Up in the air: Cleanroom HVAC systems
Finding the correct HVAC solution for each facility and location is key to cleanroom design
By Hank Hogan
Cleanrooms, like people, need air, but not just any air. It has to be the right type — filtered to an acceptable level and at the correct temperature and humidity. Advances in heating, ventilating and air conditioning (HVAC) technology have improved overall system performance, but there are still challenges to be overcome in delivering the air that`s needed in an energy efficient way. For semiconductor cleanrooms, two looming and perhaps intertwined issues are the steady downward march in process feature size and the jump up to 300 mm wafers. Whatever the situation, however, finding the correct HVAC solution is more important to a cleanroom than an air conditioner is to a Texan in summer.
“The HVAC system is probably the most critical part of a cleanroom system,” notes Gary Devloo, president of Lepco, Inc. (Houston). A 27-year-old company, Lepco designs and builds cleanrooms. The company`s customers include semiconductor and microelectronic manufacturers, disk drive makers, and others. The firm is also one of the leaders in the use of gas powered chillers.
Despite being vital, it`s clear that HVAC systems don`t exist just to move air around. The acceptable number and size of particulates, as well as the allowable temperature and humidity, are all process driven. That, along with the uniqueness of each facility and location, means that cleanroom HVAC solutions can`t be stamped from a single mold.
“I don`t really believe that there`s one size fits all or one optimum solution,” says Bill Acorn, the principal and founder of Acorn Engineering and Consulting (Tucson, AZ). AEC designs cleanrooms and also acts as a consultant on cleanroom related projects.
Nevertheless, there have been some improvements in cleanroom HVAC systems that can be, and are being, applied across the board. These enhancements have come in the three main areas of HVAC: contaminant/particulate suppression, temperature and humidity regulation, and overall system control. On the first of these, the trend is to dress to the nines. Where once state-of-the-art HEPA filters stopped 99.9 percent of all particulates 0.3 microns and larger, now routine ULPA filters reject 99.99 percent of all particulates 0.1 microns and larger. Today`s latest filter technology, polytetraflouride ethylene (PTFE) filters, halt 99.999 percent of all particulates at just below 0.1 microns in size. As an added bonus, because they`re made out of inert PTFE, filters don`t contribute any chemical contamination of their own to the air passing through them. On the issue of how to distribute the air to the filters there`s not much of a consensus. Consultants and contractors report that some sites use a plenum to move air where it`s needed; other sites use ducting. Part of the reason for this is that with a plenum there may be more of a chance for cross contamination between filters but that has to be balanced against the higher cost of running ducting to each and every filter. For high purity cleanrooms with continuous 100 percent laminar flow coverage, ducting may just be too expensive.
Less is more
On one topic there does seem to be agreement — that for air flow, less may be more. “Everybody is trying to reduce air flows,” notes Mark Jamison, manager of semiconductor projects for Black & Veatch (Kansas City, MO). “Ten years ago everybody was doing 90 feet per minute. You just had to do it if you wanted to get a Class 10 or cleaner, and people are saying they don`t have to do that now. We can get down to 70 feet per minute or less.” Black & Veatch is a large partnership with 80 or so offices worldwide. Besides work in power generation design and construction, the firm designs and builds cleanrooms for both the microelectronics and pharmaceutical industries.
Since there`s a pressure drop across any filter, forcing a large volume of air through it takes energy, and that translates to money. So part of the drive for lower air flows is a need to lower the expense of operating cleanroom HVAC systems. Just how much savings will result from the reduction of air flow from 90 feet to 70 feet per minute in a given facility depends upon the exact design of the site in question. The energy savings can be large since the relationship between air speed and energy isn`t a linear one, but offsetting this is the fact that finer filtration usually means a larger pressure drop across the filters for a given air flow.
Moving less air doesn`t necessarily mean an increase in particulates/contaminants. That`s because the cleanliness of the air is a factor of the degree of filtration, the amount of air flow, and the laminarity, or smoothness, of the air movement. Avoiding even the slightest amount of turbulence can pay big dividends.
