TOWARDS GLOBAL CLEANROOM TECHNOLOGY STANDARDS

Dr. Hans H. Schicht

Introduction

Cleanroorn technology is eminently international: most of the leading suppliers of services and hardware as well as many users operate on an international, mostly global scale. Harmonised technical standards are thus of vital importance if uniform quality and performance goals are to be met everywhere in the world. To address this need, ISO - the International Organisation for Standardisation - established, in 1993, its Technical Committee ISO/TC 209 Cleanrooms and associated controlled environment. This committee enjoys broad support from all over the world as well as from CEN, the European Committee for standardisation.
Ten different standards covering a wide range of contamination and biocontamination control topics are presently being developed, and 5 of these documents have already been, or will soon be, circulated as Draft International Standards for the parallel ISO and CEN approval procedures. The most advanced is ISO/DIS 14644-1 devoted to the new ISO classification system for cleanrooms. This document is discussed in particular detail in this article.

International Cleanroom Technology Standards - Why?

Since the mother of all cleanroom technology standards - U.S. Federal Standard 209- was first published in 1963,more than 300 normative documents, guidelines or recommended practices on contamination control topics have been published all over the world.

Without any doubt they contain valuable advice on many important subjects. However, the sheer number of documents and the fact that they have been prepared on a national rather than an international level creates a very unsatisfactory situation in a field as international as cleanroom technology: microelectronics, the pharmaceutical and medical device industries, the emerging nanotechnologies and all the other leading application areas of contamination and biocontamination control technology act and operate globally. The same applies to many of their suppliers of design, testing and qualification services, of cleanroom components and systems as well as of cleanroom disposables - and to all the other industries serving the field of cleanroom technology.

International Standardisation: The Key Objectives

International technical standardisation is driven by two key objectives:

• to establish a common basis for understanding, agreed and accepted on a global scale;
• to eliminate technical barriers to trade.

In the field of cleanroom technology, international harmonisation of standards is driven by two entities:

• on a European level by CEN, the European Committee for Standardisation;
• on a truly global level by ISO, the International Organisation for Standardisation.

ISO, established in 1947, is a world-wide federation of national standards bodies - one per nation - and it comprises at present 127 members [3]. Of these, 85 are full members enjoying voting rights, the remaining member bodies being correspondent and subscriber members. ISO's work is carried out through 2 830 technical bodies grouped together into 214 technical committees. More than 30 000 experts from all parts of the world participate in the technical work which, to date, has resulted in the publication of 11 258 ISO standards. ISO is independent from the various political and economical blocks in existence throughout the world. Its objective is to promote standardisation on a world wide basis: it aims at facilitating international exchange of goods and services, and at developing co-operation in the spheres of intellectual, scientific, technological and economic activity.

CEN, on the other hand, has been established since 1975 as a common organ of the European Community (now European Union EO) and the European Free Trade Association EFTA. Its objective is the elimination of technical barriers to trade between the CEN member nations through harmonisation of the European technical standards. It embraces the 18 standardisation bodies of the EU and EFTA nations, and its scope of work is identical with that of ISO.

ISO and CEN aim at harmonising their standardisation activities to the widest possible extent. With this in mind, they have negotiated the Vienna Agreement on Technical Co-operation between ISO and CEN. It entered into force in 1991 and establishes procedures for the mutual recognition of standards developed within one or the other of the two organisations.

What is the impact of this agreement on the adoption of standards? If ISO and CEN agree on a given technical item of standardisation, the ISO and CEN approval procedures can be triggered in parallel. If through this procedure a draft standard is approved on ISO level, it will be published in the ISO collection of standards as International Standard. Each nation is then free to decide whether it wishes also to include it in its national collection of standards. If the draft has also been approved during the parallel CEN vote, then all CEN nations are bindingly obliged to include this standard into their national standards collections as European Standard. Furthermore, all national standards conflicting with the European Standard thus adopted must be withdrawn, and no new national standards on the same subject may be elaborated henceforth.

The parallel ISO and CEN voting procedure gives tremendous weight to the standards thus approved, as the inclusion into 18 national collections of standards is guaranteed right from the beginning.

