K. Kratt

A fully MEMS-compatible process for 3D high aspect ratio micro coils obtained with an automatic wire bonder

We report the fabrication of 3D micro coils made with an automatic wire bonder. Using standard MEMS processes such as spin coating and UV lithography on silicon and Pyrex® wafers results in high aspect ratio SU-8 posts with diameters down to 100 µm that serve as mechanical stabilization yokes for the coils. The wire bonder is employed to wind 25 µm insulated gold wire around the posts in an arbitrary (e.g. solenoidal) path, yielding arrays of micro coils. Each micro coil is bonded directly on-chip, so that loose wire ends are avoided and, compared to other winding methods, coil re-soldering is unnecessary. The manufacturing time for a single coil is about 200 ms, and although the process is serial, it is batch fabrication compatible due to the high throughput of the machine. Despite the speed of manufacture we obtain high manufacturing precision and reliability. The micro air-core solenoids show an RF quality factor of over 50 when tested at 400 MHz. We present a flexible coil making method where the number of windings is only limited by the post height. The coil diameter is restricted by limits defined by lithography and the mechanical strength of the posts. Based on this technique we present coils ranging from 100 µm diameter and 1 winding up to 1000 µm diameter and 20 windings.

Microfabricated Inserts for Magic Angle Coil Spinning (MACS) Wireless NMR Spectroscopy

This article describes the development and testing of the first automatically microfabricated probes to be used in conjunction with the magic angle coil spinning (MACS) NMR technique. NMR spectroscopy is a versatile technique for a large range of applications, but its intrinsically low sensitivity poses significant difficulties in analyzing mass- and volume-limited samples. The combination of microfabrication technology and MACS addresses several well-known NMR issues in a concerted manner for the first time: (i) reproducible wafer-scale fabrication of the first-in-kind on-chip LC microresonator for inductive coupling of the NMR signal and reliable exploitation of MACS capabilities; (ii) improving the sensitivity and the spectral resolution by simultaneous spinning the detection microcoil together with the sample at the “magic angle” of 54.74° with respect to the direction of the magnetic field (magic angle spinning – MAS), accompanied by the wireless signal transmission between the microcoil and the primary circuit of the NMR spectrometer; (iii) given the high spinning rates (tens of kHz) involved in the MAS methodology, the microfabricated inserts exhibit a clear kinematic advantage over their previously demonstrated counterparts due to the inherent capability to produce small radius cylindrical geometries, thus tremendously reducing the mechanical stress and tearing forces on the sample. In order to demonstrate the versatility of the microfabrication technology, we have designed MACS probes for various Larmor frequencies (194, 500 and 700 MHz) testing several samples such as water, Drosophila pupae, adamantane solid and LiCl at different magic angle spinning speeds.

3D high aspect ratio, MEMS integrated micro-solenoids and Helmholtz micro-coils

High-aspect-ratio 3D geometrically perfect solenoidal micro-coils are fabricated for the first time in a fully MEMS-integrated technology. Vertical micro-coils with up to 15 windings and diameters down to 100µm have been wound using an automatic wirebonder around SU8 and PMMA cylindrical posts. Using this method we also fabricate Helmholtz micro-coils capable to generate magnetic fields of 1mT, according to simulations. We demonstrate the potential to use large arrays of solenoidal micro-coils for energy harvesting applications by placing them in a sinusoidal magnetic field. The induced voltage is 1.4mV at 300kHz for a 7 windings micro-coil, in agreement with theoretical calculations.

Characterization of a 3D MEMS fabricated micro-solenoid at 9.4 T.

We present for the first time a complete characterization of a micro-solenoid for high resolution MR imaging of mass- and volume-limited samples based on three-dimensional B(0), B(1) per unit current (B(1)(unit)) and SNR maps. The micro-solenoids are fabricated using a fully micro-electromechanical systems (MEMS) compatible process in conjunction with an automatic wire-bonder. We present 15 μm isotropic resolution 3D B(0) maps performed using the phase difference method. The resulting B(0) variation in the range of [-0.07 ppm to -0.157 ppm] around the coil center, compares favorably with the 0.5 ppm limit accepted for MR microscopy. 3D B(1)(unit) maps of 40 μm isotropic voxel size were acquired according to the extended multi flip angle (ExMFA) method. The results demonstrate that the characterized microcoil provides a high and uniform sensitivity distribution around its center (B(1)(unit) = 3.4 mT/A ± 3.86%) which is in agreement with the corresponding 1D theoretical data computed along the coil axis. The 3D SNR maps reveal a rather uniform signal distribution around the coil center with a mean value of 53.69 ± 19%, in good agreement with the analytical 1D data along coil axis in the axial slice. Finally, we prove the microcoil capabilities for MR microscopy by imaging Eremosphaera viridis cells with 18 μm isotropic resolution.

