Verifying the Performance of a Cleanroom

To verify that your cleanroom is operating according to design specifications, several tests can be run with the help of the Institute of Environmental Sciences Recommended Practice 006.

By William Whyte

When a cleanroom has just been built and is about to be handed over to the purchaser, or when an existing cleanroom is reopened after being shut down for modifications, it is necessary to verify that the room is working correctly and achieving the contamination standards that are expected of it.

In addition, the verification process should establish the initial performance of the room so that this can be adopted as a “benchmark.” When the room is checked in the future, either routinely or when a contamination problem is encountered, deviations from the original conditions can be found–hence, the possible reasons for the contamination.

The final and indirect reason for carrying out cleanroom verification is to familiarize and train the staff who will monitor and run the room. This may be their most important (and possibly only) opportunity to understand how their cleanroom works.

General principles of verifying a cleanroom

To verify that a cleanroom is working satisfactorily, it is necessary to show that the following principles have been satisfied:

Air supplied to the cleanroom is of sufficient quantity to dilute or remove contamination generated in the room.

Air supplied to the cleanroom is of a quality that will not significantly add to contamination within the room.

Air should move between areas in the cleanroom suite so as to control contaminated air movement; i.e., it should not move from the less-clean to the cleaner areas through the doorways and the general fabric of the room, i.e., walls and ceiling joints.

Air movement within the cleanroom should ensure that there are no areas where insufficient airflow will allow excessively high concentrations of contamination to accumulate.

To ensure that the above requirements are fulfilled, the following tests should be carried out generally in the following order. A considerable amount of assistance in how to perform these tests can be obtained from the Institute of Environmental Sciences` Recommended Practice 006 (RP-006). When these conditions are met, the cleanroom should then comply with its design requirements.

Air supply and extract quantities. The air supply and extract volumes to and from the room should be measured, and in the case of unidirectional airflow, the air velocity from the air filters.

Air movement control between areas. To demonstrate that the airflow between areas is moving in the correct direction–from the clean to less clean–it is necessary to check that: (a) differential air pressures between areas are correct; (b) airborne contamination is not entering the cleanroom through its fabric.

Filter and filter-housing integrity test. The filter and its housing should be checked to ensure that no airborne contamination passes through a damaged filter, between the filter and its housing, or any other part of the filter and housing system.

Air movement control within the room. These types of tests depend on whether the room is conventionally or unidirectionally ventilated. If it is conventionally ventilated, it is necessary to check that there are no areas with insufficient air movement. If the room has unidirectional airflow, it is necessary to check that the air velocity and direction throughout the unidirectional area meet the specifications of the room design.

Airborne particle concentrations. If the above tests are shown to be satisfactory, then final measurements should be carried out to ascertain that the airborne concentration of particles is within the room`s specifications.

Measurement of air supply volumes

The airborne contamination of a room is largely dependent on the volume of air supplied to it. To ensure the correct air movement between rooms, the room air extracts must be adjusted to obtain the correct outflow of air from the cleanroom. It is therefore important to measure the supply and extract air volumes of the rooms.

The balancing of these air quantities is a task normally carried out by the contractor prior to it being handed over to the user. However, it is common for the user to check these volumes. If the room is of the conventionally ventilated type and air is supplied through a terminal air filter in the ceiling, then the air supply at the filter face can be measured by use of an anemometer. However, because of the non-uniformity of air velocity across the filter, it is easier to measure the air volume by means of a hood held up to the ceiling and around the filter face. In a unidirectional flow system, the air velocity below the filter face can be measured with an anemometer.

Air movement between areas

Pressure difference between areas. It is necessary to ensure that air moves from a clean area to a less clean area and not vice versa. Measurement of pressure is an indirect measurement of this, as air will flow from a high pressure area to one with low pressure. The highest quality cleanrooms should, therefore, have a higher pressure than adjacent, less clean areas. Experience has shown that a pressure differential of 10 Pa is sufficient between cleanrooms and 15 Pa between a cleanroom and an uncontrolled area such as an outer corridor. Where there is a problem in achieving this pressure differential, for example, between areas connected by large openings such as a pass-through tunnel, then either very large air quantities have to be used to achieve the suggested pressure drop, or a lower pressure drop accepted. The acceptance of a lower pressure is perfectly reasonable as long as the primary requirement is achieved–i.e., the airflow is always in the correct direction. It may, however, be difficult to convince everyone of the correctness of this argument, and it may be necessary to conform to the written requirements that will normally be stated in terms of pressure.

