Assessment of nanosystems for space applications

Acta Astronautica 65 (2009) 1272 – 1283 www.elsevier.com/locate/actaastro

Assessment of nanosystems for space applications Lise Bilhaut, Laurent Duraffourg∗ Laboratoire des Composants Microsystèmes, CEA/LETI-MINATEC, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France Received 5 December 2007; accepted 10 March 2009 Available online 13 May 2009

Abstract This paper first gives an overview of the applications of micro-electro-mechanical systems (MEMS) in space. Microsystems are advertised for their extremely low size and mass, along with their low power consumption and in some case their improved performances. Examples of actual flown MEMS and future missions relying on MEMS are given. Microsystems are now enjoying a dynamic and expanding interest in the space community. This paper intends to give an idea about the next step in miniaturization, since the microelectronic industry is already looking at nano-electro-mechanical systems (NEMS) driven by the more-than-Moore philosophy. We show that the impact of nanosystems should not be reduced at a homothecy in size, weight and power consumption. New forces appear at this scale (Casimir force . . . ) which have to be considered in the system design. The example of a nano-mechanical memory is developed. We also show that performances of nanosystems are not systematically better than their microscopic counterparts through the study of the impact of dimension reduction on an accelerometer resolution and sensitivity. We conclude with the idea that nanosystems will find their greatest applications in distributed intelligent networks that will allow new mission concepts for space exploration. © 2009 Elsevier Ltd. All rights reserved.

1. Introduction While micro-electro-mechanical-systems (MEMS) have demonstrated their usefulness in commercial sectors such as automotive (e.g. accelerometer for airbag or air-pressure sensor for tires) or entertainment (e.g. Texas’s Instruments’ mirror arrays for data projectors, accelerometers by Analog Devices in Nintendo’s Wii Console), the space community has only looked at the opportunity they could offer for a little more than a decade [1,2]. Despite recent encouraging developments, MEMS technology has still to fully convince the space community of its potential benefits through more and more flight demonstrators. Meanwhile,

researchers are now looking at nano-electro-mechanical systems (NEMS), which may well be a breakthrough technology in terms of mission applications as MEMS were with the development of micro and nanosatellites. In the first part of this paper, we give a quick overview of MEMS development for space applications, from their expected advantages to the actual results. In the second part of the paper, we introduce NEMS, which as we will see should not be simply considered as reduced MEMS. In the last part, we consider advantages and drawbacks of NEMS regarding to MEMS, and examples of potential applications in space. 2. Overview of MEMS in space 2.1. Expected advantages of MEMS in space

∗ Corresponding author.

E-mail addresses: [email protected], [email protected] (L. Duraffourg). 0094-5765/$ - see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.actaastro.2009.03.066

MEMS are usually advertised for their low price, low mass and low size, while keeping or even increasing

L. Bilhaut, L. Duraffourg / Acta Astronautica 65 (2009) 1272 – 1283

performances. Of those advantages, the first one is not a driving factor for the integration of MEMS in space systems. Although cost is a major resource for space applications, only very few systems actually fly and the price is mainly determined by the price of the space qualification and the integration within the spacecraft. Actually, since it is very costly to develop a MEMS-based product, and development costs are only reduced thanks to a mass-market production, MEMS would not have been considered for space applications if it were not for their popular commercial applications. A very good overview of MEMS in aerospace applications, although mostly American, is given in [3]. It is demonstrated that MEMS can have an impact on major spacecraft subsystems: scientific instruments (sensors, spectrometer, adaptative optics) as well as navigation and guidance control subsystem (inertial sensors), propulsion (microthrusters), communication subsystem (MEMS microwave RF switches and phase shifters), and thermal subsystem (variable emittance panel, thermal interface). Because of their inherent characteristics, low mass, low size, and low power consumption, they allow for redundancy scheme or graceful degradation, which can partly mitigate their lack of reliability. 2.2. Examples of past and future MEMS Applicability of MEMS to spacecrafts has been established through various flight demonstrators. To quote a few, the first one was a MEMS microthruster array on the NASA flight STS-93 in 1999 [4], followed by a MEMS-based RF switch successful experiment on two picosatellites ejected from OPAL in 2000 [5]. A most recent example is a variable emittance coating (VEC) made of MEMS for smart thermal control that flew on the Space Technology-5 project (mission completed in June 2006). The coating was made of an array of 150×6 m gold coating shutters. When a shutter is closed, the emissivity is very low (0.1) whereas it becomes higher (0.6) when the shutter opens, exposing the underlying silicon (Si) substrate. It is reported that the VEC operated within specifications [6]. Future missions will use MEMS as enabling technology. For instance, the PLANCK surveyor mission (scheduled for launch in 2009) is intended to image the anisotropy and polarization of the cosmic microwave background. It relies on the use of micromachined bolometers [7]. Another example is the microshutter array developed at NASA GSFC for Near Infra-Red Spectrograph for the James Webb Space Telescope scheduled for launch in 2013 [8]. This microshutter array consists of 250,000 individually addressable

