Self‐Assembled On‐Chip‐Integrated Giant Magneto‐Impedance Sensorics

A novel method relying on strain engineering to realize arrays of on-chip-integrated giant magneto-impedance (GMI) sensors equipped with pick-up coils is put forth. The geometrical transformation of an initially planar layout into a tubular 3D architecture stabilizes favorable azimuthal magnetic domain patterns. This work creates a solid foundation for further development of CMOS compatible GMI sensorics for magnetoencephalography. Encephalography techniques, especially those based on the detection of electrical potential, are commonly applied in medical institutions worldwide for health monitoring, e.g., diagnosis of epilepsy,1 tremor,2 or depressions3 and are now entering the new field of interfacing the human brain with smart prosthetics.4, 5 Its magnetic counterpart, namely magnetoencephalography, relies on the detection of tiny magnetic fields generated by the electrical currents in the nervous system.6, 7 Being able to provide the same physiological information as the conventional electroencephalography,8 magnetoencephalography offers strong advantages in terms of sensitivity and the opportunity to identify diseases, e.g., epilepsy at early stages with great spatial localization.9 A standard magnetoencephalography equipment is based on superconducting quantum interference devices (SQUIDs)6 or hybrid systems combining superconducting flux transformers with magneto-resistive sensorics.10 Although SQUID-based devices have proven their relevance in neurological disease treatment,11 rather high fabrication and maintenance costs limit their wide spread applicability. Furthermore, as SQUIDs need permanent cooling with liquid helium, the resulting setup has high maintenance cost, and is bulky and heavy, thus limiting the mobility of the patients. The development of a cost-efficient yet high-performance and even portable magnetoencephalography equipment would bring these unique devices to regular medical institutions, offering early stage disease diagnostics with great spatial resolution hence helping to minimize invasivity upon surgical treatment. Furthermore, if achieved, portable as well as light-weight magnetoencephalography devices bear a great potential to revolutionize the field of smart prosthetics,4 brain–machine and brain–brain interfaces due to a precise volumetric localization and characterization of current elements that correspond to particular mental activity and that are not accessible directly by electroencephalography and electrocorticography techniques.12 The realization of this ambitious goal requires new sensor solutions offering cost-efficient room temperature access to the tiny magnetic fields stemming from the brain activity in the neural tissue.13 In this respect, atom vapor magnetometers13, 14 were recently suggested as an alternative to SQUIDs for magnetoencephalography applications. However, the need of vacuum packaging of the measurement cell and optical pumping makes these devices rather complex in fabrication and in operation. Hence, there is a need to develop simple use, cost-efficient, and all-electrical measurement schemes for magnetoencephalography equipment, which would be in the spirit of the conventional electroencephalography devices. To this end, magnetic field sensorics has benefited from the discovery of the new effect in magnetically coated nonmagnetic microwires known as the giant magneto-impedance (GMI).15-18 The GMI effect resembles itself as large variation of real and imaginary parts of the complex impedance of the sensor, driven with an alternating current (AC), when exposed to a magnetic field. The GMI devices operate at room temperatures17, 19 and reveal remarkable sensitivity to small magnetic fields down to pico-Tesla regime.18, 20-24 Although the GMI effect does not have a long history, it is already implemented in devices applied in automotive, space, and medical sectors.23, 25 Cost efficiency and high universality suggest extensive applications of GMI devices in biology and medicine for, e.g., noninvasive monitoring of biomagnetic fields.26, 27 However, the microwire GMI sensor elements cannot be produced in a CMOS compatible way due to the stringent requirements on the fabrication involving rapid quenching of molten magnetic alloys to achieve circumferential magnetization25 and the need to be combined with pick-up coils17 to enhance the sensitivity. Therefore, the state-of-the-art GMI sensorics resemble itself as hybrid microelectronic devices with the need to bond the GMI wire to the CMOS-based conditioning electronics, thus limiting the total number of sensing elements.28 This is a clear disadvantage for magnetoencephalography applications where a spatial mapping of the