ISO 14644-Part 13 ‘Cleaning of surfaces to achieve defined levels of cleanliness in terms of particle and chemical classifications’1 addresses the subject of cleaning cleanroom surfaces to remove particles (surface, in this context, refers to any solid object, work surface or item of equipment). Cleaning is an essential element of contamination control, and particles can cause significant damage to delicate materials and electronic components.
To put the new standard in context, ISO 14644 is a set of cleanroom standards (for which there are currently 12 active parts, in addition to the draft discussed in this article). The first part of the standard to appear was for the classification of cleanrooms in relation to particle air cleanliness, which was issued in 1999 (and subsequently revised in 2015).2
The standard, as one of the ISO series, can be applied to any sector. However, its foremost application is in the electronics and semiconductor industries. The standard does not, for example, refer to any microbiological aspects.
ISO 14644 Part 13 is an intended cleanroom standard concerned with surface cleanliness. Cleanliness is defined as the condition of a solid surface where the amount of contamination (which relates to the level of unwanted matter in an undesirable location – be it particle, chemical or biological) is controlled to a specific level as required for the intended use of the cleanroom.
The status of ISO 14644 Part 13 is a draft international standard (DIS). This means the document has been through the committee stage at various national standards organisations and deemed to be acceptable. The standard is then placed in the public domain for comments. It is unusual for a DIS to be withdrawn and substantially re-written; however, procedures do allow for this eventuality.
The main point of the standard is to provide guidance on cleaning methods designed to achieve the required surface cleanliness levels specified in ISO 14644–9
The main point of the standard is to provide guidance on cleaning methods designed to achieve the required surface cleanliness levels specified in ISO 14644–9 ‘Classification of surface cleanliness by particle concentration’3 and ISO 14644–10 ‘Classification of surface cleanliness by chemical concentration’4. With respect to these two standards, two initialisms are used: particle concentration (SCP), defined as the level that represents the maximum allowable surface concentrations, in particles per m2, for considered sizes of particles; and surface cleanliness by chemical concentration (SCC), which relates to common logarithm (to the base of 10) of the chemical concentration on a surface in grams/m2.
These terms are used in relation to specific cleanliness classes. ISO 14644–9, -10, and -13 should be applied in cleanrooms that have first been classified by ISO 14644–1 for particulate levels. ISO 14644–9, -10, and -11 provide levels of cleanliness for other attributes5. These attributes are particles and chemicals.
A key factor is the interactive bonding forces between contaminants and surfaces, which influences how particles are attracted to surfaces and how easily particles can be removed from surfaces
Particles in cleanrooms are generated by people, by equipment and production processes. Even under the cleanest class of cleanrooms, particles remain a risk factor. Particle generation will be both time-dependent and process-dependent. A key factor with particle contamination is the interactive bonding forces between contaminants and surfaces, which influences how particles are attracted to surfaces and how easily particles can be removed from surfaces. The factors influencing particle attachment to surfaces include both electrostatic charge and ionic charges.
Cleanroom surfaces are environments that can create high levels of electrostatic charge and such charges can remain on objects for prolonged periods of time. When a charge flows, this is termed electricity (such as charge flowing through a conductor). However, with a material that does not allow the charge to flow, this is termed an electrostatic charge – that is the charge ‘stays still’.
According to Mittal6, the air in cleanrooms is stripped of its normal air ion content, as a consequence of moving through high efficiency particulate air (HEPA) filters. The concern is that many of the materials found within cleanrooms act as insulators, such as materials fashioned from plastics, glass, metal and silicon. These materials become readily charged and these charges pull particles out of the air stream and onto surfaces. The consequence is that particles in the air become attracted to the materials in the cleanroom and become bound to surfaces through the subsequent electrostatic charge.
Removing particles attracted to surfaces through electrostatic or ionic charges is a key contamination control step in industries where particles can cause damage to the materials being processed
In relation to electrostatic charge, damage can also be caused from discharge. When two surfaces rub together an electrical charge can be created. Examples of charge creation are: moving air; people touching surfaces or walking across the floor; and operating equipment. Electrostatic discharge can damage electronic components.
With ionic charges, ions are electrically charged particles that are formed when atoms lose or gain electrons. Some materials such as metals form positive ions, whereas other materials form negative ions. The strong electrostatic forces of attraction between oppositely charged ions can create ionic bonds, and this can also influence a particle being attracted to a surface7.
Removing particles attracted to surfaces through electrostatic or ionic charges is a key contamination control step in industries where particles can cause damage to the materials being processed, such as semiconductors. The Part 13 standard provides the basis for cleaning such surfaces.
