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Integrated High Frequency RF Inductors with Nano/micro Patterned Ferromagnetic Cores

Y. Zhuang, M. Vroubel, B. Rejaei and J.N. Burghartz
Delft University of Technology, NL

Keywords: self-assembled, RF, granular, ferromagnetic, inductor

Integration of magnetic material has been long proposed to potentially improve performance and shrink size of on-chip inductive RF/microwave components, e.g. inductors, transformers, and transmission lines, which is crucial for the development of cost-effective RF/BiCMOS and RF/CMOS technologies [1]. Considerable efforts are underway to develop on-chip inductors with ferromagnetic cores having a high inductance per area (IPA) with a sufficiently high maximum quality factor (Qmax), a high operating frequency f(Qmax), and a high cut-off frequency (fcut-off) that is related to the ferromagnetic resonance (FMR) frequency [2]-[4]. However, the FM's high conductivity deteriorates the device performance at RF/Microwave frequencies manifested by the low Qmax, f(Qmax), and fcut-off, even the principal superior solenoid-type inductors have been exploited [4]. Reduction of the effective FM film conductivity and thus of eddy currents, while maintaining a sufficiently high permeability and FMR, can be achieved by nano/micro-size patterning of the FM film. Sophisticated multiple-target sputtering techniques have been proposed to deposit granular FM films [5]. In IC processing, a more cost-effective deposition method, such as electroplating, is certainly preferable, provided that nano/micro patterning is feasible. In this paper, we present a novel low-cost nano/micro structured Ni80Fe20 film that is deposited by electroplating in combination with an optimized seed layer. The superior magnetic properties of the Ni80Fe20 film at RF leads to a 3x higher Qmax and a 3x higher f(Qmax) compared to the state-of-the-art [4]. A record operating frequency f(Qmax) of >6.5 GHz and a record cut-off frequency (fcut-off) of >20 GHz are demonstrated. The configuration of a 4-turn nano/micro granular Ni80Fe20-core solenoid inductor is shown in Fig.1. Depositions of Ni80Fe20 films have been realized on three optimized types of seed layers, i.e. 100nm Ti (#a-seed), 100nm Ti covered by 10 nm TiN (#b-seed), and 100 nm Cr (#g-seed). During the electroplating deposition the FM films received the different nano/micro pattern illustrated in Fig.2. In contrast to the continuously plated homogeneous film (Fig.3c; #g- seed), a nano /micro structured film resulted from the #b- seed (Fig.2b), and the #a- seed gave a nano/micro pattern of mostly disconnected FM islands (R < 1.6 mm) (Fig.2a). Inductors having a #a-core clearly exhibited a higher FMR frequency and lower eddy current loss than inductors based on the #b-core or #g-core (Fig.3). The overall more than 3x enhancements of both Qmax and f(Qmax) have been achieved from the #a-core over the #g- core devices published by us at the past IEDM [4]. Inductance and quality factor versus frequency of three 4-turn #a-core inductors are shown in Fig.4 to have fcut-off >20 GHz. After systematically optimizing the design, the inductors with IPA>0.20 ìH/mm2, Qmax>4.5 and f(Qmax)>6.5 GHz have been achieved (Fig.5).

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