Preventing human-generated particle contamination in cleanrooms
Guidelines to help maintain an area free of human-generated particle contamination
By Ron Perry, RPA
People play a major role in generating particles inside a cleanroom. It is important that each member of the cleanroom team understands how personal hygiene and habits affect the cleanroom’s cleanliness. Following are guidelines to help maintain an environment free of human-generated particle contamination.
Maintaining personal hygiene is extremely important for conditioning behavior and preventing undesirable contamination and the bad habits of everyday life from entering the cleanroom. When your job entails working in or even just entering a cleanroom environment, you should get into the practice of wearing non-linting street clothes made of cotton or polyester blend fabrics. Avoid nylon and other synthetic fabrics, which have a natural tendency to create static and attract particles. Fuzzy sweaters, scarves, or socks are never a good idea.
When it comes to cosmetics deodorant is encouraged. The dry, unscented, non-powder brands, such as roll-on, gel, or stick types, are preferred. Lip moisteners are good, but certain lipsticks that easily transfer color or other residue, rouge, powder, eye shadow, eyebrow pencil, mascara, hairspray, perfume, or cologne are not recommended because all of these products particulate or outgas.
Daily bathing and shampooing are encouraged; this helps control flaky skin and dandruff. Fingernails should be trimmed relatively short, and if fingernail polish must be used, use only non-flaking, non-metallic, and non-glitter types. Long nails are discouraged since they can puncture gloves or finger cots.
Wash your hands before entering the cleanroom with a specially formulated cleanroom cleaner and moisturizer to remove dead cells and prevent skin from flaking. Use only the hand soaps and moisturizers provided by your employer for this purpose. Certain over-the-counter, store-bought lotions contain sodium and oils that are not cleanroom compatible. It is not recommended that you scratch or rub your skin when inside the cleanroom as this can cause exfoliation and product contamination.
Hair should be combed frequently before entering a cleanroom because combing removes dandruff and loose hair. Wear a bouffant cap; bangs must be completely covered under the bouffant-no exceptions. Never remove your bouffant inside the cleanroom or gowning room and never comb or brush your hair when inside the gowning room or cleanroom.
Chewing gum and sucking mints should never be allowed. But do drink water before entering the cleanroom to help keep your body hydrated. A water fountain can be installed just outside the cleanroom. Never bring food or drinks inside the cleanroom. Smoking should be avoided up to 15 minutes before entering the cleanroom; this will give your lungs a chance to clear.
Garments and personal items
When it comes to jewelry, wedding and engagement rings are acceptable, though rings with prongs or sharp edges should be avoided since they can puncture gloves or garments. If they must be worn, then they can be tape wrapped before donning gloves and other garments. Dangling earrings, necklaces, brooches, and other protruding or hanging jewelry are not recommended, even if normally worn under street clothes. These items may interfere with cleanroom or laboratory activities.
Figure 1. Hygiene and gowning protocol must be outlined and followed to maintain the appropriate level of cleanliness for each critical environment. Photo © Christian Delbert.
Keep personal items such as cigarettes, purses, tissues, and paper products inside your desk or locker, not in the cleanroom. Pockets in cleanroom garments should always be empty, unless authorized by your supervisor. Eyeglasses or goggles are strongly recommended because our eyes spew out mucus every time we blink. Wipe glasses and goggles clean of any visible particle contamination before entering cleanroom.
Figure 2. Examples of typical cleanroom garments. Photos courtesy of Cintas Corporation.
Dry skin flakes, exhalant, and saliva can cause product contamination. Wearing a face mask varies with each individual company, but operators with sideburns, mustaches, or beards should wear beard covers. Disposable booties are recommended and should be changed once or twice daily. Most gowning rooms have a clean and a dirty side; protocols should always be followed. Never walk outside the gowning room with booties on and return wearing the same booties.
Bouffant caps are always recommended. As mentioned previously, hair, including bangs, must be covered completely without exception. Remove the bouffant cap once outside the cleanroom or gowning room, never within the cleanroom. The same bouffant cap should not be used if worn outside the cleanroom/gowning room.