1273

100×200 m shutters magnetically actuated 90◦ out of plane and electrostatically latched. This will allow the observation of hundreds of different objects in a single field of view. 2.3. MEMS: a “to be continued” story The widespread use of MEMS is space system is impaired by their adaptation to the harsh space environment. In order to achieve a high reliability level, without which no integration of new component is possible in space, there is a strong need for advanced modelling and simulation of MEMS, as well as multiple ground testbeds and on-orbit demonstrations. Packaging is a particular sensitive matter. Unlike electronics, it is difficult to fully radiation-harden MEMS when a part of the sensor has to be in direct contact with the external environment. On the bright side, one should underline that more and more people in the space community are aware of the strong interest of MEMS for space applications: the CANEUS network was created to foster the development of micro-nano technologies for aerospace applications [9]. The real impact of MEMS onto space applications is that they are part of the enabling technology for affordable micro and nanosatellites. This brought to the space community new actors such as universities, while making possible new concepts up to now absent of the space field. The first one is the possibility of having a spacecraft where most of the subsystems are off-the-shelf products that can be inserted in the spacecraft in a plug and play fashion (the best example of this new trend is the products developed by the Swedish company Ångström Aerospace Corporation Microtec [10]). The second one is the burgeoning of new mission concepts using swarm of microsatellites which can act on a collective manner (e.g. [11,12]). 2.4. Examples of MEMS at the CEA-LETI-MINATEC The Laboratoire d’Electronique et des Technologies de l’Information (LETI) is a department of the Commissariat à l’Energie Atomique (CEA) that focuses on microelectronics, micro and nanotechnologies and integration of advanced devices into equipments and systems. In partnership with MINATEC, it enjoys more than 6500 m2 of cleanrooms and can process 200 and 300 mm wafers with more than 400 equipments, which makes it a unique facility in Europe. Microsystems developed at the Microsystems Components Laboratory (LETI/LCMS) include a gyroscope for avionic application developed with Thales and

1274

L. Bilhaut, L. Duraffourg / Acta Astronautica 65 (2009) 1272 – 1283

3. Nanosystems: from Moore’s law to more-than-Moore 3.1. Motivation

Fig. 1. Gyrometer developed by the LCMS for Thales.

Fig. 2. Accelerometer developed by the LCMS for Freescale.

using a silicon-on-insulator (SOI) technology. This gyroscope, shown in Fig. 1, has a range of ± 100◦ /s with an error of 10−2 ◦ /s h and a noise of 10−2 ◦ /s Hz0.5 ). A matrix of 16×16 microbolometers has also been developed for ESA, the noise equivalent power been 10−17 W/Hz0.5 at 0.3. Moreover, a 2-g-range accelerometer (output error: ± 32 mg and noise of 200 g/Hz0.5 ) was developed for Freescale with a SOI technology (Fig. 2). This accelerometer was in production with a volume of 107 units per year for the automotive industry.

Having followed the empirical Moore’s law1 for nearly 50 years, the semiconductor community is now looking at how to pursue its exponential growth: an obvious way is pursuing the “more Moore” paradigm to keep shrinking dimensions and increasing density (with e.g. new high dielectric constant materials). On another hand, researchers are now thinking “beyond CMOS”, that is to say using disruptive technology (e.g. nanoimprint for lithography, phase change memory, spintronics, carbon nanotubes . . . ). They are also looking at the “more-than-Moore” paradigm, which is the merging of nanoelectronics and technologies that do not scale with Moore’s law (typically MEMS components) thanks to heterogeneous integration (Fig. 3). Regarding sensor applications, in front of the combined demand for cost reduction and increased performance, potential answers are more miniaturization (NEMS), the integration of the sensor system and the electronics (co-integration) or the increase of the functionality with new materials and/or new architectures. Advances in semiconductor technology enable to move microsystems one step forward, or rather downward, to nanosystems. A NEMS is a nano mechanical structure with movable part(s), usually silicon-based. Its size does not exceed a few tens of micrometers and it has at least one sub-micrometric lateral dimension. Compared to a microresonator, a nanoresonator vibrates faster, is thousand times smaller in term of surface and more than 100 000 times lighter (see the example on Table 1). Although this reduction is less impressive transposed in term of chip surface (100–500 smaller) and mass (reduced by 1000 (from 1.5 g to 1 ng)), it is nevertheless remarkable (Fig. 4). Miniaturization may have an impact on the sensitivity (the proof mass is reduced) and the signal to noise ratio may be degraded (smaller capacitance and capacitance variation: this issue may be mitigated with sensors network) (see Section 4.3 about resolution). However, for resonant sensors, the decrease in size induces an increase in the resonance frequency ( f 0 is proportional to (E I /m L 3 )1/2 , so for a homothetic reduction of  1 It is only fair to take the occasion here to mention that Jacques Ellul stated more than 10 years before Moore (1965) that technical innovations takes place as a geometrical progression (J. Ellul, La technique, ou l’enjeu du siècle, 1954).

