3866 IEEE TRANSACTIONS ON MAGNETICS, VOL. 49, NO. 7, JULY 2013
MgO Magnetic Tunnel Junction Electrical Current SensorWith Integrated Ru Thermal Sensor
António Lopes , Susana Cardoso , Ricardo Ferreira , Elvira Paz , Francis L. Deepak , Jaime Sánchez ,Diego Ramírez , Sergio I. Ravelo , and Paulo P. Freitas
Instituto de Sistemas e Com putadores-Microsistemas e Nanotecnologias (INESC-MN) and IN, Lisbon 1000-029, PortugalPhysics Department, Instituto Superior Técnico (IST), Lisboa 1000-029, PortugalIberian International Nanotechnology Laboratory (INL), 4715-31 Braga, Portugal
Department of Electronic Engineering, University of Valencia, Valencia, 46100–Burjassot, Spain
Full Wheatstone bridge electrical current sensor incorporating 114 MgO-based magnetic tunnel junction elementsconnected in series was produced for improved electrical robustness. To that end, magnetic tunnel junctions with ,tunnelingmagnetoresistance of 200%, were produced. The sensor was designed with an integrated Ru thin film resistive thermal detector(RTD) for temperature drift monitoring and compensation. In order to achieve a full bridge signal, a U-shaped copper trace was placedunder a printed circuit board (PCB) specifically designed for this type of device. The resulting device exhibit sensitivities of 63.9 V/Oe/Ain a 75 Oe linear range biased with 1 mA current, providing a significantly advantageous alternative to AMR and GMR based bridges.
Index Terms—Magnetic tunnel junction, magnetoresistive sensor, Ru thin film temperature sensor, thermal drift, wheatstone bridgecurrent sensor.
I. INTRODUCTION
T HE ability to monitor electrical currents in power elec-tronics systems is crucial. However, the three common
choices for current measuring (resistive shunts, current trans-formers and Hall effect-based sensors) may not be suitable tointegrate in the future generation of integrated power electronicsmodules (IPEMs). They show the following disadvantages: ina shunt resistor, though small in size and inexpensive, requiresgalvanic insulation and has large insertion loss, which can beexpensive to operate; a current transformer usually needs ironcores and has large electromagnetic interference (EMI), beingan unlikely candidate for integration, deals only with ac cur-rents; a Hall Effect sensor also needs iron cores and has largeEMI [1].Nowadays magnetic tunnel junctions (MTJs) based de-
vices have become a reliable choice for sensor applicationsdue to the large Tunneling Magnetoresistance (TMR) values.CoFeB/MgO/CoFeB MTJs have shown TMR up to 500% atroom temperature (RT) and 1010% at 5 K, approaching thetheoretically predicted value, observed for sputtered MTJs[2]. Current sensing applications require large output volt-ages, therefore understanding and increasing the TMR signalsare a top priority [3]. Moreover, the stability of the sensoroutput must be ensured over a large range of temperatures.This demand is usually met by integrating the sensors in FullWheatstone Bridges which guarantees thermal stability of theoutput and provides a null-voltage output in the absence of anexternal magnetic field, while at the same time ensuring thesame full output voltage of a single device [4]. Finally, current
Manuscript received November 05, 2012; revised January 21, 2013; acceptedFebruary 01, 2013. Date of current version July 15, 2013. Corresponding author:A. Lopes (e-mail: [email protected]).Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TMAG.2013.2246550
sensing also requires the sensor’s sensitivity to be compensatedelectronically due to the temperature drift which induces aparasitic ohmic resistance when biasing current is incremented[5]. In order to study the sensor’s temperature behavior, theproposed device includes a microfabricated resistive tempera-ture detector.This work describes the optimization of a MTJ-based current
sensor integrated with a thin film Ru temperature sensor.
II. DEVICE CHARACTERIZATION
In this work, a Wheatstone bridge current sensor was fab-ricated, using 114 MTJ elements connected in series in eachbranch of the bridge as shown in Fig. 1. Series configurationincreases electrical robustness, detectivity while reducing thepower consumption when compared with isolated devices [7].A printed circuit board (PCB) was designed to sense the ex-ternal current, using a U-shaped Cu trace mounted under thePCB maintaining a 0.6 mm separation distance between sensorand trace [Fig. 1(b)]. No glue was used, and an air gap wasgrooved in the PCB for improved thermal conductivity, there-fore avoiding excessive sensor heating originated by the trace.The MgO based MTJ stack was deposited at the Iberian
International Nanotechnology Laboratory (INL) in an auto-mated sputtering system (Singulus Timaris tool), and consistsof: Si/2000 Ta/500 CuN/50 Ta/500 CuN/50 Ta/50Ru/75 IrMn/20 CoFe/8.5 Ru/26 CoFeB/ MgO 4 83 3 kW600 sccm/30 CoFeB/2.1 Ta/160 NiFe/100 Ta/300 CuN/70Ru/150 TiW(N) (thickness in ). Here CoFe, CoFeB andTiW(N) stands for , , and .Afterwards, the sample was annealed in vacuum at 330 ,for 1 hour under a magnetic field of 1 T. Bulk film propertiesmeasured with a current in plane tunneling (CIPT) indicated a200% TMR and . The samples were thenpatterned at Instituto de Sistemas e Computadores-Microsis-temas e Nanotecnologias (INESC-MN) by optical lithographyand ion beam milling, creating arrays of 114 individual MTJ
LOPES et al.: MgO MAGNETIC TUNNEL JUNCTION ELECTRICAL CURRENT SENSOR WITH INTEGRATED Ru THERMAL SENSOR 3867
Fig. 1. (a) MTJ sensor connection in series and PCBwith trace layout, with indication of the magnetic field created by the trace at each series region. (b) Schematicsof the current sensing method with a integrated MTJ Wheatstone bridge.
