Construction of a typical Clean Room envelope centres around first selecting the most suitable partition system. A ‘Monobloc’ Partition system offers the ability to fit ‘flushed in’ service points with connections routed & concealed within cavity whilst a ‘Bi-Panel’ Partition system provides the ability to access fully exposed integral services from the ‘dirty side’ without compromising Validation of the room.
Doors and Windows
Most cleanroom doors are made either from plastic covered wood, or are faced with mild steel that is suitably treated and painted. To assist cleaning and minimise hand contamination, door fittings such as door handles should be dispensed with where possible.
Doors would usually be hung so they would be kept closed by the positive pressure of the cleanroom. However, exceptions may be required so that personnel can pass through by pushing the door with their body. In that case, a self-closure should be fitted.
Doors are often manufactured with fine tolerances to minimise air leakage. This is a particularly useful property in a negatively pressurised containment room to minimise the entry of contamination from outside they room. However, in positively pressurised rooms this may be unnecessary, especially if the design requires air to flow through the doorway when open.
Doors may be glazed, which is particularly useful in the materials transfer airlock to see if it is occupied. If glazing is required, the method of glazing (I.e. gaskets) should allow easy cleaning. Doors that are completely made of glass are also available.
Windows are fitted to cleanrooms. These are useful in dissuading visitors who have some to see the cleanroom from entering; they are also necessary to allow management to see what is going on in the room without having the bother of changing into cleanroom clothing. Their number should, however be kept to a minimum. Windows should be glazed using easy-to-clean gaskets.
Concrete would be used as a common floor foundation in almost all cleanrooms. A smooth, impervious and durable surface would then be added. This should be resistant to spilled chemicals and be slip resistant. It may also be necessary in some situations to use materials that have good electrostatic or minimal outgassing properties.
A commonly used method of covering the floor is with vinyl sheeting that is welded together. A less common surface covering is terrazzo, which is very durable and is suitable in some situations.
Vinyl sheeting can be manufactured to be electrically conductive for use where this is desirable. In unidirectional flow rooms, where the air passes through the floor, it would be common to find that the floor is made of tiles placed on pedestals. If the room is used for semiconductor manufacturing, vinyl may not be an acceptable surface material because of the problem of ‘outgassing’.
The floor-to-wall joints would normally be coved with some type of corner profile. An exception to this requirement may be where machines are used to clean the floor.
It would be unusual to have anything but a false ceiling in a cleanroom. The need for access to air conditioning ducts and other gas and electricity services, and the use of terminal filters and recessed lights that are integrated with the ceiling, dictates the use of a suspended or supported ceiling.
In conventionally ventilated cleanrooms the ceiling would be either of the suspended or self-supported type. Into the space between the support channels, the light fittings and air filter housings would be placed, the remaining areas being fitted with blank panels. Ceilings are often provided suitable for walking on.
All of the light fittings, filter modules and blank panels must be well fitted to ensure a minimum of air leakage. If top-quality components are not used, mastic or other means can be used to bed down the various components. Access to change lamps can be provided from above (i.e. ouside of cleanroom).
In unidirectional flow cleanrooms, most of the ceiling is covered with filters. In that case, the ceiling is built of aluminium extruded channels and the filters are inserted into the channels.
Outgassing and Electrostatic Properties
In semiconductor and similar areas, the use of construction materials that allow chemicals to ‘outgas’ can give contamination problems. Cleanroom flooring will often be plastic sheeting. Vinyl flooring is made of PVC with other materials added, one of these being a plasticiser to make it flexible. These plasticisers will outgas and are therefore no a good choice for semiconductor cleanrooms, especially as the floor has a large surface area. Some types of wall and ceiling panels may also be considered a risk. Another material that is considered a risk is silicone sealant used for sealing various joints. More suitable sealants can be purchased. Adhesives may also be a problem
Test methods are available to ascertain the outgassing properties of materials. These can accelerate the ‘outgassing’ of contamination and assess the amount that will condense onto a surface.
If static charge is considered a problem, then the construction materials should be conductive and earthed to dissipate the electrostatic charge. However, if there is no electrical resistance there will be a danger to personnel through an electrical accident. It has been suggested that the electrical resistance should therefore be between 106 and 109 ohms/cm.