Cleanroom floors: Supporting our future
04/01/2006
Design, construction must keep pace with evolving needs
By Brian P. Mazur, independent consultant, and Pete Spransy, AIA, vice president of research and development at Daw Technologies
A cleanroom floor system does much more than provide solid footing for the people working in the space. It permits the flow of clean air through the room, supports process tools, and provides easy access for required tool service as well as a sturdy surface to make equipment changes on. The cleanroom floor also provides for grounding of electrical systems and for conductive and static dissipative features to eliminate ESD buildup.
All this makes the cleanroom floor a critically important part of the overall cleanroom. Yet, it would be hard to find a facilities manager or tool installer without at least one horror story involving the use of raised flooring in a cleanroom. For example, anyone who has ever had to gain access to the underfloor space soon realizes the amount of strength needed-due to close tolerances-to lift the floor panels; then, when trying to replace the panels afterward, likely experienced the faint hope that the panels would go back into place without the need for someone to physically stomp on them. And, of course, there’s always the possibility of the panels cracking when tools are rolled over them.
It’s clear that, going forward, those as well as a number of other factors will demand a different approach to the design and construction of cleanroom flooring.
From past to present
The traditional approach to installing a cleanroom floor system has been to build it above an “open waffle slab,” which is a formed concrete structural matrix with openings that allow services and airflow to pass through. The cleanroom floor system is then assembled above this matrix using a series of pedestals located between the slab openings, thus supporting the floor system. The pedestals are normally on a 2- by 2-ft. pitch to match the 2- by 2-ft. floor or access panels.
The airflow requirements for the cleanroom have also been changing. Since the introduction of minienvironments in the production space, the amount of clean filtered air has been decreasing. This is due to the product being contained in front-opening unified pods (FOUPs) or a wafer transport system that will move the material to each minienvironment, where the function will be performed.
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Even though the cleanroom still requires filtered air, the quantity of filters has slowly been decreasing from 100 percent coverage to 35 percent coverage. As a result, this has changed the amount of air that will be flowing through the space and through the raised floor, reducing the number of perforated tiles needed and replacing them with more-solid panels throughout the cleanroom.
The traditional way that the floor system is grounded has not changed much in the industry. The normal application would be to apply clamps to the pedestals, which would be linked together and connected to the building ground. The static dissipative floor has also been achieved by utilizing a special vinyl material that allows for chemical resistance and is impregnated with carbon, allowing for the desired conductivity. The floor panels would rest on a vinyl/wire-mesh electrical grounding pad placed on each pedestal head. This assures that the system has a proper flow of current through all the components.
The floor panels themselves have traditionally been die-cast aluminum, which are light enough to be handled in the field. There are three types of cleanroom panels used: perforated, solid, and grated. Regardless of their type, however, the floor panels must be of adequate strength to support the anticipated weight requirements.
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Over the past 15 years, however, the weight-bearing requirement of cleanroom floors has been steadily increasing to accommodate similarly increasing production process requirements. In fact, support requirements have increased from approximately 1,200 lb. rolling load to up to 3,000 lb rolling load presently. To further complicate matters, the designs of the waffle slabs have also been evolving, reducing the amount of available open space. This has meant that instead of a floor pedestal spacing of 2- by 2-ft. spacing, they must be installed on a 2- by 4-ft. spacing.
To solve this problem, a steel understructure has been introduced to provide additional structural strength, vibration control, and support for the traditional 2- by 2-ft. pedestal spacing. The problem with this, of course, is that it has also added another step to the floor installation process, increasing both construction costs and schedule time.
New approaches
In future, all indications are that the trend toward larger spans and higher-strength flooring requirements will continue. As a result, current approaches may no longer be feasible. For example, die-cast floor panels will no longer be practical if the cost of larger dies is cost-prohibitive.
Fortunately, however, some creative solutions are now being introduced to the industry. One of these is the integration of the understructure and floor panels, combining elements of both systems into one solution. Already, great strides are already being made with this approach.
The approach can be visualized as an extruded frame forming a 4- by 4-ft. box with perpendicular cross members at the center points. Extruded aluminum has the advantage of being more ductile than die-cast aluminum.
In addition, increasing the extrusion depth for the load-bearing frame members as the specified loads increase is simply a matter of replacing the extrusion die, which is a fraction of the cost of replacing a die-cast die and a quarter of the lead time. The frame system then would connect to pedestals spanning the 4 feet required by the open waffle slab. Once the frame is in place, a cast aluminum plug (four total required for a 4- by 4-ft. frame) is dropped into the extruded frames already installed.
There are several advantages to this system:
- The understructure could be eliminated since the strength of the extruded frame would be designed to handle the required loading.
- The weight of the removable panels would be greatly reduced since the tool installers would only be lifting the center plug from the floor.
- The panels would not be “wedged” into the floor, thereby making removal difficult; instead, the frame would permit easy removal and replacement of the floor panels.
- The plugs would be interchangeable to accommodate grated, solid, or perforated panels, which would permit easy modifications to airflow and the relocation of power boxes and such.
- Every floor panel becomes an access panel.
- Walls would be located over the extruded frames so they would not interfere with accessibility under the floor or the relocation of panels since only the center plug would be relocated.
- The extruded frame would be designed to hang process piping and all other services under the floor.
- The extruded frame could also be developed to accommodate retrofit stiffeners where required for extreme loading requirements.
The new system allows the solid and perforated tiles (plugs) to be easily changed for accomplishing the desired quantity and path of airflow. In fact, the industry has been using Computational Fluid Dynamics (CFD) modeling to understand the effects of the airflow in the cleanroom floor space. Traditionally, the use of dampers below each of the perforated tiles would be used to try and balance the system for certification. That’s because the amount of free area of perforated tiles had to be maximized in order to limit the pressure drop, thereby contributing to the energy-efficiency of the mechanical system. The free area previously used for that purpose required a 22 percent opening for each perforated panel. Yet, with the increased weights that are rolling across the floor, it becomes difficult to accommodate this amount of free space so that it correlates to the strength supported by the die-cast floors for each panel.
 Figure 3: Shown here are two examples of a cleanroom floor system. Images courtesy of Daw Technologies. |
Since the new system has decreased the total airflow, the ability to have 18 percent free area would be permitted, and would be placed in a uniform pattern throughout the floor system. If an odd air pattern occurs after the tools are installed, changing perforated plugs with solid plugs would be very simple to adjust for fixing this problem.
The requirement for grounding and conductivity will still be needed in future cleanrooms. However, changes will be made in the selection of materials and mastic that are used to eliminate any undesirable outgassing that would contaminate the space.
It’s clear there is room for improvement in traditional cleanroom floor systems. As cleanroom requirements and technology change, it is easy to see how important it will be for cleanroom floor systems to evolve as well.
Brian P. Mazur is an independent consultant and has been in this industry for 20 years. He has worked for companies including Industrial Design Corp. (IDC), Cleanpak International, and Daw Technologies, and has an extensive background in the cleanroom envelope.
Pete Spransy, AIA, is vice president of R&D at Daw Technologies, where he develops new products for use in semiconductor cleanrooms. At Daw for 17 years, Spransy has published a number of industry-related articles, made technical presentations at the CleanRooms CCT conference, and currently holds five utility patents and has a number of other patents pending at the United States Patent and Trademark Office.