Fig. 2. (a) Cross section TEM image of the device. Transfer curve of: (b) a single MTJ and (c) 114 MTJ array with and without PMs.
elements sensor with an area of connected in seriesand a 400 -thick Ru [Fig. 1(a)].Top contacts were done by lift-off of 300 nm-thick AlSiCu
sputtered film, patterned by optical lithography. Fig. 2(a) showsthe edge of an MTJ element, showing the MgO barrier sepa-rating the free and pinned layers, laterally isolated by afilm. The slope profile is the signature of 2-angle combined ionmilling process used for pillar definition.The application described in this paper is a good example
where the large area occupied by several elements in seriesis not an obstacle, and the ad-
vantages of series configurations can thus be profited.CoCrPt thin film permanent magnet (PM) elements
( , ) areintegrated along the sides of the MTJ-array providing a longi-tudinal bias (LB) field perpendicular to the pinned layer of thesensor [ Oe]. Unpatterned CoCrPt films exhibit aspontaneous magnetization of 997 and coercive field
of 610 Oe. Such LB field reinforces the shape anisotropyof the MTJs, promoting a low coercivity, linear response [8].Fig. 2(b) and (c) show the sensor transfer curves (R–H) forsingle MTJ elements and for the series elements connectionwith and without permanent magnets. It can be observed a
effect in a single junction and an averagein its corresponding series with linearization
set by the CoCrPt PMs. We can see a reduction in hysteresis of3.03 Oe in the arrays.
III. ELECTRICAL CHARACTERIZATION
The MTJ bridge dc electrical characterization was acquiredwith the objective to obtain the dc sensor’s sensitivity.We testedthe bridge sensor for the following biasing currents: 0.5, 1, 2, 3,4, and 5 mA. The MTJ connected in series configuration allowsthe sensor to sustain over 40 V across their terminals, for a 5 mAbiasing current without significant loss of voltage output signal,making the array configuration suitable for power applications.The base resistance value of each of the Wheatstone bridge
elements were calculated from the resistance measured at eachbranch of the bridge, at zero fields, and are: ,
, and . Thesevalues are averaging over the 114 MTJ elements in each re-sistor. Biasing the bridge with 1 mA constant current source andwith no current through the copper trace a sensor output voltageoffset of (1.9%) was measured.The dc sensor sensitivity was obtained biasing the Wheat-
stone bridge with 1 mA and acquiring the output voltage in re-sponse to an external dc current sweep between 10 and 10 Aapplied to the Cu trace (Fig. 3). From these experimental mea-surements a 63.9 V/Oe/A (42.6 mV/A) sensitivity was obtainedfor the MTJ current sensor [Fig. 3(a)]. This value is at least fivetimes greater than the sensitivity showed in [6] and one orderof magnitude greater than observed in spin-valve based currentsensors [5], [9]. At this point we also researched the dependencebetween the biasing current and the normalized sensitivity with
3868 IEEE TRANSACTIONS ON MAGNETICS, VOL. 49, NO. 7, JULY 2013
Fig. 3. (a) DC sensor sensitivity for a 10 A current sweeps with 1 mA biasingcurrent. (b) Dependence of the bridge voltage output (in V/Oe/A) and Wheat-stone bridge temperature with MTJ biasing current. (c) Sensor transfer function(transimpedance, in dB).
a current range mentioned above. We detected that when we in-crease the biasing current, the normalized sensitivity drops in-stead of displaying a constant behavior. So for the biasing cur-rent range in use we have a loss of sensitivity of about 12%.The normalized sensitivity versus biasing current is representedin Fig. 3(b), where it can be seen the relation between the de-crease of the sensitivity and the increase of the biasing current.To know the ability of the designed current sensor to mea-
sure ac current, the experimental setup described in [10] wasused. For this particular test a transconductance amplifier and ahigh-frequency high-power transformer capable of supplying toits secondary at least 10 at a frequency of 200 kHz weredeveloped. Fig. 3(c) shows the sensor transfer function (tran-simpedance, in dB) considered as the ratio between its outputvoltage over the input current, both in rms units and for frequen-cies from 50 Hz to 400 kHz. As it could be observed, the 3 dBcut-off frequency is near 90 kHz ranging input current from 0.1to 9.4 A rms. This last value improves the current capability of
Fig. 4. (a) Temperature time evolution inside the MTJ Wheatstone bridgesensor, without external current on the Cu trace, . Themaximum temperature increment (self-heating) is equal to 22.2 corre-sponding to 5 mA of biasing current. (b) Bridge sensor’s normalized sensitivityvariation with temperature. Inset shows the sensor’s temperature increase dueto biasing current and dc current passing on the copper trace.
