Integration of CaCu3Ti4O12 capacitors into LTCC multilayer modules

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Integration of CaCu3Ti4O12 capacitors into LTCC multilayer modules Romy Löhnert a , Beate Capraro b , Stefan Barth b , Heike Bartsch c , Jens Müller c , Jörg Töpfer a,∗ a

University of Applied Sciences Jena, Department of SciTec, 07745 Jena, Germany Fraunhofer Institute of Ceramic Technologies and Systems, 07629 Hermsdorf, Germany c Ilmenau University of Technology, Department of Electrical Engineering & Information Technology, 98693 Ilmenau, Germany b

Received 14 January 2015; received in revised form 30 March 2015; accepted 2 April 2015

Abstract The sintering behavior and electric properties of multilayer laminates made from tapes of CaCu3 Ti4 O12 (CCTO) with glass additive were studied. Sintering at 900 ◦ C gives a fine-grained microstructure with a permittivity of ε = 900. The co-firing behavior of hetero-laminates of CCTO and commercial CT700 glass–ceramic LTCC tapes as well as with polycrystalline zinc titanate (ZT) LTCC tapes was studied. Co-firing of both heterolaminates is possible, however, cracking due to shrinkage and thermal expansion mismatch was observed in the CCTO/CT700 combination. Co-fired CCTO/ZT multilayer laminates did not show any cracks, delaminations or other defects. CCTO multilayer capacitor elements were integrated into LTCC zinc titanate multilayer laminates and co-fired at 900 ◦ C with AgPd electrodes. © 2015 Elsevier Ltd. All rights reserved. Keywords: CCTO; LTCC; Embedded capacitor

1. Introduction The integration of dielectric materials into low-temperature co-fired ceramics (LTCC) is a convenient method to increase the integration density in electronic circuits. Usually a middle-k or high-k dielectric or relaxor material is integrated as screenprinted thick film or green tape in the multilayer architecture of the low-k LTCC tapes [1]. A variety of commercial LTCC tape systems is available which might be combined with high-k dielectric materials in order to generate embedded capacitors, enhance functionality and reduce the size of a device [2]. In order to enable cofiring of the high-k and low-k dielectric LTCC materials in a multi-material hetero-laminate, the shrinkage characteristics of both materials should be very similar. Moreover, thermal expansion mismatch and chemical reactions between materials during co-firing should be minimized. The co-firing and shrinkage

∗

Corresponding author at: University of Applied Sciences Jena, Department of SciTec, C.-Zeiss-Promenade 2, 07745 Jena, Germany. Tel.: +49 3641 205479; fax: +49 3641 205451. E-mail address: [email protected] (J. Töpfer).

behavior of several low-k and middle-k LTCC hetero-laminates has been studied [3,4]. Various titanate-based perovskites were developed as middle- and high-k dielectrics for co-firing with commercial LTCC tapes [5–8]. Screen-printed thick films of Ba/Sr titanate were also combined with a LTCC multilayer laminate [9]. A further level of LTCC-integration is the combination of high-k dielectrics with other functional (e.g. magnetic) materials embedded into a LTCC multilayer architecture that would allow to generate more complex LTCC device functions, e.g. LC filters or front/back-end modules. CaCu3 Ti4 O12 (CCTO) was intensively studied in the last years because of its large effective permittivity of about ε = 100,000 and low temperature dependency of permittivity [10–12]. It is widely accepted that the high effective permittivity originates from an internal barrier layer capacitor (IBLC) effect caused by insulating grain boundary regions and a semi-conducting bulk [13]. Polycrystalline CCTO ceramics are typically sintered at 1050–1100 ◦ C and exhibit a microstructure of large CCTO grains and grain boundaries decorated with a Cu-rich intergranular phase [14]. The microstructure of CCTO ceramics, its evolution with sintering temperature and dwell time, as well as its correlations with electrical properties was studied by several groups [15,16]. Because

