IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 10, OCTOBER 2009
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Fabrication and Characterization of Microscaled On-Chip Toroidal Inductors Jun-Yu Ou, Sen-Huei Chen, Huang-Ming Lee, and Jong Ching Wu Department of Physics, National Changhua University of Education, Changhua 500, Taiwan Microscaled on-chip toroidal inductors with ultrahigh quality factor (Q-factor) at tens of gigahertz have been successfully fabricated and characterized. The toroidal inductors with various diameters and dielectric layer thickness were designed and fabricated with two sets of inclined metal bars with a ring-shaped core of SiO2 inserted in-between. The frequency-dependent Q-factor and inductance were investigated using a 50–GHz S-parameter measurement system with standard two terminal ground–signal–ground microprobes on a radio-frequency (RF) probe station. The maximum inductance increases with increasing diameter of the inductor due to the enlargement of total magnetic flux. The highest Q-factor of 183 at a frequency of 28.8 GHz was realized in the inductor with diameter of 960 m and dielectric layers thickness of 5000 nm. In addition, the resonance frequency increases with increasing the dielectric layer thickness owing to a reduction of the parasitic capacitance. Index Terms—On-chip, quality factor, radio frequency (RF), toroidal inductor.
I. INTRODUCTION
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HE high demand for diminutive inductors operating at tens of gigahertz frequency but at low expense has been driven predominately by the wireless communication industry. Many efforts have been devoted to an on-chip method that realizes the inductor in spiral configurations for miniaturization. However, the performance of spiral inductors is restricted by loss resulting from the eddy current that is induced by magnetic flux leakage to the substrate. These induced eddy currents follow a path under the spiral metal wires causing a lower Q-factor, lower operating frequencies, and lower self-resonant frequencies [1], [2]. Inevitable loss is also due to eddy currents excited in the package metallization. Schemes that disrupt these eddy currents including tessellated ground planes and doped radial lines have been investigated [3]–[9]. Consequently, the Q-factor was slightly increased in high-resistivity substrates, such as high-resistivity silicon, GaAs, ceramic, and glass, resulting in Q-factors of 20 or higher [6], [10]. Moreover, an on-chip toroidal type of inductor was developed [11], in which a low-frequency inductance of 5.5 nH and a Q-factor of 42 at 4 GHz were achieved from a 15-turn toroidal inductor. Similarly, on-chip toroidal inductors integrated with a magnetic core were demonstrated for low-frequency power electronic applications [12], [13]. The main feature of the toroidal inductor is that the flux can be confined and thus little eddy current is induced. In this paper, we demonstrate microscaled on-chip 30-turn toroidal inductors with ultrahigh Q-factor up to 183 at an operating frequency of 28.8 GHz. II. EXPERIMENTS Fig. 1 illustrates the fabrication process flow of the microscaled on-chip toroidal inductor. First, a bilayer photo resist structure (AZ6112/LOR5B) was spin-coated on SiO -coated Si substrates. The thickness of the SiO dielectric layer ranged Manuscript received March 05, 2009. Current version published September 18, 2009. Corresponding author: J.-C. Wu (e-mail:
[email protected]). 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.2009.2023916
Fig. 1. Fabrication process flow of the microscaled on-chip toroidal inductor. (a) Bottom electrodes of the toroid and the probe pads. (b) Ring-shaped SiO core. (c) Top electrodes of the toroid. (d) The zoom-in diagram of the inductor. (f) SEM micrograph of the fabricated toroidal inductor, in which the capital denotes the outer diameter of the toroidal inductor varying linearly from 240 to 960 m.
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from 50 to 5000 nm. Then, a standard photolithography technique was used to define the bottom electrode and probe pads. Metal films of 5-nm-thick Cr and 35-nm-thick Au were thermally evaporated and transferred on the SiO -coated Si substrates through a liftoff process depicted in Fig. 1(a).
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IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 10, OCTOBER 2009
Fig. 2. Size-dependent inductances of the toroidal inductors fabricated on 50-nm-thick SiO -coated Si substrate.
Second, a 160-nm-thick ring-shaped core of SiO was fabricated using photolithography in conjunction with the SiO sputtering and liftoff process as shown in Fig. 1(b). Finally, the top electrode and probe pads were patterned and metalized with 220-nm-thick Au as shown in Fig. 1(c). A detailed image of the toroidal inductor is given in Fig. 1(d). A scanning electron microscopy (SEM) image of the fabricated toroidal inductor is as shown in Fig. 1(f), in which the capital letter denotes the outer diameter of the toroidal inductor. The mask pattern of the vary linearly from toroidal inductor was designed to make 240 to 960 m. Unless otherwise specified, in the following description, the diameter presents the outer diameter of the toroidal inductor. Note that a non-device under test (DUT) dummy pad has been made as well for the de-embedding process of the data acquisition [14]. The frequency-dependent Q-factor and inductance were calculated [15], [16] from the measured S-parameter using a 50-GHz S-parameter measurement system with standard two terminal ground–signal–ground microprobes on an RF probe station. Details of the frequency characteristics such as inductance and Q-factor as function of toroidal diameter and the thickness of dielectric layer coated on the Si substrate were extracted.
