MicroElectronics: Cleaning the Cleanroom Q&A
Why clean the room? It’s a cleanroom, isn’t it?
This was the most popular question when we began cleaning cleanrooms in 1980. Owners and operators often assumed that because their cleanrooms were equipped with state-of-the-art air filtration systems and other safeguards, normal cleaning procedures were optional or even unnecessary.
The accumulation of dust that one sees in many cleanrooms after only a day is a combination of factors. Both workers and manufacturing processes contribute to contamination. Particles too small to be filtered add contamination. Small charged particles combine with other charged particles to form particles that are eventually large enough for the human eye to see. Place a black fatigue mat on the cleanroom floor; within hours, even in the cleanest of cleanrooms, large particles will be visible on the mat. Where there are particles, there are bacteria. Each of these contaminants has the potential to destroy the product and affect batch yields.
Manufacturers concerned with viable or otherwise harmful particles (i.e. pathogens or pyrogens) know the necessity and benefits of daily decontamination. For those working in the pharmaceutical, biomedical, and cosmetics industries, it is not an option, but a requirement to ensure product safety. Clean products directly relate to higher profits.
Those concerned with nonviable particles—in the semiconductor, display, or microelectronic industries, for example—must weigh the benefits of a cleaner room against potential yield increases. In some semiconductor processes, a 1% increase in yield may be worth $50 million in annual revenues. We are often made aware of companies who are losing large sums of money in yield losses, but don’t want to invest in a professional cleaning programs.
If the particle counts taken in my cleanroom fall below or are within ISO-14644 requirements, why should I be concerned about visible particles?
The answer to this popular question is “Yield, yield, yield, and product assurance.” Many cleanrooms are overbuilt for their classification as defined in ISO-14644. Also, some particle counting systems actually monitor the particle counts within six inches of the filter face, thus never revealing the true integrity of the room. Though not recommended, during certification particle counts are often taken with the cleanroom at rest, without anyone in the room. In either case, most of the air sampled is undisturbed. During certification, when particle counts are taken at working height with no room disturbances, particles visible on the ledges may not contribute to the overall sum of particles counted. Unlike smaller particles, those large enough to be visible to the naked eye are likely to remain in place unless disturbed by physical movement, which occurs as a result of human activity, seismic events, geological settling, and temperature and/or humidity changes. Movement and molecular activities cause the particles on surfaces to move and become potential product or process contamination.
Even though room counts are within limits, a one-time, “top-down-superclean” or continuous cleaning by professional staff will always result in decreased particle counts. Top-down super-cleans, interstitial cleans, and rooms not previously professionally cleaned will show a decrease in particle counts ranging from two to ten times (see Figures 1-4).
If cleaning decreases particle counts and ensures that large surface particles are removed, even though the room met the ISO-14664-2 requirements, is the effort beneficial to the product being manufactured in the room?
The answer is always yes.
When developing a cleaning program, the size of the particles and the types of microorganisms to which the product is vulnerable and the mechanism of particle retention to the surface must to be considered.
Small particles are more difficult to remove because of electrostatic charges, ionic attraction, humidity, gravity, and van der Waal molecular forces, adhesion, and entrapment. Particles >25 microns in diameter are usually at rest and may be removed with a cleanroom approved vacuum cleaner featuring adequate efficiencies unless the particles are trapped in an uneven surface or a surface that was contaminated by an oil or adhesive. If adhesion has occurred, a cleanroom-safe detergent may be necessary. Particles that are 25 to 10 microns in diameter are removed by using solutions that will wet the surfaces to which the particles are attached. Smaller particles may require physical force, ultrasonic waves, solvents, or a combination of methods to remove them. A staff of trained cleaners can clean and/or disinfect a cleanroom to ensure that particles are removed and the room’s maximum potential is realized. This service may include daily, weekly, or monthly cleaning and disinfecting, periodic interstitial cleanings, super-cleans. and post-construction cleaning.
What is a “top-down superclean”? An “interstitial” clean? And what constitutes “continuous” cleaning?
* A top-down super-clean addresses the cleanroom envelope; T-bars, lighting, ionizing grids, return air vents, walls, windows, work stations, equipment surfaces, cabinets, sinks, shelves, furniture, doors, pass-throughs, air locks, trash cans, floor mats and floors all in a single clean.
*An interstitial clean may address rafters, interstitial spaces (between walls), duct work, plenum areas, tops of ceiling panels, subfloors, waffle decks, and sub-fabs.
*Continuous cleaning or disinfecting addresses the cleanroom envelope surfaces on a regularly scheduled basis. The frequency of cleaning and disinfecting will be determined by the vulnerability of the product being produced and by the results of validation.
All cleanroom cleaning consist of proper staff training; the use of appropriate supplies, equipment, and methodology; and knowledge of the surfaces to be cleaned, cleaning frequency, and documentation.
*Training should be tested in and applied to the field, with hands-on training by a qualified trainer. In addition to technical training, cleaners must be instructed in the “whys” of cleaning. Once they understand the “whys,” they are able to apply their skills and judgment to various cleaning challenges. A typical training program includes elements that ensure understanding of the following:
*Why cleanrooms are necessary
*Basic principles of cleanroom design
*ISO 14664-2 definitions
*Impact of human behavior
*Gowning protocols and their importance
*Principles of particle attachment/removal
*Role of static electricity
*Cleaning supplies and procedures
*Equipment and supplies must be selected to meet the requirements of the room and prevent recontamination of the room or contamination of the product. Detergents and IPA or other solvents are ULPA filtered to 0.1 micron; disinfectants are selected according to the type of biocontamination, rotation, and the commodity produced. Vacuum cleaner exhaust is filtered from 0.1 to 0.3 microns. Wipes are cleanroom approved and chosen for surface compatibility and absorption. The specific products purchased to clean and disinfect the room are selected for compatibility and cleaning performance.
*Methodology is developed to eliminate the possibility of product contamination and room recontamination. Methodology addresses cleaning procedures, process, surfaces, and frequency of application. Cleaning begins at the highest point in the room (ceilings or walls) and finishes with the floors. The process moves from the cleanest end of the cleanroom space (usually the area farthest from the gowning) to the least clean, working toward the gowning room and then out from the gowning room to the uncontrolled area. This method ensures the capture of the maximum number of particles and minimizes the risk of cross-contamination. When developing cleaning methodology, all cleanroom surfaces should be considered.
*Frequency of cleaning is generally unrelated to the class of the room because frequency is relative; the gowning level, number of air changes per hour, materials used, and number of personnel are determined by the level or Class of the cleanroom. Particle accumulation normally occurs continuously along the same ratios regardless of the class of the room. There is an exception to this rule, however: cleanrooms in which there is a presence of viable particles or pyrogens. Depending on the product, such rooms may need to be super-cleaned daily. A sample checklist developed for a top-down superclean and a weekly cleaning program is shown in Table 1.
How does one know if a room is clean?
There are several ways to know if a cleanroom is clean. Visual surface counts, fall-out counts, and particle counts are the most commonly used techniques. By examining a surface at an angle of ~10% with the naked eye, one can easily see particles that were not visible at a glance. Surface counts may be taken with a surface particle counter, which electronically measures particles on surfaces down to molecular levels. Fall-out counts may be taken using petri dishes or other surface substrates and examined under a microscope. Airborne particle counts may be taken, of course, using an air particle counter. Microbial environmental testing is used to determine the presence of viable particles (bacteria, yeasts, molds, and spores) and pyrogens. Regardless of the type of concerns, a cleaning program can be developed to keep cleanrooms operating at peak performance, with results from two to ten times cleaner, virtually effortlessly.