L. Bilhaut, L. Duraffourg / Acta Astronautica 65 (2009) 1272 – 1283

1275

Fig. 3. Technology roadmap.

Table 1 Microresonator versus nanoresonator.

L×w×t (m) Surface (m2 ) Weight (kg) (Si = 2330 kg m−3 ) Frequency (Hz) Surface/volume

MEMS

NEMS

Ratio MEMS/NEMS

100×10×5 100E−09 117E−11 3.83×106 4.60×1010

5×0.1×0.05 500E−13 583E−17 1.53×107 4.12×1014

2000 200,000 0.25 0.0001

Fig. 4. Size comparison between NEMS, MEMS, wire bonding, and hair.

( < 1), f 0 increases by 1/), hence a decrease in the response time. Moreover, the cost lowers dramatically (more systems on a single wafer), and the packaging

may be easier (lower topology and size). In addition nanosystems allow a multi-sensors approach in the direct line of the more-than-Moore philosophy.

1276

L. Bilhaut, L. Duraffourg / Acta Astronautica 65 (2009) 1272 – 1283

Fig. 5. Technological gap between sensors according to their sizes.

Co-integration is an interesting feature of MEMS and NEMS: the electronics is closer to the system, therefore parasitic effects are reduced and the integration density can be very high. Co-integration can be achieved through three different methods, all of them having in common the release of the mechanical structure at the end of the process [13]. The first strategy (“MEMS with Si”) is to define the MEMS before the complementary metal oxide semiconductor (CMOS). The advantage is that thermal budget can be high, but the surface state requirements are very stringent: non contamination, flatness, constraintless. The second approach is the “above IC” (integrated circuit) integration (post-CMOS approach). Although it can be easily integrated in a production line, this approach is limited to low temperature processes, and there is no access to the monocrystal silicon. Furthermore, the total yield, product of the IC yield and the MEMS yield, would tend to decrease. Last, an intermediate approach, “in-IC” integration, allows accessing to the monocrystal silicon and to a very high integration. Difficulties lie in detection principles, characterization and technological challenges. Indeed, feature size of in-IC integration demand that the sensor dimension be in the nanometer scale. Both above IC and in-IC approaches, based on IC processes, increase the reliability (essential when we deal with space applications) and decrease the run cycle time (increasing the yield). All in all, in-IC approach seems more suitable for reliable and high performance sensors. However, one has to be able to reduce the MEMS

dimensions without deterioration of its performance: this is where nanosystems become interesting. 3.2. Challenges faced by NEMS Fig. 5 shows the gap between different generation of miniaturized systems, each generation having specific detection schemes and manufacturing strategies. NEMS are still in their infancy and there is indeed a gap between MEMS and NEMS, first in term of design. Submicron phenomena have to be taken into account when designing a nanosystem: Casimir force, dissipative phenomena or surface states. There is also a need to use more sensitive detection schemes (MOS, tunnelling effect, new materials such as magnetic or piezoelectric). The in-IC integration will require new manufacturing processes (development of hybrid lithography (deep UV/e-beam), compatibility or packaging issues). Last but not least, new tools and methods have to be developed for NEMS characterization (AFM, SNOM, . . . ). However, we can still foresee there is a potential for a large scope of applications such as low cost sensors (inertial, chemical sensor, bio-sensor) and high integrated components (multi-sensors, sensors network, integrated switch). An example of a NEMS developed at the CEA-LETIMINATEC is given Fig. 6. The setup is based on a suspended gate MOSFET (SG-MOSFET). It consists in a movable gate (clamped–clamped beam) placed in front of a P doped channel that is placed between two

L. Bilhaut, L. Duraffourg / Acta Astronautica 65 (2009) 1272 – 1283

N regions (source and drain). Fig. 7 shows the beam deflection along the MOS channel according to the time (resonant beam at a frequency of 100 MHz). The beam deflection modulates the electrostatic potential at the surface of the MOS channel, hence controlling the drain current coming in the channel (Fig. 8) [14]. We see that this detection scheme is very sensitive.

1277

4. Nanosystems for space applications 4.1. Impact of nanosystems on size and weight Reducing size and weight of component does not linearly decrease the size and weight of the spacecraft. For instance, it is reported that a flight mass of an optical 8

x 10-7 Resonance frequency 100 MHz

7.8 7.6

Current [A]

7.4 7.2 7 6.8 6.6 6.4 6.2 6 0

0.25

0.5

0.75

1

1.25

Time [s] Fig. 6. Scheme of a SG-MOSFET (typical sizes: beam length 2 m, beam width 50 nm, gap 50 nm).

