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Paul Ahern - Overview of Micro & Nano Transducers

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Abstract— The aim of this paper is to present a review of current transducer technology, fabrication methods and materials pertinent to the nanotechnology and MEMS era. We begin with an introduction …

Abstract— The aim of this paper is to present a review of current transducer technology, fabrication methods and materials pertinent to the nanotechnology and MEMS era. We begin with an introduction to the concept of a transducer and the historical context, and then review some specific application classes of transducers where nanotechnology has already, or has the possibility in the future, to have an impact on the transducer device market. This review highlights the advantages of these MEMS approaches to promote new transducer types, especially those related to nanotechnology, and possible future research directions are discussed.

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  • 1. Overview of Micro- and Nano-Transducers (December 2013) Paul Ahern, 1School of Electronic Engineering, Dublin City University. Reliability & Eco-Environmental Engineering, Bell Labs / Alcatel Lucent. paul.ahern@alcatel-lucent.com 2 Abstract— The aim of this paper is to present a review of current transducer technology, fabrication methods and materials pertinent to the nanotechnology and MEMS era. We begin with an introduction to the concept of a transducer and the historical context, and then review some specific application classes of transducers where nanotechnology has already, or has the possibility in the future, to have an impact on the transducer device market. This review highlights the advantages of these MEMS approaches to promote new transducer types, especially those related to nanotechnology, and possible future research directions are discussed. Keywords — Accelerometer, gyroscope, inertial sensor, resonator, microfabrication, MEMS, silicon sensors, piezoelectricity, thin films, ultrasonic actuators. I. INTRODUCTION A mounting cause for concern in the growing microelectronics market is the slow pace of availability of power efficient, high sensitivity and commercially viable transducers. Thanks to generations of familiarity with silicon VLSI components attempts are on-going in many other fields to apply “Moore’s Law” type scaling improvements to transducer technology to innovate and refine new multidisciplinary transducer products. These efforts typically have a low initial success rate as the numbers of transducers with adequate performance at a suitably low price point do not exist. Novel transducers which make use of nano-sized devices and materials have the potential to allow the ultra-sensitive measurement of previously undetectable phenomena all the way down to the molecular scale. Scaling transducers down to the length scale of nanotechnology affords us the opportunity to utilise their special properties for a wide variety of measurements. Potential applications of such a shrink could be utilised in diverse fields such as nanoelectronics, biotechnology and medicine. To date, there are only a handful of commercially available transducer types which utilise micromechanical principles and fabrication techniques, however the potential for micro and eventually nano-sized transducers to have a disruptive effect on the world of sensing and control is huge. In this review paper we will undertake a whistle-stop tour of the subject, and discuss the principles of fabrication, nanomaterials properties and applications for a select sample of micro- and nanotransducer systems. II. OVERVIEW OF TRANSDUCER OPERATION & FABRICATION A transducer is at its most basic is simply a mechanical device which reads an input signal and outputs an electrical or optically detectable response1. The buttons of a computer keyboard , the quartz crystal of a watch, and the light sensor of a burglar alarm are all examples of a cheap, uncomplicated input transducers2 with the light emitting diode (LED) probably the currently most well-known example of a lowcost output transducer. Many would consider the origin of the burgeoning field of micro-transducer as being closely related to the publication of a series of important research papers by a group of scientists starting in the mid-1950’s. Charles Smith, then of Bell Labs while on sabbatical from Case Institute of Technology, published his study3 of uniaxial strain on the resistivity of silicon and germanium devices in 1954 which opened the possibility of having such a microscopic phenomenon as his “electron transfer mechanism” giving direct usable experimental data. Paul and Pearson4, of Harvard and Bell Labs respectively, continued this direction with their 1955 publication concerning single high purity silicon crystals and their behaviour of lowering their energy gap when in a high pressure environment. Feynman’s now infamous 1959 presentation at CalTech to the American Physical Society, entitled “There’s Plenty of Room at the Bottom”, with hindsight, is credited as spurring more interest into the nascent concept of manipulating single atoms to build structures and devices. In 1961, Pfann and Thurston5 again of Bell Labs probed the viability of a transducer which used the piezoresistive response of diffusion-doped silicon and germanium to evaluate longitudinal and transverse stresses. At Honeywell, Tufte, Chapman and Long6 were the first in 1962 to create a truly thin silicon diaphragm for pressure sensing by using early semiconductor processing methods such as plasma etching and selective oxidationa. a For a full and entertaining history of the explosion in research and new companies in the field from the mid-sixties onwards, a good starting point is Marc J. Madou’s “Fundamentals of Microfabrication & Nanotechnology” by CRC press.