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of its interesting properties, CCTO has been discussed as an alternative dielectric for multilayer ceramic capacitor (MLCC) applications. Barbier et al. prepared CCTO multilayer laminates by sintering at 1100 ◦ C which exhibit smaller effective permittivity (ε = 50,000) as compared to sintered compacts [17]. Sintering of green sheets [18] as well as the fabrication of multilayer capacitors has been reported recently [19]. Sintering of CCTO at low temperatures under LTCC conditions was also investigated. Glass sinter additives were used to induce liquid phase sintering and to reduce the sintering temperatures to T ≥ 950 ◦ C [20,21]. A temperature of 900 ◦ C is required to co-fire the dielectric materials with Ag electrodes to fabricate capacitors integrated into LTCC modules. CCTO multilayers with Ag electrodes were co-fired at 940 ◦ C [22]. CCTO thick films were screen printed onto alumina substrates and co-fired at 900 ◦ C [23]. Recently, the fabrication of multilayer laminates as well as MLCCs through co-firing at 900 ◦ C was demonstrated using CCTO with a glass sintering additive [19]. The combination of high-k dielectric CCTO layers that show small temperature variation of capacitance with low-k dielectric LTCC multilayer structures may offer interesting opportunities for integrated capacitive functions into LTCC module architectures. Typically, low-k dielectric LTCC tapes consist of ceramic particles embedded in a glassy matrix phase. During firing, the glass phase softens and shrinkage occurs due to viscous flow [1,2]. In some of these glass–ceramic LTCC systems the glass phase partially crystallizes and additional ceramic phases are formed during firing [24]. Another approach is to use a low-k crystalline material as low-temperature sinterable LTCC material [25,26]. This is achieved by adding a small amount (typically 0.5–3 wt%) of a crystalline or amorphous sintering aid with low melting/transformation point which results in liquid-phase sintering characteristics of the crystalline LTCC material. This might be beneficial for integrating polycrystalline functional material layers in multilayer laminates of such LTCC materials, since co-firing and shrinkage of both, functional and crystalline LTCC material, operate along the same liquid-phase sintering mechanism. In this communication we report on investigations on the co-firing behavior, microstructure development and dielectric properties of multilayer laminates of high-k CCTO in combination with two low-k dielectric LTCC materials: (i) a commercial glass–ceramic CT700 tape and (ii) a polycrystalline zinc titanate-based (ZT) tape. It will be shown that if the shrinkage and thermal expansion mismatch exceed a certain level, successful co-firing will not be possible. In the case of the CCTO/CT700 combination this mismatch obviously is too large. On the other hand, if CCTO tape is co-fired with ZT tape with very similar shrinkage and thermal expansion behaviors, defectfree hetero-laminates are observed through co-firing. Integrated capacitors were fabricated using CCTO/ZT combined multilayers and co-fired with Ag/Pd electrodes at 900 ◦ C. 2. Experimental CaCu3 Ti4 O12 powder was prepared by the mixed oxide route. Stoichiometric amounts of CaCO3 (Merck), CuO (Alfa

