Cleanroom
A cleanroom or clean room is an engineered space that maintains a very low concentration of airborne particulates. It is well-isolated, well-controlled from contamination, and actively cleansed. Such rooms are commonly needed for scientific research and in industrial production for all nanoscale processes, such as semiconductor device manufacturing. A cleanroom is designed to keep everything from dust to airborne organisms or vaporised particles away from it, and so from whatever material is being handled inside it.
A cleanroom can also prevent the escape of materials. This is often the primary aim in hazardous biology, nuclear work, pharmaceutics, and virology.
Cleanrooms typically come with a cleanliness level quantified 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, equivalent to an ISO 9 certified cleanroom. By comparison, an ISO 14644-1 level 1 certified cleanroom permits no particles in that size range, and just 12 particles for each cubic meter of 0.3 μm and smaller. Semiconductor facilities often get by with level 7 or 5, while level 1 facilities are exceedingly rare.
History
The modern cleanroom was invented by American physicist Willis Whitfield. As an employee of the Sandia National Laboratories, Whitfield created the initial plans for the cleanroom in 1960. Prior to Whitfield's invention, earlier cleanrooms often had problems with particles and unpredictable airflows. Whitfield designed his cleanroom with a constant, highly filtered airflow to flush out impurities. Within a few years of its invention in the 1960s, Whitfield's modern cleanroom had generated more than US$50 billion in sales worldwide.By mid-1963, more than 200 U.S. industrial plants had such specially constructed facilities—then using the terminology “White Rooms,” “Clean Rooms,” or “Dust-Free Rooms”—including the Radio Corporation of America, McDonnell Aircraft, Hughes Aircraft, Sperry Rand, Sylvania Electric, Western Electric, Boeing, and North American Aviation. RCA began such a conversion of part of its Cambridge, Ohio facilities in February 1961. Totalling 70,000 square feet, it was used to prepare control equipment for the Minuteman ICBM missiles.
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, cleanrooms 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 C. McElroy Jr. worked as an 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 cleanrooms.
Overview
A is a necessity in the manufacturing of semiconductors, rechargeable batteries, pharmaceutical products, and any other field that is highly sensitive to environmental contamination.Cleanrooms can range from the very small to the very large. On the one hand, a single-user laboratory can be built to cleanroom standards within several square meters, and on the other, entire manufacturing facilities can be contained within a cleanroom with factory floors covering thousands of square meters. Between the large and the small, there are also modular cleanrooms. They have been argued to lower costs of scaling the technology, and to be less susceptible to catastrophic failure.
With such a wide area of application, not every cleanroom is the same. For example, the rooms utilized in semiconductor manufacturing need not be sterile, while the ones used in biotechnology usually must be. Vice versa, operating rooms need not be absolutely pure of nanoscale inorganic salts, such as rust, while nanotechnology absolutely requires it. What then is common to all cleanrooms is strict control of airborne particulates, possibly with secondary decontamination of air, surfaces, workers entering the room, implements, chemicals, and machinery.
Sometimes particulates exiting the compartment are also of concern, such as in research into dangerous viruses, or where radioactive materials are being handled.
Basic construction
First, outside air entering a cleanroom is filtered and cooled by several outdoor air handlers using progressively finer filters to exclude dust.Within, air is constantly recirculated through fans serving high-efficiency particulate absorbing filters, and/or ultra-low particulate air filters to remove internally generated contaminants. Special lighting fixtures, walls, equipment and other materials are used to minimize the generation of airborne particles.
Air temperature and humidity levels inside a cleanroom may be tightly controlled, because they affect the process and personnel as well as the generation and retention of particles. If a particular room experiences humidity low enough to make static electricity a concern, it too may be controlled by, e.g., introducing controlled amounts of charged ions into the air using a corona discharge. Static discharge is of particular concern in the electronics industry, where it can instantly destroy components and circuitry.
Equipment inside any cleanroom is designed to generate minimal air contamination. The selection of material for the construction of a cleanroom should not generate any particulates; hence, monolithic epoxy or polyurethane floor coating or welded sheet goods are preferred. Finished stainless steel or powder-coated mild steel sandwich partition panels and ceiling panel are used instead of iron alloys prone to rusting and then flaking. Corners like the wall to wall, wall to floor, wall to ceiling are avoided by providing coved surface, and all joints need to be sealed with epoxy sealant to avoid any deposition or generation of particles at the joints, by vibration and friction. Many cleanrooms have a "tunnel" design in which there are spaces called "service chases" that serve as air plenums carrying the air from the bottom of the room to the top so that it can be recirculated and filtered at the top of the cleanroom.
Airflow principles
Cleanrooms maintain particulate-free air through the use of either HEPA or ULPA filters employing laminar or turbulent airflow principles. Laminar, or unidirectional, airflow 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 airflow 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 airflow filters and hoods to prevent excess particles entering the air. Turbulent, or non-unidirectional, airflow uses both laminar airflow 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 stringent guidelines and limits to ensure freedom from microbial contamination in pharmaceutical products. Plenums between air handlers and fan filter units, along with sticky mats, may also be used.In addition to air filters, cleanrooms can also use ultraviolet light to disinfect the air. UV devices can be fitted into ceiling light fixtures and irradiate air, killing potentially infectious particulates, including 99.99 percent of airborne microbial and fungal contaminants. UV light has previously been used to clean surface contaminants in sterile environments such as hospital operating rooms. Their use in other cleanrooms may increase as equipment becomes more affordable. Potential advantages of UV-based decontamination includes a reduced reliance on chemical disinfectants and the extension of HVAC filter life.
Cleanrooms of different kinds
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. This is most typically the case in semiconductor manufacturing, where even minute amounts of particulates leaking in could contaminate the whole process, while anything leaking out. The opposite is done, e.g., in the case of high-level bio-laboratories that handle dangerous bacteria or viruses; those are always held at negative pressure, with the exhaust being passed through high-efficiency filters, and further sterilizing procedures. Both are still cleanrooms because the particulate level inside is maintained within very low limits.Some cleanroom HVAC systems control the humidity to such low levels that extra equipment like air ionizers are required to prevent electrostatic discharge problems. This is a particular concern within the semiconductor business, because static discharge can easily damage modern circuit designs. On the other hand, active ions in the air can harm exposed components as well. Because of this, most workers in high electronics and semiconductor facilities have to wear conductive boots while working. 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, including the necessary machinery.
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 is used to put on cleanroom clothing. This practice is common in many nuclear power plants, which operate as low-grade inverse pressure cleanrooms, as a whole.
Recirculating vs. one pass cleanrooms
Recirculating cleanrooms return air to the negative pressure plenum via low wall air returns. The air then is pulled by HEPA fan filter units back into the cleanroom. The air is constantly recirculating and by continuously passing through HEPA filtration removing particles from the air each time. Another advantage of this design is that air conditioning can be incorporated.
One pass cleanrooms draw air from outside and pass it through HEPA fan filter units into the cleanroom. The air then leaves through exhaust grills. The advantage of this approach is the lower cost. The disadvantages are comparatively shorter HEPA fan filter life, worse particle counts than a recirculating cleanroom, and that it cannot accommodate air conditioning.