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Proceedings of the 7th IEEE
International Conference on Nanotechnology
August 2 - 5, 2007, Hong Kong
Spin-MTJ based Non-Volatile Flip-Flop
Weisheng Zhao, Student Member, IEEE, Eric Belhaire, Member, IEEE and Claude Chappert
the MTJ with currents compatible with standard logic gate
close the minimum size in advanced CMOS technologies.
Abstract— Spin Transfer Torque (STT) writing approach based
Magnetic Tunnel Junction (Spin-MTJ) is the excellent candidate
to be used as Spintronics device in Magnetic RAM (MRAM) and
Magnetic Logic. We present the first Non-volatile Flip-Flop based
on this device for Field Programmable Gate Array (FPGA) and
System On Chip (SOC) circuits, which can make these circuits
fully non-volatile by storing permanently all the data processed in
the Spin-MTJ memory cells. The non-volatility enables logic
circuits to decrease significantly the start-up latency of these
circuits from some micro seconds down to some hundred pico
seconds. By using STMicroelectronics 90nm CMOS technology
and a behavior Spin-MTJ simulation Model in Verilog-A
language, this non-volatile Flip-Flop has been demonstrated that
it works not only in very high speed or low propagation delay, but
also keeps low power dissipation and small cell surface.
Fig.1 Magnetic Tunnel Junction is composed of three principle layers:
an oxidation barrier, such as MgO and AlxOy; a Pinned layer and a
storage layer, which are ferromagnetic Materiaux (e.g.CoFe). The
spin direction in pinned layer is fixed, but can be changed in the
storage layer, there are so that two states of MTJ: parallel and antiparallel when the spin direction in pinned layer and free layer are in
the same direction or the opposite direction.
Index Terms—Magnetic, MRAM, Non-volatile, High speed,
Low power, SOC, FPGA, Flip-Flop, Spin-MTJ, SMNFF
I.
INTRODUCTION
SRAM based Flip-Flop is widely used as internal memory
to store and synchronize the data processed in the Field
Programmable Gate Array (FPGA) and System On Circuit
(SOC) circuits, but as the SRAM is volatile, all the data stored
in these Flip-Flops is lost when the power goes down, FlipFlop based on non-volatile memory is thus required to protect
these data from system crashes and power failures to improve
the data security and performance of FPGA and SOC circuits.
Contrary to memory plane cells, Flip-Flops work at very high
frequency [1] and the Flash and Phase Change Memory
(PCRAM) technologies can thus hardly be used here, while
Magnetic memory (MRAM) seems the best candidate thanks
to its high writing/reading speed and infinite endurance.
MRAM is based on Magnetic Tunnel Junction (MTJ)
technology, as represented in Fig.1 [2]. A MTJ device has two
permanent states with different resistance values which can
easily be read by a CMOS Sensing Amplifier circuit (S.A).
Recent rapid progresses in the MTJ writing techniques,
especially the Spin Transfer Torque (STT) technology (SpinMTJ) [3], and the use of MgO barrier [4] now allows to write
This work was supported in part by the European Community under the
sixth Framework, Contract Number 510993: MAGLOG. The views expressed
are solely those of the authors, and the other Contractors and/or the European
Community cannot be hold liable for any use that may be made of the
information contained herein.
1
IEF, Univ Paris-Sud, UMR 8622, Orsay, F-91405
2
CNRS, Orsay, F-91405
*Contacting Author: Weisheng Zhao is with Univ Paris-Sud, Bat220,
Orsay, France, 91405 (Email:[email protected], Phone: +33-0169155251,
Fax: +33-0169154000)
1-4244-0608-0/07/$20.00 © 2007 IEEE.
In this paper, we introduce Spin-MTJ based Non-Volatile
Flip-Flop (SMNFF). In the second section, the Spin-MTJ
device property, the working mode of Spin Transfer Torque
writing approach and its integration with CMOS technology
are presented. SMNFF circuit design and the evaluation of its
characteristics in the aspect of stability, speed and surface are
presented in the third section. The fourth section presents some
electrical simulation results of SMNFF based on 90nm CMOS
technology and a complete MTJ simulation model [5]. This
model is based on Spin Transfer Torque writing approach and
MgO barrier.
