Nanoscale molecularswitch crossbar
circuits
Group 2
J. R. Edwards
Pierre Emelie
Mike Logue
Zhuang Wu
Outline
Introduction and basic principles of
crossbar circuits
 Growth techniques
 Current research results
 Summary
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Introduction
Nearly a billion transistors on a silicon chip
 Lengths of the smallest chip will shrink to
nearly the molecular scale
 Major innovations are needed to reach a
length for functional features around 10 nm
(30 atoms long)
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Quantum Computing
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A classical computer has a memory made up of bits
A quantum computer maintains a set of qubits and operates by
manipulating them
A qubit can hold a “1”, a “0” or a superposition of these
Qubits can be implemented using the two spin states of an electron
If large-scale quantum computers can be built, they will be able to
solve certain problems faster than any classical computer
It is however decades away from realization
It remains unclear how useful it would be for most applications
Several groups are investigating another path (one group at the
Hewlett-Packard Laboratories was the precursor)
The crossbar architecture
Crossbar Architecture
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Configurable crossbar
architecture
Array of crossing
nanowires
Switch is formed at the
junction between two
crossing nanowires
Nanowires are separated
by a single monolayer of
molecules
Crossbar Architecture
High resistance state
“OFF” state
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Low resistance state
“ON” state
VT=0.2 V
Voltage limit |VT|<2 V
The switch remains in the
state it was last set
Positive (negative) cycling
voltages reversibly switch
the device to the “ON”
(“OFF”) state
Crossbar Architecture
“OFF” state (ohmic response)
Rd=9.2x106 Ω
VT=0.2 V
Curves are
offset for clarity
“ON” state (ohmic
response)
Clockwise IV
hysteresis
Rd=4.8x105 Ω
Counter-clockwise
IV hysteresis
“OFF” state (ohmic response)
Rd=8.1x106 Ω
Crossbar Architecture
Molecular structure of the bistable
[2]rotaxane R
Imprint lithography is used to
grow these molecular-switch
devices
Crossbar Architecture
How do we control these crossbars and link them with
external systems in order to perform memory and/or logic
functions?
 By using micron-scale silicon ICs
How do we bridge the gaps in size and number of wires
between nanoelectronics and the conventional-scale
silicon ICs?
=> By using a demultiplexer
Demultiplexer
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Demultiplexer enables
conventional wires on silicon
chips to control a great
number of nanowires
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If k is the number of
conventional wires, the
multiplexer can control 2k
nanowires
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An additional d conventional
wires provide redundancy to
work despite broken
connections
Applications and Advantages
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This configurable architecture can be used to perform memory
and/or logic function
The wires can be scales continuously down to molecular sizes while
the number of wires in the crossbar can be scaled up arbitrarily to
form large-scale generic circuits
It requires only 2N communication wires to address 2N nanowires
This allows the nano-circuit to communicate efficiently with external
circuits
It can tolerate defective elements generated during the fabrication
process by introducing redundancy
Fabrication is feasible and potentially inexpensive
Next: More details on the fabrication process and some results will be
presented
Fabrication Overview
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Pattern bottom
electrode and deposit
rotaxane monolayer
switching material
Protective Ti layer
evaporated onto film
Pattern top electrode
Remove excess
Why Nanoimprint Lithography
Pattern small feature sizes
 High throughput
 Low cost
 Precludes damage to sensitive
components
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Nanoimprint Lithography (NIL)
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But at smaller feature
sizes, resist adheres
to mold.
Double-layer UV-curable NIL
Double-layer UV-curable resist
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Highly cross-linked top
imaging layer,
mechanically strong
Bottom transfer layer with
good liftoff, also serves to
planarize the surface
Exceptional thickness
uniformity
Thickness distribution of
spin-coated UV-curable
imprint resist over a 4
inch wafer.
Double-layer UV-curable NIL
The Mold
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Build out of silicon substrate using e-beam and optical lithography
Benefits
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Hundreds of circuit patterns per mold: increases throughput because
large number of circuits created with one imprinting step
 Reusable: reduces costs
 Mechanical mechanism precludes damage
Double-layer UV-curable NIL
Imprint and Curing
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Mold pressed onto
resist layer with
homogeneous
pressure (500psi)
Heated to 80 C to
cross-link imaging
layer
Double-layer UV-curable NIL
Reactive Ion Etching (RIE)
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Etch down to
substrate layer
Oxygen RIE
Selectivity of greater
than 10 between
imaging and transfer
layer---gives margin
in over-etching
RIE
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Reactive gas and
accelerated ions increase
etch rate
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Etch product may form
passivation layer on the
side wall---preventing
lateral etching.
Anisotropic
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Double-layer UV-curable NIL
Metal and Liftoff
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Metal (Ti and Au, Pt)
evaporated onto
pattern
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Solubility of transfer
layer provides good
liftoff of resist
Wetting problem
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Adhesion and Wetting
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Hydrophobic imaging
layer reduces adhesion
forces between mold and
resist but also prevents
solvent from getting into
the feature gaps
Solution: treat the surface
with O2 plasma to
improve wetting property
of the resist.
Nanoscale cross-bar circuits
Write a bit
A positive voltage ranging from 3.5 to 7 V
would turn it ‘on’, and a negative voltage
ranging from −3.5 to −7 V would switch it ‘off’.
A voltage bias |V| < 3.5 V applied to the
devices did not change their resistance state
1)
2)
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“0”---- 106—5X108Ω
“1”---- 4X109Ω and up
Write at the cross point
Keep the voltage at other cross
points so that the error rate is
reduced
Read a bit
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a bias voltage (much smaller than the voltage used to write the bit)
was applied across the row and column of the bit to be read (e.g.
row A and column 1 for reading cross point (1, A) in figure 3(a)), but
all of the other rows and columns were grounded.
Resistivity
Reading current
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Max reading
current is of the
order of 10-10A
Results for 1kbits memory circuits
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More condensed
nanowires
Read current
Electronic characteristics
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it shows the process of
turning on and off.
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the turning on and off
processes don’t overlap
with each other.
Limitations
The current crossbar memory technology
does not allow for a very large number of
write cycles
 A voltage magnitude, |V| >=3.5 is needed
to change the resistance state between ‘0’
and ‘1’
 Lithography used in electrode and
connection fabrication
 Nanowires are so small that atomic
defects are unavoidable and serious
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Limitations
Resolution is limited currently to about
30nm half-pitch
 Have to build in redundancy to
compensate for defects
 Range of logic operations that can be
performed is limited without the NOT
function
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Conclusions
Nanoscale crossbar structures show
potential in developing new
nanoelectronics, especially high density
memory and logic
 It will take many years before the
manufacturing technology reaches the
point where the full potential of these
structures can be realized
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Future Work
Defect Tolerance
 Improvement of nanofabrication
technology for more reliable, higher
density crossbar structures
 Look at what materials are best suited for
these structures
 Increase logic capabilities of the crossbar
structure
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Summary
Nanoscale molecular crossbar circuits can
function as ultra high density memory
 Demultiplexer/multiplexer logic can be
integrated with the memory using the
crossbar structure
 There is a lot of interest in the use of
nanoscale crossbar structures for use in
high density nanoelectronics
 More work has to be done to achieve full
potential of the crossbar structure
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