Surface particulates

This is the second of two articles concerning additions to the basic group of international ISO cleanroom standards. The September issue covered the launch of the new airborne molecular specification, EN-ISO 14644-8. This second article covers the subject of Surface Particulate, which is designated part 9 of these standards, and currently in its final draft format. Its purpose is to qualify the cleanliness level of components or material moving into or out of the cleanroom.

Molysmophobia or the fear of being unclean may be traced back to Johnson’s 19th century dictionary but its principles are carried into today’s technology. “How clean is clean?” is one of the key conundrums facing the modern process engineer and is finally being addressed in terms of an ISO standard. While the US military as well as the semiconductor and motor vehicle industries have developed cleanliness stan-dards over the years, this is the first one to be non-industry specific.

Since the concept is Swiss-driven, there is a nucleus of precision engineers in the standard’s working group. Furthermore, created as a spin-off from the ISO AMC work, the group includes many of the Europeans from that team. Notable additions would be Dr Fuji from Japan, Mrs Tovena-Pecault from France and Mr Gommell from Germany. These cover a background from nuclear to semiconductor and have provided a wealth of expertise, research and practical knowledge during the past three years. The illustration below (generally attributed to K Mittal in the US) shows the range of particulates and residues surrounding a typical surface. The working group added roughness and porosity to this model to complete the full consideration.

Three deliberate and pivotal exclusions were made when considering the scope of the standard:

• That of biological contamination, since this is considered to be part of EN ISO 14698
• That of chemical contamination – in favour of an extension to the AMC standard outlined in the previous article.
• That of the size range of 0.1 to 500 microns, in order to widen usage potential into the nuclear and automotive fields.

The current draft defines the subject area and in-cludes notes on porosity, roughness and ESD. A study was also carried out to examine the exact correlation between roughness and particulate. Whilst the plastic results shown below indicate a possible trend, this work is considered inconclusive.

Measurement techniques

Measurement and sampling methods are divided into direct and indirect systems with explanation and application tables included. These are graded for material suitability and include guidance for the packaging of test samples and presentation of data. Thus the range extends from graticules, optical surface counters and ultrasonic extraction systems to devices such as SEM, ESCA, X-ray and EDX.

Band width

Eight parallel log-log slopes are used to characterise the relationship between particle size and quantity. (Shown per sq metre, as the ISO preferred unit, not per sq cm as practised). The slope of the graph is the defining criteria and has been deduced from extensive work in Japan. The resulting equation is quoted in the new standard but is excluded from this article.

As for the airborne sample system used in 14644 part 1, the slopes may only be used for guidance. The tester and supplier should defer to the equation for definitive results. The principle of the parallel slopes is shown below with numbers changed to avoid copyright infringement.

SPC indices

Surface Particle Count (SPC) numbers are used to identify each slope with the lowest number representing the highest quality level. Line extrapolation is not permitted and there are also some critical rules about the application of the minimum and maximum line extremities. Unlike the AMC slopes, it is not considered that levels lower than class 1 will be reached. Therefore a system of positive logarithmic indexes has been used. Two size ranges may be used to define the SPC level.

As can be seen from the first illustration the surface varies dramatically at a microscopic level. It is therefore essential to qualify the exact position required for sample selection before it is taken. This must also be included in the contract document together with the agreed technique.

Examples would be:

Sampling the bore of a glass tube or the exterior of a wafer cassette. The first example might state

“SPC Class 5 at 0.5 microns by liquid particle counting.”

This will allow a maximum of 20 half-micron parti-cles to be flushed from each square centimetre of the bore.

The second example may state:

“SPC class 6 at 1 and 10 microns by external sur-face particle counting.”

This would allow 100 particles at 1 micron and 10 at 10 microns per square centimetre of external box. Measurement would be qualified as surface particle counting with vacuum suction.

Target market

This document should have wide appeal in procurement contracts where cleanliness is a condition of purchase. Design, process and quality control engineers should also find the content useful in monitoring process capability. Equipment and furniture suppliers should also be aware of its potential to qualify the cleanliness level of material supplied to the cleanroom facility. However, it will have to be used in conjunction with the chemical contamination document currently in process.

We expect to complete the last of 10 drafts by the end of 2006. This will be circulated for international ISO review and subsequent translation in 2007. Publication should then follow later in the year.