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The Synchrotron Light Source as a Tool for Microtechnology |
By Assoc. Prof. E C Harvey and Dr M J Murray FTSE
We are all familiar with lathes and milling machines for shaping parts in machine shops and factories. But what if the parts we need to make are significantly smaller than a millimetre, and featuring details even smaller? Semiconductor chip manufacturers have faced these problems and have learnt to use new ways to make devices. No longer are transistors made one at a time, but rather are 'printed' millions at a time, together with their interconnection wiring in a process called photolithography. Light from an excimer laser is directed through masks that incorporate the patterns required and photoexposes surfaces positioned behind the masks. This form of photolithography is today a standard process in semiconductor FAB plants and has several critical advantages in terms of cost, reproducibility, reliability and its ability to scale towards ever smaller and more complex systems.
An alternative but less common lithographic method is to use highly focused electron beams programmed to draw patterns, as one might use a pencil, except that line widths and spot dimensions can be significantly less than one micron. This process does not have the same 'pattern transfer' advantage of photolithography and so is slower and more expensive, however details as small as 10nm can be written. When the patterned photo resist is used as a die to transfer the pattern to other materials, like electroplated nickel, for example, large numbers of identical patterns can be made from the nickel replica by embossing into plastic. Now costs of individual devices are calculated according to a batch run (embossing) rather than single component manufacture. A method similar to this is being used by CSIRO to manufacture exelgrams - anticounterfeit diffracting devices used in document (for example, paper currency) security.
Microtechnology uses these and other processes (see Table 1) to manufacture 3D mechanical devices that manipulate light, fluids and biological samples; sense pressure, acceleration and force; and even move internal parts to become physical actuators. And since their manufacturing processes are similar to those for microelectronics, many of these devices have 'smarts' built-in. Advantages of microtechnology devices include smaller size, reduced power consumption, greater speed, fewer components, increased integration, greater performance and reliability, and most significantly, reduced cost in comparison to devices made by traditional processes to perform the same tasks.
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Table 1: The Micromachining
Toolkit
| Material
Removal Methods |
Material
Deposition |
Methods
Exposure Methods |
| Electrodischarge
Machining
Plasma Etching
Reactive Ion Etching
Chemical Etching
Excimer Laser Ablation
Focused Ion Beam |
Chemical
Vapour Deposition
Physical Vapour Deposition
Electrodeposition
Dipping, Spinning, Spraying,
Printing, casting, injection moulding |
Ultra-violet
lamps
Laser (various)
Electron beam
Synchrotron Radiation |
In general the processes described so far are appropriate for manufacturing parts in which the aspect ratios of the pattern structures are low: that is, depth of pattern is about the same order of dimension as the line width. If it is required that the aspect ratio (depth : line width) is much greater than 10 : 1 it will be necessary to seek an alternative technique. This is where synchrotron radiation can be useful, and probably uniquely so.
Using the synchrotron's extremely bright, highly collimated beams, and selecting the short wavelength x-ray component to reduce diffraction effects around mask features it is possible to create deeply etched structures with precisely vertical walls and very high aspect ratios. For example a one micron width line can readily be drawn to a depth in excess of 1cm (an aspect ratio of 10,000:1). Most practical applications would not require aspect ratios of such magnitude, but the scope and speed of the technique, particularly when combined with a batch replication process, makes it potentially attractive for mass producing a new range of micro-parts. Figure 1 depicts a production micropart which requires the deep precision etching obtainable only by means of synchrotron radiation.
Figure 1: 400µm high alignment cross created by synchrotron photolithographic exposure of perspex (PMMA).
Microtechnology: the Global Picture
Although in its infancy in Australia, micromanufacturing in some sectors is more than 20 years old. In his plenary address to the October 2001 San Francisco SPIE Symposium on Micro-machinery and Micro-fabrication, Professor Wolfgang Ehrfeld, a pioneer of synchrotron micromachining (known as LIGA, see Figure 2), pointed to a vast array of products which are steadily being reduced in size, and other micro-products being invented which, because of their very functions, have to be extremely small at the outset.
