Cleanroom

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Cleanroom used for the production of microsystems. The yellow lighting is necessary for photolithography, to prevent unwanted exposure of photoresist to light of shorter wavelengths.
Cleanroom from outside
Entrance to a cleanroom with no air shower
Cleanroom for microelectronics manufacturing with fan filter units installed in the ceiling grid
Cleanroom cabin for precision measuring tools
Typical cleanroom head garment

A cleanroom or clean room is an environment, typically used in manufacturing or scientific research, that has a low level of environmental pollutants such as dust, airborne microbes, aerosol particles and chemical vapors. More accurately, a cleanroom has a controlled level of contamination that is specified by the number of particles per cubic meter at a specified particle size. To give perspective, the ambient air outside in a typical urban environment contains 35,000,000 particles per cubic meter in the size range 0.5 μm and larger in diameter, corresponding to an ISO 9 cleanroom, while an ISO 1 cleanroom allows no particles in that size range and only 12 particles per cubic meter of 0.3 μm and smaller.

Contents

Overview

Cleanrooms can be very large. Entire manufacturing facilities can be contained within a cleanroom with factory floors covering thousands of square meters. They are used extensively in semiconductor manufacturing, biotechnology, the life sciences and other fields that are very sensitive to environmental contamination.

The air entering a cleanroom from outside is filtered to exclude dust, and the air inside is constantly recirculated through high-efficiency particulate air (HEPA) and/or ultra-low penetration air (ULPA) filters to remove internally generated contaminants.

Staff enter and leave through airlocks (sometimes including an air shower stage), and wear protective clothing such as hoods, face masks, gloves, boots and coveralls.

Equipment inside the cleanroom is designed to generate minimal air contamination. Only special mops and buckets are used. Cleanroom furniture is designed to produce a minimum of particles and to be easy to clean.

Common materials such as paper, pencils, and fabrics made from natural fibers are often excluded, and alternatives used. Cleanrooms are not sterile (i.e., free of uncontrolled microbes)[1]; only airborne particles are controlled. Particle levels are usually tested using a particle counter.

Some cleanrooms are kept at a positive pressure so that if there are any leaks, air leaks out of the chamber instead of unfiltered air coming in.

Some cleanroom HVAC systems control the humidity to low levels, such that extra equipment ("ionizers") is necessary to prevent electrostatic discharge (ESD) problems.

Low-level cleanrooms may only require special shoes, with completely smooth soles that do not track in dust or dirt. However, for safety reasons, shoe soles must not create slipping hazards. Access to a cleanroom is usually restricted to those wearing a cleanroom suit.[2]

In cleanrooms in which the standards of air contamination are less rigorous, the entrance to the cleanroom may not have an air shower. There is an anteroom (known as a "gray room"), in which clean-room clothing must be put on, from which a person can walk directly into the room (as seen in the photograph on the right).

Some manufacturing facilities do not use fully classified cleanrooms, but use some cleanroom practices to maintain their contamination requirements.[3][4]

Air flow principles

Air flow pattern for "Turbulent Cleanroom"
Air flow pattern for "Laminar Flow Cleanroom"

Cleanrooms maintain particulate-free air through the use of either HEPA or ULPA filters employing laminar or turbulent air flow principles. Laminar, or unidirectional, air flow systems direct filtered air downward in a constant stream towards filters located on walls near the cleanroom floor or through raised perforated floor panels to be recirculated. Laminar air flow systems are typically employed across 80 percent of a cleanroom ceiling to maintain constant air processing. Stainless steel or other non-shed materials are used to construct laminar air flow filters and hoods to prevent excess particles entering the air. Turbulent, or non-unidirectional, air flow uses both laminar air flow hoods and non-specific velocity filters to keep air in a cleanroom in constant motion, although not all in the same direction. The rough air seeks to trap particles that may be in the air and drive them towards the floor, where they enter filters and leave the cleanroom environment.[5]

Cleanroom classifications

Cleanrooms are classified according to the number and size of particles permitted per volume of air. Large numbers like "class 100" or "class 1000" refer to FED-STD-209E, and denote the number of particles of size 0.5 µm or larger permitted per cubic foot of air. The standard also allows interpolation, so it is possible to describe, for example, "class 2000".

A discrete-particle-counting, light-scattering instrument is used to determine the concentration of airborne particles, equal to and larger than the specified sizes, at designated sampling locations.

Small numbers refer to ISO 14644-1 standards, which specify the decimal logarithm of the number of particles 0.1 µm or larger permitted per cubic metre of air. So, for example, an ISO class 5 cleanroom has at most 105 = 100,000 particles per cubic metre.

Both FS 209E and ISO 14644-1 assume log-log relationships between particle size and particle concentration. For that reason, zero particle concentration does not exist. The table locations without entries are non-applicable combinations of particle sizes and cleanliness classes, and should not be read as zero.

Because 1 m³ is approximately 35 ft³, the two standards are mostly equivalent when measuring 0.5 µm particles, although the testing standards differ. Ordinary room air is approximately class 1,000,000 or ISO 9.[6]

US FED STD 209E cleanroom standards

Classmaximum particles/ft³ISO
equivalent
≥0.1 µm≥0.2 µm≥0.3 µm≥0.5 µm≥5 µm
1357.5310.007ISO 3
103507530100.07ISO 4
1003,5007503001000.7ISO 5
1,00035,0007,50030001,0007ISO 6
10,000350,00075,00030,00010,00070ISO 7
100,0003.5×106750,000300,000100,000700ISO 8

US FED STD 209E was officially cancelled by the General Services Administration of the US Department of Commerce November 29, 2001,[7][8] but is still widely used.

ISO 14644-1 cleanroom standards

Classmaximum particles/m³FED STD 209E
equivalent
≥0.1 µm≥0.2 µm≥0.3 µm≥0.5 µm≥1 µm≥5 µm
ISO 1102.371.020.350.0830.0029
ISO 210023.710.23.50.830.029
ISO 31,000237102358.30.29Class 1
ISO 410,0002,3701,020352832.9Class 10
ISO 5100,00023,70010,2003,52083229Class 100
ISO 61.0×106237,000102,00035,2008,320293Class 1,000
ISO 71.0×1072.37×1061,020,000352,00083,2002,930Class 10,000
ISO 81.0×1082.37×1071.02×1073,520,000832,00029,300Class 100,000
ISO 91.0×1092.37×1081.02×10835,200,0008,320,000293,000Room air

BS 5295 cleanroom standards

 maximum particles/m³
Class≥0.5 µm≥1 µm≥5 µm≥10 µm≥25 µm
Class 13,000 000
Class 2300,000 2,00030 
Class 3 1,000,00020,0004,000300
Class 4  200,00040,0004,000

BS 5295 Class 1 also requires that the greatest particle present in any sample does not exceed 5 μm.[9]

GMP EU classification

Classmaximum particles/m³[10]
At RestAt RestIn OperationIn Operation
0.5 µm5 µm0.5 µm5 µm
Class A3,520203,50020
Class B3,52029352,0002,900
Class C352,0002,9003,520,00029,000
Class D3,520,00029,000n/an/a

See also

References

External links