SCIENCE CHINA Technological Sciences • RESEARCH PAPER •
January 2013 Vol.56 No.1: 84–88 doi: 10.1007/s11431-012-5039-7
Electrical and magnetic properties of electrodeposited Fe-based alloys used for thin film transformer HOU XiaoWei*, LIU ShiBin, YANG ShangLin, LI JuPing & GUO Bo School of Electronics and Information, Northwestern Polytechnical University, Xi’an 710072, China Received June 29, 2012; accepted August 23, 2012; published online October 1, 2012
Three electrodeposited Fe-Ni, Fe-Co, and Fe-Ni-Co cores of thin film transformer are prepared on silicon (1 0 0) substrates, which are sputtered a 90 nm thick film of Cu acting as the seed layer. The core films consisting of Fe-Ni 20:80, Fe-Co 60:40 and Fe-Ni-Co 10:60:30, respectively, are deposited using direct current electrodeposition. The surface texture, electrical and magnetic properties are surveyed by scanning electron microscopy (SEM), superconducting quantum interference device (SQUID), etc. The wave transmission ability and efficiency of thin film transformer with these cores, inputting the sine wave, are compared. All the analyses indicate that FeNi alloy films display the optimal magnetic properties and excellent transformer performance. electrodeposition, thin film transformer, core, magnetic properties, transmission efficiency Citation:
Hou X W, Liu S B, Yang S L, et al. Electrical and magnetic properties of electrodeposited Fe-based alloys used for thin film transformer. Sci China Tech Sci, 2013, 56: 8488, doi: 10.1007/s11431-012-5039-7
1 Introduction
saturation magnetic induction Bs, high electrical resistivity , high remanence ratio R 'r Br /Bs (Br represents rema-
As a kind of miniaturization magnetic devices, the thin film transformer is widely used in the impedance matching and signal coupling and isolation of RF integrated circuit [1, 2]. The magnetic core, conducted as one of the most important components of thin film transformer, has a high demand for the soft magnetic materials. Soft magnetic thin films, such as Fe based alloys [3] including Fe-Ni [4−9], Fe-Co [10−14], Fe-Ni-Co [15−20] alloys, are widely surveyed and intensely demanded for the high-frequency devices, for example the high-density recording media, MEMS, planar inductors and thin film transformer integrated with the Si wafers [4, 5, 21−23]. The basic property demand for the high-frequency applications operating in high frequency range (MHz) is as follows: high initial permeability i, high
nence) and low coercivity Hc [24]. Traditionally, ferrite and metal magnetic materials are used to fabricate the core of thin film transformer. However, it is found that the two magnetic substances have their respective shortcomings [1, 2, 21]. For ferrite magnetic materials, both the hysteresis loss and eddy current loss increase greatly as well as the proximity effect and skin effect of the traditional winding when the operating frequency reaches or exceeds the MHZ; on the other hand, the inherent low resistivity of metal magnetic substances makes large power consumption. All of the disadvantages restrict the further development of high-frequency applications. In this condition, the new metal magnetic thin film alloys with excellent magnetic properties are widely investigated now. It is known that the ferromagnetic 3D transition metal amorphous metal substances, including iron, nickel and cobalt, possess the excellent soft magnetic properties if the
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alloys are prepared with appropriate proportion of metal mixture [19]. Literarily, there are many kinds of methods to produce the alloy film [6−10]. The electrodeposition is widely used for its unprecedented maneuverability and good extension growth [3, 14, 16, 25]. Here three kinds of ferromagnetic 3D transition metal mixture thin films are deposited on the Si (1 0 0) substrates using the direct current electrodeposition, Fe-Ni, Fe-Co and Fe-Ni-Co alloys. The related characteristics, including electrical and magnetic, are studied by several techniques, including SEM, EDS and SQUID, etc.
