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LTCC components
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Ag precipitation at the free interface of multilayer NiCuZn ferrites/LTCC components Hsing-I Hsiang a,∗ , Bing Jyun Lyu a , Li-Then Mei a , Chi-Shiung Hsi b a b
Particulate Materials Research Center, Department of Resources Engineering, National Cheng Kung University, Tainan 70101, Taiwan, ROC Department of Materials Science and Engineering, National United University, Miaoli, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 12 August 2015 Received in revised form 16 November 2015 Accepted 5 December 2015 Available online xxx Keywords: LTCC NiCuZn ferrites Cofiring Silver diffusion Silver precipitation
a b s t r a c t The diffusion and precipitation of silver and copper in low-dielectric-constant low temperature cofired ceramics (LTCC) for multilayer NiCuZn ferrites/LTCC components are investigated in this study. Ag precipitation is formed at the free interface between NiCuZn ferrites and LTCC for multilayer NiCuZn ferrites/LTCC components after cofiring. During the heating stage Cu+ will segregate into the grain boundary from the NiCuZn ferrites due to the low oxygen partial pressure inside the sample and then dissolve into LTCC near the interface between NiCuZn ferrites and LTCC. Cu+ in glass would diffuse from the interior toward the surface and become oxidized into Cu2+ or precipitated as the CuO phase at the surface. To maintain charge neutrality this process requires an outward diffusion of Cu+ and Ag+ from the interior toward the free surface. The reduction of Ag+ into Ag on the free surface is accelerated by Cu+ oxidation. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction With the growing appetite for handheld electronic devices, these devices will continue to undergo further miniaturization, reduction in weight and portability. As portable devices continue to develop toward multi-functionality and high frequency applications the integration of inductors, capacitors and resistors into one monolithic chip component will offer better performance and save more occupied space on the printed circuit board during assembly. Therefore, the demand for small-sized integrated electromagnetic interference (EMI) filters, such as LC filters, multilayer common mode filters and DC–DC converters will continue to rapidly increase [1–3]. One of the most important processes in manufacturing defectfree multilayer integrated passive components involves cofiring the inner-electrode, dielectric and magnetic materials. Low temperature cofired ceramics (LTCC) and NiCuZn ferrites have been widely used in manufacturing LC filters and inductors due to their superior dielectric and magnetic properties. Ag is chosen as the material for the internal conductor for multilayer chip inductors due to its low resistivity resulting in components with high quality factor. Mismatched densification kinetics [4], thermal expansion [5], severe
∗ Corresponding author. Fax: +886 62380421. E-mail address: [email protected] (H.-I. Hsiang).
chemical reactions [6] and Ag diffusion [7–9] between the different materials could generate undesirable defects such as delamination, cracks, camber and reliability concerns in the final products. The cofiring behavior and interfacial structure of LTCC/NiCuZn ferrite laminates were recently investigated [2,10–13]. In our previous study large pores and air gaps occurred at the interface between NiCuZn ferrites containing Bi2 O3 and LTCC after cofiring at 900 ◦ C for 2 h. This is due to widening of the shrinkage mismatch between NiCuZn ferrites and LTCC resulting from Bi2 O3 in NiCuZn ferrites diffusion into LTCC. This leads to the formation of a new Bi–B–Si glass with a lower glass transition temperature and viscosity sandwiched between LTCC and NiCuZn ferrites [10]. Lee et al. [11] studied the interdiffusion between NiCuZn ferrite and LTCC during co-firing. They found that Cu ions from the ferrite can diffuse into LTCC at a distance of around 120 m and Mg and Al ions from LTCC also diffuse into the ferrite for a short distance during co-firing. Ag ion diffusion into LTCC may lead to reliability concerns, such as increasing leakage current and decreasing insulation resistance [14]. The interfacial reaction kinetics and mechanism between silver and ceramic-filled glass substrates, silver diffusion and microstructure development in LTCC has been reported [7–9]. The previous reports focused mostly on the Ag diffusion of the LTCC-Ag internal electrode systems. However, Ag diffusion of the embedded silver internal electrode in LTCC for multilayer NiCuZn ferrites/LTCC components during cofiring has never been reported to our best knowledge. In this study, the diffusion and precipitation of silver
http://dx.doi.org/10.1016/j.jeurceramsoc.2015.12.002 0955-2219/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: H.-I. Hsiang, et al., Ag precipitation at the free interface of multilayer NiCuZn ferrites/LTCC components, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.12.002
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Table 1 Chemical composition (wt%) of the commercial NiCuZn ferrites. Materials
Fe2 O3
ZnO
NiO
CuO
MnO
Co2 O3
NiCuZn ferrites
67.28
13.18
12.49
6.84
0.21
–
Table 2 Chemical composition (wt%) of the commercial LTCC powder. Materials
B2 O3
Al2 O3
SiO2
K2 O
Na2 O
LTCC
0.87
14.30
83.83
0.77
0.23
and copper in LTCC for multilayer NiCuZn ferrites/LTCC components are characterized and analyzed using optical microscopy, scanning electron microscopy, X-ray diffractometer and X-ray photoelectron spectroscopy.
Fig. 1. Optical micrograph of the surface of the multilayer NiCuZn ferrite/LTCC component after cofiring at 900 ◦ C.
