Humidification
by Jim Marsh
Technology that has been put to work in the pharmaceutical, medical and food processing businesses could ease progress in your 300 mm retrofit
With the advent of 300 mm wafer fabrication, controling humidity levels in semiconductor manufacturing environments is just as important today as it was in the past. This importance continues to be fixed on the larger, general cleanroom but is becoming increasingly crucial in the minienvironments being integrated for specific manufacturing processes.
Concerns involving 300 mm wafer fabrication are not unique but amplified when compared to 200 mm. The larger size of the wafer increases the potential for static charge on the material as well as electrostatic discharge (ESD) on work-area surfaces. Efficiency of production and loss of product are also more critical due to the cost per unit for the larger chips. The automated material handling systems (AMHS), front opening unified pods (FOUP) and high air changes per hour (ACH) inherent to the new manufacturing processes all create additional potential for ESD.
Older manufacturing facilities being upgraded to meet current 300 mm needs often require the application of retrofit humidification systems. During the planning and design stages of new or retrofit cleanroom projects, consider the following when selecting humidifier technology:
- System capacity
- Control parameters
- Reliability/redundancy
- Absorptive distance
- Energy efficiency
- Maintenance requirements
- First cost including installation and utilities
- Cooling load.
The humidity levels required for a typical cleanroom have not changed in recent years: a median target of 45 percent RH with control at ±5 percent RH. Photolithography, however, is one specific sub-process of wafer production that requires lower target humidity levels, typically 30-35 percent RH with control at ±0.5 percent RH.
According to Richard Pavlotsky, Ph.D., P.E., director of advanced technology at Encompass Facility Services in San Jose, CA, there are many different requirements specific to each process and manufacturer when humidity is considered in the new 300 mm cleanroom. “Semiconductor manufacturers in various countries operate cleanrooms at different temperature set points. The various temperatures make the dew point control of the air stream very important in the understanding of cleanroom relative humidity in the calculation of mechanical HVAC equipment parameters.”
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Because semiconductor fabrication requires mid-range humidity levels, refrigerant cooling systems are adequate for dehumidification in these facilities. Desiccant dehumidification is applied to processes requiring dryer air—less than 30 percent RH. Due to potential for airborne particle generation from desiccant media, post filtration is required and additional filter loading calculated for cleanroom applications.
A number of technologies are available for the addition of water vapor to increase humidity levels. Direct injection of building heating or process steam is no longer used in cleanroom design due to airborne distribution of anti-corrosive chemicals used to maintain the boiler and steam distribution system. Cleanroom facilities that use direct-injection boiler steam for humidification are prime candidates for humidifier equipment upgrade or retrofit. The energy from an existing steam source can be used to boil deionized (DI) water via heat exchangers. This is known as a “steam-to-steam” humidifier. Use of central steam requires that boilers be operated year round.
A number of electric and gas-fired steam humidifiers are available. The most common, and low-cost type of commercial-industrial humidifier uses electrode canisters that require mineral content in the water to operate; thus, periodic maintenance is carried out at the humidifier. Humidifiers that do not require conductance in the water to operate can use DI water and take advantage of centralized periodic maintenance sites for mineral removal at the water treatment system. When planning for a steam humidifier, consider the gas or electric utility allowance.
Two available nozzle technologies, also known as atomization, use either compressed air or high-pressure water supply to generate mist or fog. Atomizers produce an average droplet size of 10 microns and usually require the application of mist eliminators to reduce carryover in ducted or air-handling unit (AHU) applications. Nozzle humidification is well suited to high-capacity requirements.
In an ultrasonic humidifier, electric energy produced by an ultrasonic oscillator is converted into mechanical energy via a piezoelectric converter. A piezoelectric substance is one that becomes distorted due to its own molecular movements as a result of an electric field excitation. The piezoelectric converter in an ultrasonic humidifier is known as a transducer.
The transducer is located at the bottom of a shallow water reservoir. Through the “thickness vibration method” this transducer will change in thickness as positive and negative voltage is applied. The water tries to follow the high-frequency mechanical oscillation but cannot because of its mass inertia, a momentary vacuum and strong compression that are produced in the water.
In the negative oscillation of the transducer, the momentary vacuum causes the water to cavitate into vapor at low temperature and pressure. In the positive oscillation of the transducer, high-pressure compression waves are produced and, by focusing the pressure waves on the surface of the water, droplets of about 1 micron are generated and quickly absorbed into the airflow.
Ultrasonic humidification offers unique advantages.
 A look at cumulative savings using ultrasonic humidification. |
Lower energy requirements. The principle of operation does not use any electric water heating or boiling, so energy consumption of an ultrasonic humidifier is approximately 93 percent less than a comparable steam humidifier. The electric utility requirements are therefore far less than steam. Some electric utility companies offer rebates for the application of ultrasonic humidification in lieu of electric steam humidifiers. The payback on retrofits and new installations typically ranges from six months to two years, depending on hours per year of operation and cost of energy, when compared to electric steam humidifiers.
Clean operation. By design, ultrasonic humidifiers use clean, mineral-free DI water. This ensures protection against contaminants and bacteria, especially when ultraviolet water-purification equipment and drain cycles are used. Ultrasonic humidifiers are built from stainless- steel or high-quality ABS plastic to avoid contamination caused by corrosion. The DI water purification system can be furnished as a complimentary component of the humidification system if no DI water is available on site.
Capacity range and control. Ultrasonic humidifier systems can generate from less than 1 lb/hr to thousands of pounds per hour. With no need for boiling water to generate steam, ultrasonic humidifiers provide instantaneous response to a call for humidification. Through the use of standard proportional controller and/or transducer output adjustment, close humidity control can be achieved, in large or small capacity applications.
Redundancy by design. Each humidification module is independently wired, thus if one transducer or unit fails, redundancy is achieved through the other modules.
Economical water consumption. No mist eliminators are required, thus no DI water goes to drain.
Reduced air conditioning requirements. Ultrasonic humidifiers provide adiabatic cooling, an added benefit in facilities with high sensible heat loads or year-round cooling requirements.
For large AHU applications, ultrasonic humidifier systems are provided as individual humidifier modules mounted in stainless-steel racks with each module pre-piped and wired for single point water and power connections. Small, dedicated applications such as minienviroments can be addressed with low adjustable output units with manual turndown and/or proportional controls.
Jim Marsh is regional sales manager for Stulz Air Technology Systems (SATS), specializing in humidification applications. SATS is located in Frederick, MD. Jim Marsh can be reached via e-mail: [email protected].