A novel NiZn ferrite integrated magnetic solenoid inductor with a high quality factor at 0 . 7 – 6 GHz

Integrated inductor is one of the fundamental components and has been widely used in radio frequency integrated circuits (RFICs). It has been challenging to achieve simultaneously high inductance and quality factor, particularly at GHz frequencies. In this work, we reported a novel integrated solenoid inductor with a magnetic NiZn ferrite as the core material, which was deposited by a low-cost spin spray technique. These integrated inductors showed a significant improvement in both inductance and quality factor at GHz frequencies over their air core counterparts. A stable inductance was observed within a wide frequency ranged from 700 MHz to 6 GHz. The peak value of quality factor reached 23, a relatively higher value not reported for solenoid inductors up to date. Our results indicate that the integrated inductor are promising for applications in RFICs.


I. INTRODUCTION
Integrated inductors with a high quality factor (Q-factor) have attracted more and more interests for radio frequency (RF) applications, especially for wireless communications in smart phones, tablet computers and wearable electronics etc. High inductance density and quality factor are two essential parameters for their practical applications in radio frequency integrated circuits (RFICs).In previous works, increased inductance density and quality factor have been achieved by using sputtered or electroplated magnetic materials as an inductor core.  Fornstance, by using Veeco-CVC system to deposit Co-Zr-Ta as a single-layer magnetic material in spiral inductors, Gardner et al. 1 reported an increase in inductance (30%-60%) at a frequency up to 3GHz.Gao et al. 19 demonstrated a high quality factor integrated RF magnetic inductors based on a solenoid structure with magnetic FeGaB/Al 2 O 3 multilayers.Those inductors showed an excellent high-frequency performance with a wide operation frequency in range of 0.5-2.5GHz, in which the peak quality factor reached ∼20.The inductance showed a more than 100% enhancement compared to that of the air core inductor.
However, these typical sputtering or electroplating methods take much long time, usually more than 24 hours.Also the price are very expensive even with a low Q-factor.
7][28] Specifically, it has been demonstrated  28 The growth rate was approximately 40 nm/min. 22,23Microstructure and composition characterization of the NiZn ferrite films were studied by x-ray diffraction (XRD) with a Cu Kα source (λ = 1.541Å).The magnetization vs applied magnetic field loops were measured using a vibrating sample magnetometer (VSM), with an external magnetic field applied in the plane of the thin film.The complex permeability spectrums of films were taken by a broad band measurement technique using a coplanar waveguide network analyzer with a bandwidth in 0.5 to 5 GHz range.
The solenoid inductors were microfabricated on silicon wafer by a thermal oxide process to get a 500 nm thick SiO 2 thin layer.To reduce the substrate clamping effect, a layer of polyimide PI2611 was spin coated on the wafers.The thickness of the first layer of polyimide (PI1) was 6 µm.Physical vapor deposition (PVD) system was used to deposit the Cr(10nm)/Cu(30nm) seed layer for the Cu bottom electrode.A 6 µm thick photoresist (P4620) layer was patterned on the seed layer as the mask for the electroplating of the bottom Cu layer.The current density for electroplating was optimized to achieve a 4µm thick Cu for good quality.After electroplating, the photoresist strip was rushed off by using acetone.The coil thickness was around 4 µm measured by a profilometer.A second layer of polyimide (HD 4110, photosensitive) was patterned on the bottom Cu to form the isolation layer, at the same time, via openings were also made for the winding connection.The magnetic layer was deposited by using the spin spray.Similarly, the top seed layer of Cu was also deposited with PVD, followed by electroplating to get 4 µm thick Cu top layer.

