Figure 1. Magnetic printing heads (in background) with magnetic poles and metallic coils integrated on silicon, produced by CESM (CH)
The reduction of costs for a given performance is a major goal of space systems technologists. Extreme miniaturisation, by reducing the levels of resources required, is a possible solution. Micro/nano-technologies allow the production of multiple types of microdevices and their integration into microsystems, resulting in application-specific integrated microinstruments. In recognition of their importance, ESTEC and Industry held a Round Table on Micro/Nano-technologies for Space. Based on the conclusions and recommendations made at the meeting, activities are proposed to gradually introduce these technologies in space-based systems.
The main objective of most space agencies in the mid to long term is to reduce the costs and delays associated with space-based services. This means strongly reducing spacecraft lifecycle costs and lead time, without reducing (and most likely increasing) performance. In turn, this would allow the full potential of space to be exploited and space-based systems to be competitive with ground-based systems that provide similar services. For scientific and experimental missions, more data and data of a higher quality and interest would be produced within a shorter time from the mission approval, and that within the budgetary limitations, to satisfy the respective communities. In commercial sectors, space-based services should be considerably cheaper for the operator than the equivalent ground services in order to offset some perceived disadvantages of space, such as difficult (if not impossible) upgrades and maintenance or less direct control, and to become competitive.
Spacecraft manufacturing is currently a labour-intensive, one- off, artisanal task: few similar units are ever produced. Due to this fact, traditional cost reduction approaches such as new design and production paradigms, modularity, and pre-fabrication, will help reduce the costs, but not to the required levels. A sensible way to significantly decrease costs and enable commercialisation could be via a technology or engineering break- through, oriented towards mass production of space subsystems, and based on distributed (instead of the current centralised) systems and services. The economies of scale achieved by centralising functions would be exceeded by mass-production savings, and a new lifecycle could be initiated, resulting in more affordable space-based systems and services.
One area where innovation is proceeding at a very fast pace is miniaturisation, as may be observed in everyday life. High levels of miniaturisation may be achieved by applying micro/nano-technologies. The term micro/nano-technology is broadly defined to encompass the synthesis and integration of materials, processes and devices of sub-millimetre to submicron size. It is used to mark a distinction between micro (current state of the art in disciplines such as electronics) and nano (mostly understood as referring to molecular devices). These technologies allow the production of microsystems (the European term) and microelectromechanical systems (MEMS, the American term), understood to be the integration of microelectronics with peripherals and micromechanics, and resulting in devices such as application-specific integrated microinstruments (ASIM) and eventually nano-satellites (satellites weighing only a few kilograms).
Microengineering is a discipline dealing with the design, materials synthesis, micro-machining, assembly, integration and packaging of miniature 2-D and 3-D sensors, actuators, microelectronics and microelectro-mechanical systems. The goal of the discipline is to develop and produce intelligent micro- instruments, and some results are currently being applied, in among other areas, medicine or the automotive industry: smart (chemical, pressure, temperature) sensors to reduce emissions, smart microaccelerometers for crash detection and airbag deployment, and smart micro-gyroscopes in active suspension systems.
Microsystems manufacturing relies mostly on the equipment and fabrication techniques used for very large scale integration in the electronics industry, as for example to produce microprocessors. It is expected that the advantages shown by these techniques in terms of cost and reliability, e.g. in current computers, will also apply to microinstruments and microsystems. Other techniques are specific to microsystems, such as bulk and surface micromachining and LIGA (Lithographie mit Synchrotonstrahlung, Galvanoformung und Abformtechnik mit Kunststoffen, or deep-etch X-ray lithography, electroforming and moulding), the latter allowing moulds for mass fabrication of microstructures to be produced.
Most European countries are heavily involved in microsystems research, with public funding at high levels. As an example, the European Union will invest about 100 million ECU in the Fourth Framework Programme, through the ESPRIT and BRITE-EURAM programmes, in microsystems and related technologies. ESA member states have the capability to produce and, with high proficiency, to design new devices and to conduct research on new concepts and applications. Although the quality of fundamental research, creativity and ingenuity are high in Europe, applications are still scarce and there seems to be a gap between research on the one hand and industry or users on the other.
The main advantages offered by micro/nano-technologies are:
These advantages are even more significant for the space sector, where each of the points has a strong influence on cost. Microminiaturisation could play an essential role in Europe in designing new generation satellites, thus preparing the European space industry for the challenges of the global competitive market. The impact could be similar to that achieved by the transition from the vacuum tube to the integrated circuit, which has resulted in the huge consumer electronics market.
