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Cleanroom

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Cleanroom used for the production of microsystems. The yellow (red-green) 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 a facility ordinarily utilized as a part of specialized industrial production or scientific research, including the manufacture of pharmaceutical items and microprocessors. Cleanrooms are designed to maintain extremely low levels of particulates, such as dust, airborne organisms, or vaporized particles. Cleanrooms typically have an cleanliness level quanitified by the number of particles per cubic meter at a predetermined molecule measure. The ambient outdoor air in a typical urban area contains 35,000,000 particles for each cubic meter in the size range 0.5 μm and bigger in measurement, equivalent to an ISO 9 cleanroom, while by comparison an ISO 1 cleanroom permits no particles in that size range and just 12 particles for each cubic meter of 0.3 μm and smaller.

Contents

HistoryEdit

The modern cleanroom was invented by American physicist Willis Whitfield.[1] As employee of the Sandia National Laboratories, Whitfield created the initial plans for the cleanroom in 1960.[1] Prior to Whitfield's invention, earlier cleanrooms often had problems with particles and unpredictable airflows. Whitfield designed his cleanroom with a constant, highly filtered air flow to flush out impurities.[1] Within a few years of its invention in the 1960s, Whitfield's modern cleanroom had generated more than 50 billion USD in sales worldwide.[2][3]

The majority of the integrated circuit manufacturing facilities in Silicon Valley were made by three companies: MicroAire, PureAire, and Key Plastics. These competitors made laminar flow units, glove boxes, clean rooms and air showers, along with the chemical tanks and benches used in the 'Wet Process' building of integrated circuits. These three companies were the pioneers of the use of Teflon for airguns, chemical pumps, scrubbers, water guns, and other devices needed for the production of integrated circuits. William (Bill) C. McElroy Jr. worked as engineering manager, drafting room supervisor, QA/QC, and designer for all three companies and his designs added 45 original patents to the technology of the time. McElroy also wrote a four page article for MicroContamination Journal, wet processing training manuals, and equipment manuals for wet processing and clean rooms.[4]

OverviewEdit

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. There are also modular cleanrooms.[5]

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 particulate 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 is 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);[6] only airborne particles are controlled. Particle levels are usually tested using a particle counter and microorganisms detected and counted through environmental monitoring methods.[7][8] Polymer tools used in cleanrooms must be carefully determined to be chemically compatible with cleanroom processing fluids [9] as well as ensured to generate a low level of particle generation.[10]

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

Some cleanroom HVAC systems control the humidity to such low levels that extra equipment like air ionizers are required to prevent electrostatic discharge 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.

In cleanrooms in which the standards of air contamination are less rigorous, the entrance to the cleanroom may not have an air shower. An anteroom (known as a "gray room") is used to put on clean-room clothing.

Some manufacturing facilities do not use fully classified cleanrooms, but use some practices or technologies typical of cleanrooms to meet their contamination requirements.

In hospitals, theatres are similar to cleanrooms for surgical patients' operations with incisions to prevent any infections for the patient.

Air flow principlesEdit

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 or in horizontal direction 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% of a cleanroom ceiling to maintain constant air processing. Stainless steel or other non shedding 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 nonspecific 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. US FDA and EU have laid down guidelines and limit for microbial contamination which is very stringent to ensure freedom from microbial contamination in pharmaceutical products.[11][12]

Personnel contamination of cleanroomsEdit

The greatest threat to cleanroom contamination comes from the users themselves.[13] In the healthcare and pharmaceutical sectors, control of microorganisms is important, especially microorganisms likely to be deposited into the air stream from skin shedding. Studying cleanroom microflora is of importance for microbiologists and quality control personnel to assess changes in trends. Shifts in the types of microflora may indicate deviations from the “norm” such as resistant strains or problems with cleaning practices.

In assessing cleanroom microorganisms, the typical flora are primarily those associated with human skin (Gram-positive cocci), although microorganisms from other sources such as the environment (Gram-positive rods) and water (Gram-negative rods) are also detected, although in lower number. Common bacterial genera include Micrococcus, Staphylococcus, Corynebacterium, and Bacillus, and fungal genera include Aspergillus and Pencillin.[8]

Cleanroom classification and standardizationEdit

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; for example SNOLAB is maintained as a class 2000 cleanroom.