International Cleanroom Standards: The First Steps

The first step towards international harmonisation of standards in the field of contamination control technology took place in 1990: the establishment of the European Technical Committee CEN/TC 243 Cleanroom Technology. Without delay, it embarked upon a dynamic work programme addressing the topics of air cleanliness classification; design, construction and operation of Cleanrooms; terms and definitions; plus a wide range of biocontamination control subjects. The only drawback to these efforts was the limitation to Europe: the other driving regions in microcontamination control technology - North America and the Far East - were left out. However, the European initiative paved the way, and not much time was wasted until a proposal was submitted to ISO by the United States of America to launch an international standardisation effort in this field. This initiative was enthusiastically supported and promoted by the world-wide contamination control community including the European nations. Duly, the International Technical Committee ISO/TC 209 Cleanrooms and associated controlled environments was launched in 1993 - a mere three years after CEN/TC 243 had been created.

Presently, ISO/TC 209 membership stands as follows:

• 19 nations which, as P-members (participating members) pronounced their intention to actively participate in the elaboration of the forthcoming family of standards: Australia, Belgium, China, Denmark, Finland, France, Germany, Italy, Jamaica, Japan, the Republic of Korea, the Netherlands, Norway, Portugal, the Russian Federation, Sweden, Switzerland, the United Kingdom and the USA;
• 15 nations which, as O-members (observing members) keep themselves informed on the progress of the standardisation work: Bulgaria, the Czech Republic, Egypt, India, Ireland, Malaysia, New Zealand, the Philippines, Poland, Saudi Arabia, South Africa, Thailand, Turkey, the Ukraine and Yugoslavia.

In addition, the International Confederation of Contamination Control Societies ICCCS - the roof organisation of the various national professional societies promoting cleanroom and contamination control technology - also lends its support to ISO/ TC 209: as a Category A liaison organisation to said Technical Committee, it is entitled to participate in its deliberations and is expected to give the standardisation work items in development its explicit backing.

ISO/TC 209: Scope and Guidance Principles for Work

The scope of work established for ISO/TC 209 is ample and reads as follows: Standardisation of equipment, facilities, and operational methods for Cleanrooms and associated controlled environments. This includes procedural limits, operational limits and testing procedures to achieve desired attributes to minimise microcontamination. Topics of interest are non-viable particles, viable particles, surface cleanliness, room temperature and humidity profiles, airflow patterns and velocities, room vibration profiles, room light levels, room infiltration leakage, personnel procedures, personnel cleanroom clothing, equipment preparation, and any other topics related to optimising cleanroom operations.

Indeed, a spirit of total contamination control was to reign!

The general guidance principles to be observed in the development of the forthcoming family of standards are:

• The family of standards to be developed shall address only subjects of general applicability to all cleanroom technology usage areas.
• Applicationspecific standardisation remains outside the brief of ISO/TC 209.
• The standards to be prepared should be target oriented and establish objectives to be met.
• As much freedom as possible should be granted regarding the path leading to the goal.
• The standards should promote and encourage progress, rather than impeding it.
• The standards should contribute to the elimination of technical barriers to trade, and promote understanding between nations.
• The standards should neither favour nor prejudice individual nations.

ISO/TC 209: Structure, Projects, and Priorities

A total of 15 work items have been approved by the P-members for being addressed by l SO/TC 209, and a total of 7 Working Groups have been established for their development into standards (the nations entrusted with convenorship are given in brackets):

• WG 1: Air cleanliness classification (United Kingdom);
• WG 2: Biocontamination and biocontamination control (France);
• WG 3: Metrology and testing methods (Japan);
• WG 4: Design and construction (Germany);
• WG 5: Cleanroom operation (USA);
• WG 6: Terms, definitions and units (Switzerland);
• WG 7: Mini-environments and isolators (USA).

CEN/TC 243 has harmonised its list of work items with that of ISO/TC 209, and has transferred its earlier activities in the development of standards fully into said ISO Technical Committee. Thus, the determinations of the Vienna Agreement will apply and all standards developed by ISOITC 209 - without a single exception - will be submitted to the parallel approval procedure under ISO and CEN auspices.

The 15 internationally approved work items will be developed into a family of 10 international standards. Table 1 shows the ISO number allocated to them, their provisional title and the date when they have been approved by ISO/TC 209 - or are scheduled for approval by this body - prior to forwarding them to the ISO Central Secretariat. This body will subsequently launch them into the formal approval procedure by circulating them to the 85 voting members of ISO for the ISO enquiry as Draft International Standards (DIS). In parallel, CEN will circulate them to their 18 member bodies for the CEN enquiry.