If launderable frocks or coveralls (“bunny suits”) are utilized, then it’s recommended that they be changed once or twice a week or whenever the garment becomes soiled. This procedure varies with each company’s policies.
Always follow gowning procedures. Garments must not touch the floor. Touching the floor contaminates the garment. If the garment does touch the floor, get a clean, fresh garment.
If gloves are required, use cleanroom powder-free latex or vinyl gloves. Vinyl gloves should be worn if redness appears on hands after wearing latex gloves since some operators are allergic to the proteins in latex. Change gloves whenever they discolor, or when small holes appear-but not in the cleanroom. Gloves should always be changed in the gowning room. Never pull briskly and snap off the gloves; they should always be removed slowly.
Tacky mats are recommended for cleaning the soles of shoes before entering a clean environment. The size of the mat varies with door entry openings. They are typically placed outside gowning room doors, inside the gowning room, and at main cleanroom entry doors. They are not required at exits. Step on the mat at least twice with each foot. Changing layers frequently captures loose dirt and keeps it out of the cleanroom. Change layers at least two or three times daily (traffic dependent).
Specialized HEPA-filtered vacuums are recommended and best suited for periodic cleaning. They are available in wet/dry or dry styles and are great for cleaning up spills. Hazardous spills must be properly attended to by trained safety personnel. Walls and ceilings can be dry vacuumed.
Cleanroom wipers, wet or dry, that are certified for your cleanroom classification are recommended and should be properly disposed of whenever soiled, torn, or abraded. Floors should be mopped or vacuumed once or twice a day. (This may vary from company to company.) Use only HEPA vacuum cleaners or low-particulating cleanroom mops.
Aqueous solutions, without sodium, potassium, ammonia, iron, copper, or nickel, are recommended for periodic wet cleanups. Detergents and disinfectants may require additional rinsing with cleanroom-compatible water, such as distilled or deionized water. If lubricating oils are to be used, consult your cleanroom manager for acceptable lubricants. Typically low-outgassing oils are recommended-wipe all residual oils from surfaces after lubrication to prevent particle accumulation.
Doors, frames, and thresholds should be cleaned on a regular basis. Wipe the top of the door first, then the sides and edges. A pre-saturated wiper is well suited for this. Tables, carts, and other furnishings should also be cleaned regularly. Wipe furniture slowly using long overlapping strokes. When wiping tables or work stations start at the back and move forward. Never wipe with fast or brisk strokes as this causes air turbulence within the cleanroom. And don’t forget to wipe underneath carts and tables.
Set up an in-house cleaning routine, or use trained outside professionals for periodic cleaning. Frequency may vary from once a day to once a week. Start at the dirty end of the cleanroom, wiping down the ceiling first, and work your way down the walls toward the floor. Work in small sections at a time and change solutions and water frequently. Additional rinsing may be required, particularly when using detergents. It is not recommended that you use brisk or side-to-side strokes. Try to use a slow and deliberate, vertical and overlapping method.
Flexible vinyl walls or curtains can be wiped down with pre-saturated wipes. Start at the top of the curtain and move toward the floor. Follow up with a dry wiper to remove any moisture remaining on the vinyl. Always use a cleanroom wiper with sealed edges. Outside walls of the cleanroom can be wiped with Simple Green and a non-abrasive, non-linting cloth.
Lights should also be cleaned. Make sure wipers and solutions will not scratch or etch the lens. Before cleaning, test the wiper on a small section of lens that is typically not visible. A pre-saturated cleanroom wiper may be best suited for the job. Clean the inside of the light starting at one end and move to the other side, using long, overlapping strokes. Then clean the outside in the same manner.
When cleaning glass, acrylic, or polycarbonate windows the selection of wiper and cleaning agent is vital. Plexiglas (polycarbonate or acrylic) windows can scratch or craze. Check with the window manufacturer for compatible chemicals. Squeegees make cleaning easy. Handles should be stainless steel or plastic coated. Use blades that are low in particulate shedding. Start at the top of the window and move toward the floor in slow, overlapping strokes. Check blades frequently since old blades may have exposed metal that can scratch the windows.
