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Electrodeposition of FeCoNi thin films for magnetic-MEMS devices
Electrochimica Acta 51 (2006) 6346–6352
Electrodeposition of FeCoNi thin films for magnetic-MEMS devices B.Y. Yoo a , S.C. Hernandez a , D.-Y. Park b , N.V. Myung a,∗ a
Bourns Hall B350, Department of Chemical and Environmental Engineering, University of California, Riverside, CA 92521, USA b Department of Applied Materials Engineering, Hanbat National University, Daejeon 305-719, South Korea Received 16 February 2006; received in revised form 18 April 2006; accepted 18 April 2006 Available online 12 June 2006
Abstract The physical properties, including macro and microstructures, film stress, and corrosion resistances, along with the magnetic properties of electrodeposited FeCoNi thin films, which can be later integrated to magnetic-MEMS devices were systematically investigated by varying film composition. Increased Ni content affected both macro and microstructure of electrodeposits, switching from columnar structure to lamellar structure and from body centered cubic (BCC) to face centered cubic (FCC), respectively. The film stress of electrodeposits was increased with increasing deposit Ni content and it was inversely proportional to grain size. The corrosion resistance of films determined by polarization resistance and pitting potential initially improved with increasing deposit nickel content, followed by a maximum at ∼48 at.% deposit Ni content. After reaching an upper limit, the corrosion resistance slightly decreased with increasing deposit Ni content. The coercivity of FeCoNi alloy decreased when Ni content increased from 0 at.% to ∼13 at.% which might be due to decrease in grain size. However, from ∼13 at.% to ∼48 at.%, coercivity increased, which could be predominately affected by changes in film stress and microstructure. Fe-rich FeCoNi thin films (e.g. 68Fe29Co 3Ni) show good magnetic properties with minimum film stress for magnetic-MEMS actuated in the out-of-plane direction. © 2006 Elsevier Ltd. All rights reserved. Keywords: Electrodeposition; Magnetic thin film; FeCoNi; Film stress; Microstructure
1. Introduction The development of magnetic thin films with high magnetic moments has been greatly needed for many areas including magnetic induction writing heads [1–5], magnetic-MEMS devices [6,7], and other magnetic sensing devices. A higher magnetic moment is especially important for magnetic-MEMS actuators, which create mechanical movement by interaction between magnetic materials and external magnetic field, to obtain relatively large forces [8,9]. In addition, film stress of magnetic materials must be minimized to prevent structural failure. Furthermore, high corrosion resistance is desired to improve the reliability of magnetic-MEMS devices. Magnetic thin films can be deposited using several methods. Electrodeposition is widely used because of advantages such as ability to control the film composition, superior magnetic properties, fast deposition rate, and cost-effectiveness. Electrodeposited NiFe thin film, which is one of the most well known soft
∗
Corresponding author. Tel.: +1 951 827 7710; fax: +1 951 827 5696. E-mail address: [email protected] (N.V. Myung).
0013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2006.04.020
magnetic materials, was widely adopted in many applications [10,11], but its low magnetic moment limited the expansion of its usage in advanced devices. As a high magnetic moment material, CoFe binary alloy with a composition of 50Co50Fe was reported as having a ∼2.45 T magnetic moment as a bulk metal [12]. Even though it has such a high magnetic moment, CoFe is difficult to apply to practical devices because of its low corrosion resistance and brittleness. Therefore, many researchers have suggested adding a third element, such as Ni, Cu or B, to CoFe to overcome these limitations. CoFeCu alloy was suggested by several researchers [1,13]. Andricacos and Robertson [1] decreased coercivity of CoFe electrodeposits by adding Cu to a CoFe matrix, but magnetic saturation was degraded. Bradley et al. [13] applied pulse current to obtain CoFeCu alloys and predicted the composition variation of deposits by using an electrochemical model for the binary solution. As a CoFe based ternary alloy, FeCoNi alloy is a widely investigated magnetic material [1,2,14–28]. Osaka group studied magnetic properties and other physical properties of electrodeposited FeCoNi alloys [16–22]. Magnetic moments as high as 2.1 T were achieved at a composition of 12Ni65Co23Fe with a coercivity of 1.2 Oe [19]. The effect of sulfur containing addi-
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tives (SCA), such as saccharin and thiourea, on microstructures and corrosion properties was also studied [20]. Based on their work, they proposed that, by adding SCA, the BCC to FCC transition point would shift to a lower Fe content region, and corrosion resistance would be degraded, therefore low amounts of sulfur content should facilitate achievement of optimal high magnetic moment FeCoNi thin films. Gong et al. electrodeposited FeCoNi/Cu multilayers on n-type Si wafers and achieved high magnetoresistive sensitivity for low magnetic field (i.e. 5–10 Oe) [15]. Electroanalytical studies of FeCoNi electrodeposits were performed by several researchers [23–26]. Phan et al. demonstrated that their experimental data agreed well with the Hessami–Tobias model [23]. They also observed that abnormality of codeposition of CoNiFe was decreased when solution temperature increased. Zhuang et al. also suggested a kinetic model of electrodeposition of FeCoNi thin films [24–26]. Another important physical property of FeCoNi thin films is film stress. However, little research has been published about the film stress of FeCoNi electrodeposits. Our group showed that stress could be reduced by increasing the deposition temperature during electrodeposition of iron, Fe-rich FeCo binary alloys, Ferich FeCoNi ternary alloys in additive-free acidic chloride bath [27–29]. Tabakovic et al. investigated the effects of substrate on FeCoNi electrodeposits [30]. They compared two different substrates, PdNi and NiFe, resulting in slightly lower stress when films were deposited on NiFe. Guan and Nelson [31] investigated residual stress of CoNiMnP thin films, and they observed the stress was affected by film thickness and applied current density. For investigating the physical properties of electrodeposited FeCoNi thin films, many deposition parameters can be varied, such as metal ion concentration, temperature, current density, and pH. Among them, the effect of film composition was focused on in this work by varying the Ni ion concentration. While several studies have been performed to understand the effect of composition variation on the physical properties of FeCoNi alloy deposits [14,22,30], the relationship between composition and important physical properties, such as micro and macrostructures, stress, and corrosion, has not been systematically investigated. Therefore, in this work, a systematic study of physical properties including crystal structures, film stress, and corrosion resistance of electrodeposited FeCoNi thin films at different compositions were studied. In addition the magnetic properties, coercivity and squareness, which are important factors for magnetic-MEMS actuator, were studied with in-plane and out-of-plane directions to provide the fundamental data for the optimization of electrodeposition conditions of ternary FeCoNi magnetic thin films for magnetic MEMS and other magnetic devices. 