1.5

1.75 x 10-8

Fig. 8. Drain current coming in the channel controlled by the beam deflection (or the surface potential).

Re s 1 0 0 onan M H ce fr equ z

x 10-8

enc

y

Beam Shape [m]

2

1

0

-1

-2 2 x1 0 -6

1 Be am

2 0

1.5

len

gth

[m]

1

-1 -2

0.5 0

-8

x 10 e [s]

Tim

Fig. 7. Beam deflection along the MOS channel according to the time for a resonant beam at 100 MHz (the deflection amplitude is 5 nm).

L. Bilhaut, L. Duraffourg / Acta Astronautica 65 (2009) 1272 – 1283

Force [N]

1278

2E-11 1.8E-11 1.6E-11 1.4E-11 1.2E-11 1E-11 8E-12 6E-12 4E-12 2E-12 0 50

Casimir force Electrostatic force (V = 0.1 V) Electrostatic force (V = 0.3 V) van der Waals forces

100

150 200 gap [nm]

250

300

Fig. 9. Influence of Casimir force vs. electrostatic force for different gap.

communications subsystem is typically 55–65% of that of a conventional microwave subsystem [15]. But if one translates this numbers into the decrease of mass on the whole spacecraft (where the telemetry tracking and command subsystem is in average 7.5% of the dry mass [16]), it makes a decrease of only 3% (for a decrease of 60%) in the total satellite size. Now, if one assumes that NEMS will be able to reduce the part of the optical communications subsystem to e.g. 30% (twice as low) of a conventional microwave subsystem, the decrease will be only of 5.25% and not twice as low. This simple example shows that the use of NEMS instead of MEMS might not have straightforward advantages in term of size and mass saving.

actuation [21] does not influence the required current for a given deflexion, but it decreases it for thermal actuation using a bimorph effect [22]. It seems that only both thermal and electrostatic actuation beneficiate of reduced dimension. New phenomena appear in NEMS and other forces, negligible in MEMS, have to be taken into account. One of them is the Casimir force, which has to be taken into account when two surfaces are brought very close (50 nm to 1 m) from each other. This force can be expressed as

4.2. NEMS actuators

where c and c are, respectively, the Planck constant and the speed of light (for a more complex expression taking into account detailed geometries and material characteristic, see [23]). If we compare this force to a regular electrostatic force (Felectr ostatic = 0 SV 2 /2g 2 ) for different gaps between the moving part and the fixed part, we see that the Casimir force is not negligible when compared to low actuation voltage (Fig. 9). Therefore, it has to be carefully integrated in the design of the system. For instance, it will have a notable influence on the pull-in voltage of NEMS [24]. For smaller gap ( < 50 nm), one should rather consider the van der Waals forces, coming from the electrostatic dipolar interactions (between two polar molecules or atoms, or between a permanent dipole and an induced dipole or between two induced dipoles). As we see on Fig. 9, those (usually attractive) forces diverge when the gap tends to zero. Indeed, in this regime, one has to add repulsive contribution (Pauli’s exclusion principle).

4.2.1. Emerging forces at the nanoscale Every micro or nanosystem has to be actuated, either statically (e.g. switch) or dynamically (e.g. resonant sensors). Nanoactuators can find application in space for adaptative optics, steerable antenna or even memories. The effect of scaling on the actuation forces is not always proportional to the dimension reduction. For electrostatic actuation, the pull-in voltage decreases linearly with the dimension [17]. For a piezoelectric actuation utilizing a bimorph effect, there is on a first look no influence on the required actuation voltage [18]. However, studies show that the effective piezoelectric coefficient d31 decreases strongly with the thickness of the layer (e.g. AlN [19]). That would mean that the actuation voltage would have to increase to compensate this effect. However, promising results have already been achieved [20]. Reducing dimension for a magnetic Lorentz force

FCasimir =

Scc2 240g 4

L. Bilhaut, L. Duraffourg / Acta Astronautica 65 (2009) 1272 – 1283

Fig. 10. Nano-mechanical memory: the bit is 0 when the beam is in the position 0 and 1 when it is in the position 1.