Fig. 3. Maximum inductances as a function of the diameters of the toroidal inductors fabricated on 50-nm-thick SiO -coated Si substrate. The maximum inductances were extracted from Fig. 2. Notice that the maximum inductance increases linearly with increasing the diameter of the toroidal inductor.
Fig. 4. Q-factors as a function of the diameters of the toroidal inductors. Note that the maximum Q-factor of the toroidal inductor with diameter of 960 m is 100 at operating frequency of 16.4 GHz.
III. RESULTS AND DISCUSSION Fig. 2 shows the size dependence of the inductances of the toroidal inductors fabricated on a 50-nm-thick SiO -coated Si substrate, in which the inductances increase as the diameters of the toroidal inductors enlarge. Moreover, the maximum inductances as a function of the diameters of the toroidal inductors extracted from Fig. 2 are plotted in Fig. 3, showing that the maximum inductance increases linearly with increasing diameter of the toroidal inductor. The maximum inductance measured on the toroidal inductor with diameter of 960 m is 8.6 nH at an operating frequency of 14.4 GHz. The inductance increase can be attributed to the enlargement of the cross-sectional area of the toroidal inductor, which led to higher magnetic flux flow resulting in higher magnetic energy stored. Fig. 4 presents the Q-factors as a function of the diameters of the toroidal inductors. The Q-factor becomes larger as the diameter of the toroidal inductor increases. The increase of the Q-factor is believed to be from lower resistance resulting from the larger line width of
Fig. 5. Q-factors of the toroidal inductors with fixed diameter of 960 m made on various SiO -coated Si substrates. Notice that the Q-factor increases with increasing the SiO dielectric layers thickness.
the toroidal inductor. The maximum Q-factor of the toroidal inductor with diameter of 960 m is 100 at an operating frequency of 16.4 GHz. In addition, the trend of the Q-factor in Fig. 4 tends to saturation when the diameter of the toroidal inductor reaches 960 m. Thus, another parameter, dielectric layer thickness, to change the Q-factor was taken into account. Fig. 5 exhibits
OU et al.: FABRICATION AND CHARACTERIZATION OF MICROSCALED ON-CHIP TOROIDAL INDUCTORS
the Q-factors measured on the inductors with fixed diameter of 960 m but various SiO dielectric layers thicknesses of 50, 500, and 5000 nm. The highest Q-factor of the inductor with dielectric layer thickness of 5000 nm is 183 measured at an operating frequency of 28.8 GHz. Notice that the Q-factor increases with increasing the SiO dielectric layer thickness. This result can be attributed to the great reduction of parasitic capacitance of the toroidal inductor to substrate due to thicker dielectric layers. IV. CONCLUSION In conclusion, we have successfully fabricated microscaled on-chip 30-turn toroidal inductors which exhibit maximum Q-factor of 183 at an operating frequency of 28.8 GHz, and in which the diameter of the toroidal inductor and the dielectric layer thickness are 960 m and 5000 nm, respectively. Moreover, the size dependence and dielectric layer thickness of toroidal inductors have been characterized. The reported manufacturing technique can be used not only in the telecommunication industry but also in RF IC manufacturing. ACKNOWLEDGMENT This work was supported in part by the Ministry of Economic Affairs under Grant 96-EC-17-A-01-S1-026 and in part by the National Science Council of the Republic of China under Grant 95-2112-M-018-004-MY3. REFERENCES [1] J. Chen, J. Zou, C. Liu, J. Schutt-Aine, and S. M. Kang, “Design and modeling of a micromachined high-Q tunable capacitor with large tuning range and a vertical planar spiral inductor,” IEEE Trans. Electron Devices, vol. 50, no. 3, pp. 730–739, Mar. 2003. [2] A. C. Reyes, S. M. El-Ghazaly, S. Dorn, M. Dydyk, D. K. Schroder, and H. Patterson, “Coplanar waveguides and microwave inductors on silicon substrates,” IEEE Trans. Microw. Theory Tech., vol. 43, no. 9, pp. 2016–2022, Sep. 1995.
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