The fabrication of semiconductor microdevices takes place in cleanrooms to protect the complex circuits from contaminations which can impact operability. The cleanrooms are classified by size of the particles and their amount per cubic foot (cu.ft.) or per cubic meter (ISO standard):
|Klasse ↓||≥ 0,1 µm||≥ 0,2 µm||≥ 0,3 µm||≥ 0,5 µm||≥ 1,0 µm||≥ 5,0 µm|
In a class 3 cleanroom, for example, there may only be 1,000 particles with a diameter of ≥ 0.1 microns, 237 with a diameter of ≥ 0.2 microns and so on. In an operating room the cleanroom class is generally 2 or 3. In a volume of 1 m3 air in a city there are e.g. 400 million particles with a size of 5 microns.
In cleanrooms for the semiconductor industry the air is cleaned with super fine filters which is blown through the ceiling and exhausted through holes in the floor. This laminar flow forwards particles top down. To avoid contaminations from the outside there's always some over pressure inside the cleanroom. Since the production line heatens the air, the air rises and can cause turbulences. To avoid this, the laminar flow can be in reversed direction, from the floor to the ceiling. This is primarily done if the wafers are transported inside of sealed boxes which dock directly to the facility so that there is no possibility for contaminations to reach the wafers. This boxes are called FOUPs (Front Opening Unified Pod).
Since the wafers are completely separated from the ambient air in the cleanroom, there can be another cleanroom class inside the FOUP - a so-called mini environment. For this reason the class inside the boxes can be 1 or 2 to protect the wafers from particles, while the class in the cleanroom itself is only 5, for instance. This is much more efficient since not the entire cleanroom needs to be super clean but only the relatively tiny transport boxes.
Illustration of a class 1 cleanroom
In the illustration above the production line is only the region between the light red colored ventilating system and the yellow colored bottom area. Beneath there is the basement with supply units (pumps, generators etc.), on top there are the air filters. To minimize turbulences the huge cleanroom area often is separated into individual corridors. Many cleanrooms are surrounded by a so-called greyroom (service room) in which the production tools are standing. In this case only the locks to bring in the wafers and the control panels are accessible from inside the cleanroom.
The staff wears special cleanroom suits (often called bunny suits) which do not emit particles. Depending on the requirements the suit covers the complete body including head and face. In addition there are special boots, underclothing and gloves. Next to the entrance there are sometimes air showers to blow everything clear.