  • 2. Paul Ahern “Overview of Micro- and Nano-Transducers” These devices can be broadly categorised as passive or active; passive types operate due to the generation of some shift in material properties such as capacitance or resistance with an external power source required to give rise to the final measurable output. Active transducers on the other hand usually transforms the input signal directly into electrical energy, such as the way a piezoceramic element materials translates energy from mechanical to an electrical form. Additional signal processing can be done either on-board the transducer or elsewhere, but still the principle is using a basic mechanical element such any kind of micro-sized beam or diaphragm. Typical transducers are made possible by the harnessing of certain intrinsic solid state materials properties, and common transducer devices such as thermometers, pressure and moisture sensors all have their basis of operation in the manifestation of a suitable material band gap, with their low cost fabrication being based on very well understood silicon planar technology7. The differing current approaches used for MEMS-based transducers are to either attempt to utilise nanotechnology to fashion both the transducer and the required amplification component8, or else as in the silicon case to attempt the approach whereby an integrated circuit wafer as a 2D substrate is used and we include the nano-sized transducer parts as separate elements situated as closely as possible to the patterned transistors amplifiers9. If the mechanical transducer and the integrated circuit components can be incorporated together in such a “hybrid” arrangement, then there is the opportunity for mass production to give rise to inexpensive, high quality transducers. However there are benefits to taking a different, more disruptive approach and pursuing miniaturisation using nanotechnology principles to drive miniaturisation down closer to the theoretical limits, with improved performance and lower power consumption side benefits to the realm of new measurements concepts for Nano-transducers that can only be realised at the nanoscale. SELECTED NOTABLE APPLICATIONS III. NANO-RESONATORS Resonators are devices which utilise an element, such as a beam or a diaphragm, to detect or generate vibrations of a certain frequency. The ability to discriminate different frequencies is critical to their operation and usefulness. Traditional resonators have been with us since the midsixties when Belyaev10 and co-workers showed their prototype which was a simple diaphragm mounted on two supporting brackets. In the silicon age, micromachining and IC fabrication techniques have been used successfully to design new types of smaller resonator which contain a hollow cavity and can operate much more efficiently in vacuum environments. A typical representation of this type of device is shown in Figure 1 opposite. The resonator device can then 1 be affixed onto a printed circuit board which houses the control electronics using industry standard adhesives. Figure 1 – Schematic of the cross-section of a typical silicon micro-machined diaphragm resonator. The PZT layer is clamped between a bottom electrode of Pt/Ta and a top electrode of Cr/Ag. (Image – Deshpande & Saggere11, 2007) The inclusion of a transparent cantilever allows an optical detection scheme to be used if an electronic scheme is not suitable for the application , which means the system is now a optical resonator described as a “microoptoelectromechanical” device, or MOEMS. The further addition of lenses, prisms, mirrors and refractive media can be used with such systems to build small, fast devices for photonic waveguides, routing and modulation. An early commercially available example of such a device is a handheld barcode scanner which most readers would be familiar with, however integration with other types of free space optical components has given rise to interesting and more complex devices. One example of such a device is Tien12 and co-workers surface laser for micro positioning which is sown in Figure 2 overleaf. Built upon a silicon substrate, it makes use of freestanding MEMS components such as folding micro mirrors, micro lenses and a resonant transducer operating at a natural frequency of 29.2 kHz.