“Out of the three things you can do to the air — filtration, air flow, or laminarity — laminarity holds a lot of promise because with just small improvements in laminarity you can get tremendous improvements in contamination control,” points out Lepco`s Devloo. As an example of what`s possible, Lepco`s trademarked Membrane Diffusion system disperses air over the entire ceiling plane even when there`s only partial filter coverage. Turbulence and dead zones are consequently greatly reduced or eliminated. The result, according to independent tests cited by the company, is a ten-fold increase in cleanliness with seven to 10 times more uniformity and three to 10 times less cross contamination. Such techniques don`t help when filter coverage is already 100 percent. One development that has helped in those situations is the introduction of high quality and long-lived modular ceiling units. Custom built by such major manufacturers as CleanPak International and Hunt Air in a variety of sizes, these units combine filtration, lighting, and sprinkler systems into one assembly.
These systems are being widely adopted for the most stringent cleanrooms. With direct drive motors and no belts to fray and contaminate, such modular systems offer some of the best air filtration around but at a cost of $250 per square foot. One complication in this picture is that laminar air flow doesn`t tend to stay that way. There are obstacles, such as tools, transport systems, and people, in the way of the incoming air stream. There`s also the problem of the return air path. Exhaust systems tend to suck air out. In addition to that, other obstacles or the placement of return air vents may further imbalance the air flow. Such asymmetrical air flow can be alleviated by the use of either wall or floor dampers to adjust the return air path and to restore laminarity to the room air flow. The advantage to this approach is that as tool sets evolve adjustments can be made to accommodate a change. As far as temperature and humidity controls are concerned, the trend is toward tighter and tighter specs.
“It used to be five degrees. Now it`s two degrees, one degree. That`s becoming very critical for a lot of these semiconductor applications,” says James Smith, OEM sales manager for minienvironments for Clestra Cleanroom Inc. (North Syracuse, NY). Besides minienvironments made to enclose single tools, Clestra also makes entire cleanrooms.
Tight controls cropping up
Temperature controls as tight as ± 0.1 degrees Fahrenheit are now starting to crop up in such applications as photomask manufacturing and metrology labs. Such tight controls are not the norm, but many semiconductor facilities require half a degree control in photolithography areas and perhaps two degrees elsewhere. In the case of humidity, specifications may be ± 2 percent relative humidity or better in critical areas and ± 5 percent elsewhere. However, just because specs are tightening doesn`t mean that money to control the environment is getting any looser. Fortunately, vendors are aware of this and are addressing the issue.
“The cost of operating the HVAC equipment is always an issue that directly relates to profits,” points out Mike Hurley, marketing manager for Nortec Industries Inc. (Ogdensburg, NY). “Nortec has humidifiers that can use the least expensive source of energy available at the facility.” While a better price performance ratio in temperature and humidity equipment has certainly helped this effort to control the environment and cost, one of the keys has been the development of inexpensive and yet powerful computers and digital sensors.
“The controls are getting much cheaper. They are getting more accurate,” says Roberto Filho, an engineer with Cleanrooms West (Tustin, CA). Over the past decade the cost of controls has dropped from $10,000 to about $2,000. The shift from pneumatic to electronic technology is largely responsible for this decline. Because of the steady increase in semiconductor performance, this downward price trend should continue. Besides being less expensive, these controls from such industry leaders as Honeywell and Johnson Controls are also capable of doing much more than simply sounding an alarm.
“The control systems are getting so sophisticated that when you integrate that into your facilities management system, you`ve really got the ability to monitor what`s going on in your cleanroom, and to know when you`re out of spec, or when you`re approaching that out of spec condition and you can make adjustments,” adds Black & Veatch`s Jamison. In addition to offering greater control, these systems also make data logging much easier. This can be an important feature when a facility has to demonstrate ISO 9000 compliance. Data logging can also be useful for quality control and can contribute to yield analysis. As for future cleanroom HVAC challenges, there`s an overall problem of delivering air to the right specifications at an economical price. Some developments, such as Lepco`s use of gas powered chillers or Nortec`s humidifiers, solve the problem by finding a cheaper fuel. This trend helps, but the biggest savings comes from the drive for lower air flows. One of the ways to measure the efficiency of a cleanroom, according to Lepco`s Devloo, is to measure total system static pressure of the recirculation air side of the facility. In the past this number might be as high as four to five inches of pressure. Today, plants are being built with a scant one inch total static pressure.