2. Experimental The plating electrolytes of FeCoNi thin films were based on electroforming iron chloride baths which contain ferrous chloride (≈1–1.5 M) and calcium chloride (≈1 M) operating at high temperature (85–93 ◦ C) in very acidic environment (pH 0.5–1.5). Even though chloride baths are known to generate
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higher stress than sulfate and sulfamate bath in nickel electrodeposits [35], we previously reported that low-stress Fe-rich FeCoNi thin films can be obtained from chloride baths operating at high temperature [27]. In order to enhance the compatibility of bath to other MEMS processes, the pH of FeCoNi electrolytes was increased to four and L’acsorbic acid was added to improve the stability of bath. The effect of Ni composition on film properties, such as film stress, micro and macrostructure, corrosion resistance, and magnetic properties, was investigated by varying the concentration of Ni2+ in the baths. The electrolyte consisted of 1.275 M FeCl2 ·4H2 O + 0.225 M CoCl2 ·6H2 O + X M NiCl2 ·6H2 O + 1 M CaCl2 ·2H2 O + 0.05 M L-ascorbic acid, where the concentration of Ni ion was varied from 0.0 M to 1.6 M in 0.1 M increments. The solution pH was kept constant at four by adding HCl or NaOH. Calcium chloride was used as a supporting electrolyte, and L-ascorbic acid was added to minimize Fe2+ oxidation. Copper–beryllium strips with surface areas of 7.74 cm2 were used as substrates. Most of the thin films were deposited galvanostatically at 70 ◦ C with a constant current density of 5 mA/cm2 without agitation. Electrodeposition was performed with EG&G Princeton Applied Research Potentionstat/Galvanostat, model 173. Film stress was determined from sample curvature via profilometry after depositing the FeCoNi films on the copper–beryllium strips. The displacement between the substrate strips was measured with a deposit stress analyzer (model 683, Special Testing & Development Co.) and correlated to film stress. Structural investigation of film deposits was performed on films which were deposited galvanostatically for 24 h at 70 ◦ C, on brass cathodes with surface areas of 4 cm2 . Samples which were prepared to observe cross sectional macrostructure with optical microscopy were treated with selective chemical etching on polished cross sectional areas of the deposits with a mixture of ammonia and hydrogen peroxide (H2 O2 ) in deionized water. After 2 min of exposure to the etchant, the film was rinsed with deionized water and visually examined under a digital optical microscope (Hirox KH-3000). Analysis and identification of crystal phases and measurement of grain size in the electrodeposits was performed with an X-ray diffractometer (XRD) (model D/MAX 2500H, RIKAKU) with Cu K␣ radiation and a scanning range of 30–100◦ with 0.03◦ increments. Corrosion studies of FeCoNi films were performed by electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV). EIS was measured to determined polarization resistance and LSV was performed to determine pitting potential. Both experiments were conducted in naturally aerated 0.5 M NaCl solutions. Platinum-coated titanium sheet and saturated calomel electrode (SCE) were used as counter and reference electrode, respectively. A multichannel EG&G PAR VMP2 potentio/galvanostat was used to perform both experiments. Impedance spectra were collected from 100 kHz to 0.05 Hz at open circuit potentials, and the scan rate of LSV was fixed at 5 mV/s. Magnetic properties such as coercivity (Hc ) and squareness (S = Mr /Ms ) of the electrodeposited films were determined using
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Fig. 1. The dependence of film composition on the ratio of Ni ion concentration to total metal ion concentration.
a vibrating sample magnetometer (VSM, ADE Tech, Model 1660). 3. Results and discussion Fig. 1 shows the variation in composition when the concentration of Ni ion changed from 0 M to 1.6 M. As the Ni concentration in the electrolyte was increased from 0 M to 0.4 M, increasing the ratio of Ni concentration to the total amount of metal ion from 0% to 20%, the Ni content in the deposit was
slightly increased from 0 at.% to ∼13 at.%. However, when the Ni concentration ratio reached ∼28% (concentration = 0.6 M), the deposited Ni content jumped to ∼48 at.%. With this abrupt change in Ni content, Fe content also changed from ∼55 at.% (0.4 M of Ni2+ ) to ∼16 at.% (0.6 M Ni2+ ), whereas Co content remained relatively constant compared to Ni and Fe. Anomalous deposition phenomena, which means the less noble metal reduced preferentially [33], could be found when the ratio of Ni ions was below 28%, but disappeared when the Ni ratio was higher than 28%. In the 0–0.6 M Ni region, Co content in the deposit actually increased (from ∼30 at.% to ∼32 at.%) and Fe drastically decreased (from ∼70 at.% to ∼55 at.%). However, in the 0.6–1.6 M Ni region, Co and Fe content decreased simultaneously. Cross-sectional optical images of FeCoNi alloys at different compositions are shown in Fig. 2. To display cross sectional images clearly, films were deposited as thick as 150 m. When Ni concentration was 0 M, with only a CoFe binary alloy deposited, columnar structures were well developed (Fig. 2(a)). However, directionality of columnar structures began to display disturbance when Ni content was ∼3 at.% (Fig. 2 (b)), and almost disappeared when Ni composition reached ∼13 at.% (Fig. 2(c)). When Ni content was ∼48 at.%, only a lamellar structure was clearly shown. Fig. 3 shows the crystalline structures of each FeCoNi sample investigated with X-ray diffraction patterns and surface morphologies of the corresponding FeCoNi deposits imaged with a scanning electron microscope (SEM). As shown in the graphs, when Ni was not added, BCC(2 1 1) was predominant for the 70Fe30Co binary alloy deposit. The preferred orientation of BCC(2 1 1) corresponded to a star like hexagon shape of surface morphology which was observed in SEM micrographs.