  • 3. Paul Ahern “Overview of Micro- and Nano-Transducers” Figure 2 – Tien & co-workers design of a laser resonant scanner built on a silicon substrate which makes use of freestanding MEMS components. In the future it is very likely that optical MEMS will consume more and more of the market in this space, and that increasingly expertise from the nanotechnology and semiconductor fabrication arenas will be brought to bear on making these devices which can directly interact with the photonic packets being distributed in modern optical networks. Newly simplified spin on glass (SOG) processes such as those described by Cheng13 and co-workers which is an enabler for tighter integration of integrated photonics and MOEMS devices which critically require smooth surfaces with uniform nanometre scaled roughness values. One compound which is attractive for this purpose is biphenol A ether glycidyl due to its high optical transparency above the 400nm wavelength window coupled with its very low propagation loss profile, which makes it valuable for fabricating optical components such as resonators and MachZender diodes. Plasma treatments such as those detailed by Zebda14 and coworkers are critical in delivering interfacial surface modification methods which allow these materials to grip to plasma treated surfaces effectively to allow additive spin-on processing. And pave the development for ultra-integrated 2.5D photonics structures. The problematic high thermal budget associated with these processes can also be decreased, when piezoceramic materials are present in the device, by instead using one of the newer CVD-related deposition techniques such as plasma enhanced chemical vapour deposition (PECVD) or conformal atomic layer deposition (ALD) methods. IV. INERTIAL SENSORS Once the exclusive preserve of aircraft, space rockets, and cruise missiles, the last decade has seen MEMS-based inertial sensors such as gyroscopes and accelerometer make rapid inroads into first the luxury segment, and now the non-luxury 2 automobile supply chain. Anti-lock braking and skid control systems in the average family hatchback owe thanks to the advent of low cost, mass produced silicon based MEMS sensors. On luxury models, additional features such as adaptive active suspension systems, autosensing intelligent cruise control, and advanced airbag protection systems are all controlled by inertial transducers. While historically inertial systems such as gyroscopes and accelerometers have been composed of mainly mechanical sensors, in recent decades there was an impetus to move the foundation of such devices in to the MEMS and nanotechnology space. The main drivers for this change have been the marked performance and reliability improvements possible, easier integration directly with the on-board digital electronics as well as the lower costs attainable with semiconductor manufacturing methods which allow many thousands of devices to be created on a single silicon wafer substrate, such as shown in Figure 3 below. Figure 3 – Scanning Electron Microscope image of an early silicon MEMS comb drive tuning fork gyroscope. The comb structures allow detection of resonance along the axis normal to the vibration plane. (Image – Barbour15 and coworkers/Draper Laboratories) MEMS gyroscopes are an offshoot of the resonator type device, and similarly to resonators are usually fabricated from a monolithic substrate of silicon (or sometimes quartz). These devices operate based on the physical principle of translating an external angular change to an internal torsional force which is detectable around the sensor’s axis, called a Coriolis force16. For a silicon-based sensor, the change is directly measured as a capacitance shift, whereas for quartz materials the change is manifested as a piezoelectric perturbance. The electrical output is then demodulated by the digital electronics, amplified and fed to the digital output. Gyroscopic sensor devices require the presence of a vacuum inside an hermetically sealed device package, usually an Al2O3 ceramic casing, in order to lower losses and obtain high Q resonance conditions, and are intended to maintain stable and
  • 4. Paul Ahern “Overview of Micro- and Nano-Transducers” uncontaminated surroundings for the device for a typical 20year design lifetime17. Semiconductor manufacturers are adept at squeezing every last but of performance out of silicon devices, and inertial sensors are no exception. Supplier like Samsung use their IC experience to tune the manufacturing process to drop the noise floor of the inertial sensor to its minimum using some novel tweaks, such as employing differential drive voltage setups, application of an effective interlayer DC bias after fabrication and utilising high-aspect-ratio polysilicon structures in the device front-end to maximise the desired frequency response18. Recent work by Zandi19 and co-workers has postulated how microphotonics may be able to advance accelerometer devices by using the familiar principles of a Fabry-Perot cavity. Their scheme involved the attachment of a suspended movable mass to one Distributed Bragg reflector (DBR) mirror – when there is an acceleration, the movable mirror’s position changes and thus the cavity width changes which can be seen as a modification of the resonance conditions. This approach allows an easily-integrated solution, shown in Figure 4 below, which is independent of EMI and consumes very low power. Figure 4 – Can microphotonic devices extend the operational envelope for accelerometers? Zandi and coworkers silicon based Fabry-Perot accelerometer allows integration of all components on a single substrate. 