IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 53, NO. 9, SEPTEMBER 2006 2401 On-Chip High-Q Variable Inductor Using Wafer-Level Chip-Scale Package Technology Kenichi Okada, Member,IEEE, Hirotaka Sugawara, StudentMember,IEEE, Hiroyuki Ito, Member,IEEE, Kazuhisa Itoi, AssociateMember,IEEE, Masakazu Sato, Hiroshi Abe, Tatsuya Ito, and Kazuya Masu, Member,IEEE Abstract—In this paper, the authors propose an on-chip high-Q variable inductor embedded in wafer-level chip-scale package (WL-CSP). The variable inductor has a metal plate and a spiral inductor fabricated by the WL-CSP technology. The metal plate can be moved by a microelectromechanical systems (MEMS) actuator, and the inductance is varied according to the position of the metal plate. At the present time, the MEMS actuator has not been implemented yet. In this paper, the authors present a feasibility study on the proposed variable inductor. The inductor is evaluated with measurement results using a metal plate moved by a micromanipulator instead of the MEMS actuator. At 2 GHz, the measured inductance is varied from 4.80 to 2.27 nH, i.e., the variable ratio is 52.6%. The maximum value of quality factor is 50.1. Index Terms—Eddy current, high-Q, microelectromechani- cal systems (MEMS), variable inductor, wafer-level chip-scale package (WL-CSP). I. INTRODUCTION R ECENTLY, Si CMOS technology has provided high- density integration, high-frequency performance, and low cost, and the technology also realizes CMOS RF wireless circuits, e.g., more than 5-GHz transceivers. On the other hand, multiband wireless circuits are required due to commercial demands. Recent wireless mobile terminals have to provide several wireless communications, e.g., global system for mobile communications (GSM), universal mobile telecommunications system (UMTS), wireless LAN (WLAN) a/b/g, Bluetooth, global positioning system (GPS), etc. Such the multistandard wireless circuits require variable passive devices, so variable inductors [1]–[6] and variable capacitors [7], [8] are proposed. The variable passive devices realize multiband RF circuits. For example, wideband voltage-controlled oscillator (VCO) using the variable inductor [9], [10], VCO using the variable capacitor [11], [12], and wide-tunable low-noise amplifier (LNA) [13] have been reported. Many of the variable inductors use microelectromechani- cal systems (MEMS) processing [1]–[5]. The MEMS variable inductor uses RF switches [1], and the inductor has three- Manuscript received October 27, 2005; revised June 14, 2006. This work was supported in part by NEDO, MEXT.KAKENHI, JSPS.KAKENHI, MIC.SCOPE, and VDEC in collaboration with Cadence Design Systems, Inc., and Agilent Technologies Japan, Ltd. The review of this paper was arranged by Editor K. Najafi. K. Okada, H. Sugawara, and K. Masu are with the Integrated Research Institute, Tokyo Institute of Technology, 4259-R2-17 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan (e-mail: okada@pi.titech.ac.jp). H. Ito is with Intel, Hillsboro 97124 OR. K. Itoi, M. Sato, H. Abe, and T. Ito are with the Electron Device Laboratory, Fujikura Ltd., 1440 Mutsuzaki, Sakura, Chiba 285-8550, Japan. Digital Object Identifier 10.1109/TED.2006.880815 Fig. 1. Schematic illustration of on-chip inductor embedded in WL-CSP. dimensional (3-D) self-assembling structure [2]. A non-MEMS variable inductor using a MOSFET switch is also proposed [6], whose Q-factor is degraded by large resistive loss of the MOSFET switch. We have previously proposed the MEMS variable inductor using film metallic glass Pd 76 Cu 7 Si 17 [3], and the metal-plate-shielding variable inductor embedded in Si CMOS chip [4], [5]. However, the quality factor of the variable inductors is less than ten due to large resistive loss. In this paper, we propose an on-chip high-Q variable inductor embedded in wafer-level chip-scale package (WL-CSP). The fixed inductors using WL-CSP have been reported [14], [15], which achieve more than 30 of Q-factor. The WL-CSP induc- tors can be fabricated above Si CMOS chips, and fabrication cost is fairly reasonable because it is a kind of packaging process [14]. The previously proposed variable inductor [4], [5] consists of an on-chip spiral inductor, fabricated by multilevel interconnects in Si CMOS chip, and a metal plate, moved by a MEMS actuator. It is indispensable issue to improve large resistive loss of the spiral inductor. Therefore, the proposed inductor in this paper uses the WL-CSP inductor and the metal plate to achieve higher Q-factor. Section II explains the fabrication processes of WL-CSP spiral inductor. Section III describes the principle of the pro- posed variable inductor. The measured results are presented in Section IV. II. FABRICATION PROCESS Here, the fabrication process of WL-CSP inductor is pre- sented [14]. The WL-CSP is a kind of above IC process, which consists of two Cu electroplating layers and two resin layers, as shown in Fig. 1. It has originally been used for rerouting of pads and bumps. Cu electroplating layer is thick and has low resistivity, and it is suitable to embed inductors. The proposed variable inductor consists of a WL-CSP in- ductor and a metal plate above the inductor. The movable 0018-9383/$20.00 © 2006 IEEE