Engineering the invisible

20 November, 2006

Most design engineers struggle to imagine what a micron looks like. To get an idea, the full stop at the end of that first sentence would cover 600 square microns. When nanoscale engineering is concerned, we must go a full order of magnitude smaller.

By the time we reach a single nanometre measurement we are nine orders of magnitude smaller than a metre (10-9m), and at this level we approach atomic measurements – a single atom of gold, for instance, is 0.1441nm. The naked eye cannot detect particles below 10-5 or, to put it another way, the eye loses its efficacy four orders of magnitude before we reach the units we now propose to adopt for manufacturing tolerances.

Yet, such scales are increasingly being deployed in everyday engineering designs. While it is fair to state that nano sized machines remain fixed in laboratories at present, nano tolerances are now specified in all sorts of small components for medical, electronics and other applications. Here we are talking not about the physical size of the components, but the machined features within them.

At such scales, when trying to attain – and maintain – accuracy, every single factor imaginable counts. The machines used to cut the materials must be rigid, stiff and impervious to vibration, temperature and moisture. The ambient atmosphere must be controlled and even the presence of human operators can create resonant frequencies in the machine to throw it off tolerance. The motion control electronics, which is where Delta Tau comes in, must be accurate, fast and stable. Feedback from encoders or resolvers must have extremely high resolution, but the controls must also be capable of creating nanomotions at the tool point.

Why nanoscale engineering?
This begs the question why anyone would need to control machining at nanoscale. The answer lies not just in the increasing miniaturisation of systems, but also in new features, functions and benefits micro surfaces can deliver. These include anything from sophisticated lubrication designs in which oil can be retained and directed to the surfaces needing it using highly accurate micro grooves, to the manufacture of miniature filters for portable blood testing apparatus in which the micropores are laser machined to incredible accuracies.

Recently, new demands in the fabrication of miniature/micro products have appeared, such as the manufacture of microstructures and components with 3D complex shapes or free-form surfaces. It is found that those microstructures possess some special functions including light guiding, anti-reflective and self-clean among others for microlenses, micromoulds, laser targets and so forth. If one considers the example of the matrix of tiny prismatic lenses that reflects light within a mobile telephone’s screen, this functionality exists before finishing the part at a nanoscale, but the efficiency the light spreads after nanomachining is greatly enhanced. This, in turn, reduces the number of LEDs required to light the screen and lowers power consumption accordingly.

These microstructures will further improve the performance of miniature and micro products. Furthermore, fabrication of real 3D miniaturised structures and free-form surfaces are also driven by the integration of multiple functions in one product, for example adding light guides to existing mechanical components or aiding lubrication methods by using micromachined surface geometries. As one example, the smoothness of lenses in a camera or telescope increases the accuracy, effectiveness and quality of the resultant image. Glass lenses are traditionally produced by a tedious manual process of machining (lapping), grinding and polishing. In ultraprecision machining, using a single-point diamond cutting tool, a surface roughness of less than 10 nanometers can be achieved. This is especially important today in producing optical microstructures such as DVD and camera lenses, lenses for photonics and optical fibre for telecommunications. Many of these lenses can now be moulded from plastics materials as a result of the ultra smooth finish that can be achieved within the mould tools.

Currently, the advancement of designs in Micro Electro Mechanical Systems (MEMS) devices, together with their increasing acceptance by industry, are one of the major driving forces for making micro components. Silicon is the traditional material for making MEMS or microsystems, but many other materials including a range of polymers and even some metals have emerged for the increasing number of applications that are becoming relevant for micro products. While early MEMS relied primarily on laser micromachining the silicon surfaces (largely a 21/2D application – i.e. interpolated geometry in the horizontal plane, but with a fixed depth of cut), more recently complex 3D geometries have been deployed in devices.

For example, life sciences are an emerging application area of MEMS requiring the manufacture of disposable blood testing cartridges that utilise glass, ceramics, metal and plastics rather than just silicon as raw materials for micro components. Although conventional mechanical ultraprecision machining is used as the most common means to fabricate micro components the use of such machinery continues to pose problems with regards to their ability to produce components with predictability and sufficient productivity – especially for those components with complex 3D surface forms.

At nanoscale, current high accuracy machines cannot produce 3D components with sufficiently reliable repetition – which leads to high scrap yield and expensive parts. One reason for this is that the targeted dimensions or surface finish depend not just on the machine accuracy but in large part on the material itself and what happens at its structural level. Adding a third machining dimension makes it ever more difficult to attain accuracy consistently because the atoms or crystals are being altered in three dimensions.

The requirement on positioning systems to control accuracies of motion in nanometers is increasing at an exponential rate. But the challenge in meeting such super accurate systems means that no single element in that system can be treated autonomously. Feedback impinges on gain, gain on stability, inertia on dynamics, friction on stiffness and so on.

To take a quantum leap into the world of nano machining requires a whole new design and approach. For example, imagine trying to put a lump of jelly onto a milling machine and then produce an accurate cylinder block geometry. Ridiculous though the analogy may seem, the research team at Leeds Metropolitan University has observed hard ceramic materials that exhibit ductility at the machining face when removing material at nanoscale!

Machine construction
As one might imagine, the machines used to make such miniscule accurate cuts are huge and extremely expensive. Hence, the components that have nanoscale features are priced accordingly. As a result, the MASMICRO project was implemented in 2005 to develop nanoscale machining technology.

The MASMICRO project aims to develop a machine tool capable of attaining 50nm accuracy with staggering 5nm repeatability. But, the aim of the project is to make that machine perhaps one-tenth of the size of anything that’s gone before, at one tenth of the price! It must also remain suitable for mass production of micro components within small and medium enterprises (SME) in the manufacturing sector.

The machine most advanced in its development under MASMICRO is the preliminary three axis turning machine development. The current construction is mounted on a high density granite platform and uses a direct drive rotary servo for the C-axis, a linear motor for the cross slide (Y-axis) and then a piezo electric actuator to give fine motion in the X-axis.

A new motor has been designed for the spindle; the use of an air-bearing and linear motor has been adopted to obtain the level of stiffness required for nanoscale machining. In parallel, the MASMICRO team at Brunel University, under the guidance of project leader professor Kai Cheng, has provided extensive FE analysis of the machine design and critical parts in order to come up with a final design. Again, the MASMICRO project is trying to combine industrial experience and academic and scientific knowledge to achieve nano/micro machining on a bench-top machine.

The direct drive spindle motor came from precision engineering company Loadpoint and was selected because it has zero cogging and good dynamic response. These high accuracy spindle motors are used in precision lens manufacturing. The linear motor was supplied by Anorad and is a U-channel type with high accuracy, exceptional acceleration/deceleration rates and good velocity stability. The piezo electric actuator comes in stroke lengths of 8µm, 20µm to 40µm and forces up to 200N. The frequency response of the piezo actuator is 1800Hz while its stiffness is 25N/µm.

Conclusion
The MASMICRO project is making great headway in developing not just new machine tools, but also new tooling technology, greater understanding of material behaviours at nanoscales and new measurement and inspection technology. The results could place European manufacturers at the forefront of emerging technology with expertise that will be hard to match anywhere else in the world.

Technology leaders, such as Delta Tau, have a major role to play in such developments, not just by providing equipment, but also important experience and know how. This is why the company has been so closely involved. I ndeed, this year Delta Tau provided a bursary for one of the project engineers at Brunel University.