3 A common and non-trivial problem with generally all types of accelerometer devices is the question of how to try and limit cross-talk from other directions which may be mistaken as input signals in the axis of interest. Maximisation of sensitivity and simultaneous suppression of unwanted forces and excitations is a large and on-going area for research. These optical types of accelerometer however allow a very low value of unwanted cross-axis sensitivity, with Zandi and co-workers reporting that their initial prototype devices achieved <0.5% on the orthogonal (in-plane) axis and <0.01% for the z-axis (out-of-plane); to put this in perspective, this is performance that could simply not be attained by traditional mechanical acceleration sensors, no matter how much they are shrunk in size. For very high resolution sensing, another approach is to harness quantum mechanical tunnelling effects in the device. First envisaged at the Jet Propulsion Laboratory, in this case a constant tunnelling current can be maintained between a fixed tip and a moveable microstructural element which is within a few Angstroms of separation, with a counter-electrode employed to sense the displacement in a closed-loop system20. Preservation of the closed-loop mode is vital, as the ratio of tunnelling current: distance is exceptionally large and critically limits the usable measurement range if employed in the open loop arrangement, and the tunnelling barrier height for the opposing electrodes can vary by one order of magnitude in air, which hinders device sensitivity21. An example of one such tunnelling device can be seen in Figure 5 overleaf. Using tunnelling as a sensing mechanism, a device can measure very small sized displacements with high sensitivity in a small footprint, however they fall down in the area of their low-frequency noise levels and their high voltage requirements.
  • 5. Paul Ahern “Overview of Micro- and Nano-Transducers” 4 frequency noise which has so far generally been noticeably high in measurements made using these types of nanotubebased devices. A proposed workaround for these limitations is the proposed inclusion of AC bridging rectifier, such as the wellunderstood Wheatstone bridge, onto the transducer in order to resolve the typically non-linear output of the transducer. Once this response has been accounted for however, these devices offer excellent sensitivity as shown in a study carried out by Minot23 and co-workers where a device was successfully fabricated with a measured sensitivity limit of 0.1nN/Hz 1/2 at low frequencies. Figure 5 – Yeh & Najafi’s design22 (top) and as-fabricated electron micrograph (bottom) of their micro-machined accelerometer which operates based on the principles of tunnelling. The device boasts their high resolution with a wide dynamic range contained within a small footprint. V. NANO-STRAIN GAUGES In the physical world, gradual miniaturisation of macrosized transducers for sensing applications has a finite limit, dictated by the device signal to noise (SNR) value which becomes larger as the device geometry shrinks. For transducers such as strain gauges, SWCNTs have an advantage due to their high strain sensitivity caused by changes to the band gap energy due to distortion of the atomic lattice. This atomic-level effect is a clear example of the type of quantum mechanical phenomena that govern the properties of materials at the nanosystem level, and gives rise to a detectable resistance change which encompasses up to two orders of magnitude for even minor strain energies in the region of 1-3%. A diagrammatical illustration of this mechanism in terms of the band gap of the CNT can be seen in Figure 6 opposite. A significant impediment to the ready use of CNTs as a strain sensitive material is their non-linear behaviour due to temperature. The strain response has the effect of limiting the strain sensitivity at very small magnitudes. Other issues are surmounting contact resistance issues which have been shown to limit SNR due to thermal noise, as well as controlling low Figure 6 – Diagrammatical illustration of how pressure can be measured in a nano-sized strain transducer. At (a) there is no strain so the CNT acts as a p-type semiconductor. At (b) transport in the CNT is interrupted by the creation of a depletion region. At (c) effectively a p-n-p junction has formed in the middle of the CNT and tunnelling can now increase as the size of  decreases. (Image from Minot24 & co-workers) In terms of the fabrication method, we once again encounter the question of a top-down vs. bottom-up approach when it comes to the question of how to fabricate the CNT, whilst the tried and trusted method of AFM manipulation has been used to position the CNT beam on the silicon substrate. Large metallic contacting pads for the device can be grown by conventional Chemical Vapour Deposition (CVD) based methods, with gold pads providing the best contact and lowest resistance. The SWCNT based transducer created by Stampfer and co-workers can be seen in Figure 7 opposite.
