A co-fireable material system for ceramics and ferrites hetero-laminates in LTCC substrates

Journal of Alloys and Compounds 737 (2018) 144e151

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A co-fireable material system for ceramics and ferrites heterolaminates in LTCC substrates Yuanxun Li a, **, 1, Yunsong Xie b, *, 1, Ru Xie c, Daming Chen d, Huaiwu Zhang a a State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu, Sichuan 610054, China b Department of Physics and Astronomy, University of Delaware, Newark, DE 19716, USA c Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, DE 19716, USA d College of Materials and Chemical Engineering, Hainan University, Haikou, Hainan 570228, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 July 2017 Received in revised form 30 November 2017 Accepted 7 December 2017 Available online 9 December 2017

A method for solving the current challenges in manufacturing hetero-laminated material systems using Low Temperature Co-firing Ceramics (LTCC) technique is introduced in this paper. The system contains all three major materials that widely used in the LTCC industry, Zn2SiO4 ceramic, NiCuZn ferrite and silver. Previous reported methods either involve additional buffer layers which would increase the device thickness, or apply mechanical pressure during the co-firing which is not compatible to the standard LTCC process. Our method provides a facile route to manufacture hetero-laminated material systems that demands no buffer layer and follows the standard LTCC process. In this paper, four glass-free heterolaminated material systems using LTCC technology, i.e. NiCuZn/Zn2SiO4/NiCuZn/Zn2SiO4/NiCuZn (termed “LTCC-5”), NiCuZn/Zn2SiO4-Ag/NiCuZn (termed “LTCC-3-NZAN”), NiCuZn/Ag/NiCuZn (termed “LTCC-3NAN”) and NiCuZn/Zn2SiO4/NiCuZn (termed “LTCC-3-NZN”), have be were designed, manufactured and characterized. The shrinkage rates of these material systems show well-matched temperature spectrum. The sintering properties and interfacial diffusions of the material systems were characterized using optical microscopy, electron microscopy and Energy Dispersive Spectrometer (EDS) element mapping. Optical microscopy shows that all the material systems have well-defined multi-layered structure with controllable and tunable layer thickness. Electron microscopy images prove no microstructural cracks or defects at any of the hetero material interfaces. EDS analysis further proves that the inter-diffusions of the magnetic elements (Fe and Ni) at the NiCuZn/Zn2SiO4 interface is minimized. Such property is essential for achieving high performance devices for applications such as power inductors and DC/DC converters. © 2017 Elsevier B.V. All rights reserved.

Keywords: LTCC Mathcing co-firing shrinkage crack

1. Introduction Low Temperature Co-fired Ceramics (LTCC) technology is recognized as one of the ideal candidates for bringing together the functions of interconnected, packaged and integrated components in mass manufacturing production. LTCC enables one to achieve a hybrid system within a minimal volume. It combines individual layers with different functionalities (high permittivity, low loss,

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Li), [email protected] (Y. Xie). 1 These two authors contribute equally to this work. https://doi.org/10.1016/j.jallcom.2017.12.074 0925-8388/© 2017 Elsevier B.V. All rights reserved.

high permeability or gyromagnetic property) into single multilayer hetero-laminate. Many materials have been developed for LTCC technology to realize these desired functionalities, for example, high permeability ferrite for inductor applications [1,2], high frequency ferrite [3e5] and low loss dielectric ceramics for microwave device applications [6,7], and high permittivity ceramics for capacitor and resonator applications [8,9]. The future development of the electronic technology demands multi-functionality, miniaturization and high reliability. However, most of the current LTCC manufacturing processes manufacture multi-layered laminates contains only one functional material, i.e. multi-layered homolaminate. However, a hetero-laminated system is the better solutions to achieve multi-functionality and high intergration. It offers the possibility of incorporating multiple components, such as