III. RESULTS AND DISCUSSION
A typical XRD pattern for the spinel ferrite is given in Fig. 1 the permeability spectra of the NiZn ferrite film.A large initial real permeability of µ r >200 has been achieved at 0.5 GHz and µ r >80 at 1 GHz.While a low magnetic loss of tanδ m (µ r /µ r ) is less than 0.03 at 3-5GHz range.Figure 3(a) and (b) give optical photos of integrated air core and magnetic core.Figure 3(c) shows the focused-ion-beam (FIB) milled cross section of devices imaged in-situ with a scanning electron microscopy (SEM).From the imag of crossing section as shown Figure 3(c), the thickness of the NiZn Ferrtie of 2.1µm was obtained.
Such parameters of the fabricated inductor as inductance and Q-factor can be described by the following relation: The total inductance of an inductor Lac comes from the main inductance Lm(ac) and the winding leakage inductance Ll(ac) Lac = Lm(ac) + Ll(ac) (1) The Lm(ac) is given by (2) FIG. 3. Optical photos of inductors with the (a) air core and (b) magnetic core and the (c) cross-sectional SEM image of the magnetic core.

056606-5
Wang et al.AIP Advances 7, 056606 (2017)   where s is the thickness of a single laminated core and δ t is the skin depth of the core, given as The dc inductance is expressed as where N is the number of turns, u e and l c are the relative permeability and length of the core, respectively.While ac inductance is written as where Rw(dc) is the dc resistance of the conductor coil.A is a dimensionless quantity that depends on the winding conductor geometry.For integrated magnetic solenoid inductors, the inductance can be approximated by where N is the total number of turns; and w, t m , and l are the width, the thickness, and the length of the magnetic film.The total ac resistance is Rac = Rw + Rc, which Rw is the ac winding resistance and Rc is the core's equivalent series resistance.The ac winding resistance of an inductor is The resistance can be written by Accordingto Q = (ωL/R), the quality factor of an integrated solnoid inductor can be calculated using ( 5) and ( 7) a  Based on the above analysis model, we used a HFSS software to simulate the parameters of a 3.5turn inductor and the simulation results are shown in Figure 4(a) and (b).The measured inductance and Q-factor are presented in Figure 4(c) and (d).It shows that simulation results agree well with the measured ones.The results show a very good high-frequency performance over a wide frequency ranged 0.5-6 GHz.The inductance can be over 1.04 nH and the peak quality factor is more than 23 at 3 GHz, which is a highest Q obtained up to date at this frequency.The ferrite magnetic inductor showed about a 20% inductance increase than the similar air core inductor.An increase in Q was observed at a relatively low frequency owing to the magnetic core, demonstrating a benefit by using the ferrite magnetic layer.
Table I compared the published results with our measurements.According to Table I, the working frequency of published inductors were only MHz.Some design inductors are working at a high frequency, however, the quality factor is very low.From Gao's work, they obtained significantly enhanced inductance and quality factor at 1GHz, but the quality factor decreased a lot.That is because FeGaB/AlOx multilayers were very lossy at high frequency.These working frequency range is very narrow with the addition of magnetic layers.In this work, we obtained very wide working frequency from 0.5 -6 GHz with a relatively high quality factor, which can be potentially used for smart phone, tablet computers and wearable electronics etc.

IV. CONCLUSION
In this work, solenoid inductors were fabricated by using spin sprayed NiZn ferrites as magnetic cores.The magnetization of 7000Oe, the real permeability of >200 at 0.5 GHz and loss of <0.03 at 3-5GHz with FMR linewidth of 140 Oe were demonstrated for the fabricated solenoid inductors.It showed that both inductance and Q factor have an excellent performance at the frequency of 700MHz-6GHz.Moreover, the spin spray procedure can reduce time and cost for the fabrication of integrated inductors, which is of great potential for applications in RFICs.

FIG. 4 . 6 Wang
FIG. 4. Comparison of the simulated results of (a) inductance and (b) Q-factor with the measured ones of (c) indcance and (d) Q-factor.
that the NiZn spinel ferrite thin film is a great candidate for high frequency applications regarding its high permeability, resistivity and saturation magnetization.In this work, we report a Ni 0.27 Zn 0.1 Fe 2.63 O 4 thin film with the thickness of ∼2.1 µm.The film was firstly deposited by the spin spray technique and then applied as the magnetic cores of inductors.

TABLE I .
Operation frequency, inductance density and peak q-factor of published design and this work.