No ‘absolutes’ for cleanliness in ISO 5 design
02/01/2008
A cleanroom should be planned as an environmental control system for best results
By Thomas E. Hansz, Facility Planning & Resources, Inc.
Not too long ago, I sat down with a prominent researcher in advanced materials to discuss his upcoming cleanroom requirements. He was currently working in an ISO 6 or Class 1000 laboratory. I asked him to describe the critical nature of the contamination control required for his research. His response was, “I need Class 100.” All right, but what about the type, size, and concentration of “killer particles” that would negatively affect his work? Again, his response was, “I need Class 100.” It took quite a while to convince him that cleanroom design does not begin with designing to the next higher level of cleanliness, but that understanding the contamination characteristics detrimental to the process leads to determining the appropriate level of cleanroom classification. We finally concluded with a sound understanding of his needs.
This example is not uncommon. When one equates cleanroom quality with cleanroom classification levels, it can become a very expensive trap. One of the main objectives of a cleanroom design should be to establish and maintain the appropriate level of contamination control. This will be a recurring theme in both this article and the next part on ISO 7 cleanrooms.
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Like art, the condition of “cleanliness” depends on the point of view of the cleanroom user. So it is important to bear in mind that there are no absolutes for cleanliness. Think of a cleanroom as an environmental control system (part fixed facility and part continuous process) that, once in use, requires regular monitoring and strict adherence to defined operational protocols for the life of the facility.
Therefore, unlike other facility types, there are three critical aspects required in establishing a successfully functioning cleanroom:
- Defining the cleanroom program (identifying contaminants; determining size and concentrations; establishing contamination control criteria; identifying process services; and defining facility systems)
- Developing design and construction (proceeding from a thoroughly developed definition; coordinating process systems with facility systems; developing operational protocols; and coordinating intent with realization)
- Implementing/operating the cleanroom (finalizing operational protocols; developing an appropriate cleaning and maintenance program; and instituting effective management over cleanroom operations)
Who needs an ISO 5 cleanroom?
So, how does all this apply to an ISO 5 cleanroom and who needs one? Generally, the aerospace/defense, microelectronics, and semiconductor industries have been the primary users of ISO 5 cleanrooms. This is by and large due to the scale of their work with silicon-based wafer fabrication. During the 1970s and into the 1980s, protecting silicon-based products during the fabrication process relied primarily on reducing the number of 0.5-??m size particles from coming into contact with the product. As technology advanced, the circuit geometries and the sophistication of chip design drove cleanrooms to even higher standards of contamination control. Another major factor in performing to higher standards was the competitiveness of that market. Production yields of 60 percent were no longer acceptable and since then have been dramatically increased due to improved cleanroom and process designs.
 Figure 2. Increases in equipment height generate taller ceiling heights and greater air volumes. Photo courtesy of Facility Planning & Resources, Inc. |
Those activities that are concerned primarily with materials such as silicon, gallium arsenide, germanium, and indium are usually the ones that will generally require an ISO 5 (or better) controlled environment. However, many bio-pharmaceutical processes are now carried out in ISO 5 cleanrooms. Traditionally, bacterial and fungal contaminants have been successfully filtered out at ISO 7 levels. Today, critical work with human hormones, vaccines, and processes that cannot be terminally sterilized are more often than not carried out in aseptic, ISO 5 environments. So, the once-held adage of “Semiconductor cleanrooms are Class 100 (ISO 5) and above and pharmaceutical cleanrooms are Class 1000 (ISO 6) and below” no longer holds true.
Determine contaminant size and concentration
Keep in mind that the level of contamination control is determined by the process and not by the industry. It all depends upon the research or production process involved and what will contaminate the product. Let’s go back to establishing the basic criteria, and consider the destructive elements to the process involved.
For an ISO 5 level of cleanliness, the size and distribution of contaminating particles will fall within the range of 0.1 ??m at a density of 100,000 particles per cubic meter to as large as 5.0 ??m at the density of 29 particles per cubic meter. A type C HEPA filter is constructed to filter out 99.97 percent of particles 0.3 ??m or larger.
 Figure 3. Complete coverage with cleanroom garments is essential at ISO 5. Photo courtesy of Facility Planning & Resources, Inc. |
At the same time, the rate of air change per hour must be sufficient to keep the contamination below the required range. Typically, an ISO 5 cleanroom operates with type C HEPA filters with filter face air velocities between 40 and 80 fpm. This rate will generate between 240 and 480 air changes per hour in the cleanroom.
