Power and Radio Frequency Inductors Using a Hybrid Ferrite-Flex Foil Technology
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0969-W06-04
Power and Radio Frequency Inductors Using a Hybrid Ferrite-Flex Foil Technology Martin Gijs and Menouer Saidani Institute of Microelectronics and Microsystems, Ecole Polytechnique Fédérale de Lausanne, Station 17, Lausanne, 1015, Switzerland
ABSTRACT We present a hybrid technology for the realization of three-dimensional miniaturized power inductors and RF inductors on silicon. The power inductors consist of planar Cu coils on polyimide substrates, and mm-size ferrite magnetic cores, obtained by three-dimensional micropatterning of ferrite wafers using powder blasting. 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. Inductive and resistive properties are characterized as a function of frequency; inductance values in the 100 µH range have been obtained. We also have realized millimetre-size RF inductors on silicon using the same polyimide mould - Cu electroplating coil technology. Subsequently 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.
INTRODUCTION Many of today’s electronic applications are based on miniaturized electronic devices and components. The basic electronic element that is least compatible with miniaturization and integration often is the inductor. A high inductance value can be realized by allocating a sufficiently large chip area to the inductor, or by combining current carrying circuits with lowloss high permeability magnetic materials. Such microfabrication processes are not always wanted or compatible with standard CMOS technology. In the important field of application of small-size inductive switch mode power converters, switching frequencies in the MHz range are common, posing stringent requirements on the losses in magnetic materials that are used in the inductive components. In parallel, today’s boom in communication products at radio frequencies (RF) has strongly motivated research on integration of low-loss inductors directly on lowresistivity silicon substrates.
An ideal inductor or transformer would have no losses and would therefore be 100 % efficient. Energy is dissipated either by currents flowing through the windings, or by magnetic loss mechanisms that are associated with the magnet materials used within the inductors. The choice of the magnetic material depends on the frequency of the application. One is interested in an inductor with a highest possible quality factor Q ≡ ωL / R , with L the inductance and R the resistance of the inductor at the circular frequency ω. In practice, for a high quality factor MicroElectroMechanical System (MEMS) inductor, Q > 10 − 20 . A small value for R is a prerequisite for a high quality factor. The three most
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