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This edition contains two articles providing new insights in a relevant field for future nano-electronics applications, i.e. molecular computing at the atomic scale. Since 1974, Molecular electronics ...

This edition contains two articles providing new insights in a relevant field for future nano-electronics applications, i.e. molecular computing at the atomic scale. Since 1974, Molecular electronics had been always associated to the possible future of computers. A new European
Integrated Project AtMol was proposed and accepted by the European Commission after the FP7 ICT Call 6 to create this new technology (www.atmol.eu). The nanoICT “Mono-Molecular Electronics” Working Group was also set-up in 2008 to reach this objective.
In 2010, the nanoICT project launched its first call for exchange visits for PhD students with the following main objectives: 1. To perform joint work or to be trained in the leading European industrial and academic research institutions; 2. To enhance long-term collaborations within
the ERA; 3. To generate high-skilled personnel and to facilitate technology transfer;
The first outcome report (“Heat dissipation in nanometer-scale ridges”) of this call investigates experimentally the effect of lateral confinement of acoustic phonons in silicon ridges as a function of the temperature.
We would like to thank all the authors who contributed to this issue as well as the European Commission for the financial support (projects nanoICT No. 216165 and AtMol No. 270028).

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Enano newsletter issue22 Enano newsletter issue22 Document Transcript

  • No. 22 /// August 2011 www.phantomsnet.net Atomic Scale and Single Molecule Logic Gate Technologies (AtMol) Heat dissipation in nanometer-scale ridges The raise up of UHV atomic scale interconnection machines
  • dear readers,This edition contains two articles providing new insights in a relevant field for future nano-electronics applications, i.e. molecular computing at the atomic scale. Since 1974, Molecularelectronics had been always associated to the possible future of computers. A new EuropeanIntegrated Project AtMol was proposed and accepted by the European Commission afterthe FP7 ICT Call 6 to create this new technology (www.atmol.eu). The nanoICT “Mono-Molecular Electronics” Working Group was also set-up in 2008 to reach this objective.In 2010, the nanoICT project launched its first call for exchange visits for PhD students withthe following main objectives: 1. To perform joint work or to be trained in the leading Europeanindustrial and academic research institutions; 2. To enhance long-term collaborations withinthe ERA; 3. To generate high-skilled personnel and to facilitate technology transfer;The first outcome report (“Heat dissipation in nanometer-scale ridges”) of this callinvestigates experimentally the effect of lateral confinement of acoustic phonons in siliconridges as a function of the temperature.We would like to thank all the authors who contributed to this issue as well as the EuropeanCommission for the financial support (projects nanoICT No. 216165 and AtMol No. 270028). > Dr. Antonio Correia Editor - Phantoms Foundationcontents05 > nanoresearch. Atomic Scale and Single Molecule Logic Gate Technologies(AtMol) /// C. Joachim12 > nanoresearch. Heat dissipation in nanometer-scale ridges /// P.-O. Chapuis,A. Shchepetov, M. Prunnila, S. Laakso, J. Ahopelto and C. M. Sotomayor Torres18 > nanojobs21 > nanoICT Conf Report23 > nanoresearch. The raise up of UHV atomic scale interconnection machines /// J. S.Prauzner-Bechcicki, D. Martrou, C. Troadec, S. Gauthier, M. Szymonski and C. Joachimeditorial informationNo 22. August 2011. Published by Phantoms Foundation (Spain)editor > Dr. Antonio Correia > antonio@phantomsnet.netassistant editors > José Luis Roldán, Maite Fernández, Conchi Narros,Carmen Chacón and Viviana Estêvão.1500 copies of this issue have been printed. Full color newsletteravailable at: www.phantomsnet.net/Foundation/newsletter.phpFor any question please contact the editor at: antonio@phantomsnet.neteditorial board > Adriana Gil (Nanotec S.l., Spain), Christian Joachim (CEMES-CNRS,France), Ron Reifengerger (Purdue University, USA), Stephan Roche (ICN-CIN2,Spain), Juan José Saenz (UAM, Spain), Pedro A. Serena (ICMM-CSIC, Spain), DidierTonneau (CNRS-CINaM Université de la Méditerranée, France) and Rainer Waser(Research Center Julich, Germany).deadline for manuscript submission depósito legal printingIssue No 24: October 31, 2011. legal deposit GráficasIssue No 25: December 30, 2011. BI-2194/2011 Valdés, S.L. 03
  • nanoresearch Atomic Scale and Single Molecule Logic Gate Technologies (AtMol) FET ICT Integrated Project (2011-2014)C. Joachim CEMES-CNRS, France. on very specific fabrication and measurementAtMol Coordinator know-hows like the break junction or LT-UHV STM techniques.According to the ITRS roadmap, the transistor The new European Integrated Projecti.e. the basic switching element of any AtMol was proposed and accepted by theprocessor Arithmetic and Logic Unit (ALU) will European Commission after the FP7 ICTreach its bottom source – drain distance limit Call 6 to create this new technology for(between 5 nm to 10 nm) in the years 2020’s. molecular computing at the atomic scale.This will be one limit more after the switching The members of the AtMol consortiumenergy problem which started to stop the started from the observation that for singleincrease of the computer performance in molecule electronics to diffuse towardsyears 2005-2006. Our global need for more applications and at the same time be usedand more computing power in portable and as epistemological devices to explore the limitsustainable forms is now pushing the semi- of calculating machines, it is required to startconductor industry to explore new avenues for from the best of surface science. We need toproducing machines to transmit and process create the ultra clean technology required toinformation. On the other side of the road, this construct atomic scale calculating circuits, toextreme miniaturisation of the ALU and its interconnect them to the external world andassociated memory bring again on the table at the same time to invent a proper packagingthe problem of the limits of the machine in term technology to protect the constructed atomicof size and power consumption whatever the scale circuit when ready to be extracted frommachine: a mechanical machine, a calculator, its native UHV environment.an emitter… The technical and fundamentallimits of machines from the gears to the The AtMol molecular chip conceptheat engine, from the relay to the transistor For AtMol, a molecular chip is a fullyhave always been a fantastic playground for packaged and interconnected planarphysicist and chemists to make progresses in atomic scale complex logic circuit where theour understanding of the laws of physics. ALU is constructed with a set of complexSince 1974, Molecular electronics had been molecule logic gates which may be onealways associated to the possible future day embedded in a single molecule. Theseof computers. It is now one of the options are interconnected by surface atomic oramong others like quantum computers on the molecular wires constructed with atomicway to bring the next computing technology precision on a substrate with a large enoughafter our fantastic transistor era. Step by step, surface electronic band gap. The centralMolecular electronics evolves and gives rise and entirely new concept to be exploreto different branches like organic electronics, within AtMol is the separation in space ofsingle molecular devices and molecular the atomic scale structures of the ALU fromlogics. Organic electronics had developed the nano/mesoscopic scale interconnects.its own dedicated technologies like printed This new interconnect concept is motivatedelectronics. This had not been the case yet for by the fact that, whatever the architecture ofthe others branches which remain dependent the planar atomic scale complex logic circuit 05
  • nanoresearch to be prepared on the surface, there is an incompatibility to deal at the same time and on the same surface with all the interconnection scales from the atomic to the mesoscopic scales (and beyond). All the known nano-scale fabrication techniques, including e-beam nano- lithography, nano-stencil, nano-imprint, and Focused Ion Beam (FIB), are not atomically clean techniques. For example, in e-beam Fig. 1 > An LT UHV STM image (5.8 nm x 6.7 nm) lithography a resist is used which is very difficult of a Ge(100)H surface carefully prepared by the to remove entirely after the nanofabrication Krakow AtMol partner. Large terraces of Ge(100)H step and thus precludes the use of atomically in Krakow, Si(100)H in Dresden, Nottingham and clean surfaces and the associated state-of- Singapore, MoS2 in Singapore and AlN in Toulouse the-art surface science characterisation and AtMol partners are now prepared to become the atomic-scale manipulation tools. Similarly, supporting surface of the AtMol atomic scale atoms can diffuse laterally and in a random interconnects./ manner in the nano-stencil technique and when engraving using a FIB. On the other hand, while LT-UHV STM or UHV NC-AFM microscopes are capable of manipulating single atoms, these instruments are not capable of constructing interconnects from the atomic scale (0.1 nm) to the mesoscopic scale (100 nm). A spatial separation of the interconnects between the two faces of a same wafer is the solution proposed by the Fig. 2 > The detail configuration of the targeted AtMol consortium. It has the great advantage AtMol molecular chip structure with its back that the top surface of the wafer is reserved for interconnects, its vias through the surface stopped the planar atomic scale circuit constructions. just before disturbing it and its top packaging chip. In this drawing, the top packaging chip is laterally The AtMol process flow cross cut to help in locating the atomic scale circuit supported by an Si(100)H surface in this case. The AtMol is proposing a comprehensive process insert is presenting this circuit with its 16 Au nano- flow, spanning the atomic to mesoscopic islands interconnection pads and the location of the scale for processing and fabricating a active circuit. The question mark is an indication that molecular chip. the exact optimal architecture of this circuit is not yet determined in AtMol (see the main text for the 1) The Atomic scale logic gates and atomic different possible choices). The surface size of the scale circuits are going to be constructed atomic chip is 54 x 54 surface SiH dimers. This is an on the front side of the wafer whose atomic indication of the simulation target of the Toulouse, scale surface is going to be prepared Singapore and Barcelona AtMol partners to succeed with care (Fig. 1). Then, the nanoscale to in associating semi-empirical (N-ESQC) and DFT mesoscopic (and, indeed, macroscopic) (TranSiesta) surface transport calculations to predict interconnects will be fabricated on the back the best surface atomic scale surface architecture./ side of the same wafer (See Fig. 2). From atomic scale structures of the top surface wafer from the back to the front side. Of to mesoscopic connections of the back, course, the piercing is going to be stopped solid and rigid nano-vias will be fabricated just before the perturbation of the top surface06 by piercing, with nanoscale precision, the atomic order.
