by Mike Fitzpatrick and Ken Goldstein, Ph.D.
Scarcely a week goes by that we don't receive a question regarding the testing of cleanrooms. The queries usually regard airflow properties such as air velocity or parallelism and the writers often exhibit a sense of frustration, having searched numerous documents and sources for answers, all to no avail.
In most cases, the standard response to these questions has been to direct them to Institute of Environmental Sciences and Technology (IEST; Rolling Meadows, IL) Recommended Practice (RP)-006. A study of this document will provide answers for many of the questions regarding cleanroom-testing procedures.
Before getting into the nuts and bolts of RP-006, we should digress for a moment to clear up a common misconception. Many people seeking information regarding cleanroom testing will search International Organization for Standardization (ISO) 14644-1 or U.S. Federal Standard (FED STD) 209E for answers. This is much like looking for fish in a tree—they're not there.
ISO 14644 and FED STD 209E describe the methodology for classifying cleanrooms based upon airborne particle counts only. The documents do not discuss airflow velocities, parallelism, pressurization or any other property of cleanroom air other than particle concentration. Don't search these documents frantically looking for cleanroom testing information (other than airborne particle concentration) because it's not there. It's in IEST RP-006.
Since its first publication in 1984, RP-006 has languished in the shadow of the more widely known FED STD 209E. RP-006 has remained so anonymous that many of its methodologies are wrongly attributed as being part of FED STD 209E. As a result, RP-006 may be the cleanroom industry's best-kept secret.
IEST RP-006 was first published by the Institute of Environmental Sciences and Technology in 1984. Although it has been a mainstay for those who test and certify cleanrooms, RP-006 appears to be little known outside that small circle. RP-006 offers testing methodologies for the following:
As with other IEST documents, RP-006 is a “recommended practice” rather than a standard. In other words, use of and adherence to the document is voluntary.
Like many standards, RP-006 requires a certain amount of user input. For example, the document states: “In contractual agreements, the buyer selects the specific tests…” and “acceptance limits are also defined in contractual agreements.”
What this means is that to properly specify the testing methodologies of RP-006, the user must specify which tests are to be conducted and also the acceptance levels for the tests specified. An example might be: “The average airflow throughout the cleanroom shall be 90 (±10) fpm, when tested in accordance with Section 6.1 of IEST RP-CC-006.2.”
Although the document lists fourteen tests that may be performed, it is left to the user to decide which tests are appropriate for any particular cleanroom. To assist in determining which tests are appropriate for your cleanroom, let's briefly examine the tests contained in the RP-006.
6.1 Airflow volume and uniformity
This test determines the average velocity throughout the cleanroom and if the velocity is uniform throughout. Determining the average velocity is appropriate for unidirectional airflow rooms, while in non-unidirectional airflow rooms (with non-uniform velocities), measurement of the airflow volume or the air change rate is often used.
RP-006 recommends that the relative standard deviation of the points tested (uniformity) not exceed 15 percent—although another value may be agreed upon.
6.2 Filter leak test
This procedure tests for leaks in the filters as well as for system integrity and the absence of bypass leakage in non-ducted installations. These tests have been superseded by those contained in RP-CC-034 HEPA and ultra-low penetration air (ULPA) leak tests.
6.3 Particle count
Procedures to obtain particle counts for the certification of a cleanroom are described. These numerical results are used in calculations described in FED STD 209E and ISO 14644-1.
Verifies the ability of the cleanroom systems to maintain the desired pressure differentials. Cleanroom pressurization levels can affect particle infiltration, noise and energy levels, as well as construction materials and methods.
Provides tests to verify that the airstreams in unidirectional cleanrooms are, in general, parallel.
6.6 Enclosure integrity test
Tests the cleanroom envelope for infiltration of unfiltered air.
6.7 Recovery test
Tests the ability of a cleanroom to return to its specified cleanliness level within a finite time after the room has been exposed to a source of airborne contamination.
Both the enclosure integrity test and the recovery test tend to be industry-specific. Commonly, the microelectronics crowd will not perform these tests because they are considered to be contaminating.
6.8 Particle fallout
Used to determine the concentration of large airborne particles not readily detected using discrete particle counters.
Also called a “witness plate test,” this test looks for “large” particles that are almost large enough to see with the unaided eye. By looking at the plate under ultraviolet light, particles composed of organic materials fluoresce, making them easier to detect.
Tests the lighting level and uniformity. Typically, this is done to assure that operators have adequate task lighting to perform their daily activities. But in other cases, the process and materials used are photosensitive, making this one of the critical tests.
Measures the airborne sound pressure levels produced by the cleanroom mechanical systems.
With a large number of fans, motors, pumps and a lot of air moving about, cleanrooms tend to be somewhat noisy places. To promote better working conditions, upper bounds on the noise level are sometimes specified.
6.11-6.13 Temperature and moisture uniformity
These tests demonstrate the capability of the cleanroom to maintain the desired temperature and moisture (humidity or dewpoint) levels. Two series of tests are offered, general tests and comprehensive tests. The later tests are for use in cleanrooms with very stringent requirements.
Not surprisingly, relative humidity levels tend to be industry- and process-specific. Photolithography areas in the microelectronics facilities can normally be expected to have the tightest specifications in terms of allowable drift. At the same time, pharmaceutical facilities involved with powder products tend to be the most stringent in terms of the maximum allowable value.
This test is used to determine the vibration characteristics of the cleanroom structure and components. A more detailed (and technical) methodology can be found in IEST RP-CC-024, “Measuring and Reporting Vibration in Microelectronics Facilities.” Some products and processes are very sensitive to vibration, while for others, any vibration beneath the threshold of human detection is of no concern.
Overall, RP-006 provides an excellent discussion of cleanroom testing and should be considered a “must” read. It is available at the IEST Web site (www.iest.org). Another good source of information is Procedural Standards for Certified Testing of Cleanrooms from the National Environmental Balancing Bureau (NEBB; Rockville, MD).
Michael A. Fitzpatrick is program director of microelectronics for Lockwood Greene. A senior member of the IEST, he is chairman for WG012 (Considerations in Cleanroom Design) and WG028 (Minienvironments). Ken Goldstein is principal of Cleanroom Consultants Inc. in Phoenix, Arizona, and is a member of the CleanRooms Editorial Advisory Board.
If you seek information regarding cleanroom testing, look to IEST RP-006 instead of ISO 14644 or FED STD 209E. Testing involves information about airflow velocities, parallelism, pressurization and other properties of cleanroom air, not just particle counts.