Aesar) and TiO2 (rutile, Tronox) were homogenized in deionized water for 6 h using stabilized zirconia media. The powder mixture was calcined at 970 ◦ C for 6 h and wet milled to a mean particle size of d50 = 1.0 ␮m in a planetary ball mill (Pulverisette 5, Fritsch) using zirconia vessel and media. To enable low temperature sintering 3 wt% of a Bi2 O3 –B2 O3 –SiO2 –ZnO glass powder (BBSZ) with a Tg = 400 ◦ C was added before milling. Cu-substituted zinc titanate of composition Cu0.2 Zn1.8 TiO4 was synthesized by mixing CuO, ZnO and TiO2 for 16 h in an aqueous suspension. The mixture was calcined at 750 ◦ C for 2 h. With the addition of 0.5 wt% Bi2 O3 fine milling was performed in a NETZSCH lab star attrition mill for 2 h to a mean particle size of d50 = 0.6 ␮m. The dried powders were mixed in organic solvents (MEK and ethanol) and dispersants. Next, a binder (PVB) and phthalatebased plasticizer were added to form slurries for tape casting. The de-aired slurry was cast on a Mylar carrier tape using a doctor-blade tape caster of 12 m casting length. High-k (CCTO) and low-k dielectric (zinc titanate, ZT) green tapes were prepared with thickness of 197 ␮m and 105 ␮m after solvent evaporation, respectively. A commercial low-k dielectric LTCC tape CT700 (Heraeus) of thickness 310 ␮m was used. Multilayer laminates of single materials (CCTO: 6 layers, ZT: 10 layers, CT700: 4 layers) as well as symmetrical heterolaminates of CCTO (2 inner layers) with ZT or CT700 tapes (2 layers on top and bottom) were prepared by stacking and laminating at 80 ◦ C and 24 MPa in an isostatic lamination press. Samples of 10 mm diameter and of 20 mm × 20 mm were cut from the single-material and integrated-material multilayer laminates, respectively. The laminates were slowly heated to 500 ◦ C with a heating rate of 0.5 K/min for binder and plasticizer burnout and then heated at a rate of 4 K/min to 900 ◦ C and sintered for 4 h. A LTCC multilayer module with integrated capacitor was fabricated by combining a high-k CCTO layer with low-k dielectric LTCC zinc titanate tapes. Electrodes were screen-printed using AgPd TC 7402 metallization paste and vias were filled with AgPd TC 7602 (both Heraeus). At first two bottom ZT tapes were laminated uniaxially for 10 min at 70 ◦ C using a pressure of 5 MPa. The CCTO tape was laser cut into patches of 3.5 × 3.5 mm2 . Next, the CCTO patches were positioned top down onto the printed electrodes and laminated at the same conditions. After removing the carrier tape from the CCTO patches, the low-k ZT top layer was stacked and laminated at 70 ◦ C and 10 MPa. Two substrates of size 8.5 cm × 8.5 cm with 20 single capacitors each were processed. The LTCC multilayers with integrated capacitor were slowly heated to 500 ◦ C with 0.5 K/min and then to 900 ◦ C at a rate of 4 K/min and held for 4 h. A weight of 20 g was placed on the assembly during co-firing. The shrinkage and the coefficient of thermal expansion (CTE) between 140 ◦ C and 740 ◦ C were measured on single-material laminates by dilatometry (DIL502E, Netzsch). The density of sintered laminates was determined by Archimedes principle. X-ray diffraction (XRD) was used to investigate the phase

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composition (D8 ADVANCE, Bruker AXS). Cross sections of sintered laminates and capacitors were polished and thermally etched at 900 ◦ C for 10 min. The microstructure was investigated using an Ultra 55 Field Emission Scanning Electron Microscope (Zeiss). Energy dispersive X-ray (EDX) mappings and point measurements were performed with an X-Flash® 6|30, Bruker system; analysis of the spectra was performed using the Quantax software package (Bruker). For dielectric measurements the sintered single-material laminates of CCTO and ZT were covered with silver paste (TC 7307, Heraeus) and fired at 850 ◦ C for 30 min. Room temperature relative permittivity was measured with an impedance analyzer (4192A, HP) in the frequency range f = 10–107 Hz. The integrated capacitors were investigated with X-ray tomography (Phoenix nanome|x, GE). The dimensions of the embedded capacitors after co-firing were determined from tomograms; a single layer capacitor covers an area of 4 mm2 . The thickness of the sintered dielectric layer was determined from micro-sections of selected components. The measurement of the capacitance of the integrated capacitors was carried out with a RCL meter at 0.25 kHz, 1 kHz and 1 MHz, respectively. The capacity and loss tangent were measured for an alternating voltage without bias (amplitude 1 V). The permittivity ε of the dielectric was calculated using the measured dimensions and capacity. 3. Results and discussion 3.1. Low-temperature firing of CCTO CCTO compacts show poor densification if fired at 900 ◦ C. The addition of 3 wt% BBSZ glass to CCTO powder improves the low-temperature firing behavior; shrinkage already sets in at 680 ◦ C and the temperature of maximum shrinkage rate is

Fig. 1. Shrinkage and shrinkage rate of single-material laminates of CCTO with 3 wt% BBSZ glass, low-k zinc titanate with 0.5 wt% Bi2 O3 additive (ZT) and CT700.

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Table 1 Temperature of maximum shrinkage rate TMSR , coefficient of thermal expansion CTE, and density after sintering of single-material laminates at 900 ◦ C 4 h for CCTO, zinc titanate ZT and commercial tape CT700.

