Densification and microwave dielectric properties of CaO–B2O3–SiO2 system glass–ceramics

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Densification and microwave dielectric properties of

CaO–B

2

O

3

–SiO

2

system glass–ceramics

Chuang-Chung Chiang

a

, Sea-Fue Wang

a,

*

, Yuh-Ruey Wang

a

, Wen-Cheng J. Wei

b

a_{Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, 1, Sec. 3,}

Chung-Hsiao E. Rd., Taipei 106, Taiwan, ROC

b

Department of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan, ROC Received 4 September 2006; received in revised form 11 December 2006; accepted 21 December 2006

Available online 30 January 2007

Abstract

In order to relieve the narrow processing window and poor material compatibility in practical applications as well as understand the microwave dielectric properties, investigation on the formulations of CaO/B2O3/SiO2glasses on their structure, thermal properties, and microwave properties

were performed in this study. Six glasses with different molar ratios of CaO/B2O3/SiO2(designed as CBS-3, CBS-5, CBS-7, CBS-8, CBS-9, and

CBS-10) were prepared and pulverized. Results indicate that most softening points of glasses are ranging from 680 to 710 8C. They were sintered at different temperatures to reach maximum densification. Among various glass formulations, CBS-9 glass–ceramic containing the largest amount of SiO2has the lowest CTE. The dielectric constants can be divided into two groups including around 4–5 and 7–8, and the dielectric losses (tan d) are

all below 0.005 in the frequency of10 GHz. The dielectric constants and dielectric losses are generally frequency dependent. For CBS-9 glass– ceramic, the dielectric constant and dielectric loss at 4.7 and 18.6 GHz are 4.13 and 0.0018, and 4.20 and 0.0063, respectively.

Keywords: Glass–ceramics; LTCC; Microwave properties; CaO–B2O3–SiO2

1. Introduction

In additional to the conventional printed circuit board technology, low-temperature co-firable ceramic (LTCC) tech-nology is one of the new technologies to manufacture integrated multifunctional electronic chips[1]. It can be utilized for the integration of passive components into a monolithic, high reliable and robust LTCC module. These modules consist of several layers of substrate material with buried components such as capacitors, inductors, resistors, resonators, and filters. They are interconnected with 3D stripline circuitry. Utilization of low dielectric constant (K) LTCC substrate materials and associated high conductivity metallizations such as Au and Ag was found to have potential applications in the area of wireless communication[2].

A targeted LTCC substrate materials should possess several characteristics such as low dielectric constant and dielectric

loss, high thermal conductivity, low thermal expansion coefficient (close to Si and GaAs), very robust against environmental stress, and low cost. LTCC substrate materials usually contain multiple phases. The properties of the multiphase systems not only depend on the properties and the extent of each phase present, but also the morphology, continuity, and connectivity of each phase[3,4]. In fabrication of desirable LTCC substrates, a complete densification and sufficient crystallization are generally necessary for the required mechanical properties and dielectric properties such as high Q value. Porosity and low degree of crystallinity would lead to relatively poor mechanical properties, and residual glass would detrimental to the Q value at microwave frequency. Approaches to achieve the above requirements are the use of binary glass systems including a high softening point glass and a low softening glass, nucleating agents, sintering aids, and properties and structure modifiers[5–8].

Among several LTCC glass–ceramic systems [6,9–11], CaO–B2O3–SiO2 system has been reported as a promising material for use in microelectronic packaging because of low firing temperature and low dielectric loss[12,13]. Commercial

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* Corresponding author.

E-mail address:[email protected](S.-F. Wang).

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available Ferro A6 LTCC system is a typical example. However, utilization of CaO–B2O3–SiO2 glass–ceramic as LTCC substrate often encountered obstacles including narrow process window that is sensitive to firing profile and atmosphere, poor compatibility with the metallizations such as silver, and limited compatible materials for RLC compo-nents. Though crystallizations and physical properties of some specific CaO–B2O3–SiO2compositions were discussed in the literature, rare information regarding the compositional effects on the physical properties, particularly for microwave dielectric properties, of the CaO–B2O3–SiO2glass–ceramics have been reported. Optimization of glass–ceramic composition and further understanding of the structure–property relations of the CaO–B2O3–SiO2glass–ceramics for LTCC applications are required to relieve the obstacles encountered in practical applications. In this study, the effects of the compositions of CaO–B2O3–SiO2 glass–ceramics on their thermal behaviors and microwave dielectric properties were systematically investigated and discussed.