  • 6. Paul Ahern “Overview of Micro- and Nano-Transducers” resonance conditions, simulations. 5 in agreement with numerical Figure 8 – Lee and co-workers experimental results showing the Raman shift observed when a SWCNT is subjected to increasing pressure from a probing AFM tip. The G mode at 1590cm-1 shows marked changes as the strain is applied. Figure 7 – High magnification SEM images showing topdown (a) and isometric (b) views of the nano strain gauge fabricated by Stampfer25 and co-workers which uses a SWCNT draped across CVD gold pads. The conductance of the CNT beam strongly depends on the chirality (armchair or zigzag) of the carbon atoms in the wrapped sheet. Investigations by Maki 26 and co-workers of photoluminescence (PL) spectrum measurements taken from individual SWCNT demonstrated the presence of an energy shift which is directly due to the band gap change caused by the elastic strain in the system. The band gap change is understood to be caused by the displacement of elastic strain of the SWCNT under stretching, and immediately before the ultimate failure stress is reached a distinct emission intensity reduction event can be seen. It is envisaged that these type of devices could be useful in the future when applied to the potential application of nano-sized tunable LEDs. Lee27 and co-workers also looked at SWCNTs under strain, but this time using in-situ Raman spectroscopy as reproduced in Figure 8 opposite. They also recognised changes in the intensity, specifically in the radial breathing mode frequency, due to increases in strain which in turn leads to a change in One somewhat counterintuitive finding was that increasing the length of the nanotube did not give rise to an increase in the axial bond length or a decrease in the resonant frequency. When strains grew above a level of ∼2% some permanent damage was seen to occur in the lattice structure of the CNT which was distinct from other effects in the band structure which were recoverable in materials very close in chirality to the “armchair” geometries of (11,10) and (10,9). It could be argued that previous work by Reich28 had predicted this somewhat, as it found that there are differences in chiral versus achiral tubes in terms of mixing of dominant optical modes, as it was postulated that high-energy Eigen modes can no longer be neatly categorised into purely circumferential or axial modes which are used to respectively describe the “armchair” and “zig-zag” nanotubes geometries, as illustrated in Figure 9 on the following page in Wildoer and co-workers seminal “Nature” article from 1998.