  • 07
  • nanoresearch system under a scanning electron microscope (or an optical for insulating surface) navigation system. Uniquely, AtMol will use and develop further the only N-probe UHV interconnection machines which are currently existing in the world i.e. in Singapore, Krakow and Toulouse (Fig. 4). These machines are the ultimate UHV compatible multi-probe testers reaching the nanoscale precision. Before encapsulating the front side under UHV conditions, electrical characterisation will be carried out in parallel with the electrical testing of the back Fig. 3 > A detail atomic scale representation of interconnects. a top surface contact LT-UHV experiment using multiple STM tips, each one contacting one Au nano-pads. The molecule logic gate represented is here a starphene molecule interconnected in a classical way to 3 quite large surface dangling bond atomic wires, each reaching an Au nano- pad. The 3 AtMol UHV interconnection machines able to perform such multi-probes experiments are discussed in Fig. 4. The insert is presenting the LT-UHV STM image of the starphene electronic ground state obtained by the Singapore and Toulouse AtMol LT-UHV STM. This starphene molecule was synthesized by the Tarragona AtMol partner. Long molecular wires, new molecule logic gates and latching molecules are going to be synthesized by the Berlin, Tarragona and Toulouse AtMol Chemists./ 2) The back-side mesoscopic interconnection circuitry and the nano-via through the wafer indicated in Fig. 2 are prepared and UHV cleaned for example before the atomic scale construction step. In this case, the front side is going to be encapsulated using a wafer bonding technique without modifying the back mesoscopic face of the wafer. Fig. 4 > The 3 AtMol UHV atomic scale interconnection machines in construction. The first one is now being 3) The top surface planar atomic scale logic tested in Singapore, the second one in Krakow circuits will be tested using N probes (see and the third one in Toulouse. Each one has is the testing principle presented in Fig. 3) own specific characteristics. The Singapore one is located within a UHV-compatible atomic fully LT and equipped with a UHV transfer printer. scale interconnection machine (Fig. 4). The Krakow one is equipped by a hemispherical These interconnection instruments are electron energy analyser (Auger microscope). Both integrated within one large UHV system which are equipped by the required high resolution (4 nm) incorporates a surface preparation chamber, UHV-SEM. The Toulouse one is more dedicated a UHV-transfer printing device, an FIM atomic towards large electronic gap surfaces which scale tip preparation device, an LT-UHV-STM explained this peculiar multi-probes contacting08 (or NC-AFM) microscope, and a N-nanoprobe approach based on metallic cantilevers./
  • nanoresearch4) New atomic scale construction and Exploring the different molecule logicfabrication techniques will be fully developed architectureswithout fear of seeing the atomically precise At the atomic scale, the main advantage ofcircuits being destroyed by any subsequent the AtMol atomic scale chip technology is thatmesoscopic scale interconnection fabrication it offer a definitive working bench to determinestep. The atomic scale fabrication techniques the optimal atomic scale architecture forare under development before being constructing a complex logic gate able, forincorporated into back interconnected example, to add two binary numbers. The fantastic advantage of the front-back sidewafers. For example, AtMol will exploit this interconnection AtMol innovation describedcapability to develop a unique UHV atomic above is that any circuit architecture can bescale transfer printing technique able to constructed and tested in full planar and UHVintegrate nano-scale contacting metallic technology with an atomic scale technologypads, long molecular wires, active molecule and with the possibility to determine thelogic gates, and latch molecules on the front exact atomic scale structure of the ALU being(atomically clean) surface (Fig. 5). This will be constructed. It can be expanded to singlesupported by the objective of improving the molecule mechanics and transmission ofconstruction of long dangling bond atomic mechanical motion.wires on a semiconductor surfaces using local For “single molecule” molecular logic, theforces instead of inelastic electronic effects of standard solution coming from the 70’s isthe STM. an hybrid molecular electronics architecture where each molecule in the circuit acts as a switch (or, better, as a transistor). Of course, there are significant problems with this conventional scheme. In particular, it requires a command (for example an electric field or a mechanical push) to be applied on each molecule-switch in the circuit. As a consequence, the distance between each molecule-switch has to be larger than the electron mean free path of the interconnection materials for the electric field to be well defined on each molecule of the circuit. This also requires bringing the command electrodes on many points of the atomic scale circuit.Fig. 5 > A cartoon indicating that the Berlin and For AtMol, this is not the way to go. But thisSingapore AtMol partners are exploring UHV type of design can be well tested on the AtMoltransfer printing to avoid any nano-lithography wafer top.steps. Initially developed in Singapore to transfermetallic nano-pads on the Si(100)H surface, the The first AtMol objective is to test semi-UHV transfer printing technique is now generalized classical electronic circuit laws at the atomicin AtMol to molecular wires and molecule logic scale. Distinct from the well-known Kirchhoffgates. This cartoon is also presented here to circuit laws, these laws were demonstratedillustrate how AtMol via its Madrid partner is pro- theoretically at the end of the 90s butactive in diffusing information to a wide public remain to be experimentally tested. Be it byabout AtMol and about the development of Atom synthesising specific long molecules havingTech in general. This cartoon belongs to a series the shape of an electronic circuit or byof 30 drawn by the famous cartoon drawer G. constructing atom by atom such a circuit atCousseau invited for the AtMol Kick-off meeting./ the surface of a passivated semiconductor, 09
  • nanoresearch atomic and molecular manipulation and very precise dI/dV spectroscopy using scanning probes (LT-UHV-STM/AFM) are going to provide the definitive experimental testing of those laws. Having verified atomic scale electronic circuit laws, AtMol designer and chemists can generate complete designs of fully integrated ALU atomic scale circuits. More complex semi-classical circuits will be designed theoretically and tested up to the point where the maximum reachable complexity will be attained due, for example, to the output current intensity falling below a minimum detectable threshold. Fig. 6 > The detail atomic scale representation of a LT-UHV STM experiment performed by the The second AtMol objective is to determine Singapore AtMol Partner to demonstrate how the how the very new Quantum Hamiltonian Boolean truth table of a QHC molecule NOR logic Computing (QHC) concept introduced in gate can be measured. Each Au individual atom the European Pico-Inside project can reach is STM manipulated to interact (or not) with one a larger Boolean logic function complexity branch of the conjugated starphene molecule. as compared with semi-classical atomic The positioning of an Au atom nearby the scale circuits. In QHC, logic operations are molecular branch is a classical input converted in quantum information by the molecule itself. The carried out not via charge processing but starphene ground state position is determined through quantum information manipulation by performing an STS spectrum on the output inside the atomic scale circuit or inside the branch of the molecule. The Berlin AtMol partner molecule. In QHC, the electron transfer will perform single molecule pulling experiments rate between a drive electrode and an to determine the conductance of a single output electrode is controlled by locally molecular wire (or intramolecular circuit). The changing the Hamiltonian of the molecule Singapore, Barcelona and Toulouse molecule (or of the surface atom circuit). In QHC, the (or surface atomic circuit) AtMol designers are decoherence phenomenon is used to build now in full interactions with the Berlin, Tarragona and Toulouse AtMol chemists to create new up a measurable output current averaging molecule logic gates and with the Dresden, all the quantum fluctuations. One major Nottingham, Krakow and Singapore AtMol architectural innovation is that the inputs are surface scientists to create the first surface atomic basically classical but locally converted in scale fully interconnected logic gate circuits./ quantum information (Fig. 6). This conversion is performed by the energy and phase surface supporting the logic gates. A changes of some of the electronic states of detail theory of the surface atomic scale the molecule (or of the surface atomic scale structure, its relaxation while contructing circuit). Two strategies will be considered: surface atomic wire or adsorbing and either the molecule(s) can do everything or interconnecting molecule logic gate and a surface atomic scale circuit completed molecular wire is going to be developed by molecule latches to handle the inputs together with the calculation of the running is preferable to reach large output current current (and its associated electronic (perhaps up to the microampere range). effects) through the atomic scale surface This design effort will be supported by an circuit. New quantum design rules may intense theory of surface science efforts to emerged from those developments which take into account for the first time the full will be immediate feed backed to the10 electronic contribution of the underneath AtMol chemists.
