4770 IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 10, OCTOBER 2009 Fabrication of Toroidal Microinductors for RF Applications Ulrich Schürmann , Andreas Gerber , Amit Kulkarni , Falk Hettstedt , Vladimir Zaporojtchenko , Reinhard Knöchel , Franz Faupel , and Eckhard Quandt Chair for Inorganic Functional Materials, Faculty of Engineering, Christian-Albrechts-University, 24143 Kiel, Germany Chair for Multicomponent Materials, Faculty of Engineering, Christian-Albrechts-University, 24143 Kiel, Germany Faculty of Engineering, Institute of Electrical and Information Engineering, Christian-Albrechts-University, 24143 Kiel, Germany Microdevices like microinductors for modern mobile communication electronics working at high frequencies and with sufficient quality factors are of great interest due to their potential benefits in terms of miniaturization and integration into the overall fabrication process. Toroidal microinductors with a magnetic core with high cut-off frequencies and permeabilities consisting of nanostructured composite materials of a magnetic alloy and a polymer fulfill these demands. This paper presents the fabrication of the microinductors by use of thin film deposition techniques, photolithography, electroplating, and etching processes and the integration of sputtered magnetic cores consisting of material with high cut-off frequencies. Index Terms—Microdevice fabrication, microinductors, polymer-metal nanocomposite, RF applications. I. INTRODUCTION T HE current demands for further miniaturization and in- creasing cut-off frequencies in the field of mobile commu- nication devices have intensified the investigations on high-fre- quency magnetic components [1]–[3]. The inductance and the quality factor of inductors can be increased by the integration of a highly permeable material provided that extra losses due to eddy currents are avoided. Current integrated microinductor devices for commercial ap- plications are realized as planar spiral inductors without a mag- netic core. The drawback of these types of microinductors is related with their large lateral size and, thus, higher costs. An additional disadvantage is the spatial propagation of stray fields leading to eddy currents in the substrate and possibly to cou- pling effects between neighboring devices. To increase the in- ductivity, inductors with an integrated magnetic material are promising candidates to fulfill these demands. Different microinductor designs have been suggested, each with advantages and disadvantages. Spiral and strip inductors with magnetic sandwich layers are easy to fabricate but show a relative low inductivity L and quality factor [4], [5]. Another problem of the spiral type is the difficult alignment of the easy axis. This is not the case for solenoid structures [6], but the field lines of the core cause large stray fields. In this investigation a toroid design (Fig. 1) was used, which avoids stray fields due to a closed magnetic core ring [7], [8]. II. MICROINDUCTOR PROCESSING The devices were fabricated in several process steps in a clean room facility. Borosilicate glass wafer were used as substrate. The bottom gold coil wiring and the connections (VIAs) be- tween the upper and lower metallization were structured by pho- tolithography and deposited by electroplating. Therefore, a Au Manuscript received March 06, 2009. Current version published September 18, 2009. Corresponding author: E. Quandt (e-mail: eq@tf.uni-kiel.de). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2009.2023618 Fig. 1. Light microscope image of a microinductor with a composite magnetic core (winding number 30, diameter 1 mm). seed layer (100 nm) with a Cr adhesion layer (20 nm) were de- posited by magnetron sputtering in an Ardenne CS 730 cluster system. The negative photoresist ma-N 440 (provided by Mi- croresist) was spin coated at 2500 rpm to achieve a resist thick- ness of approximately 6 m. The resist was structured with UV lithography using a SUSS Microtec mask aligner MA6. AZ 825 MIF (provided by Microchemicals) was used as developer [Fig. 2(a)]. The structures were filled with gold by electroplating up to a height of 5 m and the resist is removed afterwards. For the deposition of the vias, the connections between upper and lower metallization, the procedure was repeated with a re- duced rotation speed of 1000 rpm to enable a higher resist thick- ness including both the height of the lower metallization and the vias [Fig. 2(b)]. The seed layer was removed by ion beam etching (IBE) using an Oxford Ionfab 300 plus with Ar gas at a power of 400 W [Fig. 2(c)]. A dielectric layer of the negative photoresist Cyclotene (cy- clobutene, BCB provided by Dow) was spin coated in two steps on the gold wiring as insulation between core and wiring. In 0018-9464/$26.00 © 2009 IEEE