Magnetic Quantum Dot Cellular Automata (MQCA) have been recently proposed as an attractive implementation of QCA as a possible CMOS technology substitute. Marking a difference with respect to previous contributions, in this work we show that it is possible to develop and describe complex MQCA computational blocks strongly linking technology and having in mind a feasible realization. Thus, we propose a practicable clock structure for MQCA baptised "snake-clock", we stick to this while developing a system level Hardware Description Language (HDL) based description of an architectural block, and we suggest a delay insensitive Null Convention Logic (NCL) implementation for the magnetic case so that the "layout=timing" problem can be solved. Furthermore we include in our model aspects critically related to technology and real production, that is timing, power and layout, and we present the preliminary steps of our experiments, the results of which will be included in the architecture descriptio

Chiolerio, Alessandro

Graziano, Mariagrazia

Zamboni, Maurizio

English

Magnetic Quantum Dot Cellular Automata (MQCA) have been recently proposed as an attractive implementation of QCA as a possible CMOS technology substitute. Marking a difference with respect to previous contributions, in this work we show that it is possible to develop and describe complex MQCA computational blocks strongly linking technology and having in mind a feasible realization.
Thus, we propose a practicable clock structure for MQCA baptised "snake-clock", we stick to this while developing a system level Hardware Description Language (HDL) based description of an architectural block, and we suggest a delay insensitive Null Convention Logic (NCL) implementation for the magnetic case  so that the "layout=timing" problem can be solved.
Furthermore we include in our model aspects critically related to technology and real production, that is timing, power and layout, and we present the preliminary steps of our experiments, the results of which will be included in the architecture description

GRAZIANO, MARIAGRAZIA

CHIOLERIO, ALESSANDRO

ZAMBONI, Maurizio

PORTO@iris (Publications Open Repository TOrino - Politecnico di Torino)