  • nanoresearchAtMol in 2014 January 2012 and the next one on “UHVThe AtMol consortium members are very surface chemistry and UHV transfer printing”committed by the fact that at the same in Berlin 6 months after. Output from thesetime chemistry, surface science, single workshops (lecture notes, etc.) concerningatom manipulation, new UHV multi-channel atomic scale technology will be regroupedinterconnection machines, new UHV printing in a new series of books entitled “Advances in Atom and Single Molecule Machine”and packaging techniques need to be published by Springer.developed to construct the first ever singlemolecule chip. After 2014, AtMol is expecting The AtMol consortium (www.atmol.eu)that its “concept Chip” will be the pivot of thedevelopment of real molecular chips and at CEMES-CNRS (Toulouse)the same time will point forward the material LETI-CEA (Grenoble)and technological down limit of a calculator. The Phantoms Foundation (Madrid)To accompany its efforts and to associate ICIQ Institute (Tarragona)more academic and industrial groups to the CIN2 Institute (Barcelona)creation of its new atomic scale technology,AtMol is organizing each 6 months a F. Haber Institute (Berlin)workshop on topics of high concerns. Humbolt University (Berlin)The 1st AtMol workshop on “Atomic scale TU Dresden (Dresden)interconnection machines” ran in Singapore Nottingham University (Nottingham)from the 28th to 29th of June 2011. The 2ndworkshop on “Molecular logic architecture at Jagiellonian University (Krakow)the atomic scale” will be running in Barcelona, IMRE A*STAR (Singapore) Impact Factor 7.333 2010 Journal Citation Reports® (Thomson Reuters, 2011) provides the very best forum for experimental For subscription details please and theoretical studies contact Wiley Customer Service: of fundamental and >> cs-journals@wiley.com applied interdisciplinary (Americas, Europe, Middle East and Africa, Asia Pacific) research at the micro- >> service@wiley-vch.de and nanoscales (Germany/Austria/Switzerland) 2011. Volume 7, 24 issues. >> cs-japan@wiley.com (Japan) Print ISSN 1613-6810 / Online ISSN 1613-6829 For more information please visit www.small-journal.com or contact us at small@wiley-vch.de 11
  • nanoresearch Heat dissipation in nanometer-scale ridges Pierre-Olivier Chapuis(a)*, Andrey to electric and heat conduction, namely Shchepetov(b), Mika Prunnila(b), Sampo particles free path and even wavelength in Laakso(b), Jouni Ahopelto(b) and Clivia M. some cases. The case of the heat carriers Sotomayor Torres(a)(c)(d)* is particularly of interest because it involves (a) Institut Catala de Nanotecnologia (ICN), Centre phonons that have free paths estimated d’Investigacio en Nanociencia e Nanotecnologia to be larger than the device characteristic (CIN2-CSIC), Edificio CM3, Campus de la length in some cases, undergoing fewer Universitat Autonoma de Barcelona, collisions as a consequence. The so-called 08193 Bellaterra (Barcelona), Spain. (b) VTT Technical Research Center of Finland, phonon rarefaction effect is then responsible PO Box 1000, 02044 VTT, Espoo, Finland. for higher temperature in the heat source (c) Department of Physics, Universitat Autonoma de than what can be estimated with the usual Barcelona, 08193 Bellaterra (Barcelona), Spain. Fourier-based heat conduction equation (d) Institució Catalana de Recerca i Estudis Avançats (ICREA), Passeig Lluís Companys, 23 [2]. Larger temperature gradients are then 08010 Barcelona, Spain. responsible for cracks as differential thermal expansion at material interfaces is not manageable. Abstract > Heat management in today’s A key parameter in the improvement of the electronic devices is critical to prevent device design is therefore the understanding possible failures due particularly to cracks. of heat conduction in the devices. Here Heat is carried in such devices by electrons we focus on one way to measure the in electrical conductors and mainly by implications of the particle behaviour (mean acoustic phonons in electrical insulators. free path) of acoustic phonons when they We explain the first steps of our work are confined in tiny structures. We propose a aiming to investigate experimentally the method to analyse the heat flux propagation effect of lateral confinement of acoustic in these structures and show our first efforts phonons in silicon ridges as a function of towards the efficient use of the designed the temperature. Inspired by the electrical samples. 3ω method, we design a setup that can be used as a mean to generate phonons in 2. Heat flux measurements in low- ~100 nm wide ridge nanostructures and dimensional samples as a thermometer that allows tracking the generated heat flux. A. The macroscopic 3ω method The 3ω method has been developed since 1. Introduction the 1980s, in particular by Cahill [3], with the Works performed over the last decades goal of studying the thermal conductivity have shown that electronic devices with of planar materials. Thin films have been nanometre-scale dimensions are subject investigated widely as well as the thermal to larger temperature-driven stresses in boundary resistance between the films [4]. comparison to what had been estimated in It was extended for multilayer materials or the past [1]. In particular, the size of different particle-based materials [5, 6]. The method components of transistors and electronics is based on the Joule heating of a metallic devices present in printed circuits are now wire of micrometric size deposited on top of12 comparable to the key scales associated the substrate that “steals” part of the heat
  • nanoresearchflux generated. As a consequence, the substrate thermal conductivity has to betemperature of the wire gives an indication found. A 2D cross-section based modelon the ability of the sample to conduct heat. has been used extensively over the pastThe use of a harmonic current to heat the 20 years. It is based on frequency sweeps.wire allows the excitation of the temperature The slope of the temperature variation giveshigher-order harmonics. If the heating is not the thermal conductivity [3]. Some authorstoo high, only the third harmonic is excited have underlined that better models can beas will be seen in the following. One can used [7, 8]. Usually, the width of the wire iswrite for a current I = I0 cos(ωt) the generated in the micrometric range, which is reachedpower due to the Joule heating as with standard optical lithography in the fabrication process. P(t) = R I(t)2 = ½ R (1 + cos(2ωt))and the total temperature reads including B. Implementation at the nanoscalethe heating TDC by DC current : The case of nanoelectronic devices is very different to the micrometre-scale one. Even T(t) = T0 + TDC + T2ω cos(2ωt+ 2ω). if the heating/sensing system that can beThe key point now is the dependence of used has the same principle, the sizes arethe wire resistivity to temperature, that is much smaller. We fabricated nanostructureslinear in first approximation for low heating: where the heater/sensor lies on top of siliconR(T) = R0 (1+α ∆T). Finally, the voltage of the substrates as represented in Fig. 1. The topwire can be written as of a ridge is a wire, either a metal or doped silicon, which acts as a heater and as aU = RI = R0I0 [1+α(TDC+T2ωcos(2ωt+ 2ω))] cos(ωt) thermometer at the same time. The doped- = R0I0 [(1+αTDC) cos(ωt)+½αT2ωcos(ωt- 2ω) layer structure requires epitaxial growth of +½αT2ωcos(3ωt+ doped silicon. 2ω)]The use of a lock-in amplifier at the thirdharmonic enables to measure the amplitudeand the phase of the third harmonic and thusextract the local temperature. The amplitudeis U3ω = ½ αR0I0 T2ω. The frequency rangeto be used here is generally between 10 and5000 Hz. Care has to be taken with the wirewidth and thickness that should be smaller Fig. 1 > Two types of resistive heater for the ridgethan the thermal diffusion wavelength in order experiments./to prevent from a possible nonhomogeneity The substrates can be made of high-of the heat generation in the wire. resistivity silicon. The submicrometer ridgesThis experimental part of the work permits are fabricated with electron beam lithographyonly to get a qualitative idea of the material and ICP dry etching (see Fig. 2, page 14).thermal properties or to make an estimate This type of structure enables to generatebased on comparisons with reference phonons in the ridge and to measure the heatmaterials the thermal conductivity of which is flux flowing to the substrate. An adaptationknown. This is not an easy task as a heating of the 3ω method is then used to heat thedevice has to be deposited on top of each wire and measure the wire temperature. Assample. it has been previously explained, a harmonicIf one wants to find the thermal conductivity electrical current generates the heat at 2ωdirectly from the sample, a physical model due to Joule effect and lock-in detectionlinking the measured temperature and the allows measuring the in-phase 3ω voltage 13