Table 1:Standards being developed by ISO/TC 209 and the date of approval for subsequent international circulation as draft international standards(DIS)

The document in pole position - ISO/DIS 14644-1 [4] - has already successfully passed the ISO and CEN enquiry stage, with not a single negative vote having been cast! It is now being readied for the final and formal ISO and CEN vote of approval as Final Draft International Standard (FYIS), Another four documents have been liberated by ISO/TC 209 into the DIS stage, and the remaining ones are to follow in quick succession.

The approval procedure of international standards is tedious - even more so if they are submitted to the parallel ISO and CEN approval procedures which requires them to be circulated in three languages: English, French and German. In order to bridge this gap in time, CEN has decided to publish an interim document prepared under the auspices of CEN/TC 243 prior to the integration of its activities into ISO/TC 209: the European Prestandard ENV 1631 Cleanroom technology - Design, construction and operation of cleanrooms and clean air devices [5]. One of the highlights of this prestandard is the orientation given in respect to the various qualification steps of a cleanroom system, i.e. design, installation, operational and performance qualification. This is - as far as this author is aware - the first time that this issue is addressed exhaustively in any cleanroom and contamination control technology standard. ENV 1631 will automatically be withdrawn as soon as the corresponding ISO documents - ISO 14644-4 and ISO 14644-5 - are approved and published.

Some of the forthcoming ISO cleanroom technology standards have been developed from earlier national standards. Others, such as the biocontamination control documents, are pioneering efforts without national precedents. Will not quality suffer if documents of this complexity are developed by international teams of experts, with cultural differences and language problems,-and obeying a very tough schedule of deadlines? The opposite is the case: a high degree of perfection is striven for and is being achieved. Where comparison can be made, the documents are perceived by professionals to be clear improvements in substance and clarity in comparison with their national predecessors.

Towards Global Standards (part 2)

THE ISO AIR CLEANLINESS CLASSIFICATION

Hans Schicht concludes his look at the development of international standards

Final approval of the international standard ISO 14644-1 on air cleanliness classification is now imminent, well ahead of the remaining documents of the ISO family of cleanroom technology standards. This has to be so: after all, air cleanliness classification is the point of departure for all subsequent standardisation efforts in contamination and biocontamination control.

Why is air cleanliness classification so important a subject? In order to guarantee the necessary protection of product and man, the air in the cleanroom should be as clean as necessary - not as clean as possible: the better the air cleanliness level, the more expensive the protection scheme will become. Classifying cleanrooms according to the air cleanliness level maintained in them is a convenient tool for allotting the correct air cleanliness level to a given protection task - and for striking the appropriate balance between the level of protection and cost.

A - The point of departure

For 35 years, US. Federal Standard 209 has offered a widely accepted scheme for air cleanliness classification, and was - and is - recognised throughout the world for conveniently fulfilling this task. The edition presently in force - U. S. Federal Standard 209E [6] was published in 1992. Whereas earlier editions indicated air volumes in Imperial units (i.e. cu.ft.) only, the 209E edition introduced metric SI units (SI = Systeme International) for the air volume (i.e. m³) in parallel to the Imperial units. Unfortunately, the adopted denomination scheme for the metric air cleanliness classes is lacking in elegance: the new denomination class M3.5 instead of class 100, for instance, is not exactly convincing. As a consequence, acceptance of the new metric air cleanliness classes was far from enthusiastic, and many professionals continued to give preference to the earlier Imperial air cleanliness class denominations even when indicating particle concentrations in metric volume units.

Many nations have followed the lead of U.S. Federal Standard 209 and developed their own air cleanliness classification schemes from this point of departure. Although they tended to follow the same classification principles, subtle variations galored and full co-incidence of determinations was non-existent. Moreover, nations were most ingenious in the invention of new air cleanliness class designations. An example may help to illustrate this point: an air cleanliness level corresponding with class 100 (or M3.5) according to U.S. Federal Standard 209E is denominated class 3.5 in Australia, class 4 000 in France, class 3 in Germany, class E or F in Great Britain, class 5 in Japan and class M10 000 in the Republic of Korea. This babylonian variety of terms complicated things still more for the international player and caused a lot of confusion. A need for international harmonisation was clearly apparent!

B - The ISO air cleanliness classification scheme

In elaborating the forthcoming international airborne particulate cleanliness classification according to ISO 14644-1, the objective was to combine the best features of the different base documents available, above all those from Europe, Japan and the United States of America.

The new ISO air cleanliness classification scheme is based on a formula as follows.

Table 2 below shows the ISO air cleanliness class limits in tabular form, and Figure 1 as a graph. Both the table and the graph possess informative character only: in case of doubt, the formula prevails.

