Vlad Badilita

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.

Rate-equation model for coupled-cavity surface-emitting lasers

We present a detailed theoretical study of a vertical-cavity surface-emitting laser (VCSEL) with two optically coupled, active cavities. The study is based on a rate-equation model written for carriers and photons under steady-state conditions. The model allows one to determine all the relevant parameters-carrier densities, gains, and output powers-starting from two input parameters: the injection currents in each cavity. The system of equations is solved for different operating regimes of the device and the results provided by the model are shown to be in very good qualitative and quantitative agreement with the experimental data.

Control of polarization switching in vertical coupled-cavities surface emitting lasers

We report a novel three-contact vertical-cavity surface-emitting laser (VCSEL) for use in polarization-sensitive optical applications. The device consists of two vertical cavities, coupled by a common mirror. We demonstrate that one can independently choose both the power of the output beam-through the current in the first cavity-and the polarization state-through the bias applied to the second cavity. The control of the polarization state is performed with a control voltage in the range -2 to 0 V. Within this interval, the structure exhibits a bistable behavior. We present the very first experimental proof of nonthermal, electrically induced polarization switching in coupled-cavities VCSELs.

Microscale nuclear magnetic resonance: a tool for soft matter research

Nuclear magnetic resonance spectroscopy (NMR) and imaging (MRI) are important non-destructive investigative techniques for soft matter research. Continuous advancements have not only lead to more sensitive detection, and new applications, but have also enabled the shrinking of the detectable volume of sample, and a reduction in time needed to acquire a spectrum or image. At the same time, advances in microstructuring and on-chip laboratories have also continued unabated. In recent years these two broad areas have been productively joined into what we term micro nuclear magnetic resonance (μMR), an exciting development that includes miniaturized detectors and hyphenation with other laboratory techniques, for it opens up a range of new possibilities for the soft matter scientist. In this paper we review the available miniaturization technologies for NMR and MRI detection, and also suggest a way to compare the performance of the detectors. The paper also takes a close look at chip-laboratory augmented μMR, and applications within the broad soft matter area. The review aims to contribute to a better understanding of both the scientific potential and the actual limits of μMR tools in the various interdisciplinary soft matter research fields.

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.

Micro-fabricated Helmholtz coil featuring disposable microfluidic sample inserts for applications in nuclear magnetic resonance

In this study, we report on a novel, multi-use, high-resolution NMR/MRI micro-detection probe for the screening of flat samples. It is based on a Helmholtz coil pair in the centre of the probe, built out of two 1.5?mm diameter wirebonded copper coils, resulting in a homogeneous distribution of the magnetic field. For liquids and suspensions, custom fabricated, disposable sample inserts are placed inside the pair and aligned automatically, preventing the sensor and the samples from contamination. The sensor was successfully tested in a 500?MHz (11.7 T) spectrometer where we achieved a linewidth of 1.79?Hz (3.58?ppb) of a water phantom. Nutation experiments revealed an overall B1-field uniformity of 92% (ratio in signal intensity at flip angles of 810?/90?), leading to a homogeneous excitation of concentration limited samples. To demonstrate the imaging capabilities of the detector, we acquired images of a solid and a liquid sample?of a piece of leaf, directly inserted into the probe and of a sample insert, filled with a suspension of 50 ?m diameter polymer beads and deionized water, with in-plane resolutions of 20 ? 20 ??m2 and 10 ? 10 ??m2, respectively.

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.

Heteronuclear Micro-Helmholtz Coil Facilitates µm-Range Spatial and Sub-Hz Spectral Resolution NMR of nL-Volume Samples on Customisable Microfluidic Chips.

We present a completely revised generation of a modular micro-NMR detector, featuring an active sample volume of ∼ 100 nL, and an improvement of 87% in probe efficiency. The detector is capable of rapidly screening different samples using exchangeable, application-specific, MEMS-fabricated, microfluidic sample containers. In contrast to our previous design, the sample holder chips can be simply sealed with adhesive tape, with excellent adhesion due to the smooth surfaces surrounding the fluidic ports, and so withstand pressures of ∼2.5 bar, while simultaneously enabling high spectral resolution up to 0.62 Hz for H2O, due to its optimised geometry. We have additionally reworked the coil design and fabrication processes, replacing liquid photoresists by dry film stock, whose final thickness does not depend on accurate volume dispensing or precise levelling during curing. We further introduced mechanical alignment structures to avoid time-intensive optical alignment of the chip stacks during assembly, while we exchanged the laser-cut, PMMA spacers by diced glass spacers, which are not susceptible to melting during cutting. Doing so led to an overall simplification of the entire fabrication chain, while simultaneously increasing the yield, due to an improved uniformity of thickness of the individual layers, and in addition, due to more accurate vertical positioning of the wirebonded coils, now delimited by a post base plateau. We demonstrate the capability of the design by acquiring a 1H spectrum of ∼ 11 nmol sucrose dissolved in D2O, where we achieved a linewidth of 1.25 Hz for the TSP reference peak. Chemical shift imaging experiments were further recorded from voxel volumes of only ∼ 1.5nL, which corresponded to amounts of just 1.5 nmol per voxel for a 1 M concentration. To extend the micro-detector to other nuclei of interest, we have implemented a trap circuit, enabling heteronuclear spectroscopy, demonstrated by two 1H/13C 2D HSQC experiments.

Microfluidic integration of wirebonded microcoils for on-chip applications in nuclear magnetic resonance

We present an integrated microfluidic device for on-chip nuclear magnetic resonance (NMR) studies of microscopic samples. The devices are fabricated by means of a MEMS compatible process, which joins the automatic wirebond winding of solenoidal microcoils and the manufacturing of a complex microfluidic network using dry-photoresist lamination. The wafer-scale cleanroom process is potentially capable of mass fabrication. Since the non-invasive NMR analysis technique is rather insensitive, particularly when microscopic sample volumes are to be investigated, we also focus on the optimization of the wirebonded microcoil for this purpose. The on-chip measurement of NMR signals from a 20 nl sample are evaluated for imaging analysis of microparticles, as well as for spectroscopy. Whereas the latter revealed that the sensitivity of the MEMS microcoil is comparable with hand-wound devices and achieves a full-width-half-maximum linewidth of 8 Hz, the imaging experiment demonstrated 10 μm isotropic spatial resolution within an experiment time of 38 min for a 3D image with a field of view of 1 mm × 1 mm × 0.5 mm (500 000 voxels).

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.