2. Experimental procedure

The design of the glass compositions in this study is based on the phase diagram of CaO–B2O3–SiO2system [14], and the formulations are marked inFig. 1, in which the CaO, B2O3, and SiO2are ranging from 10 to 70%, 7 to 32%, and 14 to 67%, respectively. Glasses of CaO–B2O3–SiO2system were prepared using the solid-state reaction technique, with high purity (>99.9% pure) of SiO2, B2O3, and CaO (Wako, Reagent grade) as raw materials. Mixtures based on the weight percentages shown in Table 1 were mixed and milled in methyl alcohol solution using polyethylene jars and zirconia balls for 2 h and then oven-dried overnight at 80 8C. After drying, the powders were melt in an alumina crucible at the temperatures ranging from 1500 to 1650 8C for 1 h, depending on the glass composition. To prevent the occurrence of any crystallization, the melt was quenched into the de-ion water. Subsequently, the glasses were then crushed in the mortar and pestle and then re-milled in methyl alcohol for 10 h. Though some glasses have phase separation during melting, homogeneous distribution of

the species was ensured through the milling. The pulverized glass powders were then subjected to particle size analysis and phase analysis using X-ray diffraction (XRD, Rigaku DMX-2200) with Cu Ka radiation to confirm their amorphous nature. Differential thermal analysis (DTA) was performed using Perkin-Elmer calorimeter, Series 1700 DTA, on the glasses to evaluate the possible reactions during heating.

The powder was added with a 5 wt% of 15%-PVA solution and pressed into disc-shaped compacts using uniaxial pressure of 1 t/cm2. The samples were then heat treated at 550 8C for 2 h to eliminate the PVA, followed by sintering at temperatures between 800 and 925 8C for 15 min with a heating rate of 5 8C/ min. In order to understand the effects of glass compositions on the sintering characteristic, dilatometric analyses were per-formed to characterize the shrinkage of the glass with respect to temperature. Experiments were performed using a DIL 402C dilatometer in air and at a heating rate of 5 8C/min. This method makes it possible to trace the onset temperature of the densification during sintering.

Density measurements were done using a liquid displace-ment method, with water as a media. Phase identification on the sintered glass–ceramics was performed using XRD. Scanning electron microscopy (SEM, Hitachi S4700), and energy dispersive spectroscopy (EDS) studies were used to reveal the microstructures of sintered samples. The normal (z) components of the dielectric constants (er) and dielectric losses (tan d) of the glass–ceramics values at microwave frequencies were measured in the TM0l0 mode using a Damaskos Model 400 [15] with a network analyzer (HP 8722ES). A cylindrical shaped unmetallized sample with an aspect ratio (diameter/height)1 was used to chock off many interfering modes. The sample was positioned inside the circular cavity with good contact to top and bottom plates. Dielectric constant and tan d were calculated from the frequency of the TM0l0 resonant mode.

3. Results and discussion 3.1. Densification behavior

The sintering shrinkage curves of the CaO–B2O3–SiO2 powder compacts measured using dilatometer are revealed in

Fig. 2. The densification behaviors of these systems behave differently due to their significant dissimilarity in the molar ratios of CaO/B2O3/SiO2. The glass softening points obtained from DTA measurement are summarized inTable 2, which are also

Table 1

The formulations of various CaO–B2O3–SiO2glass–ceramics

Formulation CaO B2O3 SiO2

mol% wt% mol% wt% mol% wt%

CBS-3 69.7 66.4 16.2 19.2 14.1 14.4 CBS-5 38.3 34.9 31.5 35.6 30.2 29.5 CBS-7 29.3 27.5 9.3 10.8 61.4 61.7 CBS-8 19.8 17.8 30.9 34.6 49.3 47.6 CBS-9 10.5 9.5 22.2 25.0 67.3 65.5 CBS-10 50.1 47.8 7.3 8.6 42.6 43.6

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evident in the sintering shrinkage curves. The results indicate that most glass softening points are ranging from 680 to 710 8C, except CBS-3 at 830 8C. At the temperatures above the glass softening point, the viscous flow causes the shrinkage of glass (Fig. 2).

The level of the shrinkage is correlated to the locations of the glass softening point and the crystallization point [16]. The formation of the crystalline phase in the early stage generally

retards the densification. Among the six compositions, CBS-3, containing rich CaO, has the highest onset temperature of the shrinkage and the smallest dimensional change, due to its high melting point (1400 8C). CBS-3 starts to crystallize at 920 8C, which ceases the densification and leads to the shrinkage concluding at less than 5%.