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capacitors, resistors, inductors, and microwave devices [10e13], in to single device. The processes to achieve such multi-layered hetero-laminated material system has been one of the most popular research topics in the LTCC community. There are three major challenge to achieve a multi-layered hetero-laminated material system. In LTCC manufacturing process, one of the most crucial steps is the co-firing process, where the multi-layered laminates are co-fired and solidified at around 900
C for several hours. The 1st challenge is to minimize the structural cracks and defects, despite the fact that different materials usually have considerably un-matched thermal expansion profiles. The magnetic/non-magnetic layered configuration is commonly used to construct power inductors and DC/DC converters. A strong magnetic ion diffusion that commonly happens at the high temperature co-firing will reduce the effective air-gap thickness and eventually deteriorate the power handling capability of the device [14e16]. Therefore, the 2nd challenge is to reduce the magnetic ion diffusion between the magnetic and nonmagnetic layers during the sintering process. Last but not the least, from chemical reaction perspective, high temperature may trigger undesired chemical reactions during co-firing. This not only leads to material degradation, but also residual stresses and/or co-firing failures [17], which will eventually cause device performance deterioration or structural disintegration [18,19]. Therefore, the 3rd challenge is to eliminate the undesired chemical reaction among the materials in the hetero-laminated material system. Many researches have been conducted to explore the methods of co-firing both non-magnetic ceramic and magnetic ferrite as individual functional layers in single hetero-laminate. One of the most investigated methods is inserting a buffer layer between these two functional layers. Since different functional layer often have mismatched thermal shrinkage properties [20e23]. The inserted layer is designed to be chemically inert to both functional materials, and has a thermal shrinkage coefficient lying between these two materials in order to reduce the residual stress at the interface and eventually reduce the risk of structural cracks. The disadvantage of this method, however, is that the sizes of the devices will be inevitably increased because of the additional non-functional layer. In the case where the two functional layers become thicker, the demanded thickness of buffer layer needs to be increased, which leads to further device size increasing. The other popular methods for manufacturing hetero-laminate is mechanical compressing cofiring. This method implement a large mechanical pressure on the laminate during the co-firing process to eliminate the structural deformation [24e27]. This method indeed effectively suppresses the potential appearance of the structural cracks in the heterolaminates stacks. However, because this method is not compatible to the conventional LTCC manufacturing process, it dramatically increase the manufacturing cost. This manuscript reports four LTCC manufactured heterolaminated material systems that comprise widely used functional materials including Ag, NiCuZn ferrite and Zn2SiO4 ceramic. Unlike the previous reports, this method not only demands no buffer layer, but also follows the conventional LTCC manufacturing process. We chose NiCuZn ferrite and Zn2SiO4 to be the ferrite and ceramic layer that enclosed in the material systems, respectively. NiCuZn ferrite is one of the most commonly used magnetic materials in LTCC magnetic devices due to its high permeability in the RF frequency region, high electrical resistivity, and environmental stability [28e30]. Zn2SiO4 has been chosen for its low permittivity and dielectric high-quality factor at RF frequency region [31,32]. Furthermore, it is noteworthy to point out that, unlike most of the previous reported materials or commercial available products, no glass additive is required in our material system to complete the densification. Such property effectively reduces the probability of

