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Molecular Electronics aKIn MIcroelectronics

Final Report Summary - MEKIMI (Molecular Electronics aKIn MIcroelectronics)

Nanoelectronics is predicted to match in the years to come the relevance of Microelectronics. Several nanostructures are being studied and experimentally demonstrated. However, physicists, chemists, technologists and engineers work on the demonstration of the feasibility of a single structure or device, oriented to their own discipline and using methodologies and models derived from their own background. There is no uniformity of approach nor a common methodology and there is not a common vision about the possible direction to be followed. MEKIMI (Molecular Electronics AKIn MIcroelectronics) project aimed at mixing the research fellow background in microelectronics and new knowledges at the nanoscale and in particular at the molecular scale to help her to set up a methodology for the design of molecular devices, nanocircuits and nanoexperiments that could fill the gap between micro and nanoelectronics in the near future. MEKIMI addressed three key-points:
Keypoint-I) concentrates on a particular molecular electronics device: a molecular Quantum Doc Cellular Automata Wire (MOCAW) as a promising technology for computation for the medium/long term future (20 years),and used as precursor of other molecular systems to come. Keypoint-II) developed a methodology to fabricate, verify the functionality and simulate the system considered that, though specific for this molecular system, can be a starting point to be expanded and generalised to the molecular electronics scenario as a whole. Keypoint-III) contributed to compensate the deficiencies that molecular electronics is suffering with respect to the well established and successful microelectronics approach.

The OBJECTIVES identified were: OB1) Study and set-up the method: study the molecule for MQCAW and set-up a solid and reproducible technique to fabricate the MQCAW and learn techniques and methods to be used in other future devices. OB2) Set-up the MOCAW: experimentally attempt the fabrication of sub-parts of the wire. i.e. the sub-elements propaedeutic for ensuring write-in of a logic value to the wire input, propagation of the information along the wire and read-out of the logic value at the output, in absence or presence of a clocking system. OB3) Demonstration of the system functionality via simulation: model the behaviour of the MQCAW system and set up a dedicated and systematic simulation methodology and CAD tool to be used for MOCAW system, but to be considered as a precursor of future molecular system simulation tool.

TASK ORGANISATION. The scientific activity has been organised in five main tasks.
TASK 1. Methodology. The analysis of the MOCAW system and of its components required to define a new methodology, as MOCAW study and realisation inherently includes several disciplines. In several cases a totally new approach has been set up, as existing methods and tools simply cannot tackle the analysis of the subsystem involved and, especially, of their joint relations. A specifically developed suite was conceived in order to overcome the lack of tools suitable for this purpose (MoSQuiTo). Its general claim is the capability to study a system of molecules and to analyse their potential for transferring information according to the QCA principle. This package is organised in two stages. In Stage I accurate ab-initio simulations are performed based on the molecular system under different conditions as detailed herein. In Stage II, starting from Stage I data, electrostatic equations and models are set up. New figures of merits are defined to describe the QCA molecular system and to understand its potentials maintaining an electronic point of view.
TASK 2. Molecule analysis. The molecule was studied using the developed methodology is the bis-ferrocene. Both the effect of the switching field due to input or to other molecules, and due to a clock filed have been studied and accurately simulated and modelled. Data on oxidised molecules show excellent behaviour not only in terms of information reception but also of propagation. The basic molecular cell transcaracteristic has been identified considering the impact of fabrication and synthesis possible variations.
TASK 3. Technology. The first analysis performed in relation with the experimental phase was aimed at considering the behaviour of a bis-ferrocene molecule deposited on a real gold substrate. Results are essential to derive constraints on the quality of the surface to be obtained when fabricating the MOCAW sub-elements. Subsystems of the MOCAW have been simulated (COMSOL Multiphysics) to understand the possible structures that might influence the molecule for write-in and or clocking. The goal was to obtain as much as possible relaxed sizes and distances in order to facilitate the fabrication phase that are quite challenging due to the extremely tiny sizes of the molecules and of the required intermolecular distance. A second important step concentrated on the development of a method to conceive and fabricate the system of electrodes aimed at hosting the molecule, at forcing an input field and an external clock filed in multiple phases (pipeline). The fabrication facilities allowed to demonstrate the structure as from simulations with a good resolution, though not the resolution needed by the molecules sizes (few nanometers). For this reason a second step focused on conceiving a totally new method to fabricate nanowires of nanometers size overcoming the current limit in resolution of up-to-date fabrication facilities. The conceived idea, confirmed by very interesting results, has a huge potential directed not only towards the MOCAW system, but also towards the whole nanoelectronics scenario. In fact in several cases, devices at the nanoscale (e.g. molecular) can be conceived and implemented, but then they lack the possibility to be reached, and as a consequence used, at a practical more systemic level because of the extreme difficulty in applying currents or voltages using electrodes of comparable sizes. The results obtained at this stage, even though requiring further study and optimisation phases, already promise to achieve very good resolution without the need of extremely expensive procedures and of nanometers level resolution.
TASK 4. Information propagation algorithm. ON the basis of task 1,2 and 3, a complete computation algorithm including technological parameters has been developed. Complex logic circuits based on sequence of molecules placed in the correct layout and subjected to the correct sequence of clock phases have been implemented and tested. A simulation system for molecules that merges at the same time this level of complexity and of accuracy has never been implemented, not only in the MQCA scenario, but in general in the molecular electronic scenario. Results are rich and show interesting behaviour of the molecular system analysed, very promising features, and, especially, important feedbacks for the technological level. When attempting to implement the equivalent molecular circuit at the fabrication stage, the real constraints and degree of freedom available are now known: distance between consecutive molecules assuring the correct results, distance between clock electrodes for assuring the correct clock system delivery, characteristics of the wire in terms of sizes and roughness that can assure the correct information propagation and correct attaching of the molecules, etc...
TASK 5. Architecture. Generalising the filed Coupled Computation principle, but keeping a strong connection with technology and technological constraints, the project aims at analysing the possible behaviour of MQCA (and similar FCN structures) when complex circuits and architectures are conceived. The goal is to identify the potentials and to assess the possible drawbacks. One of the examples studied is an architecture for bio-sequence analysis implemented in VHDL. Several results
are obtained by simulations related to different conceived implementations of the circuit sub-parts, each solving one particular problem arising and taking into account technological constraints.