4.2.2. Example of a breakthrough system Those attractive surface forces can create new systems. For instance, in the area of emerging memory scheme, one thinks about nano-mechanical memories: here, the information bit would be stored in the mechanical state of a moving element (Fig. 10). To realize a high density non volatile memory, one would need bistable nanoswitches. Several routes have been already taken, all of them taking advantage of the equilibrium between the mechanical force of the system and the attractive surface forces. The mechanical part can be made of silicon nanobeams [25], nanowires [26] or even carbon nanotubes [27]. The main envisioned advantage for space application is a natural radiation-hardness while keeping a high integration density [28]. Bistability arises from the equilibrium between the mechanical forces and the attractive surface forces (van der Waals). If we look at the interactions between a silicon beam and a silicon substrate, we can write equilibrium between the van der Waals forces FvdW , the elastic force Felastic and the actuation force Factuation : FvdW = Felastic ± Factuation From an energetic point of view, if we take as actuation force an electrostatic bias, we can write the total energy E total as a function of the beam position z (the origin being at the substrate level, see Fig. 10) k 0 SV 2 E total (z) = (g − z)2 + 2   2z    12  6 + 4 − z z where k is the spring constant of the beam, g is the gap between the beam and the substrate, z is the position of the beam (we have g =  + z, with  standing for the beam deflexion), 0 is the air dielectric constant, V is the bias between the substrate and the beam, and  and

1279

 are the Lennard-Jones potential constants. The three terms in the right-hand member represent, respectively, the elastic energy, the electrostatic energy and the energy coming from the van der Waals interaction. The total energy is represented on the Fig. 11 for different values of the bias V. We can see there are two energetic minimums corresponding to two stable positions of the beam: one at about 0.2 nm (corresponding to the beam in contact with the substrate (inset of Fig. 11) and the other at 50 nm, corresponding to the steady state position of the beam. We can go from a position to another by increasing and then decreasing the value of the electrostatic force. 4.3. NEMS sensors Very basic MEMS and NEMS sensors are made of a sensing mass having 1◦ of freedom (it is attached to a fixed support via a beam represented as a spring (Fig. 12)). An external parameter, such as a force (pressure, acceleration, yaw . . . ), a mass (chemical species, photon . . . ), or a gradient (magnetic, temperature, . . . ), induces a change in the position (static sensors) or the frequency resonance (dynamic sensors) of the proof mass. This change is detected thanks to an electronic circuit. The system can be either an open loop (i.e. the change output is directly related to the sensed parameter) or a closed-loop (i.e. the change is maintained to zero with a control feedback loop, and the measure of the provided actuation energy gives the value of the sensed parameter). We consider the influence of reducing dimension on the resolution of an accelerometer. The resolution is the smallest difference between two measured output values of the sensed parameter, here the acceleration. It can be calculated taking into consideration all the noise sources of the system. In this study, in the limit of low frequency, we consider only the thermomechanical noise of the proof mass (i.e. the Brownian noise) anBr ownian and the electrical noise of the low noise amplifier (LNA) circuit used for the readout anElectrical . The acceleration noise an is thus defined by 2

an2 = anBr ownian + anElectrical

2

anBr ownian can be expressed as 2

anBr ownian =

4k B T 0 BW MQ

where k B is the Boltzman constant, T is the temperature, 0 is the resonance frequency, BW is the bandwidth of the sensor, M is the mass of the proof mass and Q is

1280

L. Bilhaut, L. Duraffourg / Acta Astronautica 65 (2009) 1272 – 1283 1E-15 V = 0.1 V V=1V V = 10 V

Energie (J)

1E-16

1E-17 1E-16

1E-17

1E-18 1E-18

1E-19

1E-19

1E-20 0

0.1

0.2

0.3

0.4

0.5

1E-20 5

0

10

15

20

25

30

35

40

45

50

Distance (nm)

Fig. 11. Total energy of an electrostatically actuated Si beam (3000 × 10 × 10 nm3 ) whose steady state position is at 50 nm from a Si substrate (inset: zoom between 0 and 0.5 nm).

z Beam Proof mass (moving electrode) C+ΔC

F Fixed electrode

Fig. 12. Working principle of a simple micro and nano sensor.

the quality factor of the beam (one can find this result using the dissipation/fluctuation theorem, that states the kinetic energy of the beam deformation modes equals the dissipated thermal energy). The LNA used to compute anElectrical is a simple MOS transistor with a feedback loop 2

anElectrical =

2 BW i ds

|gm |2 2Br ownian 2Electrical

where i ds is the noise density in current of the channel coming from the flicker and thermal noise, gm is the transconductance of the LNA, Br ownian is the mechanical response ( Br ownian ≈ M/k) and Electrical is the electrical response (Electrical =2C0 VM /((2C0 + C p + C gs + Cs )g)). If we scaled down the proof mass by a factor  ( < 1) while changing the spring constant so the frequency resonance 0 does not change (so M and k scale down by 3 , Q by 2 , and in the expression of anElectrical only