magnetic field requires sensor arrays of at least 200 GMI devices to be compatible with the current SQUID-based technology. Thin film fabrication technologies emerged as an integrated circuit friendly alternative to the wire-based technology, potentially allowing the fabrication of sensor arrays.29, 30 However, due to the planar configuration of magnetic layers, it is still required to define a transversal to the wire axis magnetization by annealing the planar structures at temperatures beyond 200 °C at applied electric currents of at least 30 mA. This process is difficult to realize on an integrated circuit for a single sensor and even more difficult for array configurations.31, 32 Hence, a viable technology platform should be developed to produce arrays of compact GMI sensing elements with defined circumferential anisotropy in a CMOS compatible way. Here, we apply strain engineering33-36 to realize arrays of GMI sensors, which are directly on-chip integratable in a CMOS compatible process. We put forth a new platform37 that is relying on photopatternable, thermally and chemically stable imide- and acrylic-based polymers allowing for the stimuli-controlled self-assembly of initially planar NiFe-/Cu/NiFe-based heterostructures into 3D tubular architectures possessing the GMI functionality. In this way, we overcome the most crucial aspect in GMI fabrication technology, i.e., the formation of a circular magnetization, thus avoiding high temperature processing with applied electric current and allowing for the direct fabrication of sensor arrays on the integrated circuit. The NiFe/Cu/NiFe stacks are chosen as model prototype GMI systems revealing a stable effect with an amplitude in the range of 50%/Oe below 100 MHz excitation frequency and characteristics suitable for lab investigations.38, 39 The developed technology platform supports the integration of multiple functional elements, including pick-up coils and GMI sensors, into a single tubular architecture with a typical length of 2 mm and a diameter of some tens of micrometer. A characterization of the GMI sensing elements before and after the self-assembly process reveals that their magneto-electrical performance is drastically improved up to 80 times when transformed into a 3D architecture. This improvement is due to a geometrically induced circumferential magnetization achieved upon transforming the initial planar sensor into the tubular architecture. Furthermore, the integrated pick-up coil allows for a bipolar response of the devices. These compact GMI sensors equipped with pick-up coils operate at ambient condition and demonstrate rather strong responsiveness to small magnetic fields of 45 μV Oe−1 at a remarkably small excitation current of 1 mA. Independent of its compactness, the performance of the rolled-up GMI sensors is in the range of the state-of-the-art GMI devices. We note that the low excitation current of 1 mA and frequencies below 100 MHz are chosen to comply with the safety regulations accordingly to the “IEEE Standard for Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields, 3 kHz to 300 GHz.”40 With these parameters, the maximum AC magnetic field produced by the GMI element with a typical length below 10 mm is in the range of 10 nT, i.e., well within the safety limits of 100 nT. The realization of complex electronic components, e.g., sensors with pick-up coils, requires multiple fabrication steps including deposition and lithography to be carried out. Microelectronics processing as typically used in CMOS technology relies on photopatterning of polymers offering immediate batch fabrication while processing the structures on planar 2D substrates like silicon and glass. To allow for the self-assembly of the initially planar devices into compact 3D tubular structures, we propose to use stimuli-responsive polymers,41, 42 which develop a differential strain during water uptake relying on reinforcement layers.35, 43 Similar complex polymeric heterostructures were already successfully applied to fabricate self-assembled 3D objects including pyramids,35 stars,44 cubes,45 as well as tubular architectures.46, 47 Despite a broad variety of technological solutions, the state-of-the-art polymeric platforms applied for strain engineering44, 48-50 are typically relying on a single-step lithography. Hence, they do not allow the fabrication of microelectronic devices owing to a chemical incompatibility among the fabrication steps, especially the use of various polar and nonpolar solvents including strong base and acidic solutions during lithographic processing. In this respect, the largest concern is about the stability of the functional polymeric layer stack (sacrificial, stimuli-responsive layer, and reinforcing layers). The sacrificial