A cleanroom requires the higher standards of construction than many other buildings. Construction materials used to build cleanrooms differ from those used in non-cleanroom construction for the following reasons:
Construction
- A cleanroom should be built with an airtight structure.
- The internal surface finish should be smooth and suitable for cleaning.
- The internal surface finish should be sufficiently tough to resist chipping or powdering when impacted or abraded.
- Some process chemicals, cleaning agents, disinfectants and water may attack or penetrate conventional finishes.
- In come cleanrooms, electro-dissipative construction materials will be required.
- In some cleanrooms, construction materials that give a minimum of ‘outgassing’ will be required.
Air Pressures
Cleanrooms are maintained at a positive pressure with respect to adjacent areas. If the construction is poor the joints not well sealed, then the structure may leak too much. It may then be necessary to pressurise the room by excessive amounts of ‘make up’ outside air. It is not good economic sense to waste air that has be to expensively filtered and air-conditioned. Attempting to seal up the structure during the ‘snagging’ part of the construction will not be as successful as making it airtight during construction. Containment rooms that are maintained at a negative pressure must be airtight, as dirty unfiltered air will be drawn into the room trough cracks, joints and at service penetration points in the fabric.
Materials
The materials that are used for the construction of a cleanroom should be smooth on the surface facing the inside of the cleanroom. There should be no pores or roughness that will retain contamination. The surface should be free of ledges and easily wiped free of any contamination that is deposited. The butts and joints, as seem from the inside of the cleanroom, should not show openings that may harbour, and then disperse, dirt.
Surfaces
The surface finish in a cleanroom must not break up easily and disperse chips or particles of material. A conventional material that is often used is houses and offices is plasterboard that is nailed to stud partitions and then painted. If this material is struck heavily the plaster powder will be released. This is unacceptable in a cleanroom and surface finishes must be suitably resistant to impact.
Cleanroom surfaces, especially floors, should be able to withstand liquids used in cleanrooms. Some processes use strong acids or solvents that will attack surfaces. Cleanrooms, where micro-organisms cause contamination, will require disinfection. Disinfectants are in an aqueous solution and to correctly disinfect the surface the contact time should be several minutes; water penetration can occur if construction materials are not suitable. Similar problems can occur when cleaning the cleanroom with surfactant solutions. It is therefore necessary to ensure that penetration of water does not occur.
Outgassing & Electrostatics
An electrostatic charge can be generated by rubbing two dissimilar surfaces together and can give two problems. Firstly, the charge will attract particles from the air and those deposited particles may give a contamination problem. Secondly, electrostatic discharge can cause some components to fail. Construction materials that minimise the problem may be necessary in some cleanrooms.
Some chemicals can ‘outgas’ from the materials used in construction of cleanrooms. These outgassed chemicals are often called ‘molecular contamination’. In some cleanrooms, such as those required for manufacturing optical surfaces or semiconductors, the deposition of these materials onto the product may be unacceptable. Where this is a problem, construction materials that do not outgas will be required.