3512 IEEE TRANSACTIONS ON MAGNETICS, VOL. 49, NO. 7, JULY 2013
Integration of TMR Sensors in Silicon Microneedlesfor Magnetic Measurements of Neurons
José Amaral , Vítor Pinto , Tiago Costa , João Gaspar , Ricardo Ferreira , Elvira Paz ,Susana Cardoso , and Paulo P. Freitas
Instituto de Sistemas e Computadores-Microsistemas e Nanotecnologias (INESC-MN), Lisbon, PortugalInstituto Superior Técnico (IST), 1000-029 Lisbon, PortugalLife and Health Sciences Research Institute, Braga, Portugal
INESC-Investigação e Desenvolvimento, 1000-029 Lisbon, PortugalINL-International Iberian Nanotechnology Laboratory, 4715-31 Braga, Portugal
In this work an alternative neuroscience tool for electromagnetic measurements of neurons at the level of individual cells is developed.To perform such measurements we propose the integration of an array of magnetoresistive sensors on micro-machined Si probes capableof being inserted within the brain without further damage. Si-etch based micromachining process for neural probes is demonstrated inthe manufacture of a probe with 15 magnetoresistive sensors in the tip of each shaft. Magnetic tunnel junction sensors with dimensionsof , sensitivities of 3.32 V/T and detectivity of 13 nT/Sqrt (Hz) are placed in the end of the sharply defined probe tips.In order to measure the small signals coming from the neurons, a homemade signal amplifying system was used with a noise level of 240
for the system bandwidth. The full system noise is 2772 .
Index Terms—Magnetic tunnel junction sensors, magnetoresistive devices, neuroscience tool.
I. INTRODUCTION
U NDERSTANDING how the brain works is a challengingtask in life sciences and in neurosciences in particular.
The most important means for rapid information transfer is byelectrical currents flowing in the neurons, the latter formingan extremely dense and entangled network. Although the gen-esis of neural electric fields is well known [1], [2], the sameis not true for neural magnetic fields, in part due to the tech-nical difficulty of recording the extremely weak signals. In thiswork, we aim to tackle these challenges by conceiving, realizingand testing an integrated device to record the created magneticfields.In this paper, we design sensing elements with micrometer
size, exhibiting picoTesla sensitivity at a micrometer scale andin the low frequency range [3]. These elements are incorporatedinto Si probes [4], [5] allowing them to enter the extracellularspace and there probe the activity on smaller distances (few )at locations few to tens micrometers from each other. In thiscase, due to the reduced distance, the local field variations willbe accessible, and various source depths can be targeted.Previous work could demonstrate direct probing of local
synaptic and cellular currents with micrometer size spatial res-olution using integrated magnetoresistive (MR) sensor arrays.First results [6], [7] measured with spin valve MR sensors atthe bottom surface of a hippocampus slice [8] were promisingbut showed the necessity of penetrating the hippocampus slicewith microelectrodes to avoid the dead layers at the sliced
Manuscript received November 05, 2012; accepted January 02, 2013. Dateof current version July 15, 2013. Corresponding author: J. Amaral (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.2013.2239274
interface. Therefore, integrating MR sensors into microma-chined Si needles becomes necessary. In this work, a hybridsystem combining Si needles and MR sensors was developedallowing the detection of magnetic field generated by distinctcurrent sources (dendritic synaptic activity) in a hippocampusbrain slice. With this approach, we aim to improve the existentextracellular local field potential (LFP) [9] by offering a localmeasurement of extracellular currents with micrometer spatialresolution. This system (LFP MR sensors) provides a com-parison between the electrical potential and the magnetic fieldproduced by neurons in different regions. Our design incorpo-rates needles capable of being physically inserted within thecurrent sources of the brain slice without causing significantdamage (avoiding the destruction of the neuronal connections)and enabling larger sensitivity measurement of the magneticfields generated in the brain slice. Furthermore, an MR sensoris placed at the end of each needle-tip to measure the magneticfields generated by electrical currents flowing from differentareas of the brain slice.
II. EXPERIMENTAL DETAIL
A. Magnetoresistive Sensor Fabrication
This fabrication of the hybrid MR needles requires 8 lithog-raphy steps, 3 in the wafer backside in addition to 5 more stepsin the wafer frontside, 7 deposition steps and 6 etching steps.Fig. 1 outlines the Si-MR probe front side fabrication process.The full process is described herein.A double-side polished (DSP) Si wafer (720 thick) was
used. In the wafer backside, a 420 reactive ion etch (RIE)was performed using an SPTS Pegasus system, which will leadto a lower thickness in the needles region (300 thick).At this point the sample is ready to deposit the MR ma-
terial on the wafer front side. In this work, state-of-the-artmagnetic tunnel junction (MTJ) thin-films based on
AMARAL et al.: INTEGRATION OF TMR SENSORS IN SILICON MICRONEEDLES FOR MAGNETIC MEASUREMENTS OF NEURONS 3513
Fig. 1. Schematic illustration of the fabrication process: a) Alignment markslithographs to the front side; b) MTJ stack deposition and then bottom electrodeand pillar lithography; c) Electrical leads patterned by lithography and liftoff;d) Sensor passivation layer deposition; e) SEM image of the detached probesafter the final DRIE etch.
