Current status of cleanroom certification

ISO TC 209 – "Cleanrooms and associated controlled environments" has shifted its focus from the traditional emphasis on air cleanliness to the broader issue of contamination control, with particular attention to clean surfaces. Two new classification systems for surface cleanliness (ISO 14644-9 Surface cleanliness by particle concentration; ISO 14644-10 Surface cleanliness by chemical concentration) were published in 2012. Since there are now four classification systems (for airborne and surface contamination, each for particles and chemicals, with potential future systems for microbiological cleanliness), a coding system was established to identify these specific classification standards. To incorporate the nanometer range into the existing classification framework, a preliminary standard for classifying air particle cleanliness based on nanoparticle concentration was developed. In the next revision of ISO 14644-1, the nanorange will be integrated, covering the entire size spectrum from nano to micrometer. 
The significant revision of ISO 14644-1 will introduce many changes compared to the previous edition, which are discussed here. 
Two new projects were initiated in 2012: 
- "Cleaning of Surfaces" 
- "Assessment of Suitability of Equipment and Materials for Cleanrooms" 
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The ICCCS 2012, the biannual international symposium of cleanroom societies (September 2012, Zurich, Switzerland), marked a turning point in international standards development in the field of cleanroom technology. After years of effort, a paradigm shift was achieved: from a focus solely on airborne particle contamination to a comprehensive approach encompassing all aspects of contamination control, especially surface cleanliness. The recognition that nanotechnology exists had to be fought for and accepted as well.

ISO TC 209 "Cleanrooms and associated controlled environments"

The technical committee ISO 209 "Cleanrooms and associated controlled environments" is responsible for standardization. This committee has issued several international standards so far (see Table 1).

Table 1: List and status of all published standards of ISO TC 209 "Cleanrooms and associated controlled environments"

ISO	English Title	DIN EN ISO 14644-	Status
14644-1:1999	Classification of air cleanliness	Classification of air cleanliness based on particle concentration	Revision
14644-2:2000	Specifications for testing and monitoring to demonstrate continued compliance with ISO 14644-1 New: Monitoring to provide evidence of cleanroom performance by airborne particle cleanliness	Specifications for testing and monitoring to demonstrate ongoing compliance with ISO 14644-1	Revision
14644-3:2005	Metrology and test methods	Testing methods	Revision
14644-4:2001	Design and construction	Planning, execution, testing procedures
14644-5:2004	Cleanroom operations	Operation
14644-6:2007	Vocabulary	Terminology	Revision
14644-7:2004	Separative devices (clean air hoods, glove boxes, isolators and mini-environments)	Separation devices (clean air hoods, glove boxes, isolators, and mini-environments)	Standard
14644-8:2006	Classification of airborne molecular contamination New: Classification of air cleanliness by chemical concentration – ACC	Classification of airborne molecular contamination New: Classification of air cleanliness by chemical concentration – ACC	Revision
ISO 14698 Biocontamination Control
14698-1:2003	General principles	General principles	Revision
14698-2 2003	Evaluation and interpretation of biocontamination data	Evaluation and interpretation of biocontamination data	Revision

 
Table 1 shows that most published standards are currently under revision. The key revisions of Part 1: Airborne Particle Cleanliness and Part 3: Testing Methods will be discussed further.

Table 2: Newly published standards (2012) and ongoing projects of ISO 14644

Tab. 2: New standards, standards in progress, and new projects
14644-9: 2012	Classification of surface cleanliness by particle concentration	Classification of particulate surface cleanliness	Newly published
14644-10: 2012	Classification of surface cleanliness by chemical concentration	Classification of chemical surface cleanliness	Newly published
14644-12	Classification of air cleanliness by nanoscale particle concentration	-	CD
14644-X	Cleaning		Project
14644-Y	Suitability of Equipment and Materials for Cleanrooms		Started

 

A new coding system for classification standards has been introduced. The original series of standards (Table 1, Parts 1-7) included only a classification standard for airborne particle cleanliness. Later, classifications for air chemical purity (Part 8) were added. After the publication of the standards for particle and chemical surface cleanliness (and later biological cleanliness), a system was needed to clearly identify each classification system. Based on a Swiss proposal, a coding system was introduced (Table 3). The code consists of three letters:
1: A (Air) for airborne or S (Surface) for surface cleanliness
2: C (cleanliness) for purity
3: P for particles, C for chemicals, or V (viable) for microbiological purity