Fig. 2. Cross-sectional optical images of FeCoNi electrodeposits with different film compositions: (a) 30Co70Fe; (b) 68Fe29Co3Ni; (c) 55Fe32Co13Ni; and (d) 16Fe36Co48Ni.
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Fig. 4. The dependence of FeCoNi grain size on deposit Ni content. The grain size was determined based on preferred plane of FeCoNi.
Fig. 3. X-ray diffraction patterns and SEM surface morphologies of FeCoNi electrodeposits with different film compositions: (a) 30Co70Fe; (b) 68Fe29Co3Ni; (c) 55Fe32Co13Ni; (d) 16Fe36Co48Ni; and (e) 12Fe15Co73Ni. Sub indicates the X-ray diffraction peaks from substrate.
As Ni content increased from 0 through ∼3 at.% to ∼13 at.%, the preferred orientation changed to BCC(1 1 0) and the intensity of BCC(2 0 0) increased. Rectangularly shaped facets became visible in surface morphology as BCC(1 1 0) became predominant, but when the Ni content reached ∼13 at.% the surface morphology was drastically changed. SEM images also reveal the facets shown at Ni ∼3 at.% disappeared, and very fine crystallite structures were observed, which corresponds to a broader peak in the XRD results. As shown in Fig. 1, when Ni concentration increased to 0.6 M, Ni content of the deposit was ∼48 at.% and FCC structures began to emerge. For the 48Ni36Co16Fe alloy deposit, FCC(1 1 1) became predominant and FCC(2 0 0) and FCC(3 1 1) planes were also detected. Surface morphology of this deposit showed similar structure to the sample with ∼13 at.% Ni content. The point of drastic change from a body centered structure to a face centered structure with increasing Ni content, which occurred at a Ni concentration of 0.6 M, was concurrent with the disappearance of columnar macro-structure as shown in Fig. 2. Variation in grain size of predominant planes with changing Ni content is presented in Fig. 4. Grain sizes for each condition were calculated from XRD results by using the Debye–Scherrer
method. Grain size rapidly decreased as Ni content increased from 0 at.% to ∼13 at.% and then slightly decreased when Ni content was higher than ∼48 at.%. The film stress values of the FeCoNi deposits, which were measured via profilometry for different Ni contents, are shown in Fig. 5. As shown in the graph, over the whole range film stress increased with increasing Ni content. Intrinsic film stress of the thin films showed high correlation with grain size. As shown in Fig. 4, grain size decreased with increasing Ni content. The inset graph in Fig. 5 clearly shows that stress decreased with increasing grain size. The stress of thin films which had grain sizes from 110 nm to 40 nm linearly decreased with grain size; however, below 40 nm, the relationship between stress and grain size was less clear. This ambiguity might be caused by phase changes in the FeCoNi thin films. As shown in Fig. 5, the stress of the FeCoNi thin films linearly increased with increasing Ni content up to ∼13 at.% Ni, which corresponds to grain size changes from 110 nm to 40 nm, but when Ni content was higher than ∼13 at.%,
Fig. 5. The dependence of FeCoNi film stress on deposit Ni content.
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the rate of film stress increase was hindered. This change in the stress variation tendency might be related to the micro and macrostructures of FeCoNi. As previously described, well developed columnar structures were drastically disturbed in the cross sectional view, when Ni content was increased from 0 at.% to ∼13 at.%, and grain size also decreased rapidly. Above ∼48 at.% Ni content, columnar structures were supplanted by the more stable lamellar macrostructure. Moreover, at this transition point (deposit Ni content ∼13 at.% to ∼48 at.%), the microstructure changed from BCC to FCC. Therefore, abrupt variation of stress from the low Ni content region (13 at.%) to the high Ni content region (48 at.%) corresponds to the massive change of the columnar structure (BCC) to a lamellar structure (FCC) in the electrodeposits. Slight increases in film stress were seen at subsequent higher Ni contents (deposit Ni content >48 at.%). Polarization resistance of the FeCoNi deposits was investigated by electrochemical impedance spectroscopy (EIS). Polarization resistance also seems to be related to the macro and microstructure of the deposits. As shown in Fig. 6(a), even though data were scattered, when Ni content was lower than ∼13 at.%, polarization resistance was below 8 × 103 cm, whereas at 48Ni36Co16Fe polarization resistance reached a maximum of 4.5 × 104 cm, and slightly decreased from there with increasing Ni content. In the low Ni content region (deposit Ni content 13 at.%), it is apparent that polarization resistance of the FeCoNi deposits was enhanced by not only increasing amounts of Ni, which can make strong passivation layers, but also by weakening of the columnar structure, which is comparatively vulnerable to corrosion, and decreasing grain size. When deposit Ni content was higher than 48 at.%, polarization resistance reached a maximum and then decreased with increasing Ni content. In this region, the macrostructure was stable as a lamellar structure, and remained predominately FCC(1 1 1). Size also slightly decreased compared to the low Ni content region, leading to the conclusion that structural effect on polarization resistance may be minimal. According to Myung and Nobe [32], for Ni-rich binary transition metal alloys, such as NiCo and NiFe, polarization resistance did not improve with increasing Ni content when the microstructure was stabilized as an FCC structure in high Ni content regions. This means that, in FCC structures, increasing Ni content may not improve polarization resistance. Therefore, in FeCoNi alloys, the decrease of polarization resistance with increasing Ni content in a Nirich phase may not be caused by the Ni itself, but is probably related to the increasing Fe/Co content ratio in alloy deposits. As Ni content increased from ∼48 at.% to ∼73 at.%, the Fe/Co ratio in deposits was increased from 0.43 to 1.11. Pitting potentials of FeCoNi thin films with different composition were also investigated with anodic polarization curve (Fig. 6(b)). When Ni content was zero, i.e. CoFe binary alloy, passivation was not observed. The pitting potential of the FeCoNi ternary alloy thin film was shift positively with increasing Ni content until reaching the negatively maximum (∼40 mV) when film composition was 16Fe36Co48Ni, which is similar tend with EIS results. Coercivities of FeCoNi thin films were measured in both inplane and out-of-plane directions (Fig. 7). Whereas in-plane coercivity is only concerned for magnetic head application,
Fig. 6. Polarization resistance (a) and pitting potential (b) as function of deposited Ni content. Polarization resistances and pitting potentials were determined from EIS and LSV, respectively.