Figure 2: Professor Ehrfeld and others developed the Synchrotron LIGA process at the Forschungszentrum Karlsruhe, Germany, to produce a nozzle used for uranium enrichment. The hydrodynamic flow of uranium hexafluoride gas through the channels causes the isotopes to separate. LIGA is a German acronym meaning Lithography, Galvanoformung (electroplating) and Abformung (replication).
He began with communication technology where the extension of optical network components through micro-optical-electro-mechanical systems, or MOEMS, (such as star-couplers, connectors, matrix switches, and wavelength division multiplexers and de-multiplexers) failed at first because of high production costs, but are now made using Microtechnology.
In transportation a major goal currently is the development of miniaturised, highly integrated micro-electro-mechanical systems (MEMS) for use in vehicle safety, performance, and comfort. Early technologies are already in everyday use controlling airbags, antilock brakes, motor management, road holding and collision warning systems.
Professor Ehrfeld finally pointed to the fields of chemistry and molecular biology where micro-reactors are used to provide information, assist production development, and to produce chemicals. In the case of information provision a huge number of reaction units perform parallel reactions under differing conditions. For chemicals production, the reaction units work in parallel under similar conditions to produce required volumes.
At the same meeting Dr Marc Madou, Vice President of Advanced Technology at Nanogen in San Diego further expanded the microtechnology list to include compact-disc-based micro-fluidic platforms, merging of fluidics with active DNA arrays, responsive drug delivery systems, and semicontinuous fabrication of electrochemical sensor arrays.
At the 4th World Micro-machine Summit held in Melbourne in April 1998, Professor Ron Lawes of the Rutherford Appleton Laboratory in the UK spoke on micro-engineering for industry in Europe. Professor Lawes estimated that by 2005 the world wide Micro System Technology (MST) industry will turnover $US40 billion per year and be typified by such products as micro-sensors for chemical and biological/biomedical analysis, drug delivery systems, waveguides and switches for telecommunications and micro-machined flat panel displays. In an update at the Commercialising MEMS conference in Oxford in September 2001 delegates acknowledged that this survey has greatly underestimated the market growth, with bio-MEMS and telecommunications growing more rapidly than expected.
Figure 3: Microspectrometer where the plastic elements, including curved grating, mirrors and fibre optic coupling channels are created by synchrotron LIGA. The device is less than 25mm long. (Courtesy: Microparts GmbH).
Whether our micotechnologies are referred to as MEMS, MOEMS, MST or whatever, there is a driving economic force to create a wide range of less expensive micro devices using the machining methods listed in Table 1. As device sizes, costs and tolerances further reduce, the availability of synchrotron light will place LIGA as an increasingly important and cost effective method of micro-manufacture.
The first use of synchrotron light in microtechnology occurred in the manufacture of nozzles for the separation of uranium isotopes. One of these nozzles is shown in Figure 2. A more recent example of synchrotron radiation usage is found in the manufacture of microparts incorporated in miniaturised spectrometers as depicted in Figure 3.
The Growth of Microtechnology in Australia
A high profile success of Australian microtechnology has been the 'bionic ear' or cochlear implant invented in Melbourne by Professor Graeme Clark. An outstanding part of the success of this device has been the huge improvement in the quality of life of its users. New generations of the cochlear implant will include more electrodes better placed within the cochlear and the manufacturing technologies used will be based upon microtechnology principles.
Other successful examples of Australian developed microtechnology include the CSIRO-developed optically variable device (OVD) exelgram©!=, hearing aids built by Crystalaid, polymer banknotes, new food packaging technology developed by AMCOR and Swinburne University, thermal imaging arrays and smart airframe corrosion detectors developed by DSTO, photonic devices developed by Redfern Optical Components, communication chips invented by Radiata, weld crack sensors from SMS in Perth. The list continues to grow, albeit rather slowly.
In June 1999 the CRC for Microtechnology was set up with the intention of bringing together in a collaborative way all the major micro-manufacturing methods and skills available locally. Currently the CRC is working with a number of companies on a range of micro-component developments for communications, biotechnology, health, agriculture, defence and sporting industries. One of the CRC projects, supported by the Australian Synchrotron Research Program (ASRP), is a collaboration with the Chicago APS Synchrotron to produce micromachined vacuum pumps. Another ASRP and ARC supported LIGA activity, with the University of Melbourne and Swinburne University, involves the design and fabrication of advanced lobster-eye x-ray imaging lenses for space applications.
