Nanotechnology – How big is small

To the average man in the street, the building next to him is very large but compared to the city the building is quite small. This city, compared to the country is smaller still, and then to the planet and the universe and so on. We can go on forever – quite literally due to the nature of infinity.

Taking that in the other direction, we have the basis of a much more relevant question, at what point does small actually become big?

Cleanroom classifications specify a permitted concentration of particles of a certain size per unit volume. For the semiconductor and microelectronics industries, this has meant dealing with circuitry on the micron scale and hence cleanliness is generally given in terms of a concentration in particles greater than or equal to 0.5µm – although the necessary standards do specify at 0.1µm if preferred – which is generally smaller than the working scales.

The founder of Intel, Gordon Moore stated that the speed of a computer will approximately double every 18-24 months. This is now referred to as Moore's Law and has meant ever decreasing scales of semiconductor devices to allow more processing power and speed on a chip.

Striving to achieve the dictates of Moore's Law and even go beyond them has seen the advent of nanotechnology in which we are now dealing on the nano-scale, i.e. sub micron.

This raises the issue of whether our standard cleanroom filtration levels are acceptable as our 0.5µm particle is actually 500nm and potentially problematic on the nanoscale. Diagramatically if the red ball is a 50nm structure, the blue ball is a 0.5µm particle.

With current technologies, for example the use of molybdenum-silicon mirrors to project chip design onto wafers, there is potential to form components as small as 32nm across so the particle size could prove disastrous, couldn't it?

The latest standard, ISO14644-1, has gone some way towards accommodating this with the 2 new classifications, class 1 and 2. However, these standards only deal with particles down to 0.1µm (100nm), which is still potentially double the size of the components we are trying to protect. Surely, one would think that this is a huge problem? Well in actual fact it shouldn't be.

The reason is that we are now working on the sort of scale where the low concentration of such small particles per m³ will mean that the distances between particles are relatively vast. Assuming a steady state and like charged particles to limit attraction, the probability of such a particle being in the exact place that you do not want it to be in a class 1 room are 1:3.2x10¹⁸. This means that you are approximately 2.3x10¹¹ more likely to win the UK national lottery jackpot than to contaminate your component.

Bear in mind that the figures quoted reflect a steady state environment which a cleanroom is not. There is a constant air change rate that should lessen the particle counts significantly as long as the correct gowning regimes are followed.

These are still pretty good odds and due to the astronomical cost of installing an ISO14644-1 class 1 room, there has to be an alternative that fits in with the budgetary and operational requirements of most organisations. As the majority of such research is carried out by academic institutions with limited funding but seemingly limitless competition for that funding, we need to ask: What is the ideal specification for a nanotechnology cleanroom?

Ideal specification

There is no precise answer to this as nanotechnology covers such a wide scope and certain applications will require ever more stringent environments. There is also a greater need for flexibility as the technology is still in its infancy and could lead to a number of currently unforeseen requirements.

However, having considerable experience in the design and construction of nanotechnology rooms and associated areas for a number of universities such as Cambridge, University College London, Newcastle, Sussex and Huddersfield as well as defence organisations and industry, there seems to be a pattern emerging.

Class 6 areas with localised class 4 or 5 laminar flow above specific work benches and wet process stations seem to be adopted by most institutions. class 7 and above facilities are not controlled at 0.1µm and so are not really suitable for nanoscale work although there are some in existence as nanotechnology rooms.

These facilities will generally offer good cleanliness standards with enhanced flexibility due to the cleaner local environments without the increased capital outlay that would be expected for higher classification rooms. They are also much more straightforward to modify where later changes in technology require.

The second question to ask is how does this lower classification affect our performance and yield quality?

Performance

In a class 6 environment there are a total of 1,382,813 permissible particles of sizes from 5µm to 0.1µm as a worst case scenario. Assuming regular shaped particles, the maximum volume of the space that can be occupied by these particles is 54,985µm³ in a volume of 1m³, which gives a ratio of 1:1.8x1013. Looking at the potential spacings of these, that lottery win is still looking about as likely as a particle being where you don't want it. By carrying out more sensitive work under class 5 or class 4 hoods, these odds are lessened further by approximately a factor of 10 each time, creating reasonably safe environments in which to carry out nano-scale works.

Fig. 2 demonstrates such areas within the University of Newcastle Nanotechnology cleanroom constructed in 2001. This shows a wet process area with integral laminar flow and class 5 hoods above all potential work areas to give maximum flexibility. Although these rooms were designed to operate overall at class 7, the emphasis was on the laminar flow hoods and the wet benches to carry out the intricate work and the overall operational parameters far exceeded the class 7 design.

These concepts are taken further still at the London Centre for Nanotechnology, which is a joint venture between Imperial College and UCL and is currently under construction in the heart of London.

This state of the art cleanroom facility has been designed to specifically meet user needs and is segregated into tunnels to divide the different processes and disciplines involved. It is in the central tunnel (see Fig. 3) where the main fabrication is carried out and all works take place either in one of the specialist wet process benches or beneath class 5 hoods. When this facility opens in early 2005 it will be one of the UK's premier nanotechnology research facilities and will bring together many of the industries leading figures.

The emphasis here is on flexibility with ample service areas accessible from outside and a higher than average ratio of small power to all areas.

There is also a huge accent on the aesthetics of the room and the building as a whole. With a high profile central London location, the appearance is very important, especially with full height glazing to the external wall which needs to match precisely with the structural windows to give a world class appearance from both inside and out.

As important as appearance is, we should not get bogged down with this as the starting point for our design. Priority number one is functionality followed closely by flexibility for any nanotechnology facility.

And the future?

This article has taken a somewhat simplistic view of the cleanroom environment with regards to nanotechnology with the intention of demonstrating to those involved the relative sizes that we are dealing with. It is often difficult for the user to appreciate just how low a concentration of particles they actually have compared with the amount of "free air".

From the user's point of view as well as the contractor's, it should be reassuring to know that our current standards will meet the requirements of the nanotechnology industry as it stands at present and probably into the foreseeable future. However, we as an industry need to keep a close eye on the technology that we make possible as this may change.

After all we do not know what the future will hold but we do know it will be small....very small.

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