Fabrication of planar and three-dimensional microcoils on flexible substrates

Inductors are basic components of magnetic sensors. Generally, with those sensors, a weak magnetic variation has to be detected. As the sensitivity increases with the inductance value, our objectives are to design inductors with a maximum of turns while keeping millimetric sizes for the sensor. In this work, we present two microcoil fabrication processes compatible with rigid and flexible substrates. The first one is used for the realization of planar microcoils with one step of copper micromoulding. For example, a 40-turn microcoil of 1 mm external diameter and 5 μm copper width and spacing wires has been obtained. The second process allows the fabrication of three-dimensional microcoils (microsolenoids). It is based on two steps of copper micromoulding. In this process, a grey-tone photolithography step is implemented. Microsolenoids with 10–13 wires have been realized.

     

Fabrication of planar and three-dimensional microcoils on flexible substrates

Inductors are basic components of magnetic sensors. Generally, with those sensors, a weak magnetic variation has to be detected. As the sensitivity increases with the inductance value, our objectives are to design inductors with a maximum of turns while keeping millimetric sizes for the sensor. In this work, we present two microcoil fabrication processes compatible with rigid and flexible substrates. The first one is used for the realization of planar microcoils with one step of copper micromoulding. For example, a 40-turn microcoil of 1 mm external diameter and 5 μm copper width and spacing wires has been obtained. The second process allows the fabrication of three-dimensional microcoils (microsolenoids). It is based on two steps of copper micromoulding. In this process, a grey-tone photolithography step is implemented. Microsolenoids with 10–13 wires have been realized.     

High current densities in copper microcoils: influence of substrate on failure mode

Copper planar microcoils were processed by U.V. lithography on SiO₂/Si and Kapton^®. They were packaged on different types of support in order to study the influence of thermal exchange conditions on device functioning. An electric current was injected in the coil and step by step increased until the electric connection broke, while the temperature of the microcoils remained free. This latter was estimated from the copper resistivity. It allowed demonstrating that the thermal exchange mode of the wire-bonded microcoils is conductive. The current density was calculated taking into account the deterioration of the coils by oxidation. Its maximum value is linearly decreasing with the thermal exchange ability of the support. The failure modes of the microcoils are related to track melting and oxidation, the current density remaining one order too weak to induce electromigration.     

Analysis of an in-plane micro-generator with various microcoil shapes

This study presents an analysis of an in-plane micro-generator with various microcoil shapes and multiple aspects of coupling, and reports the fabrication of a prototype micro-generator. It is important to establish analytical solutions for the micro-generator to predict the induced voltage. These analytical solutions can be used to estimate the micro-generator power to reduce the experimental time and the cost. Understanding the physical meanings of the variables can optimize the structure of the micro-electromagnetic generator. This model considers electromagnetism, kinematics, and geometry. The proposed in-plane rotary electromagnetic micro-generator was fabricated using low-temperature co-fired ceramic technology to co-fire the silver microcoils on the ceramic substrate with different shaped coils (e.g., square-shaped, circle-shaped and sector-shaped) both with the printing linewidth and 100 μm spacing of these microcoils. A planar permanent magnet with an outer diameter of 9 mm and a thickness of 700 μm was sintered by Nd/Fe/B. Its residual induction is 1.4 T. The experimental data in this study can be compared with analytical solutions. Analytical results show that the micro-generator with a sector-shaped microcoil generates a maximum effective value of 218.127 mV induced voltage at 1395.34 rad/s. Experimental measurements show a close agreement with these analytical solutions.     