layer should withstand all the processing steps without degrading its ability to be selectively dissolved after all the processing is finished. Furthermore, the stimuli-responsive layer must not be activated during all the processing steps of the devices. In turn, it should be assured that the self-assembly process, which include etching of the sacrificial layer and swelling of the stimuli-responsive layer does not disturb the magnetic, electrical as well as the mechanical properties of the functional layer stack to assure a high fabrication yield. These strict requirements impose strict limitations on the choice of the sacrificial layer as well as strained bilayer. To overcome these limitations, we put forth an alternative strain engineering technology platform relying on photopatternable, thermally, and chemically stable imide- and acrylic-based polymers. To this end, we formulated and synthesized a metal-organic sacrificial layer, a novel stimuli-responsive hydrogel, stiff polyimides as reinforcement layers, and a polychloroprene-based passivation polymer. These layers are stable at high temperatures up to 270 °C and inert in common organic nonpolar, polar protic, and aprotic solvents, as well as in moderate bases and acids allowing for multiple lithography steps. The stack of the three polymers, namely the sacrificial layer (thickness: 160 nm), hydrogel-based swelling layer (thickness: 180 nm), and stiff polyimide-based reinforcement layer (thickness: 700 nm), is prepared onto a 22 × 22 mm2 glass substrate. Using optical lithography, we define sites for the GMI sensors by patterning the functional stack to achieve arrays of squares or rectangles with a typical size in the range of 1 × 1 mm2 (Figure 1a1). Our technology allows to process up to 400 devices over the entire 22 × 22 mm2 substrate in a single fabrication run. Onto these predetermined areas, we prepare a lift-off photoresist mask to define in a single-step electrical contacts and a pick-up coil with a conductor width of 40 μm by depositing Cr(5 nm)/Au(50 nm) bilayers using electron beam evaporation (Figure 1a2). Then, another lift-off photoresist mask is brought onto the sample and the GMI stack consisting of Ni81Fe19(100 nm)/Cu(50 nm)/Ni81Fe19(100 nm) is grown using magnetron sputtering (Figure 1a3). To improve adhesion and protect the magnetic stack during lithographic processing, it is sandwiched by 2-nm-thick Ta layers. After the sensor layout is finalized, the sensor locations are covered by the photolithographically defined layer of polychloroprene rubber (thickness: 40 nm) leaving contact of for the electrical access for characterization (Figure the 2D of the and magnetic layers be as in the optical in The GMI response of the sensors is in a equipped with an allowing to apply magnetic fields of up to in the substrate the of the electrical the magnetic field at various frequencies of the current in the frequency range from 5 to we the 3D of the GMI (Figure high-performance we are in the sensitivity of the of the GMI Hence, the GMI is as where is the the of the in an applied and the at applied field. the planar sensors, we a standard GMI with a maximum of about which is when the is exposed to a magnetic field of 5 and driven by an AC current with MHz excitation frequency of the AC current is 1 The sensors are magnetically in the field of about The GMI performance of the planar devices is with the After the arrays of the planar sensor devices are by a layer of and self-assembled into 3D magnetic architectures by selectively etching the sacrificial layer in a of In the the hydrogel-based mechanical in the to the reinforcing This to an of the functional layer stack and a up process (Figure After the etching process, the sample is in water with in and at ambient to arrays of GMI sensors (Figure the entire substrate with an of 22 × 22 mm2 (Figure on substrates were not as those cannot be using the deposition However, the fabrication limitations on the processing to realize arrays of devices. the of the self-assembly process over 400 rolled-up GMI devices allows to the fabrication of the be The chosen of the and layers allow to realize tubular structures with a diameter in the range of μm and 2 (Figure The GMI response of the rolled-up is in with its planar reveals that the rolled-up GMI sensor a remarkable more 80 times GMI The tubular sensor reveals a maximum GMI of about at a 10 applied field and MHz the magnetic field the axis does not bring the GMI response to in the magnetic field range of 40 at frequencies This the of the circumferential which from the geometrical transformation from the planar layout to the GMI To this and the of the GMI response in the we of the magnetization and magnetic domain of the in planar and