By Michael Fitzpatrick, Consultant, and Chair of IEST Contamination Control Working Group 028 (Minienvironments)
This Recommended Practice (RP) published by the Institute of Environmental Sciences and Technology (IEST) is considered essential for individuals in industries that might benefit from the use of localized, enhanced contamination control. IEST-RP-CC028.1 describes the various uses, configurations, and benefits of minienvironments. It discusses their various design features and facility integration requirements. The document also provides a series of minienvironment testing and evaluation procedures.
Initially, Working Group 028 found that the lack of common terminology applied to minienvironments had left most potential users confused. No one had yet performed the tedious work of actually writing down what we collectively knew of the subject and integrating that with the contamination-control and cleanroom concepts that we work with daily. The group realized that minienvironments could not simply be installed within cleanrooms, but required integration into the entire cleanroom system, including the manufacturing equipment and the materials-handling systems.
The Working Group spent six years developing the RP, a time interval that was driven by the evolutionary nature of the topic. Basic concepts, terminology and definitions were still being forged in industry, and it was difficult to find consensus until some time had passed and designers and users began to agree on some common principles. Difficulties also arose with initial attempts to produce a document that encompassed both minienvironments and isolators. Where we initially referred to “Microelectronics Minienvironments” and “Pharmaceutical Minienvironments,” we now talk of “Minienvironments” and “Isolators.” The group eventually found the two topics sufficiently unique that a common document would serve neither adequately. Isolation technology has since diverged from minienvironment technology and taken its own path to better meet the needs of the pharmaceutical industry. This RP will, however, still be of significant interest to those in the pharmaceutical industry who use minienvironments for enhanced contamination control.
Several members of Working Group 028 also participated in the writing of IEST-RP-CC012.1 Considerations In Cleanroom Design. Because of this overlap in membership, the resulting documents share a common approach. Each is intended to be of use to both novices and experienced users. In each case, the Recommended Practice does not actually tell the user how to design, build, install and operate cleanrooms or minienvironments; rather, it provides a review of the design and operational criteria a user should carefully consider.
IEST-RP-CC028.1 contains material common to all IEST Recommended Practices, such as References, and Terms and Definitions. Additionally, it discusses applications and concepts, planning, design considerations, and evaluation of minienvironment systems.
Applications and concepts
Minienvironments are used to protect contamination-sensitive processes from the ambient environment or to protect the environment and personnel from hazardous products or processes. The intended use will determine the selection, configuration, and complexity of the appropriate minienvironment.
Minienvironments are available in many different models ranging from simple flow shields and gloveboxes to the more complex, environmentally controlled, facility-integrated versions. Which to use is a decision that will be based upon process requirements, available facility support, desired control parameters and economics. The RP guides the user through this selection process with a thorough discussion of minienvironments, including their uses, applications and characteristics.
Minienvironments may be configured as stand-alone installations or integrated into the facility. The type of airflow required, the desired degree of environmental control and the availability of facility utilities determine the appropriate selection. Cost will also play a major role in this decision.
This section of the document also discusses the minienvironment airflow path and the ability of the minienvironment to provide localized environmental control. Airflow paths are either open, where the air mixes with the ambient air, or closed, where the air is isolated from the ambient air. The selection of which to use is determined by the degree of process isolation required and the need to control the properties of the air inside the minienvironment. Open-path systems often track the conditions present in a room, while closed-path systems offer the capability of controlling the environmental parameters inside the minienvironment.
This section concludes with a review of input/output configurations and discusses the need to integrate the minienvironment into other facility systems. It guides the user through the various configurations and options available and offers guidance in their selection. It also provides a series of sketches that illustrate the various configurations.
Planning
The planning section of IEST-RP-CC028.1 approaches minienvironments from a manufacturing and facilities perspective. It reviews the advantages and disadvantages of using minienvironments and discusses facility support requirements. Contamination-control strategies, material flow issues and life-safety requirements are also addressed.
Much has been written regarding the benefits of using minienvironments, such as enhanced contamination control, reduction in cost, lower energy consumption, and reduced facility size. Considerably less has been offered regarding disadvantages associated with minienvironments. The RP discusses the pros and cons of using minienvironments and enables the reader to make informed choices.