05 August 2020POLITECNICO DI TORINORepository ISTITUZIONALEA Technology Aware Magnetic QCA NCL-HDL Architecture / GRAZIANO M.; CHIOLERIO A; ZAMBONI M. - 1(2009), pp.951-954. ((Intervento presentato al convegno IEEE International Nanotechnology Conference tenutosi a Genova, Italynel 26-30 August, 2009.OriginalA Technology Aware Magnetic QCA NCL-HDL ArchitecturePublisher:PublishedDOI:Terms of use:openAccessPublisher copyright(Article begins on next page)This article is made available under terms and conditions as specified in the  corresponding bibliographic description inthe repositoryAvailability:This version is available at: 11583/2279307 since:IEEEA Technology Aware Magnetic QCANCL-HDL ArchitectureMariagrazia Graziano, Member IEEEDipartimento di ElettronicaPolitecnico di TorinoTorino, ItalyEmail: mariagrazia.graziano@polito.itAlessandro ChiolerioDipartimento di FisicaPolitecnico di TorinoTorino, ItalyEmail: alessandro.chiolerio@polito.itMaurizio ZamboniDipartimento di ElettronicaPolitecnico di Torinoorino, ItalyEmail: maurizio.zamboni@polito.itAbstract—Magnetic Quantum Dot Cellular Automata (MQCA)have been recently proposed as an attractive implementationof QCA as a possible CMOS technology substitute. Marking adifference with respect to previous contributions, in this work weshow that it is possible to develop and describe complex MQCAcomputational blocks strongly linking technology and havingin mind a feasible realization. Thus, we propose a practicableclock structure for MQCA baptised “snake-clock”, we stickto this while developing a system level Hardware DescriptionLanguage (HDL) based description of an architectural block,and we suggest a delay insensitive Null Convention LogicTM(NCL, [1]) implementation for the magnetic case so that the“layout=timing” problem can be solved. Furthermore we includein our model aspects critically related to technology and realproduction, that is timing, power and layout, and we present thepreliminary steps of our experiments, the results of which willbe included in the architecture description.I. INTRODUCTION AND MOTIVATIONQCA, in their general meaning, are bistable cells coupledthrough electromagnetic forces. In the micro-magnetic im-plementation, single-domain nanometer pills-shaped magnetsexhibit two stable magnetic states, “up” and “down”, carryingthus a binary information thanks to their reciprocal influencebased on a “domino” effect. Many works have been proposedin recent years either on circuits and architectures (see for all[2]) or on detailed physical nanomagnets behavior analysis(see for an example [3]). In the former case it is shownthat, should technology be of support, computation wouldbe feasible, and many of the traditional CMOS based digitalimplementations could be adopted. In the latter case, experi-ments are carried on to demonstrate the feasibility of the ideaadopting to date production phases. Anyway, the two aspectsare seldom integrated, thus the design approach is splitted intwo averted points of view, while, as we have experiencedespecially in the recent CMOS technology success story, thestrength of a design methodology is based on the ability oflinking them as much as possible. We believe thus that, atthe light of the current scientific MQCA scenario MQCA, thetime is mature for entangling circuit design with technologywhen the aim is demonstrating the feasibility of the MQCAcomputation paradigm.In this work we show our approach for twisting computationand implementation. The idea is to assess a practicable andnot theoretical technological implementation for MQCA; toconstraint the circuit design to such a base, to solve atthe architectural level the critical limitations due to it; todescribe the circuit behavior so that implementation detailscan be taken into account, thus circuit performance can beestimated and feedbacks to technologists suggested; to stepprogressively to the physical implementation and to enrich thecircuit description with on the field data.In this paper we show the preliminary results of such approach.In section II the foundamental technological hypotesys isexplained. In section III an example of circuit descriptionis given with a specific architectural solution adopted andwith the “low-level” details added to it. In section IV thepreliminary technological implementation and performanceanalysis are described.II. A FEASIBLE THREE PHASES CLOCKIt has been demonstrated (see [4]) that for MQCA as wellas for molecular QCA, an adiabatic switching is suggestedto assure a correct information propagation. This means thatswitching of a nanomagnet from state “up” to state “down” isfavoured if an intermediate state is reached first. That is, anexternal filed is applied so that the pill “memory” (previousmagnetization state to “up” or “down”) is erased, and, at thispoint, as soon the external field is released, an input canmore easily and with lower power loss, force the new “up”or “down” to the pill. This is particularly important when theinput of nanomagnet-B is another nanomanet-A, which canforce on the coupled nanomagnet-B only a limited magneticfield due to its intrinsic characteristics (shape and material).Such external field is meant as a clock, as it is iterativelyswitched on and off and allows the evaluation phase, eventhough it has not the “traditional” function of a clock signal.One of the related aspects is the clock organization in complexcomputational structures. In this work, thus, we propose, start-ing from [5], a solution to clock distribution, “snake-clock”,which we judge more feasible for the multiple-phases clockdistribution, necessary to allow the information propagationwithout losses in complex nanomagnets arrays. First, phasesare three, differently from the four ones introduced for themolecular case: in figure 1.a the RESET, SWITCH and HOLDsequence is shown both in time and space. In figure 1.b thebehaviour of nanomagnets grouped (each clock phase shouldserve a group of pills for the unavoidable size