  • 7. Paul Ahern “Overview of Micro- and Nano-Transducers” 6 Figure 9 – Illustration of how a planar sheet of grapheme is folded into a nanotube by rolling the sheet along a wrapping vector, C. (Image – Wildoer29 and co-workers, Nature) VI. PIEZOELECTIRC NANO-TRANSDUCERS Piezoelectric films are a staple part of many different types of transducer device. One specific role they play is in the field of high frequency ultrasonic transducers for use in a wide variety of applications. In the medical field, high frequency ultrasound scanners are used routinely to give detailed viewing of anatomical structures such as the skin, eyes and circulatory system, with high spatial resolution traded off against penetration depth. Piezoceramic films such as PZT have shown promise as a possible ultrasound transducer which could be fabricated in a MEMS array to allow high frequency readout over a large focal area with low losses30. These layers can be fabricated by a variety of methods, from sintering to modified sol-gel reactions to aerosol deposition31, however the effective response of these very thin piezoelectric films can be markedly different to the properties it exhibits in the bulk. As can be seen in figure 10 opposite, subtle changes in the deposition process can have a large effect on the surface properties of the as-deposited film which in turn can limit the feasibility of later nano-scale patterning and lithography. Polymeric electrostrictives such as PVDF have also been evaluated32 as potential transducers for ultrasound scanning due to their low dielectric constant and the added benefits of high flexibility and low acoustic impedance which is very compatible with biological specimens – however its low coupling factor at present does not make it a suitably attractive choice as a transmitting material, unless modified as a copolymer with trifuoroethylene (TrFE) where it has found success for uncomplicated annular array transducers 33. Figure 10 – Electron micrographs from Zhu34 and coworkers showing the influence of sintering temperature and separation method from the titanium substrate. Since a variety of factors such as film density, crack volume, porosity and grain size critically determine the transducer’s operating frequency, processing conditions must be very carefully controlled to give a film suitable for the desired application. Zinc Oxide (ZnO) is another material with many thin-film transducer applications. Specifically within the realm of biomedical imaging, a novel approach suggested by Feng35 and co-workers (among others) suggests that acoustic focussing results can be achieved by fabricating dome-shaped diaphragms on a sapphire or silicon substrate, with the transducer’s effective focal region dictated by the diameter of the domes in the array. One successful method which has been described to create an array of these dome-shaped Parylene diaphragms is by a low temperature lost wax moulding processing route, where toluene is used to remove the wax balls from the domes at room temperature leaving an array of stress-free Parylene
  • 8. Paul Ahern “Overview of Micro- and Nano-Transducers” domes as shown in Figure 11 below. In the final step, the piezoelectric ZnO layer is then deposited on top of the supporting Parylene domes. 7 VII. NANOSCALE POLYMER-BASED ACTUATORS There are numerous benefits to creating artificial elements which can actuate at the nanoscale. One approach has been to utilise semiconducting polymeric materials to this end as they possess large strain fields when correctly stimulated, and also this allows the possibility of incorporating the actuating mechanism directly with the control system in one monolithic package. While these Electro Active Polymer (EAP) materials such as Trans-polyacetylene (PA), Poly(para-phenylene) (PPP) and poly(pyrrole) (PPy) are somewhat conductive in their natural state, success has been reported with doping methods which allows them to display metal-like characteristics, with the incorporating of the doping species38 into the polymer chain giving rise to a net change in volume which increases the actuating properties of the substrate. A summary of their principles of operation can be seen in Figure 13 below. Figure 11 – A novel fabrication method by Feng and coworkers for creating ZnO transducers on a silcon substrate which are dome-shaped to take advantage of acoustic focussing enhancement. (Image – Feng et al) Piezoelectric ultrasonic transducers are an essential part of the semiconductor manufacturing process; they are intrinsic to the thermosonic wire bonding process which is central to the packaging of silicon integrated circuits as they allow transmission of acoustic energy from an electrical input to the bonding interface, as shown diagrammatically in Figure 12 below. This has the effect of greatly improving the bond quality of the solder interface36 and thus is an enabling technology which allows faster, stronger and more repeatable bonding technology with higher packaging pitches. An experimental study by Wang37 and co-workers showed that moving to smaller sizes of transducer design had the advantage of allowing better frequency selection and minimising detrimental vibration modes which can occur close to the