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structural cracks caused by the mismatch of the thermal expansion coefficients between the ceramic and glass phases [33e35]. Optical and electron microscopy results illustrate that no significant crack and defect can be found in either material interface or interior of the material systems. More importantly, minimized magnetic ion inter-diffusion at the ferrite/ceramic interfaces is proved by element mapping. 2. Experiment The low temperature sintered Zn2SiO4 ceramic and NiCuZn ferrite materials were synthesized by a conventional solid-state reaction method [36]. The production of Zn2SiO4 casting slurry started with mixing ZnO and SiO2 powders at a molar ratio of 2:1 by using planetary ball milling for 8 h. Both powders were purchase from Alfa Aesar and had a purity of 99%. The milling was followed by calcined at 1300
C for 3 h with ramping rate of 10
C/min. The obtained material was then mixed with 3 wt% analytical grade sintering agent (a mixture of Bi2O3 and V2O5 in molar ratio of 2:1), with addition of methyl ethyl ketone as solvent, citric acid as dispersant, polyvinyl butyral as binder, and polyethylene glycol as plasticizer. Production was finished by planetary ball milling for 48 h. The produced Zn2SiO4 slurry had a median particle size (d50) of 1.6 mm, measured by laser particle size analyzer LA-960 from Horiba. The production of NiCuZn ferrite casting slurry used NiO, Fe2O3, CuO and ZnO powders, with purity of 99% from Alfa Aesar. In order to make spinel structured NiCuZn ferrite with formula Ni0.3Cu0.2Zn0.5Fe1.9O4, the powders were mixed in molar ratio of 15:47:10:25 by planetary ball milling for 8 h and calcined at 750
C for 3 h with a ramping rate of 10
C/min. The obtained material was then mixed with analytical grade of 2 wt% sintering agent (Bi2O3), with the addition of the same solvent, dispersant, binder, and plasticizer comparing to the Zn2SiO4 manufacturing recipe. Production was finished by planetary ball milling for 48 h. The produced NiCuZn ferrite slurry had a median particle size (d50) of 1.8 mm according to LA-960 measurement. In the casting process, the thickness of the tapes (14 mm for Zn2SiO4 and 38 mm for NiCuZn ferrite) was controlled by doctor blades. In the case where an Ag pattern needs to be deposited on either Zn2SiO4 or NiCuZn ferrite tape, the Ag slurry (LL602 from DuPont) was screen printed onto the tape. To produce the laminates, the ceramic and ferrite tapes were laminated (15 MPa at 35
C, pressure held for 4 s) and isostatic pressed (25 MPa at 70
C for 30 min). The binder was burnt at a heating rate of 1
C/min to 350
C, and kept at 350
C for 6 h. The multi-layered samples were later sintered at 890
C for 4 h with a ramping rate of 5
C/min. The microstructural and composition distribution analyses were conducted using a Scanning Electron Microscope (SEM) with Energy Dispersive Spectrometer (EDS) (Jeol JSM-6400). 20 kV acceleration voltage was used for all EDS measurements. Shrinkage curves were determined by the SETSYS Evolution TMA, SETARAM. Densities were characterized by Archimedes' Principle via Density kit MLDNY-43 (Mettler-Toledo). The crystal structure information was obtained by X-ray diffraction (XRD) technique using Rigaku IV Xray diffractometer and analyzed using Jade software package. 3. Results and discussions 3.1. The low temperature sintering properties of NiCuZn ferrite and Zn2SiO4 homo-laminates Homo-laminates comprising NiCuZn ferrite and Zn2SiO4 are prepared separately. SEM images of the NiCuZn ferrite and Zn2SiO4 laminates have been measured and shown in Fig. 1 (a) and (b),

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Fig. 1. SEM images of the sintered (a) NiCuZn ferrite homo-laminate and (b) Zn2SiO4 ceramic homo-laminate. (c) The shrinkage curves in X-Y direction with respect to temperature of Zn2SiO4 (black) and NiCuZn (red). Both curves were obtained at the sintering rate of 5
C/min. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