Liquid metal microcoils for sensing and actuation in lab-on-a-chip applications

The use of metals and alloys with melting point near room temperature, called here as liquid metals, allows the integration of complex three-dimensional metallic micro structures in lab-on-chip devices. The process involves the injection molten liquid metal into microchannels and subsequent solidification at room temperature. The paper reports a technique for the fabrication of three-dimensional multilayer liquid-metal microcoils by lamination of dry adhesive films. The adhesive-based liquid metal microcoil could be used for magnetic resonance relaxometry (MRR) measurement in a lab-on-a-chip platform. Not only that the coil has a low direct-current resistance, it also has a high quality factor. In this paper, we investigate the sensing and actuating capabilities of the liquid metal microcoil. The sensing capability of the microcoil is demonstrated with the coil working as a blood hematocrit level sensor. In a MRR measurement, the transverse relaxation rate of the blood sample increases quadratically with the hematocrit level due to higher magnetic susceptibility. Furthermore, a vibrating adhesive membrane with the embedded coil was realized for electromagnetic actuation. A maximum deflection of approximately 50 μ at a low resonance frequency of 15 Hz can be achieved with a maximum driving current of 300 mA.     

Fabrication of three-dimensional magnetic microdevices with embedded microcoils for magnetic potential concentration

Novel magnetic microdevices were developed for magnetic field generation and concentration and successfully characterized and tested for magnetic potential focusing which is very important for various MEMS applications such as magnetic particles manipulation. These microdevices have been fabricated using an innovative processing sequence which eliminates many problems associated with other fabrication techniques and provides a platform for adding other subsequent fabrication steps required to integrate the microcoils with other microcomponents. They consist of high aspect ratio planar coils made of electroplated copper embedded in the silicon substrate, with ferromagnetic pillars and backside plates made of a CoNiP ternary alloy. A large magnetic field gradient is generated and enhanced by two structural parameters: the small width and high aspect ratio of each single conductor and the ferromagnetic pillars positioned at high flux density locations. This arrangement creates very steep magnetic potential wells, in particular at the vicinity of the pillars. The manipulation of micromagnetic particles in a static and continuous flow conditions has been demonstrated.     

The design and fabrication of a low-field NMR probe based on a multilayer planar microcoil

The nuclear magnetic resonance (NMR) probe has great influence on signal transmission and reception in NMR technology applications. In this paper, we present a design, fabrication, and test of an NMR probe comprised of a multilayer planar microcoil with a polydimethylsiloxane (PDMS) microchannel. First, geometric parameters of the probe are determined through theoretical analysis. Second, based on a glass substrate, the multilayer planar microcoil is manufactured using repeated photolithography and electroplating processes. During the fabrication process, the polyimide layer is used to package the coil, and the PDMS interlayer is used to adjust the distance from centerlines between the coil and the sample chamber. Third, the resistance and the quality factor of the coil are found to be 1.2158 Ω and 7.217, respectively, at a Larmor frequency of 28.1 MHz. Finally, the NMR probe is tested in an NMR experiment. The transverse relaxation time T₂ for the solid PDMS is 20.6 ± 0.4 ms, which is in agreement with 21.1 ± 0.2 ms obtained by a Bruker Minispec MQ60. Results show that the design and fabrication of this NMR probe are feasible for time-domain NMR applications.     

Characterization of flexible RF microcoils dedicated to local MRI

In magnetic resonance imaging (MRI), the electrical performance of the RF coil is critical to achieve sufficient signal to noise ratio (SNR), especially when microscopic structures [about (100 μm)³] have to be observed. In this field of application, we have developed a device (microcoil) based on the original concept of monolithic resonator, dedicated to superficial region imaging (human skin) or small animal imaging. This paper presents the developed process based on micromoulding. Flexible thin films of polymer have been used as dielectric substrate so that the microcoil could be form-fitted to non-plane surfaces. First, electrical characterizations of the RF coils have been performed. The results were compared to the expected values. A flexible RF coil of 15 mm diameter was then used to perform proton MRI of a saline phantom. When the coil was form-fitted to the phantom surface, a maximum SNR gain of 2 was achieved with respect to identical but plane RF coil. Finally, the flexible coil was used to perform MRI in vivo with high spatial resolution on a mouse using a small animal dedicated scanner operating at 2.35 T.     