working frequency, due to a decrease in the vibration coupling between the radial and longitudinal directions. Figure 12 – Schematic diagram of a typical ultrasonic transducer used in microelectronic bonding (Image from Wang et al). Figure 13 – Illustration of the mechanism of operation of a flexible bilayer Electro Active Polymer (EAP) actuating element. Oxidation and reduction mechanisms are harnessed to cause motion as a result of expanding and contracting the opposing polymeric layers. These transitions are complemented by modification of the polymer chain, driving the introduction and termination of molecular entanglements. (Image: Ryhanen et al,2010. “Nanotechnologies for Future Mobile Devices”, Cambridge University Press) When compared to familiar piezoelectric materials which are typically used in this area, the polymer based solutions have the major advantage of requiring an order of magnitude less voltage with a much higher associated displacement distance. The downside is that current designs will only operate in the presence of a suitable electrolyte which provides the ionic contact between the EAP layers, normally a lithium salt, which currently limits the commercial applications of such devices. With that said, one material which has garnered significant interest is free-standing films of poly-pyrrole (PPy) which are fused with double sided adhesives to form a simple flexible
  • 9. Paul Ahern “Overview of Micro- and Nano-Transducers” bilayer actuator39 with platinum wires used as the connecting electrode. Providing that the poly-pyrrole film is uniform and smooth enough, the actuators response can be modulated solely using the magnitude of the applied charge, with no current influence. Heating of the Pt electrode lowers efficiency and response in the system, so a further refinement of a trilayer design was developed where two layers of EAP are employed and the Pt electrode is no longer needed, with the encouraging result that the ensuing actuator was able to move an object thousands of times its own mass whilst also giving a feedback response signal of the required power needed40. Further research which removed the need for a liquid electrolyte employed the use of a solid electrolytic layer such as poly(epichlorohydrin-co-ethylene oxide) and showed some promising results41. The angular velocity of the actuator in air was comparable to that achieved in the electrolytic medium, as was the electrical energy consumed by the motion with the same broadly linear relationship between movement speed and applied current noted. Taking matters into the nanotechnology realm, SWCNTs with their high Young’s modulus and high electric conductivity have been touted as ideal replacement actuator materials using the same vein of fabrication, lamination with adhesives to give arrays of nanofibres. Challenges due to separating the nanotubes were been surmounted by Fukushima and co-workers42 successful preparation of “Bucky Gels” such as those shown in Figure 14 below, using room temperature ionic liquids that contain dispersed high-purity HiPco SWCNTs and are built up using a rudimentary layer by layer addition casting process, which has no stringent pre-requisites save for a agate mortar and a hot plate which can be held at 80C. Figure 14 – Scanning Electron Microscope image of Fukushima & co-workers’ “Bucky Gel” polymeric nanoactuator. Layer (a) is the polymeric electrode composed of dispersed SWCNTs, and (b) the ionic-liquid electrolyte material. 8 The recorded displacement performance and operating lifetimes of these actuators are best in class in terms of their in air, low-voltage characteristics, and this is before they are further fine-tuned by material optimisation by tweaking the properties of both the underlying polymer network and the ionic liquid electrolyte layer. VIII. SMART BUILDING MATERIALS A further interesting use of carbon nanotube-based transducers is where cement-based sensors can be utilised for monitoring the dynamic strain in building structures. The data provided is useful as it contains all the dynamic character of the input and thus provides a useful sensor for the emerging market of "SHM" or “Structural Health Monitoring" applications. The use of nanomaterials as an additive to building materials has shown a lot of promise in the last two decades providing new materials which have strain sensitive properties and can allow smart sensing of changes in mechanical properties, even if typically this has been done only under static loads. The smart sensing effect is due to the change in the electrical signal generated by the increase in the concrete’s mass resistivity during crack initiation and growth, and an attendant decrease in the resistivity during subsequent crack closure43. Research by Materazzi44 and co-workers showed that the frequency response of cement with added CNTs (Figure 15 below) showed an approximately linear relationship as the load frequency increased, which suggested that this mechanism could be used to measure dynamic strain responses in buildings. Figure 15 – SEM images from research by Materazzi and co-workers of cement with added MWCNTs, demonstrating the efficacy of mixing nan tubes’ in water (a) and in cement paste(b) after curing.