2. Experimental procedures A commercial NiCuZn ferrite powder without the addition of Bi2 O3 and a commercial low-dielectric-constant LTCC powder (borosilicate glass added with alumina and quartz fillers) were used as the raw materials. The specific surface area values of NiCuZn ferrites and LTCC powders were measured using a surface area analyzer (Micrometrics, ASAP 2020, Norcross, GA, USA) are 7.48, and 6.36 m2 /g, respectively. The chemical compositions determined by X-ray fluorescence spectrometer (Rigaku, ZSX100E, Tokyo, Japan) of the raw materials are listed in Tables 1 and 2. The chemical formula of NiCuZn ferrites can be represented as Ni0.4 Zn0.38 Cu0.2 Mn0.02 Fe2 O4 . The multilayer LTCC/NiCuZn ferrite components were prepared using tape casting and screen-printing methods. The LTCC and NiCuZn ferrites powders were mixed with a commercial organic vehicle (Ferro, MSI 73210) and then fabricated using the doctor–blade technique for the tape preparation. The NiCuZn ferrites and LTCC green sheet thicknesses were all about 50 m. Silver conducting paste (Shoei, Japan) was screen-printed onto the LTCC green sheet. The printed LTCC green sheets (5 layers) were sandwiched between NiCuZn ferrite green sheets (5 layers) or blank LTCC green sheet (5 layers). The NiCuZn ferrite/LTCC/NiCuZn ferrite sandwiched composites and LTCC/Ag composites were prepared by stacking and then hot isostatic laminated at 70 ◦ C and 60 MPa for 10 min. Binder burnout was carried out at 450 ◦ C for 12 h. The samples were sintered at 900 ◦ C for 2 h in air or nitrogen atmosphere at a heating rate of 5 ◦ C/min. In order to determine the copper diffusion behavior in LTCC, 10wt% reagent-grade CuO was mixed with LTCC powder and then melted at 1400 ◦ C for 1 h. The melt was then quenched in water to form (B–Si–Al–Cu glass) BSACG glass. The BSACG glass was powdered homogeneously and dry pressed at 100 MPa into pellets. These specimens were then sintered at 900 ◦ C for 1 h. The microstructure was observed using optical microscopy (Nikon, 50iPOL) and scanning electron microscopy (Hitachi, S4100, Tokyo, Japan). The element distribution was measured using electron probe microanalysis (EPMA) (JEOL, JXA-8900R, Tokyo, Japan). The crystalline phase identification was determined using X-ray diffractometry (Dandong Fangyuan, DX-2700, Sandong, China) with CuK␣ radiation. X-ray photoelectron spectroscopy (XPS) (ESCA PHI 1600, Physical Electronics, Chanhassan, MN) measurements were performed on both surface and polished specimens using Mg K␣ (1253.5 eV) radiation. The sample charge produced by irradiation was determined by measuring the shift in the C 1s signal given by a binding energy of 284.5 eV. The specimen chemical compositions were compared through XPS.
Fig. 2. X-ray photon spectroscopy (XPS) results of (a) the surface and (b) interior (about 0.02 mm below the surface) of the multilayer NiCuZn ferrite/LTCC component after cofiring at 900 ◦ C.
3. Results and discussion Fig. 1 shows the optical micrograph of the surface of the multilayer NiCuZn ferrite/LTCC component after cofiring at 900 ◦ C, indicating that a different phase was formed at the interface between NiCuZn ferrites and LTCC. Note that a surface-segregated phase was not formed in the component interior, indicating that the phase observed at the interface between the NiCuZn ferrite and LTCC concentrated only near the free interface. Serious overelectroplating was observed near the interface between the NiCuZn ferrites and LTCC, suggesting the segregated phase would exhibit a low resistivity. This will lead to a reliability concern involving the increase in leakage current and shorting between conductors. Based on the EDS result (not shown) the segregated phase can be identified as an Ag-rich phase. Fig. 2(a) and (b) show the X-ray photon spectroscopy (XPS) results of the surface and interior (about 0.02 mm below the surface) of the multilayer NiCuZn ferrite/LTCC component after cofiring at 900 ◦ C, respectively. Two characteristic binding energy peaks occurred at 368.4 eV and 366.6 eV for the component surface cofired at 900 ◦ C assigned as Ag◦ and Ag2 O, respectively [15–17]. However, at the interior only one characteristic peak around 369.36 eV was observed due to Ag+ [18], indicating that Ag+ may diffuse from the inner silver electrode into the LTCC
Please cite this article in press as: H.-I. Hsiang, et al., Ag precipitation at the free interface of multilayer NiCuZn ferrites/LTCC components, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.12.002
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Fig. 3. X-ray diffraction patterns of (a) the surface and (b) interior of the BSACG pellet sintered at 900 ◦ C and (c) BSACG powders.