MgO barriers were deposited at INL on a Timaris sput-tering tool. The material optimization was done previ-ously [10], aiming linear response and large magnetore-sistance. The MTJ stack consists of (thickness in nm):
,then annealed in vacuum for 1 h at 330 under 1 T, showing forunpatterned samples a and .Afterwards, a 15 nm layer was deposited bymagnetron sputtering in a Nordiko 7000 tool, for passivationpurposes during the sensor microfabrication at INESC-MN.The MTJ bottom electrode was defined by optical lithographyand ion milling (Nordiko 3000 system). In the second stepthe junction pillar is defined into a rectangular shape (sensingarea of ) also by optical lithography andbeam milling performed in two steps: first, an beam at60 etches down to the MgO barrier, then a 30 etch is used todefine the MgO barrier down to CuN [Fig. 1(b)]. Afterwards, a100 nm layer of is deposited for lateral side insulation,followed by a lift off process.Afterwards, Cr (5)/Au (200)/Cr (10) (thickness in nm) elec-
trical leads were deposited in an Alcatel SCM 450 sputtering
system and patterned by optical lithography and liftoff in a2-point probe (2pp) geometry [Fig. 1(c)].The sensors are then passivated with (200 nm)
layer deposited by sputtering from an ceramic target[Fig. 1(d)].
B. Si Needles Fabrication
To finalize the process, a Deep RIE (DRIE) of 300 thickSi probe shafts is performed on the wafer frontside at an SPTSPegasus system at INL—to etch the remaining Si thickness leftin the first wafer backside RIE. After the final DRIE etch, theprobes are detached manually from the sample [Fig. 1(e)].The needles are then passivated with an (200
nm)/ (200 nm) bilayer to protect the device (needlessensors) for a reasonable period of time allowing us to performa full experiment without destroying the sensors. Thelayer is deposited by reactive sputtering (using Ar and basedplasma) from an target.
C. Experimental Setup
The microfabricated needles were mounted on a ribbon flatcable, where the MR sensors were wire bonded (wires are pro-tected with silicone gel). The flat cable connects the MR nee-dles to the signal amplifying system, which consists on a printedcircuit board system designed to provide an electrical interfaceto the magnetic sensors. The circuit is divided in two parts:1) current generation for sensor biasing and 2) amplificationand filtering. The current generation circuit converts a stable,low noise voltage reference into a set of multiple currents thatcan bias multiple magnetic sensors simultaneously. The currentvalue was set to 0.5 mA; nevertheless, it can be tuned to accom-modate different sensor resistance values. The noise of the cur-rent source is also taken in consideration, since it affects directlythe device signal-to-noise ratio. Therefore, large noise filteringcapacitors (100 to 5 mF) were added to the current biasingcircuit, in order to remove current AC disturbances. The am-plification and filtering parts of the circuit are responsible forthe amplification of the weak AC signals produced by the smallsensor resistance variationmultiplied by the biasing current, andfor filtering out-of-band noise. An ultra-low-noise instrumenta-tion amplifier is used as a front-end, with a fixed gain of 100,in order to make the noise of the following amplification andfiltering stages negligible. A second amplification stage givesan extra gain, from 2 to 128, which can be controlled accordingwith the input signal amplitude. Finally, a band-pass filter withtunable 2nd-order high-pass and 5th-order low-pass cutoff fre-quencies was also implemented. The bandwidths are from 20Hzto 400 Hz, 300 Hz to 2.5 kHz, and 20 Hz to 2.5 kHz, in orderto amplify a specific frequency range, or the complete neuronalsignal spectrum. The PCB was inserted into a shielded housing,in order to minimize external interferences.During the experiments, sensor voltage output is amplified
and filtered, and acquired into a computer using an externalADC. The data is then post-processed with a homemade soft-ware, which includes a 50-Hz artifact removal notch filter andallows the measurement of the system noise continuously.
3514 IEEE TRANSACTIONS ON MAGNETICS, VOL. 49, NO. 7, JULY 2013
Fig. 2. Transfer curve of an MTJ sensor with a sensitivity of 3.32 V/T. Insetsshow theMTJ structure and a top view of the contact leads with the MTJ sensorsand the field sensitivity direction.