The second attribute i.e. chemicals, relates to any unwanted chemical material that can exert a deleterious effect on the product, process or equipment.8
Contamination removal
With particle and chemical contamination, methods need to be deployed to remove the contamination. This is not always straightforward since different variables can affect the cleaning of cleanroom surfaces. These include the type of surface (e.g. whether it is hydrophobic or hydrophilic), together with its geometrical complexity and size. The state of the surface influences both chemical and particle contamination levels. A hydrophilic surface is water wettable, although it will have a lower electrostatic charge; and a hydrophobic surface will repel water (although it may be lipophilic and attract oil), and will carry a higher electrostatic charge. Several chemical factors that influence surface activity are listed in the draft standard.
Other variables include the types of contamination likely to be present on the surface; the cleaning techniques used; and material compatibility (some surfaces will be sensitive to different cleaning agents). Also relevant is the surface type and the presence of thin surface layers that can provide protective layers for contamination; and, as indicated above, other physical properties such as electrostatic phenomena that will attract particles to the surface.
Here the level of contamination is also an unknown variable. This will depend upon opportunities for particles to be generated; and for chemical reactions to occur. It is recommended that a baseline be established to assess surface contamination levels. Each of the variables, together with the contamination levels, should be factored into a risk assessment so that the optimal approach for cleaning can be established.
Once cleaning methods have been selected it is important that they are validated
The standard allows for different cleaning methods to be used, according to the specific level of cleanliness required. Once cleaning methods have been selected it is important that they are validated. Validation will need to embrace the measurement techniques used; the extraction techniques deployed (focusing on cleaning efficiency and cleaning efficacy); and the method used to assess surface cleanliness.
Assessment of cleanliness can include microscopic methods, in relation to particles; and spectroscopic methods in relation to chemical contaminants. Factors to be decided when implementing the standard include cleaning frequencies and which cleaning agents to deploy. Cleaning techniques are divided into physical methods (such as wiping; thermal processes; or ultrasonication) and chemical methods (such as the application of a detergent; a solvent; or plasma cleaning). Cleaning techniques can further be subdivided into ‘dry’ and ‘wet’ methods. A dry method might include the use of a soft cloth, whereas a wet method might include surface rinsing. A range of different cleaning methods are listed in the draft standard. The cleaning technique should not be so harsh as to damage the surface and any agent used must be demonstrated not to react chemically with the surface.
What is not included?
While the new standard is useful, there are a number of aspects excluded. These include any reference to the type of product produced within a cleanroom; any specific surface-related cleaning methods; and any detailed description of cleaning mechanisms, methods and procedures of various cleaning methods.
In summary, ISO 14644 provides advice for those concerned about particle cleanliness and fits in well with existing standards. In all likelihood the document will be adopted, and cleanroom users in the appropriate sectors should begin to familiarise themselves with the main points of the document in preparation for the final version being issued.
References
1. ISO/DIS 14644-13: Cleaning of surfaces to achieve defined levels of cleanliness in terms of particle and chemical classifications, ISO, Geneva, Switzerland, 2016
2. Sandle, T. and Vijayakumar, R. Cleanroom Microbiology, DHI/PDA: Bethesda, MD, USA, pp1–5, 2014
3. ISO 14644–9:2012 – Part 9: Classification of surface cleanliness by particle concentration, ISO, Geneva, Switzerland, 2012
4. ISO 14644–10:2013 – Part 10: Classification of surface cleanliness by chemical concentration, ISO, Geneva, Switzerland, 2013
5. Ensor, D. S. ISO Technical Committee on Cleanrooms and Associated Controlled Environments Enters Third Decade with New Changes on Horizon, Journal of the IEST, V. 57, (1) pp29–34, 2014
6. Mittal, K.L. Particles on Surfaces: Detection: Adhesion, and Removal, CRC Press: Roca Raton, 1994, pp140–148
7. Lieberman, A. Contamination Control and Cleanrooms: Problems, Engineering Solutions, and Applications, Springer, New York, pp94–98
8. Saiki, A. Particle contamination and chemical contamination in cleanrooms, In (Anon.) Proceedings of the 15th symposium on Aerosol Science and Technology, Tokyo, Japan, pp104–105
The author
Dr Tim Sandle is Head of Microbiology at Bio Products Laboratory Limited. He is an expert in cleanroom standards and has written several books including: Pharmaceutical Microbiology: Essentials for Quality Assurance and Quality Control for pharmaceutical microbiologists (Elsevier) and Cleanroom Management in Pharmaceuticals and Healthcare (Euromed), which covers all aspects of cleanrooms from design and qualification to cleaning and microbiological monitoring. He is also an active member of Pharmig, the UK-based association for pharmaceutical microbiologists (www.pharmig.org.uk).