Electrical scales up by 1/ (the electrical readout circuit is not modified)), we see that anBr owian scales up much faster (by −5/2 ) than anElectrical scales down (by ). We clearly see that accelerometer resolution is not enhanced by scaling down the system. Indeed, Brownian noise always limits NEMS resolution and the only solution to mitigate this is to put the system at cryogenic temperature. We consider now the sensor sensitivity (defined here as the linearized frequency variation regarding to the measured parameter variation). For a sensor measuring a force Fmeasured , the frequency shift  f around the initial frequency resonance f0 can be expressed as (for a clamped–clamped beam) [29]  L2 Fmeasur ed − f 0  f = f0 1 +

Etw3 where is a constant depending on the vibration mode, E is the Young modulus, and L, w and t are, respectively, the length, the width and the thickness of the resonant beam. If we applied the same homothetic reduction  as previously (but with f0 nonconstant) in the case of an accelerometer, we see that for a same acceleration a, the frequency shift around the initial resonance frequency is lower. However, this is the case only because the measured parameter is proportional to the proof mass, which reduces as 3 . Some sensors do beneficiate of a size reduction, for instance chemical sensors. In that case the resonator is functionalized so that a specified chemical species can

L. Bilhaut, L. Duraffourg / Acta Astronautica 65 (2009) 1272 – 1283 mean free path of N2 Acoustic wavelength (f = 1 Mz) Acoustic wavelength (f = 100 MHz) Acoustic wavelength (f = 1 GHz ) lamba_a = 1 um lamba_a = 100 nm lamba_a = 10 nm

1000000

Mean free path (thick curves) [nm] Knudsen number (thin curves)

1281

100000 10000 1000 100 10 1 100 0.1

1000

10000

100000

1000000

10000000

0.01 0.001 Pressure [Pa]

Fig. 13. Mean free path of a N2 molecule and acoustic wavelength coming from the vibration of the beam (thick lines) and variation of the Knudsen number (thin lines).

bound to it, adding a mass and shifting the resonance frequency. The sensitivity S can be expressed as [30] S=

f0 df =− dm 2m

When we decrease dimension, the resonance frequency increases by 1/, and the mass decreases by 3 , which increases the sensitivity by 1/4 ! Tens of zeptograms (10−21 ) have already been measured [30]. Chemical sensors can be used especially in biological applications which in space can be applied to human exploration: astronauts monitoring, or general biological monitoring, especially important for closed-loop life-support systems. To finish this section, the attention of the reader is one more time drawn to the fact that physics changes at those scale compared to MEMS. Intrinsic losses such as surface losses are more important in nanosystems than in microsystems since the ratio surface/volume is higher (see Table 1). Extrinsic losses change as well because of dimension reduction. This has an impact on the quality factor of the system. We illustrate this fact with fluidic losses. To correctly model those losses, one has to determine in which fluidic regime the system stands. To that purpose, we compare the acoustic wavelength a of the system with the free mean path of a molecule of the ambient gas lm .  cp R T a = f cv Mmolair e f is the frequency, cp and cv are, respectively, the specific heat capacity under constant pressure and constant

volume, R is the molar gas constant and M is the molar mass. lm is approximated by lm = √

kB T 2d 2 P

where d is the diameter of a molecule and P is the pressure. If we have a ?lm , then we are in a classic viscous regime. In the reverse case, we are in a molecular regime: one has to consider the interaction of each molecule with the structure. We see on the Fig. 13 that the more the beam size decreases (i.e. the frequency increases), the higher the pressure at which the classical regime is not valid anymore. This result is the same when considering the Knudsen number Kn (K n = lm /a (the denominator is a characteristic value of the considered system, a can be replaced for instance by the gap)): the lowest the critical parameter, the highest Kn (also on Fig. 13). It is generally considered that the freemolecule regime begins when Kn > 10. 4.4. Autonomous smart sensors networks If the usefulness of NEMS for space is still to demonstrate on middle term, with application-based technology development, on the long term, NEMS will certainly find applications in sensor networks. An AUtonomous Smart Sensors Network (AUSSN) is a system of submillimetre systems, each one integrating sensor(s), data processing capability, communication system, power source, and possibly locomotion means. A well known development of AUSSN is the so-called “Smart Dust” [31,32]. It is expected that those systems