Microsystems are a growth sector, although one that is still in the early development phase. As such, there are a number of critical issues that would require specific actions for their correction, such as:
Launch is one of the highest cost factors for space-based systems and is directly related to mass. Payload and platform are the two other major contributors to cost. The platform mass and cost are related to the payload mass, power requirements and volume. Any reduction in mass, volume and power requirements is thus desirable and will have a significant effect on cost. Microsystems are considered an excellent means of obtaining these reductions. They could become the means to implement decentralisation, whereby a given number of dispersed components could be used in place of a larger centralised unit, thus achieving greater efficiency, redundancy and economies of scale. This leads to cost reduction on space- based systems not only at the payload and platform levels but also in launchers and ground facilities down to the end-user through the economies of scale achieved by batch production, and the replacement of some high-performance units with multiple standard-performance parts, each based on a microsystem. This is because cost increases exponentially with performance after a certain point. This requires new ways of thinking about the services demanded, and the systems needed to provide those services. In the space sector, other changes would be necessary, such as in the path from data acquisition on-board the spacecraft to delivery of information to the end-user, spacecraft networking, launch, de-orbiting, operations, ground infrastructure, tracking and station-keeping, amongst others.
The Technical Directorate of the European Space Agency started considering micro/nano-technologies and the resulting micro- systems to solve specific problems in the early 1980s. Since then, activities have taken place in areas such as micro-sensors, -optics, -lasers, -mechanics, -electronics, life support, bio- reactors, robotics, thus showing the multidisciplinary nature and potential of micro/nano-technologies. European microsys-tems have flown on several space missions, including Olympus and several Space Shuttle missions.
Based on the developing interest, the increasing internal activity, the perceived potential of micro/nano-technologies and a growing system awareness, an informal working group was formed in ESA's Technical Directorate. Contacts were made with European industry, research centres and organisations, as well as the European Union. A preliminary assessment showed that micro/ nano- technologies constitute a good target for cooperative applied research under the coordination of an organisation such as ESA, since space may provide good opportunities to demonstrate the potential of microsystems, which then may help the space sector in opening ways to reduce costs and broaden applications. In addition, the innovative concepts required to create microsystems may permeate into the space sector and stimulate new mission ideas. Activities could be coordinated amongst the different interested parties, including funding institutions, to avoid duplication of efforts and increase the overall effectiveness.
The ESA Working Group on Micro/Nano-Technologies recommended holding a Round Table on Micro/Nano-Technologies for Space together with Industry. This Round Table, which was organised by the System Studies Division of the Technical Directorate, took place on 27 - 28 March 1995 at ESTEC in The Netherlands. The support of the national delegations was essential to the success of the Round Table. The main objectives, framed by the generic goal of reducing the costs of access to space and of space-based services, were to better understand micro/nano-technologies and how these technologies may benefit the space sector, to establish working contacts with the micro/nano-technologies community, and to generate ideas on the potential uses and new missions which micro/nano-technologies may enable (*).
* The Round Table presentations have been published as ESA WPP-91. To order a copy, contact the author by fax (31.71.565.5184) or by e-mail (amartine @vmprofs.estec.esa.nl).
The Round Table was organised around the following points:
The general conclusion was that micro/nano-technologies could, in due time:
One basic view that emerged from the Round Table was that the competence to manage the technologies required to produce micro- devices (in the fields of sensors, optics, lasers, mechanisms, electronics, etc) and to combine them into one fully working microsystem had to be fully developed in Europe. The space sector may offer possibilities for demonstrating the usefulness of these technologies and ESA should help develop the competence to design and produce micro-systems adapted to the space environment and capable of satisfying the space sector requirements.
It was agreed that micro/nano-technologies should be seen as a key factor within the cost reduction and miniaturisation efforts. These technologies could be a means to produce very small payloads and spacecraft with all the related benefits. Micro/nano-technologies are suitable for building distributed systems consisting of several similar or identical small units, whose combined effect would be equivalent to that of one large system unit. This type of concept is the basis, among others, of very long baseline interferometry, synthetic aperture radar and phased arrays.
It was also recommended that a detailed analysis should be initiated to derive specific requirements based on common space missions requirements which could be presented to the micro/nano- technologies community for evaluation. Instruments for Earth observation, planetary exploration, space science and communications missions should be specified. Increases in the platform efficiency per unit mass could be based on advances in areas such as management of highly distributed systems and the data they generate, decentralised control and support (e.g. power regulation at the user level), autonomy (of each subsystem and of the total spacecraft), on-board intelligence, spacecraft networks, inter-spacecraft communications. These are some of the subjects to consider at the space segment level, together with product assurance and qualification, in addition to how the subsystems and payload mass, volume and power consumption should be reduced. Similar analysis should be applied to the ground equipment and the launchers.