Pressures are measured using either an inclined liquid manometer, an electronic gauge or a magnehelic-type gauge. To check the differential pressures between areas, the air-conditioning plant must have been balanced and, hence, supplies and extracts the correct volumes of air. All doors in the cleanroom suite must be fully closed. A tube from the pressure gauge is passed under the door, or through a bypass grill or damper, connecting the two areas being tested, and into the adjacent room. It must be well clear of the door so that no pressure is registered from air movement along the floor. A differential pressure reading may then be taken. In some ventilation systems, the differential pressure throughout the suite is maintained automatically by a building management system. In this system, the pressure differential is often maintained with respect to one reference point. When this type of system is being checked, the differential pressure must be checked against this reference point.

Infiltration through the cleanroom structure is the movement of airborne contamination through doors and pass-through hatches, as well as holes and cracks in the walls, ceilings, etc., from areas adjacent to the cleanroom. If a cleanroom is pressurized with respect to all other adjacent areas, then air should flow to areas of lower pressure and contamination should not infiltrate from adjacent areas. However, there can be adjacent areas, such as service ducts, which may be of a higher pressure and can be overlooked. An example of such a problem is an air supply plenum in a vertical unidirectional flow system. This is at a higher pressure than the room, and unfiltered air can penetrate the room at the filter housing-to-ceiling interface, the ceiling-to-wall interface and through the cladding of the ceiling support pillars in the cleanroom.

It is a relatively simple matter to check that air is flowing in the correct direction through the cracks of a closed door, observation of smoke will be sufficient. To ensure that there is no unwanted flow of airborne contamination into a cleanroom through its walls, ceiling and floors, it is necessary to check the overlapping edges of the cleanroom fabric. A test dust or smoke should be injected into the areas outside the room being tested and the joints of the room scanned with a particle counter. This is not an easy task because it is often difficult to release a test smoke behind all the walls, ceiling, etc. It may be sufficient to scan for particles, relying on the naturally occurring contamination in the adjacent areas to expose any problem.

Air supply quality (Filter integrity test)

The quality of air supplied to a cleanroom through its high efficiency air filters should be appropriate to the task being carried out in the room. The efficiency of the filters to achieve this will have been specified in the design of the cleanroom, and it is not appropriate to discuss this within this section.

The high efficiency filters installed in the cleanroom will have been tested in the manufacturer`s factory and packed in such a way that they should arrive at the construction site undamaged. This is not always so, and damage can also occur when the filter is unpacked and fitted into the filter housing. This damage can occur in areas such as:

filter paper-to-casing cement area

filter paper (often at the fold)

casing joints.

If the damage does occur in the specified areas, then contaminated air can pass through the filters. Leakage problems also occur if the filter case does not fit well into its housing. Using a design with neoprene rubber gaskets can cause problems. A filter housing that uses fluid seals will generally prevent this problem. To check that a high efficiency filter is not damaged, that the filter is seated in its housing, and any other problems that allow leakage of unfiltered air into the room, the integrity of the filter/filter housing is tested.

Although the overall efficiency of a high efficiency filter should have been tested at the manufacturer and the correct filters dispatched, a secondary function of this test is to check that no mistakes have been made and the correct efficiency of filters have been installed.

Aerosol test challenges

The integrity of a high efficiency filter/filter housing is normally tested by challenging it with an artificial test dust or smoke. This is generated by a dust or smoke generator and injected into the ductwork system so that there is a suitable concentration before the high efficiency filter. Any problems with the filter are shown by scanning the filter face for a sign of the test challenge that has leaked.

Before discussing the choice of test dusts or smokes, it is appropriate to consider two issues: The necessity of carrying out filter integrity tests in cleanrooms and the need for an artificial smoke or test challenge.

It is fairly common in poorer quality cleanrooms (occasionally in Class 10,000 and often in Class 100,000 or worse) not to carry out filter integrity tests. If the required particle air standard is easily achieved in the cleanroom, then the leakage of unfiltered air through the high efficiency filter system must be small and acceptable. This viewpoint is more acceptable in Class 10,000 or poorer rooms, where a conventional air supply will mix any particles passing through a damaged filter with the room air and any localized high concentrations of contamination arising will be avoided.

In some cleanrooms, an artificial test challenge is not used, instead the airborne particles normally present in the air before the filter are used. This occurs in rooms where the artificial loading of the filters and outgassing of oil derived from the test smoke is not acceptable. A point of debate is that if the filter is scanned with a particle counter and no significant particle concentrations are detected, then a test with abnormally high concentrations of particles is unnecessary.

Outside or recirculated air as a test challenge. Fresh air drawn from outside the building, or air that has been recirculated from the cleanroom and mixed with outside air, is a useful test dust challenge, as it has the clear attribute of being non-toxic. Air recirculated from the cleanroom will have few particles, but fresh make-up air has a considerable number of particles. However, the fresh makeup air should not be filtered with high efficiency filters. If this occurs, the filters can be removed for the test period. The accuracy of this method will be increased by measuring smaller particles as the concentration of particles in air increases as their size diminishes.

