Process Variability and Electrostatic Analysis of Molecular QCA

Molecular quantum-dot cellular automata (mQCA) is an emerging paradigm for nanoscale computation. Its revolutionary features are the expected operating frequencies (THz), the high device densities, the noncryogenic working temperature, and, above all, the limited power densities. The main drawback of this technology is a consequence of one of its very main advantages, that is, the extremely small size of a single molecule. Device prototyping and the fabrication of a simple circuit are limited by lack of control in the technological process [Pulimeno et al. 2013a]. Moreover, high defectivity might strongly impact the correct behavior of mQCA devices. Another challenging point is the lack of a solid method for analyzing and simulating mQCA behavior and performance, either in ideal or defective conditions. Our contribution in this article is threefold: (i) We identify a methodology based on both ab-initio simulations and post-processing of data for analyzing an mQCA system adopting an electronic point of view (we baptized this method as "MoSQuiTo"); (ii) we assess the performance of an mQCA device (in this case, a bis- ferrocene molecule) working in nonideal conditions, using as a reference the information on fabrication-critical issues and on the possible defects that we are obtaining while conducting our own ongoing experiments on mQCA: (iii) we determine and assess the electrostatic energy stored in a bis-ferrocene molecule both in an oxidized and reduced form. Results presented here consist of quantitative information for an mQCA device working in manifold driving conditions and subjected to defects. This information is given in terms of: (a) output voltage; (b) safe operating area (SOA); (c) electrostatic energy; and (d) relation between SOA and energy, that is, possible energy reduction subject to reliability and functionality constraints. The whole analysis is a first fundamental step toward the study of a complex mQCA circuit. It gives important suggestions on possible improvements of the technological processes. Moreover, it starts an interesting assessment on the energy of an mQCA, one of the most promising features of this technolog

SCERPA Simulation of Clocked Molecular Field-Coupling Nanocomputing

Among all the possible technologies proposed for post-CMOS computing, molecular field-coupled nanocomputing (FCN) is one of the most promising technologies. The information propagation relies on electrostatic interactions among single molecules, overcoming the need for electron transport, significantly reducing energy dissipation. The expected working frequency is very high, and high throughput may be achieved by introducing an efficient pipeline of information propagation. The pipeline could be realized by adding an external clock signal that controls the propagation of data and makes the transmission adiabatic. In this article, we extend the Self-Consistent Electrostatic Potential Algorithm (SCERPA), previously introduced to analyze molecular circuits with a uniform clock field, to clocked molecular devices. The single-molecule is analyzed by ab initio calculations and modeled as an electronic device. Several clocked devices have been partitioned into clock zones and analyzed: the binary wire, the bus, the inverter, and the majority voter. The proposed modification of SCERPA enables linking the functional behavior of the clocked devices to molecular physics, becoming a possible tool for the eventual physical design verification of emerging FCN devices. The algorithm provides some first quantitative results that highlight the clocked propagation characteristics and provide significant feedback for the future implementation of molecular FCN circuits

Robustness of the In-Plane Data Crossing for Molecular Field-Coupled Nanocomputing

Molecular Field-Coupled Nanocomputing (molFCN) offers several advantages compared to other beyond-CMOS technologies, such as the cut of the power dissipation, thanks to the absence of charge transport, and the possibility to work at room temperature. Several circuits have been investigated for molFCN, primarily analyzed from a behavioral standpoint. Also, researchers proposed a few solutions to cross two signals and analyzed them from a logical and ideal perspective. Crossing information is an essential and delicate operation since molFCN is an in-plane technology. Besides, previous works demonstrated the need to consider molecule physics to predict the behavior of a molFCN circuit. This work examines different implementations of the in-plane information crossing interconnection, considering the punctual molecule physics to predict the interconnection functioning. We tune the electrostatic feature of the involved molecules to determine the robustness of the cross-wire against static electrostatic variations, thus providing valuable information for the synthesis of ad-hoc molecules for molFCN

A Roadmap for Molecular Field-Coupled Nanocomputing Actualization

In molecular field-coupled nanocomputing, electrostatically-coupled molecules encode the logic information in their charge distribution, promising extremely low power consumption, room temperature operation, and THz frequency operations. Besides the impressive theoretical predictions and simulation confirmations, a working prototype must still be fabricated. This work discusses the most crucial aspects that hinder the fabrication of a working prototype and defines a roadmap to address and guide the procedure to fabricate a working proof of concept. Accurate physical simulations support each point of the roadmap

Ab initio Molecular Dynamics Simulations of Field-Coupled Nanocomputing Molecules