  • nanoresearch (a) (b) in certain cases. The standard wire method deals in general with a few Ohms. Lithography mask 3. Phonons in silicon and some size effects A. Silicon properties Silicon is a semiconductor where electrons (c) (d) are the charge carriers but most of the heat is carried through phonons. The thermal conductivity of pure silicon is at room temperature, which is high in comparison to amorphous materials such as SiO2 with thermal conductivities two orders of magnitude below. Note that gold, one of the best metallic heat conductors, is only conducting heat two times better than silicon. Despite this high thermal conductivity, it can be shown with the Wiedemann-Franz law Fig. 2 > Examples of the fabricated structures: (a) Overall view of one layout (b) Zoom on a ridge that the electronic contribution to thermal with a metal wire before mask removal (c) Zoom conductivity is negligible even at moderate on a ridge with a doped silicon layer showing a doping levels. The phononic thermal nonrectangular shape after reactive ion etching conductivity of a crystal can be written (d) Connection between the measured wire and electrical access./ component proportional to the wire 2ω temperature. Note that one needs generally where the integration spans over the to filter the spurious signal generated by frequency ω and the discrete sum over the the source at 3ω. A 4 points measurement three different polarizations. is the reduced is better usually, but 2 points can be also Planck constant; f is the Bose-Einstein used in some cases. The difference with distribution; T the temperature; g is the the macroscopic method is that a different phononic density of states; vg the phonon physical model has to be used to link the mode group velocity; τ is the phonon wire temperature to the heat flux transmitted relaxation time and (vg τ) is the phonon to the substrate. mode mean free path. The high thermal The first test is to measure the electrical conductivity is therefore due to either high resistance of the device as a function of velocities, large density of states or large the temperature, as varying this parameter phonons mean free path. allows determining the value of the temperature coefficient α needed for the B. Acoustic phonons, mean free paths measurements. It can be found that α In general, mean free paths are parameters is positive or negative depending on the that are not very-well known as they temperature and the type of heater/sensor. (1) depend strongly on the frequency The major experimental difference with the whereas they are generally calculated macroscopic method is the value of the as an average and (2) are very difficult to14 resistances that can be as high as 20 kΩ measure at room temperature. Some early
  • nanoresearchmeasurements were reported in the 1960s reader to the mentioned references fromat lower temperature [9], and more recent the group of Goodson for the study of suchexperiments using electrical methods [10] phenomena.and time-domain thermoreflectance haveshown that part of the phonon mean free 4. Heat conduction in electric tracks andpath distribution should lie at lengths above ridges500 nm. An alternative way to get insights In electronic devices with deposited metalin the issue of mean free path is the use of lines or doped silicon tracks, the electronicmolecular dynamics simulation. Henry and path lies on top of planar substrates.Chen [11] recently calculated a distribution Considering a phonon mean free path onof the mean free paths for silicon, finding the order of 100 nm, we present in Fig. 3indeed that around 30% of the thermal three types of possible devices that consistconductivity was due to mean free paths of a ridge on a planar substrate of the samelarger than 1 micrometer. This is consistent or different material. For simplification, wewith the estimation [10] that the mean free start with only similar materials. The leftpath should be around 300 nm for silicon. device can be treated with the usual FourierHere one should keep in mind that the heat conduction, the middle one is differentwidespread evaluation of the mean free as even if the nanostructure on top is largepath vgτ from is delicate and is in a thermal equilibrium the thermalin the sense that it counts all the optical constriction resistance to the cold bath hasmodes in the specific heat cp, whereas they to be described by a subcontinuum heatare not expected to play a key role in the conduction. The right device is even furtherthermal conductivity due to the flatness of complicated as the size of the structure doestheir dispersion relation (vg≈0). Here ρ is the not allow an equilibrium inside due to its smallmaterial density as usual. This approximation size and the fact that phonons are not trappedunderestimates the effect of the phonon in the cavity but can also escape. The centrerarefaction in small devices. Recent works figure is typical of the rarefaction effect [19],performed with nanowires [12, 13, 14] and when the phonon statistics impinging thewith embedded nanoparticles [15, 16], constriction is different than the equilibriumtargeting thermal conductivity reduction one. The right one has been tackled inin thermoelectric materials, have also a theoretical paper [20]. In principle, thehighlighted the effect of roughness [14, Boltzmann transport equation has to be used15] in addition to the phonon-particle for calculating the heat flux in structures suchconfinement effect. Here we do not discuss as the centre and right ones but approximatethe suspended wire issue as it is for the methods have been developed such as themoment less relevant in nanoelectronics. ballistic-diffusive equation [21, 22].We need also to underline the role ofthe interaction of electrons and optical (a) (b) (c)phonons with the acoustic ones [17, 18].Even if optical phonons do not carry heatsignificantly they interact with the acousticones, therefore impacting the thermalconductivity through the scattering mean Fig. 3 > Three types of electrical conductors on afree path. Note also that electron scattering planar substrate. The substrate can be either anwith optical phonons is significant, and the electric conductor or an insulator. The Fourierheat redistribution to acoustic phonons description of heat conduction is not adequatetakes place through optical/acoustic for the two last devices (b,c) if the phonon meanphonons scattering interaction. We refer the free path is of the order of 100 nm or more./ 15
  • nanoresearch Transient Heat Conduction Problems using Ballistic-Diffusive Equations and Phonon Boltzmann Equation”, Journal of Heat Transfer, Vol. 127, pp.298-306 (2005). [3] D. Cahill, “Thermal conductivity measurements from 30 to 750 K: The 3ω method”, Review of Scientific Instruments, Vol. 61, p802 (1990). [4] S.-M. Lee and David G. Cahill, “Heat transport in thin dielectric films,” Journal of Applied Physics, Vol. 81, 2590 (1997). [5] S.-M. Lee, David G. Cahill, and R. Venkatasubramanian, “Thermal conductivity of Si-Ge superlattices,” Applied Phyics. Letters, Fig. 4 > Different regimes of heat conduction as a Vol. 70, 2957 (1997). function of the shape of the body in contact with [6] D.-A. Borca-Tasciuc and G. Chen, “Thermal the substrate. Adapted from Ref [20]./ Properties of Nanochanneled Alumina Templates,” Journal of Applied Physics, Vol. The purpose of our work is to observe 79, pp. 084303-1-9 (2005). these kinds of subcontinuum effects [7] T. Borca-Tasciuc, R. Kumar, and G. Chen, experimentally. We have already measured “Data Reduction in 3ω Method for Thin Film [23] the expected strong reduction of the Thermal Conductivity Measurements,” Review thermal conductance in the ballistic regime of Scientific Instruments, Vol. 72, o. 4, pp. with respect to Fourier’s prediction. Our 2139-2147 (2001). first results indicate in addition a different [8] T. Tong and A. Majumdar, “Reexamining behavior than what can be calculated in the the 3-omega technique for thin film thermal purely ballistic case, which is exactly what characterization”, Review of Scientific is pointed out in the analysis developed in Instruments, Vol. 77, 104902-104902-9 (2006). Figs. 3 and 4. [9] R. Gereth and K. Hubner, “Phonon MeAn Free Path in Silicon Between 77 and 250°K”, Acknowledgements Physical Review, Vol. 134, pp A235–A240 (1964). We thank M. Tilli for providing high ohmic [10] M. Asheghi, Y.K. Leung, S.S. Wong, and K.E. 8” Si wafers. M. Myronov and V. Shah are Goodson., “Phonon-Boundary Scattering in acknowledged for doing the n+ Si epitaxial Thin Silicon Layers,” Applied Physics Letters, growth. Vol. 71, pp. 1798-1800 (1007). We acknowledge the partial support [11] A. Henry and G. Chen, “Spectral Phonon of the EU projects NANOPACK and Properties of Silicon Based Molecular Dynamics NANOPOWER. P.O.C. acknowledges the and Lattice Dynamics Simulations,” Journal of support of EU project nanoICT for the partial Computational and Theoretical Nanosciences, funding of a stay at VTT. Vol. 5, pp. 141-152 (2008). [12] D. Li, Y. Wu, P. Kim, L. Shi, P. Yang, A. References Majumdar, “Thermal conductivity of individual [1] D.G. Cahill, W. K. Ford, K.E. Goodson, G.D. silicon nanowires,” Applied Physics Letters, Vol. Mahan, A. Majumdar, H.J. Maris, R. Merlin, and 83, pp. 2934-2936 (2003). S.R. Phillpot, “Nanoscale thermal transport,” [13] A. I. Hochbaum, R. K. Chen, R. D. Delgado, Applied Physics Reviews, Journal of Applied W. J. Liang, E. C. Garnett, M. Najarian, A. Physics, Vol. 93, p793 (2003). Majumdar, P.D. Yang. “Enhanced thermoelectric [2] R.G. Yang, G. Chen, M. Laroche, and Y. Taur, performance of rough silicon nanowires,”16 “Simulation of Nanoscale Multidimensional Nature, Vol. 451, pp. 163-167 (2008).