Both CBS-5 and CBS-10 follow a similar densification path and have a substantial shrinkage of over 20% below 800 8C and, afterward, the shrinkage of CBS-10 levels off. This is due to the facts that CBS-10 starts to crystallize CaSiO3above 800 8C, and CBS-5 has the lowest melting point (980 8C) among the systems in the phase diagram. CBS-10 has an exothermic peaks at temperatures around 820 8C, which is corresponding to the formation of CaSiO3. CBS-7 and CBS-8 have a similar shrinkage route in the early stage of sintering. However, the densification of CBS-7 ended in 8% of shrinkage at 840 8C and CBS-8 concluded in a 14% of shrinkage at 870 8C. This is due to the crystallization process, which causes the expansion of the matrix. Though the above systems show different percentages of sintering shrinkage in Fig. 2, high densification was achieved based on the SEM micrographs shown in Fig. 3. Particularly, microstructures of CBS-5 and CBS-9 evidently indicate almost no porosity.

Fig. 4shows the sintering densities of the CaO–B2O3–SiO2 system glass–ceramics andFig. 5shows the XRD patterns of

Fig. 2. Sintering shrinkage curves of the CaO–B2O3–SiO2powder compacts

measured using dilatometer.

Table 2

Physical properties of various CaO–B2O3–SiO2glass–ceramics

Formulation Glass softening point (8C)

Thermal expansion coefficient (10 6_8C 1 ) (from 25 to 500 8C) Measuring frequency (GHz) Dielectric constant Dielectric loss

Temperature coeff. of dielectric constant (ppm/8C; at 10 kHz) CBS-3 830 8.7 9.59 7.3 0.0041 52 CBS-5 705 8.2 9.59 7.3 0.0045 1690 CBS-7 700 7.3 9.97 3.9 0.0055 530 CBS-8 680 6.4 9.93 4.1 0.0048 214 CBS-9 710 3.5 9.92 4.1 0.0038 127 CBS-10 710 9.3 9.55 7.9 0.0045 29

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various systems after maximum density were obtained. The sintering temperatures required to achieve maximum bulk density are coincidence with the shrinkage behaviors shown in

Fig. 2. The maximum bulk densities are ranging from 2.3 to 2.9 g/cm3. CaO-rich systems including CBS-3, CBS-5, and CBS-10 systems possess higher theoretical densities. SiO2-rich

systems including CBS-7, CBS-8, and CBS-9 are expected to have lower densities, particularly, for CBS-9 which only consists of SiO2 (quartz structure) after sintering (Fig. 5). Coincide to the results shown inFig. 2, CBS-3 densifies at the highest sintering temperature (1300 8C) with the evolution of Ca11Si4B2O22 and CaSiO3 phases. CBS-5 and CBS-10 achieved the maximum density at 775 and 800 8C, respectively. CBS-5 remains amorphous at 780 8C and CBS-10 produces some crystalline phase of CaSiO3 when they were densified. CBS-8 has a small mount of cristobalite SiO2phase in the early stage of densification (795 8C). Coexistence of cristobalite SiO2 and trace CaSiO3are found when maximum densification was attained. CBS-9 densified with simultaneous formation of quartz SiO2phase in the glass matrix.

The linear dimensional changes with temperature for various glass–ceramics are shown in Fig. 6 and the coefficients of thermal expansion (CTE) averaging from 50 to 500 8C are listed in Table 2. The dimensional changes for all system, except CBS-8, show almost linearity with temperature. There is an abrupt change in slope for CBS-8 system at200 8C, which is corresponding to the phase change of SiO2cristobalite phase. The displacive transformation from the low cristobalite phase to the high cristobalite phase leads to the discontinuity in the thermal expansion[5]. The CTEs of the glass–ceramic systems are ranging from 3.5 to 9.7 ppm/8C, similar to those of gallium-arsenide and silicon. CBS-10 glass ceramic possesses the highest CTE and CBS-9 glass–ceramic, containing the largest amount of quartz SiO2phase among the various compositions studied, has the lowest CTE. The temperatures of displacive transformation for quartz and cristobalite phases are at 573 and 200–270 8C, respectively[17], that is why CBS-8 encounters discontinuity in thermal expansion in the temperature range of the CTE measurement, but CBS-9 not.

Table 2also shows the dielectric properties of various CaO– B2O3–SiO2 glass–ceramics. The dielectric constants can be divided into two groups including around 4–5 and 7–8, and the dielectric losses (tan d) are all below 0.005 in the frequency of 10 GHz. The CaO-rich systems, including CBS-3, CBS-5, and CBS-10 glass–ceramics, are expected to have a higher dielectric constant, due to the polarizability of the Ca2+ ions

Fig. 4. The sintering densities of various CaO–B2O3–SiO2glass–ceramics.

Fig. 5. XRD patterns of various CaO–B2O3–SiO2 systems after maximum

density were obtained.

Fig. 6. Thermal expansions vs. temperature for the various glass–ceramics after densification.