differencebetween the pills and the line generating such a phase) inthe correspondent clock zones is depicted: when a cell groupis in the HOLD phase the stable “up” and “down” states ownthe digital information; thus they can force it toward the neargroup which is in the SWITCH state, that is, its previous statehas been previously “cancelled” thanks to a reset, and nowthey are ready to be influenced. The group in the followingregion is itself in the RESET state. In figure 1.c the top view1 32nanomagnetsoxide21 3I/Imax0+1+1+100RESETRESETRESETSWITCHSWITCHHOLDHOLDHOLDHOLDspacetimetimeRESET SWITCH HOLDHOLDHOLDHOLD RESET SWITCHRESETe)b)a)c) d)1 321 3 2Fig. 1. Proposed clock organisation: a) Threes phases sequence, b) MQCAbehaviour in three zones, c) Snake clock logic zones view, d) Snake clocklayout top view e) Snake clock physical distributionof the clock zones is sketched together with the informationflow: this style still allows the information flow in both thehorizontal and vertical directions, but, as the correct phasesequence (1,2,3 in figure) must be assured, only a “snake”like propagation is possible. Even if this seems a limitation, itis feasible with current technology, differently from previouslyproposed solutions. This is also evident in the layout andphysical views 1.d and 1.e respectively. The nanomagnetsarrays can be sandwiched between two thin oxide layers, andon the top and bottom of this structure metal wires carryingthe clock signal can be routed. One stripe (phase 1) can bestraight, while the others (phases 2 and 3) should be routed ina zig-zig style, interlaced as they were twisted but belongingto two different metal layers. Active nanomagnet pills cannotbe present in zones where metal wires are oblique.III. SNAKE-CLOCK NCL-HDL MQCA DESCRIPTIONThe proposed structure influences the circuit model wedescribe. We use VHDL, as proposed in [6] for generalQCA, to model the circuit behaviour, but we also adaptthe description specifically to the magnetic implementationand to the “snake-clock” related information propagation. Anexample is in figure 2 detail on the right, where registers areassociated to phase transitions (each register is indeed placedon a different phase zone), and combinational gates (in thiscase a Majority Voter, that is the basic QCA component) to the“computational” part of a clock zone (in this example for spacereason phase three only has a computational block). Wires andblocks are composed of arrays and structures of nanomagnetsas in the “arrow” example in figure 1.c detail.One of the most critical problem in QCA is the “lay-out=timing” one, that is the overall timing and correct be-haviour of QCA is dependent on the circuit layout. As anexample, inputs to the Majority Voter (MV) should changesynchronously to assure a correct computation. This can beeasily assured in simple circuits by equalizing the number ofmagnets carrying the three signals to the MV so that no skewis sensed. But in complex circuits this could be impossible toassure. In [7] NCL is proposed as a possible solution. Here weadapt it to the magnetic and snake-clock case with our HDLrepresentation and build a meaningful circuit to show that thesethree aspects can be well combined. The chosen circuit hereis a counter, which allows to test both the combinational andthe sequential aspects of the NCL synthesis, and, as shown infigure 2, which is asynchronous. The information propagatesfrom one NCL block to the next only when “acknowledged”,thus, even if a delay occurs as the layout is such that thenumber of magnets carrying signal F0 and F1 (inputs to thecircuit on the left) is different, the computation is correct. Morein detail, the information is not a sequence of data, but, asvisible in the simulation bottom line, is an alternative sequenceof data (D) and null state (N). This is possible as each logicsignal is coded using two bits so that for each couple a 0-0 is a null state, while a 1-0 or 0-1 is meant as a data state(logic 1 or logic 0 respectively). State 1-1 is not admitted. TheN-D-N sequence assures that before a new data is evaluated,both inputs must go to 0, independently on their delay. Thissolves the layout=timing problem. NCL blocks are registers(not the phase transition registers which are related to thesnake-clock organization and used in the HDL description)and combinational (an example of the simplest, the TH22 isin the right detail, already adapted to the magnetic-snake-clock organization). In the counter example, three registersare needed as this is a 3-bit counter, while the combinationalgates assure the correct counter evolution (they represent thefuture state network of this simple finite state machine). At theblock level point of view, the NCL organization implies thatan acknowledge is needed for an input evaluation to occur; thisis supported by the AIN and AOUT control signal included ineach register and assures that only when each block has dataready the next stage is allowed to evaluate them.Fig. 2. NCL counter architecture, simulation (delay effects in the left details) and port NCL-TH22 model for the snake clock implementation (right detail).The simulation waveforms in figure 2 show how the countingsequence is correct, even if the entire structure is organizedon phases correspondent to the the snake clock and the basiccells are reorganized and based on the MV block. Thus a fullmagnetic and snake-clock NCL structure has been demon-strated here and assures promising potentialities for furtherarchitectures developments, as it solves a critical limitation ofMQCA and is also based on a practicable clock structure.The description has been also enriched with other physicalrelated information as delay (see A and B details on thesimulation waveforms in figure 2), power dissipation basedon values in [9] and using a VHDL-AMS description (a detailof power variation during counting is in figure 3), and layoutas a dependency on the number of magnets necessary to carrya signal from a gate to the