Other general recommendations, amongst the major suggestions of the Round Table, included those to reconsider product assurance and the space qualification process, and to follow standardisation efforts in industry and specialised organisations, within the objective of defining reusable modules.
The Round Table considered that ESA should endeavour to:
Actions proposed for future activities on micro/nano- technologies for space include:
The motivation behind the application of microsystems to space is manifold: significant cost reductions, the possibility of enabling new functions and improving the performance of existing ones, better ways of doing a job, the ability to accommodate data proliferation and increases in data quality, shorter development times, controlled risk, with all of these reasons resulting in better performances per unit cost and mass.
Microsystems are being developed at a fast pace for applications in areas such as automobiles, medicine, aeronautics, defence, and consumer electronics. The space sector should follow closely these developments in search of synergies, and be ready to adapt and integrate available elements, and profit from the industrial push. Joint actions could be contemplated with other European and national organisations. In this respect, most welcome is the EuroPractice action, funded by the European Union's ESPRIT programme, where four clusters of manufacturing and testing companies have been selected in Europe to facilitate the access to design and production facilities for microsystems as well as multichip modules and application-specific integrated circuits to users interested in accessing advanced technologies.
The photos shown here present some current and foreseen space applications. Some, as is the case for life sciences studies confined to small volumes, have been made possible by the high miniaturisation levels micro-systems permit. Other available microsystems having a potential use in space are, for instance, spectroradiometers, mass spectro-meters, microcameras, multiparameter logistic sensors, distributed unattended sensors, embedded sensors and actuators, inertial navigation units, GPS receivers, propellant leak detectors and wear monitors for ball bearings.
Essential for fully exploiting microsystems potential are new ways of addressing the problems to be solved, and the search for different solutions. Applications and devices based on principles different to those on which conventional units are based have to appear for the full potential of micro/nano-technologies to be exploited. An example from the past is the replacement of electro-mechanical scan imagers with CCDs. Currently, rotating masses are being replaced by vibrating combs and tuning forks in microgyroscopes and angular rate sensors, microaccelerometers could be used instead of Earth sensors, microresonators could be used in place of much bigger SAW filters, and microelectromechanical RF switches could provide better isolation than PIN diodes.
There are, however, some space-specific limitations of microsystems, and the space sector will have to devote resources to overcome them if it wishes to make widespread use of microsystems in all possible types of mission. The two most critical limitations are the high costs of development or adaptation to space and the high susceptibility to radiation. As for any other devices having a high degree of integration (e.g. microprocessors), the costs will increase by the need to qualify for space use. The risk of single event upsets (which are non- destructive but disrupt operations) and also of destructive effects such as latch-up currents or low-rate dose degradation and, in the case of micro-mechanisms, radiation-induced deposition are factors to be considered. These problems are not unique to microsystems but affect most high-density and low- voltage microelectronic devices, a proton being a big particle at these scales. The above issues are aggravated by space not being perceived as a market by industrial device producers.
The integration of microsystems into conventional systems is an issue demanding an urgent solution for the first applications of micro-systems in space to be able to demonstrate the maximum advantages. Of particular importance are the respective interfaces: microsystems require very low voltages (around 3 V) whilst standard bus voltages are much higher (28 V), the dimensions of standard mechanical connectors are similar to those of microsystems, and other disfunctionalities might be mentioned. Adding to the microsystem all the devices required for an acceptable interface may result, in some cases, in a box not much smaller than the conventional unit to be replaced by the microsystem. To flight-qualify microsystems, and given their low impact on the spacecraft budgets (mass, volume, power), microsystems could be piggy-backed on the modules they would eventually replace, operating in parallel for observation and comparison purposes, and offering an additional redundancy.
The gradual introduction of micro-systems in space-based systems may be phased-in in several different ways, and the following is proposed:
ESA's technology programmes are well adapted to the peculiarities of research on emerging technologies such as micro- systems. They allow prospective research and development to be performed and research to be conducted both in support of new projects and on existing technologies to reinforce industrial competitiveness (applied research). Both the second phases of the Basic Technology Research Programme (TRP) and the General Support Technology Programme (GSTP) will explicitly include micro/nano-technologies related activities. ESA's In-Orbit Technology Demonstration Programme (TDP), offering flight opportunities in low Earth orbit with short duration missions and thus resulting in a limited accumulated radiation dose, is well suited to show in orbit the achievable performances of the devices produced using micro/nano-technologies.
