Electronics. Pharmaceuticals. Medical devices. Cosmetics. Foods and beverages. These are just some of the products requiring careful control of relative humidity levels in controlled environments. Failure to properly measure and control relative humidity in the cleanroom can result in lower yields, increased scrap and waste, contaminated product inadvertently reaching consumers, customer lines down, increased liabilities and decreased revenues—among other situations best avoided. Carefully monitoring and controlling the relative humidity in a cleanroom is an absolute requirement—with no options.
What is relative humidity?
The amount of water vapor in the air at any given time is usually less than that required to saturate the air. According to Georgia State University1,the relative humidity is the percent of saturation humidity, calculated in relation to saturated vapor density.
Relative humidity (RH) can be defined as “the amount of moisture in the air compared to what the air can ‘hold’ at that temperature. When the air can’t ‘hold’ all the moisture, then it condenses as dew.”1 However, the argument continues that, “The air doesn’t ‘hold’ water vapor in the sense of having some attractive force or capturing influence. Water molecules are actually lighter and higher speed than the nitrogen and oxygen molecules that make up the bulk of the air, and they certainly don’t stick to them and are not in any sense held by them.”1
Why is relative humidity important?
Particulate count. Temperature. Airflow. Humidity. These five words are among the environmental factors that must be measured and controlled in the cleanroom environment. Sometimes the ‘stickiest’ of these is humidity. Measuring and controlling it within prescribed parameters can be a challenge. Too little or too much RH can impact much more than the personal comfort of cleanroom employees. Too little humidity can be quite electrifying—creating issues of static build-up and discharge. Too much humidity brings its own woes: encouraging the growth of bacteria and microbes, corroding sensitive metals whether in products or equipment, and manifesting itself in moisture condensation and water absorption. Then there’s photolithographic degradation. Photoresist processes are among the most sensitive to humidity, and can be among the most costly to control for, due to their tightly required parameters. The bottom line: any of these conditions can result in cost overruns, scrapped products, and shortened equipment life. In short, the diminution of cleanroom performance, which is costly in itself.
Some of these evils deserve a bit more discussion, as the cleanroom facilities staff finds itself in a role not unlike that of the carnival plate spinner—trying to balance sometimes conflicting, and certainly not aligned, humidity requirements for different control factors.
For employee comfort working in a controlled environment, the generally accepted ideal range for humidity levels is 40 to 60 percent. The ‘bunny suits’ worn by cleanroom employees exacerbate warmth and moisture effects so it’s important not to discount employee comfort and the impact that can have on productivity. High levels of humidity can depress even the most energetic employee; levels that are too low are clinically proven to increase the rate of respiratory infections, skin issues, and general discomfort.
While humans typically thrive in 40 to 60 percent humidity ranges, most microscopic nasties 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.
The holy trinity of measuring RH
Accurately measuring relative humidity is the initial step to control. Selecting the most appropriate sensors and transmitters to monitor and measure RH is the first item of business in ensuring trouble-free operations. The second critical factor is ensuring the selected equipment performs in a stable manner over the long term. The third factor in the holy trinity of RH monitoring is proper placement of the monitors; it’s important to avoid background environmental conditions that can impact accuracy. While RH monitors probably aren’t the first thing most facilities professionals would name among their top concerns, their failure can create huge headaches. A quick overview of each of these factors:
• Equipment selection: Required accuracy specifications are the first items to consider and delineate when comparing RH monitoring equipment. This is the time to review process and product requirements, narrowing the field of potential equipment under consideration. However, equipment selection based solely on performance claims can fall short. “Accuracy” can be in the mind of the salesperson—as a qualitative term in the ambiguity of measurement.
• Accurate performance over the long term: Monitoring relative humidity is a never-ending requirement—a marathon, not a 5k. Consistent accuracy, consistent accuracy, consistent accuracy—over an extended period of time. This is the most important factor in selecting RH monitoring
equipment. Chemical vapors lurk in controlled environments and pose the biggest threats to stability over the long haul. Because vapors can keep water molecules from the sensor, they can cause inaccurately low RH readings when the humidity is high. Conversely, they can also cause falsely high readings in low humidity settings. Features on some monitors, including a heat-driven chemical purge function, protect some RH monitors.
• Monitor placement: RH monitors are extremely sensitive to factors in the surrounding environment—temperature, heat generation, isolated moisture—any of which can produce inaccurate measurement.
RH is ruled by temperature; requiring careful consideration when placing and installing RH sensors. Some factors to consider:
• Install devices away from heat sources, include heat generating equipment. It’s important to avoid false readings created by temperature variations localized to heat radiated from isolated equipment. While good airflow throughout the controlled environment can minimize this impact, discrepancies around equipment can occur.
• Make sure the RH measuring device doesn’t become its own worst enemy. Some, when contained in enclosures, will heat up, resulting in an erroneous RH reading. Segregating the humidity sensing element from the monitor’s electronics will circumvent this problem.
• Given that the RH monitor is measuring moisture, avoiding environments that falsely increase or decrease detection of RH is critical. A few places to avoid when placing monitors: humidification systems – including ultrasonic and steam injection, and too close to cooling coils. While
integrated filters shield the sensors from water, microclimates can be created from water in the filter, creating an erroneous reading.
Calibration
Maintaining the integrity of the RH measurements is an ongoing effort, requiring careful and consistent calibration. Whether manufacturing to GMP rules in the pharmaceutical industry or developing parameters in new product development, careful calibration will ensure accuracy.
Maintaining a data log of RH readings verifies cleanroom conditions—particularly important if product integrity questions arise.
Conclusion
While the measurement and control of relative humidity can literally be a ‘sticky wicket’ the central role it plays in process and product integrity, as well as equipment life and employee comfort, demands careful attention. A carefully designed monitoring program is the best insurance policy to avoiding operations and quality problems.
References
1. http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/relhum.html
Brad Hodges, PE, LEED AP, CxA, is Principal, Science, Technology and Industry Group at SMRT Architects and Engineers (www.smrtinc.com). He has more than 20 years of experience engineering controlled environments and labs for clients in life sciences, electronics, pharmaceutical, education, government, and healthcare sectors. Brad can be reached at [email protected].
This article appeared in the November/December 2014 issue of Controlled Environments.
