Miniaturized hybrid inductors and transformers

In the frame of this thesis, we have developed a novel batch-type technology for the realization of three-dimensional millimeter-size transformers for ultra-small low-power (0.1-1 Watt) applications. The technology is based on the three-dimensional micropatterning of 0.5, 0.75 and 1 mm thick ferrite wafers by powder blasting to form the magnetic cores of the inductive devices, and the combination of these cores with electrical windings realized in flex-foil printed circuit board technology. Microfabrication and assembly of the parts can be done in a batch-process on a wafer/foil level, opening the way to further size reduction of the components. The coils are realized using an in-house developed high-resolution polyimide spinning and Cu electroplating process. Winding widths down to 5 μm have been obtained and total device volumes are ranging between 1.5 and 10 mm3. We have measured the inductive and resistive properties of our devices as a function of frequency and device geometry. The results clearly show the high potential of our technology for power applications, in which miniaturization and small-size is an issue. In the domain of RF frequencies, efforts are being made to improve the quality factor by multilayer techniques and leaving a space between the substrate and the conducting lines. Magnetic components operating at high frequencies rapidly increase eddy current and hysteresis losses in the magnetic core as the operating frequency increases. The micromachining techniques provide several approaches for the miniaturization of inductors operating at high frequencies. In the present work, we have realized millimeter-size RF inductors on silicon using the same coil as before and Ni-Zn ferrite plates. The coils are assembled with magnetic cover plates of commercially available bulk Ni-Zn ferrites of high resistivity. Using the magnetic flux-amplifying ferrite plates, we obtain a 40 % enhancement of the inductance and a 25 % enhancement of the quality factor (Q=10-20) for frequencies up to 0.2 GHz.

Hybrid flex foil-ferrite technology for miniaturized power and RF applications

Martinus Gijs

A hybrid technology for the realization of threedimensional miniaturized power inductors, transformers and RF inductors is presented. This technology consists of combining planar Cu coils on polyimide substrates with high permeability and high resistivity ferrite cores. For miniaturized power inductors and transformers, high permeability (µr ~2000 Mn-Zn ferrite cores are used. These cores operate up to 10 MHz and are obtained by threedimensional micro-patterning of ferrite wafers using the powder-blasting technique. High resistivity (107 Ohm.cm) Ni-Zn ferrite plates cores are used for the realization of RF inductors. The plates are deposited on top of the coils to enhance the magnetic properties of the inductor at GHz frequencies. The planar Cu coils are realized using an inhouse developed high-resolution polyimide spinning and Cu electroplating process on a flexible support of Kapton. Inductive and resistive properties of the power inductors are characterized as a function of frequency; inductance values in the 1 mH range have been obtained. RF inductors show a 40 % enhancement of the inductance and a 25 % enhancement of the quality factor for frequencies up to 0.2 GHz. Our results indicate the potential of using bulk ferrites in a hybrid assembly process for power and RF applications.

2004

Properties and Printability of Inkjet and Screen-Printed Silver Patterns for RFID Antennas