glass. This is consistent with the observations of Jean and Chang [7] who investigated the interfacial reaction kinetics between silver and ceramic-filled glass (CFG) substrates and reported that Ag+ diffused from the silver and incorporated into the glass accompanied with Al3+ ions dissolved from the CFG. The main compositions in the LTCC are silicon and aluminum. The NiCuZn ferrite is composed mainly of nickel, copper, zinc and iron. In our previous study [10], the chemical composition across the interface between NiCuZn ferrites and LTCC after cofiring at 900 ◦ C for 2 h was investigated and observed that no obvious element inter-diffusion of Ni, Zn, Fe, Al and Si was observed at the interface. Only a small concentration of copper ions from the ferrite can diffuse into the LTCC at a distance of around 10 m, which is consistent with the observations of Lee et al. [11]. This may be due to the formation of copper-rich precipitates during NiCuZn ferrite sintering attributed to the sintering process being faster than oxygen diffusion from the outside into the pores [19]. The absence of oxygen inside the pores leads to a reduction of Cu2+ into Cu+ , promoting the precipitation of Cu-rich phase onto the NiCuZn ferrite grain boundary [20–22] and dissolving into the glass at the interface between the ferrites and LTCC. Silver precipitation was not found on the LTCC surface for the LTCC-Ag cofired components. Moreover, the silver precipitation position occurred at the interface between NiCuZn ferrites and LTCC (Fig. 1), where was overlapped with that enriched with copper for the LTCC-NiCuZn ferrite composites. These suggest that the silver precipitation onto the surface near the interface between the ferrites and LTCC may be related to the copper diffusion. To further confirm whether copper segregation and diffusion in LTCC can lead to silver precipitation onto the surface near the interface, the valence state and behavior of copper ions in boron alumino-silicate glass were investigated. Fig. 3 shows the X-ray diffraction patterns of BSACG powders and the surface and interior of the BSACG pellet sintered at 900 ◦ C. BSACG glass powder exhibited amorphous characteristics. After sintering at 900 ◦ C, quartz, corundum and CuO phases were observed. Note that CuO XRD intensities at the surface were higher than those in the interior for the sintered BSACG glass, indicating that more CuO was formed on the surface than in the interior. The XPS spectra of the surface and polished surface of the sintered BSACG glass were simulated using XPSPEAK41, as shown in Fig. 4. The simulation results indicate that the three peaks at 932.8, 933.8 and 935 eV for the surface sample can be assigned as Cu+ , CuO and Cu2+ , respectively [23]. However, for the polished samples, only two peaks attributed to Cu+ (932.8 eV) and Cu2+ (934.5 eV) were
3
Fig. 4. XPS spectra of (a) the surface and (b) polished surface of the sintered BSACG glass.
Table 3 Ratios between the integrated area of XPS peaks of Cu 2p3/2 (Cu2+ , Cu+ and CuO) and Si 2p (as an internal standard) for the surface and interior of the sintered BSACG glass. Integrated area ratio
Surface
Interior
Cu2+ /Si Cu+ /Si CuO/Si (Cu+ + Cu2+ + CuO)/Si
0.054 0.035 0.127 0.216
0.134 0.025 – 0.159
observed. Table 3 summarizes the ratios between the integrated XPS peak areas of Cu 2p3/2 (CuO, Cu2+ and Cu+ ) and Si4+ (as an internal standard) for the surface and interior (polished surface) of the sintered BSACG glass. Note that the copper existed mainly as Cu2+ and Cu+ in the interior of the sintered BSACG glass. However, the CuO 2p3/2 peak became dominant for the surface sample. Moreover, the ratio of the total CuO, Cu2+ and Cu+ 2p3/2 peaks and Si 2p peak area for the surface sample is larger than that for the polished sample, indicating that copper is concentrated at the sample surface. This suggests that copper ions in the boron aluminosilicate glass would diffuse from the interior toward the surface and precipitated as the CuO phase at the surface. This is consistent with the observations of Kamiya et al. [24] who reported that CuO layers were formed at the CuO–Al2 O3 –4SiO2 glass surface after annealing in air. The fraction of about 90% total copper ions was Cu+ ions and Cu+ ions located in the interior of the glass migrated to the surface and reacted with oxygen. Fig. 5 shows the SEM micrographs and electron probe microanalysis (EPMA) mapping result of the interface between NiCuZn ferrites and LTCC for the NiCuZn ferrite-LTCC component after cofiring in air and nitrogen atmosphere at 900 ◦ C for 2 h. It shows that Ag diffused into LTCC and concentrated at the interface between the ferrites and LTCC for the sample sintered in air. In contrast, the diffusion of silver and the Ag-enrichment at the interface seems to be suppressed for the sample sintered in nitrogen atmosphere, compared with the sample sintered in air. Fig. 6 shows optical micrograph of the side surfaces for the NiCuZn ferrite-LTCC component after cofiring in nitrogen atmosphere at 900 ◦ C for 2 h. In contrast to the sample cofired in air (Fig. 1), no silver was found at the side surfaces for the samples cofired in nitrogen atmosphere. This means that the outward diffusion and enrichment of Ag+ near the interface and Ag precipitation at the free interface can be ascribed to Cu+ oxidation into Cu2+ .
Please cite this article in press as: H.-I. Hsiang, et al., Ag precipitation at the free interface of multilayer NiCuZn ferrites/LTCC components, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.12.002
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Fig. 5. SEM micrographs and electron probe microanalysis (EPMA) mapping result of the interface between NiCuZn ferrites and LTCC for the NiCuZn ferrite-LTCC component after cofiring in air (a) and nitrogen atmosphere (b) at 900 ◦ C for 2 h.
Fig. 6. Optical micrograph of the side surfaces for the NiCuZn ferrite-LTCC components after cofiring in nitrogen atmosphere at 900 ◦ C for 2 h.