Table 3: Coding system for the classification standards of ISO TC 209

ISO Standard	Name of Standard	ISO Code	Status 2012
14644-1	Classification of Air Cleanliness by Particle concentration	ACP	DIS (Revision)
14644-12	Classification of Air Cleanliness by  Nanoscale Particle concentration (later fully integrated into 14644-1)	nACP	WD
14644-8	Classification of Air Cleanliness by Chemical Concentration	ACC	DIS (Revision)
14644-9	Classification of Surface Cleanliness by Particle Concentration	SCP	STD 2012
14644-10	Classification of Surface Cleanliness by Chemical Concentration	SCC	STD 2012
Microbiological cleanliness classification (planned)
14698-x	Classification of Airborne Cleanliness by Viable Particle Concentration (Colony forming units)	ACV	Proposal
14698-y	Classification of Surface Cleanliness by viable particle concentration	SCV	Not started yet

Therefore, each cleanliness class designation within TC 209 must be paired with a code that identifies the classification system, e.g.: 
ISO ACP Class 6 indicates air cleanliness by particle concentration, Class 6. 
ISO SCP Class 6 indicates surface cleanliness by chemical concentration, Class 6. 
This coding system allows for unambiguous identification of each classification and avoids confusion caused by multiple classification systems within the same standards series.

Major changes in revisions of existing standards

Revision of ISO 14644-1: Classification of air cleanliness by particle concentration ("Old title: Classification of air cleanliness")

ISO 14644-1: 1999 was the first and fundamental standard in the ISO 14644 series, primarily based on US Fed. Std 209 E from 1988. Updating and modernizing this standard was necessary. One major issue was the statistical basis used. Additionally, the standard completely ignored nanotechnology. The revision proved highly complex, with over 200 comments, some fundamental, on the draft proposal. To address these, the working group (WG 1) produced a new (second) draft (DIS - Draft International Standard), leading to significant delays. Following the new naming convention for classification standards, the name of the standard was changed, and the abbreviation ACP (Air Cleanliness by Particle Concentration) was introduced. Key changes include:

Classification via table instead of diagram
The basis for class determination is now Table 1 instead of the previous diagram. This is simpler and more accurate.

Elimination of particles ≥ 5 µm in ACP Class 5:
Another important change is the removal of particles ≥ 5 µm in ACP Class 5. The low count (29 particles ≥ 5 µm) is statistically insufficient for evaluation with current optical particle counters. However, this change conflicts with GMP requirements, especially in life sciences, which demand such classifications.

Sampling procedure:
Most discussions focused on the sampling method in Annex A, Table 1, for determining the number of measurement points based on room size. The minimum number is statistically derived to guarantee with 95% confidence that at least 90% of measurement points are within class limits. This is only valid if each measurement point within a 2x2 m area is randomly selected. To meet this condition, a random generator would be needed, which is impractical. The current table A.1 will be expanded to 1000 m², and alternative solutions will be developed for larger rooms.

Very large cleanrooms: 
Another challenge involves enormous cleanrooms up to 40,000 m² (mostly in China). Constructive proposals for modifying Table A.1, combined with new solutions for large cleanrooms, are under consideration. The definitive solution remains open.

U-Descriptor: 
The so-called U-Descriptor for the number of ultrafine particles in a cleanroom is no longer used after the revision. Today, these ultrafine particles are called nanoparticles and are specified in a new classification standard for the nanometer range (see further).

Risk locations: 
Another debated topic was the handling of risk locations when assessing the cleanliness of critical points within a cleanroom or clean zone. No consensus has been reached yet.

Inclusion of measurement methods: 
Particle measurement methods are being separated from ISO 14644-3 (Test methods) and integrated into ISO 14644-1.

Summary of the revision of ISO 14644-1: 

Many open questions remain, mainly regarding the classification approach and the number of measurement points relative to room size. However, consensus has been (probably) reached on some issues, such as abandoning the random selection of measurement points within the surface unit.

Other important revisions:

Revision of ISO 14644-2: (Specifications for testing and monitoring to demonstrate ongoing compliance with ISO 14644-1) 
Now titled "Monitoring to provide evidence of cleanroom performance by airborne particle cleanliness," focusing solely on monitoring.

Revision of ISO 14644-3 (Test methods): 
Post-revision, this standard will include general measurement methods such as leak testing, pressure difference measurement, airflow measurement, etc. Specific methods for classifying air or surface cleanliness are detailed in the respective standards.

Revision of biological standards (ISO 14698 series): 
Microbiological cleanliness, proposal: 
Following Swiss initiative, WG 2 "Biocontamination Control" will develop a microbiological cleanliness classification for the first time. This is a revolutionary step, as the existing GMP classification with classes A, B, C, D is outdated and does not allow precise specification of requirements. A proposal for classifying microbiological cleanliness of air and surfaces exists. For air cleanliness, four classes are defined with units "CFU (Colony Forming Units) / m³": the highest (cleanest) class is ACV1X (Air Cleanliness by Viable concentration) with a maximum of 10 CFU/m³, and the lowest (least clean) is ACV4X with 10,000 CFU/m³. For surface cleanliness (units CFU per surface area - m² or dm²), five classes are proposed, starting with SCV1X (< 4 CFU/m²) up to SCV5X (> 300 CFU/dm²). The "X" index indicates the microbiological contamination type, e.g., SCV 4E - E. coli. This microbiological classification is still under discussion.