CCTO + 3 wt% BBSZ ZT + 0.5 wt% Bi2 O3 CT700

TMSR (◦ C)

CTE (10−6 K−1 )

Density (g/cm3 )

920 915 830

10.6 10.3 6.7

4.3 (86%) 5.16 (97.0%) 3.05

decreased to 920 ◦ C (Fig. 1). The shrinkage characteristics as well as CTE are summarized in Table 1. Sintering of BBSZcontaining CCTO laminates at 900 ◦ C for 4 h leads to a relative density of 86% ± 1%. X-ray diffraction of the sintered samples confirms that a single-phase CCTO material is obtained; no secondary phase formation or interaction between CCTO and BBSZ glass was observed within the limits of resolution of XRD (Fig. 2). The microstructure of CCTO laminates sintered at 900 ◦ C consists of small grains of size 1–2 ␮m and a considerable amount of pores (Fig. 3a). Some small platelets of BBSZ glass are visible in the SEM image as the glass phase accumulates at the sample’s surface during thermal etching (Fig. 3a). However, the glass phase is distributed homogeneously in the ceramic CCTO matrix as proven by SEM of the cross-section of fractured samples. Grain growth seems to be very limited at these sintering conditions. Densification in combination with significant grain growth that is typical for CCTO sintered at higher temperatures has not set in yet. Sintering at 1050 ◦ C or 1100 ◦ C triggers growth of large grains of 20–30 ␮m size separated by a Cu- and Ti-rich intergranular phase with a varying composition that depends on temperature and dwell time [14,16]. The dielectric properties of CCTO laminates sintered at 900 ◦ C are shown in Fig. 4. An effective permittivity of ε = 900 is observed at 1 kHz. At f > 1 MHz the permittivity drops to ε = 100. At low frequency, a slight increase in permittivity occurs which might be due to a sample-electrode interface polarization effect. Dielectric losses are relatively large and

Fig. 2. XRD of single-material laminates sintered at 900 ◦ C of CCTO with 3 wt% BBSZ glass (top) and low-k dielectric zinc titanate (ZT) with 0.5 wt% Bi2 O3 additive (bottom).

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Fig. 3. SEM micrographs of single-material laminates of CCTO with 3 wt% BBSZ glass (a) and low-k zinc titanate (ZT) with 0.5 wt% Bi2 O3 additive (b) sintered at 900 ◦ C for 4 h.

3.2. Low-k dielectric LTCC materials

Fig. 4. Permittivity and loss tangent vs. frequency of single-material laminates of CCTO with 3 wt% BBSZ glass sintered at 900 ◦ C for 4 h.

tan δ < 0.12 is achieved in the frequency range 103 –105 Hz (Fig. 4). At low and high frequencies, dielectric losses increase due to electrode-sample interactions and a Debye-like relaxation process, respectively. These results demonstrate that CCTO is processible under low-temperature firing conditions at 900 ◦ C using BBSZ as liquid phase sintering additive. Contrarily to high temperature sintered CCTO with coarse grains and large permittivity, laminates sintered at 900 ◦ C exhibit a smaller permittivity. This is very likely due to increased porosity and smaller grain size with less well developed intergranular and grain boundary phases as compared to bulk CCTO sintered at 1100 ◦ C. However, the use of BBSZ additive gives rise to significant densification after sintering at 900 ◦ C, and the effective permittivity reaches a still impressive value of ε = 900 at 1 kHz. This might offer some potential for using CCTO to integrate capacitive components into LTCC modules. Moreover, it was demonstrated that CCTO with BBSZ additive exhibits a small temperature dependence of permittivity and the requirements of the X7R-characteristic are fulfilled in the frequency range from 100 Hz to 60 kHz [19].