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(3.16 A˚3), which are much higher than those of Si4+(0.87 A˚3) and B3+ (0.05 A˚3) ions [18]. The SiO2-rich glass–ceramics, such as CBS-7, CBS-8, and CBS-9, are expected to possess lower dielectric constants since the dielectric constant of SiO2 is the lowest among the ceramics. Particularly, most glass– ceramics in this study contain CaSiO3phase which has a low dielectric constant of 5. Tan d of glass is dependent on a variety of factors. At least three types of dielectric losses for glasses have been distinguished: resonance-type vibrational losses at very high frequency, migration losses caused by the movement of mobile ions (mainly Na+), and deformation losses by defect or deformation of the basic silicon oxide network

[19]. Resonance-type vibrational losses are particularly important in the microwave region. Among the glasses, silica glass has the lowest tan d in the microwave region. Although the loss level is attractive, silica is not a low-temperature co-firable ceramic if used alone. To lower the melting point, the rigid bonds in SiO2may be broken by modifiers, particularly alkali ions, but results in higher losses. It is expected that the CBS-9 would be the best in term of dielectric loss, since CBS-9 is rich in SiO2content. The dielectric constants and dielectric losses are generally frequency dependent. Typical example is shown

in Fig. 7 for CBS-9 glass–ceramic. The dielectric constant increases very slightly (1.7%) while the dielectric loss raises nearly 250% as frequency increased from 4.7 GHz to 18.6 GHz (4.13 and 0.0018 at 4.7 GHz, and 4.20 and 0.0063 at 18.6 GHz). The low dielectric constant provides an attractive feature for minimizing cross-talk and increasing signal transmission speeds. The temperature coefficient of dielectric constant (TCe) for various CaO–B2O3–SiO2 glass–ceramics listed in

Table 2indicated that the changes in dielectric constant with temperature (TCC) for the glass–ceramics are all less than 1.4% based on the measurement at 10 kHz, as also plotted inFig. 8. The calculated temperature coefficients of dielectric constant (TCe) based on the equation of (eT e25 8C)/Te25 8Care listed in

Table 2. It is expected that the TCe will reduce significantly as the frequency increase since the space charge and ionic polarizations would disappear at microwave frequency. The temperature stable behavior of the CaO–B2O3–SiO2 glass– ceramics meets the requirement for LTCC substrate applica-tions.

It can be concluded that each constituent in the glass composition is crucial to the crystallization, densification, and thus microwave dielectric properties. However, their effects are intercorrelated and, sometimes, unable to distinguish the contributions from an individual constituent. Higher SiO2favor the formation of wollastonite (CaSiO3) associated with crystallization of cristobalite, tridymite or quartz, depending on the sintering temperatures and the cooling rate, such as CBS-8 (61.7 wt% SiO2). When SiO2 exceed 65 wt%, wollastonite formation is prevented (CBS-9). The substrates with high SiO2 content tend to possess a lower thermal expansion coefficient and lower dielectric constant at microwave frequency.

Roles of the B2O3in the glass are to facilitate the vitrification and lower the melting point of the glass. The content of the B2O3is limited at a maximum value of35 wt% in this study, since it is detrimental to the resistance of acid and humidity erosion, as reported in the literature. Comparing CBS-5 and CBS-10, devitrification becomes difficult and formation of wollastonite is limited after heat treatment for glass with high B2O3content.

Addition of CaO in the samples increases the melting point of the samples and makes the vitrification difficult. The higher the CaO content in the samples, the higher the firing temperature for densification and the dielectric constant is. CBS-3 is a typical example for the glass–ceramic with high CaO content.

4. Summary

In this study, the effects of the compositions of CaO–B2O3– SiO2glass–ceramics on the thermal behavior and microwave dielectric properties were investigated. The results indicate that most glass softening points are ranging from 680 to 710 8C, except CBS-3 at 830 8C. The level of the sintering shrinkage is correlated to the locations of the glass softening point and the crystallization point. CBS-3, containing rich CaO, has the highest onset temperature of the shrinkage and the smallest dimensional change. The maximum sintered bulk densities are

Fig. 7. Dielectric constants and dielectric losses vs. frequency for CBS-9 glass– ceramic.

Fig. 8. Change in dielectric constant with temperature for various glass– ceramics.

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ranging from 2.3 to 2.9 g/cm3. There is an abrupt change in slope of thermal expansion for CBS-8 system at200 8C, due to the displacive transformation of the high temperature and the low-temperature SiO2 cristobalite phase. The dielectric constants can be divided into two groups including around 4–5 and 7–8, and the dielectric losses (tan d) are all below 0.005 in the frequency of10 GHz.

Acknowledgement

This work is supported by National Science Council of Republic of China (NSC-94-2216-E-027-006), which is gratefully appreciated.

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