other is inserted in the model (detailin figure 3 right).We plan to further improve these descriptions related tothe physical implementation as we believe that not only thearchitecture is more physically meaningful and variations to itcan be decided on a technology basis, but also feedbacks totechnology can be generated, so that it can be evolved towardsa usable solution.Fig. 3. Power simulation for the NCL counter; detail: power dependency onthe number of magnets due to layout.This is the reason why we are setting up our own ex-periments, still at the preliminary phase at present time anddriven by results partially presented in literature (e.g. [?]),with the aim of jointly proof the feasibility of architecturesand specifying meaningful objectives to physical experiments.IV. PRELIMINARY EXPERIMENTSMQCA arrays have been produced using RF magnetronsputtering and Electron Beam Lithography (EBL). The arraysfeature a square-lattice arrangement of single elliptical cylin-ders, composed by a soft ferromagnetic material (face centeredcubic cobalt). Such arrays are realized over an electricallyconductive layer (either gold or copper) deposited on anadhesion promoter (titanium) over a stiff substrate (100 siliconwafer). The electrically conductive layer plays an importantrole both in extracting the secondary electrons generatedduring the EBL exposure and in controlling the magnetizationby the dc-current-induced Oersted field. The samples thatso far have been tested consist of 30-nm-thick Co MQCAsfeaturing various aspect ratios, ranging from 1:2 up to 1:5, witha major axis ranging from 2.5 down to 1 µm (see figure 4), tobe reduced in the near future. The aspect ratio induces differentshape magnetic anisotropies and is used either to facilitate orto hinder the magnetic switching. The magnetic configurationassociated to the logic state has been characterised by field-assisted Magnetic Force Microscopy (MFM) imaging. Themagnetic field acting on the MQCAs was produced either byan electrical current flowing in the metallic substrate or bytwo Helmholtz coils.Fig. 4. FESEM picture of a Co MQCA array precursor; thickness: 30 nm;major axis: 1 µm; minor axis: 500 nm; spacing: 5 µm along major axis, 2µm along minor axis.In our most recent analysis however we demonstrated a tip-induced magnetization reversal, with an effect dependent onthe relative orientation of the fast scan axis with respect tothe ellipses major axis. We developed a numerical algorithmin order to increase the signal to noise ratio of the MFM datamatrices and to let information connected with the magneticstatus of each elliptical pillar emerge, starting from a regis-tering routine described elsewhere [10] and implementing anew differential approach that allows to completely excludethe effects of a tip magnetization reversal occurred during thesubmission of an external magnetic field. This means that adifference in the MFM maps taken with different values ofmagnetic field may be correctly addressed either to the sampleor to the tip.The image shown in figure 5 is a collection of differentialframes of a single MQCA element taken with a fast scan axisorthogonal with respect to the major axis (a, 90o) and parallelto it (b, 0o), with an external magnetic field of −100 Oe(left elements) and +100 Oe (right elements), after registeringthe images and subtracting the zero field reference state. Themagnetization alignment with respect to the ellipse axes isdefined by the grey scale contrast anisotropy: the (a) elementsfeature a white spot respectively at bottom and on top of thepillar, while the (b) elements feature a white line runningrespectively on the right and on the left borders. This contrastcorresponds to a magnetization lying along the major axis(a) and along the minor one (b). The switching was obtainedby the application of an external magnetic field and stronglyinfluenced by the acquisition configuration.Fig. 5. Registered differential MFM frames of a single MQCA pillar; fromleft to right: 90o and −100 Oe external field, 90o and +100 Oe, 0o and−100 Oe, 0o and +100 Oe.REFERENCES[1] K.M. Fant and S.A. Brandt., NULL Convention LogicTM : “A Completeand Consistent Logic for Asynchronous Digital Circuit Synthesis”, Proc.Int. Conf. on Application Specific Systems, Architectures, and Processors,1996.[2] H. Cho and E.E. Swartzlander, “Adder Designs and Analyses forQuantum-Dot Cellular Automata”, IEEE Transactions on Nanotechnol-ogy, vol. 6, no. 3, May 2007.[3] A. Imre, G. Csabaa, G.H. Bernstein, W. Porod and V. Metlushkob,“Investigation of shape-dependent switching of coupled nanomagnets”,Superlattices and Microstructures, 34, 513-518, 2003.[4] G. Csaba and W. Porod, “Simulation of iled Coupled Computing Ar-chitectures based on Magnetic Dot Arrays”, Journal of ComputationalElectronics, Kluwer, 1:87-91, 2002[5] M.T. Alam, J.DeAngelis, M. Putney, X.S. Hu, W. Porod, M. Niemier andG.H. Bernstein, “Clock Scheme for Nanomagnet QCA”, Proc. of IEEEInt. Conf on Nanotechnology, 2007.[6] M. Ottavi, L. Schiano abd F. Lombardi, HDLQ: A HDL Environmentfor QCA Design, ACM Journal on Emerging Technologies in ComputingSystems, Vol.2, No.4, 2006[7] M. Choi, M. Choi, Z. Patiz and N. Park, “Efficient and Robust Delay-Insensitive QCA (Quantum-Dot-Cellular-Automata) Design”, Proc. IEEEInt. Symp. on Defect and Foult-Tolerance in VLSI Systems, 2006[8] S. henderson, E.W.Johnson, J.R.Janulis and P.D. Tourgaw, “IncorporatingStandard CMOS Design Process Methodologies into the QCA LogicDesign Process”, IEEE Trans. on Nanotechnology, Vol. 3, No.1, 2004.[9] G. Csaba, P. Lugli, A. Csurgay and W. Porod, “Simulation of PowerGain and Dissipation in Field-Coupled Nanomagnets”, Journal of Com-putational Electronics, Springer, Vol. 4, 2005.[10] A. Chiolerio, E. Celasco, F. Celegato, S. Guastella, P. Martino, P. Allia,P. Tiberto and F. Pirri, “Enhanced imaging of magnetic structures inmicropatterned arrays of Co dots and antidots”, J. of Magnetism andMagnetic Materials, Vol. 320, e669-e673, 2008.

