By Robert L. Mielke, Principal Metrology Engineer, Abbott Laboratories
The principal purpose of testing cleanrooms and clean zones is to determine operational performance. The current IEST RP-CC006.3, Testing Cleanrooms, provides a set of recognized and standardized test procedures for determining cleanroom and clean zone performance. The test procedures are divided into two categories: primary tests and secondary tests. The primary tests are closely related to classification or verification of airborne particulate cleanliness in the cleanroom and consist of the following: airflow volume, velocity and uniformity tests; HEPA and ULPA filter installation tests; airborne particle count test; and room pressurization test. The secondary tests involve other cleanroom or clean zone characteristics, for which acceptance limits might be specified and consist of the following: visual airflow characterization test; lighting level and uniformity test; noise level test; temperature and moisture uniformity test; and vibration test. The performance tests may be carried out in one or more of the operational phases: as built, at rest, or operational (see Table 1). The order in which the tests are performed is not defined in the document however, and in general, certain tests should be performed in a preferred sequence. For example, for unidirectional cleanrooms, the following order is recommended:
- Airflow volume or airflow velocity (section 6.1)
- Visual airflow characterization (section 6.5)
- Room pressurization (section 6.4)
The Recommended Practice (RP) is intended to assist planners, designers, manufacturers, and customers in preparing detailed specifications for cleanroom testing and for assuring cleanroom operational compliance. In contractual agreements, the customer would select the tests appropriate for their cleanroom(s) and specify the acceptance limits required for the use of the cleanroom(s). The RP may also be used to write in-house test procedures for the testing of proprietary cleanrooms.
Table 1: Index of tests by cleanroom (or clean zone) airflow type
The RP also provides recommended test equipment for each test method. Where the test method is affected by the type of cleanroom (i.e., unidirectional or nonunidirectional), alternative procedures are given. For some test methods, more than one method and instrumentation are described so that the end-user considerations may be accommodated. When these alternative methods are given, the end user should be aware that the methods might not give equivalent results.
In general, the RP does not present acceptance limits, but does provide standard terminology for specifying appropriate limits. Also, the RP does not address all tests that may be used to characterize the operating parameters of a cleanroom such as, but not limited to, particle deposition rates, airborne molecular contamination, microorganisms, electrostatic discharge (ESD), recovery testing, and containment leak testing.
Airflow volume, velocity, and uniformity: These tests determine average airflow velocity and uniformity for unidirectional cleanrooms and clean zones and air supply volume flow rate and volume uniformity for cleanrooms. Typically either the airflow velocity or airflow volume is requested and the results given in average velocity, average volume, or total volume. From the total volume, the air exchange rate can be determined.
HEPA and ULPA filter installation leak test: These tests confirm the integrity of the HEPA or ULPA filter systems. The test are performed by introducing a challenge aerosol upstream of the HEPA or ULPA filters and performing a scan integrity test on the downstream side of the filters, including the filter-to-frame seal. The RP references IEST-RP-CC034, HEPA and ULPA Filter Leak Tests, where the tests are described in detail.
Airborne particle count test: This test determines whether the completed as-built, at-rest, or operational cleanroom meets the specified air cleanliness classification as detailed in ISO 14644-1:1999. In general, the document refers to the test method specified in the ISO standard.
Room pressurization test: This test verifies the ability of the cleanroom to maintain the specified static pressure differential between the cleanroom and its surrounding rooms, including contiguous spaces above and below the cleanroom. In general, the cleanroom will have a positive pressure; however, for certain applications involving toxic, biological, or radioactive materials that require containment, the pressure differential will be specified to be negative.
Visual airflow characterization test: The purpose of this test is to visually characterize the predominant airflow direction (flow vector) at multiple locations throughout, and contiguous to, the work zone. The test verifies that a cleanroom is capable of controlling dispersion of the air stream and therefore airborne contaminants of a local origin. There are two test methods provided: the dispersion string test and the aerosol dispersion test.