II. SPIN-MTJ AND HYBRID MTL/CMOS DESIGN
Spin Transfer Torque (STT) writing phenomena [6] has
been observed in MTJ whose length or width is inferior to
100nm. A current, passing through the MTJ, superior to the
density threshold Ic, can switch the MTJ from one state to the
other, while a current superior to Ic but in the opposite
direction can switch it back (see Fig.2). Because Spin-MTJ cell
surface is very small (e.g. 75nm×113nm), the threshold current
can be as low as 221uA [3]. STT writing approach resolves
some major disadvantages of conventional Field Induced
Magnetic Switching (FIMS) writing mode, such as high power
dissipation, easy selection disturbance and large transistors in
of the CMOS writing circuit etc.
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III. SPIN-MTJ BASED NON-VOLATILE FLIP-FLOP
(SMNFF)
When used in Flip-Flop circuit, the MTJs are spread on the
entire chip surface. So, SRAM based sense amplifier,
represented in Fig.4, is used to sense the state of MTJs. This
sensing structure is much simpler and less precise than a
standard MRAM sense amplifier and so two complementary
MTJs are used per bit for more robustness. The cell sense the
magnetic information by briefly turning on the NMOS
transistor MN2 switch and then promptly turning it off (“SEN”
is the reading control signal). This structure has been
demonstrated to read the states of MTJs in very high speed
about 200 ps [10], which allows the FPGA circuits to realize a
real “instant-on”. The use of MgO barrier in MTJs makes this
sense amplifier robust to CMOS process variation and
mismatch. As represented in Fig.5, Monte-Carlo statistic
simulations with pessimistic mismatch parameters has shown
an error percentage inferior to 0.1% if the TMR of Spin-MTJ
is about 230% and all the transistors in the sense amplifier are
in the minimum dimension.
Fig.2 The Spin-MTJ state changes from Parallel (P) to Anti-parallel
(AP) if the positive direction current density I>Ic, on the contrast, its
state will return if the negative direction current density I > Ic.
As mentioned in the first section, Spin-MTJ shows different
resistance between its two states (low resistance in parallel P
state and high resistance in anti-parallel AP state). The ratio
between these two resistance values, named Tunnel
MagnetoResistance (TMR) [2] is an important factor for
design of the reading circuit. TMR as high as 230% has been
obtained at the room temperature with MgO barriers [4] while
it was limited to around 70% with AlxOy barriers [7]. The
high TMR enables the sensing of the different states in lower
disturbance of process variation and mismatch.
Combining the STT effect and MgO barrier, the Spin-MTJ
has the excellent writing and reading performance and makes
hybrid MTJ/CMOS logic circuit design much more compact.
In the writing circuit, a bi-direction current source with the
signal “EN” is required to generate the currents to switch the
states of MTJs; in the reading circuit, a sense amplifier is used
to identify the state registered in the MTJs and export the logic
information '1' or '0'.
Fig.4 SRAM based sense amplifier used in the SMNFF
Error percentage of Mismatch %
250
Fig.3 Spin-MTJ memory cells are implemented above the CMOS
circuits
200
150
100
50
0
Another advantage of this hybrid Spin-MTJ/CMOS design
is that the storage element MTJ does not take much die area,
because it is processed over the chip surface (see Fig.3).
Unlike the conventional FIMS-MTJ, which requires very high
current (>20mA) [8] and thus impose the use of thick metal
layers to avoid the electromigration effect, a thin metal layer
can be used to connect the electrode of Spin-MTJ. This makes
that the Spin-MTJ/CMOS technology is less costly.
0
50
1 00
1 50
20 0
25 0
TM R %
Fig.5 The error percentage of mismatch decreases down to 0.1% if
MgO is used as barrier in MTJs, which can produce the TMR up to
230% maximum at room temperature.
The Spin Transfer Torque (STT) writing approach needs a
bi-directional current to write the information in the MTJs, so
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that a bi-directional current generator is used in this SMNFF,
as represented in Fig. 6. Four transistors NMOS MN3-6
generate the current and every time two of them are active, for
example, MN3, MN6 are active and MN4, MN5 are inactive,
the current passing through the two MTJs will be from the
right to the left, contrarily the current will pass from the left to
the right; two logic gates NOR control how to activate the four
transistors. The signal “EN” is required to enable this current
source and reduce the power dissipation as the circuit is in
static mode. “Input” determines the current direction and
writes the couple of MTJs. The low writing current required in
STT writing approach allows to use small size transistors in
this current source.
