Ringing in the changes
While the ink on the wording of the ISO 14644 group of standards is still wet, SEE contamination group chairman Dick Gibbons reviews their implications and – in this, the first of a two part series – the requirements for airborne molecular contamination
Cleanroom professionals will be aware that British Standard 5295 and Federal Specification 209 cleanroom standards have been replaced by the 14644 group of international ISO standards. This is in order to unify the specification, design and validation of cleanrooms at a global level and involves the preparation, review and publication of nine new documents prefixed by EN- ISO 14644.
Some of these documents are already in the public domain and are available from BSI in Chiswick, UK, others are in the final stages of approval and will be seen in the next few months. Note, however, that several of the new documents have been written to address issues other than room design or classification. Part 7, for example, deals with mini environments and glove boxes, while new standards for molecular contamination and surface cleanliness are currently being finalised as sections 8 and 9 respectively. These two articles discuss the background to these particular specifications and present an insight into their content, need and application. The author is the convenor of working group 8 and the UK expert to working group 9.
14644 - 8 Airborne Molecular Contamination (AMC), released in August 2006, defines cleanroom limits and test methods for outgassing and ingressing chemicals.
This contamination mechanism involves the exposure of process material to molecular levels of chemical, which have either penetrated the room filters, or are escaping from the fabric of the cleanroom or its processes. Loosely described as "outgassing", the chemical molecules are either deposited as a surface impurity or react with the substrate to form complex discrete films.
For example, diesel fume is known to easily penetrate unprotected HEPA via the fresh air ducting, which if badly positioned may also pick up chemical exhaust from adjacent fume stacks. Other principal sources are the adhesives or wall coverings used in the building fabric and the detergents and buffing agents used in floor polish.
The deposited film may inhibit subsequent deposition processes, effecting process yield, or may shorten product life by creating corrosion or stiction (static friction) products. Detection is often hindered by delayed reaction of the absorbed chemical during the shipping or storage phase. Processes at risk include medical device, microelectronics, disc drive, precision lenses and silicon wafers. Ancillary processes, such as tooling or furniture manufacture, wafer cassette, or product cleaning, are also vulnerable to this problem.
Examples would be wafer haze due to acid from adjacent exhaust stacks, circuit corrosion from chloride release in PVC sheeting, or read/write heads stuck to data discs by organic fume compounds.
Since the AMC may also arrive during filter leak testing with DOP (Dioctyl Phthalate) or smoke, new test procedures in 14644-3 include latex sphere counting, up and down stream of the filter as an alternative for sensitive rooms.
The team behind the new AMC standard consists of a working group of European, Japanese and US experts in the field of molecular contamination chaired by the UK. This includes eminent people such as Drs. Fujimoto, Forslund and Veillerot as well as practitioners, such as Egon Hollander, who are well versed in ISO protocol. Work has proceeded over the past five years under three different UK chairman starting with the sadly deceased Martin Reeves. A further document is now being prepared to cover the effect of AMC deposition on the product surface.
Experts from the European life science, aerospace and nuclear industries have been added to the original team convened by the author. This document will be known under the heading of Surface Chemical Contamination (SCC) and defines the chemical residue in terms of nanograms/sq m of deposit.
Part 8
Meanwhile, the recently released document introduces 13 control levels for four separate forms of molecular problem; acid, base, organic and inorganic chemical. The chemicals are also listed in terms of effect, ie, bio-toxic, condensable, corrosive or dopant.
An unusual system of negative logarithmic group descriptors is used to control their concentration in terms of g/m3 of air. The lowest is class -12 representing 10 –12g/m3. While the dirtiest is class 0 for 10-0 g/m3
This system has been used to enable additional groups to be added as lower working levels are established. When appropriate these can be combined with the required airborne particulate level in the form of a total room specification for use by the constructor and client.
An example would be:
ISO 4, AMC class - 5 S base, class - 6 S ammonia.
Interpreted as:
Class 4 for particles, 10 –5 g/m3 total base chemicals, and 10 –6 g/m3 of total ammonia.
The specification includes tables and ratings for most industrial problem chemicals. Guidelines on sampling procedure and a range of test techniques that can be used for finite measurement are included in the standard. Due recognition is also paid in the specification to the decay period for these chemicals which can often run to several months.
Although the specification lists procedures that can minimise AMC burden, it does not detail the type of filter that can be used to counter the external threat.
Target market
Process or quality control engineers involved in the preparation of high purity surfaces or working with reactive materials inside cleanrooms would be well advised to obtain a copy of this document. Procurement agents, architects and constructors should also become familiar with its scope. There is a considerable application for its use in new cleanroom contracts. Test engineers and analytical chemists should also become familiar with the procedures defined within.