Table 2: Selected airborne particulate cleanliness classes for cleanrooms and clean zones

Note:Uncertainties related to the measurement process require that concentration data with no more than three significant figures be used in determining the classification level.

Figure 1: Graphical representation of the ISO airborne particulate cleanliness classes according to Formula 1. C_n represents the maximum permitted concentration (in particles/m³ of air) of airborne particles equal to and larger than the considered particle size. N represents the specified ISO class number.

With the selection of 0,1 mm as the reference particle diameter for air cleanliness classification a very straightforward denomination scheme results - thus overcoming elegantly the principal drawback of the metric air cleanliness classes according to U.S. Federal Standard 209E. Simple, single-digit class denominations now correspond with the traditional classes of said standard: ISO 5, for example, replaces class 100, and ISO 8 substitutes class 100 000.

The exponent 2,08 of the correlation between particle concentration and particle diameter ensures the best possible co-incidence with the particle concentrations according to U.S. Federal Standard 209E at that standard's reference particle diameter of 0,5 mm. Thus, a harmonious connection to previous generations of standards is assured.

C - Demonstrating compliance

In harmony with U.S. Federal Standard 209E, the range of particle diameters for air cleanliness classification extends from 0,1 to 5 mm. Thus, it coincides with the detection capabilities of the discrete particle counter which has been identified as the reference test method for demonstrating compliance with ISO 14644-1. These tests shall be conducted with calibrated instruments.

Within the range established above, the particle diameter selected for demonstrating compliance with the stipulated air cleanliness class is to be agreed between customer and supplier. (The customer is defined as the organisation - or the agent thereof - responsible for specifying the requirements for the cleanroom or clean zone, and the supplier is the organisation engaged to satisfy the specified requirements.)

If measurements are to be made at more than one particle size, each larger particle size (e.g. D₂) shall be at least 1,5 times the next smaller particle size (e.g. D₁):

A complete air cleanliness class indication must comprise three informations: the air cleanliness class itself, the particle diameter (or particle diameters) as well as the occupancy state of the cleanroom at which it should be met. The occupancy states are defined as follows in ISO 14644-1:

• As built: The condition where the installation is complete with all services connected and functioning but with no production equipment, materials, or personnel present.
• At rest: The condition where the installation is complete with equipment installed and operating in a manner agreed between the customer and supplier, but with no personnel present.
• Operational: The condition where the installation is functioning in the specified manner, with the specified number of personnel present and working in the manner agreed upon.

The minimum number of sampling locations is given by the formula:

where:

Note: In the case of unidirectional airflow, the area A may be considered as the cross section perpendicular to the airflow.

Sampling locations shall be evenly distributed throughout the area of the cleanroom and positioned at the height of work activity. During measurement, the sampling probe shall be positioned pointing into the airflow. If the airflow direction is uncontrolled or unpredictable, as in turbulent airflow, the inlet of the sampling probe shall be oriented vertically upwards. In comparison with U.S. Federal Standard 209E, the minimum number of sampling locations is generally lower, especially so in the case of large-area cleanrooms. Also, it is independent both of the air flow pattern and of the specified air cleanliness class.

The minimum single sampling volume per location should guarantee that at least 20 particles would be counted at the class limit - a determination already found in U.S. Federal Standard 209E. This requirement may, however, lead to undesirably high sample volumes in cleanrooms with a high level of air cleanliness, i.e. cleanrooms of ISO class 4 or less. In such cases, the sample volume and consequently the time required for taking a single sample may be appreciably shortened if the sequential sampling procedure as described in Annex F of ISO 14644-1 is used. Again, this procedure has been adopted from U.S. Federal Standard 209E without substantial change.

• the name and address of the testing organisation, and the date on which the test was performed;
• the number and date of the standard according to which the test was performed i.e. ISO 14644-1: 199x;
• a clear indication of the physical location of the cleanroom tested (including reference to adjacent areas if necessary), and the indication of the co-ordinates of all sampling locations;
• the specified ISO air cleanliness class, the corresponding occupancy state(s), and the considered particle size(s);
• details of the test method used, comprising also any specific conditions relating to the test, or departures from the test method;
• identification of the test instrument(s) and its (their) current calibration certificate(s);
• the test results, including particle concentration data for all sampling location co-ordinates;
• a statement of compliance or non-compliance with the specified ISO air cleanliness class.