respectively. Both laminates appear to be dense bodies and contain little porosity. To illustrate the matching co-fireability of Zn2SiO4 ceramic and NiCuZn ferrite, the shrinkage curves on temperature spectrum of both materials have been measured between 600
C and 900
C and shown in Fig. 1 (c). The two materials have very similar shrinkage profiles. They both start to shrink at around 700
C and have a similar slope as temperature increases. The largest difference in shrinkage profiles of the two materials is smaller than 0.8% throughout the measured temperature range. This shrinkage profile similarity suggests that if the two layers were co-fired to form a hetero-laminate, there will be minimal stress at the interface [20e26,37]. As a key factor to affect both electrical and mechanical performances of the final device, the densities of both sintered NiCuZn ferrite and Zn2SiO4 laminates were measured to be 4.91 and 3.86 g/cm3, respectively, which are over 90% to their corresponding theoretical densities of 5.38 [38] and 4.25 g/cm3 [39]. The crystallization property of NiCuZn ferrite and Zn2SiO4 was characterized by XRD measurements shown in Fig. 2. Both of NiCuZn ferrite and Zn2SiO4 were well crystallized after being sintered at 890
C. All the major peaks of NiCuZn ferrite marked by square symbols in Fig. 2 (a) match its standard pattern, JCPDS 80234. The crystal structure of the fabricated NiCuZn ferrite corresponds to FD-3m (227) space group, with unit cell parameter of 0.840(8) nm. Considering the existence of additives and imperfect sintering conditions, such result is already very close with the reported bulk value of 0.8375 nm [39]. The major peaks of Zn2SiO4 were highlighted using triangular symbols. Analysis shows that all the measured peaks match with the corresponding standard pattern, JCPDS 37-1485. In addition, Zn2SiO4 has a space group of R-

Fig. 2. Measured XRD pattern for manufactured (a) NiCuZn and (b) Zn2SiO4 stacks after being laminated, isostatic pressed and sintered. The major peaks of both materials between 20
and 60
have been highlighted using square (NiCuZn) and triangular (Zn2SiO4) symbols.

3 (148), with unit cell parameters of a ¼ b ¼ 1.391(9) nm, c ¼ 0.929(2) nm, which is close to the reported bulk value of a ¼ b ¼ 1.3948 nm, c ¼ 0.9315 nm [41].

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3.2. Microstructure of five-layered and three-layered heterolaminates To test the co-firing performance of Zn2SiO4 and NiCuZn ferrite in LTCC technology, a five-layered NiCuZn/Zn2SiO4/NiCuZn/ Zn2SiO4/NiCuZn hetero-laminated material system (termed “LTCC5”) were manufactured. Fig. 3 displays the appearances of LTCC-5 using optical microscope. The dimension of the displayed heterolaminate in Fig. 3 is 2.0 mm  1.2 mm  0.5 mm. The thickness of the two inserted Zn2SiO4 ceramic layers are 0.02 mm and 0.08 mm, respectively. Both top and bottom NiCuZn ferrite layers have thicknesses of 0.15 mm. The center NiCuZn ferrite layer has a thickness of 0.1 mm. As shown in Fig. 3, all the Zn2SiO4/NiCuZn hetero-laminated material system maintain good physical integrity, have well-defined multi-layered structures and contain no structural cracks or defects. SEM has been used to investigate the microstructure of the interface between Zn2SiO4/NiCuZn layers in LTCC-5 and the result is shown in Fig. 4. It appears that the heterogeneous layers are in good contact with each other with no signs of crack or defects. The welldefined multi-layered structure is due to the thermal shrinkage resemblance between these two materials both in X-Y direction and Z direction as previously explained for homo-laminates. Ag is a common choice to form the metallic circuits inside the LTCC device, therefore, a LTCC hetero-laminated material system demands good co-fireability between Ag and other functional materials, such as ferrites and ceramics. A hetero-laminated material system containing all these three materials, three-layered NiCuZn/Zn2SiO4-Ag/NiCuZn (termed “LTCC-3-NZAN”), has been designed and manufactured. The structure of the three-layered LTCC-3-NZAN hetero-laminated material system is analogous to an inductor with air gap, which is often used in inductors to enhance the current handling capability of the device. The current handling capability improvement of this design comes from the fact that the non-magnetic material will reduce the magnetic flux so that magnetic material would not be saturated. In this case, the magnetic material (NiCuZn ferrite) could maintain at high permeability despite the high device operating current [42e44]. In these hetero-laminated material systems, the Zn2SiO4 and Ag are

Fig. 3. The picture of the LTCC fabricated five-layed NiCuZn/Zn2SiO4/NiCuZn/Zn2SiO4/ NiCuZn hetero-laminates (termed “LTCC-5”) containing two layers of Zn2SiO4 ceramics (shown as white layer) and three layers of NiCuZn ferrites (shown as grey layers) with different thickness.