The next step
Although Australian microtechnology is now slowly establishing, there remains a large gap between public sector research and industrial investment. Part of this problem is a consequence of Australian industry having opted out of the microelectronics revolution.
Since each new industry is generally based upon a sound foundation in the previous ones, the almost complete lack of industrial investment in cleanrooms, wafer processing equipment, deposition gear and the like has resulted in a serious lack of trained technical officers, poor service and maintenance infrastructure, and weakly developed design skills. This has lead in turn to a spiralling problem where local businesses are cautious about investing in the technology and multinationals are wary of bringing it to Australia due to the low skill base.
With several Australian States positioning themselves for strategic investment in biotechnology and nanotechnology, the Victorian commitment to the Synchrotron, and the recent bid for an integrated micromanufacturing facility, miniFAB, to be located nearby are key elements in filling this gap and lowering the entry barrier for industrial participation in the next manufacturing revolution.
Bibliography
- 'Fundamentals of Micro-fabrication', Marc Madou, CRC Press, Boca Raton and New York, 1997.
- 'Micro-machines † A new era in Mechanical Engineering', Iwao Fujimasa, Oxford Science Publications, New York, 1996.
- 'Sounds from Silence', Graeme Clark, Allen and Unwin, St. Leonards NSW, 2000.
- 'Microtechnology for Anti-counterfeiting', Robert A. Lee, Journal of Microelectronic Engineering Vol. 53 513-516. 2000.
Useful links
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Associate Professor Erol Harvey is the Deputy Director (R&D) at the Industrial Research Institute Swinburne (IRIS) and program leader for Microfluidics in the CRC for Microtechnology. Originally trained as a physicist, his postdoctoral work was as Operations Manager of the SPRITE Ultra high power excimer laser facility of the SERC Rutherford Appleton Laboratory near Oxford. In 1990 he moved to Exitech Limited, a spin-out company from the Rutherford Laser Facility, where he was Principal Development Engineer and Head of System Design and Fabrication of the Advanced Excimer Laser Micromachining systems. In this role he worked closely with customers around the world developing commercial applications of excimer laser micromachining. The first tool to be installed at the Institute of Micromachining in Mainz (IMM), an institute founded by Professor Ehrfeld one of the inventors of synchrotron LIGA, was designed and built by Dr Harvey. Examples of some industrial development projects Dr Harvey worked on include thin flim transistor annealing for advanced lap-top computer LCD displays, ink jet printer nozzles, ultra-high density electronic packaging, biomedical devices, the first Hubble Space Telescope Rescue Mission, 248 / 193 nm lithography for sub-0.18µm semiconductor fabrication, hard disk fabrication, thin film deposition, solar panels and packaging applications for the pharmaceutical industry.
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Dr Mike Murray, having spent 26 years with CSIRO, recently left the organisation to follow new initiatives of his own. During his association with CSIRO he spent 10 years as Chief of the Division of Materials Science and Technology, and five years subsequently as Corporate Executive seeking major initiatives with industry and academia. Dr Murray was a founder of Ceramic Fuel Cells Ltd, and X-Ray Technologies Ltd, and led several other commercialising ventures, including that of the optically variable security device (exelgram) that appears on American Express travellers cheques as well as many banknotes and security documents worldwide. He was founder, and is co-chairman, of the Industrial Synchrotron Roundtable (ISRt), the group whose work over five years provided the critical input to the Victorian Government's decision last year to establish a synchrotron in the Monash Precinct. Apart from the synchrotron and its multifarious uses, Dr Murray has ongoing interests in microtechnologies (the CRC for Microtechnology was initiated by the ISRt), major environmental projects, and projects relating to sustainable energy generation and distribution. Dr Murray (a physicist) was educated in the UK at the universities of Durham (King's College) and Cambridge (Emmanuel College and the Cavendish Laboratory).
ATSE Focus is a non-refereed publication. The views expressed in the above article are those of the author(s) and do not necessarily represent the views of the Academy.
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