1282

L. Bilhaut, L. Duraffourg / Acta Astronautica 65 (2009) 1272 – 1283

will change our way of living as much as personal computers did. Emerging collective behavior will give rise to ambient intelligence. J. Attali call them “watcher” and applications range from health monitoring, entertainment, art, defense application to yet unexpected appliances [33]. It is predicted here that just as MEMS enabled micro and nanosatellites and new missions concepts such as swarm of satellites, from which we are just starting to witness the full potentiality, NEMS will be part of the technology development that will enable a totally new type of mission concepts using AUSSN. In this development, the batch processing capability of MEMS that is not essential for space applications will be a requirement that will made NEMS even more essential (cost reduction brought by NEMS is even more dramatic that the one brought by MEMS). But just as MEMS development was not driven by the space community, and unless strong commitment from this community, AUSSN will not be developed for space. It is thus very likely that motes composing the AUSSN will have to face similar challenges as MEMS when adapting them to the stringent requirements of the peculiar environment of space. Reliability issues might be mitigated by the multiplicity of the systems, in a sort of graceful degradation concept taken one step further. As for the harsh space environment, one should consider that unlike MEMS components today, AUSS motes will be considered like a full system, since each one of them has to be autonomous and relies on itself for survival. If we concentrate solely on the mission environment, the motes will have to withstand extreme high and low temperature effects. It is probable that temperature cycling may lead to numerous failure modes (e.g. thermal expansion mismatch). Mechanical effects (acceleration, random vibration, acoustic vibration and shock, most of them during the launch phase and the landing) might be mitigated by packaging motes in a protective box. Chemical effect such as moisture could be avoided by sealing the AUSSN in a microelectronic-grade cleanroom (class 10 or 100) instead of a space qualified cleanroom. This would also eradicate the issues of loose or floating particles under zero gravity condition. Outgassing happening during release will depend on the materials used for AUSS motes. It may set condition on the material choice. Electrical stresses and damages due to plasma and radiation will be a concern of importance since shielding is difficult due to the low size and mass requirement and the need for the sensors to communicate with the environment. Zero pressure will be an issue, as well as atomic oxygen. Achieving an appropriate packaging

that does not impair the functionality of the motes will be of the greatest challenges. Eventually one should consider ethical issue: because of their low size, retrieving sensor motes may be well impossible. This issue will be first addressed for Earthbased applications. But in case of planetary exploration, we should assess our moral right to “pollute” other celestial bodies. Another concern will be the release of such systems in outer space, creating potential huge clouds of space debris impossible to retrieve or even to track. 5. Conclusion Microsystems are actually more and more integrated in space missions, either as technology enhancer or technology enabler. Mini to picosatellites can be built thanks to miniaturization and mission concepts using swarm of satellites are flourishing. Meanwhile, driven by a demand in cost reduction and performance increase, the microelectronic industry starts to be interested in nanosystems. Advantages of those systems cannot be reduced to a gain in mass, size or power consumption since those very characteristics will enable new systems especially in term of integration. We have given a glimpse of the complexity of NEMS design, with concrete examples of new phenomena which have to be taken into account. Because we are at a scale between molecular and macroscopic physic, modelling is very difficult and this field is wide open to mesoscopic physicists. We have also shown that the actual performance of nanosystems have to be carefully investigated on a case by case basis. In conclusion, we can say that today’s nanosystems might be to microsystems what the CMOS transistors are now to the bulky transistors of the sixties: they will allow new system concepts, especially through autonomous smart sensors networks. Although those networks will certainly find wide applications on Earthly systems, the challenge for the space community will be to adapt them to the space environment. But potentialities are so huge that it is certainly worth the investment.

References [1] H. Helvajian, E.Y. Robinson, Micro- and Nanotechnology for Space Systems, The Aerospace Press, 1997 Monograph 97-01. [2] H. Helvajian, Microengineering Technology for Space Systems, The Aerospace Press, 1997 Monograph 97-02. [3] R. Osiander, M.A. Garrison Darrin, J.L. Champion, MEMS and Microstructures in Aerospace Applications, Taylor and Francis Group, 2006.