Danick Briand, Nico de Rooij, Francisco Molina Lopez, Jinyu Jason Ruan

We report the modeling, and geometrical and electrical characterization, of inkjet and screen-printed patterns on different polymeric substrates for use as antennas in radio-frequency identification (RFID) applications. We compared the physical and electrical characteristics of two silver nanoparticle-based commercial inkjet-printable inks and one screen-printable silver paste, when deposited on polyimide (PI), polyethylene terephthalate (PET), and polyetherimide (PEI) substrates. First, the thickness of the inkjet-printed patterns was predicted by use of an analytical model based on printing conditions and ink composition. The predicted thickness was confirmed experimentally, and geometrical characterization of the lines was completed by measuring the root-mean-square roughness of the patterns. Second, direct-current electrical characterization was performed to identify the printing conditions yielding the lowest resistivity and sheet resistance. The minimum resistivity for the inkjet-printing method was 8.6 +/- A 0.8 mu I (c) cm, obtained by printing four stacked layers of one of the commercial inks on PEI, whereas minimum resistivity of 44 +/- A 7 mu I (c) cm and 39 +/- A 4 mu I (c) cm were obtained for a single layer of screen-printed ink on polyimide (PI) with 140 threads/cm mesh and 90 threads/cm mesh, respectively. In every case, these minimum values of resistivity were obtained for the largest tested thickness. Coplanar waveguide transmission lines were then designed and characterized to analyze the radio-frequency (RF) performance of the printed patterns; minimum transmission losses of 0.0022 +/- A 0.0012 dB/mm and 0.0016 +/- A 0.0012 dB/mm measured at 13.56 MHz, in the high-frequency (HF) band, were achieved by inkjet printing on PEI and screen printing on PI, respectively. At 868 MHz, in the ultra-high-frequency band, the minimum values of transmission loss were 0.0130 +/- A 0.0014 dB/mm for inkjet printing on PEI and 0.0100 +/- A 0.0014 dB/mm for screen printing on PI. Although the resistivity achieved is lower for inkjet printing than for screen printing, RF losses for inkjetted patterns were larger than for screen-printed patterns, because thicker layers were obtained by screen printing. Finally, several coil inductors for the HF band were also fabricated by use of both printing techniques, and were used as antennas for semi-passive smart RFID tags on plastic foil capable of measuring temperature and humidity.

Springer Verlag2014

Single-Layer MoS2 Electronics

Simone Bertolazzi, Andras Kis, Dominik Sebastian Lembke

CONSPECTUS: Atomic crystals of two-dimensional materials consisting of single sheets extracted from layered materials are gaining increasing attention. The most well-known material from this group is graphene, a single layer of graphite that can be extracted from the bulk material or grown on a suitable substrate. Its discovery has given rise to intense research effort culminating in the 2010 Nobel Prize in physics awarded to Andre Geim and Konstantin Novoselov. Graphene however represents only the proverbial tip of the iceberg, and increasing attention of researchers is now turning towards the veritable zoo of so-called "other 2D materials". They have properties complementary to graphene, which in its pristine form lacks a bandgap: MoS2, for example, is a semiconductor, while NbSe2 is a superconductor. They could hold the key to important practical applications and new scientific discoveries in the two-dimensional limit. This family of materials has been studied since the 1960s, but most of the research focused on their tribological applications: MoS2 is best known today as a high-performance dry lubricant for ultrahigh-vacuum applications and in car engines. The realization that single layers of MoS2 and related materials could also be used in functional electronic devices where they could offer advantages compared with silicon or graphene created a renewed interest in these materials. MoS2 is currently gaining the most attention because the material is easily available in the form of a mineral, molybdenite, but other 2D transition metal dichalcogenide (TMD) semiconductors are expected to have qualitatively similar properties.In this Account, we describe recent progress in the area of single-layer MoS2-based devices for electronic circuits. We will start with MoS2 transistors, which showed for the first time that devices based on MoS2 and related TMDs could have electrical properties on the same level as other, more established semiconducting materials. This allowed rapid progress in this area and was followed by demonstrations of basic digital circuits and transistors operating in the technologically relevant gigahertz range of frequencies, showing that the mobility of MoS2 and TMD materials is sufficiently high to allow device operation at such high frequencies.Monolayer MoS2 and other TMDs are also direct band gap semiconductors making them interesting for realizing optoelectronic devices. These range from simple phototransistors showing high sensitivity and low noise, to light emitting diodes and solar cells. All the electronic and optoelectronic properties of MoS2 and TMDs are accompanied by interesting mechanical properties with monolayer MoS2 being as stiff as steel and 30x stronger. This makes it especially interesting in the context of flexible electronics where it could combine the high degree of mechanical flexibility commonly associated with organic semiconductors with high levels of electrical performance. All these results show that MoS2 and TMDs are promising materials for electronic and optoelectronic applications.