Fig. 4. The SEM image at the interface between the Zn2SiO4/NiCuZn layers of a LTCC-5 from Fig. 3.

embedded in the center of laminates, surrounded by the NiCuZn ferrite on the top, bottom and the lateral sides. The dimension of the stacks can be found in Fig. 5 (a), where the black, white and yellow components represent NiCuZn ferrite, Zn2SiO4 ceramics and Ag, respectively. The dimensions are a ¼ 1.2 mm, b ¼ 2 mm, t ¼ 0.2 mm, h ¼ 0.1 mm, c ¼ 1.4 mm, d ¼ 0.8 mm, e ¼ 0.15 mm and the thickness of the Ag is 0.02 mm. According to the optical images in Fig. 5 (b) and (c), all Zn2SiO4, NiCuZn ferrite and Ag regions maintain their own physical integrity, and no crack or defects can be observed in the captured images.

3.3. The inter-diffusion and co-firing behaviors among Ag, NiCuZn ferrite and Zn2SiO4 First, co-fireability between NiCuZn ferrite and Ag was investigated. In this demonstration, NiCuZn/Ag/NiCuZn hetero-laminated material system (termed “LTCC-3-NAN”) with Ag embedded in ferrite body was fabricated. Detailed investigation on the interdiffusion of elements was carried out by examining the interior microstructure near the Ag region. A clean, smooth NiCuZn/Ag surface was obtained by breaking LTCC-3-NAN laminate followed by polishing with fine sandpaper (grit P1200). According to the microscopic image in Fig. 6 (a), no crack or structural deformation appears at the Ag/NiCuZn interfaces. The EDS element mapping of Ag in Fig. 6 (b) further proves that no observable Ag element diffused into the NiCuZn ferrite body. Element distribution boundaries can be clearly identified in the EDS element mapping of Fe, Ni, Cu and Zn according to Fig. 6 (c)e(f). This result indicates that these elements, similar to Ag, also undergone very weak iondiffusion during the co-firing procedure. According to the Cu mapping imaging in Fig. 6 (e), island shaped region with higher Cu concentrations can only be observed in NiCuZn ferrite layers. The size of these islands is approximately 2 mm, which is similar to the particle size in the NiCuZn ferrite casting precursor, evidencing that these islands are caused by the incomplete diffusion of the Cu in the NiCuZn ferrite phase. A lot of work has been done to understand the inter-diffusion of magnetic elements between the magnetic and non-magnetic layers [45,46]. Such inter-diffusion often needs to be suppressed between the magnetic and non-magnetic layers in the hetero-laminates for many applications. Taking power inductor as an example, when the ceramic material, such as Zn2SiO4, is used as an air gap spacer layer in the power inductor. It will be sandwiched by two magnetic ferrite layers, such as NiCuZn ferrite. The magnetic elements, such

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Fig. 5. (a) A cartoon of the three-layered NiCuZn/Zn2SiO4-Ag/NiCuZn hetero-laminate (termed “LTCC-3-NZAN”), where Ag traces appears as yellow color. The co-firing performance of NiCuZn/Zn2SiO4/NiCuZn hetero-laminates taken by (b) a scaled reticle integrated optical microscope at magnification of 40 and (c) an optical microscopy at higher magnification. The Zn2SiO4 ceramic layer, NiCuZn ferrite layer and Ag are shown as white, grey and shinning bright colors in both (b) and (c). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 6. The images measured at the NiCuZn/Ag/NiCuZn hetero-laminate (termed “LTCC-3-NAN”) region by (a) SEM and EDS element mapping regarding (b) Ag, (c) Fe, (d) Ni, (e) Cu and (f) Zn. From top to bottom, the layers are NiCuZn, Ag and NiCuZn.