3D solenoidal microcoil arrays with CMOS integrated amplifiers for parallel MR imaging and spectroscopy

We present high-performance MR imaging and spectroscopy results obtained with wirebonded solenoidal microcoils manufactured in a MEMS-integrated technology. We report MR testing of 400µm inner diameter solenoidal microcoils for imaging of Eremosphaera Viridis algal cells with 10µm isotropic resolution. NMR spectroscopy has been performed on a water sample obtaining a linewidth of 0.04ppm. The newly introduced MEMS technology naturally lends itself to the fabrication of microcoil arrays, thus enabling parallel high-throughput MR investigation. As a proof of concept we report flip-chip integration of a microcoil-array with a CMOS amplifier array. “Teflon grease” has been used as phantom and three spectra have been acquired simultaneously yielding a linewidth of 0.5 kHz and a spin sensitivity in the frequency domain of 1015 spins/Hz½.

Solenoidal micro coils manufactured with a wire bonder

We present for the first time the development of 3-D solenoidal micro coils using an automatic wire bonder. By developing a stable and repeatable bond process with insulated wire, micro coils with sub-millimeter diameter have been manufactured. The winding process for a single coil takes about 200 ms, whereas the manufacture of a 100-coil-array takes less than a minute. Micro coils with 4 windings and a diameter of 690 mum exhibit an inductance of 12.7 nH and a resistance of 580 muOmega at 300 MHz. These values correspond to a quality factor of 41 and compare favorably lo state-of-the-art micro coil manufacturing technologies.

Three-dimensional microcoils as terahertz metamaterial with electric and magnetic response

A metamaterial consisting of three-dimensional submillimeter solenoidal coils is presented. The structures have been fabricated by an automated wire-bonding technique and are characterized by terahertz time-domain spectroscopy and numerical simulations. We demonstrate the excitation of electric and magnetic resonances in the microcoils at frequencies between 0.05 and 0.6 THz. The implementation of these microstructures provides a promising route for the design of nonplanar and intrinsically chiral metamaterials for gigahertz to terahertz frequencies.

Electromagnetic micro generator array consisting of 3D micro coils opposing a magnetic PDMS membrane

This work describes the fabrication and characterization of an electromagnetic micro generator array for energy harvesting purposes. The array consists of a magnetic polydimethylsiloxane (PDMS) membrane with 3x3 magnets which is aligned over a PCB with 3x3 coils. Different coil cores are tested and compared. The parameter for comparison is the voltage induced in the coil array by the vibrating membrane. The maximum measured voltage is 1.2 mV when exciting the converter array with an acceleration of 60 m/s2.

Multilayer phased microcoil array for magnetic resonance imaging

We present for the first time wirebonded microcoils arranged in a planar phased-array configuration for large field of view (FOV) microscale magnetic resonance imaging (MRI) of 2D samples. The phased array consists of seven microcoils providing a sensitive area of 18.3 mm2. We demonstrate successful high-resolution imaging of a water phantom with 16 × 16 µm2 in-plane resolution and an signal-to-noise ratio (SNR) of 27 in 12 min 48 s acquisition time.

Towards Piezoresistive CMOS Sensors for Out-of-Plane Stress

This paper presents a novel piezoresistive sensor detecting a temperature compensated sum of the three mechanical normal stress components including the Out-of-plane stress ¿zz. The sensor is based on CMOS-compatible diffusions designed to exploit vertical currents. We show that the temperature compensated stress sum can be extracted via two different sensor designs and even with a single device exploiting the strong influence of the junction field effect in the sensor. For this purpose, an experimental setup that induces homogenous vertical normal stress ¿zz by applying a vertical force via a gold bump to the sensor is presented. For the characterization, the resulting mechanical stress at the sensor location was evaluated using finite element simulations. Sensitivities, depending on gold bump shape and bias current, were experimentally determined and vertical forces were successfully extracted between 10°C and 60°C, independent of temperature.