Lighting level and uniformity test: The purpose of this test is to verify that the lighting level and lighting uniformity within the cleanroom meet specified requirements.
Noise level test: The purpose of this test is to measure the sound pressure levels produced by the cleanroom mechanical and electrical systems as well as by adjacent external areas to verify that the end user’s criteria are met.
Temperature and moisture uniformity tests: The purpose of these tests is to determine whether the air handler can maintain both the temperature and moisture levels (expressed as relative humidity or dew point) within the requirements for the time period specified by the end user. Two levels of the test methods are presented: general tests and comprehensive tests. The comprehensive test requires that the cleanroom be profiled to determine uniformity, while the general tests only require one test location per temperature control zone.
Vibration test: The purpose of this test is to determine vibration levels of the cleanroom structure and its components to verify that the cleanroom meets the end user’s requirements. The RP presents a general test procedure and references IEST-RP-CC024, Measuring and Reporting Vibration in Microelectronics Facilities, where more detailed test applications may be found.
Current work on the document
Working Group 006 has evaluated IEST-RP-CC006.3 to determine which areas of the document require revision. As a consequence, the WG is currently discussing the following topics: recovery rate test; airflow visualization; surface conductivity test; and the relationship of the test instrumentation accuracy when conducting airflow volume, velocity, and uniformity tests and room pressurization testing.
ISO 14644, Cleanrooms and associated controlled environments – Part 3: Test Methods is currently available as a Final Draft International Standard (FDIS) and covers a number of test methods that are currently included in IEST-RP-CC006.3. The FDIS also cites a large amount of additional material from IEST-RP-CC006.3 and IEST-RP-CC034.2 documents. It is anticipated that ISO 14644-3 will be available as a standard in late 2005.
Robert L. Mielke is a principal metrology engineer at Abbott Laboratories, North Chicago, Illinois. He is a Fellow of the Institute of Environmental Sciences and Technology (IEST). Mielke served the IEST as Standards and Practices chairman for eight years, and as Contamination Control technical vice president from 1995 to 1997 and 1999 to 2001, respectively. He was instrumental in securing the Secretariat of ISO/TC 209 for IEST, as well as for the United States, and continues to serve as its secretary. Mielke is the chair of IEST Working Group 006.
IEST is an international technical society of engineers, scientists, and educators that serves its members and the industries they represent (simulating, testing, controlling, and teaching the environments of earth and space) through education and the development of recommended practices and standards. IEST is the Secretariat for ISO Technical Committee 209, Cleanrooms and associated controlled environments, charged with writing a family of international cleanroom standards. IEST is also an ANSI-accredited standards-development organization. For more information, visit the IEST Web site at www.iest.org.
Evolution of cleanroom testing standards
The subject of testing cleanrooms has been the focus of standards and recommended practices efforts since the 1960s. One of the first standards to discuss the testing of cleanrooms was Federal Standard 209 (FED-STD-209), published in 1963, which had a portion of its non-mandatory supplemental guidance Appendix devoted to cleanroom testing. Revisions A and B of the standard continued to have cleanroom testing in the Appendix. There also were many private-sector or company-specific procedures on how to test cleanrooms. The first recommended practice in the United States to cover cleanroom testing was the American Association of Contamination Control’s (A2C2) CS-6 document entitled Clean Rooms, which was published in 1970. When A2C2 was folded into the then Institute of Environmental Sciences (IES), their documents became part of the IES.
The Contamination Control Division of what is now the Institute of Environmental Sciences and Technology (IEST) first formed Working Groups (WG) to write RPs in 1982. One of the first working groups formed was WG-CC006, Testing Cleanrooms. Since its inception, the WG has published three documents, IES-RP-CC-006-84T in 1984, IEST-RP-CC006.2 in 1997, and IEST RP-CC006.3 in 2004.
When ISO Technical Committee 209 (ISO/TC 209) was formed in 1992, it was decided that it would develop a document to cover cleanroom testing. As a result, Working Group 3 was formed within ISO/TC 209 in 1993, resulting in publication of ISO 14644-3.