 Figure 4. ISO 5 cleanroom prior to tool installation using conventional airflow design. Photo courtesy of Facility Planning & Resources, Inc. |
This large amount of recirculating air will require air-handling units and several of them may be located on a mechanical mezzanine. Air will generally be returned through a series of ducts rather than a return air plenum. A cleanroom operating at ISO 5 will require anywhere from four to nine times the amount of recirculated air as the next lower level of ISO 6. This large amount of recirculated air not only requires more constructed floor area for the mechanical mezzanine, it also necessitates a much taller building overall. To accommodate a 9- or 10-floor ceiling height in the cleanroom, the total building height can be an additional 15 to 20 ft higher. The photo of an ISO 5 chase space (Fig. 2) illustrates how much flexible ductwork is required for such an installation. Notice too, how open and uncluttered the chase floor is. The process cooling water, deionized (DI) water, and scrubbed exhaust ducts are located high so as not to interfere with the change-out of equipment and/or utilities. Consideration of the chase is as important as the cleanroom itself.
 Figure 5. Cleanrooms with an open layout promote worker safety and are more visually appealing. Photo courtesy of Facility Planning & Resources, Inc. |
Unlike many other facilities, a cleanroom must be designed from the inside out. That is, the process is the main driver of the design. The flow of filtered air keeps exposed products free from people-generated and environmental particles. Typically, the large volume of recirculating air has required ISO 5 cleanrooms to be on raised floors, allowing the air patterns to be vertical???also known as laminar flow. Where the process is such that the production space can be relatively narrow, side wall returns can be used in place of a raised floor for recirculating the air back to the fan units. To maintain a relatively uniform laminar flow, the distance between the side wall returns typically should be limited to 12 to 14 ft.
Develop operations and protocols
The design of an ISO 5 cleanroom cannot be successfully undertaken without including the workflow and the movement of people within the contamination controlled environment. It may sound overly simplistic, but the greatest amount of contamination comes from people and their degree of contamination increases in direct relationship to their movement. Yet this is perhaps one of the most overlooked aspects of cleanroom design. The process within the cleanroom should have a layout that minimizes movement.
It should also be afforded the proper ancillary spaces that support the level of contamination control. At a minimum three support spaces are required for an ISO 5 cleanroom: a spacious gown room, a materials and equipment wipe-down room, and a finished goods and waste removal room. A gown room should be provided with ample space for people entering and leaving simultaneously. Cleanroom workers should be able to enter, obtain their garments, and change without difficulty. Materials and equipment should enter the cleanroom through HEPA filtered spaces rather than through the gown room or primarily through pass-throughs. Finished work and wastes should also leave the cleanroom through a separate HEPA filtered space. This follows the concept of segregation as explained in ISO 14644-4, Design, Construction and Start-Up of Cleanrooms. The greater the extent to which these “levels of cleanliness” are built into the cleanroom and facility design, the more reliable the cleanroom design will be.
Several years ago, our firm was asked to bring an ISO 5 cleanroom into compliance with a company’s local fire regulations and, while we were doing that, to see if there was any way we could help the staff reduce the high level of particle counts they were recording. Our staff had set up make-shift workstations in a large, unused space just outside the gown room entrance. The noise from the gown room automatic door opening and closing soon became an annoyance. As we watched, company engineers went into the gown room, sticking their heads into the cleanroom and asking questions of the technicians. The particle problem became clear. What this facility needed was an ancillary space that would allow the engineers to communicate with the cleanroom workers without coming into the clean space. It didn’t have to be HEPA filtered; it just had to be adjacent with communicating capabilities. This is just another example of how a cleanroom is more than just an enclosed HEPA filtered space. It is process, facility, and operations.
Needless to say, the focus on the process then should necessitate the development of the operational protocols concurrently with the development of the cleanroom design. Again, this point is too often overlooked, and the protocols are developed after the design has been completed. The result is either some type of redesign to modify the cleanroom or having protocols that cannot be effectively implemented. In next month’s article, the importance of operational protocols and a maintenance program will be emphasized.
Today, the reliability of HEPA filters is practically taken for granted. Airborne particulates can be adequately removed from the air stream as a matter of routine. Other sources of contaminating particles, however, can include the process water and specialty gases. This aspect of an ISO 5 cleanroom design includes the utilities required for supporting the process on an ongoing basis. Electrical power, data systems, DI water, and high-purity gas delivery systems all need to be coordinated with the facility design. While it is not always true, generally speaking, the higher the level of contamination controls required, more associated process utilities will be needed and at a higher level of purity.
Therefore, the aspects of designing and building an ISO 5 cleanroom are characterized by a great deal of coordination and pre-planning. Providing the contamination controlled environment is paramount; however, it is but one part of the complete cleanroom project.
Thomas E. Hansz, AIA, is founder of Facility Planning & Resources, Inc., the director of advanced technology projects for Flad & Associates, and a member of the CleanRooms editorial advisory board.