Molecular Field-Coupled Nanocomputing (FCN) represents one of the most promising solutions to overcome the issues introduced by CMOS scaling. It encodes the information in the molecule charge distribution and propagates it through electrostatic intermolecular interaction. The need for charge transport is overcome, hugely reducing power dissipation. At the current state-of-the-art, the analysis of molecular FCN is mostly based on quantum mechanics techniques, or ab initio evaluated transcharacteristics. In all the cases, studies mainly consider the position of charges/atoms to be fixed. In a realistic situation, the position of atoms, thus the geometry, is subjected to molecular vibrations. In this work, we analyse the impact of molecular vibrations on the charge distribution of the 1,4-diallyl butane. We employ Ab Initio Molecular Dynamics to provide qualitative and quantitative results which describe the effects of temperature and electric fields on molecule charge distribution, taking into account the effects of molecular vibrations. The molecules are studied at near-absolute zero, cryogenic and ambient temperature conditions, showing promising results which proceed towards the assessment of the molecular FCN technology as a possible candidate for future low-power digital electronics. From a modelling perspective, the diallyl butane demonstrates good robustness against molecular vibrations, further confirming the possibility to use static transcharacteristics to analyse molecular circuits

Multi-Molecule Field-Coupled Nanocomputing for the Implementation of a Neuron

In recent years, several alternatives have been proposed to face CMOS scaling problems. Among these, molecular Field-Coupled Nanocomputing is a paradigm that encodes information in the spatial charges distribution and promises to consume a minimal amount of power. In this technology, circuits have always been designed using the same molecule type, and logic functions are obtained through specific layouts. This work demonstrates that multi-molecule circuits, which use different kinds of molecules in the same layout, enhance the circuit features and set up a new way to conceive molecular Field-Coupled Nanocomputing. In particular, by inserting different molecules with specific characteristics into appropriate layout positions, it is possible to obtain an artificial neuron behavior using the Majority Voter layout

SCERPA: a Self-Consistent Algorithm for the Evaluation of the Information Propagation in Molecular Field-Coupled Nanocomputing

Among the emerging technologies that are intended to outperform the current CMOS technology, the field-coupled nanocomputing (FCN) paradigm is one of the most promising. The molecular quantum-dot cellular automata (MQCA) has been proposed as possible FCN implementation for the expected very high device density and possible room temperature operations. The digital computation is performed via electrostatic interactions among nearby molecular cells, without the need for charge transport, extremely reducing the power dissipation. Due to the lack of mature analysis and design methods, especially from an electronics standpoint, few attempts have been made to study the behavior of logic circuits based on real molecules, and this reduces the design capability. In this article, we propose a novel algorithm, named self-consistent electrostatic potential algorithm (SCERPA), dedicated to the analysis of molecular FCN circuits. The algorithm evaluates the interaction among all molecules in the system using an iterative procedure. It exploits two optimizations modes named Interaction Radius and Active Region which reduce the computational cost of the evaluation, enabling SCERPA to support the simulation of complex molecular FCN circuits and to characterize consequentially the technology potentials. The proposed algorithm fulfills the need for modeling the molecular structures as electronic devices and provides important quantitative results to analyze the information propagation, motivating and supporting further research regarding molecular FCN circuits and eventual prototype fabrication

A Model for the Evaluation of Monostable Molecule Signal Energy in Molecular Field-Coupled Nanocomputing

Molecular Field-Coupled Nanocomputing (FCN) is a computational paradigm promising high-frequency information elaboration at ambient temperature. This work proposes a model to evaluate the signal energy involved in propagating and elaborating the information. It splits the evaluation into several energy contributions calculated with closed-form expressions without computationally expensive calculation. The essential features of the 1,4-diallylbutane cation are evaluated with Density Functional Theory (DFT) and used in the model to evaluate circuit energy. This model enables understanding the information propagation mechanism in the FCN paradigm based on monostable molecules. We use the model to verify the bistable factor theory, describing the information propagation in molecular FCN based on monostable molecules, analyzed so far only from an electrostatic standpoint. Finally, the model is integrated into the SCERPA tool and used to quantify the information encoding stability and possible memory effects. The obtained results are consistent with state-of-the-art considerations and comparable with DFT calculation

Investigation of Molecular FCN for Beyond-CMOS: Technology, design, and modeling for nanocomputing

L'abstract è presente nell'allegato / the abstract is in the attachmen

ANALYSIS AND MODULATION OF MOLECULAR QUANTUM-DOT CELLULAR AUTOMATA (QCA) DEVICES

Field-Coupled nanocomputing (FCN) paradigms offer fundamentally new approaches for digital computing without involving current transistors. Such paradigms perform computations using local field interactions between nanoscale building blocks which are organized with purposes. Among several FCN paradigms currently under active investigation, the Molecular Quantum-dot Cellular Automata (MQCA) is found to be the most promising and its unique features make it attractive as a candidate for post-CMOS nanocomputing. MQCA is based on electrostatic interactions among quantum cells with nanometer scale eliminating the need of charge transportation, hence its energy consumption is significantly decreased. Meanwhile it also possesses the potential of high throughput if efficient pipelining of information propagation is introduced. This could be realized adopting external clock signals which precisely control the adiabatic switching and direction of data flow in MQCA circuits. In this work, in order to model MQCA as electronic devices and analyze its information propagation with clock taken into account, an effective algorithm based on ab-initio simulations and modelling of molecular interactions has been applied in presence of a proposed clock mechanism for MQCA, including the binary wire, the wire bus and the majority voter. The quantitative results generated depict compelling clocked information propagation phenomena of MQCA devices and most importantly, provide crucial feedback for future MQCA experimental implementation