On-chip manipulation and trapping of microorganisms using a patterned magnetic pathway

We demonstrate on-chip manipulation and trapping of individual microorganisms at designated positions on a silicon surface within a microfluidic channel. Superparamagnetic beads acted as microorganism carriers. Cyanobacterium Synechocystis sp. PCC 6803 microorganisms were immobilized on amine-functionalized magnetic beads (Dynabead^® M-270 Amine) by 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC)–N-hydroxysulfosuccinimide coupling chemistry. The magnetic pathway was patterned lithographically such that half-disk Ni₈₀Fe₂₀ (permalloy) 5 μm elements were arranged sequentially for a length of 400 micrometers. An external rotating magnetic field of 10 mT was used to drive a translational force (maximum 70 pN) on the magnetic bead carriers proportional to the product of the field strength and its gradient along the patterned edge. Individual microorganisms immobilized on the magnetic beads (transporting objects) were directionally manipulated using a magnetic rail track, which was able to manipulate particles as a result of asymmetric forces from the curved and flat edges of the pattern on the disk. Transporting objects were then successfully trapped in a magnetic trapping station pathway. The transporting object moves two half-disk lengths in one field rotation, resulting in movement at ~24 μm s^?1 for 1 Hz rotational frequency with 5 μm pattern elements spaced with a 1 μm gap between elements.     

Magnetically actuated micromechanical tweezers

Two planar actuators with magnetic thin films are used for magnetic tweezers. The planar actuators consisting of a pair of a 75 × 0.8 × 0.3 μm³ silicon oxide beam and a 72 × 13 × 0.3 μm³ silicon oxide plate deposited with a 65 × 4 × 0.1 μm³ Ni magnetic thin film are successfully fabricated and successfully gripped to a single NPC-tw01 cell consisting of Fe₃O₄ magnetic nanoparticles under a vertical magnetic field. The planar actuator bends under an external magnetic field because of the high shape magnetic anisotropy of the Ni magnetic thin film and a highly sensitive microcantilever. NPC-tw01 cells, which are adherent cells, are cultivated in a culture solution. The two planar actuators are placed in water to move and grip a living cell.     

Fabrication of microcoils with narrow and high aspect ratio coil lines

Recently, there has been a growing requirement to reduce their size of actuators. However, the miniaturization of actuators has made little progress since this requires micro-fabrication, processing, and other new techniques that are not compatible with traditional machining technologies. We have forcused on the fabrication of electromagnetic type microactuators that could be driven at low voltage and with high efficiency but it is well known that existing technologies for miniaturization of these devices are unsuitable because the allowable current path would be too small in microscopic applications. Therefore, we have proposed the development of a spiral microcoil with narrow width and high aspect ratio lines that can be fabricated using X-ray lithography and metallization techniques. We have fabricated spiral structures consisting of coil lines with widths of 10 μm and with aspect ratios of over 5. We have also succeeded in electroforming copper onto seed layers and have demonstrated isotropic copper etching in order to form narrow width coil lines to act as current paths. In addition, we have estimated the suction forces that can be generated by electromagnetic actuators fabricated using these coils. These results give rise to the expectation that practical high performance spiral microcoils could be manufactured using these techniques, in spite of their miniature size.     

Multilayer high-aspect-ratio RF coil for NMR applications

Microcoils offer a high degree of mass sensitivity and high magnetic field gradient strength in magnetic resonance microscopy applications. This paper presents a novel multilayer high-aspect-ratio metal fabrication process that can be used to fabricate a nanoliter-volume radio frequency (RF) saddle coil with an embedded flow-through fluidic channel for nuclear magnetic resonance (NMR) applications. The fabrication process is based on repeated electroplating processes and structure release processes. The achieved aspect ratio of the developed RF saddle coil is 4 with a structure line width of 25 μm. The resistance of the RF coil and the ¹H spectrum line width have been measured and are found to be 0.7 Ω and 350 Hz, respectively. Our results indicate that this novel fabrication process for RF microcoils is feasible for NMR applications.     