Based on the above results a schematic of the Ag+ diffusion, enrichment and silver precipitation mechanism at the interface between NiCuZn ferrites and LTCC is proposed as shown in Fig. 7:
During the heating stage Cu+ will segregate into the grain boundary from the NiCuZn ferrites due to the low oxygen partial
Please cite this article in press as: H.-I. Hsiang, et al., Ag precipitation at the free interface of multilayer NiCuZn ferrites/LTCC components, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.12.002
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References
Fig. 7. Schematic mechanism of Ag+ diffusion, enrichment and silver precipitation at the interface between NiCuZn ferrites and LTCC.
pressure inside the sample and then dissolve into LTCC near the interface between the NiCuZn ferrites and LTCC. In the meantime, the silver embedded in LTCC will dissolve into the glass and exist as Ag+ . At the free surface Cu+ ions are oxidized into Cu2+ ions, and the oxygen is reduced by electrons into oxygen ions. Cu2+ ions react with ionic oxygen at the surface, which leads to a crystalline CuO forming onto the glass surface. To maintain charge neutrality, this process requires an outward diffusion of Cu+ and Ag+ from the interior toward the free surface. Ag+ ions are thermally unstable in glass. The reduction of Ag+ into Ag on the free surface may be accelerated by the oxidation of Cu+ . 4. Conclusion Ag precipitation was formed at the free interface between NiCuZn ferrites and LTCC for the multilayer NiCuZn ferrite/LTCC component after cofiring. Serious over-electroplating was observed after the electroplating process, leading to a reliability concern involving the increase in leakage current and shorting between conductors. Ag diffused into LTCC and concentrated at the interface between the ferrites and LTCC for the sample sintered in air. In contrast, the diffusion of silver and the Ag-enriched at the interface can be suppressed for the sample sintered in nitrogen atmosphere. The outward diffusion and enrichment of Ag+ near the interface and Ag precipitation at the free interface can be ascribed to Cu+ oxidation into Cu2+ . At the free surface Cu+ ions are oxidized into Cu2+ ions and the oxygen is reduced by electrons into oxygen ions. Cu2+ ions react with ionic oxygen at the surface, which leads to a crystalline CuO forming onto the glass surface. To maintain charge neutrality this process requires an outward diffusion of Cu+ and Ag+ from the interior toward the free surface. The reduction of Ag+ into Ag on the free surface then occurs, accelerated by the oxidation of Cu+ .
[1] Y. Li, Y. Liu, H. Zhang, L. Han, Z. Yang, A multilayer low pass filter fabricated by ferrites and ceramic cofiring system based on ltcc technology, Int. Conf. Electron. Packag. Technol. High Density Packag. (ICEPT-HDP) (2009) 961–964. [2] T. Rabe, H. Naghib-Zadeh, C. Glitzky, J. Topfer, Integration of Ni–Cu–Zn ferrite in low temperature co-fired ceramics (LTCC) modules, Int. J. Appl. Ceram. Soc. 9 (2012) 18–28. [3] A. Roesler, J. Schare, C. Hettler, D. Abel, G. Slama, D. Schofield, Integrated power electronics using a ferrite-based low-temperature co-fired ceramic materials system, Electron. Compon. Technol. Conf. (2010) 720–726. [4] Y.J. Choi, J.H. Park, W.J. Ko, I.S. Hwang, J.H. Park, J.G. Park, S. Nahm, Co-firing and shrinkage matching in low- and middle- permittivity dielectric compositions for a low-temperature co-fired ceramics system, J. Am. Ceram. Soc. 89 (2006) 562–567. [5] T. Rabe, W.A. Schiller, T. Hochheimer, C. Modes, A. Kipka, Zero shrinkage of LTCC by self-constrained sintering, Int. J. Appl. Ceram. Technol. 2 (2005) 374–382. [6] H.I. Hsiang, W.C. Liao, Y.J. Wang, Y.F. Cheng, Interfacial reaction of TiO2 /NiCuZn ferrites in multilayer composite, J. Eur. Ceram. Soc. 24 (2004) 2015–2021. [7] J.H. Jean, C.R. Chang, Interfacial reaction kinetics between silver and ceramic-filled glass substrate, J. Am. Ceram. Soc. 87 (2004) 1287–1293. [8] K.B. Shim, N.T. Cho, S.W. Lee, Silver diffusion and microstructure in LTCC multilayer couplers for high frequency applications, J. Mater. Sci. 35 (2000) 813–820. [9] C.S. Hsi, Y.R. Chen, H.I. Hsiang, Diffusivity of silver ions in the low temperature co-fired ceramic (LTCC) substrates, J. Mater. Sci. 46 (2011) 4695–4700. [10] H.I. Hsiang, B.J. Lyu, L.T. Mei, C.S. Hsi, Interfacial reaction between LTCC/NiCuZn ferrites in multilayer composites, Int. J. Appl. Ceram. Technol. 11 (2014) 496–501. [11] Y.H. Lee, W.C. Kuan, W.H. Tuan, Inter-diffusion between NiCuZn-ferrite and LTCC and its influence on magnetic performance, J. Eur. Ceram. Soc. 33 (2013) 95–103. [12] J.C. Jao, P. Li, S.F. Wang, Characterization of inductor with Ni–Zn–Cu ferrite embedded in B2 O3 –SiO2 glass, Jpn. J. Appl. Phys. 46 (2007) 5792–5796. [13] X.M. Cui, J. Zhou, B. Li, Z.F. Tong, Co-firing behavior and interfacial structure of BaO–TiO2 –B2 O3 –SiO2 glass–ceramics/NiCuZn ferrite composites, Mater. Manuf. Proc. 22 (2007) 251–255. [14] R.E. Doty, J.J. Vajo, A study of field-assisted silver migration in a low temperature cofirable ceramic. in ISHM 1995 proceedings, Int. Soc. Hybrid Microelectron. Rest. VA (1995) 465–474. [15] K. Xu, J. Heo, Effect of silver ion-exchange on the precipitation of lead sulfide quantum dots in glasses, J. Am. Ceram. Soc. 95 (2012) 2880–2884. [16] S. Thomas, S.K. Nair, E.M.A. Jamal, S.H.A. Harthi, M.R. Varma, M.R. Anantharaman, Size-dependent surface plasmon resonance in silver silica nanocomposites, Nanotechnology 19 (7) (2008) 075710–075711. [17] S. Mohapatra, Tunable surface plasmon resonance of silver nanoclusters in ion exchange soda lime glass, J. Alloys Compd. 