Al2O3 composite for LTCC applications

Accepted Manuscript Ultralow-permittivity glass /Al2O3 composite for LTCC applications Yong Shang, Chaowei Zhong, Huajing Xiong, Xinyuan Li, Hao Li, Xian Jian PII:

S0272-8842(19)30887-9

DOI:

https://doi.org/10.1016/j.ceramint.2019.04.066

Reference:

CERI 21260

To appear in:

Ceramics International

Received Date: 23 February 2019 Revised Date:

4 April 2019

Accepted Date: 8 April 2019

Please cite this article as: Y. Shang, C. Zhong, H. Xiong, X. Li, H. Li, X. Jian, Ultralow-permittivity glass / Al2O3 composite for LTCC applications, Ceramics International (2019), doi: https://doi.org/10.1016/ j.ceramint.2019.04.066. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Ultralow-permittivity glass /Al2O3 composite for LTCC applications Yong Shanga, Chaowei Zhonga,*, Huajing Xiongb, Xinyuan Lia, Hao Lic,*, Xian Jianb

a. State Key Laboratory of Electronic Thin Films and Integrated Devices, School of

Technology of China, Chengdu 610054, China

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Electronic Science and Engineering, University of Electronic Science and

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b. School of Materials and Energy, National Engineering Research Center of

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Electromagnetic Radiation Control Materials, Center for Applied Chemistry, University of Electronic Science and Technology of China, Chengdu 610054, China

c. College of Electrical and Information Engineering, Hunan University, Changsha

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410082, China

* Corresponding authors. E-mail addresses: [email protected] (Chaowei Zhong),

Abstract

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[email protected] (Hao Li)

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In the field of low temperature co-fired ceramic (LTCC), it remains a challenge to design the performance of LTCC with low permittivity less than 5. Here, a novel glass mixture of K-Al-B-Si-O (KABS) and Zn-B-Si-O (ZBS) is introduced as a sintering aid of alumina to obtain ultralow-permittivity glass/Al2O3 composite. Meanwhile, the factors of glass mixture component on microstructure, phase structure and dielectric properties of the composites are considered systematically. The crystal structure measured by X-ray diffraction (XRD) shows that pure crystalline phase of ZnAl2O4 1

ACCEPTED MANUSCRIPT spinel can be attained by tailoring the component of the glass mixture. In case of mass ratio of KABS: ZBS equal to 6:4, it favors to efficiently increase the sintering densification of composite, and significantly benefit the low dielectric loss, good

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mechanical and thermal performances. In detail, the optimal glass/ceramic composites sintered at 850 °C for 2 h exhibit the bulk density of 2.89 g/cm3, εr of 4.92 at 14 GHz and Q × f of 6873 GHZ, flexural strength of 202 MPa, thermal expansion coefficient of

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5.5 ppm/°C. The above study provides an effective approach for preparing the novel

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composites as a promising candidate for LTCC applications.

Keywords: glass/Al2O3 composite, low temperature co-fired ceramic, low permittivity, dense composite

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1. Introduction

Low temperature co-fired ceramic (LTCC) technology with superior dielectric, mechanical and thermal performances has been widely applied for multilayer

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microelectronic devices[1-3]. For fast signal transmission, low εr and high quality factor

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(Q) are required for practical LTCC applications[4]. Additionally, high flexural strength allows passive components to be embedded into LTCC modules, contributing to integrated electronic circuits and systems[5]. Importantly, it should be taken into consideration that thermal expansion mismatches cause soldering failures between LTCC modules and the substrate[1, 5]. In order to improve the reliability of the system designs, the thermal expansion of LTCC materials should be accurately matched with the thermal expansion coefficient of the substrate materials. 2

ACCEPTED MANUSCRIPT It is well known that Al2O3 has εr of 9.8[4], making Al2O3 ceramic-filler a potential glass/ceramic candidate. Al2O3 also plays an important role in maintaining the mechanical strength of the composite[6]. However, since Al2O3 ceramic has high

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sintering temperatures of over 1400 °C, Al2O3 ceramic is not cofireable with Ag electrodes. The addition of glass can effectively reduce the sintering temperature of Al2O3 ceramic through liquid phase sintering[6-11], making glass/Al2O3 composites a