The SRAM based sense amplifier use the minimum
transistors in width, here 0.12um; because of the low threshold
current also the current source occupy also a small surface and
the total surface of the SMNFF cell is as small as
5.65um×10.15um. Its layout is shown in Fig.8. Metal 1-3
layers are used to connect the transistors and Metal 4 layers are
connected with the electrode of MTJs. Contrary to the FIMS
writing approach, here a low inter disturbance is found
between two MTJs and their distance can be small barely
affecting the final layout.
Fig.8 Full layout (5.65um×10.15um) of Spin-MTJ based Flip-Flop,
MTJs are placed above the two points ML and MR, see also Fig.2
IV. THE SIMULATION OF SMNFF
Fig.6 Bi-directional current source used in the SMNFF
By using STMicroelectronics 90nm CMOS low power
technology and a behavior Spin-MTJ simulation model [5],
under the Cadence Spectre 5.0.32 simulator environment and
1.2v power supply, the simulations of SMNFF have been done
[see Fig.9 and Fig.10]. The width of four NMOS transistors
MN3-6 in the writing circuit is 1um and all the others
transistors are with the minimum dimension: 0.12um. The
TMR of MTJ0-1 is 170%, the height of barrier between two
ferromagnetic layers is 1.2nm and the surface is 75nm*113nm
in the Spin-MTJ model. The simulation results demonstrate
that this Flip-Flop stores and synchronizes well the input data
as classical Master-Slave Flip-Flop, moreover, the propagation
delay of SMNFF is as low as about 300ps, which includes the
input set-up time, sense amplifier reading time, logic delay and
the Spin-MTJ writing time. This last time is very rapid and
writing times around 100ps has been observed with a
significant switching current [11], therefore SMNFF can work
in high speed up to 3.3GHz maximum. The non-volatility of
SMNFF enables to decrease largely the start-up latency of
FPGA and SOC circuits from some micro seconds down to
some hundred pico seconds [10]. By allowing frequent power
down of the circuits, it would allow to reduce significantly the
standby power dissipation which takes more and more
proportion in the total power dissipation as the minimizing
development of CMOS technology [12].
Fig.7 Full schematic of Spin-MTJ based Non-Volatile Flip-Flop
Combining the SRAM based sense amplifier with the bidirectional current source, the full schematic of the SMNFF is
represented in Fig.7.An additional NMOS transistor MN7 is
introduced to switch between the writing and reading mode
and a slave classical SRAM based register is connected at the
output of sense amplifier. The writing and reading process of
SMNFF is controlled by the signal “Clk”, when it is active as
‘1’, the input data is stored in the couple of MTJs (e.g. MTJ1:
P state and MTJ0: AP state), meanwhile the slave register
keeps the precedent data; when it is ‘0’, the sense amplifier
reads the data stored and the slave register becomes transparent
and updates its output data from Qm.
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aviation and space where the security of information is one of
the most important considerations.
VI. ACKNOWLEDGMENT
The authors would like to thank Dr. P. Mazoyer and F.
Jacquet of STMicroelectronics for the CMOS 90nm
technology design kit, their support and some valuable
suggestions.
REFERENCES
Fig.9 Simulation of Spin-MTJ based Flip-Flop; Qm is the output of
sense amplifier. (Clk: 1GHz and Input data: 500MHz)
Fig.10 Simulation of Spin-MTJ based Flip-Flop; the points A
(10.091ns) and B (10.396ns) show that the propagation delay is abut
300 ps.
V. CONCLUSIONS AND PERSPECTIVES
Spin-MTJ based Non-Volatile Flip-Flop is presented in this
paper. This technology is very promising and would allow
logic circuit to work in low power dissipation and high speed
with the same surface than SRAM based Flip-Flop.
Nevertheless, all the results obtained here are still from the
simulations, the first SMNFF demonstrator with CMOS 90nm
low power process is under design in our laboratory in
cooperation with STMicroelectronics and the University of
Bielefeld. As the MTJ is intrinsically radiation-hard and nonvolatile, the application of SMNFF in FPGA and SOC
(System-on-chip) then makes these chips non-volatile and
secure, therefore it could be advantageously used in the field of
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