Standard LTCC tape materials typically are glass–ceramic composites. Commercial CT700 LTCC dielectric tapes consist of a glass phase mixed with alumina and silica as basic ceramic components [2]. The shrinkage characteristics of CT700 is shown in Fig. 1; the shrinkage rate peaks at 830 ◦ C and the shrinkage is finished at 900 ◦ C. Cu-substituted zinc titanate (ZT) was developed as an alternative, poly-crystalline low-k dielectric LTCC material. 0.5 wt% Bi2 O3 was added as liquid-phase sintering aid. Shrinkage sets in at 750 ◦ C and terminates at about 950 ◦ C; the absolute shrinkage at 950 ◦ C amounts to about 20%. The shrinkage rate has a sharp maximum at 915 ◦ C (Fig. 1). Characteristics of shrinkage, thermal expansion and density are summarized in Table 1 in comparison to commercial LTCC tape CT700 and low temperature sintered CCTO. X-ray diffraction demonstrates that the main crystalline phase in the sintered samples is a Cu-substituted spinel Zn2 TiO4 (Fig. 2). XRD peaks of lower intensity of hexagonal ZnO are also detected. A SEM micrograph of the sintered sample (Fig. 3b) shows a dense microstructure with grains of 10–20 ␮m size, grain boundaries decorated with another phase and inclusions in the grains. EDX analysis reveals that the composition of the grains corresponds to that of the Cu-substituted Zn-titanate main phase. The rounded inclusions within the grains were identified as the ZnO wurtzite second phase. The composition of the grain boundary phase at the tripel junction between grains was analyzed as Cu- and Zn-containing Bi-rich oxide phase. This seems plausible, since Bi2 O3 with its low-melting point of Ts = 823 ◦ C acts as liquid-phase sintering aid and forms thin films between the grains of the spinel main phase. It might also partially dissolve Ti, Cu and Zn and form nucleates of ternary oxides as intergranular phase. Dielectric measurements of ZT laminates sintered at 900 ◦ C show that the permittivity is ε = 21 in the frequency range from 100 Hz to 10 MHz. The shrinkage characteristics and CTE qualify the ZT tapes as an interesting alternative LTCC system that might be used for co-firing

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Fig. 5. Cross-section SEM micrographs of co-fired hetero-laminates of CCTO/CT700 (a) and CCTO/ZT (b).

with other functional materials. A drawback of ZT as (low k) dielectric LTCC material is the relatively high permittivity. 3.3. Co-firing of CCTO/LTCC hetero-laminates Hetero-laminates of CCTO/CT700 and CCTO/ZT were cofired at 900 ◦ C. SEM micrographs (Fig. 5) demonstrate that co-firing of the high-k CCTO layers with low-k CT700- and ZTlayers gives a multilayer structure without significant defects such as warping or delamination. Comparing the shrinkage curves (Fig. 1) of the materials one can conclude that the shrinkage behaviors of CCTO and ZT are very similar, whereas CT700 shrinks at lower temperature compared to CCTO. Nevertheless, regardless this substantial shrinkage mismatch, co-firing of the CCTO/CT700 hetero-laminate seems to be possible. However, closer inspection reveals vertical cracks in the CCTO layer of the CCTO/CT700 multilayers (Fig. 5a) which might originate from

the large thermal expansion mismatch between the two materials. EDX analysis of the interface between CCTO and CT700 (Fig. 5a, inset) shows that Si migrates into the CCTO layer and accumulates with Ca. EDX point scans reveal a significant concentration of Si within the CCTO layer, even at positions far from the interface. It is well known, that Si has a tremendous effect on the microstructure formation and electrical properties of CCTO at sintering temperatures of 1050 ◦ C and above [27]. Here, under LTCC conditions, it seems that Si is very mobile in the glass phase of the CCTO layer. On the other hand, co-firing of CCTO with ZT tapes results in a multilayer without delamination or cracks. In that heterolaminate, both materials are well adapted for co-firing and have almost identical shrinkage and thermal expansion behaviors (Fig. 1, Table 1). At the interface between the two materials (Fig. 5b, inset) a reaction zone of few microns width is found with needle-shaped precipitations of a Bi-rich phase. However,

Fig. 6. CCTO capacitor integrated in LTCC multilayer; schematic drawing of embedded capacitor (a), photograph of substrate with embedded CCTO capacitors in green state (b) and X-ray tomogram of an embedded capacitor element with electrodes and contact pads (c).

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not applicable for integration into glass–ceramic CT700 LTCC multilayers. 4. Conclusions

Fig. 7. SEM micrograph of cross-section of co-fired CCTO layer embedded in ZT multilayer and AgPd electrodes.