Development and characterization of a microthermoelectric generator with plated copper/constantan thermocouples

This work reports the development and the characterization of a microthermoelectric generator (μTEG) based on planar technology using electrochemically deposited constantan and copper thermocouples on a micro machined silicon substrate with a SiO₂/Si₃N₄/SiO₂ thermally insulating membrane to create a thermal gradient. The μTEG has been designed and optimized by finite element simulation in order to exploit the different thermal conductivity of silicon and membrane in order to obtain the maximum temperature difference on the planar surface between the hot and cold junctions of the thermocouples. The temperature difference was dependent on the nitrogen (N₂) flow velocity applied to the upper part of the device. The fabricated thermoelectric generator presented maximum output voltage and power of 118 mV/cm² and of 1.1 μW/cm², respectively, for a device with 180 thermocouples, 3 kΩ of internal resistance, and under a N₂ flow velocity of 6 m/s. The maximum efficiency (performance) was 2 × 10^?3 μW/cm² K².     

Geometrical analysis of planar coil design for fluxgate magnetometer

This paper presents geometrical analysis on the design of planar coupled coils for use as magnetic sensors. Inductance and magnetic coupling of the coil are analyzed using field solver analyzer ASITIC. Effects of different coil parameters, such as winding number, spacing, and width are discussed in detail. As results, the coil design considers not only the inductance value and the height of magnetic coupling, but also the geometrical area consumed. The analysis is verified by experimental data from coupled coil fabricated using surface micromachining techniques. An outer coil area having a typical dimension of 1.8 × 1.8 mm² and a fix inner coil diameter of 500 μm was fabricated. Coupling factor of about 0.7 and self inductance of 12.7 nH were achieved, which show a reasonably good agreement with the simulated results. The coil platform developed offers an integrated solution for the design of fluxgate magnetometer.     

Broadband simultaneously RHCP‐LHCP radiating microstrip fed rectangular slot antenna for polarization diversity applications

In this article, for the first time, an antenna that can radiate LHCP and RHCP waves simultaneously is presented. The antenna enables simultaneous transmission of both right‐handed (RH) and left‐handed (LH) circular polarized (CP) waves separately over an elevation range from ?45 ^° to ?5 ^° and 5 ^° to 45 ^° from the zenith. The simultaneous radiation of dual sense CP in the different spatial directions enables the antenna to act as polarization diversity transmitter. The mechanism of virtual sequential rotation of magnetic currents inside the different parts of the slot, excited with uniform phase fields results in dual CP generation. The uniform phase orthogonal fields are generated in the different parts of the slot essentially by exciting the full wavelength rectangular slot loaded with grounded stubs, symmetrically, with a shorted microstrip line. The final design of the slot antenna arrived with a rigorous parametric study on different dimensional parameters of slot and grounded stub. The measured impedance bandwidth of 22.5% centered around 7 GHz and axial ratio bandwidth of 19% is achieved. An overlapping bandwidth of 17% is achieved where both matching and AR are very good. The measured isolation between the RHCP and LHCP in the above‐mentioned elevation ranges is maintained above 10 dB. The simulated and experimental results are matching very well.     

Fabrication and test of multilayer microcoils with a high packaging density

In magnetic microactuation, the energy converted by a magnetic microactuator is proportional to its volume (Kohlmeier et al., Microsystem Technol 8(4–5):304–307, 2002a). For that reason, it is desirable not only to choose an actuator’s active part with a sufficient footprint size, but also with a substantial building height. By applying UV depth lithography, such a requirement may be fulfilled. This paper investigates the application of high aspect ratio systems technology to fabricating a compact microcoil as field generating element. The coil has four layers. To take optimal advantage of the building height, the insulation film between the coil layers was kept small by applying a thin Si₃N₄ layer. The coils are intended for the integration in a magnetic microlevitation system serving as an electromagnetic guide for a linear microhybrid step motor (Budde et al., In: Proceedings of the 9th int. conf. on new actuators, pp 665–668, 2004).     

