While humans typically thrive in 40 to 60 percent humidity ranges, most microscopic nas-ties do not. Viruses, mold, bacteria, fungi, and mites tend to flourish when the relative humidity exceeds 60 percent. While some types of biological contaminants can grow robustly above
30 percent, the majority do not.
On the flip side, the build-up of static electrical charge (and the resulting discharges
that can be extremely destructive in a cleanroom) accelerates at lower levels of humidity. Cleanroom surfaces below 30 percent relative humidity are particularly fraught with
the build-up of static electrical charges, particularly if ungrounded. Taken alone, relative
humidity exceeding 50 percent is the sweet spot for minimizing electrical static discharge
problems. Typically in the management of electrostatic discharges (ESD), additional standard processes ranging from static dissipative shoes and cleanroom garments, ESD protective flooring, air ionizers, and insulative grounders, among other tools, are deployed. ESD
is not conquered by humidity levels alone.
Other selected impacts
The point of a cleanroom is clean, with particulate counts strictly controlled. When the
humidity is too high, the adhesion of particles to surfaces can increase. Kelvin condensation can become the dominant factor in particle adhesion issues when the RH is 70
percent or more, and becomes generally unimportant below 50 percent. Higher humidity
leads to capillary forces—creating a bonding bridge between the surface and the contaminant, increasing particle adhesion to substances such as silicon.
Controlling for resist stability and resulting dimensional precision are impacted not
only by temperature but also relative humidity. Increasing RH, even without a change
in temperature, can quickly decrease photoresist viscosity. This can cause changes to the
thickness of a resist film using a fixed coating recipe. Too high humidity can also intensify
water absorption, increasing resist swelling after a bake cycle. Low humidity aids resist
adhesion, while it can be negatively impacted by high relative humidity.
Cost considerations in controlling RH
Simply put, because humidity is relative to temperature, controlling RH within very tight
tolerances or at extremely low levels can end up costing you more money in both construction and operating budgets. It’s important to understand that target humidity and
temperature control decisions impact costs.
A cleanroom target temperature of 65 degrees will have a lower relative humidity
than a target temperature of 60 degrees. The lower your controlled temperature goes,
more is required to “dry out” the air to reach a set RH level. Driving lower moisture
content drives cost.
Similarly, driving RH to tight tolerances can be costly, as you need to take your chilled water
down to pull the moisture from the cleanroom. Tighter tolerances could require a larger chiller,
or the requirement to derate the chiller by introducing glycol to maintain flow. That reduces heat
transfer and drives the requirement for a larger chiller to due to decreased efficiencies.
The outside climate impacts humidity levels, and how best to control it, inside the
cleanroom. Tropical climes with consistently high humidity will pose quite a different
challenge than that found in an area with warm summers and brutally cold, dry winters—
dishing up a double whammy of humidity and below freezing temperatures, with an
added bonus of possible snow intrusion through the air systems.
of RH measurements is an ongoing effort.