598 (2014) 11–15. [18] R.C. Lucace, T. Radu, A.S. Tãtar, I. Lupan, O. Ponta, V. Simon, The influence of local structure and surface morphology on the antibacterial activity of silver-containing calcium borosilicate glasses, J. Non-cryst. Solids 404 (2014) 98–103. [19] A. Barba, C. Clausell, J.C. Jarque, M. Monzó, ZnO and CuO crystal precipitation in sintering Cu-doped Ni–Zn ferrites: I. Influence of dry relative density and cooling rate, J. Eur. Ceram. Soc. 31 (2011) 2119–2128. [20] H.I. Hsiang, J.L. Wu, Copper-rich phase segregation effects on the magnetic properties and dc-bias-superposition characteristic of NiCuZn ferrites, J. Magn. Magn. Mater. 374 (2015) 367–371. [21] H.I. Hsiang, J.F. Chueh, Low-pressure-assisted constrained sintering of low-temperature-fire NiCuZn ferrites, Int. J. Appl. Ceram. Technol. 12 (2015) E194–E201. [22] H.I. Hsiang, J.L. Wu, Cooling rate effects on the microstructure, magnetic properties, and DC superposition behavior of NiCuZn ferrites, Int. J. Appl. Ceram. Technol. 12 (2015) 1065–1070. [23] T. Yano, M. Ebizuka, S. Shibata, M. Yamane, Anomalous chemical shifts of Cu 2p and Cu LMM auger spectra of silicate glasses, J. Electron Spectrosc. Relat. Phenom. 131-132 (2003) 133–144. [24] K. Kamiya, T. Yoko, S. Sakka, Migration of Cu+ Ions in Cu2 O–Al2 O3 –SiO2 glass on heating air, J. Non-Cryst. Solids 80 (1986) 405–411.
Acknowledgment This work was financially sponsored by the Ministry of Science and Technology of the Republic of China(NSC 101-2221-E-006-125MY3 and 102-2622-E-006-013-CC2).
Please cite this article in press as: H.-I. Hsiang, et al., Ag precipitation at the free interface of multilayer NiCuZn ferrites/LTCC components, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.12.002
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Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Ag precipitation at the free interface of multilayer NiCuZn ferrites/LTCC components Hsing-I Hsiang a,∗ , Bing Jyun Lyu a , Li-Then Mei a , Chi-Shiung Hsi b a b
Particulate Materials Research Center, Department of Resources Engineering, National Cheng Kung University, Tainan 70101, Taiwan, ROC Department of Materials Science and Engineering, National United University, Miaoli, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 12 August 2015 Received in revised form 16 November 2015 Accepted 5 December 2015 Available online xxx Keywords: LTCC NiCuZn ferrites Cofiring Silver diffusion Silver precipitation
a b s t r a c t The diffusion and precipitation of silver and copper in low-dielectric-constant low temperature cofired ceramics (LTCC) for multilayer NiCuZn ferrites/LTCC components are investigated in this study. Ag precipitation is formed at the free interface between NiCuZn ferrites and LTCC for multilayer NiCuZn ferrites/LTCC components after cofiring. During the heating stage Cu+ will segregate into the grain boundary from the NiCuZn ferrites due to the low oxygen partial pressure inside the sample and then dissolve into LTCC near the interface between NiCuZn ferrites and LTCC. Cu+ in glass would diffuse from the interior toward the surface and become oxidized into Cu2+ or precipitated as the CuO phase at the surface. To maintain charge neutrality this process requires an outward diffusion of Cu+ and Ag+ from the interior toward the free surface. The reduction of Ag+ into Ag on the free surface is accelerated by Cu+ oxidation. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction With the growing appetite for handheld electronic devices, these devices will continue to undergo further miniaturization, reduction in weight and portability. As portable devices continue to develop toward multi-functionality and high frequency applications the integration of inductors, capacitors and resistors into one monolithic chip component will offer better performance and save more occupied space on the printed circuit board during assembly. Therefore, the demand for small-sized integrated electromagnetic interference (EMI) filters, such as LC filters, multilayer common mode filters and DC–DC converters will continue to rapidly increase [1–3]. One of the most important processes in manufacturing defectfree multilayer integrated passive components involves cofiring the inner-electrode, dielectric and magnetic materials. Low temperature cofired ceramics (LTCC) and NiCuZn ferrites have been widely used in manufacturing LC filters and inductors due to their superior dielectric and magnetic properties. Ag is chosen as the material for the internal conductor for multilayer chip inductors due to its low resistivity resulting in components with high quality factor. Mismatched densification kinetics [4], thermal expansion [5], severe
∗ Corresponding author. Fax: +886 62380421. E-mail address: [email protected] (H.-I. Hsiang).