  • nanoresearch[14] I. Akram Boukai, Y. Bunimovich, J. Tahir-Kheli, Phonon Dispersion,” Journal of Applied J.-K. Yu, W.A. Goddard III, and J.R. Heath, Physics, Vol. 96, no. 9, pp. 4998-5005 (2004). “Silicon nanowires as efficient thermoelectric [19] G. Chen, G. Chen, “Nonlocal and materials”, Nature, Vol. 451, pp168-171 (2008). Nonequilibrium Heat Conduction in the Vicinity[15] W. Kim, J. Zide, A. Gossard, D. Klenov, S. of Nanoparticles,” ASME Journal of Heat Stemmer, A. Shakouri, A. Majumdar, “Thermal Transfer, Vol. 118, pp. 539-545 (1006). conductivity reduction and thermoelectric figure [20] S. Volz and P.-O. Chapuis, “Increase of thermal of merit increase by embedding nanoparticles resistance between a nanostructure and a surface due to phonon multireflections”, Journal in crystalline semiconductors,” Physical Review of Applied Physics, Vol. 103(3), p034306 (2008). Letters, Vol. 96, p045901 (2006). [21] G. Chen, “Ballistic-Diffusive Heat Conduction[16] W. Kim, S. L. Singer, A. Majumdar, J. M. O. Equations,” Physical Review Letters, Vol. 85, Zide, D. Klenov, A. C. Gossard, S. Stemmer, pp. 2297-2300 (2001). “Reducing thermal conductivity of crystalline [22] G. Chen, “Ballistic-Diffusive Equations for solids at high temperature using embedded Transient Heat Conduction from Nano to nanostructures,” Nano Letters, Vol. 8, pp. Macroscales”, Journal of Heat Transfer, Vol. 2097-2099 (2008). 124, pp. 320-328 (2002).[17] S. Sinha, E. Pop, R.W. Dutton, K.E. Goodson, [23] P.O. Chapuis, M. Prunnila, A. Shchepetov, L. “Non-Equilibrium Phonon Distributions in Sub- Schneider, S. Laakso, J. Ahopelto, and C.M. 100 nm Silicon Transistors,” ASME Journal of Sotomayor, “Effect of phonon confinement Heat Transfer, Vol. 128, pp. 638-647 (2006). on heat dissipation in ridges”, Proceedings of[18] E. Pop, B. Dutton, and K.E. Goodson, “Analytic the 16th International Worshop on THERMal Band Monte Carlo Model for Electron Transport INvestigations of ICs and Systems (THERMINIC), Modeling in Si Including Acoustic and Optical B. Courtois and M. Rencz (ed), (2010). 17
  • nanojobs • PostDoctoral Position (CEA-Léti, We are looking for talented chemists France): ”Theory and modeling of NEMS motivated to pursue a PhD in the area of based digital functions” Materials Science. The students will be CMOS technology scaling has enabled able to join to a pioneer, dynamic and significantly reduced energy per operation active group from the Department of in integrated circuits. However, these NanoScience and Organic Materials (www. improvements are not in line with the icmab.es/nmmo). This research group expected performances of future approaches some of the most exciting and challenging fields that a chemist and autonomous systems. Autonomous a materials scientist can explore nowadays systems that use energy harvesting are — the study of advanced organic functional attractive for many applications (medical materials and nanoscopic systems with implants, micro-sensors, internet of things useful electronic (superconductors, metallic devices…). Today technologies used in conductors, semiconductors), magnetic autonomous systems are not efficient (ferromagnets, superparamagnets, single enough for ultra low power applications. The molecule magnets, nanoporous magnets, transistor threshold voltage has been scaled etc), biological and/or optical properties. to optimally balance leakage and dynamic We also involved on the development of power but optimized performances are materials processing techniques, molecular below autonomous systems specifications. self-assembly and on the preparation of The deadline for submitting applications functional nanostructured materials. is October 13, 2011 The deadline for submitting applications For further information about the position, is October 15, 2011 please contact: For further information about the position, Hervé Fanet (herve.fanet@cea.fr) please contact: Jaume Veciana (vecianaj@icmab.es) • PostDoctoral Position (CEA-Léti, France): ”Characterization of a flexible array • PhD Position (IMM - CSIC, Spain): of tactile sensors” ”New paradigms and New Devices based We aim to develop a flexible tactile sensor on Nanomechanics” based on MEMS technology developed The aim of this PhD project is the at CEA Léti. Three-axis force sensors development of new NEMS devices and developed at Léti and already tested for new sensing paradigms to achieve the texture measurements will be integrated in ultimate limits in biological detection based an array. The work proposed will include on nanomechanics. Silicon nanowires both the integration and characterization of together carbon nanotubes represent the flexible sensor array. the ultimate limit in the minituarization of The deadline for submitting applications nanomechanical resonators. It is expected is October 13, 2011 that these devices can be applied for ultrasensitive mass sensing at the sub- For further information about the position, zeptogram level and for mass spectroscopy please contact: of single biomolecules. However, the Caroline Coutier achievement of the optimal performance (caroline.coutier@cea.fr) of these devices requires a detailed understanding of the nanomechanical • PhD Position (ICMAB - CSIC, Spain): response and a major development of the “Functionalisation of surfaces with optical instrumentation for the detection of functional organic molecules for electronic the picometer scale vibrations. In this PhD18 or biological applications” project advanced optical instrumentation
  • nanojobsand modeling of the nanomechanical and Group and ETSF Scientific Developmentoptical response of the silicon nanowires Centre in Spain and the Theory group ofwill be developed. Finite element simulations the Fritz-Haber-Institut in Berlin. The aimand analytical models will be developed in of the research project is to develop neworder to describe how the static and dynamic concepts for understanding, identifying,response of nanomechanical systems with and quantifying the different contributionsdifferent geometries behaves when subject to energy harvesting and storage as well asto biological adsorption. The final aim will be describing transport mechanisms in naturalto establish the potential for weighing single light harvesting complexes, photovoltaicbiomolecules and measuring molecular materials, fluorescent proteins and artificialrecognition at the level of few events. (nanostructured) devices by means ofThe deadline for submitting applications theories of open quantum systems, non-is October 18, 2011 equilibrium processes and electronicFor further information about the position, structure.please contact: The deadline for submitting applicationsMontserrat Calleja is October 31, 2011(mcalleja@imm.cnm.csic.es) For further information about the position, please contact:• PostDoctoral Position (ICFO, Spain):”Optics and Photonics” Angel Rubio (Angel.Rubio@ehu.es)ICFO – The Institute of Photonic Sciences is • PostDoctoral Position (CEA-Léti,a center based in Castelldefels (Barcelona), France): ”CMOS electro-optical bridge forSpain, devoted to the research and network-on-chip and optical network”education of the optical and photonicsciences, at the highest international level. The forecasted developments of high-No restrictions of citizenship apply to the performance computing (HPC) andICFO post-doctoral contracts. Candidates Cloud computing induce new needs formust hold an internationally-recognized computation density and data mining. ThePhD-equivalent degree in a field of science architectural model is built from a largeand engineering related to optics and number of processors sharing a hugephotonics. Suitable backgrounds include memory (eventually several terabytes).optics, physics, mathematics, electronics Besides, while networks-on-chip (NoC) areand telecommunications engineering. becoming the dominant interconnectionThe deadline for submitting applications paradigm within chips, the connections tois October 20, 2011 large memories are still point-to-point. TheFor further information about the position, gap between the theoretical computingplease contact: power and the effective or real computingAriadna García power is hence widening because of(ariadnag@heuristica.org) bandwidth limitations to shared memory and increasing communication latency.• Postdoctoral and PhD positions Emerging high-bandwidth connection(University of the Basque Country UPV/ standards (DDR3, WideIO…) remainEHU, Spain): ”Dynamical processes in incremental solutions and do not allowOpen Quantum Systems” concurrent accesses to a large number ofApplications are invited for postdoctoral and memory banks.PhD positions link to a five year project on The deadline for submitting applicationsthe topic of Dynamical processes in open is October 31, 2011quantum systems as part of an EuropeanResearch Council Advanced grant (DYNamo For further information about the position,project). The project will be conducted please contact:between the NanoBio Spectroscopy Yvain Thonnart (yvain.thonnart@cea.fr) 19
