Hal Amick, Ph.D., P.E., Colin Gordon & Associates, San Bruno, CA, USA
Vibration has long been recognized as a contaminant in a fab. The vibrations can come from many sources both within and exterior to the facility. The most significant interior sources are the plant’s mechanical systems, personnel activities, and sometimes the tools themselves. Exterior sources include nearby traffic, rail lines, construction, and mechanical equipment in neighboring buildings. To the extent possible, vibration control features, such as spring isolators, are built into the mechanical systems themselves at the time the facility is designed. The exterior vibrations are considered during site selection. Typically, it is possible to achieve a very quiet vibration environment in the as-built state of a fab simply through careful design and construction. However, it is not unusual for the owner and the users themselves to degrade the vibration environment over time with user-installed equipment or certain process tools. Gendreau and Amick (2004) call this tendency “maturation,” and have demonstrated that the vibrations in the fab can more than double due to this effect. For quite some time, designers and consultants have discussed means by which future vibrations from “unplanned” sources might be mitigated via design or construction. This article presents one possible approach for using the building itself to mitigate vibrations. Concrete is the structural material of choice for the vibration-sensitive areas in a fab, via waffle slabs or concrete two-way grillages. Some benefits arise from altering the vibration damping characteristics of the concrete itself. A recent research project examined various options for concrete damping modification, finding that the use of a particular family of admixtures was the most straightforward approach. We will look at damping itself and the role it can play in vibration control. Several methods of altering the damping of concrete have been studied, but the most efficacious is the use of a particular group of polymer admixtures. These impart some significant improvements under certain circumstances, but concrete damping modification will not resolve all problems. We will look at where this approach is useful, and where it is not justified or cost-effective.