chemical reactions [6] and Ag diffusion [7–9] between the different materials could generate undesirable defects such as delamination, cracks, camber and reliability concerns in the final products. The cofiring behavior and interfacial structure of LTCC/NiCuZn ferrite laminates were recently investigated [2,10–13]. In our previous study large pores and air gaps occurred at the interface between NiCuZn ferrites containing Bi2 O3 and LTCC after cofiring at 900 ◦ C for 2 h. This is due to widening of the shrinkage mismatch between NiCuZn ferrites and LTCC resulting from Bi2 O3 in NiCuZn ferrites diffusion into LTCC. This leads to the formation of a new Bi–B–Si glass with a lower glass transition temperature and viscosity sandwiched between LTCC and NiCuZn ferrites [10]. Lee et al. [11] studied the interdiffusion between NiCuZn ferrite and LTCC during co-firing. They found that Cu ions from the ferrite can diffuse into LTCC at a distance of around 120 m and Mg and Al ions from LTCC also diffuse into the ferrite for a short distance during co-firing. Ag ion diffusion into LTCC may lead to reliability concerns, such as increasing leakage current and decreasing insulation resistance [14]. The interfacial reaction kinetics and mechanism between silver and ceramic-filled glass substrates, silver diffusion and microstructure development in LTCC has been reported [7–9]. The previous reports focused mostly on the Ag diffusion of the LTCC-Ag internal electrode systems. However, Ag diffusion of the embedded silver internal electrode in LTCC for multilayer NiCuZn ferrites/LTCC components during cofiring has never been reported to our best knowledge. In this study, the diffusion and precipitation of silver
http://dx.doi.org/10.1016/j.jeurceramsoc.2015.12.002 0955-2219/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: H.-I. Hsiang, et al., Ag precipitation at the free interface of multilayer NiCuZn ferrites/LTCC components, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.12.002
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2
Table 1 Chemical composition (wt%) of the commercial NiCuZn ferrites. Materials
Fe2 O3
ZnO
NiO
CuO
MnO
Co2 O3
NiCuZn ferrites
67.28
13.18
12.49
6.84
0.21
–
Table 2 Chemical composition (wt%) of the commercial LTCC powder. Materials
B2 O3
Al2 O3
SiO2
K2 O
Na2 O
LTCC
0.87
14.30
83.83
0.77
0.23
and copper in LTCC for multilayer NiCuZn ferrites/LTCC components are characterized and analyzed using optical microscopy, scanning electron microscopy, X-ray diffractometer and X-ray photoelectron spectroscopy.
Fig. 1. Optical micrograph of the surface of the multilayer NiCuZn ferrite/LTCC component after cofiring at 900 ◦ C.
2. Experimental procedures A commercial NiCuZn ferrite powder without the addition of Bi2 O3 and a commercial low-dielectric-constant LTCC powder (borosilicate glass added with alumina and quartz fillers) were used as the raw materials. The specific surface area values of NiCuZn ferrites and LTCC powders were measured using a surface area analyzer (Micrometrics, ASAP 2020, Norcross, GA, USA) are 7.48, and 6.36 m2 /g, respectively. The chemical compositions determined by X-ray fluorescence spectrometer (Rigaku, ZSX100E, Tokyo, Japan) of the raw materials are listed in Tables 1 and 2. The chemical formula of NiCuZn ferrites can be represented as Ni0.4 Zn0.38 Cu0.2 Mn0.02 Fe2 O4 . The multilayer LTCC/NiCuZn ferrite components were prepared using tape casting and screen-printing methods. The LTCC and NiCuZn ferrites powders were mixed with a commercial organic vehicle (Ferro, MSI 73210) and then fabricated using the doctor–blade technique for the tape preparation. The NiCuZn ferrites and LTCC green sheet thicknesses were all about 50 m. Silver conducting paste (Shoei, Japan) was screen-printed onto the LTCC green sheet. The printed LTCC green sheets (5 layers) were sandwiched between NiCuZn ferrite green sheets (5 layers) or blank LTCC green sheet (5 layers). The NiCuZn ferrite/LTCC/NiCuZn ferrite sandwiched composites and LTCC/Ag composites were prepared by stacking and then hot isostatic laminated at 70 ◦ C and 60 MPa for 10 min. Binder burnout was carried out at 450 ◦ C for 12 h. The samples were sintered at 900 ◦ C for 2 h in air or nitrogen atmosphere at a heating rate of 5 ◦ C/min. In order to determine the copper diffusion behavior in LTCC, 10wt% reagent-grade CuO was mixed with LTCC powder and then melted at 1400 ◦ C for 1 h. The melt was then quenched in water to form (B–Si–Al–Cu glass) BSACG glass. The BSACG glass was powdered homogeneously and dry pressed at 100 MPa into pellets. These specimens were then sintered at 900 ◦ C for 1 h. The microstructure was observed using optical microscopy (Nikon, 50iPOL) and scanning electron microscopy (Hitachi, S4100, Tokyo, Japan). The element distribution was measured using electron probe microanalysis (EPMA) (JEOL, JXA-8900R, Tokyo, Japan). The crystalline phase identification was determined using X-ray diffractometry (Dandong Fangyuan, DX-2700, Sandong, China) with CuK␣ radiation. X-ray photoelectron spectroscopy (XPS) (ESCA PHI 1600, Physical Electronics, Chanhassan, MN) measurements were performed on both surface and polished specimens using Mg K␣ (1253.5 eV) radiation. The sample charge produced by irradiation was determined by measuring the shift in the C 1s signal given by a binding energy of 284.5 eV. The specimen chemical compositions were compared through XPS.