The European space sector should note developments from other industries that are currently more competitive, and adapt its culture accordingly. An imaginative and flexible effort is required to produce new paradigms which, although different to those traditionally in use, include the new concepts and result in similar or improved functionalities at lower costs. ESA's role would be to foster these changes.
The contributions and support of the members of the Working Group on Micro/Nano-Technologies and of the chairmen and participants in the Round Table on Micro/ Nano- Technologies for Space have been essential in the preparation of this article. P. Cordero, M. Garvin, D. Kassing, P. Plancke, P. Roussel, and G. Scoon provided input, and A. Atzei, F. Ongaro and D. Raitt provided valuable comments.
Special thanks are due to the active researchers in micro/nano-technologies who provided the photographs of their work to illustrate this article.
Figure 1. Magnetic printing heads (in background) with magnetic
poles and metallic coils integrated on silicon, produced by CESM (CH)
Figure 2. Flight models of the Space Bioreactor, mounted on the
base of a Type II experiment container (black box at the back).
Dimensions: 84 x 60 x 60 mm 3 . Produced under ESA contract
10080/92 by Mecanex S.A. (CH) in collaboration with the Institute
of Microtechnology (IMT) at Neuchâtel and the Space Biology Group
ETH Zurich. The limited space available in the experiment
container made essential the use of miniaturised components. Key
elements are a micropump for fluid circulation and microsensors
to monitor vital parameters in the reactor chamber, manufactured
at IMT. Four units flew on STS-65 (1994), and it is scheduled for
reflight in March 1996
Figure 3. Complete microspectrometer system: A LIGA
microspectrometer is placed on a photodiode array, in the upper
left corner (orange rectangle). Its dimensions are 18 x 7 x 6 mm
3. The critical dimensions are the grating teeth with 3 m length
and 0.2 m height which extends over the full LIGA height of 125
m. The system's characteristics are: sensible to energy in
wavelenghts from 400 to 1100 nm (other ranges also available:
near UV, IR), transmission of 25%, spectral resolution 7 nm,
dynamic range up to 20000; ADC, micro-controller and serial port
included. The overall dimensions of the system are 70 x 60 x 15
mm 3. The signal is received from the collecting optics via
fibre-optics. Developed at the Institut für
Microstrukturtechnik (FZK/IMT), Forschungszentrum Karlsruhe GmbH
(D) with internal funds. Being considered as instrument in
preliminary analysis
Figure 4. Quartz angular rate sensor (tuning fork) developed by
Jan Söderkvist, Colibri Pro Development AB (S), for
automotive and other applications. The sensor is well suited to
space applications due to its small size and ability to withstand
harsh conditions. The tines of the tuning fork sensor element
shown are 2.5 mm long
Figure 5. The Fuga 15 video camera chip is the basis of the
visual telemetry system on-board Envisat. It is being developed
by MMS (F), IMEC (B) and OIP (B). The microcamera features
include random addressable pixels, logarithmic response and a
digital interface
Figure 6. Thermal switch for temperature control. The emissivity
may be changed to any value, which allows dynamic temperature
control. Internally funded development conducted by Verhaert D&D
(Kruibeke, B), IMEC (Leuven, B) and Wildcat Micromachining (USA).
Its adaptation to space is under study
Figure 7. 3-D multi-chip-module (MCM) associating 8 x 8 silicon
chips in 8 levels connected by fusible microbumps and wire
bonding. Developed by LETI (Laboratoire d'électronique, de
technologie et d'instrumentation, F), this MCM has 1 Gigabit
memory capacity
Figure 8. Micromachined accelerometer developed at LETI and
commercialised by Sextant Avionique for aeronautic
applications
Figure 9. Pressure sensor integrated with CMOS circuitry,
developed by CNM (Centro Nacional de Microelectrónica, E)
Figure 10. Active endoscope actuated by modular shape memory
alloy microactuators, produced at MitechLab, Scuola Superiore
Sant'Anna (I)
Figure 11. Electromagnetic micromotor developed at IMM (Institut
für Mikrotechnik Mainz, D). Such micromotors use various
electromagnetic principles and are composed of precision
mechanical parts and elements, fabricated by the LIGA method.
Ball bearings guarantee long lifetime, and planetary gearboxes
allow higher torques. The micromotor's diameter is about 2 mm,
and it generates a torque of 0.1 µN
ESA Bulletin Nr. 85.