as Ni and Fe in the ferrite layer, is not supposed to diffuse into the ceramic materials during the co-firing process. Otherwise, the effective thickness of the air gap will be reduced and the current handling performance will be deteriorated. To evaluate the level of the inter-diffusion of magnetic elements in our proposed method, NiCuZn/Zn2SiO4/NiCuZn heterolaminated material system (termed “LTCC-3-NZN”) with center

layer thickness of 12 mm was manufactured. The overall dimension of the stack is 2.0 mm  1.2 mm  0.8 mm. EDS measurement was applied on the polished broken surface. The SEM image is shown in Fig. 7 (a) and the element mapping of Fe, Ni and Cu are shown in Fig. 7 (b), (c) and (d), respectively. The boundaries between Zn2SiO4 ceramic and NiCuZn ferrite can be recognized from both Ni and Fe element mapping in Fig. 7 (b) and (c). It means that the Fe and Ni

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Fig. 7. (a) SEM image at the interface of the NiCuZn/Zn2SiO4/NiCuZn hetero-laminate (termed “LTCC-3-NZN”). The material of the center region is Zn2SiO4. The element mapping regarding (b) Fe, (c) Ni and (d) Cu obtained by EDS at the same area. From top to bottom, the layers are NiCuZn, Zn2SiO4 and NiCuZn.

elements undergone no significant inter-diffusion during the 890
C sintering process. No clear boundary can be observed at the Zn2SiO4/NiCuZn interfaces, which indicates that Cu element has diffused into the Zn2SiO4 during the co-firing process. An element composition analysis has been carried out using EDS for obtaining the quantitative information of the element interdiffusion between the NiCuZn ferrite and Zn2SiO4 layers. The data in Table 1 is obtained by analyzing the EDS spectrum collected from a region of 10 mm  8 mm for both NiCuZn ferrite (yellow dash enclosed region in Fig. 8 (a)) and Zn2SiO4 (red dash enclosed region in Fig. 8 (a)) layers analysis. It is noticeable that the oxygen concentration is lower than the raw materials in the recipe, where Zn:O ¼ 1:1. This is believed to be caused by EDS detector insensitivity element as light as oxygen. The atomic ratio between Zn and Si in Zn2SiO4 is more than 3:1, which could be a result of Si having suffered from severe diffusion during the sintering, so that

Table 1 The quantitative EDS composition analysis results for LTCC-3-NZN at both NiCuZn layer and Zn2SiO4 layer. Element

Wt%

NiCuZn layer OK 13.33 SiK 0.27 BiM 1.21 FeK 46.57 NiK 9.73 CuK 7.63 ZnK 21.26 Zn2SiO4 layer OK 11.76 SiK 8.94 BiM 0.37 FeK 1.75 NiK 0.58 CuK 1.09 ZnK 75.51

considerable amount of Si has gone into NiCuZn ferrite layer. At the same time, because the Zn2SiO4 layer is only 12 mm thick, the Si concentration in NiCuZn ferrite layer does not change much. According to this Table, the atomic composition of Fe and Ni drops from 36.34% to 7.23% in NiCuZn ferrite layer to 1.38% and 0.43% in Zn2SiO4 layer, respectively. The Fe, Ni and Cu atomic ratio in NiCuZn ferrite and Zn2SiO4 layers are about 100:19:5 and 100:29:57, respectively. Several conclusions can be drawn from this measurement. First, both Fe and Ni undergone minimum inter-diffusion between NiCuZn ferrite and Zn2SiO4 layer in the co-firing process. Second, the inter-diffusion of Cu element was much stronger than that of Ni and Fe. Such observation is in consistence with the previously reported investigations, where it has been suggested that the diffusion capability of Cu is better than both Fe and Ni in a ceramic matrix [48]. Further quantitative information can be found in the EDS element lines in Fig. 8 (b)e(d) regarding Fe, Ni and Cu, respectively. As shown in the figure, sharp concentration drop at the NiCuZn/Zn2SiO4 interface occurs within 2e3 mm for all three chosen elements, which is in good agreement with the element mapping measurement results in Fig. 7.

