
polymer sensor to measure the moisture, as well as a platinum
RTD to measure the temperature. The capacitive moisture
sensor and RTD combine to provide relative humidity
readings, with temperature compensation for maximum RH
accuracy over the entire operating temperature range. A
hydrophobic sintered stainless steel filter protects the sensor
from water droplets and prevents sensor contamination. The
rugged half-inch 316 stainless steel probe can be mounted in a
variety of ways, using adjustable tube compression fittings.
Various probe lengths are available, as is an optional flange
mount probe, also with adjustable insertion depth.
The MDR series RH probe can be used with either the
HygroTwin 2850 or RH-Plus 2250 moisture analyzers. The 2850
has two current outputs that can be assigned to the relative
humidity or temperature values. The 2250 can be ordered with
current outputs for both relative humidity and temperature.
The current outputs of either analyzer may be utilized to
continuously monitor clean room airstream conditions. The
MMR 31 can be ordered with either one or two outputs for the
same functions as above. This information could be used as
feedback to the process control or the HVAC system to
optimize the clean room environmental parameters. The alarm
contacts of either analyzer may be set for relative humidity and
temperature to indicate a system malfunction or warn of a
condition that could be detrimental to production.

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