New standards, standards in progress

Beginning of a new era: Surface cleanliness
After long discussions and strong resistance from TC 109, the Swiss proposal to broaden the scope from airborne contamination to all aspects of contamination control, including surface cleanliness, was accepted. This marks the start of a new era in TC 209, which now develops classification standards for surface cleanliness. Previously, it was not possible to specify cleanliness requirements for surfaces in the same way as for air.

Almost simultaneously, standards for particle cleanliness (ISO 14644-9) and chemical cleanliness (ISO 14644-10) of surfaces are being developed. Both standards are parallel to the existing classification standards for air cleanliness (ACP ISO 14644-1 for particles and ACC 14644-8 for chemicals). 

ISO 14644-9: 2012: Classification of Surface Cleanliness by Particle Concentration (SCP): 
Classification is based on the number of particles ≥ 1 µm per unit area (square meter). The logarithm (base 10) of the particle count ≥ 1 µm determines the SCP class. For example, SCP Class 2 indicates a maximum of 100 (10²) particles ≥ 1 µm. The table specifies the maximum allowed number of particles for sizes from 0.05 to 500 µm (nine size categories). The table is similar to the one used for airborne particle classification (ACP).

ISO 14644-10: 2012: Classification of Surface Cleanliness by Chemical Concentration (SCC): 
Classification is based on the logarithm (base 10) of the concentration of the chemical component per square meter. There are 13 classes, from Class 0 (100 g/m² = 1 g/m²) to Class -12 (10⁻¹² g/m²), mainly for applications in microelectronics and optics. The chemical substance must be specified along with the class, e.g., 
SCC Class -8 (TOC, Total Organic Carbon), SCC Class -6 (Ammonia, NH₄).

The new surface cleanliness standards enable, for the first time, objective specification of surface cleanliness, a capability previously limited to air cleanliness.

ISO 14644-12: Classification of air cleanliness by nanoparticle concentration (ACnP)
The lower limit of the current ISO 14644-1 standard for airborne particles (and other published standards in the series 14644) is 0.1 µm, corresponding to 100 nm. This limit originates from early standardization efforts in 1999, before the advent of nanotechnology. Recognizing the importance of nanotechnology, TC 209 accepted the existence of nanomaterials and decided to initiate a nanometer-range standard. A Swiss proposal allowed a simple integration of the nanometer range (1-100 nm) into the existing ISO 14644-1 base standard. Due to the lengthy and complex revision process, it was decided to first develop a provisional standard for the nanometer range, with full integration into the revised 14644-1 planned for the next revision. The revised standard will cover the entire particle size range from nano- to micrometer. It will differ from the previous only by the inclusion of four new particle sizes (0.05, 0.01, 0.005, and 0.001 µm, i.e., 50, 10, 5, and 1 nm).

New standards (in development)

"Assessment of Suitability of Equipment and Materials for Cleanrooms" WG 11 
This important standard provides fundamental instructions regarding the suitability of various materials and equipment for cleanrooms, selection of appropriate material combinations, and guidance on potential cross-contaminations.

Surface cleanliness: Cleaning methods 
While the new surface cleanliness standards include necessary measurement and analytical procedures, they do not specify cleaning methods to achieve a particular cleanliness class. Since cleaning methods for removing particles, chemicals, and biological contaminants are mostly similar or identical, it was deemed practical to extract cleaning procedures from individual standards and describe them in a common standard.

Summary: 
With the publication of standards for surface cleanliness classification, ISO TC 209 has transitioned from traditional airborne cleanliness issues to the technically relevant field of surface cleanliness. The revision of the somewhat outdated ISO 14644-1 is of great importance for the technical community. Extending the classification into the nanometer range benefits not only cleanroom technology but also the entire field of nanotechnology. Unfortunately, the integration of the nanometer range into the existing standard occurs in two stages: first, a separate standard for the nanometer range is created; only in the next revision of ISO 14644-1 will the nanometer range be fully incorporated into the main standard. 

Egon Hollaender, Dipl. Ing., is one of the pioneers in contamination control in semiconductor technology. He is actively involved in international standardization in nanotechnology and cleanroom technology, chairs Swiss committees for standards in these fields, and has contributed significantly to the development of several international standards and VDI guidelines.