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viable option for LTCC applications. Xianfu Luo et al.[12] outlined the properties of

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CaO–Al2O3–B2O3–SiO2 glass/Al2O3 composite with different CaO contents. They found that an appropriate addition of CaO into CABS glass enhances glass/ceramic density. The composites exhibited bulk density of 3.1 g/cm3, flexural strength of 185 MPa, dielectric constant of 7.99 at 7 GHz, and dielectric loss of 0.00189. N. Santha et al.[13]

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obtained 30BaO–60B2O3–10SiO2 glass/Al2O3 composite with dielectric constant of 8 at 1 MHz, and argued that its physical and thermal properties are due to the structural changes associated with the addition of Al2O3 into the glass. J. Induja et al.[14]

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described 40 wt% Al2O3–60 wt% BBSZ (35Bi2O3:32ZnO:27B2O3:6SiO2) glass/Al2O3

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composites with dielectric constant of 11.3 and dielectric loss of 0.001 at 1 MHz. Their work indicated that additional ZnAl2O4 phase was formed by the reaction between Al2O3 ceramic-filler and ZnO in BBSZ glass. Xingyu Chen et al.[15] described SiO2– B2O3–CaO–MgO glass/Al2O3 composite sintered at 875 °C with dielectric constant of 7.3, dielectric loss of 0.0015, TEC of 5.41 ppm/°C, and flexural strength of 184 MPa. Their work indicated the appearance of additional phases due to the chemical reactions between glass and Al2O3. The outstanding research on glass/Al2O3 composites carried 3

ACCEPTED MANUSCRIPT out by the above-mentioned researchers has spearheaded progress in this field. However, glass/Al2O3 composites with low permittivity less than 5 deserve in-depth research. Qin Xia et al.[16] had reported the mechanical strength of K2O–B2O3–SiO2–Al2O3

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glass/ceramics strengthened by adding Al2O3 into glass structure, indicating that the addition of Al2O3 into the glass structure resulted in enhanced strength. K.P. Surendran et al.[17] had reported that ZnAl2O4 with the spinel structure exhibited dielectric

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constant of 8.5 and Q × f of 56139 GHZ, making ZnAl2O4 possible candidates for

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packaging applications. However, K.P. Surendran et al.[17] synthesized ZnAl2O4 sintered at the high temperature of 1425 °C, unsuitable for LTCC applications. Sharing Thomas and Mailadil Thomas Sebastian[18] used BBSZ glass to lower the sintering temperature of 0.83ZnAl2O4–0.17TiO2 (ZAT) ceramics to about 950 °C with Q × f >

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10000 GHz and εr of 10, indicating that the sintering temperature of ZnAl2O4 ceramic could be lowered through the addition of BBSZ glass. Sang Ok Yoon et al.[19] prepared ZBS glass/Al2O3 composites sintered at 900 °C with Q × f of 17757 GHz, εr of 5.72.

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The XRD analysis showed that ZnAl2O4 and Al2O3 crystal phases were present in the

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composites, but the dielectric constant of ZBS glass/Al2O3 composite was above 5. Furthermore, the thermal expansion coefficient and flexural strength of ZBS glass/Al2O3 composite were not discussed. Few studies were conducted on glass /Al2O3 composite with low permittivity of 4~5 and pure crystal phase of ZnAl2O4. Herein, we design and prepare glass /Al2O3 composite by using KABS glass and ZBS glass as sintering aid of Al2O3. The purpose of obtaining low permittivity of 4~5 and pure crystal phase of ZnAl2O4 was achieved. The effects of glass mixture on 4

ACCEPTED MANUSCRIPT microstructure, phase compositions, dielectric properties, thermal expansion coefficient, and flexural strength are discussed separately. 2. Experimental procedure

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As shown in Table 1, the melting glass was prepared via high temperature melting technique. The raw materials (including SiO2, B2O3, K2CO3, Al2O3 and ZnO) were

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blended for 4 h using the planetary ball mill. The dried K-Al-B-Si-O powders and Zn-B-Si-O powders were then heated at the high-temperature of 1450 °C and 1400°C,

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respectively. The melts were quenched into the deionized water, and the quenched glass was ball milled using alumina ball and pot.