  • www.nanociencia.imdea.org RESEARCH PROGRAMMES • Molecular nanoscience IMDEA-Nanociencia is a private Foundation created by joint initia- tive of the Comunidad de Madrid and the Ministry of Education of • Scanning probe microscopies the Government of Spain in February 2007 to manage a new and surfaces research Institute in Nanoscience and Nanotechnology (IMDEA- Nanociencia). The Institute is located at the campus of the Univer- • Nanomagnetism sidad Autónoma de Madrid in Cantoblanco. The Institute aims at performing research of excellence in selected • Nanobiosystems: biomachines and manipulation of macromolecules areas and offers attractive opportunities to develop a career in sci- ence at various levels from Ph.D. students to senior staff positions. • Nanoelectronics and superconductivity The Madrid Institute for Advanced Studies in Nanoscience also develops an important program of technology transfer and creation of spin-off companies. • Semiconducting nanostructures and nanophotonics E-mail contacto.nanociencia@imdea.org Phone 34 91 497 68 49 / 68 51 Fax 34 91 497 68 55 • Nanofabrication and advanced instrumentation [Nanociencia y Nanotecnología: lo pequeño es diferente small is different Nanoscience and Nanotechnology: ]
  • nanoICT Conf Report 7th International Thin Film Transistor Conference-ITC 2011 3-4 March 2011, Clare College, CambridgeOrganisers were presentations on CNTs and nanowiresW.I.Milne > Engineering Dept, University of for use in TFTs. In terms of distribution, anCambridge, UK. equal balance in presentations was achievedArokia Nathan > Electrical & Electronic EngineeringUniversity College, London, UK. between materials and applications, fulfillingwww-g.eng.cam.ac.uk/edm/itc2011/ the primary theme of ITC2011. Much of the meeting concentrated on theSponsored by: production, characterisation and application of metal oxide based semiconductors although there were also several reports on the use of organic based material systems for TFTs. Metal Oxide transistors are becoming increasingly important as their mobility is much higher than those of amorphousThe aim of this meeting was to highlight silicon based TFTs and, as their stabilitythe on-going work on Thin Film Transistors is improved, their use in practical systems(TFTs), including a-Si:H and related materials including flat panel displays, sensors andsystems such as nano, micro and poly LEDs cannot be far away.crystalline silicon. Sessions however alsoincluded work on metal oxides, organics, There were 7 oral sessions and a postersemiconducting nanowires, carbon session on both afternoons. The first sessionnanotubes (CNTs) and naturally the new was based on Materials & Processing“material of choice” graphene. Thin-film and the invited papers in this session wereTransistors (TFTs) have become increasingly presented by Hiroshi Tanabe from NECimportant since amorphous silicon (a-Si:H) and Richard Wilson from CDT. Dr Tanabe’sTFTs were first incorporated in the backplanes presentation was on TFT technologies forin AMLCD TVs. They of course are now being Flexible Displays based on the productionconsidered for a variety of other applications of metal oxide TFTs at low temperaturesincluding RFID tags, sensors, smart tags, using an excimer laser annealing technique.etc. and increasingly in flexible electronics. Richard Wilson’s talk concentrated onHowever the electronic properties of a-Si:H solution processing of organic TFTs with fieldlimit its possible applications and a variety effect mobilities in excess of 1 cm2V-1 cm-2.of different material systems are now being The optimisation of the solvent selectioninvestigated as alternatives. from which the material is deposited is key toThis year the conference theme was on enhancing and controlling crystalline domainNovel Materials, Processing and Device- formation.Circuit Integration. There were 110 abstracts Session 2 and 8 concentrated on Thin Filmsubmitted and 150 attendees many from the Transistors themselves and the invited talksFar East. here were given by Kenji Nomura from TokyoThere were 17 invited speakers and 29 Tech and Elvira Fortunato from FCT-UNL,contributed papers who presented their work Portugal. In his presentation Prof Nomuraon a variety of thin film material systems. There described the work they have been doing to 21
  • nanoICTConfReport improve the stability of a-In-Ga-Zn-O TFTs graphene by Markuu Rouval from the Nokia and Prof Fortunato’s talk concentrated on Research Centre. transparent electronics with emphasis on 74 posters were presented in the two the production of both p-type and n-type sessions and the banquet was held in TFTs. Gilles Horowitz from the Université Clare College which is the second oldest Denis-Diderot covered the modelling Cambridge College, having been founded of organic TFTs and Simon Ogier from in 1326. PeTEC presented their work on backplane technologies for flexible displays. All the sessions were exceedingly well attended despite a tight two-day program Novel devices and their applications were described in Session 3. Sigurd Wagner from with back-to-back talks and posters. Princeton reviewed their work on self aligned Excellent feedback was received from the amorphous silicon transistors and Yue Kuo attendees on the technical quality of the from Texas A&M described his work on program and the general organization. non-volatile memory based devices based ITC 2012 will be held in Lisbon, hosted by on floating gate amorphous silicon TFTs. Uninova, in January 2012. This was followed by Mutsuku Hatano from Tokyo Tech who gave her vision of the future- nanoICT Coordination Action (nanoICT) integration of wireless-communication www.nanoict.org functions on Display Panels using TFT technology. Sessions 4 and 6 looked at TFT circuits and System Integration and involved 4 further invited talks. Prof Takao Someya from Tokyo The nanoICT plan to strengthen scientific University gave an excellent presentation and technological excellence will go beyond on his work on foldable and stretchable the organisation of conferences, workshops, electronics using organic based transistors exchange of personnel, WEB site, etc. and memories and this was followed by Prof developing the following activities: Jin Jang from Kyung Hee University in Korea who presented their research on the stability 1. Consolidation and visibility of the and flexibility of a-IGZO Transistors on plastic research community in ICT nanoscale and their application to circuits. devices The second session (Session 5) on Materials 2. Mapping and benchmarking of research and Processing was held on the morning of at European level, and its comparison the second day and mostly concentrated on with other continents metal oxide materials and devices. Andrew 3. Identification of drivers and measures Flewitt from Cambridge University and to assess research in ICT nanoscale Thomas Anthopoulos from I.C. were the devices, and to assess the potential invited speakers and covered respectively of results to be taken up in industrial insulators and semiconducting materials research deposited at low temperature using a novel sputtering method and spray pyrolysis 4. Coordination of research agendas and processed ZnO for use in TFT manufacture. development of research roadmaps Session 7 was sponsored by nanoICT EU 5. The coordination of national or regional project and the invited talks were on CNTs for research programmes or activities, with TFTs by Prof Didier Pribat of Sungkyunkwan the aim to involve funding authorities in22 University in Korea and Circuits based on building the ERA around this topic.