Fig. 2. X-ray photon spectroscopy (XPS) results of (a) the surface and (b) interior (about 0.02 mm below the surface) of the multilayer NiCuZn ferrite/LTCC component after cofiring at 900 ◦ C.
3. Results and discussion Fig. 1 shows the optical micrograph of the surface of the multilayer NiCuZn ferrite/LTCC component after cofiring at 900 ◦ C, indicating that a different phase was formed at the interface between NiCuZn ferrites and LTCC. Note that a surface-segregated phase was not formed in the component interior, indicating that the phase observed at the interface between the NiCuZn ferrite and LTCC concentrated only near the free interface. Serious overelectroplating was observed near the interface between the NiCuZn ferrites and LTCC, suggesting the segregated phase would exhibit a low resistivity. This will lead to a reliability concern involving the increase in leakage current and shorting between conductors. Based on the EDS result (not shown) the segregated phase can be identified as an Ag-rich phase. Fig. 2(a) and (b) show the X-ray photon spectroscopy (XPS) results of the surface and interior (about 0.02 mm below the surface) of the multilayer NiCuZn ferrite/LTCC component after cofiring at 900 ◦ C, respectively. Two characteristic binding energy peaks occurred at 368.4 eV and 366.6 eV for the component surface cofired at 900 ◦ C assigned as Ag◦ and Ag2 O, respectively [15–17]. However, at the interior only one characteristic peak around 369.36 eV was observed due to Ag+ [18], indicating that Ag+ may diffuse from the inner silver electrode into the LTCC
Please cite this article in press as: H.-I. Hsiang, et al., Ag precipitation at the free interface of multilayer NiCuZn ferrites/LTCC components, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.12.002
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Fig. 3. X-ray diffraction patterns of (a) the surface and (b) interior of the BSACG pellet sintered at 900 ◦ C and (c) BSACG powders.
glass. This is consistent with the observations of Jean and Chang [7] who investigated the interfacial reaction kinetics between silver and ceramic-filled glass (CFG) substrates and reported that Ag+ diffused from the silver and incorporated into the glass accompanied with Al3+ ions dissolved from the CFG. The main compositions in the LTCC are silicon and aluminum. The NiCuZn ferrite is composed mainly of nickel, copper, zinc and iron. In our previous study [10], the chemical composition across the interface between NiCuZn ferrites and LTCC after cofiring at 900 ◦ C for 2 h was investigated and observed that no obvious element inter-diffusion of Ni, Zn, Fe, Al and Si was observed at the interface. Only a small concentration of copper ions from the ferrite can diffuse into the LTCC at a distance of around 10 m, which is consistent with the observations of Lee et al. [11]. This may be due to the formation of copper-rich precipitates during NiCuZn ferrite sintering attributed to the sintering process being faster than oxygen diffusion from the outside into the pores [19]. The absence of oxygen inside the pores leads to a reduction of Cu2+ into Cu+ , promoting the precipitation of Cu-rich phase onto the NiCuZn ferrite grain boundary [20–22] and dissolving into the glass at the interface between the ferrites and LTCC. Silver precipitation was not found on the LTCC surface for the LTCC-Ag cofired components. Moreover, the silver precipitation position occurred at the interface between NiCuZn ferrites and LTCC (Fig. 1), where was overlapped with that enriched with copper for the LTCC-NiCuZn ferrite composites. These suggest that the silver precipitation onto the surface near the interface between the ferrites and LTCC may be related to the copper diffusion. To further confirm whether copper segregation and diffusion in LTCC can lead to silver precipitation onto the surface near the interface, the valence state and behavior of copper ions in boron alumino-silicate glass were investigated. Fig. 3 shows the X-ray diffraction patterns of BSACG powders and the surface and interior of the BSACG pellet sintered at 900 ◦ C. BSACG glass powder exhibited amorphous characteristics. After sintering at 900 ◦ C, quartz, corundum and CuO phases were observed. Note that CuO XRD intensities at the surface were higher than those in the interior for the sintered BSACG glass, indicating that more CuO was formed on the surface than in the interior. The XPS spectra of the surface and polished surface of the sintered BSACG glass were simulated using XPSPEAK41, as shown in Fig. 4. The simulation results indicate that the three peaks at 932.8, 933.8 and 935 eV for the surface sample can be assigned as Cu+ , CuO and Cu2+ , respectively [23]. However, for the polished samples, only two peaks attributed to Cu+ (932.8 eV) and Cu2+ (934.5 eV) were
3
Fig. 4. XPS spectra of (a) the surface and (b) polished surface of the sintered BSACG glass.