Cold-generated oils. Di-octyl phthalate (DOP) is an oil-like substance that has been used for many years to test filters. Because of possible toxic effects that may occur from the breathing of DOP, a variety of replacement oils such as DOS, DEHS, Shell Odina EL and Emprey 3004 are used. To create a test challenge, air is passed at high pressure through a nozzle designed for this purpose (known as a Laskin nozzle). One Laskin nozzle will produce a sufficient test challenge for a ventilation system using about 1,000 cfm (3m s-1) of air when the filter penetration is measured by a photometer. Multiple nozzle systems are therefore necessary for larger volume air systems, although it is not possible to generate sufficient challenge for a cleanroom supplied with large volumes of air. When this occurs, a single particle counter can be used in place of a photometer. Another alternative is to generate higher quantities of a test challenge using a hot-generated system.

Hot-generated oils and smokes. These generators use an inert gas such as CO2 to inject a suitable oil into a heated evaporation chamber, that oil being condensed as a fine aerosol at the exit nozzle. Such generators will produce sufficient aerosol to test (in association with a photometer) an air ventilation system of up to 28m s-1 (60,000 ft3 min-1).

Latex spheres. Latex spheres are available for testing filters in size ranges of approximately 0.1&#181m and upwards. These particles are dispersed into the supply air and the filters/filter housings scanned for particle penetration. This method is free from the possible problems of outgassing, but is much more expensive than hot- or cold-generated systems.

Methods of testing filters and housings

Scanning methods. The normal method used with a photometer is to scan the filter face with overlapping strokes. The perimeter of the filter must also be scanned for leaks between the filter paper and the filter case, and the filter case and its filter housing. The probe is normally held about 2 to 3 cm from the filter. Suggestions are made in RP-006 as to the speed of the scan. If a single particle counter is used, the scan speed can be determined by referencing RP-006. A leak in excess of 0.01 percent is normally considered a problem.

Unidirectional flow rooms have a large air filter area. Testing, as described above, can take a considerable time (several days for a large semiconductor fabrication cleanroom) and the high efficiency filters are subjected to a significant loading of the test particles. It is possible to reduce the time by the use of several particle samplers on a trolley. Their adjoining inlet nozzles should be at a suitable distance from the filter bank. Scanning can then be done by moving the trolley about the room. It is also possible to test each filter by scanning it on a rig in the cleanroom, then taking the scanned filter and placing it into its housing in the ceiling. One would have confidence in this method only if the filter were placed into a fluid type of ceiling housing.

In a conventionally ventilated room, if the air supply issues direct from HEPA filters in the ceiling, particles may be entrained from the room into the clean air supply. It is difficult at the perimeter of the filter to distinguish between a leak from the case/housing interface and entrainment of particles from the room. Use of a box a few inches deep, which is placed against the ceiling and round the filter, can prevent entrainment. If there is a problem with leaks coming from the filter case, then it is difficult to distinguish them from leaks between the filters case and the filter housing. It may be possible to resolve this by removing the filter and testing it on a rig. If a leak is detected from the filter medium, it is often found at the paper fold. This can be repaired with silicon mastic from a caulking gun. It is generally accepted that up to 3 percent of the filter area can be repaired (see RP-006.)

Air movement within the cleanroom

Ensuring sufficient air movement within a cleanroom for diluting or removing airborne contamination, which prevents contamination buildup, is necessary. In a conventionally ventilated room, the air supply should be sufficient to ensure good mixing and removing of contaminants from all parts of the room. Areas with low airflow and where contamination is generated in that area (or being fed into that area), will have a higher than normal concentration of contamination. Areas with poor airflow but with no sources of contamination within that area (or fed into it) i.e., areas away from people or machinery, are unlikely to constitute a hazard. Critical areas where the product is exposed to contamination should be investigated thoroughly and good air dilution achieved.

Air movement visualization. Generators that vaporize oils and produce smoke to test the integrity of high efficiency filters can be used for visualizing air in a cleanroom, although the amount of smoke produced can be excessive. Also available for observing airflow are “puffer and smoke tubes.” By pressing the puffer, air passes through the tube, removing TiCl4, which then reacts with water in the air to produce a white smoke (TiO2). It is possible to puff smoke in the area to be checked and observe the smoke movement. An acidic vapor is produced which may be harmful to sensitive machinery in the cleanroom. In rooms such as microelectronic fabrication areas, the visualization of air movement can be carried out by use of water vapor streams. There are a variety of methods that will produce water vapor, e.g. CO2/water vapor and water nebulizer systems. Using one of these methods, the airflow in the room can be visualized and areas sought in which the air movement is poor. A permanent record can be obtained with a video camera.