A Technology Aware Magnetic QCA NCL-HDL Architecture

PORTO Publications Open Repository TOrino

A Technology Aware Magnetic QCA
NCL-HDL Architecture
Mariagrazia Graziano, Member IEEE
Dipartimento di Elettronica
Politecnico di Torino
Torino, Italy
Email: mariagrazia.graziano@polito.it
Alessandro Chiolerio
Dipartimento di Fisica
Politecnico di Torino
Torino, Italy
Email: alessandro.chiolerio@polito.it
Maurizio Zamboni
Dipartimento di Elettronica
Politecnico di Torino
orino, Italy
Email: maurizio.zamboni@polito.it
Abstract—Magnetic Quantum Dot Cellular Automata (MQCA)
have been recently proposed as an attractive implementation
of QCA as a possible CMOS technology substitute. Marking a
difference with respect to previous contributions, in this work we
show that it is possible to develop and describe complex MQCA
computational blocks strongly linking technology and having
in mind a feasible realization. Thus, we propose a practicable
clock structure for MQCA baptised “snake-clock”, we stick
to this while developing a system level Hardware Description
Language (HDL) based description of an architectural block,
and we suggest a delay insensitive Null Convention LogicTM
(NCL, [1]) implementation for the magnetic case so that the
“layout=timing” problem can be solved. Furthermore we include
in our model aspects critically related to technology and real
production, that is timing, power and layout, and we present the
preliminary steps of our experiments, the results of which will
be included in the architecture description.
I. INTRODUCTION AND MOTIVATION
QCA, in their general meaning, are bistable cells coupled
through electromagnetic forces. In the micro-magnetic im-
plementation, single-domain nanometer pills-shaped magnets
exhibit two stable magnetic states, “up” and “down”, carrying
thus a binary information thanks to their reciprocal influence
based on a “domino” effect. Many works have been proposed
in recent years either on circuits and architectures (see for all
[2]) or on detailed physical nanomagnets behavior analysis
(see for an example [3]). In the former case it is shown
that, should technology be of support, computation would
be feasible, and many of the traditional CMOS based digital
implementations could be adopted. In the latter case, experi-
ments are carried on to demonstrate the feasibility of the idea
adopting to date production phases. Anyway, the two aspects
are seldom integrated, thus the design approach is splitted in
two averted points of view, while, as we have experienced
especially in the recent CMOS technology success story, the
strength of a design methodology is based on the ability of
linking them as much as possible. We believe thus that, at
the light of the current scientific MQCA scenario MQCA, the
time is mature for entangling circuit design with technology
when the aim is demonstrating the feasibility of the MQCA
computation paradigm.
In this work we show our approach for twisting computation
and implementation. The idea is to assess a practicable and
not theoretical technological implementation for MQCA; to
constraint the circuit design to such a base, to solve at
the architectural level the critical limitations due to it; to
describe the circuit behavior so that implementation details
can be taken into account, thus circuit performance can be
estimated and feedbacks to technologists suggested; to step
progressively to the physical implementation and to enrich the
circuit description with on the field data.
In this paper we show the preliminary results of such approach.
In section II the foundamental technological hypotesys is
explained. In section III an example of circuit description
is given with a specific architectural solution adopted and
with the “low-level” details added to it. In section IV the
preliminary technological implementation and performance
analysis are described.
II. A FEASIBLE THREE PHASES CLOCK
It has been demonstrated (see [4]) that for MQCA as well
as for molecular QCA, an adiabatic switching is suggested
to assure a correct information propagation. This means that
switching of a nanomagnet from state “up” to state “down” is
favoured if an intermediate state is reached first. That is, an
external filed is applied so that the pill “memory” (previous
magnetization state to “up” or “down”) is erased, and, at this
point, as soon the external field is released, an input can
more easily and with lower power loss, force the new “up”
or “down” to the pill. This is particularly important when the
input of nanomagnet-B is another nanomanet-A, which can
force on the coupled nanomagnet-B only a limited magnetic
