Constructing Vibration-Free Cleanrooms
Construction will be a key component as vibration control becomes an increasingly important factor for cleanrooms which house photolithography processes.
By Dan Mahoney
Amajor topic of discussion among the world`s leading semiconductor manufacturers is the possibility of mass producing an advanced 300-mm wafer by the year 2000. To accomplish this, semiconductor manufacturers will need to build smaller chips using deepultraviolet
photolithography and other light sources that have the ability to emit shorter wave lengths to accurately reproduce the chip`s features. The slightest vibration in or surrounding a cleanroom that uses photolithography in the manufacturing process could severely alter the final image transposed onto the wafer.
As a result of this advancement in technology, there has been a surge of super-cleanroom facilities being designed with low-level degrees of vibration. In fact, producers of equipment and chemicals used in the semiconductor manufacturing process, are now constructing state-of-the-art, vibration-free cleanrooms as testing facilities to qualify their products.
As the emphasis on vibration control becomes a more prevalent feature in the construction of today`s cleanrooms, facility managers, engineers and cleanroom contractors must work closely together to ensure that the smallest degree of vibration doesn`t affect the laboratories. Therefore, contractors must determine how vibration is generated; the proximity of that vibration; how to eliminate vibration, and how to construct the facility without jeopardizing the regulation of temperature controls, relative humidity, and gas handling/delivery systems. To accomplish this, today`s construction manager must eliminate external and internal sources of vibration through accurate site analysis, solid construction, and a thorough understanding of today`s and tomorrow`s technologies.
Determining tolerance levels
Obtaining an understanding as to what is and what is not a tolerable level of vibration based upon the specifications of the project is the first step construction managers must take before designing a super-cleanroom. Knowing these provisions determines what problem areas need to be uncovered and resolved on the site before construction begins.
The optimum method for establishing a tolerable vibration level is to determine what internal equipment such as steppers and coaters will be affected by any surrounding motion. Another goal is to uncover what new products equipment manufacturers are developing in order to predict how the cleanroom should be constructed to adapt to those potential developments.
Investigating the site
Once the tolerance level is determined, the next step is to investigate the site for sources of vibration that will disrupt any recommended levels. These sources are broken down into external and internal categories. An internal source of vibration is one that exists within the actual facility, and includes foot traffic, robotics, power systems, elevators, carts, as well as sources directly affecting the tool such as operations, vacuum pumps, sounds, the flow of clean air. Whereas, external sources of vibration include environmental equipment, ground vibrations, infrasound, traffic vibrations, wind, and seismic vibrations. The construction process has to manage each of these vibration sources and their path to the cleanroom table. Certain vibration sources could be isolated at or near their source while others are minimized to acceptable levels based on the cleanroom`s structural construction.
Vibration isolation equipment
Before construction begins, we also must evaluate which sources of vibration can be eliminated or reduced through the use of vibration isolation equipment. This provides guidelines for the kinds of materials needed for the actual construction. Vibration isolation equipment uses platform with springs which are placed under vibrating equipment to absorb the resonance before it escapes into the structure. This technique is not only used for operating equipment just outside the actual lab, it is also a technique used on elements inside the lab, such as air ducts and cleanroom air handling systems. Though it is easy to find large areas of vibration such as emergency generators, chillers, compressors, and other equipment used to operate and control the environment of the cleanroom, the project becomes more complex as more areas of vibration that can`t be eliminated through vibration isolation are discovered. This is where the construction process comes in.
The most complicated area in the construction process is discovering how methods and materials used to construct a vibration-free cleanroom affect the standard requirements for outgassing, humidity, and temperature control specifications.
Tight room temperature and humidity controls require sealing the cleanroom from outside environments and not importing vibration to the cleanroom floors and walls. For instance, hot and cold water valves emit vibration as water passes through them as they throttle cooling water to maintain room temperature and humidity. Vibration levels of 5,000 micro-inches per second were measured at control valves of existing facilities, yet they were too numerous to install vibration control systems. Therefore, the walls and floors of the cleanroom must be constructed to ensure no transfer of vibration to production tools. This is handled in the construction process through selecting vibration absorbing materials for all room penetrations. Each penetration required different sealing systems that would not cause wall vibration (see Photo on page 33).
Another obstacle, especially for Class 1 cleanrooms, is the higher standards or the elimination of outgassing. Materials that are often used in a Class 10 cleanroom, for instance, can`t be used in a Class 1 cleanroom. This restriction eliminates a number of standard materials such as standard gel seals, gas heating materials, and construction adhesives. As a result, the construction manager must investigate alternative materials which will keep the room clean and vibration free according to the client`s standards and specifications.
To construct a solid, vibration-free structure, the cleanroom`s process floor needs to be built as its own free-standing structure. When the foundation is poured, vertical steel beams are placed into four feet of concrete to ensure a solid structure. These steel beams are then connected at the top by additional beams. The beams are then cross-braced to keep the structure steady, since the frame will be supporting a thick concrete floor.
Once there`s a solid steel frame in position, the floor of the cleanroom is poured into steel trays that lay on top of the steel structure. This provides the floor of the cleanroom. By constructing this table-like structure, the cleanroom now stands on the second floor. Under the table is the first floor that could house the equipment needed to maintain the lab, such as the generators and chillers. Or the space could be used for another laboratory.
Traditionally, cleanrooms have been built on the first floor. However, now with the use of new technology like photolithography, it`s essentially impossible to build a vibration-free cleanroom that sits on a compressible surface as opposed to sitting in non-compressible surfaces such as granite, bedrock and concrete.
Now that the floor is in place, the walls of the cleanroom are constructed. The walls cannot touch the floor because they are also a source of vibration. Instead, vertical steel is again placed into the foundation surrounding the cleanroom and the walls are hung from the beams, just like a standard skeletal construction process. The walls are close enough to the floor to create a box structure, but not close enough to touch.
As discussed earlier, any activity that emits vibration, such as opening or shutting a door can be a trouble area in the construction process. This means that any nearby building which houses equipment such as generators and fork trucks are also intolerable to the site. This creates an unusual problem for the construction process because cleanroom additions are traditionally attached to an existing building. For example, in a standard addition project you are essentially building only three walls — the fourth is the wall of the existing building. Since cleanrooms can`t come into contact with any neighboring buildings, an alternative strategy is needed. Building the cleanroom with four walls, within close proximity to the existing building, but not touching it, ensures that people can enter the cleanroom from the existing structure without the cleanroom being effected by the vibration. This is done via a platform on the second floor which connects the two individual structures.
It is also important to take into consideration that structures settle and expand over time. By leaving enough room between the buildings, the buildings will never come into contact with each other. Thus, a vibration-free structure is created.
The more commonplace vibration control becomes, the more construction becomes a critical component to cleanroom facility stability. This is not just a requirement for end manufacturers of semiconductor chips, it will soon be a requirement for those companies producing processing tools and chemicals used in the manufacturing process. n
Dan Mahoney is vice president of sales engineering at JMC Environmental Systems in Concord, MA, and has over 25 years of experience in the HVAC mechanical systems industry. At JMC Environmental Systems, Mahoney manages the technical systems design and procurement functions associated with cleanrooms, laboratories and industrial facilities and assists customers in defining their project requirements.
Sealing systems prevent vibration. All photos courtesy of JMC Environmental Systems.
Cleanroom mechanical systems chase.
Initial stage of cleanroom construction.
Final stage of construction of Class 1 cleanroom.