both in-plane and out-of-plane coercivities must be considered for magnetic-MEMS device, since they can be actuated in both in-plane and out-of-plane direction [8,9]. Both coercivities were gradually decreased with increasing Ni content, however abrupt changes occurred when Ni content was varied from ∼3 at.% to ∼13 at.%. Coercivity is also highly affected by the grain size of thin films. If the grain size is large enough to have multiple domains, magnetization of ferromagnetic materials occurs via domain wall movement; coercivity decreases as grain size increases. However, if the grain size is in the nanometer range, where it is smaller than the effective domain-wall width, the magnetic properties of ferromagnetic materials can change. Below the magnetic exchange length (Lex ), the smallest grain size for magneto-crystalline anisotropy governing the magnetization process, the coercivity shows a steep decrease with decreasing grain size, following the D6 law [34]: Hc ≈ pc
K14 D6 J s A2
(1)
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changed from BCC to FCC. Therefore, the increase in coercivity in this region might be related to stress and crystallographical change. When the deposit Ni content was higher than 48 at.%, both coercivities decreased as grain size decreased by possessing the same predominant microstructure (FCC(1 1 1)). Squarenesses at in-plane and out-of-plane positions are also shown in Fig. 7(b). Across the entire range, squarenesses in both directions were varied following a similar trend as coercivity. As expected, the magnetic saturation of FeCoNi electrodeposits decreased with increased in Ni content (data not shown). 4. Conclusion
Fig. 7. Coercivities and squarenesses of FeCoNi electrodeposits with varying deposit Ni content in parallel and perpendicular directions: (a) parallel and perpendicular coercivity; (b) parallel and perpendicular squarenesses.
where A, K1 , and Js are the exchange constant, anisotropy energy, and magnetic moment, respectively. Pc is a constant and is typically 0.64 for randomly oriented cubic particles. As shown in Fig. 3, the grain size of the predominant planes decreased from ∼110 nm to ∼40 nm when Ni content increased from 0 at.% to ∼13 at.%, which corresponds to decreasing coercivity in this region. When deposit Ni content further increased to ∼48 at.% coercivity slightly increased (in-plane: ∼26 Oe to ∼31 Oe, out-of-plane: ∼160 Oe to 305 Oe), even though grain size did not change. Coercivity could be affected by not only grain size but also several other factors such as impurities and variation of magnetic anisotropy energy governed by shape, film stress, and crystalline anisotropy. Among these, film stress usually increases coercivity by increasing magnetoelastic anisotropy energy. When the deposit Ni content was increased from ∼13 at.% to ∼48 at.%, there was not a significant change in grain size. However, in this region, film stress slightly increased from ∼130 MPa to ∼150 MPa, and the predominant structure
Micro and macro-structural change, film stress, and corrosion resistance of electrodeposited FeCoNi thin films were investigated by varying the Ni concentration in a deposition bath with fixed Fe and Co concentrations. From the cross sectional optical images, a transition of columnar structure to lamellar structure with increasing deposit Ni content was observed when deposit Ni content was increased from ∼13 at.% to ∼48 at.%. As the lamellar structure became more pronounced, the predominant phase changed from BCC to FCC. In the low Ni content region (Ni content 13 at.%), corrosion resistance was observed with impedance measurements and anodic polarization curves. The corrosion resistance improved with increasing Ni content and decreasing grain size, but it slightly degraded when Ni content was over ∼48 at.%. The variation of coercivity of FeCoNi alloy was mostly affected by grain size in the low Ni content region (Ni content 13 at.%). However, in the phase transition region, from deposit Ni content ∼13 at.% to ∼48 at.%, coercivity seemed to be affected by stress and structural variations. Outof-plane squarness behaves similar to out-of-plane coercivities with a maximum of 0.45 at ∼3 at.% of Ni. For magnetic-MEMS devices, which are actuated by out-of-plane external magnetic field, Fe-rich FeCoNi thin films (e.g. 68Fe29Co3Ni) will be excellent magnetic material. Acknowledgements This work was supported by American Electroplaters and Surface Finishers (AESF) (Project #114), the Basic Research Program of the Korea Science & Engineering Foundation (Grant #R01-2003-000-10597-0), and Defense Microelectronic Activity (DMEA) under agreement H94003-05-2-0505. The United State government is authorized to reproduce and distribute reprints for government purpose, not withstanding any copyright notation thereon. References [1] P.C. Andricacos, N. Robertson, IBM J. Res. Develop. 42 (1998) 671. [2] E.I. Cooper, C. Bonhote, J. Heidmann, Y. Hsu, P. Kern, J.W. Lam, M. Ramasubramanian, N. Robertson, L.T. Romankiw, H. Xu, IBM J. Res. Develop. 49 (2005) 103. [3] E. Gomez, E. Pellicer, E. Valles, Electrochem. Commun. 7 (2005) 275. [4] K. Ohashi, N. Morita, T. Tsuda, Y. Nonaka, IEEE Trans. Magn. 35 (1999) 2538. [5] T. Osaka, Electrochim. Acta 44 (1999) 3885.