A discrete, light-scattering airborne particle counter 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 m3 of air. So, for example, an ISO class 5 cleanroom has at most 105 particles/m3.

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. Some classes do not require testing some particle sizes, because the concentration is too low or too high to be practical to test for, but such blanks should not be read as zero.

Because 1 m3 is about 35 ft3, the two standards are mostly equivalent when measuring 0.5 µm particles, although the testing standards differ. Ordinary room air is around class 1,000,000 or ISO 9.[14]

ISO 14644-1 and ISO 14698Edit

ISO 14644-1 and ISO 14698 are non-governmental standards developed by the International Organization for Standardization (ISO).[15] The former applies to clean rooms in general (see table below); the latter to cleanrooms where biocontamination may be an issue.

ISO 14644-1 defines the maximum concentration of particles per class and per particle size with the following formula[16]

 

Where   is the maximum concentration of particles in a volume of 1m  of airborne particles that are equal to, or larger, than the considered particle size which is rounded to the nearest whole number, using no more than three significant figures,   is the ISO class number,   is the size of the particle in  m and 0.1 is a constant expressed in  m. The result for standard particle sizes is expressed in the following table.

Class Maximum particles/m3 a FED STD 209E
equivalent
≥0.1 µm ≥0.2 µm ≥0.3 µm ≥0.5 µm ≥1 µm ≥5 µm
ISO 1 10b d d d d e
ISO 2 100 24b 10b d d e
ISO 3 1,000 237 102 35b d e Class 1
ISO 4 10,000 2,370 1,020 352 83b e Class 10
ISO 5 100,000 23,700 10,200 3,520 832 d,e,f Class 100
ISO 6 1,000,000 237,000 102,000 35,200 8,320 293 Class 1,000
ISO 7 c c c 352,000 83,200 2,930 Class 10,000
ISO 8 c c c 3,520,000 832,000 29,300 Class 100,000
ISO 9 c c c 35,200,000 8,320,000 293,000 Room air
a All concentrations in the table are cumulative, e.g. for ISO Class 5, the 10 200 particles shown at 0,3 μm include all particles equal to and greater than this size.

b These concentrations will lead to large air sample volumes for classification. Sequential sampling procedure may be applied; see Annex D.
c Concentration limits are not applicable in this region of the table due to very high particle concentration.
d Sampling and statistical limitations for particles in low concentrations make classification inappropriate.
e Sample collection limitations for both particles in low concentrations and sizes greater than 1 μm make classification at this particle size inappropriate, due to potential particle losses in the sampling system.
f In order to specify this particle size in association with ISO Class 5, the macroparticle descriptor M may be adapted and used in conjunction with at least one other particle size. (See C.7.)

US FED STD 209EEdit

US FED-STD-209E was a United States federal standard. It was officially cancelled by the General Services Administration on November 29, 2001,[17][18] but is still widely used.[19]

Class Maximum particles/ft3 ISO
equivalent
≥0.1 µm ≥0.2 µm ≥0.3 µm ≥0.5 µm ≥5 µm
1 35 7.5 3 1 0.007 ISO 3
10 350 75 30 10 0.07 ISO 4
100 3,500 750 300 100 0.7 ISO 5
1,000 35,000 7,500 3000 1,000 7 ISO 6
10,000 350,000 75,000 30,000 10,000 70 ISO 7
100,000 3.5×106 750,000 300,000 100,000 830 ISO 8

EU GMP classificationEdit

EU GMP guidelines are more stringent than others, requiring cleanrooms to meet particle counts at operation (during manufacturing process) and at rest (when manufacturing process is not carried out, but room AHU is on).

Class Maximum particles/m3[20]
At Rest In Operation
0.5 µm 5 µm 0.5 µm 5 µm
Grade A 3,520 20 3,520 20
Grade B 3,520 29 352,000 2,900
Grade C 352,000 2,900 3,520,000 29,000
Grade D 3,520,000 29,000 Not defined Not defined

BS 5295Edit

BS 5295 is a British Standard.