L. Bilhaut, L. Duraffourg / Acta Astronautica 65 (2009) 1272 – 1283 [4] D.G. Sutton, R.S. Smith, P. Chaffee, S. Janson, L. Kumar, N., Marquez, J. Osborn, B.H. Weiller, L. Wiedeman, MEMS spaceflight tested, AIAA-1999-4604, 1999. [5] J. Jason Yao, et al., Microelectromechanical system radio frequency switches in a picosatellite mission, Smart Mater. Struct. 10 (2001) 1196–1203. [6] http://nmp.nasa.gov/st5/TECHNOLOGY/enabling.html. [7] M. Yun, J. Bock, W. Holmes, T. Koch, J. Mulder, R.P. Vasquez, L. Wild, A. Lange, Microfabrication of silicon–nitride micromesh bolometric detectors for planck high frequency instrument, J. Vac. Sci. Technol. B 22 (1) (2004) 220–225. [8] R.S. Winsor, J.W. MacKenty, M. Stiavelli, M.A. Greenhouse, J.E. Mentzell, R.G. Ohl, R.F. Green, Optical design for an infrared imaging multi-object spectrometer (IRMOS), in: SPIE Proceedings, vol. 4092, 2000, pp. 102–108. [9] http://www.caneus.org/. [10] http://www.aaerospace.com/. [11] H. Lübberstedt, D. Koebel, F. Hansen, P. Brauer, MAGNAS—magnetic nanoprobe swarm, Acta Astronaut. 56 (2005) 209–212. [12] P. D’Arrigo, S. Santandrea, APIES: a mission for the exploration of the main asteroid belt using a swarm of microsatellites, Acta Astronaut. 59 (2006) 689–699. [13] S. Ghosh, M. Bayoumi, On integrated CMOS-MEMS system-on-Chip, in: The Third International IEEE-NEWCAS Conference, 2005, pp. 31–34. [14] E. Ollier, L. Duraffourg, E. Colinet, C. Durand, D. Renaud, A.S. Royet, P. Renaux, F. Casset, P. Robert, Lateral MOSFET transistor with movable gate for NEMS devices compatible with "In-IC" integration, in: Proceedings of the 3rd IEEE Int. Conf. on Nano/Micro Engineered and Molecular Systems, 2008, pp. 764–769. [15] K. Wilson, M. Enoch, Optical communications for deep space missions, in: IEEE Communications Magazine August, 2000, pp. 134–139. [16] W.J. Larson, J.R. Wertz, Space Mission Analysis and Design, third ed., Microcosm, Inc. and W.J. Larson, 1999 p. 896. [17] H.C. Nathanson, W.E. Newell, R.A. Wickstrom, J. Ransford Davis, The resonant gate transistor, IEEE Trans. Electron Devices 14 (1967) 117–133. [18] D.L. DeVoe, Thin film zinc oxide microsensors and microactuators, Ph.D. Thesis, 1997. [19] F. Martin, P. Muralt, M.-A. Dubois, A. Pezous, Thickness dependence of the properties of highly c-axis textured AlN thin films, J. Vac. Sci. Technol. A 22 (2004) 361–365.

1283

[20] S.C. Masmanidis, R.B. Karabalin, I. De Vlaminck, G. Borghs, M.R. Freeman, M.L. Roukes, Multifunctional nanomechanical systems via tunably coupled piezoelectric actuation, Science 317 (2007) 780–783. [21] F.-H. Cheng, Statics and Strength of Materials, second ed., McGraw-Hill International Editions, 1998. [22] S. Timoshenko, Analysis of bi-metal thermostats, J. Optical Soc. Am. 11 (1925) 233. [23] L. Duraffourg, P. Andreucci, Casimir force between doped silicon slabs, Phys. Lett. A 359 (2006) 406–411. [24] P. Andreucci, L. Duraffourg, E. Ollier, V. Nguyen, M.T. Delay, P. Robert, Impact of Casimir force on nano accelerometers Modelling, in: Proceedings of the IEEE Sensors, 2006, pp. 1057–1060. [25] M.A. Beunder, R. van Kampen, D. Lacey, M. Renault, C.G. Smith, A new embedded NVM technology for low-power, high temperature, rad-hard applications, in: IEEE Non-Volatile Memory Technology Symposium, 2005, pp. 65–68. [26] K.J. Ziegler, D.M. Lyons, J.D. Holmes, D. Erts, B. Polyakov, H. Olin, K. Svensson, E. Olsson, Bistable nanoelectromechanical devices, Appl. Phys. Lett. 84 (2004) 4074–4076. [27] J.W. Ward, M. Meinhold, B.M. Segal, J. Berg, R. Sen, R. Sivarajan, D.K. Brock, T. Rueckes, A non-volatile nanoelectromechanical memory element utilizing a fabric of carbon nanotubes, in: IEEE Non-Volatile Memory Technology Symposium, 2004, pp. 34–38. [28] M.N. Lovellette, et al., Nanotube memories for space applications, IEEE Aerosp. Conf. Proc. 4 (2004) 2300–2305. [29] H.W.Ch. Postma, I. Kozinsky, A. Husain, M.L. Roukes, Dynamic range of nanotube- and nanowire-based electromechanical systems, Appl. Phys. Lett. 86 (2005) 223105. [30] Y.T. Yang, C. Callegari, X.L. Feng, K.L. Ekinci, M.L. Roukes, Zeptogram-scale nanomechanical mass sensing, Nano Lett. 6 (2006) 583–586. [31] B. Warneke, M. Last, B. Liebowitz, K.S.J. Pister, Smart dust: communicating with a cubic-millimeter computer, IEEE Comput. 34 (2001) 44–51. [32] B.A. Warneke, M.D. Scott, B.S. Leibowitz, L. Zhou, C.L. Bellew, J.A. Chediak, J.M. Kahn, B.E. Boser, K.S.J. Pister, An Autonomous 16 mm3 solar-powered node for distributed wireless sensor networks, Proc. IEEE Sensors 2 (2002) 1510– 1515. [33] J. Attali, Une brève histoire de l’avenir, Fayard, 2006 (in French).