field due to its intrinsic characteristics (shape and material).
Such external field is meant as a clock, as it is iteratively
switched on and off and allows the evaluation phase, even
though it has not the “traditional” function of a clock signal.
One of the related aspects is the clock organization in complex
computational structures. In this work, thus, we propose, start-
ing from [5], a solution to clock distribution, “snake-clock”,
which we judge more feasible for the multiple-phases clock
distribution, necessary to allow the information propagation
without losses in complex nanomagnets arrays. First, phases
are three, differently from the four ones introduced for the
molecular case: in figure 1.a the RESET, SWITCH and HOLD
sequence is shown both in time and space. In figure 1.b the
behaviour of nanomagnets grouped (each clock phase should
serve a group of pills for the unavoidable size difference
between the pills and the line generating such a phase) in
the correspondent clock zones is depicted: when a cell group
is in the HOLD phase the stable “up” and “down” states own
the digital information; thus they can force it toward the near
group which is in the SWITCH state, that is, its previous state
has been previously “cancelled” thanks to a reset, and now
they are ready to be influenced. The group in the following
region is itself in the RESET state. In figure 1.c the top view
1 3
2
nanomagnets
oxide
21 3
I/I
m
ax
0
+1
+1
+1
0
0
RESET
RESET
RESET
SWITCH
SWITCH
HOLD
HOLD
HOLD
HOLD
sp
ac
e
timetime
RESET SWITCH HOLD
HOLDHOLD
HOLD RESET SWITCH
RESET
e)
b)a)
c) d)
1 32
1 3 2
Fig. 1. Proposed clock organisation: a) Threes phases sequence, b) MQCA
behaviour in three zones, c) Snake clock logic zones view, d) Snake clock
layout top view e) Snake clock physical distribution
of the clock zones is sketched together with the information
flow: this style still allows the information flow in both the
horizontal and vertical directions, but, as the correct phase
sequence (1,2,3 in figure) must be assured, only a “snake”
like propagation is possible. Even if this seems a limitation, it
is feasible with current technology, differently from previously
proposed solutions. This is also evident in the layout and
physical views 1.d and 1.e respectively. The nanomagnets
arrays can be sandwiched between two thin oxide layers, and
on the top and bottom of this structure metal wires carrying
the clock signal can be routed. One stripe (phase 1) can be
straight, while the others (phases 2 and 3) should be routed in
a zig-zig style, interlaced as they were twisted but belonging
to two different metal layers. Active nanomagnet pills cannot
be present in zones where metal wires are oblique.
III. SNAKE-CLOCK NCL-HDL MQCA DESCRIPTION
The proposed structure influences the circuit model we
describe. We use VHDL, as proposed in [6] for general
QCA, to model the circuit behaviour, but we also adapt
the description specifically to the magnetic implementation
and to the “snake-clock” related information propagation. An
example is in figure 2 detail on the right, where registers are
associated to phase transitions (each register is indeed placed
on a different phase zone), and combinational gates (in this
case a Majority Voter, that is the basic QCA component) to the
“computational” part of a clock zone (in this example for space
reason phase three only has a computational block). Wires and
blocks are composed of arrays and structures of nanomagnets
as in the “arrow” example in figure 1.c detail.
One of the most critical problem in QCA is the “lay-
out=timing” one, that is the overall timing and correct be-
haviour of QCA is dependent on the circuit layout. As an
example, inputs to the Majority Voter (MV) should change
synchronously to assure a correct computation. This can be
easily assured in simple circuits by equalizing the number of
magnets carrying the three signals to the MV so that no skew
is sensed. But in complex circuits this could be impossible to
assure. In [7] NCL is proposed as a possible solution. Here we
adapt it to the magnetic and snake-clock case with our HDL
representation and build a meaningful circuit to show that these
three aspects can be well combined. The chosen circuit here
is a counter, which allows to test both the combinational and
the sequential aspects of the NCL synthesis, and, as shown in
figure 2, which is asynchronous. The information propagates
from one NCL block to the next only when “acknowledged”,
thus, even if a delay occurs as the layout is such that the
number of magnets carrying signal F0 and F1 (inputs to the
circuit on the left) is different, the computation is correct. More