Two test methods relevant to air movement are documented in cleanroom testing procedures. These are the “airflow parallelism test” and the “recovery test.”

Airflow parallelism test. This test verifies the unidirectional nature of the airflow in the room and demonstrates its capability of preventing the transfer of contamination generated within the room. In addition, the test is described in the Institute of Environmental Science`s RP-006. The room is divided into squares of about 3 meters in length. The outlet tube of a smoke generator is set up with its exit facing the direction of the flow. Its flow rate is then adjusted so that the supply velocity is the same as the air velocity of the unidirectional flow–isokinetic. The smoke should preferably be at the same temperature as the room. The distance that the smoke has dispersed horizontally in a given vertical distance should be measured. This angle of offset should not be greater than 14&#176. This test should be carried out at the other nodes of the grid.

Recovery test. Smoke is generated in the area being studied and then shut off. The airborne particle count is taken for six-second intervals every minute until the original count in the room is registered. The time from “shut-off” to recovery is noted.

Airborne particle counts within a cleanroom

One of the final tasks that must be carried out prior to the handover of the cleanroom is the measurement of the airborne particle concentration to ensure that it is within the standards set at the design stage. These standards are defined in Federal Standard 209E. The airborne contamination in a cleanroom can be measured in three occupancy states. These are defined as:

“As-built” cleanroom: A cleanroom that is complete and ready for operation, with all services connected and functional, but without equipment or operating personnel in the facility.

“At-rest” cleanroom: A cleanroom that is complete, with all services functioning and with equipment installed and operable or operating, as specified, but without operating personnel in the facility.

Operational cleanroom: A cleanroom in normal operation, with all services functioning and with equipment and personnel, if applicable, present and performing their normal work functions in the facility.

There can often be a considerable time between the cleanroom being completed by the building contractor and products manufactured in the room. However, the contractor will wish to be paid for building the room, and it is common practice to check the room in its as-built condition–without any equipment installed–if this is satisfactory, settle with the contractor. The problem with checking the as-built condition is that if there is no equipment working and no personnel in the room, there will be no generation of particles and the count will be very much lower than when the room is operational.

If a cleanroom has been designed properly and the room tested when empty, it should be possible by the use of the additional tests described above to predict that it will perform correctly in the “at-rest” and fully operational states. The most important tests that ensure the operational room will give the correct standard are the quality and quantity of air supplied. In unidirectional flow, because of the nature of the airflow, a reasonable amount of confidence can be put in the correct standards being achieved if the filters are functioning correctly and the designed air velocities are correct. In a conventionally ventilated room, this is less certain, as the turbulent mixing of room air makes it necessary to ensure that the designed air supply will give sufficient dilution of the contamination dispersed from the machines and people. However, most turbulently ventilated cleanrooms are overspecified and should therefore achieve the desired standard.

To check the airborne particle count in a cleanroom, it is necessary to take sufficient samples to have confidence that the room is performing with the limits set by the standards. The number of sampling locations must also reflect the size of the room and its cleanliness, the larger and cleaner the room, the more sampling locations that must be taken.

The air sample must also be of sufficient volume to give confidence in the results. The methods of selection of the number and location of samples and the minimum air sampling volume are given in the Federal Standard 209E. The acceptance criteria is also given in Federal Standard 209E. It is stated that the cleanroom will pass the test if (a) the average particle concentration at each location tested falls below the class limit, and (b) the mean average of the samples should be below the upper class limit, with a 95 percent confidence limit. The methods used to analyze the test results are given in Fed-Std-209E.n

Editor`s Note: For more information on the Institute of Environmental Sciences Recommended Practice 006, contact the Institute at 940 East Northwest Highway, Mount Prospect, IL 60056, (847) 255-1561, fax (847) 255-1699.

William Whyte is an expert in contamination control and cleanroom design. He has a degree in bacteriology and an engineering background as well as experience as an industrial consultant. In addition, Whyte is a Research Fellow in the Building Services Research Unit at Glasgow University, Scotland. He has published over 100 reports and papers on contamination control and cleanroom design in the field of pharmaceutical manufacturing, electronics and health care. Whyte also edited “Cleanroom Design” a book published by J. Wiley & Sons. He is a founder, former chairman, and now general secretary of the Scottish Society for Contamination Control; and he is a member of the British Standards Technical Committee overseeing the standards on cleanrooms, and a member of the European CEN Technical Committee and International Standards Organization, which is now writing new European and international cleanroom standards.