As shown in Table 2, the glass mixture was directly blended with Al2O3 for 4 hours, using a planetary ball mill. The slurry was dried for 24 hours before being sifted using

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100 mesh sieves to guarantee uniform particle size. Later, the powders were thoroughly ground by adding acrylic acid as a binder until it became uniform and fluid. Finally, the

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powders were shaped via a pressing process at 20 MPa, and the samples were sintered for 2 hours at 810 °C, 830 °C, 850 °C, 870 °C and 890 °C, respectively.

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The phase analysis of the samples sintered at 850 °C was carried out by XRD using Cu Kα radiation (Philips X’Pert Pro MPD). The microstructures of the samples were examined via electron microscopy (SEM, FEI Inspect F, UK). X-ray Photoelectron Spectroscopy (XPS) measurements were performed on an AXIS ULTRA spectrometer (Kratos Analytical Ltd., Japan) using a monochromatic Al source. The differential scanning calorimetry (DSC) analyses of the composites were carried out by using a thermal analyzer (NETZSCHSTA449c, Germany). The densities of the samples were 5

ACCEPTED MANUSCRIPT measured using a GF-3000D Density Meter. A network analyzer (Agilent Technologies E5071C, USA) and a temperature chamber (DELTA 9023, Delta Design, USA) were used to measure the microwave dielectric properties. Flexural strengths and TEC values

machine and a NETZSCH DIL402PC, respectively.

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3. Results and discussion

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were measured using a CMT6104 microcomputer control electronic universal testing

The results of particle sizes measured by laser particle size analysis are presented in

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Fig. 1 (a). The real particle size ratio with 1.32:1:1.37 of ZBS glass, KABS glass and Al2O3 can be calculated by using the average particle size. In order to deepen our understanding of sintering mechanism of glass/Al2O3 composite, Fig. 1 (b) illustrates the process without regard to the shape of the Al2O3 and glass. The densification of

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glass/ceramic is described in four stages, including particle rearrangement, glass softening with particle dissolution, viscosity flow and the reaction with the

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crystallization of ZnAl2O4. Under the driving force produced by viscosity flow of melting glass[1], the formation of ZnAl2O4 phase results from the reaction created when

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sliding Al2O3 particles and ZnO into ZBS glass. The remarkable changes in the element distributions of the composites sintered at 850 °C are shown in Fig. 2 (a)-(d). Crystals with maximal sizes of up to 7 µm are clearly discernable in the mappings (a) and (b). Mappings in Fig. 2 (c) and (d) present a more equally-distributed and smaller-size crystals (4µm) than the mappings Fig. 2 (a) and (b). Because Si element is enriched in the glass phase, the decreasing enrichment of Si element indicates that liquid phase suffers from massive reduction across samples S1 to 6

ACCEPTED MANUSCRIPT S4. Contrary to the decreasing enrichment of Si element, an increasing enrichment of the O, Al and Zn elements can be detected. Table 3 presents the atomic counts of the composites, demonstrating the variation tendency of elemental map distributions, which

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is agreements with the change of Si, O, Al, Zn shown in Fig. 2. The microstructural changes of composites sintered at 850 °C are shown in Fig. 3 (a)-(d). It appears that the amounts of glass phase begin to decrease with larger grains

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corresponding to ZnAl2O4, indicating a reaction between ZBS glass and Al2O3 filler.

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Micrographs of the composites show a softening and fluidization of glass particles bonding with ceramic particles. From the observation on the element distributions and the SEM micrographs of composites, it can be seen that the increasing ZBS glass and decreasing KABS glass leads to a reduction of liquid phase, indicating that the volume

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of liquid phase can be influenced by adjusting the composition of the glass mixture. Fig. 4reveals the comparison between varying densities and dielectric constant. Firstly, the densities of these composites with different sintering temperature are

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discussed. The maximum densities in Fig. 4 range from 2.75 to 2.96 g/cm3. KABS-rich

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systems (including S1 and S2) are expected to exhibit lower densities. S1, S2 and S3 achieve optimum density at 830 °C, 830 °C and 850 °C, respectively. Apparently, a sintering temperature beyond 890 °C is required in order to achieve the maximum density for S4. From the results in Fig. 3, it is observed that the glass phase decreasing as ZBS glass content increases. ZBS glass may therefore have a negative wettability on Al2O3 fillers, resulting in a higher sintering temperature. The densities of S1, S2 and S3 decline on the account of the fact that increasing sintering temperature leads to 7