in detail, the information is not a sequence of data, but, as
visible in the simulation bottom line, is an alternative sequence
of data (D) and null state (N). This is possible as each logic
signal is coded using two bits so that for each couple a 0-
0 is a null state, while a 1-0 or 0-1 is meant as a data state
(logic 1 or logic 0 respectively). State 1-1 is not admitted. The
N-D-N sequence assures that before a new data is evaluated,
both inputs must go to 0, independently on their delay. This
solves the layout=timing problem. NCL blocks are registers
(not the phase transition registers which are related to the
snake-clock organization and used in the HDL description)
and combinational (an example of the simplest, the TH22 is
in the right detail, already adapted to the magnetic-snake-
clock organization). In the counter example, three registers
are needed as this is a 3-bit counter, while the combinational
gates assure the correct counter evolution (they represent the
future state network of this simple finite state machine). At the
block level point of view, the NCL organization implies that
an acknowledge is needed for an input evaluation to occur; this
is supported by the AIN and AOUT control signal included in
each register and assures that only when each block has data
ready the next stage is allowed to evaluate them.
Fig. 2. NCL counter architecture, simulation (delay effects in the left details) and port NCL-TH22 model for the snake clock implementation (right detail).
The simulation waveforms in figure 2 show how the counting
sequence is correct, even if the entire structure is organized
on phases correspondent to the the snake clock and the basic
cells are reorganized and based on the MV block. Thus a full
magnetic and snake-clock NCL structure has been demon-
strated here and assures promising potentialities for further
architectures developments, as it solves a critical limitation of
MQCA and is also based on a practicable clock structure.
The description has been also enriched with other physical
related information as delay (see A and B details on the
simulation waveforms in figure 2), power dissipation based
on values in [9] and using a VHDL-AMS description (a detail
of power variation during counting is in figure 3), and layout
as a dependency on the number of magnets necessary to carry
a signal from a gate to the other is inserted in the model (detail
in figure 3 right).
We plan to further improve these descriptions related to
the physical implementation as we believe that not only the
architecture is more physically meaningful and variations to it
can be decided on a technology basis, but also feedbacks to
technology can be generated, so that it can be evolved towards
a usable solution.
Fig. 3. Power simulation for the NCL counter; detail: power dependency on
the number of magnets due to layout.
This is the reason why we are setting up our own ex-
periments, still at the preliminary phase at present time and
driven by results partially presented in literature (e.g. [?]),
with the aim of jointly proof the feasibility of architectures
and specifying meaningful objectives to physical experiments.
IV. PRELIMINARY EXPERIMENTS
MQCA arrays have been produced using RF magnetron
sputtering and Electron Beam Lithography (EBL). The arrays
feature a square-lattice arrangement of single elliptical cylin-
ders, composed by a soft ferromagnetic material (face centered
cubic cobalt). Such arrays are realized over an electrically
conductive layer (either gold or copper) deposited on an
adhesion promoter (titanium) over a stiff substrate (100 silicon
wafer). The electrically conductive layer plays an important
role both in extracting the secondary electrons generated
during the EBL exposure and in controlling the magnetization
by the dc-current-induced Oersted field. The samples that
so far have been tested consist of 30-nm-thick Co MQCAs
featuring various aspect ratios, ranging from 1:2 up to 1:5, with
a major axis ranging from 2.5 down to 1 µm (see figure 4), to
be reduced in the near future. The aspect ratio induces different
shape magnetic anisotropies and is used either to facilitate or
to hinder the magnetic switching. The magnetic configuration
associated to the logic state has been characterised by field-
assisted Magnetic Force Microscopy (MFM) imaging. The
magnetic field acting on the MQCAs was produced either by
an electrical current flowing in the metallic substrate or by
two Helmholtz coils.
Fig. 4. FESEM picture of a Co MQCA array precursor; thickness: 30 nm;
major axis: 1 µm; minor axis: 500 nm; spacing: 5 µm along major axis, 2
µm along minor axis.
