Small Area High Inductance LTCC Spiral Inductor with High Q Factor Ikechi A. Ukaegbu 1* , Kwang-Seong Choi 2 , Brownson O. Obele 1 , Jamshid Sangirov 1 , and Hyo-Hoon Park 1 1 Department of Electrical Engineering, IT Convergence Campus, Korea Adv. Institute of Science & Tech. (KAIST), 119 Munjiro, Yuseong-Gu, Daejeon, 305-732, Korea (Phone: 82-42-866-6269; fax: 82-42-866-6223; e-mail: aus20@kaist.ac.kr ) 2 Electronics and Telecommunications Research Institute (ETRI), 161 Gajeong-dong Yuseong-gu Daejeon 305700, Korea. Abstract—In this paper, we describe the characterization of spiral inductors and the design of a novel spiral inductor based on low temperature co-fired ceramics (LTCC). Owing to its semi-stacked nature, we obtained a peak quality factor (Q max ) of 43.3 and an effective inductance L eff of 14.41 nH. Our novel spiral inductor occupies a smaller area when compared with the conventional planar inductor of similar inductance. Index Terms—Semi-stacked spiral inductor, LTCC, quality factor, self resonance frequency. I. INTRODUCTION HERE is a continuous need for increasing the packaging density in radio frequency (RF) devices leading to continuous interest and research in the development of materials, their design and manufacturing techniques. It is well known that inductors occupy large space in radio frequency integrated circuits (RFICs). Therefore, in inductor design, it is important to reduce the area occupied. Spiral inductors are important components of RF circuits such as RF amplifiers, impedance matching networks and voltage-controlled oscillators (VCOs) [1]. Some RF front-end building blocks such as power amplifier often require off-chip matching and biasing components [2][3], since it is difficult to accommodate them on-chip due to their Q factor and current handling capability limitation. Low temperature co-fired ceramics (LTCC) technology offers one of the solutions to this due to its packaging capabilities. In inductor design, quality factor (Q) and self resonance frequency (F self ) are the two most important parameters of the spiral inductor [4][5]. Therefore in inductor design, it is important to enhance the Q and F self of an inductor in a small area by lowering parasitic effects such as skin effect, coupling capacitance and line resistance. In order to improve the performance of spiral inductors, many approaches have been applied. Thick metal or stacked layers can increase the Q factor by reducing the metal losses (series resistance) [6] and by reducing the number and area of overlapping layers; the SRF can be improved by reducing parasitic capacitance. It is difficult to improve Q and F self at the same time. Thus the designer is faced with making trade-offs between performance (Q, F self ), inductance (L), and area. Some circuits are usually designed to work in differential modes to enhance their performance. In such cases, inductors are under differential excitation where the voltages or the currents of the two ports are of the same magnitude but 180º out of phase. Symmetrical inductors under differential excitation provide a higher Q factor and F self when compared with single-ended cases [7]. Sometimes it is difficult to design inductors with high inductance while still maintaining a high Q-factor and F self within a small area. Therefore, in this paper, we propose a novel small area high inductance semi-stacked spiral inductor structure with high Q factor. Also, three single- ended spiral inductors were fabricated and characterized. II. Q FACTOR DEFINITIONS AND INDUCTOR LIBRARY A. Q factor definitions The quality factor, Q of an inductor constitutes a very important figure of merit in discussing the performance of spiral inductor. The quality factor is defined as cycle one in energy stored energy Q ⋅ = π 2 (1) Two different definitions are often used for an inductor’s Q factor which basically depends on how we define the energy stored. Equation (1) also defines the Q factor of an LC tank. For an inductor, the energy stored in the magnetic field is of more importance than the energy stored in the electric field as parasitic capacitances. This Q factor can be represented as Q L as shown in (2). T