4384 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 28, NO. 9, SEPTEMBER 2013 Microfabricated V-Groove Power Inductors Using Multilayer Co–Zr–O Thin Films for Very-High-Frequency DC–DC Converters Di Yao, Member, IEEE, Christopher G. Levey, Member, IEEE, Rui Tian, and Charles R. Sullivan, Senior Member, IEEE Abstract—V-groove microinductors are designed, fabricated, and tested for operation above 10 MHz. Multilayer nanogranular Co–Zr–O/ZrO 2 magnetic thin films are used as the core material of these inductors, to improve the magnetic performance of the films deposited on the sidewalls of V-grooves and to control eddy-current loss in the core. Prototype V-groove inductors are fabricated in a Si substrate based on optimization results for 7 to 3.3-V, 1-A dc–dc buck converters. The inductors exhibit an inductance of 3.4 nH from 10 to 100 MHz, a dc resistance of 3.83 mΩ, and a quality factor of up to at least 50. The prototype inductors are a promising candidate for high-power-density high-efficiency dc–dc converters. The measured inductor performance indicates that they could be used to make a 7 to 3.3-V, 1-A converter exhibiting a power density of 2.5 W/mm 2 and an efficiency of 86% at 100 MHz; or a power density of 0.36 W/mm 2 and an efficiency of 91% at 11 MHz. Index Terms—High-frequency dc–dc converter, inductors, mi- crofabricated inductor, thin films, V-groove, very-high-frequency (VHF) power conversion. I. INTRODUCTION M OST efficient power conversion circuits are switching circuits, and most switching circuits require magnetic components (inductors and transformers). In switching power conversion circuits, the fundamental function of the magnetic components is to periodically store and release energy to achieve smooth output [1]. These energy-storage components generally occupy more space and dissipate more heat than other compo- nents in the circuits. In most designs, inductors and transformers are the largest components in the circuits, and are external com- ponents, whereas other components are more easily and more often integrated [2]–[5]. Increasing the switching frequency can decrease the energy storage requirement per switching period for the same amount of power, enabling the use of smaller mag- netic devices. This fundamental principle has driven past and Manuscript received June 10, 2012; revised September 30, 2012; accepted November 7, 2012. Date of current version February 15, 2013. This work was supported by Draper Laboratories and the Interconnect Focus Center, one of the six research centers funded under the Focus Center Research Program, a Semiconductor Research Corporation program. Recommended for publication by Associate Editor C. O’Mathuna. The authors are with the Thayer School of Engineering at Dartmouth, Hanover, NH 03755 USA (e-mail: ncdigua@gmail.com; christopher.levey@ dartmouth.edu; Rui.Tian@dartmouth.edu; charles.r.sullivan@dartmouth.edu). 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/TPEL.2012.2233760 Fig. 1. Schematic of a V-groove inductor [46]. continuing advances in the miniaturization of power electron- ics [5], [6]. In recent research, converters have been developed to operate at switching frequencies above 10 MHz [7]– [14], and up to hun- dreds of megahertz [15], [16], to advance the level of miniatur- ization of passive components. Typically, when such converters are implemented with on-chip magnetics, they demonstrate low efficiencies (less than 80%), compared to those using off-chip air-core inductors [2]. To improve the efficiency of monolithic integrated converters, improvements in integrated magnetics are required, and microfabricated magnetic devices using low-loss magnetic material are believed to be a promising choice [1]– [5], [17]–[29]. Nanogranular thin-film magnetic materials, with nanosize magnetic metallic particles surrounded by dielectric materi- als, exhibit high resistivity as well as good magnetic properties (high saturation flux density, low hysteresis, and good high- frequency performance), and have become a competitive option for applications above 10 MHz [30]–[36]. Prototype microfab- ricated magnetic inductors have been designed and fabricated using nanogranular thin-film magnetic materials to achieve high power density, low eddy-current loss, and low hysteresis loss [37]–[45]. V-groove inductors are microinductors fabricated on silicon substrates for the realization of monolithic integration of power conversion circuits [35], [40]–[44], [46], [47]. V-groove induc- tors, as shown in Fig. 1, consist of a triangular conductor embed- ded in silicon substrate with magnetic material wrapped around it [42]. The inductors feature conductors that have a large cross- sectional area in order to provide low dc resistance, and use a low-permeability magnetic material to distribute ac current ap- proximately uniformly around the perimeter of the conductor to provide low ac resistance as well. Because of these features, they 0885-8993/$31.00 © 2012 IEEE