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Electrodeposition of FeCoNi thin films for magnetic-MEMS devices B.Y. Yoo a , S.C. Hernandez a , D.-Y. Park b , N.V. Myung a,∗ a
Bourns Hall B350, Department of Chemical and Environmental Engineering, University of California, Riverside, CA 92521, USA b Department of Applied Materials Engineering, Hanbat National University, Daejeon 305-719, South Korea Received 16 February 2006; received in revised form 18 April 2006; accepted 18 April 2006 Available online 12 June 2006
Abstract The physical properties, including macro and microstructures, film stress, and corrosion resistances, along with the magnetic properties of electrodeposited FeCoNi thin films, which can be later integrated to magnetic-MEMS devices were systematically investigated by varying film composition. Increased Ni content affected both macro and microstructure of electrodeposits, switching from columnar structure to lamellar structure and from body centered cubic (BCC) to face centered cubic (FCC), respectively. The film stress of electrodeposits was increased with increasing deposit Ni content and it was inversely proportional to grain size. The corrosion resistance of films determined by polarization resistance and pitting potential initially improved with increasing deposit nickel content, followed by a maximum at ∼48 at.% deposit Ni content. After reaching an upper limit, the corrosion resistance slightly decreased with increasing deposit Ni content. The coercivity of FeCoNi alloy decreased when Ni content increased from 0 at.% to ∼13 at.% which might be due to decrease in grain size. However, from ∼13 at.% to ∼48 at.%, coercivity increased, which could be predominately affected by changes in film stress and microstructure. Fe-rich FeCoNi thin films (e.g. 68Fe29Co 3Ni) show good magnetic properties with minimum film stress for magnetic-MEMS actuated in the out-of-plane direction. © 2006 Elsevier Ltd. All rights reserved. Keywords: Electrodeposition; Magnetic thin film; FeCoNi; Film stress; Microstructure
1. Introduction The development of magnetic thin films with high magnetic moments has been greatly needed for many areas including magnetic induction writing heads [1–5], magnetic-MEMS devices [6,7], and other magnetic sensing devices. A higher magnetic moment is especially important for magnetic-MEMS actuators, which create mechanical movement by interaction between magnetic materials and external magnetic field, to obtain relatively large forces [8,9]. In addition, film stress of magnetic materials must be minimized to prevent structural failure. Furthermore, high corrosion resistance is desired to improve the reliability of magnetic-MEMS devices. Magnetic thin films can be deposited using several methods. Electrodeposition is widely used because of advantages such as ability to control the film composition, superior magnetic properties, fast deposition rate, and cost-effectiveness. Electrodeposited NiFe thin film, which is one of the most well known soft
∗
Corresponding author. Tel.: +1 951 827 7710; fax: +1 951 827 5696. E-mail address: [email protected] (N.V. Myung).
0013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2006.04.020
magnetic materials, was widely adopted in many applications [10,11], but its low magnetic moment limited the expansion of its usage in advanced devices. As a high magnetic moment material, CoFe binary alloy with a composition of 50Co50Fe was reported as having a ∼2.45 T magnetic moment as a bulk metal [12]. Even though it has such a high magnetic moment, CoFe is difficult to apply to practical devices because of its low corrosion resistance and brittleness. Therefore, many researchers have suggested adding a third element, such as Ni, Cu or B, to CoFe to overcome these limitations. CoFeCu alloy was suggested by several researchers [1,13]. Andricacos and Robertson [1] decreased coercivity of CoFe electrodeposits by adding Cu to a CoFe matrix, but magnetic saturation was degraded. Bradley et al. [13] applied pulse current to obtain CoFeCu alloys and predicted the composition variation of deposits by using an electrochemical model for the binary solution. As a CoFe based ternary alloy, FeCoNi alloy is a widely investigated magnetic material [1,2,14–28]. Osaka group studied magnetic properties and other physical properties of electrodeposited FeCoNi alloys [16–22]. Magnetic moments as high as 2.1 T were achieved at a composition of 12Ni65Co23Fe with a coercivity of 1.2 Oe [19]. The effect of sulfur containing addi-
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tives (SCA), such as saccharin and thiourea, on microstructures and corrosion properties was also studied [20]. Based on their work, they proposed that, by adding SCA, the BCC to FCC transition point would shift to a lower Fe content region, and corrosion resistance would be degraded, therefore low amounts of sulfur content should facilitate achievement of optimal high magnetic moment FeCoNi thin films. Gong et al. electrodeposited FeCoNi/Cu multilayers on n-type Si wafers and achieved high magnetoresistive sensitivity for low magnetic field (i.e. 5–10 Oe) [15]. Electroanalytical studies of FeCoNi electrodeposits were performed by several researchers [23–26]. Phan et al. demonstrated that their experimental data agreed well with the Hessami–Tobias model [23]. They also observed that abnormality of codeposition of CoNiFe was decreased when solution temperature increased. Zhuang et al. also suggested a kinetic model of electrodeposition of FeCoNi thin films [24–26]. Another important physical property of FeCoNi thin films is film stress. However, little research has been published about the film stress of FeCoNi electrodeposits. Our group showed that stress could be reduced by increasing the deposition temperature during electrodeposition of iron, Fe-rich FeCo binary alloys, Ferich FeCoNi ternary alloys in additive-free acidic chloride bath [27–29]. Tabakovic et al. investigated the effects of substrate on FeCoNi electrodeposits [30]. They compared two different substrates, PdNi and NiFe, resulting in slightly lower stress when films were deposited on NiFe. Guan and Nelson [31] investigated residual stress of CoNiMnP thin films, and they observed the stress was affected by film thickness and applied current density. For investigating the physical properties of electrodeposited FeCoNi thin films, many deposition parameters can be varied, such as metal ion concentration, temperature, current density, and pH. Among them, the effect of film composition was focused on in this work by varying the Ni ion concentration. While several studies have been performed to understand the effect of composition variation on the physical properties of FeCoNi alloy deposits [14,22,30], the relationship between composition and important physical properties, such as micro and macrostructures, stress, and corrosion, has not been systematically investigated. Therefore, in this work, a systematic study of physical properties including crystal structures, film stress, and corrosion resistance of electrodeposited FeCoNi thin films at different compositions were studied. In addition the magnetic properties, coercivity and squareness, which are important factors for magnetic-MEMS actuator, were studied with in-plane and out-of-plane directions to provide the fundamental data for the optimization of electrodeposition conditions of ternary FeCoNi magnetic thin films for magnetic MEMS and other magnetic devices. 