Table 3 Ratios between the integrated area of XPS peaks of Cu 2p3/2 (Cu2+ , Cu+ and CuO) and Si 2p (as an internal standard) for the surface and interior of the sintered BSACG glass. Integrated area ratio
Surface
Interior
Cu2+ /Si Cu+ /Si CuO/Si (Cu+ + Cu2+ + CuO)/Si
0.054 0.035 0.127 0.216
0.134 0.025 – 0.159
observed. Table 3 summarizes the ratios between the integrated XPS peak areas of Cu 2p3/2 (CuO, Cu2+ and Cu+ ) and Si4+ (as an internal standard) for the surface and interior (polished surface) of the sintered BSACG glass. Note that the copper existed mainly as Cu2+ and Cu+ in the interior of the sintered BSACG glass. However, the CuO 2p3/2 peak became dominant for the surface sample. Moreover, the ratio of the total CuO, Cu2+ and Cu+ 2p3/2 peaks and Si 2p peak area for the surface sample is larger than that for the polished sample, indicating that copper is concentrated at the sample surface. This suggests that copper ions in the boron aluminosilicate glass would diffuse from the interior toward the surface and precipitated as the CuO phase at the surface. This is consistent with the observations of Kamiya et al. [24] who reported that CuO layers were formed at the CuO–Al2 O3 –4SiO2 glass surface after annealing in air. The fraction of about 90% total copper ions was Cu+ ions and Cu+ ions located in the interior of the glass migrated to the surface and reacted with oxygen. Fig. 5 shows the SEM micrographs and electron probe microanalysis (EPMA) mapping result of the interface between NiCuZn ferrites and LTCC for the NiCuZn ferrite-LTCC component after cofiring in air and nitrogen atmosphere at 900 ◦ C for 2 h. It shows that Ag diffused into LTCC and concentrated at the interface between the ferrites and LTCC for the sample sintered in air. In contrast, the diffusion of silver and the Ag-enrichment at the interface seems to be suppressed for the sample sintered in nitrogen atmosphere, compared with the sample sintered in air. Fig. 6 shows optical micrograph of the side surfaces for the NiCuZn ferrite-LTCC component after cofiring in nitrogen atmosphere at 900 ◦ C for 2 h. In contrast to the sample cofired in air (Fig. 1), no silver was found at the side surfaces for the samples cofired in nitrogen atmosphere. This means that the outward diffusion and enrichment of Ag+ near the interface and Ag precipitation at the free interface can be ascribed to Cu+ oxidation into Cu2+ .
Please cite this article in press as: H.-I. Hsiang, et al., Ag precipitation at the free interface of multilayer NiCuZn ferrites/LTCC components, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.12.002
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Fig. 5. SEM micrographs and electron probe microanalysis (EPMA) mapping result of the interface between NiCuZn ferrites and LTCC for the NiCuZn ferrite-LTCC component after cofiring in air (a) and nitrogen atmosphere (b) at 900 ◦ C for 2 h.
Fig. 6. Optical micrograph of the side surfaces for the NiCuZn ferrite-LTCC components after cofiring in nitrogen atmosphere at 900 ◦ C for 2 h.
Based on the above results a schematic of the Ag+ diffusion, enrichment and silver precipitation mechanism at the interface between NiCuZn ferrites and LTCC is proposed as shown in Fig. 7:
During the heating stage Cu+ will segregate into the grain boundary from the NiCuZn ferrites due to the low oxygen partial
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5
References
Fig. 7. Schematic mechanism of Ag+ diffusion, enrichment and silver precipitation at the interface between NiCuZn ferrites and LTCC.
pressure inside the sample and then dissolve into LTCC near the interface between the NiCuZn ferrites and LTCC. In the meantime, the silver embedded in LTCC will dissolve into the glass and exist as Ag+ . At the free surface Cu+ ions are oxidized into Cu2+ ions, and the oxygen is reduced by electrons into oxygen ions. Cu2+ ions react with ionic oxygen at the surface, which leads to a crystalline CuO forming onto the glass surface. To maintain charge neutrality, this process requires an outward diffusion of Cu+ and Ag+ from the interior toward the free surface. Ag+ ions are thermally unstable in glass. The reduction of Ag+ into Ag on the free surface may be accelerated by the oxidation of Cu+ . 4. Conclusion Ag precipitation was formed at the free interface between NiCuZn ferrites and LTCC for the multilayer NiCuZn ferrite/LTCC component after cofiring. Serious over-electroplating was observed after the electroplating process, leading to a reliability concern involving the increase in leakage current and shorting between conductors. Ag diffused into LTCC and concentrated at the interface between the ferrites and LTCC for the sample sintered in air. In contrast, the diffusion of silver and the Ag-enriched at the interface can be suppressed for the sample sintered in nitrogen atmosphere. The outward diffusion and enrichment of Ag+ near the interface and Ag precipitation at the free interface can be ascribed to Cu+ oxidation into Cu2+ . At the free surface Cu+ ions are oxidized into Cu2+ ions and the oxygen is reduced by electrons into oxygen ions. Cu2+ ions react with ionic oxygen at the surface, which leads to a crystalline CuO forming onto the glass surface. To maintain charge neutrality this process requires an outward diffusion of Cu+ and Ag+ from the interior toward the free surface. The reduction of Ag+ into Ag on the free surface then occurs, accelerated by the oxidation of Cu+ .
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Acknowledgment This work was financially sponsored by the Ministry of Science and Technology of the Republic of China(NSC 101-2221-E-006-125MY3 and 102-2622-E-006-013-CC2).
Please cite this article in press as: H.-I. Hsiang, et al., Ag precipitation at the free interface of multilayer NiCuZn ferrites/LTCC components, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.12.002