TechArchive

Original article date: January 1996

Moving components smaller than the head of a pin? The potential for mechanical or electromechanical devices with dimensions measurable in microns – micromachines – is huge, ranging from simple disposable blood pressure sensors to integrated industrial process control systems. Microscopic ‘engines’ may even be developed, able to operate autonomously in harsh and confined environments, even inside the human body.

But what is available today? Almost all micromachined products currently sold are sensors, used for detecting acceleration, pressure, flow and – increasingly – the presence of chemicals and other physical parameters. The largest end-used market is the automotive industry. Some actuators are available, but the revenues from these is unlikely to match that from sensors in the foreseeable future.

The availability of inexpensive sensors has huge potential for changing the availability of information at a particular point. By integrating sensors with control circuitry and – potentially – microscopic moving actuators, it should become possible to use cheap and autonomous microsystems in a variety of applications. There is also a market for stand-alone micro-actuators, for example, as arrays of tiny mirrors which could be used in displays.

A separate strand of microengineering relates to the development of tiny structures, in silicon, plastic or other materials. Products such as minute gears and motors have already been demonstrated. A parallel development is in the field of optical systems, within which microengineered components may play a part in enabling faster uptake of fibre-based communications and computing systems.

There are two schools of thought within the microengineering fraternity with regard to the technologies and processes involved in fabricating practical devices. The first is based around modifying the techniques used in conventional semiconductor fabrication, such as lithography, etching and metal deposition. This approach is predominant in the US, where the industry refers to MEMS (microelectromechanical systems). Whilst this has the advantage of utilising generally well-understood techniques, it is possibly constraining in the long term in terms of the range of applications for which it could be used. However, such techniques account for the bulk of the present commercial engineering marketplace.

The second technique involves investigation of a much wider range of technologies, each with the common element of micron-sized products. Typical processes include precision milling and grinding and ‘photomoulding’ solid objects from powder with a laser. The Japanese research community favours this approach.

Surface micromachining is the technique employed in manufacturing most of the existing commercially available MEMS, such as pressure sensors and accelerometers. It essentially involves depositing a flexible structure (which forms the mechanical part of the electromechanical device) onto a rigid platform, which is then removed to allow motion of the other component. The platform is often referred to as a sacrificial layer. It is suited to producing relatively complicated structures – and particularly those designed to move during their usage. Typical devices might involve manufacturing cantilever beams, joints, gears and pumps.

One of the main new processes under development is the use of lasers to create microstructures. The most promising technique employs an excimer laser, which produces ultra-violet (UV) beams. UV does not burn or heat the base material in the same fashion as visible lasers and thus effects such as melting are avoided. By using carefully timed pulses of measured intensity, it is possible to remove material to precise depths of up to hundreds of microns. It is also possible, using a lens system, to produce structures with more flexible shapes, such as grooves with tapered walls.

Ideally it would be possible to fabricate solid objects in three dimensions, irrespective of shape. A long-term research goal has been the development of a technique for producing items from a base material (usually in liquid or powder form), which solidifies in the presence of two intersecting laser beams of appropriate colours. At present, no such system exists.

Current technology in the field of photomoulding, however, is approaching this ideal. Japanese researchers have developed a similar technique, which involves mixing a fine ceramic or metal powder with resin to form a slurry, which hardens when irradiated when irradiated with an ultra-violet laser. By controller the aim of the laser, a three-dimensional object is generated. Burning off the resin leaves a shell made of the other material in the slurry, which is then finished by sintering at a high temperature. One of the main functions of this technology is the fabrication of complex shapes at a micro level.

Injection moulding is, of course, a common fabrication technique in macro-scale plastics manufacture. It is a flexible, relatively inexpensive method of producing polymer products, especially those involving complex shapes. As such, there are strong advantages in transferring the technology to the micro-scale if possible, especially for microscopic moving parts.

In order to produce an item by injection moulding, it is necessary to manufacture a mould. One of the most promising technologies in this respect is LIGA (lithography, electroplating and moulding). This can be used to fabricate a metal mould which can then be used as a basis for plastic component fabrication. Injection moulding of plastics to achieve mass fabrication of plastic microstructures is cost-effective. However, there are issues regarding durability in some applications.

As a result, there is considerable development activity aimed at extending the range of polymers which can be processed in this fashion, whilst research is also being conducted into using similar techniques for ceramics and metals. One of the problems involved in injection moulding small components relates to the effects of viscosity on molten metals. It is thought that this may limit the usefulness of such techniques at micron level, although it is less important for components with dimensions closer to the millimetre scale.

Micromachines: Applications, markets and trends, available from FT Newsletters and Management Reports, 149 Tottenham Court Road, London W1P 9LL.

Contact Melanie Stewart on 0171 896 2222.

Building micromachines The dominant material used at present in micromachines is silicon and the whole field of MEMS is essentially based around it.

However, silicon is only useful for certain applications. For example, silicon-based pressure sensors will not work at high temperatures and consequently are unsuitable for measurements inside jet engines. Its other main limitation, itself a function of lithography, is that silicon is essentially a two-dimensional medium. The only means of achieving significant depth in silicon microdevices is to use successive layers, bonded together. For applications requiring fully three-dimensional shapes, alternative approaches, such as LIGA offer the flexibility of using metals or polymers instead.

Metals used in the LIGA process are typically nickel, copper or gold. Nickel-cobalt alloys are used for hardness and elasticity, nickel-iron permits greater flexibility of magnetic and thermal properties. Palladium and nickel-phosphor have also been used. Such components are also ideal for durable miniatures of macro-scale mechanical devices such as gears and springs. In medical applications, stainless steels, tungsten and platinum are used.

Various plastics are under development. PMMA, one of the more commonly used, offers good structural accuracy for LIGA-type processes, but is susceptible to stress cracking and has relatively low radiation sensitivity. PMI, POM and PASO are thought to be potentially more useful. For higher temperature applications, fluorinated polymers may be better suited. Certain ceramics have properties which make them suitable as the basis for actuators and sensors, such as piezoresistivity – materials which change shape in the presence of an electric current. Many ceramics are well known to be brittle and difficult to machine, but on a micro level it is possible to use techniques such as LIGA or photomoulding to manufacture them.

January 1996