履带式爬壁机器人磁吸附单元的磁场及运动分析

介绍了履带式磁吸附爬壁机器人的磁吸附单元的结构和工作原理．对单个磁块建立了有限元模型,并利用ANSYS软件对不同工作状态的磁块进行了磁场求解．进而,通过对一系列电机的扭矩测量确立了履带的平面力学方程,为机器人的动力学和运动学奠定了基础．     

Magnetic tunnel junction sensors with pTesla sensitivity

Ultrasensitive magnetic field sensors at low frequencies are necessary for several biomedical applications. Suitable devices can be achieved by using large area magnetic tunnel junction (MTJ) sensors combined with permanent magnets to stabilize the magnetic configuration of the free layer and improve linearity. However, further increase in sensitivity and consequently detectivity are achieved by incorporating also soft ferromagnetic flux guides (FG). A detailed study of tunnel junction sensors with variable areas and aspect ratios is presented in this work. In addition, the effect in the sensors transfer curve, namely in their coercivity and sensitivity, as a consequence of the incorporation of permanent magnets and FG is also thoroughly discussed. Devices consisting of MgO-based MTJ with magnetoresistance levels of ~200 %, and incorporating thin film permanent magnets (CoCrPt) and CoZrNb flux guides, could reach sensitivities of ~2,000 %/mT at room temperature, in a non-shielded environment. The noise levels of the final device measured at 10 Hz yield 3.9 × 10^?17 V²/Hz, leading to the lowest detectable field of ~49 pT/Hz^0.5. This value is half of the best value we obtained with MTJ-based devices, and represents a step further towards integration in a biomedical device for magnetocardiography.     

Principles of nonlinear MEMS-resonators regarding magnetic-field detection and the interaction with a capacitive read-out system

This paper presents a magnetic field sensor with capacitive read-out, whose active element is a micromachined mechanical resonator. The MEMS magnetic field sensor exploits the Lorentz force to detect external magnetic flux density through the displacement of the resonant structure, which can be measured with optical and capacitive sensing techniques. The micromachined U-shaped cantilever features a length of 2 mm, a base width of 90 μm and a thickness of 20 μm, and is manufactured in SOI technology. The designed sensor has a measured resonant frequency of 4.359 kHz for the fundamental mode and a calculated mass of the flexible structure of 24.5 ng. A quality factor in the order of 10⁴ at an ambient pressure of 0.3 Pa has been measured where a magnetic field resolution of 15 nT can be achieved. Although these arrangements are well suited to capacitively sense the vibrations caused by the Lorentz force on the current lead on the silicon part, care has to be taken to avoid undesired mutual interferences. A serious interference was observed in case of a DC bias voltage at the readout capacitance and a significant voltage drop caused by the current needed for the generation of the Lorentz force. This work investigates in detail this phenomenon as well as the complete physical transduction chain and improves the understanding of such microelectromechanical systems significantly. An analytical model of the electrostatic system is established including all relevant components and their interactions as well as the motion of the MEMS part. The importance of electrostatic back-action for a feasible detection limit for magnetic fields was recognized for the first time.     

Thermal stability and switching field distribution of CoNi/Pt patterned media

The thermal dependence and distribution of the switching fields of arrays of magnetic Co₅₀Ni₅₀/Pt nanodots has been studied. These dots, with a diameter of 90 nm, are arranged in a hexagonal pattern with a periodicity of 300 nm. Field-dependent magnetic force microscopy was used to measure the switching field distribution of the array, which was found to range from 80 to 192 kA/m, a value which is confirmed by vibrating sample magnetometer measurements. Additionally, the temperature dependence on the collective behaviour of the switching fields of the array has been investigated. The energy barrier at zero field was estimated to have a value in between 1.8×10⁻¹⁹ and 2.1×10⁻¹⁹ J. Combining this value with the effective anisotropy determined by torque measurements, the switching volume can be estimated to lie in between 1.2×10³ and 1.4×10³ nm³.