Impact of pending developments
Working against this lowering of both energy and pressure in the semiconductor industry are two pending developments, which may happen at about the same time. The first is the leap from today`s 200 mm standard wafers to tomorrow`s 300 mm variety. While the tools for the larger wafer size are currently under development, the desire not to enlarge factory footprints may mean that the tools have no place to go but up, literally. That could have an impact on cleanroom HVAC requirements.
“Current rooms are typically 9 to 10 feet high,” comments Don Isaacson, president of the cleanroom design and construction firm ICOM Mechanical, Inc. (San Jose, CA). “New rooms may require 12- to 14-foot ceilings. This will cause an increase in air flow requirements to maintain the same level of cleanliness.”
Adding to these demands may be an increase in exhaust requirements. Venting more air increases the load on the make up air and getting it into the cleanroom adds to needed air flow. Tests run at Sematech as part of the I300I effort have reportedly not indicated any need for greater facilities, but a key point in this is that these tools are aimed at the current process technology. When actual production starts on 300 mm wafers, it`s likely to be done using a more advanced process. As a result there may be increased HVAC demands that are not related to the size of the wafers themselves. A process with a smaller feature size not only calls for finer filtration and tighter environmental controls but it may also require a different kind of particle/contaminant suppression. As feature sizes shrink, transistors and other electrical elements shrink. As a result, they may be increasingly susceptible to process contaminants, which can come from process chemicals as a result of cross contamination or from outgassing in such things as filter materials.
“There is a trend, and we will see more and more of this as days go on, to control the contaminants — the chemical impurity contaminants — that exist in a cleanroom environment,” says Paul L. Fowler, site and facilities engineering manager at Sematech (Austin, TX). One consequence of this may be that exhaust systems will be required to abate such contaminants and that in turn may drive up the exhaust load and hence the HVAC demands. Partly as a result of concerns like this there`s an increasing amount of attention being paid to mini- and micro environments where only the critical area immediately around wafers is stringently controlled for contaminants, temperature, and humidity, while the rest of the cleanroom adheres to a much looser standard. Interestingly, a similar idea is being bandied about for the pharmaceutical and medical device industry, which is struggling with its own issues of the need for increasingly antiseptic conditions. Such a switch would have a major impact on cleanroom HVAC systems. Some in the industry think this may be the way clean processing is done in the next century.
However, don`t look for a change anytime soon. With a modern semiconductor plant worth anywhere from $2 million to $20 million per day, no one wants to do anything that will make the plant miss any production or cause yields to drop. That potential dollar impact perhaps explains best the real challenge for those in the industry. The HVAC system is central to a facilities operation, and making sure that it`s always available may be the biggest task facing those in the industry. CR
Hank Hogan is a freelance writer based in Austin, TX. He has written for New Scientist, High Technology Careers, Electronic Components, and Multichannel News International. Prior to laying hands on a keyboard, he was a semiconductor process engineer and holds a process technology patent.
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The integration of the various air handling systems in a semiconductor cleanroom includes makeup air, clean recirculating air and the various process exhaust streams.
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View of the space envelope above a flat panel display cleanroom that moves 1 million cfm of air operating at one inch of total statistic pressure, designed and constructed by Lepco, Inc.
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Central plant with energy efficient gas fuel-based chillers are utilized in many of Lepco`s advanced technology cleanrooms.
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A mechanical mezzanine for a photomask shop with primary and secondary units for close tolerance temperature control (designed and constructed by Lepco Inc.).
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Clestra Cleanroom`s Asyst HVAC unit is among several modular units on the market being adopted in the most stringent cleanrooms.