2. Experimental The plating electrolytes of FeCoNi thin films were based on electroforming iron chloride baths which contain ferrous chloride (≈1–1.5 M) and calcium chloride (≈1 M) operating at high temperature (85–93 ◦ C) in very acidic environment (pH 0.5–1.5). Even though chloride baths are known to generate
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higher stress than sulfate and sulfamate bath in nickel electrodeposits [35], we previously reported that low-stress Fe-rich FeCoNi thin films can be obtained from chloride baths operating at high temperature [27]. In order to enhance the compatibility of bath to other MEMS processes, the pH of FeCoNi electrolytes was increased to four and L’acsorbic acid was added to improve the stability of bath. The effect of Ni composition on film properties, such as film stress, micro and macrostructure, corrosion resistance, and magnetic properties, was investigated by varying the concentration of Ni2+ in the baths. The electrolyte consisted of 1.275 M FeCl2 ·4H2 O + 0.225 M CoCl2 ·6H2 O + X M NiCl2 ·6H2 O + 1 M CaCl2 ·2H2 O + 0.05 M L-ascorbic acid, where the concentration of Ni ion was varied from 0.0 M to 1.6 M in 0.1 M increments. The solution pH was kept constant at four by adding HCl or NaOH. Calcium chloride was used as a supporting electrolyte, and L-ascorbic acid was added to minimize Fe2+ oxidation. Copper–beryllium strips with surface areas of 7.74 cm2 were used as substrates. Most of the thin films were deposited galvanostatically at 70 ◦ C with a constant current density of 5 mA/cm2 without agitation. Electrodeposition was performed with EG&G Princeton Applied Research Potentionstat/Galvanostat, model 173. Film stress was determined from sample curvature via profilometry after depositing the FeCoNi films on the copper–beryllium strips. The displacement between the substrate strips was measured with a deposit stress analyzer (model 683, Special Testing & Development Co.) and correlated to film stress. Structural investigation of film deposits was performed on films which were deposited galvanostatically for 24 h at 70 ◦ C, on brass cathodes with surface areas of 4 cm2 . Samples which were prepared to observe cross sectional macrostructure with optical microscopy were treated with selective chemical etching on polished cross sectional areas of the deposits with a mixture of ammonia and hydrogen peroxide (H2 O2 ) in deionized water. After 2 min of exposure to the etchant, the film was rinsed with deionized water and visually examined under a digital optical microscope (Hirox KH-3000). Analysis and identification of crystal phases and measurement of grain size in the electrodeposits was performed with an X-ray diffractometer (XRD) (model D/MAX 2500H, RIKAKU) with Cu K␣ radiation and a scanning range of 30–100◦ with 0.03◦ increments. Corrosion studies of FeCoNi films were performed by electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV). EIS was measured to determined polarization resistance and LSV was performed to determine pitting potential. Both experiments were conducted in naturally aerated 0.5 M NaCl solutions. Platinum-coated titanium sheet and saturated calomel electrode (SCE) were used as counter and reference electrode, respectively. A multichannel EG&G PAR VMP2 potentio/galvanostat was used to perform both experiments. Impedance spectra were collected from 100 kHz to 0.05 Hz at open circuit potentials, and the scan rate of LSV was fixed at 5 mV/s. Magnetic properties such as coercivity (Hc ) and squareness (S = Mr /Ms ) of the electrodeposited films were determined using
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Fig. 1. The dependence of film composition on the ratio of Ni ion concentration to total metal ion concentration.
a vibrating sample magnetometer (VSM, ADE Tech, Model 1660). 3. Results and discussion Fig. 1 shows the variation in composition when the concentration of Ni ion changed from 0 M to 1.6 M. As the Ni concentration in the electrolyte was increased from 0 M to 0.4 M, increasing the ratio of Ni concentration to the total amount of metal ion from 0% to 20%, the Ni content in the deposit was
slightly increased from 0 at.% to ∼13 at.%. However, when the Ni concentration ratio reached ∼28% (concentration = 0.6 M), the deposited Ni content jumped to ∼48 at.%. With this abrupt change in Ni content, Fe content also changed from ∼55 at.% (0.4 M of Ni2+ ) to ∼16 at.% (0.6 M Ni2+ ), whereas Co content remained relatively constant compared to Ni and Fe. Anomalous deposition phenomena, which means the less noble metal reduced preferentially [33], could be found when the ratio of Ni ions was below 28%, but disappeared when the Ni ratio was higher than 28%. In the 0–0.6 M Ni region, Co content in the deposit actually increased (from ∼30 at.% to ∼32 at.%) and Fe drastically decreased (from ∼70 at.% to ∼55 at.%). However, in the 0.6–1.6 M Ni region, Co and Fe content decreased simultaneously. Cross-sectional optical images of FeCoNi alloys at different compositions are shown in Fig. 2. To display cross sectional images clearly, films were deposited as thick as 150 m. When Ni concentration was 0 M, with only a CoFe binary alloy deposited, columnar structures were well developed (Fig. 2(a)). However, directionality of columnar structures began to display disturbance when Ni content was ∼3 at.% (Fig. 2 (b)), and almost disappeared when Ni composition reached ∼13 at.% (Fig. 2(c)). When Ni content was ∼48 at.%, only a lamellar structure was clearly shown. Fig. 3 shows the crystalline structures of each FeCoNi sample investigated with X-ray diffraction patterns and surface morphologies of the corresponding FeCoNi deposits imaged with a scanning electron microscope (SEM). As shown in the graphs, when Ni was not added, BCC(2 1 1) was predominant for the 70Fe30Co binary alloy deposit. The preferred orientation of BCC(2 1 1) corresponded to a star like hexagon shape of surface morphology which was observed in SEM micrographs.