D- Determinations for micro and macro particles

In some situations, typically related to specific process requirements, alternative levels of air cleanliness may have to be specified outside the size range of particles applicable to classification. Descriptors have been introduced for coping with such situations as follows:

• the U descriptor for ultrafine particles below 0,1 m m;
  
• the M descriptor for particles above 5 m m

The U descriptor is expressed in the format:

where:
x= the maximum permitted concentration of ultrafine particles, expressed as the number of ultrafine particles per m³ of air
y = the lower detection limit, i.e. the particle size in m m at which the applicable discrete particle counter - for example, a condensation nucleus counter - is capable of detecting such particles with 50 % counting efficiency

The M descriptor is expressed in the format:

where:
a = the maximum permitted concentration of macroparticles, expressed as the number of macroparticles per m³of air.
b = the equivalent diameter (or diameters) associated with the specified method for measuring macroparticles
c = the specified method for measuring macroparticles

The concept of the U descriptor is not new - it already forms part of U.S. Federal Standard 209E. On the other hand, the concept of the M descriptor is new. In determining M descriptors, the difficulties of sampling and assessing large particles has to be taken into consideration as well as the fact that large particles are normally process -generated. For these reasons the identification of the sampling device and evaluation procedure should be addressed on an application-specific basis. Factors such as the density, shape, volume and aerodynamic behaviour of the particles need to be taken into account. For describing for instance, an airborne macroparticle concentration of 1 000 particles/m³ in the particle size range of 10 to 20m m using a cascade impactor for sampling and a microscopic sizing and counting procedure for evaluation, the designation would be:

"M (1 000; 10-20 m m): cascade impactor
followed by microscopic sizing and counting"

Under certain circumstances it may be necessary, to put special emphasis on specific components of the total airborne particle population, such as fibres. Fibres for instance, may be accounted for by supplementing the M descriptor with a separate descriptor for fibres, having the format "Mfibre (a;b); c".

REQUALIFICATION OF CLEANROOMS

• verification of the air cleanliness class;
• verification of pressure differences between rooms;
• a verification of the air velocity (for displacement airflow) or of the airflow rate (for turbulent air-flow).

Other tests may be included in the requalification programme as agreed between customer and supplier such as:

• a leakage or integrity test for the HEPA filters of the cleanroom system;
• a recovery test for cleanrooms with turbulent airflow;
• a visualisation of the airflow in the cleanroom;
• a containment leakage test for the cleanroom enclosure, i.e. its walls and ceiling.

The test for demonstrating continued particle count compliance should be performed at intervals not exceeding 6 months for cleanrooms of ISO class 5 and below, and at intervals not exceeding 12 months for cleanrooms of class 6 and above. This maximum interval of 12 months also applies for the other normative compliance tests listed above. Where the installation is equipped with facilities for continuous or frequent monitoring, of the airborne particulate concentrations and of the differential pressure between rooms, the maximum time interval for the normative tests may be extended to 24 months. In the context of ISO 14644-2, frequent monitoring means that measurements should be updated at specified intervals not exceeding 60 minutes during utilisation of the cleanroom. i.e. in its operational state.

REFERENCES

[1]A.L. Moller, "International standards for the design of cleanrooms" in W. Whyte (editor), Cleanroom Design, John Wiley & Sons, Chichester, 1991, pp. 121-162.

[2]Document IES-RD-CC009.2 "Compendium of standards, practices, methods, and similar documents relating to contamination control", Institute of Environmental Sciences and Technology IEST, Mount Prospect, IL/USA, 1993.

[3]Anon., ISO Memento 1998, International Organisation for Standardisation, Geneva, 1998.

[4]Draft International Standard ISO/DIS 14644-1, "Cleanrooms and associated controlled environments - Part 1: Classification of air cleanliness". International Organisation for Standardisation, Geneva, December 1996.

[5]European Prestandard ENV 1631:1996, "Cleanroom technology - Design, construction and operation of cleanrooms and clean air devices". European Committee for Standardisation, Brussels, July 1996.

[6]U.S. Federal Standard 209E "Airborne particulate cleanliness classes for cleanrooms and clean zones". Washington, DC/USA, 11 September 1992.

[7]Draft International Standard ISO/DIS 14644-2 "Cleanrooms and associated controlled environments - Part 2: Specifications for testing and monitoring cleanrooms and clean zones to prove continued compliance with ISO 14644-1", International Organisation for Standardisation, Geneva, approved for circulation 14 April 1997.