Ladders and step stools are often vital tools inside the cleanroom; cleanroom-compatible units are available in the market. Never bring a wooden or rusty ladder or stool into an active cleanroom. To maintain safe practices, never stand on a table, chair, or cart.
When it comes to cleaning buckets and wringers, units with casters of stainless steel or plastic construction are preferred. Keep buckets, mops, and other cleaning tools inside a designated area within the cleanroom or gowning room. It’s best to designate a specific cleaning cabinet within the area. It is not recommended that you remove cleaning tools from the cleanroom area after each cleaning. This may cause cross-contamination. Note that some utensils may contain galvanized parts that are subject to corrosion. Remove them from cleanroom use immediately if corrosion appears.
If hand tools or other materials are required, then keep a cleanroom-dedicated set of tools inside the area to perform routine maintenance. Wipe down all tools with a cleanroom-compatible pre-saturated wiper before entering the gowning room. Wipe again inside the gowning room, prior to entering the main cleanroom. All cleaning tools must be cleaned, rinsed, and dried before placing them into the storage cabinet to prevent mold and mildew.
Paper and containers
Use only cleanroom-compatible notebooks, paper, binders, and clipboards within the cleanroom. Consult the manufacturers for cleanroom ratings. Before putting cleanroom paper into a copy machine or printer, check with the manufacturer for office equipment and thermal compatibility. Some paper types may have a tendency to melt, leaving a residue on the rollers of the machines, creating contamination and possibly destroying your equipment.
One-piece ballpoint “stick” pens are best for cleanroom use. Click-type pens typically generate metal and plastic particles when they retract. Never use a graphite (lead) pencil in a clean environment.
Waste receptacles (trash cans) should be stainless steel or one-piece molded plastic, and flexible plastic liners should be utilized. Remove trash liners by folding all four corners into the center of the waste receptacle. Close the liner, taking care not to let air escape into the cleanroom. Take the receptacle into the gowning room to wipe down; wipe the receptacle inside and out daily.
Figure 3. Cleanroom-compatible notebook, clipboard, paper, and stick pen. Photo courtesy of RPA.
Standard and ESD plastic totes, boxes, and containers are commercially available in all sizes. Never take cardboard or wooden containers into the gowning room or cleanroom. These fibrous containers are large particle generators. If adhesive tape is to be used, use only conductive or standard cleanroom-compatible tape.
Reviewing and following these guidelines will ensure that they become second nature. Although particle generation will still need to be monitored and analyzed, these management protocols will go a long way toward minimizing the human element in controlling the particulate levels in your critical environment.
Ron Perry is president of RPA, a company that offers state-of-the-art cleanroom designs, construction, monitors, and furnishings. Perry has more than 25 years of experience selling products to the Southern California semiconductor, aerospace, microelectronics, biomedical, and pharmaceutical industries.
Procedure for putting on coveralls (bunny suits)
- Hold cuffs of sleeves in palms of hands.
- Roll up coverall pant legs and put one leg inside coverall at a time.
- Stand up straight and remain holding sleeves.
- Let go of one sleeve and put one arm inside the sleeve, then let go of the other sleeve and put the second arm in the sleeve.
- Zip up garment and you’re ready to go to work.
This procedure requires practice and balance. You may need to sit down to put on your garment.
Procedure for removal of film layers
- Look for color tab on end of mat.
- Lift up tab and slowly peel layer away from mat.
- Continue peeling film upward, separating it from the mat until all four corners are separated.
- Bring all four corners together in the center of the mat, forming a tent- like appearance.
- Slowly separate film from the center of the mat.
- Fold film inward until it is small enough to discard. This procedure ensures that dust particles will be captured inside the film. Soiled sheets should be discarded outside the gowning room area.
It is not recommended that you pull the tab briskly and rip the film from the mat. This method allows particles to become airborne and enter the cleanroom. Go slowly and deliberately.