The planning section, similar to that found in IEST-RP-CC012.1, urges the reader to prepare a contamination-control strategy as an initial step in the planning of a minienvironment project. The strategy would eventually take the form of a matrix where the various processes are listed and their sensitivities to various contaminants quantified. Only when this is done is it possible to structure an airborne cleanliness hierarchy that will maximize efficient facility design and reduce costs. A key consideration in this effort is the development of an efficient material flow that includes both product and production materials. IEST-RP-CC028.1 offers guidance in this very critical planning activity.
Two key considerations in the application of minienvironments are the efficient integration of the minienvironment with the environment external to it, and the adequacy of the surrounding facility to support the use of minienvironments. The facility must be able to provide the utilities required and contain adequate space for the efficient operation and maintenance of the minienvironment.
This facility integration would include addressing any life-safety issues related to the use of minienvironments, including the isolation of hazardous materials and inclusion of required fire protection systems.
Design considerations
The design considerations section of the RP discusses those factors that should be taken into consideration in developing minienvironment specifications and design, including filtration, air management, environmental control and materials of construction. It can be considered the “nuts and bolts” of the document and contains detailed information that will be of particular use to those involved in the design of minienvironments.
A primary goal of minienvironments is to provide enhanced contamination control. This requires that strict attention be given to the “killer contaminants” identified during the planning process and to developing the means of minimizing those contaminants. HEPA and ULPA filters are usually sufficient to reduce airborne particulate contamination, especially in closed-loop systems or open-loop systems situated in a cleanroom environment. Airborne molecular contamination (AMC) can pose a significant challenge to some minienvironment applications. Processes that are extremely sensitive to all forms of contamination (including AMC) often require the use of closed-loop systems. In these installations, the minienvironment itself may be a major source of contamination. The RP discusses the issue of AMC, both external to the minienvironment and internal, and recommends methods for treating them.
The discussion of air management found in this section addresses airflow characteristics within the minienvironment. Due to the relatively small volumes of many closed-loop systems, disruptions in airflow that would be of no consequence in larger cleanrooms often present a significant challenge. The effects of airflow patterns, air velocities and temperature gradients can be magnified substantially within a minienvironment and the internal geometry of the minienvironment itself can pose significant challenges. Additional air management topics, such as pressurization, air supply, air return/exhaust, and temperature and humidity control, are also discussed.
This section continues with a review of materials of construction, offering guidance on the selection of materials compatible with use in minienvironments. Other topics of interest in this portion of the RP include: vibration, noise and sound; lighting; electromagnetic compatibility (EMC); safety, including inert and hazardous gases, fire protection and grounding; ergonomics; and monitoring.
Evaluation and testing
This section of the RP provides a methodology for testing and evaluating minienvironments and is of particular interest to purchasers and operators. It recommends tests and procedures to be used to assess system performance and ensure conformance to design specifications. The material offered is applicable for minienvironments of all complexities. Appropriate tests are listed in a format that can be taken straight from the document and placed into service. The description of each test includes a list of equipment required, the testing procedure, a format for reporting results, and the criteria for acceptance.
Summary
The RP provides a comprehensive discussion of minienvironments, their definitions, applications, design criteria and testing methods. It is of special interest to those firms contemplating the use of minienvironments to achieve enhanced contamination control. For more information on obtaining IEST-RP-CC028.1, please visit the IEST Web site (www.iest.org).
Another document for those with an interest in isolation technology is the international standard ISO 14644-7 Cleanrooms and associated controlled environments-Part 7: Separative devices (Clean air hoods, gloveboxes, isolators, minienvironments), also available from IEST.
Michael A. Fitzpatrick has participated in the design and construction of semiconductor facilities for over 24 years. He is a senior member of the Institute of Environmental Sciences and Technology, Chairman for WG 012 (Considerations in Cleanroom Design) and WG 028 (Minienvironments), and a past Vice President of the IEST. He is a member of the Arizona State University ACE Cleanroom Task Force and lectures as part of the ASU Cleanroom Construction Program. He also presents tutorials on cleanroom design for IEST and at the CleanRooms conferences in the United States and in Asia. He may be contacted at: [email protected].
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, contact IEST at [email protected] or visit the IEST website at www.iest.org.