The three kinds of thin film alloys, including Fe-Ni, Fe-Co, and Fe-Co-Ni alloys, were prepared by the technique of direct current electrodeposition. Before the electrodeposition, a 90 nm thick Cu thin film was sputtered with magnetron sputtering instrument on the substrate Si (1 0 0) wafers playing as the seed layer. The wafers were rectangular being 30 mm in length and 1.5 mm in width. They were cleaned by ultrasonic for 20 min, then rinsed and dried by deionized water and nitrogen gas. The DC electrodeposition was used under the different deposited conditions to fabricate the thin film specimens. The deposition was carried out in a 267 mL Hull Cell, with a closed atmosphere at the room temperature. In order to get the excellent soft magnetic properties, a great many experiments have been conducted. The optimal plating conditions for each alloy are shown in Table 1, which include the addition of the inorganic and organic additives as well as the control parameters. The alloys of Fe-Ni, Fe-Co and Fe-Ni-Co were fabricated taking the given plating conditions (shown in Table 1). The related characteristics of the thin film alloys were tested by several equipments. The surface microstructure and composition were investigated by the SEM and EDS, respectively. The electrical features, mainly referring to the electrical resistivity, were tested by the four-probe resistance measuring instrument. The static magnetic properties, such as hysTable 1 Solution composition and plating conditions for optimal soft magnetic properties Fe-Ni film Fe-Co film Fe-Ni-Co film 150 28 15 15 260 30 20 10 7.5 8 45 25 30 2.5 1 1.5 1.5 0.02 1.2 3 0.15 0.1 3.3 3.0 3.5 57 40 60 10 10 10
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teresis loop, Bs, Hc, remanence Br, were measured by SQUID. At last, the alloy films were used as cores for the thin film transformer. The coil of thin film transformer was designed the same as each other, thus to improve the comparability. The turns for primary and secondary were 1000 and 1000, respectively. Then a sine wave, with the voltage amplitude of 1 V and excitation frequency 400 kHz was imported. And the wave quality, input and output voltage were obtained and compared by the high precision oscilloscope.
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2 Experimental
FeSO4·7H2O (g L1) NiSO4·7H2O (g L1) CoSO4·7H2O (g L1) NaCl (g L1) H3BO3 (g L1) Saccharin (g L1) Additives (g L1) Cathode current density (A dm2) PH Temperature (°C) Time (min)
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Results and discussion
The specific composition of thin films measured by the EDS is as follows: Fe-Ni 20:80, Fe-Co 60:40, and Fe-Ni-Co 10:60:30. The thickness of the deposited films is appropriate 3 m. The microstructure was analyzed by EDS with the amplification factors of 2000 and 5000, respectively. It is known from Figures 1(a)−1(f) that the Fe-Co alloys are more homogeneous, compact and smooth than the two other films. The color of Fe-Ni-Co alloy looks light, for it contains a lower content of iron element. Amongst the three microstructures, the particle of the Fe-Co alloy is more uniform and compact than the two other thin films. The reason presumably is that the compatibility and combinability of Fe-Co alloy are superior to the other two films. Table 2 shows the comparison of the resistivity, magnetic properties and transformer performance among the three samples. The electrical resistivity for the three alloys is 75, 103 and 124 μΩ cm, respectively. The value of electrical resistivity of the three deposited films is not high (the largest 124 μΩ cm). Although the 3D transition metals of Fe, Ni and Co are good conductive substances, they can present high electrical resistivity if they are prepared with the appropriate composition. And there are many feasible ways to improve further the electrical resistivity. Typically, the trace amount of gas oxygen is doped during the deposition, thus a very small proportion of thin film alloys is oxidized and the resistivity has a considerable enhancement [7]. Seen from Figures 2(a), 3(a), and 4(a) of three hysteresis loops of Fe-Ni, Fe-Co and Fe-Ni-Co, the Bs is 1.67 T, 1.93 T and 1.42 T, respectively. The Hc is 2.5 Oe, 12.5 Oe and 6.19 Oe. The Br is 1.39 T, 1.84 T and 1.12 T. The R'r is 0.8323, 0.9519 and 0.7887. The i is 12330, 8280 and 4735. The voltages of transformer for both input and output are 1.15 V & 0.668 V, 1.033 V & 0.415 V, and 1.059 V & 0.498 V, respectively (shown in Figures 2(b), 3(b), and 4(b)). And the phase difference between voltage and current was ignored here to increase comparability when the transmission efficiency was calculated. It has been found that the Fe-Co alloys exhibit the largest values of Bs, remanence ratio R'r and Hc, the smallest Bs and i for Fe-Ni-Co, the largest i and the lowest Hc for Fe-Ni.