ACCEPTED MANUSCRIPT decreased viscosity of molten glass, which may generate more pores[20, 21]. Lower viscosity corresponds to excessive fluidity of glass, enabling the air to be wrapped inside liquid phase which results in the formation of closed pores. Hence, the decreasing

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densities of S1, S2 and S3 composites at high sintering temperature are attributed to the excessive fluidity of glass. With the reasonably desirable microstructural characteristics in Fig. 3 (c), a higher density of 2.89 g/cm3 for S3 system sintered at 850 °C is

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acceptable for the LTCC applications.

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With regard to dielectric properties, Fig. 4 shows dielectric constant of the glass/Al2O3 composites at various sintering temperature. S1, S2 and S3 achieve their maximum dielectric constant at 830 °C, 830 °C and 850 °C, respectively. Apparently, changes in dielectric constant coincidences with bulk density. According to previous

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studies, dielectric constant is related to the densification of fired samples[6, 22]. Thus, as shown in Fig. 4, variations in dielectric constant conform to the density curves observed at different temperatures.

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Fig. 5 shows the XRD patterns of the composites sintered at 850 °C. ZnAl2O4 is the

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main crystalline phase, whereas the additional Al2O3 crystalline phase disappears as ZBS glass contents increase up to 28 wt. % across samples S1 to S3. The enhanced reaction between the ZBS glass and Al2O3 filler results in the formation of ZnAl2O4 phase, and the intensity of ZnAl2O4 crystalline phase gradually is enhanced with increased ZBS glass content. Apparently, as ZBS glass content increases up to 28 wt%, the reaction between the ZBS glass and Al2O3 filler takes place until the appearance of ZnAl2O4 single phase. The reaction can be described through the following equation (1): 8

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(1)



The XPS measurement results (Fig. 6) further confirm the presence of ZnAl2O4. It can be seen that S3 composite contains Al, Si, O, Zn, and B elements. The photoelectron

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binding energy peaks for each element are as follows: Al2p is at 73.98 eV, B1s is at 192.12 eV, O1s is at 532.15 eV, Si2p is at 102.18 eV, Zn2p3 is at 1022.08 eV and Zn2p1 is at 1043.27 eV. From the comparison between our experimental data and the XPS

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standard database[23], the spectrums of Al and Zn elements confirm the chemical state

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of ZnAl2O4. Here, XPS measurements results and XRD patterns of the composites indicate that ZnAl2O4 is successfully synthesized by tailoring the composition of glass mixture.

The DSC curves of the glass/Al2O3 composites are shown in Fig. 7. From the DSC

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analyses, glass transition (Tg) and the glass melting temperature (Tmelt) are obtained based on the previous reports [24-27]. In detail, the inflection point in the rapidly dropping part of heat flow corresponds to Tg [24]. Herein, the Tg and Tmelt for the

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composites are figured out to be in the range of 560~573 oC and 778~791 oC,

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respectively. The endothermic peaks positioned at the range of 644~653 oC were recorded, which is attributed to the change of ZBS glass structure. Two kinds of coordination forms, namely, [ZnO4] tetrahedron and [ZnO6] octahedron in ZBS glass structure are instability and disappeared in case of temperature at around 645 oC resulting in the endothermic peaks, enabling the occurrence of chemical reactions between ZnO and Al2O3. The mentioned reaction was also reported by Sang et al. in case of ZBS glass/Al2O3 composites [19]. Therefore, the ZnAl2O4 phase generates as the 9

ACCEPTED MANUSCRIPT temperature increases up to 673~680 oC, accompanied with an exothermic peak. The phenomena is agreement with the previous report that the ZnAl2O4 is formed at about 700 oC and the solution precipitation stage of the liquid phase sintering occurs below

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700 oC [19]. It can be seen that the exothermic peak at 673~680 oC and endothermic peak at 644~653 oC are obviously sharpened as the component of ZBS glass increases in the composites, indicating that increasing ZBS glass promotes the crystallization of

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ZnAl2O4.