Fig. 2. Cross-sectional optical images of FeCoNi electrodeposits with different film compositions: (a) 30Co70Fe; (b) 68Fe29Co3Ni; (c) 55Fe32Co13Ni; and (d) 16Fe36Co48Ni.
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Fig. 4. The dependence of FeCoNi grain size on deposit Ni content. The grain size was determined based on preferred plane of FeCoNi.
Fig. 3. X-ray diffraction patterns and SEM surface morphologies of FeCoNi electrodeposits with different film compositions: (a) 30Co70Fe; (b) 68Fe29Co3Ni; (c) 55Fe32Co13Ni; (d) 16Fe36Co48Ni; and (e) 12Fe15Co73Ni. Sub indicates the X-ray diffraction peaks from substrate.
As Ni content increased from 0 through ∼3 at.% to ∼13 at.%, the preferred orientation changed to BCC(1 1 0) and the intensity of BCC(2 0 0) increased. Rectangularly shaped facets became visible in surface morphology as BCC(1 1 0) became predominant, but when the Ni content reached ∼13 at.% the surface morphology was drastically changed. SEM images also reveal the facets shown at Ni ∼3 at.% disappeared, and very fine crystallite structures were observed, which corresponds to a broader peak in the XRD results. As shown in Fig. 1, when Ni concentration increased to 0.6 M, Ni content of the deposit was ∼48 at.% and FCC structures began to emerge. For the 48Ni36Co16Fe alloy deposit, FCC(1 1 1) became predominant and FCC(2 0 0) and FCC(3 1 1) planes were also detected. Surface morphology of this deposit showed similar structure to the sample with ∼13 at.% Ni content. The point of drastic change from a body centered structure to a face centered structure with increasing Ni content, which occurred at a Ni concentration of 0.6 M, was concurrent with the disappearance of columnar macro-structure as shown in Fig. 2. Variation in grain size of predominant planes with changing Ni content is presented in Fig. 4. Grain sizes for each condition were calculated from XRD results by using the Debye–Scherrer
method. Grain size rapidly decreased as Ni content increased from 0 at.% to ∼13 at.% and then slightly decreased when Ni content was higher than ∼48 at.%. The film stress values of the FeCoNi deposits, which were measured via profilometry for different Ni contents, are shown in Fig. 5. As shown in the graph, over the whole range film stress increased with increasing Ni content. Intrinsic film stress of the thin films showed high correlation with grain size. As shown in Fig. 4, grain size decreased with increasing Ni content. The inset graph in Fig. 5 clearly shows that stress decreased with increasing grain size. The stress of thin films which had grain sizes from 110 nm to 40 nm linearly decreased with grain size; however, below 40 nm, the relationship between stress and grain size was less clear. This ambiguity might be caused by phase changes in the FeCoNi thin films. As shown in Fig. 5, the stress of the FeCoNi thin films linearly increased with increasing Ni content up to ∼13 at.% Ni, which corresponds to grain size changes from 110 nm to 40 nm, but when Ni content was higher than ∼13 at.%,
Fig. 5. The dependence of FeCoNi film stress on deposit Ni content.
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the rate of film stress increase was hindered. This change in the stress variation tendency might be related to the micro and macrostructures of FeCoNi. As previously described, well developed columnar structures were drastically disturbed in the cross sectional view, when Ni content was increased from 0 at.% to ∼13 at.%, and grain size also decreased rapidly. Above ∼48 at.% Ni content, columnar structures were supplanted by the more stable lamellar macrostructure. Moreover, at this transition point (deposit Ni content ∼13 at.% to ∼48 at.%), the microstructure changed from BCC to FCC. Therefore, abrupt variation of stress from the low Ni content region (13 at.%) to the high Ni content region (48 at.%) corresponds to the massive change of the columnar structure (BCC) to a lamellar structure (FCC) in the electrodeposits. Slight increases in film stress were seen at subsequent higher Ni contents (deposit Ni content >48 at.%). Polarization resistance of the FeCoNi deposits was investigated by electrochemical impedance spectroscopy (EIS). Polarization resistance also seems to be related to the macro and microstructure of the deposits. As shown in Fig. 6(a), even though data were scattered, when Ni content was lower than ∼13 at.%, polarization resistance was below 8 × 103 cm, whereas at 48Ni36Co16Fe polarization resistance reached a maximum of 4.5 × 104 cm, and slightly decreased from there with increasing Ni content. In the low Ni content region (deposit Ni content 13 at.%), it is apparent that polarization resistance of the FeCoNi deposits was enhanced by not only increasing amounts of Ni, which can make strong passivation layers, but also by weakening of the columnar structure, which is comparatively vulnerable to corrosion, and decreasing grain size. When deposit Ni content was higher than 48 at.%, polarization resistance reached a maximum and then decreased with increasing Ni content. In this region, the macrostructure was stable as a lamellar structure, and remained predominately FCC(1 1 1). Size also slightly decreased compared to the low Ni content region, leading to the conclusion that structural effect on polarization resistance may be minimal. According to Myung and Nobe [32], for Ni-rich binary transition metal alloys, such as NiCo and NiFe, polarization resistance did not improve with increasing Ni content when the microstructure was stabilized as an FCC structure in high Ni content regions. This means that, in FCC structures, increasing Ni content may not improve polarization resistance. Therefore, in FeCoNi alloys, the decrease of polarization resistance with increasing Ni content in a Nirich phase may not be caused by the Ni itself, but is probably related to the increasing Fe/Co content ratio in alloy deposits. As Ni content increased from ∼48 at.% to ∼73 at.%, the Fe/Co ratio in deposits was increased from 0.43 to 1.11. Pitting potentials of FeCoNi thin films with different composition were also investigated with anodic polarization curve (Fig. 6(b)). When Ni content was zero, i.e. CoFe binary alloy, passivation was not observed. The pitting potential of the FeCoNi ternary alloy thin film was shift positively with increasing Ni content until reaching the negatively maximum (∼40 mV) when film composition was 16Fe36Co48Ni, which is similar tend with EIS results. Coercivities of FeCoNi thin films were measured in both inplane and out-of-plane directions (Fig. 7). Whereas in-plane coercivity is only concerned for magnetic head application,
Fig. 6. Polarization resistance (a) and pitting potential (b) as function of deposited Ni content. Polarization resistances and pitting potentials were determined from EIS and LSV, respectively.