  • nanoresearchThe raise up of UHV atomic scaleinterconnection machinesJ. S. Prauzner-Bechcicki1, D. Martrou2,C. Troadec3, S. Gauthier2, M. Szymonski1 andC. Joachim2,31Center for nanometer-Scale Science and AdvancedMaterials (NANOSAM), Faculty of Physics,Astronomy and Applied Computer ScienceJagiellonian University, Reymonta 4, Krakow, Poland.2Centre d’Elaboration de Matériaux et d’Etudes Fig. 1 > A single five wings molecule-motor [1]Structurales (CEMES-CNRS), 29, rue Jeanne Marvig, positioned between a 4 Au nano pads junctionBP 94347, 31055 Toulouse Cedex 4, France. constructed at the Si(100)-H surface. The 4 black3Institute of Materials Research and Engineering, wires getting out of the surface are indicative of theA*STAR (Agency for Science, Technology and interconnections step 3 discussed in the text dependingResearch), 3 Research Link, Singapore 117602. on the electronic gap of the supporting surface./1. Introduction In section 2, the general principles of theSingle molecule mechanics [1], mono- few UHV atomic scale interconnectionmolecular electronics [2] and surface machines under test to solve the problem areatomic scale circuits [3] [4] are all requiring a described. Depending on the electronic gapspecific surface interconnection technology of the surface where the atomic scale deviceswith an atomic precision and cleanness and machineries are supposed to work, two[5]. In a planar configuration, this surface families of interconnections machines aretechnology must be able to provide multiple being explored. Section 3 is providing oneaccess electronic channels to the atomic (or example of an atomic scale interconnectionmolecular) scale machinery constructed on a machine designed for the surface of widesurface (see for example Fig. 1). At the end gap semi-conductor and insulator materials.of the 80’s, it was expected that the e-beam Section 4 is giving the example of twonano-lithography technique would be able interconnection machines for moderate gapto provide such a technology [6]. But with its semi-conductor surfaces. The design andresist based approach, e-beam technique instrumentation works reported here arewill not face the challenge [7] because it is the consequence of the EU ICT integratednot able to respect at the same time the project Pico-Inside in Krakow and Toulouseatomic scale precision, the cleanness and together with the A*STAR VIP Atom Techthe expected large number N of access Phase 2 project in Singapore. It is now furtherchannels to the atomic scale machinery developed in the new EU ICT integrated[8]. Alternative nanolithography techniques project AtMol and in the Phase 3 of thesuch as nano-imprint [9] or nano-stencil A*STAR VIP Atom Tech project in Singapore.[10] are neither adapted to encompassall the interconnection stages from the 2. Atomically precise electricalmacroscopic to the atomic scale nor clean interconnection machineenough down to the atomic scale. At the turn An atomic scale precision, multiple access,of the century, this problem triggers a new electrical interconnection instrument mustapproach to planar electrical interconnects provide N conducting wires convergingstarting from the bottom that is from the toward a very small surface area wherefundamentals of surface science. an active machinery (see Fig. 1 for a N=4 23
  • example) has been constructed with an atomic scale precision. Those N interconnects are positioned somewhere on a large wafer surface. As a consequence, a very efficient navigation system must be designed to locate this very small active area from a macroscopic perspective while keeping the local atomic precision of the interconnection. The solution to this navigation requirement is to combine two types of microscopy: a far field one (optical, scanning electron microscope (SEM)) for large scale navigation and a near field one (Scanning Tunneling Microscope (STM), Atomic Force Microscope (AFM)) for the atomic scale part with a full overlay between those 2 types of microscopy. An UHV atomic scale interconnection machine is Fig. 2 > Scheme of the atomic scale interconnection designed to follow a dedicated interconnection machines for (a) wide and (b) moderate surface protocol. On an atomically clean well-prepared band gap substrates. A: Atomic scale circuitry, surface, an atomic scale circuitry is fabricated B: Contacting metallic nanopads, C1: Ultrasharp (A). To reach a large number N of interconnects metallic tips, C2: Nanowires, D: Microelectrodes, E: and to be able to interconnect each atomic Metallic microcantilevers./ wire to the external world, there is a necessary lateral extension of this circuit to reach N 3. UHV interconnection machine for large contacting metallic nanopads (B) that are surface gap positioned around the atomic scale circuit. In the example of Fig. 2, a molecule is connected For a large valence-conduction band to these nanopads by atomic metallic wires. electronic surface gap (more than a few eV Depending on the electronic surface gap of up to 8 eV for standard insulators), SEM the supporting material, the nanopads (B) is difficult to use as a navigation far field have to be contacted from the top by a series microscope because its electron beam of N atomically sharp metallic tips (C1) or by will charge the surface. In this case, an a series of N nano-scale wires (C2) up to the optical microscope is natural candidate for point where mesoscopic metallic wiring or coarse-grained positioning. It determines microelectrodes (D) can be surface fabricated the minimum length of metallic surface and contacted by a series of N micro-scale wiring which must be fabricated starting metallic cantilevers (E) also from the top of from the nano-pads (B) in Fig. 2a toward the wafer. During the process, the sequence the next contact stage based on metallic of those different steps depends on the micro-cantilever. Fortunately enough, with a machine and on the supporting material. What large surface gap, the surface area of those is triggering the choice of the interconnection interconnects can be expanded horizontally technology between C1 and C2 (and after the without too much lateral leakage current need for the D and E interconnection steps between the different electrodes. This is the in Fig. 2a) is the electronic gap of the surface basis of the UHV interconnection machine that in turn will determine the kind of far field described in this section where a low microscopy to be used for navigation over the temperature approach is not compulsory24 wafer surface. but preferable.
  • nanoresearchTo realize the 5 levels of interconnect 1. a flexural-hinge guided (XY) nano-described in Fig. 2a in UHV, the deposition positioner stage (100 μm x 100 μm,of molecules, their observation by NC-AFM repeatability 5 nm) with a closed loopand the measurement of their electrical control based on capacitive sensors,properties, the Toulouse group has designed 2. an evaporation system highly collimatedand constructed a dedicated UHV equipment on the cantilever to perform nano-stencilcalled DUF (DiNaMo UHV Factory). This deposition,equipment allows transferring samples underUHV between five complementary UHV 3. a (XYZ) piezo driven table for positioningchambers (see Fig. 3): the metallic micro-combs for the electrical contacts,(1) an MBE growth chamber dedicated to nitride semiconductors growth, 4. an optical microscope to control the metallic nano-pads growth and stencil positioning of the micro-combs. evaporation for microelectrodes These modifications were introduced by(2) a room temperature AFM/STM chamber the mechanical workshop of the Toulouse for surface characterization by STM and laboratory. The main advantage of using NC AFM a commercial UHV AFM/STM is to benefit from the good characteristics for SPM(3) an AFM/STM chamber modified for imaging. But the piezo tube used to scan nano-stenciling experiments and has a range of a few μm only. The addition of electrical measurements a piezo table to move the sample offers the(4) a preparation chamber for cleaning possibility to perform wide range scanning, substrates, STM tips and AFM cantilevers up to 80 μm SPM images, while keeping the(5) a mass spectrometer chamber possibility to realize atomic sale imaging with transformed in a molecular ions source. the piezo tube.For (3) a UHV Omicron Nanotechnology One of the disadvantages is the smallVT STM/AFM head has been modified to accessible space around the SPM head.accommodate different tools, namely [11]: Indeed, it is not possible to place an opticalFig. 3 > The DUF (DiNaMo UHV Factory) equipment allows to transfer samples between 5complementary UHV chambers in order to realize the 5 levels of interconnect on wide band gapsemiconductors (GaN, AlN)./ 25
  • nanoresearch microscope with normal incidence with Krakow and the other in Singapore. The respect of the substrate, and an atomic Krakow’s system consists of three basic source for the nano-stencil experiments with segments: multi-probe, low-temperature normal incidence with respect to the AFM scanning probe microscope (LT-SPM) cantilever. In our case, the image obtained and preparation chambers. Multi-probe by the optical microscope comes from a segment is composed of 4-probe scanning mirror with an angle of 30° with the substrate tunnelling microscope (STM) combined plane. This gives distorted images, with with high resolution scanning electron a loss of resolution: only 3 μm instead of microscope (HR-SEM) and hemispherical 1 μm in normal incidence. The effusion cell electron energy analyser (scanning Auger is fixed on a port of the UHV chamber that microscope, SAM) (see Fig. 4). The Auger makes an angle of 33° with the horizontal microscope part is the element not present plane, and another angle of 28° between in Singapore’s setups. Composition of the the two vertical planes passing through the multi-probe segment allows surface element evaporation beam and the central axis of analysis, imaging and measurements of the cantilever. This orientation of the atom nanostructures conductance with very beam induces distortion, which should be high-resolution. In accord with the Fig. 2b taken into account in the design of the nano- principle, HR-SEM may act as a navigation pattern to be drilled into the pyramidal tip of to precisely position each of the 4 STM tips the cantilever [11]. (a) 4. UHV interconnection machine for moderate semi-conductor surface gap For a moderated valence band-conduction band electronic surface gap (around a few eV), it is not possible to use very long surface metallic circuitry due to the possible lateral surface leakage current between the surface electrodes. In this case, one solution is to use ultra sharp STM like tips positioned from the top on the surface as microelectrodes (Fig. 2b). In this case, the core of the tips will (b) not be in contact with the supporting surface and one can go continuously from a tip apex radius of curvature of a few nanometer up to a 100 microns or more section for the tip body. In this case, navigation on the surface can be performed using an UHV- SEM (Fig. 2b) by grounding the sample during the SEM imaging to avoid the surface charging effect. This is the basics of the UHV interconnection machines described here. A low temperature approach is compulsory with those systems because of Fig. 4 > (a) View on sample stage of 4-probe the low electronic gap at the surface of the microscope; in upper part one can see SEM column and next to it an entrance to hemispherical supporting material. electron energy analyser; below SEM column There are two apparatuses that realize the there are three of four STM probes. (b) SEM image26 above described design, one is housed in of four STM probes./
