Bit parts

As the world gets ever smaller the demand for mini electronic devices grows ever larger. Jon Excell reviews the techniques of the burgeoning micro-electronic components industry.

In the 1949 film noir classic The Third Man, Orson Welles’s character Harry Lime is famously disparaging about the Swiss. While warring nations have inspired all manner of useful developments, 500 years of Swiss democracy, peace and brotherly love have, he said, produced nothing more remarkable than the cuckoo clock.

If Harry Lime were around today he might be forced to reconsider this view. The Swiss clock industry is the foundation upon which one of the world’s most promising industries rests: the manufacture of MEMS (micro electro-mechanical systems).

Micro-machining methods have been used in the semiconductor industry for a number of years. With tiny mechanical components increasingly being combined with electronics, the demand for smaller and smaller gears, valves, cantilevers and a host of other components is rising. One company that has grown to fulfil this demand, and encapsulates the link between the traditional and the cutting edge is Swiss organisation ALC Precision.

The firm has its roots in the 1846 invention of the automatic Swiss screw machine, a turning machine used to produce tiny components for the watch industry. Now a giant in the world of small components, ALC offers a range of techniques for making tiny components for the medical, telecoms, machinery and automotive industries. The company produces parts only a few microns in size, with tolerances (or margins of error) ranging from the sub-micron to the more conventional.

For large tolerances and non-critical features the company uses fairly traditional mechanical machining methods. However, in the search for reduced size and improved tolerance these ‘external’ methods hit a ‘glass floor’, a point beyond which these methods are no longer accurate: the mechanical, thermal and even chemical properties of the material being formed cannot withstand the forces being applied to them. Metal shears instead of being cut, and adjacent features can be influenced or even destroyed during the cutting operation.

For smaller features and smaller tolerances ALC therefore recommends turning to ‘light-based’ technologies: lithographic tooling processes such as LIGA and UV LIGA. LIGA, an acronym for the German words for lithography, electroforming and injection moulding, is essentially a process by which a component can be grown.

Lithographic tooling is based on the same concept as used in the semiconductor industry to produce silicon chips: light energy is accurately focused so that patterns can be written into silicon or polymer materials. Using this kind of process ALC said that it has been able to produce parts of just a few microns, with occasionally tolerances of under 1u.

However, while this glass floor undoubtedly exists, ALC chief executive Peter Schupbach said the manufacturing process used is dictated by application rather than size. He cited the example of a tiny gear for a watch: ‘If you use stamping you can go down very small, likewise with turning, or ceramic injection moulding. It’s basically the feature of the application that dictates the technology you use.’

For example, a small gear destined for a low-cost watch would, said Schupbach, probably be produced using a stamping technology. However, the same-sized gear for a top-of-the-range watch would require a process like ceramic injection moulding, which gives a higher-quality part with less friction.

One of the problems with the much vaunted LIGA technique is its high cost. However, another Swiss company, mouldmaker and parts fabricator Mimotec, recently produced well over 50 million parts using a proprietary process similar to LIGA yet far less expensive. While the LIGA technique employs expensive, high-resolution X-ray lasers, Mimotec’s process uses a much less expensive UV light source.

Chief executive and founder Hubert Lorenz has initially targeted his technology at the Swiss watch industry for micro-moulding watch components, but is also looking into future applications in the manufacture of medical bio-sensors.

Strong UK presence

While ALC and Mimotec represent the tip of the iceberg in terms of Swiss involvement in micro components, there is plenty of activity elsewhere. Refreshingly, the UK has a strong presence in this industry courtesy of Dorset-based Tecan Components. Tecan, which last year launched a dedicated micro-structures division, has seen its whole year sales forecast for the division satisfied in the first few months of this year.

Sales vice-president Noel Cherowbrier explained how, in its search for tiny components, Tecan began by using chemical etching. This was fine as a process when up to ±10 per cent of the material thickness was being etched into, but beyond this, he said, the process hits a brick wall. So the demand for smaller features led the company to look at photo electroforming. This is essentially the exact opposite of chemical etching. Instead of starting with a flat sheet of metal and etching into it you begin with a substrate, put a resist on to the substrate and expose an image into the resist.

‘Here the image you’re exposing is where you put metal not where you etch,’ said Cherowbrier. ‘So where we’ve exposed we’re going to electroplate on to the substrate in between those pillars of resist. Normally when you plate something you put plating on because you want it to stick to that material. In this case you want to be able to take it off – which gives us, because we’re plating in nickel, a part or product which is the shape of the area that we’ve exposed in that resist.’

Although this was more accurate Cherowbrier explained that it was limited by the catastrophic effect of dirt particles at such a small size. This led to the creation of Tecan’s micro-structure facility, and the development of a process that Cherowbrier described as a hybrid of ideas from the semiconductor and audio CD industries mixed with micro-embossing techniques used to produce, for example, holograms. This new process has given the company the ability to electroform micro-structured parts in nickel to the sort of tolerances that people are used to working with in silicon.Cherowbrier added that the process can be used to produce either tiny individual components or larger 300mm2 parts with very accurate features. An example of one of the firm’s smallest features is a component used to control airflow in hearing aid microphones. It looks like a shoe box without a lid, has dimensions of around 1 x 0.5mm and its base is in the form of a mesh, the holes of which are about 40µ across.

Cherowbrier said that Tecan technology’s chief advantage is cost. While LIGA, for example, can produce smaller components, it is expensive and the small working area means that it is difficult to make in volume. ‘It’s a fantastic process,’ said Cherowbrier, ‘but has very, very severe limitations if you want to put it into any production race.’

Fledgling technologies

There is still plenty to be done in the world of micro-component manufacture. Some technologies still in their infancy are a long way from realising their full potential, while others, shoehorned into inappropriate applications, will surely fall by the wayside. One thing, however, is certain. While fear and hope are focused in equal measure on the high-profile, yet largely lab-based world of nanotechnology, the MEMS industry is alive and kicking.