At%

Intensity

Intensity St. Error

Background

4. Conclusion

36.33 0.42 0.26 36.34 7.23 5.19 14.23

386.01 10.71 17.71 593.05 74.63 40.29 85.35

1.26 14.79 10.6 1.03 3.24 13.13 2.93

2.5 15.28 20.09 12.78 10.71 17.76 8.77

32.39 14.03 0.08 1.38 0.43 0.76 50.93

313.31 342.06 5.31 26.15 6.13 6.75 339.74

1.3 1.31 30.13 4.97 19.44 17.05 1.26

3.45 23.34 22.16 11.41 10.72 9.48 8.24

This paper reports the design, manufacturing and characterization of four glass-free co-fired hetero-laminated material systems comprising Ag, Zn2SiO4 ceramics and NiCuZn ferrites using LTCC technology. Four Ag, Zn2SiO4 ceramics and NiCuZn ferrites hetero-laminates were designed, manufactured and characterized. Both Zn2SiO4 and NiCuZn ferrite have very similar thermal shrinkage profiles. The characterization results show that no structural cracks or defects in the NiCuZn/Zn2SiO4 and NiCuZn/Ag interfaces according to SEM images, indicating good co-firability among all three materials. Minimum magnetic ion inter-diffusion was found at the Zn2SiO4/NiCuZn interfaces according to EDS element mapping, evidencing that such material system can be used in power magnetic device such as power inductors and DC/DC

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Fig. 8. (a) The SEM image at the interface of LTCC-3-NZN. The blue line indicates the trace of the EDS element composition line scan test. The detailed quantitative EDS composition analysis results for red dash enclosed region at the Zn2SiO4 layer and yellow dash enclosed region at the NiCuZn layer are tabulated in Table 1. The K shell EDS signal from the line scan test is shown for elements (b) Fe, (c) Ni and (d) Cu. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

converters. This method is of special significances because, first it inherits the minimal size of LTCC technology since no buffer layer is needed for a successful manufacturing of hetero-laminated material systems. Second, it needs no special treatment the co-firing process, such as compressing, that will increase the manufacturing cost. This work proves that this co-fired material system could have wide and potential applications in LTCC technology for the fabrication of sub-miniature components and devices exhibiting high-density packaging with multifunctionality. Acknowledgement The authors would like to thank Prof. John Xiao from University of Delaware for his fruitful discussion. R.X. acknowledges the support from the National Science Foundation Graduate Research Fellowship Program under Grant No. 1247394. Y.L. would like to thank National key R&D program No. 2017YFB0406300, Sichuan science and technology supporting program 2016GZ0245 and 2016GZ0261, Guizhou science and technology major projects [2016] 3011. References [1] J. Hsu, W. Ko, H. Shen, C. Chen, Low temperature fired NiCuZn ferrite, IEEE Trans. Magn. 30 (1994) 4875e4877. [2] B. Li, Z. Yue, X. Qi, J. Zhou, Z. Gui, L. Li, High Mn content NiCuZn ferrite for multiplayer chip inductor application, Mater. Sci. Eng. B 99 (2003) 252e254. [3] Q. Yang, H. Zhang, Y. Liu, Q. Wen, L. Jia, The magnetic and dielectric properties of microwave sintered yttrium iron garnet (YIG), Mater. Lett. 62 (2008) 2647e2650. [4] T. Jensen, V. Krozer, C. Kjærgaard, Realisation of microstrip junction circulator using LTCC technology, Electron. Lett. 47 (2011) 111e113. [5] L. Peng, L. Li, X. Zhong, Y. Hu, X. Tu, R. Wang, Magnetic, electrical, and dielectric properties of LaeCu substituted Sr-hexaferrites for use in microwave LTCC devices, J. Alloys Compd. 665 (2016) 31e36.

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