  • nanoresearchthat will be used as microelectrodes. Firstmeasurements of conductance of goldnanostructures on Ge(001) surface are inprogress.Next, LT-SPM segment consist of scanningprobe microscope that may work both asSTM and NC-AFM in a range of temperatures Fig. 5 > InSb surface imaged with q-sensor NC-from 4K up to room temperature. Thanks AFM in temperature 4K./to use of scanner and sample holderembedded in a cryostat, the LT-UHV STM SAM images of metallic nano-mesa grownallows for a very high resolution imaging, as on semiconductor substrate are shown.well as, stable spectroscopic measurements The Ag/Ge(111) is an example of a systemand atomic scale manipulations. for which depending on the depositionFurthermore, NC-AFM mode is based on conditions, on the successive thermalq-sensor device (tuning fork) that enables annealing and on the amount of depositedimaging of conducting, semiconducting and material the resulting overlayer morphologyinsulating samples at low temperatures (see can be switched from an atomically smoothFig. 5) and, if required, also simultaneous to a columnar-like [12]. Sample is prepared inmeasurements of tunnelling current. This the following way: silver in amount of nominaloption makes the Krakow’s system a very 5 ML is deposited on the germanium surfacepowerful tool. Last but not least, is the kept at low temperature. Low energy electronpreparation segment that consist of typical diffraction studies performed immediatelypreparation equipment such as a XYZ after deposition reveal that compact silvermanipulator with electric contact allowing for film is crated. Such a conclusion follows fromresistive heating up to 1000K (using a direct the fact that reflections characteristic forheating mode 1200K may be achieved), the unreconstructed Ag(111) overlayer arefurthermore the manipulator allows for observed exclusively on LEED image. Onecooling the sample down to 100K with may assume it is a clear and direct indicationnitrogen vapour, an ion gun, a low energy that Ge substrate is completely buried.electron diffraction system for quick sample In the next step the sample is annealed toquality tests and several ports allowing room temperature. After annealing a massivefor incorporation of additional elements(for instance effusion cells or quartzmicrobalance thickness monitor) into thechamber. All segments are composed of thehighest quality elements all of them beingcompatible to work in UHV environment(less than 3×10-10mbar), and thus allowing Fig. 6 > STM, HR-SEM and SAM images of the Ag/for conducting very complex experiments Ge(111) sample. Image size: 220nm × 270nm. Leftin a single set-up in a very controlled panel: STM image; Middle panel: HR-SEM image;way, starting from sophisticated sample Right panel: SAM image. STM image reveals brightpreparation and ending with extensive and nanostructures 2.5nm high. The same regionscomplete characterisation. are marked red in HR-SEM image. In SAM image those structures are black. SAM image was takenAs the Auger microscopic part of the for Ge line (E=1144 eV), thus exposing as brightKrakow’s nano-probe instrument is not regions containing Ge. Therefore, it is possible topresent in the other setups (see above and identify the black structures in SAM image (brightbelow) its potential is briefly described in the and red regions on STM and HR-SEM images,following. In Fig. 6, the STM, HR-SEM and respectively) as silver islands./ 27
  • nanoresearch morphological reorganization of the Ag film is (E = 352 eV). A strong signal contrast in observed that leads to formation of metallic the Ge-SAM image (Fig. 6, right panel) is islands. Left panel in Fig. 6 contains STM observed. It clearly indicates the metallic image of the Ag/Ge(111) sample annealed composition of the nanomesa against the to room temperature. As clearly seen, the semiconductor substrate. Bright regions Ag continuous film has been converted into on right panel in Fig. 6, reveal a germanium a network of “nano-mesa” of 10 ML height content, whereas black regions prove lack with atomically flat tops and extended pits of the germanium. Those black regions exposing the underlying wetting layer (bright nicely correspond to red structures seen and dark structures on left panel in Fig. 6, in HR-SEM image (Fig. 6, middle panel) respectively). The atomic steps bounding and to bright regions in SEM image (Fig. the edges are developing into considerably 6, left panel). Thus analysis with Auger elongated ridges, and appear to be spectroscopy strengthens the conclusion predominantly oriented along the threefold that observed nanostructures are truly silver symmetry of the (111) substrate as proved nanomesa. Furthermore, in the image, the by the HR-SEM analysis. Ag ridges of the width as low as 20 nm can be resolved providing the capability of high To further confirm metallic composition lateral resolution of our Auger microscope of the observed topographic structures based on the Gemini electron column. Here Auger spectroscopy in scanning mode is it is worth noting, that in the Ag-SAM image performed. The corresponding Scanning (not shown), together with the high lateral Auger Microscopy images were taken resolution a lower contrast of the Auger at the Ge line (E = 1144 eV) and Ag line signal is observed. The reason for that is28
  • nanoresearchthe composition of the wetting layer whichis actually the intermixed layer of Ag and Ge.Following the Fig. 2b principle, the Singaporesystem consists of five UHV chambersinterconnected in a star configuration. A firstchamber is dedicated to the surface waferpreparation. A second one is dedicated tometallic nano-islands deposition and transfer(when necessary). A third chamber isequipped with a LT-UHV STM for fabricatingthe atomic wires, manipulating the moleculelogic gate and positioning the metallicnano-islands, one on each termination Fig. 7 > The final IMRE Singapore interconnection machine designed in Singapore and constructedof an atomic wire. The fourth chamber is by Omicron in 2010. Five main UHV chambers:equipped with a very efficient UHV-SEM with (A) The preparation chamber with the FIM tipa resolution better than 5 nm to be able to preparation chamber, (B) the LT-UHV-STM fromimage the contacting metallic nano-islands Omicron Nanotechnology and (C) the nano-and to position a tip apex on each of these probe system from Omicron Nanotechnologynano-islands. Finally, a lateral chamber is with its UHV-SEM (3 nm resolution) and the 4dedicated to the UHV tip preparation, integrated STM. (D) is the delicate LT transfersharpening and cleaning. The details of the from the LT-UHV-STM to the UHV-SEM. TheIMRE Singapore interconnection machine UHV transfer print chamber is behind the LT-are presented in Fig. 7. UHV-STM chamber B./ 29
  • nanoresearch atomic scale interconnection machines housed in Toulouse, Krakow and Singapore possess potential to meet requirements and demands arising upon realization of such a connection [13]. Furthermore, experience and knowledge gained by Krakow’s and Toulouse’s groups through EU ICT integrated project Pico-Inside and by Singapore’s group in parallel through A *STAR VIP Atom Tech Phase 2 set a very Fig. 8 > SEM image of the tip apex of 4 chemically etched tungsten tips converging toward 4 Au fortunate starting point for that quest which nano-islands which have been manipulated one by is now in full development in the new ICT one to enable a 4 points like surface conductance integrated project AtMol (www.atmol.eu). measurement. This image has been recorded on a multi-probe system from Zyvex./ References [1] Rapenne G, Launay J-P and Joachim C 2006 Under the UHV-SEM, each nano-island of a J. Phys.: Condens. Matter 18 S1797. contacting nanostructure can be electrically [2] Joachim C, Gimzewski J K and Aviram A 2000 contacted from the top using one ultra- Nature 408 541. sharp tip per nano-island. Each tip apex is [3] Wada Y 1996 Microelectronic Engineering 30 positioned using the UHV-SEM for a precise 375. navigation. The soft contact between a nano- [4] Ample F, Ami S, Joachim C, Thieman F and island and its tip apex is controlled by the Rapenne G 2007 Chem. Phys. Lett. 434, 280. feedback loop system of an STM meaning [5] Joachim C 2002 Nanotechnology 13 R1. that there is one STM control electronic per [6] Itoua S, Joachim C, Rousset B and Fabre N tip. A SEM image of the contact preparation 1992 Nanotechnology 3 10. of 4 nano-islands manipulated on purpose in [7] Saifullah M S M, Ondarcuhu T, Koltsov a row on a MoS2 surface is presented in Fig. D F, Joachim C and Welland M 2002 8 together where the 4 tip apex approaching Nanotechnology 13, 659. for the contact are also imaged. Electrical [8] Cacciollati O, Joachim C, Martinez J-P and measurements are now in progress to first Carsenac F 2004 Int. Journ. Nanosci. 3 233. record the “nano-pad – MoS2 surface – [9] Chou S Y, Krauss P R, Renstrom P J 1996 nano-pad” I-V characteristics for an inter Science 272 85. nano-pad distance lower than 10 nm. Then, [10] Thet N T, Lwin M H, Kim H H, Chandrasekhar metallic nano-islands will be transfer printed N and Joachim C 2007 Nanotechnology 18, on Si(100)H surface for surface atomic wire 335301. conductance measurements. [11] Guo H, Martrou D, Zambelli T, Dujardin E and Gauthier S 2008 Rev. Sci. Instrum. 79 103904. 5. Conclusion [12] Krok F., Buatier de Mongeot F., Goryl M., Connecting the atomic (or molecular) scale Kolodziej J.J., and Szymonski M., Phys. Rev. B machinery to the outer world respecting the 81, 235414 (2010). atomic scale precision of the construted [13] C. Joachim, D. Martrou, M. Rezeq, C. Troadec, machinery is a very challenging and Deng Jie, N. Chandrasekhar and S. Gauthier J.30 complex task. Each of the three described Phys. CM, 22, 084025 (2010).