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Figure 1 SEM images of Fe-based films. (a), (b) Fe-Ni film; (c), (d) Fe-Co film; (e), (f) Fe-Ni-Co film.
Figure 2
Figure 3
(a) Hysteresis loop of Fe-Ni alloy film; (b) input/output voltage for Fe-Ni core.
(a) Hysteresis loop Fe-Co alloy film; (b) input/output voltage for Fe-Co core.
It is known that the content of Fe in alloys determines the Bs, so Bs increases with Fe content to some extent. Experimentally, the electrodeposition conditions for Fe-Co films were changed, and the Bs increased with the high level of Fe. During the test, no significant variation of composition was found for the three films with the increase of the depos-
ited time, which is in accordance with the results reported by Spada et al. [3]. When the deposited films were used as the magnetic cores of thin film transformer, we found that the Fe-Ni alloy thin film had a better performance considering the transmission efficiency (47% for Fe-Ni, 35% for Fe-Co, 41% for
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Figure 4
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Fe-Ni alloy
Fe-Co alloy
Fe-Ni-Co alloy
75 12330
103 8280
124 4735
1.67
1.93
1.42
2.5
12.5
6.19
1.39 0.8323 47%
1.84 0.9519 35%
1.12 0.7887 41%
Fe-Ni-Co) and wave transmission ability. We observed that the transmission efficiency of all films showed a relatively low value and not more than 50%. The possible reason is that the resistance of winding is too large, for the small cross-sectional area of coil ( electrical resistivity is about 3 Ω m1 for copper coil), which causes a substantial power loss. The other possible reason is the large core loss (hysteresis loss and eddy loss), which is proportional to the operating frequency. We also found that the actual primary voltage was a little higher than the input voltage. This phenomenon is normal. We can use the mutual inductance between the primary and secondary coils to explain it. The voltage induced by the secondary winding is stacked to the primary voltage. Considering the operating frequency, the three films cannot work normally for the nonlinear property below low frequency. It was also found that the Fe-Co alloy had a wider operating frequency than the two other ones though the transmission efficiency was not high, and the Fe-Ni-Co came second. The preliminary explanation is the addition of Co element.
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(a) Hysteresis loop of Fe-Ni-Co alloy film; (b) input/output voltage for Fe-Ni-Co core.
Table 2 Comparison of the resistivity, magnetic properties and transformer performance among the three alloys Sample Parameter Resistivity (μΩ cm) Initial permeability i Saturation magnetic induction Bs (T) Coercivity Hs (Oe) Remanence Br (T) High remanence ratio R'r Transmission efficiency
January (2013) Vol.56 No.1
Conclusions
We fabricated three kinds of alloy films on the silicon (1 0 0) wafers by DC current electrodeposition under the optimal
plating conditions, Fe20Ni80, Fe60Co40 and Fe10Ni60Co30. The microstructure and composing component were tested by SEM and EDS. SQUID was used to analyze the magnetic properties. And the wave transmission ability and transmission efficiency for thin film transformer with the magnetic cores of deposited films were measured. The analysis for these alloys shows that the Fe20Ni80 film provides the largest initial permeability i 12330, the smallest coercivity Hc 2.5 Oe and the highest transmission efficiency 47%. Amongst the three alloys, Fe20Ni80 films are more suitable for the field of thin film transformer. This work is supported by the National Natural Science Foundation of China (Grant No. 60874101).
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