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Fig. 8 shows the comparison between density and dielectric constant, the Q × f values, flexural strength and thermal expansion coefficient of the glass / Al2O3 composite sintered at 850°C. Fig. 8 (a) shows the comparison between density and dielectric constant with different mass ratio of glass mixture. The result indicates that the increase

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of KABS glass contributes to the lowered dielectric constant of glass/ceramic. The KABS glass powder shown in Table 1 is rich in silicon (εr =3.8), making it desirable for preparing a glass/ceramic composite with a dielectric constant of less than 5. The results

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presented in Table 3 and the mapping distributions of the Si element (shown in Fig. 2)

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indicate that silicon content decreases alongside the decrease of KABS glass. Hence Si-rich systems (including S1, S2 and S3) have lower dielectric constant. The results displayed in Fig. 8 (a) indicate that permittivity can be adjusted by tailoring the composition of glass mixture. In this work, S1, S2 and S3 composites each have a low dielectric constant of below 5 at approximately 14 GHz, which is desirable for high-frequency applications with fast signal transmission. From Fig. 8 (b), we can observe that increasing ZBS glass content corresponds to 10

ACCEPTED MANUSCRIPT higher Q × f values. The Q × f values range from 4800 to 7200 GHz. There are several reasons for the tendency of the Q × f values to increase alongside glass content. It is widely known that borosilicate glass modified by alkali metal oxides has a higher

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dielectric loss due to the movement of alkali ions. In this study, since the K+ ion migrates intensely within glass structure[16], increasing KABS glass content results in low Q × f values. It has been reported that the ZnAl2O4 crystalline phase has a high Q ×

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f values[17]. The intensity of ZnAl2O4 crystalline phase is gradually enhanced, as is

across samples S1 to S4.

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shown in Fig. 5. Hence, Q × f values gradually rise with increasing ZBS glass content

With respect to physical properties shown in Fig. 8 (c), it is clear that a maximal flexural strength of 202 MPa is achieved for the S3 composite sintered at 850°C. The

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flexural strength increases initially, and then decreases slightly to 170 MPa with excess ZBS glass content. Usually, the flexural strength of composites depends on the microstructure and major crystal phases[28]. According to the theory of liquid phase

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sintering, the densification of glass/ ceramic composites occurs through the viscous

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flow mechanism[4]. As shown in Fig. 1 (b), the densification mechanism can be explained by considering that Al2O3 powders are uniformly dispersed in the glass phase. Glass materials with the superior wettability play a critical role in facilitating the reaction between ZBS glass and Al2O3 fillers, which promotes the formation of ZnAl2O4 crystalline phase. The SEM micrographs of composites (shown in Fig. 3) demonstrate their wettability. The XRD pattern of S3 sample in Fig. 5 shows an entirely singular ZnAl2O4 spinel crystalline phase. The appearance of ZnAl2O4 phase with stable crystal 11

ACCEPTED MANUSCRIPT structure enhances mechanical strength of the composites. However, excess ZBS glass is undesirable for achieving the optimum densification. Thus, as shown in Fig. 3 (c) and Fig. 5, the following properties are desirable for achieving a high flexural strength of

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glass/ceramic: firstly, entirely singular ZnAl2O4 spinel crystalline phase is achieved; secondly, there is a homogenous, dense microstructure and the crystalline grains firmly embedded within the glass phase.

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As for the thermal properties of the composites sintered at 850 °C, it is observed in

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Fig. 8 (d) that the TEC values of the composites ranging between temperatures of 25°C~300°C are 4.8~5.5 ppm/°C, which is close to that of GaAs (5.6 ppm/°C). It appears that the thermal expansion behaviors of S1, S2, S3 and S4 composites are growing as ZBS glass content increases. It is believed that phase components affect the

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values of TEC[29]. In this work, phase structures differ significantly when adjusting the glass mixture composition. Decreased liquid phase and increased ZnAl2O4 phase are deemed to dominate the TEC of the composites. The composites with TEC values

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between 4.8-5.5 ppm/°C are suitable for application in LTCC substrate and GaAs chips.