R1) Molecule Modelling for computation - MosQuiTo. Bisferrocene molecule has been analysed through ab-uniti simulation. Post simulation data processing allow to define as set of figures of merit to be used for understanding how and how much reliably a molecule can propagate information for nanocomputation. Bisferrocene demonstrates good behavior in the oxidised form, and show good reaction to clock signal. Possible improvements are identified on the internal structure to reduce constraints at the fabrication level.
R2) Study of guiding nanowires and clock nanowires for the molecular wire. Multi-physics simulations of electrodes positioned to host molecules; electric fields are applied to evaluate technological and electrical parameters impact on molecules characteristics. Optimal geometries are identified in the case of the bisferrocene molecule. However a generalised study allow to adapt the same study to other molecules.
R3) Architectures based on Field Coupling Nanocomputing devices. Complex circuits are implemented for practical applications using models of Field Coupling Nanocomputing (molecular of magnetic). Studies shows potentials and possible issues as well as points of intervention at the technological level. Architectures that reduce the use of long interconnects are to be favoured. Solutions to overcome intrinsic pipelining have been identified.
R4) Impact of clocking on molecules for FCN. Ab-initio simulations of molecules reaction to vertical electric field E-field parameters and molecular chemical characteristic are varied and associated to molecule tans-characteristics and information propagation. Realistic vertical electric field generated by real electrodes is studied. Consequences on the fabrication side are identified.
R5) Algorithm for computation in two dimensional circuits based on Molecular FCN. On the bases of the single molecule modelling and of the clocking impact on nanocomputation an algorithm is developed for computing and verifying effective information propagation in logic gates (wires, majority voters, and, or, half adders, full adders, inverters), using multiphase organisation. The algorithm include: noisy environment (from the electrostatic point of view), defects due to the fabrication of the system f electrodes, defects related to the molecule syntheses and to the molecules attachment to the substrate (process variations).
R6) Experimental implementation of nanowires and clock structures. Nanowires and clock electrodes are fabricated for hosting MOCAW based on state of the art nanotechnology fabrication capabilities (mainly FIB)
R6) Trench filling nanowires fabrication. Fabrication of nanowires based on new technological processes based on trench filling. A new method for fabrication 2-5nm wire nanowires is studied in order to overcome the limits of up to date technological capabilities. Results show very interesting outcomes and encourage to follow the path or further reduce the sizes of the nanowires. Studies and currently going on even after the end of the project.

OUTCOMES and IMPLICATIONS. The project outcomes will have a disruptive effect on research in the field in the following terms.
I) A fundamental step forward in understanding the behaviour of Molecular QCA has been done bringing new light on the potentialities of this technology, so that a new direction for molecular computation will emerge; logic gates behaviour have been simulated and demonstrated at very accurate simulation level and results pave the way to new important outcomes on the potentials of this type of computational systems. Date on the functionality and on the performance have been derived for the first time at a system level. II) The methodology and procedures to pattern molecular structures are be at disposal also for other molecular based technologies. III) The fabricated test structure constitutes a fundamental element for other more complex MQCAW structures and for any molecular based device in need of very narrow electrodes. IV) The combination of several simulation tools for understanding the molecular self-assembly, the relation between the molecular behaviour and the control electrodes, implemented for the first time, will have a disruptive effect on the way in which not only MQCA, but also other molecular structures will be analysed. V) The holistic modelling approach toward the use of a hierarchical tool enabling the simulation of the MQCAW plus write-in system and of its functionality will break through the conventional way to simulate these systems, and will represent the foundation of a new way to design, predict and analyse these systems based on
the combinations of molecular structures, control electrodes, static and dynamic electric fields. VI) The nanoelectronics designer needs a unique method to work with different circuits and devices, a unique environment for comparison, efficiency and effectiveness. In other words nanoelectronics needs an equivalent of SPICE. MEKIMI results then have clearly impact not only at academic level but at industrial level as well. VII) With the implementation of this project, the fellow acquired crucial competencies and extensive expertise in the field of nanoelectronics and molecular electronics, as well as skills related to funding request, project management, applications to real-world problems, etc., fostering her career in order to be able to
develop stable, high-quality research within the European scientific community, thus contributing to strengthen the research and development system at local, national and European levels. VIII) Being a female scientist the fellow contributes to the gender equalities balance.