Class Maximum particles/m3
≥0.5 µm ≥1 µm ≥5 µm ≥10 µm ≥25 µm
Class 1 3,000   0 0 0
Class 2 300,000   2,000 30  
Class 3   1,000,000 20,000 4,000 300
Class 4     200,000 40,000 4,000

BS 5295 Class 1 also requires that the greatest particle present in any sample can not exceed 5 μm.[21] BS 5295 has been superseded, withdrawn since the year 2007 and replaced with "BS EN ISO 14644-6:2007".[22]

See alsoEdit

ReferencesEdit

  1. ^ a b c Yardley, William (2012-12-04). "Willis Whitfield, Clean Room Inventor, Dies at 92". The New York Times. Retrieved 2013-06-22.
  2. ^ "Sandia physicist, cleanroom inventor dies at 92". KWES. Associated Press. 2012-11-26. Retrieved 2012-12-03.
  3. ^ "Willis Whitfield - Father of the Cleanroom" (PDF). Cleanroom online. September 2015. Retrieved 2016-05-18.
  4. ^ William (Bill) C. McElroy Jr., MicroAire Engineering Manager and acting VP; Kay Plastics Engineering Manager; PureAire Drafting Room Manager
  5. ^ What is a modular cleanroom?
  6. ^ In NASA’s Sterile Areas, Plenty of Robust Bacteria New York Times, 9. October 2007
  7. ^ Sandle, T (November 2012). "Application of quality risk management to set viable environmental monitoring frequencies in biotechnology processing and support areas". PDA J Pharm Sci Technol. 66 (6): 560–79. doi:10.5731/pdajpst.2012.00891.
  8. ^ a b Sandle, T (November 2011). "A review of cleanroom microflora: types, trends, and patterns". PDA J Pharm Sci Technol. 65 (4): 392–403. doi:10.5731/pdajpst.2011.00765.
  9. ^ Heikkinen, Ismo T.S.; Kauppinen, Christoffer; Liu, Zhengjun; Asikainen, Sanja M.; Spoljaric, Steven; Seppälä, Jukka V.; Savin, Hele (2018-10). "Chemical compatibility of fused filament fabrication-based 3-D printed components with solutions commonly used in semiconductor wet processing". Additive Manufacturing. 23: 99–107. doi:10.1016/j.addma.2018.07.015. ISSN 2214-8604. Check date values in: |date= (help)
  10. ^ Pasanen, T.P.; von Gastrow, G.; Heikkinen, I.T.S.; Vähänissi, V.; Savin, H.; Pearce, J.M. (2019-01). "Compatibility of 3-D printed devices in cleanroom environments for semiconductor processing". Materials Science in Semiconductor Processing. 89: 59–67. doi:10.1016/j.mssp.2018.08.027. ISSN 1369-8001. Check date values in: |date= (help)
  11. ^ Limits for Microbial load for clean room as per US FDA and EU Guidelines for pharmaceutical products
  12. ^ Cleanroom Air Flow Principles
  13. ^ "Cleanroom and Controlled Environment Attire - ANSI Blog". The ANSI Blog. 2015-07-15. Retrieved 2018-11-24.
  14. ^ Cleanroom Classification / Particle Count / FS209E / ISO TC209 /
  15. ^ "ISO 14644-1:2015 - Cleanrooms and associated controlled environments -- Part 1: Classification of air cleanliness by particle concentration". ISO. Retrieved 2016-09-12.
  16. ^ W. Whyte (17 October 2001). Cleanroom Technology: Fundamentals of Design, Testing and Operation. John Wiley & Sons. ISBN 978-0-471-86842-2.
  17. ^ Cancellation of FED-STD-209E - Institute of Environmental Sciences and Technology
  18. ^ "Archived copy" (PDF). Archived from the original (PDF) on 2008-05-28. Retrieved 2008-04-17., page 148
  19. ^ "NUFAB SAFETY & PROTOCOL" (PDF). Retrieved 24 February 2016.
  20. ^ Understanding Cleanroom Classifications
  21. ^ Market Venture Philippines Inc. web site (2006-04-19). "What is a Clean Room?". Archived from the original on 2012-08-28. Retrieved 2007-06-02.
  22. ^ "BS 5295-0:1989 - Environmental cleanliness in enclosed spaces. General introduction, terms and definitions for clean rooms and clean air devices". 2016. Retrieved 2016-04-18.

External linksEdit