  • 10. Paul Ahern “Overview of Micro- and Nano-Transducers” Early work by Fu45 and co-workers had showed that cyclic load and unloading causes damage to the cement matrix which means that the short CNTs used have a great probability of touching each another which decreases the overall resistivity of the composite material. Further investigation into this effect continues today, with the cause of this enhanced dispersion phenomenon now understood to be due to surface oxidation46, leading to the production of new functional groups on the surface of the nanotubes. The addition of carbon nanotubes serves to create ideal smart materials from the point of view of self-monitoring, but an issue is ensuring that there is adequate dispersion of the fibres in the aggregate mixture. This can be helped somewhat by ozone treatment of the fibres before they are introduced into the mixture, a phenomenon which has broader importance outside the sphere of smart construction materials and applies to all nano transducers. Najafi 47 and co-workers showed that exposing samples of MWCNTs to a UV – ozone treatment for an exposure time of 60 minutes increased their solubility in polar organic solvents by up to 320% compared to an untreated CNT. Recent work by Chang & Liu48 investigated and compared some suggested functionalisation mechanisms and concluded that this “ozone-mediated” process is an effective way to functionalize MWCNTs with a wide range of polymer layers, including traditionally non-reactive polymers which were considered to be not readily amalgamated with MWCNTs. Figure 16 below shows some high resolution TEM of functionalised MWCNTs, a method which has applications outside just the smart buildings research area. Figure 16 – Chang & Lu’s high resolution transmission electron microscopy (HR-TEM) analysis of ozone-mediated MWCNTs which have been functionalised with polymer layers. 9 IX. CONCLUSIONS & FUTURE DIRECTIONS Throughout this review paper it has been illustrated that the current nanotechnologic era affords us with some very exciting opportunities to improve traditional transducer designs and materials. Completely new transducer schemes and principles of operation and fabrication can be enabled by harnessing the unique attributes of nanoscale systems, effects and materials. This is especially true in the field of carbon nanotubes due to their unique convergence of desirable material properties. We can envision a future where new complex transducer arrays could be merged at the near atomic level with functionalised surfaces, metamaterials and advanced digital signal electronics to give rise to faster, most sensitive devices which we can utilise in ever more meaningful and adaptive ways. Indeed, it is around this very idea of boundless integration, energy efficiency and ubiquity that we can sense a way to use nanotechnology to drive new solutions in a variety of fields. However, further advances in synthesis, fabrication and signal processing will be needed in order to harness the potential benefits for low cost commercial nano-transducers, as well as research groups which straddle the different interdisciplinary areas where MEMS-based transducers can have such a large impact. X. ACKNOWLEDGMENT The author would like to thank Dr. Patrick McNally of the School of Electronic Engineering in Dublin City University for his proposal of this review topic, as well as his patience and gusto in explaining many of the important theoretical principles and concepts that underlie the field of micro- and nano-transducers.
  • 11. Paul Ahern “Overview of Micro- and Nano-Transducers” 10 XI. REFERENCES 20 1 S. D. Senturia, 2002. “Microsystem Design”, Kluwer. 2 Gerard Meijer (Editor), 2008. “Smart Sensor Systems” Wiley Interscience. 3 Smith, C.S., 1954. “Piezoresistance effect in germanium and silicon”. Physical Review, 94(1), P. 42. 4 Paul, W. & Pearson, G., 1955. “Pressure dependence of the resistivity of silicon”. Physical Review, 98, pp.1755–1757. 5 Pfann, W. & Thurston, R., 1961. “Semiconducting stress transducers utilizing the transverse and shear piezoresistance effects”. Journal of Applied Physics, 32(10), pp.2008–2019. 6 Tufte, O., Chapman, P. & Long, D., 1962. “Silicon diffusedelement piezoresistive diaphragms”. Journal of Applied Physics, 33(11), pp.3322–3327. 7 Vladimir V. Mitin, Viatcheslav A. Kochelap, Michael A. 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