rolled-up (Figure this effect and carried to the of the planar layout 2 × 1 the magnetic are the long axis of the sample (Figure which is the axis of magnetization as from the of the magnetic (Figure The field of the planar is (Figure which is typical for due to their The magnetic are using the The size of the beam in the 100 × 100 which allows to the magnetic response of rolled-up Similar to using and into the magnetic of the 3D possessing with a size in the by an magnetic field to the into the of the effect of and the of the information about the magnetic of the layer a on of the be The magnetization within the and is by The geometrical transformation of the planar GMI stack into the 3D tubular architecture in the of the magnetic domain (Figure to the of azimuthal with a size of about μm by domain to the This circular domain which flux the tubular is due to of the of This is in with the transformation of the of the magnetic into a typical axis when in a field applied the axis (Figure The of the azimuthal domain with circumferential magnetization, which is to the of the electrical the of the GMI effect in the rolled-up to their planar The of the azimuthal domain which is of for the of the GMI the geometrical transformation upon self-assembly into a tubular architecture be used to realize GMI devices in a CMOS compatible process. the annealing which is to circumferential magnetization in magnetic microwires and 32 be The of the developed technology platform is the to multiple functional elements in a single architecture. for on-chip-integrated GMI this is of great as we produce a pick-up coil during the self-assembly of the GMI layer stack (Figure The pick-up coil is produced in the same lithographic process with the formation of the electrical contacts (Figure 1a2). This the of the sensor as the number of processing steps be The of the pick-up coil requires lithography in to the lithography typically used for its The planar layout for the pick-up coil is in a way to use a single conductor a coil with about the GMI the self-assembled GMI this that coils are as the contact are on of the rolled-up the as pick-up in is the magnetic and the is the nonmagnetic of the GMI In this the coils are from their by the of the GMI stack within of the Therefore, we the of the magnetization The measurement is as the excitation AC current of 1 mA at frequencies is to the GMI element and the pick-up from the pick-up coil is as a of magnetic field in the range of to with the excitation frequency as 5 and MHz (Figure pick-up we define the in and Independent of the rolled-up the of the GMI effect is to the for microwires using pick-up we a strong of the effect upon a current the sensor We characteristics at and mA pick-up from the GMI element is This is by the of the magnetic field of the GMI at the excitation frequencies of 7 and MHz (Figure This is with the by where the of the pick-up GMI response to the of the stemming from This is typical for the where the magnetic domain size is the width of the pick-up coil (Figure a the domain of the magnetic is by the field. on the current a or circular magnetic field be which the to of the In large currents of the size of is the width of the pick-up coil to a GMI at the pick-up coil as it for magnetic using by from the current of mA a strong which the tubular in circumferential in a single domain This to rather strong GMI response as in The sensitivity of the sensor at the excitation frequency of MHz is 45 μV Oe−1 pick-up is to the of with the current of 1 mA and number of of the coil the state-of-the-art devices that are using with a of reveal in the range from μV Oe−1 up to 1 In we put forth a novel method relying on strain engineering to realize on-chip-integrated GMI The advantages by the self-assembly to realize high-performance GMI devices a geometrical transformation from the initially planar layout into a tubular 3D architecture allows to achieve favorable circular magnetic domain without the need of rapid quenching of the magnetic layer stack in a magnetic we multiple functional elements in a single architecture including GMI sensors and pick-up coils, which are produced in a single fabrication run. the high temperature processing of the layer stack be this technology platform CMOS the used polymeric and the fabrication steps, e.g., film deposition and optical lithography at comply with the requirements on conventional CMOS This work creates a solid foundation for further development of CMOS compatible GMI sensorics for magnetoencephalography applications. GMI sensors are compact and do not Hence, the sensing element be at a of 10 mm from the current element current of a of typically 2 to 100 the magnetic field. 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