the designed MTJ bridge (with PM) compared with the experi-mental ac frequency response obtained [6] (MTJ bridge with noPM). The MTJ with PM sensor provides an interesting way tomeasure current in fields like power electronics, domestic appli-ances or automotive applications where higher bandwidths andcurrent levels are required in the sensor.
IV. THERMAL CHARACTERIZATION
The temperature of the MTJ bridge could change due tothree coexisting sources: changes in ambient temperature;self-heating (biasing current) and heating in the main copperline. The objective of this experiment was to obtain the de-pendence of the Wheatstone bridge sensitivity (ratio betweenoutput voltage and input current) with the temperature increasecaused by the bridge biasing current (Joule effect). For thisthermal characterization, the MTJ bridge was measured atdifferent biasing currents at room temperature. During the ex-periment the internal MTJ Wheatstone bridge temperature wasmonitored using the integrated Ruthenium line (microfabricatedduring MTJ bridge processing), which showed a 2.24thermal coefficient. In order to obtain the Wheatstone bridgesensor’s sensitivity, the external copper trace was powered withcurrent sweeps from 10 to 10 A with 1 A steps. Measure-ments were done for several MTJ biasing currents, (from 5to 5 mA).We monitored the room temperature, the MTJ Wheatstone
bridge temperature and its offset voltage along 277 s (100 sam-ples with a 2.7 s/sample ratio), still without current on the Cu
LOPES et al.: MgO MAGNETIC TUNNEL JUNCTION ELECTRICAL CURRENT SENSOR WITH INTEGRATED Ru THERMAL SENSOR 3869
trace, to let the bridge sensor reach a stable temperature value.Fig. 4(a) represents the temperature time evolution whereis the internal Wheatstone bridge and Ru sensor’s temperature,
is the room temperature acquired using a platinum Pt100resistance temperature, and . The max-imum temperature increment is equal to 22.2 correspondingto 5 mA at the MTJ bridge terminals.Fig. 3(b) shows theMTJ bridge sensitivity dependence on the
temperature. For biasing currents ranging from 0.5 to 5 mA, theloss of sensitivity is about 12%. This value can be compensatedwith external electronics.So for an optimized sensor’s performance, a tradeoff between
three parameters must be done: 1) sensitivity (which dependson the magnetic behavior but also on the current bias), 2) cur-rent bias (the larger the current increase, the output voltage getshigher) and 3) temperature (increases with the current bias) mustbe done.When high current flows through the external copper trace,
one would expect a significant increase of the MTJ bridge tem-perature. However, inset of Fig. 4(b) demonstrates a negligibleeffect of the external current in heating up the sensor, being thebiasing current the main responsible for the self-heating prop-erty, therefore decreasing the MTJ bridge sensor sensitivity.This is understandable due to the TMR decrease as the biasingcurrent gets higher, having a great increase in voltage value pereach MTJ element, promoting changes in the MgO barrier.
V. CONCLUSIONS
An improved MTJ current sensor is presented to measure dccurrents in a 10 A range. The work shows that MTJ sensortechnology is a reliable tool in applications where large cur-rent measurement are required such as energy metering, indus-trial current transducers or smart wattmeters, due to its elec-trical robustness (40 V for across the bridge’sterminals). Furthermore an efficient linearization of 144 MTJelements in series with CoCrPt thin film integrated PM wasalso achieved. MTJ bridge sensor shows a reduced sensitivityupon temperature increase, caused mainly by the biasing cur-rent. Using a 5 mV biasing current, we got 40 V at the bridgeterminals, presenting a 12% sensitivity loss along the used bi-asing current range. This temperature dependence can be com-pensated with external electronics. Finally, the temperature in-crease due to large dc currents passing on the Cu trace line is
negligible , showing a successful thermal dissipationof the air-gap PCB architecture used for this work, when com-paring with conventional PCB-chip integration, where glue isused.The proposed sensor has a temperature drift sensitivity equal
to 0.04%/ , being an improvement regarding the results shownin spin-valve sensors.
ACKNOWLEDGMENT
This work was supported in part by FCT projectsTRAIN2-SOE2/P1/E280, PTDC/EEA-ELC/108555/2008,and PTDC/CTM-NAN/110793/2009; and by the Universityof Valencia through the funds from the Valoritza i Trans-fereix-VLC Campus and the Prometeo/2012/044 programs.INL acknowledges partial funding from the ON2 project fromPO-Norte. INESC-MN acknowledges FCT funding throughthe Instituto de Nanociência e Nanotecnologia (IN) AssociatedLaboratory.
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