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Finally, we make a comparison between the commercial LTCC materials and the glass/Al2O3 composites in this work. As shown in Table 4, the composites we prepared possess lower CTE, higher flexural strength, lower permittivity, and lower dielectric loss, which provides a good reliability for electronic packaging. 4. Conclusions Glass/Al2O3 composites with low dielectric constant less than 5 were successfully prepared by liquid phase sintering using a novel glass mixture of K-Al-B-Si-O (KABS) 12

ACCEPTED MANUSCRIPT and Zn-B-Si-O (ZBS). The XRD results shows a crystalline phase of ZnAl2O4 spinel by adjusting the component of the glass mixture. The dielectric constant shows low values between 4.56 and 5.14 due to the addition of Si-rich KABS glass as well as the

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formation of ZnAl2O4 having a low dielectric constant. The formation of ZnAl2O4 and the decreasing amount of KABS glass are corresponded to the increase of Q × f. The dense microstructure and ZnAl2O4 crystal phases are deemed to enhance the flexural

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strength. Meanwhile, the increasing values of TEC is due to the enhanced ZnAl2O4

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crystal phases as well as decreasing liquid phase. The composite with mass ratio of KABS: ZBS equal to 6:4 exhibits good sintering capability at 850 °C, with high densification (2.89 g/cm3), low dielectric constant (4.92) and Q × f (6873 GHZ), thermal expansion (5.5 ppm/°C), which makes the novel composites a promising

Acknowledgements

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candidate for LTCC applications.

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This work was supported by the National Natural Science Foundation of China (No. 51202021), the National Natural Science Foundation of China (Grant No. 21603203) Sichuan Science and Technology Program (No. 2014RZ0041).

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and

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[28] X. Zhou, E. Li, S. Yang, B. Li, B. Tang, Y. Yuan, S. Zhang, Effects of La2O3–B2O3

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on the flexural strength and microwave dielectric properties of low temperature co-fired CaO–B2O3–SiO2 glass–ceramic, Ceram.Int, 38 (2012) 5551-5555. [29] Y. Imanaka, Multilayered Low Temperature Cofired Ceramics (LTCC) Technology,

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Springer, New York, 2005.

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ACCEPTED MANUSCRIPT Table 1 Composition of glass contents (mass fraction, wt%). SiO2

B2O3

ZnO

K2O

Al2O3

K-Al-B-Si-O

83

13

16.10

1

3

Zn-B-Si-O

1.36

4.91

14.40

0

0

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Table 2 Composition of different glass/ceramics contents (mass fraction, wt%). ZBS

S1

56

14

S2

49

S3

42

S4

35

Al2O3

KABS/ ZBS

30

8:2

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30

7:3

28

30

6:4

35

30

5:5

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Table 3 Atomic counts of the composites measured by EDS analysis. Sample

O (%)

Si (%)

Al (%)

Zn (%)

74.02

16.07

8.79

1.13

S2

73.19

15.93

9.26

1.62

S3

71.75

12.70

12.04

3.51

S4

70.22

9.10

14.48

6.20

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ACCEPTED MANUSCRIPT Table 4. Properties of the glass/Al2O3 composites in comparison with commercial LTCC materials. εr

Loss (10-3)

Strength (MPa)

TEC (ppm/°C)

Sintering temperature (°C)

DuPont 951

7.8

1.5@1KHz

320

5.8

<900

Ferro A6

5.9

2@10MHz

130

7

Heraeus CT700

7

2@1KHz

240

6.7

This work

4.92

2@14GHz

202

5.5

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<900

<900 850

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composite.

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Fig. 1 (a) Particle size distribution, (b) Densification mechanism of glass/Al2O3

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Fig. 2 Elemental map distributions of composites sintered at 850 °C measured by back scattered electron mode. (a) S1, (b) S2, (c) S3, (d) S4.

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Fig. 3 SEM micrograph of composites sintered at 850 °C measured by secondary electrons mode. (a) S1, (b) S2, (c) S3, (d) S4.

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Fig. 4 The comparison between dielectric constant and density when the composites are sintered at different temperature. a. S1, b. S2, c. S3, d. S4.

Fig. 5 XRD patterns of samples sintered at 850 °C.

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Fig. 6 XPS patterns of S3 composite sintered at 850°C.

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Fig. 7 Differential scanning calorimetry (DSC) thermograph of the composites.

Fig. 8 The properties of composites sintered at 850 °C with different mass ratio of glass mixture. (a) dielectric constant, (b). Q × f, (c). flexural strength, (d). TEC