no significant diffusion or gradients of elements across the interface are observed by EDX analysis. 3.4. Cofiring of an integrated capacitor A high-k CCTO tape was embedded as capacitor element in a LTCC multilayer module as shown schematically in Fig. 6a. The substrates made of low-k dielectric LTCC material ZT with integrated patches of high-k CCTO and AgPd electrodes (Fig. 6b) were co-fired at 900 ◦ C. Upon free sintering of the substrates with several modules warpage was observed frequently. Therefore, the co-firing experiments were performed by applying a uniaxial pressure on the substrates using a weight. Fig. 6c shows an X-ray tomogram of a singularized sintered module with integrated capacitor element including electrodes, vias and contact pads. The integrated CCTO dielectric layer of 90 ␮m thickness was free of cracks and delamination as shown by SEM micrograph of a cross-section of a multilayer (Fig. 7). The CCTO layer has not sintered to complete density as discussed above. From electric measurements an effective permittivity of ε = 1200 ± 300 at 1 kHz was found which is slightly larger than the permittivity measured from CCTO laminates (Fig. 4). Reason might be an enhanced electrode-sample interaction at low frequency of AgPd compared to Ag electrode material used for singlematerial evaluation with ε = 900 at 1 kHz. Dielectric losses of the integrated capacitors were found to be high (tan δ = 0.3 at 1 kHz). Analogous experiments to integrate a CCTO capacitor layer in commercial CT700 low-k LTCC tapes with electrodes were not successful; cracks in the CCTO layer occurred and electric measurements did show a small capacitance resulting in permittivity of only 100; the insulation resistance of these samples was one order of magnitude higher than those integrated in ZT. This might be caused by the interaction between the glass phase of the CT700 and CCTO material. Consequently, CCTO is

The addition of BBSZ glass enables low-temperature sintering of CCTO at 900 ◦ C and an effective permittivity of ε = 900 is observed at 1 kHz. Cu-substituted zinc titanate (ZT) with sinter additive Bi2 O3 shows low permittivity and adequate shrinkage behavior for use as LTCC low-k material. The shrinkage and co-firing behavior of hetero-laminates of high-k CCTO tapes and (i) commercial glass–ceramic tape CT700 or (ii) ceramic zinc titanate tape were studied. Co-firing of CCTO and CT700 resulted in cracks in the high-k dielectric because of the large shrinkage and thermal expansion mismatch. Co-firing of a combination of CCTO and ZT tapes resulted in multilayers without defects, since shrinkage and thermal expansion of both materials are well adjusted. A layer of CCTO containing sinter additive BBSZ was embedded as an integrated capacitor with AgPd electrodes into a multilayer module made from ZT tapes. The module showed a permittivity of ε = 1200 and good compatibility with metallization pastes. It is shown for the first time that CCTO can be integrated as capacitor into a LTCC module. CCTO co-fired at 900 ◦ C exhibits much smaller permittivity compared to the huge permittivity of CCTO sintered at higher temperature, but the observed level of permittivity, cofirability and small temperature dependence of permittivity (X7R) [19] make it interesting for capacitor applications. The electric properties of CCTO need to be further studied and optimized, dielectric loss and voltage stability are critical. Correlations between microstructure and permittivity still need to be clarified in more detail to enable improvement of performance at low temperature sintering. Acknowledgement This work was supported by the State of Thuringia, Germany, through a grant in the ProExzellenz network (Kerfunmat, PE 214). References [1] Imanaka Y. Multilayered low temperature cofired ceramics technology. New York: Springer Science; 2010. [2] Sebastian MT, Jantunen H. Low-loss dielectric materials for LTCC applications: a review. Int Mater Rev 2008;53:57–90. [3] Choi YJ, Park JH, Ko WJ, Hwang IS, Park JH, Park JG. Cofiring and shrinkage matching in low- and middle-permittivity dielectric compositions for a low-temperature cofired ceramics system. J Am Ceram Soc 2006;89:562–7. [4] Lim WB, Cho YS, Seo YJ, Park JG. Shrinkage behavior of LTCC heterolaminates. J Eur Ceram Soc 2009;29:711–6. [5] Choi YJ, Park JH, Park JH, Nahm S, Park JG. Middle- and high-permittivity dielectric compositions for low-temperature cofired ceramics. J Eur Ceram Soc 2007;27:2017–24. [6] Hsiang HI, His CS, Huang CC, Fu SL. Low-temperature sintering and dielectric properties of BaTiO3 with glass addition. Mater Chem Phys 2009;113:658–63.

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Please cite this article in press as: Löhnert R, et al. Integration of CaCu3 Ti4 O12 capacitors into LTCC multilayer modules. J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.04.001