In our most recent analysis however we demonstrated a tip-
induced magnetization reversal, with an effect dependent on
the relative orientation of the fast scan axis with respect to
the ellipses major axis. We developed a numerical algorithm
in order to increase the signal to noise ratio of the MFM data
matrices and to let information connected with the magnetic
status of each elliptical pillar emerge, starting from a regis-
tering routine described elsewhere [10] and implementing a
new differential approach that allows to completely exclude
the effects of a tip magnetization reversal occurred during the
submission of an external magnetic field. This means that a
difference in the MFM maps taken with different values of
magnetic field may be correctly addressed either to the sample
or to the tip.
The image shown in figure 5 is a collection of differential
frames of a single MQCA element taken with a fast scan axis
orthogonal with respect to the major axis (a, 90o) and parallel
to it (b, 0o), with an external magnetic field of −100 Oe
(left elements) and +100 Oe (right elements), after registering
the images and subtracting the zero field reference state. The
magnetization alignment with respect to the ellipse axes is
defined by the grey scale contrast anisotropy: the (a) elements
feature a white spot respectively at bottom and on top of the
pillar, while the (b) elements feature a white line running
respectively on the right and on the left borders. This contrast
corresponds to a magnetization lying along the major axis
(a) and along the minor one (b). The switching was obtained
by the application of an external magnetic field and strongly
influenced by the acquisition configuration.
Fig. 5. Registered differential MFM frames of a single MQCA pillar; from
left to right: 90o and −100 Oe external field, 90o and +100 Oe, 0o and
−100 Oe, 0o and +100 Oe.
REFERENCES
[1] K.M. Fant and S.A. Brandt., NULL Convention LogicTM : “A Complete
and Consistent Logic for Asynchronous Digital Circuit Synthesis”, Proc.
Int. Conf. on Application Specific Systems, Architectures, and Processors,
1996.
[2] H. Cho and E.E. Swartzlander, “Adder Designs and Analyses for
Quantum-Dot Cellular Automata”, IEEE Transactions on Nanotechnol-
ogy, vol. 6, no. 3, May 2007.
[3] A. Imre, G. Csabaa, G.H. Bernstein, W. Porod and V. Metlushkob,
“Investigation of shape-dependent switching of coupled nanomagnets”,
Superlattices and Microstructures, 34, 513-518, 2003.
[4] G. Csaba and W. Porod, “Simulation of iled Coupled Computing Ar-
chitectures based on Magnetic Dot Arrays”, Journal of Computational
Electronics, Kluwer, 1:87-91, 2002
[5] M.T. Alam, J.DeAngelis, M. Putney, X.S. Hu, W. Porod, M. Niemier and
G.H. Bernstein, “Clock Scheme for Nanomagnet QCA”, Proc. of IEEE
Int. Conf on Nanotechnology, 2007.
[6] M. Ottavi, L. Schiano abd F. Lombardi, HDLQ: A HDL Environment
for QCA Design, ACM Journal on Emerging Technologies in Computing
Systems, Vol.2, No.4, 2006
[7] M. Choi, M. Choi, Z. Patiz and N. Park, “Efficient and Robust Delay-
Insensitive QCA (Quantum-Dot-Cellular-Automata) Design”, Proc. IEEE
Int. Symp. on Defect and Foult-Tolerance in VLSI Systems, 2006
[8] S. henderson, E.W.Johnson, J.R.Janulis and P.D. Tourgaw, “Incorporating
Standard CMOS Design Process Methodologies into the QCA Logic
Design Process”, IEEE Trans. on Nanotechnology, Vol. 3, No.1, 2004.
[9] G. Csaba, P. Lugli, A. Csurgay and W. Porod, “Simulation of Power
Gain and Dissipation in Field-Coupled Nanomagnets”, Journal of Com-
putational Electronics, Springer, Vol. 4, 2005.
[10] A. Chiolerio, E. Celasco, F. Celegato, S. Guastella, P. Martino, P. Allia,
P. Tiberto and F. Pirri, “Enhanced imaging of magnetic structures in
micropatterned arrays of Co dots and antidots”, J. of Magnetism and
Magnetic Materials, Vol. 320, e669-e673, 2008.


Adder Designs and Analyses for Quantum-Dot Cellular Automata”,

Clock Scheme for Nanomagnet QCA”,

Efﬁcient and Robust DelayInsensitive QCA (Quantum-Dot-Cellular-Automata) Design”,

Enhanced imaging of magnetic structures in micropatterned arrays of Co dots and antidots”,

HDLQ: A HDL Environment for QCA Design,

Investigation of shape-dependent switching of coupled nanomagnets”,

NULL Convention LogicTM: “A Complete and Consistent Logic for Asynchronous Digital Circuit Synthesis”,

Simulation of iled Coupled Computing Architectures based on Magnetic Dot Arrays”,

Simulation of Power Gain and Dissipation in Field-Coupled Nanomagnets”,

https://iris.polito.it/retrieve/handle/11583/2279307/51607/magnetic-QCA-VHDL.pdf

A Technology Aware Magnetic QCA NCL-HDL Architecture

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