both in-plane and out-of-plane coercivities must be considered for magnetic-MEMS device, since they can be actuated in both in-plane and out-of-plane direction [8,9]. Both coercivities were gradually decreased with increasing Ni content, however abrupt changes occurred when Ni content was varied from ∼3 at.% to ∼13 at.%. Coercivity is also highly affected by the grain size of thin films. If the grain size is large enough to have multiple domains, magnetization of ferromagnetic materials occurs via domain wall movement; coercivity decreases as grain size increases. However, if the grain size is in the nanometer range, where it is smaller than the effective domain-wall width, the magnetic properties of ferromagnetic materials can change. Below the magnetic exchange length (Lex ), the smallest grain size for magneto-crystalline anisotropy governing the magnetization process, the coercivity shows a steep decrease with decreasing grain size, following the D6 law [34]: Hc ≈ pc
K14 D6 J s A2
(1)
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changed from BCC to FCC. Therefore, the increase in coercivity in this region might be related to stress and crystallographical change. When the deposit Ni content was higher than 48 at.%, both coercivities decreased as grain size decreased by possessing the same predominant microstructure (FCC(1 1 1)). Squarenesses at in-plane and out-of-plane positions are also shown in Fig. 7(b). Across the entire range, squarenesses in both directions were varied following a similar trend as coercivity. As expected, the magnetic saturation of FeCoNi electrodeposits decreased with increased in Ni content (data not shown). 4. Conclusion
Fig. 7. Coercivities and squarenesses of FeCoNi electrodeposits with varying deposit Ni content in parallel and perpendicular directions: (a) parallel and perpendicular coercivity; (b) parallel and perpendicular squarenesses.
where A, K1 , and Js are the exchange constant, anisotropy energy, and magnetic moment, respectively. Pc is a constant and is typically 0.64 for randomly oriented cubic particles. As shown in Fig. 3, the grain size of the predominant planes decreased from ∼110 nm to ∼40 nm when Ni content increased from 0 at.% to ∼13 at.%, which corresponds to decreasing coercivity in this region. When deposit Ni content further increased to ∼48 at.% coercivity slightly increased (in-plane: ∼26 Oe to ∼31 Oe, out-of-plane: ∼160 Oe to 305 Oe), even though grain size did not change. Coercivity could be affected by not only grain size but also several other factors such as impurities and variation of magnetic anisotropy energy governed by shape, film stress, and crystalline anisotropy. Among these, film stress usually increases coercivity by increasing magnetoelastic anisotropy energy. When the deposit Ni content was increased from ∼13 at.% to ∼48 at.%, there was not a significant change in grain size. However, in this region, film stress slightly increased from ∼130 MPa to ∼150 MPa, and the predominant structure
Micro and macro-structural change, film stress, and corrosion resistance of electrodeposited FeCoNi thin films were investigated by varying the Ni concentration in a deposition bath with fixed Fe and Co concentrations. From the cross sectional optical images, a transition of columnar structure to lamellar structure with increasing deposit Ni content was observed when deposit Ni content was increased from ∼13 at.% to ∼48 at.%. As the lamellar structure became more pronounced, the predominant phase changed from BCC to FCC. In the low Ni content region (Ni content 13 at.%), corrosion resistance was observed with impedance measurements and anodic polarization curves. The corrosion resistance improved with increasing Ni content and decreasing grain size, but it slightly degraded when Ni content was over ∼48 at.%. The variation of coercivity of FeCoNi alloy was mostly affected by grain size in the low Ni content region (Ni content 13 at.%). However, in the phase transition region, from deposit Ni content ∼13 at.% to ∼48 at.%, coercivity seemed to be affected by stress and structural variations. Outof-plane squarness behaves similar to out-of-plane coercivities with a maximum of 0.45 at ∼3 at.% of Ni. For magnetic-MEMS devices, which are actuated by out-of-plane external magnetic field, Fe-rich FeCoNi thin films (e.g. 68Fe29Co3Ni) will be excellent magnetic material. Acknowledgements This work was supported by American Electroplaters and Surface Finishers (AESF) (Project #114), the Basic Research Program of the Korea Science & Engineering Foundation (Grant #R01-2003-000-10597-0), and Defense Microelectronic Activity (DMEA) under agreement H94003-05-2-0505. The United State government is authorized to reproduce and distribute reprints for government purpose, not withstanding any copyright notation thereon. References [1] P.C. Andricacos, N. Robertson, IBM J. Res. Develop. 42 (1998) 671. [2] E.I. Cooper, C. Bonhote, J. Heidmann, Y. Hsu, P. Kern, J.W. Lam, M. Ramasubramanian, N. Robertson, L.T. Romankiw, H. Xu, IBM J. Res. Develop. 49 (2005) 103. [3] E. Gomez, E. Pellicer, E. Valles, Electrochem. Commun. 7 (2005) 275. [4] K. Ohashi, N. Morita, T. Tsuda, Y. Nonaka, IEEE Trans. Magn. 35 (1999) 2538. [5] T. Osaka, Electrochim. Acta 44 (1999) 3885.
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