III. EXPERIMENTAL DETAILS
A. MTJ Sensor Characterization
MTJ based on MgO crystalline barrier (coherent tunneling)[11] is used as the sensing element in the proposed device. Fig. 2shows the characterization response of the microfabricatedMTJsensors, where the magnetoresistance MR signal of
leads to a sensitivity of 3.32 V/T for a 0.5 mA bias current,displaying a linear range between (free ofhysteresis).The magnetic anisotropy axis of the free layer is set along the
sensor length (i.e., along the needles) while the pinned layer di-rection is set perpendicular to the free layer upon annealing. Thislinearization strategy combined with shape anisotropy promoteshysteresis-free curves in these sensor dimensions,as shown in Fig. 2.
B. Sensor and Platform Noise Analysis
When the MTJ sensors are integrated with other devices, thenoise of the system may increase. Therefore, in this section, thenoise of the amplification system and the MTJ are measured, toassess the limiting noise source for magnetic detection.The amplification system set with a high-pass filter at 20 Hz
and a low-pass filter at 2.5 kHz with a gain of 88.2 dB was char-acterized using a Rohde and Schwarz Baseband Signal AnalyzerFMU36. Then the MTJ with the amplification system was char-acterized for the same bandwidth and gain and with a 0.5 mAcurrent bias. Fig. 3(a) presents the noise spectrum for the indi-vidual components of the system. The MTJ sensor integratednoise in the system bandwidth is 2618 , while the totalintegrated noise is 2772 . It can be seen that no excessnoise is coming from the electronics, when comparing with thenoise of the standalone MTJ. Together with the noise analysis, adetectivity study was performed, converting minimum voltage
Fig. 3. a) Noise of an MTJ with the instrumentation amplifier, a standaloneMTJ and the instrumentation amplifier in the range of 10 Hz–5 kHz. Sensornoise dominates the total noise. b) Detectivity at 1 kHz of an MTJ with theinstrumentation amplifier and a standalone MTJ.
to minimum magnetic field detectable. Fig. 3(b) shows a detec-tivity of about 13 nT/Sqrt(Hz) at 1 kHz for the integrated system.
C. Application and Results
The MR sensors integrated into Si needles were then used inexperiments performed on hippocampus slice samples obtainedfrom mice, with the aim of recording field potentials upon stim-ulus. The tissues were transferred to a recording chamber andcontinuously superfused with gassed Krebs solution1 at a flowrate of 3 ml/min.The MR-needle device is then mounted on a micro positioner
and placed in the area of interest within the mouse hippocampusbrain slice [Fig. 4(a)], in this case the CA1 region of the hip-pocampus [8].Fig. 4(b) shows a typical pulse measured in one of the MTJ
sensors from the array located in the pyramidal cell bodies re-gion, after hippocampus excitation [redline in Fig. 4(b)] in theCA1 region. The biological signal detected by theMTJ sensor is
1Krebs solution is an artificial cerebrospinal fluid solution used to keep thetissue alive during the experiments with the following composition (mM): 124NaCl, 3 KCl, 1.25 , 26 , 1 , 2 , and 10glucose previously gassed with , pH 7.4.
AMARAL et al.: INTEGRATION OF TMR SENSORS IN SILICON MICRONEEDLES FOR MAGNETIC MEASUREMENTS OF NEURONS 3515
Fig. 4. a) MR needles array with respect to the different hippocampus struc-tures. b)MTJ sensor output after an impulse in the CA1 region of a hippocampusslice.
probably being masked by the stimulus artifact, and thus a moredetailed analysis is required. In addition, differential measure-ment schemes using one non-active sensor are presently understudy.Additionally, Si needles devices integrating large areasMgO-
based sensors together with magnetic flux concentrators andpermanent magnets are being developed to improve the sensor’sdetectivity and to reduce its noise level [12], [13].
IV. CONCLUSION
In this paper we describe how MgO-based MTJ sensors weresuccessfully integrated into microfabricated Si needles, to beused as a neuroscience tool for biological tissue analysis. Inaddition, minimum tissue damage during needle penetrationis required; therefore, the Si needles and sensors are micro-fabricated under geometric constrains (tip sharpness, sensorwidth, needle thickness) that met these requirements. Sensor
TMR signals of 97% and field sensitivity of 3.32 V/T weremeasured upon microfabrication. The needle and MR sensorintegrated system was then used on mice brain slices. Uponstimulation, the device could measure clearly electro-magneticactivity; however, the source of signals needs to be furtherinvestigated, aiming to separate artifact stimulus signals frompurely magnetic contributions.
ACKNOWLEDGMENT
This work was supported in part by FCT projectsNanoSciEra/0002/2008, PTDC/EEA-ELC/108555/2008 andPTDC/CTM-NAN/110793/2009. J Amaral thanks FCT forgrant SFRH/BD/45488/2008. INL acknowledges partialfunding from the ON2 project from PO-Norte. INESC-MN ac-knowledges FCT funding through the Instituto de Nanociênciae Nanotecnologia (IN) Associated Laboratory.
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