Materials Letters 65 (2011) 565–567
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Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Low permittivity SrCuSi4O10–LMZBS glass composite for LTCC applications K.M. Manu, P.S. Anjana, M.T. Sebastian ⁎ Materials and Minerals Division, National Institute for Interdisciplinary Science and Technology, CSIR, Thiruvananthapuram, 695019, India
a r t i c l e
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Article history: Received 1 September 2010 Accepted 20 October 2010 Available online 29 October 2010 Keywords: Ceramics Dielectrics Relative permittivity Dielectric loss LTCC Densification
a b s t r a c t The tetragonal gillespite type SrCuSi4O10 (SCS) was prepared by the conventional solid-state ceramic route. The SCS sintered at 1100 °C/6 h showed εr = 4.0 and tan δ = 1.1 × 10−3 at 5 GHz. The SCS has poor sinterability and the addition of lithium magnesium zinc borosilicate glass (20: Li2O, 20: MgO, 20: ZnO, 20: B2O3, 20: SiO2) lowered the sintering temperature and improved densification. The SCS ceramic with 5 wt.% LMZBS glass sintered at 900 °C has εr = 5.0 and tan δ = 1.9 × 10−3 at 5 GHz. The composite is chemically compatible with the common electrode material silver. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Rapid developments in the microelectronic industry demand miniature microwave devices with a high processing speed [1]. The combined use of low permittivity dielectrics with low loss glasses enabling low temperature co-fired ceramic (LTCC) technology can meet this requirement. Compared with printed resin boards, LTCC's are superior due to their high frequency characteristics, thermal stability and their capacity for integrating passive components [2]. The LTCC substrate should possess several characteristics such as (i) low relative permittivity (εr) to increase signal speed, (ii) low dielectric loss (tan δ) for selectivity, (iii) low temperature coefficient of relative permittivity for thermal stability, (iv) low or matching coefficient of thermal expansion to that of materials attached to it and (v) high thermal conductivity to dissipate heat. The predominant covalent nature in silicates restricts the rattling of atoms, which leads to low permittivity and low dielectric loss [3]. Several low loss, low permittivity materials have been developed with good dielectric properties [3]. However, high processing temperatures limit their applications in LTCC based devices. The sintering temperature of the ceramic should be less than the melting point of electrode materials for LTCC applications. Moreover, the electrode materials such as silver or gold should be chemically compatible with the ceramics. Addition of low melting glasses is known to be the effective low cost method to lower the sintering temperature without considerably affecting the dielectric properties [4].
⁎ Corresponding author. Tel.: +91 471 2515294; fax: +91 471 249 1712. E-mail address:
[email protected] (M.T. Sebastian). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.10.062
The layered copper silicate, Wesselsite (SrCuSi4O10) has been known from ancient times. The structural properties of SrCuSi4O10 have been reported earlier [5,6]. It possesses a gillespite structure which is tetragonal with space group P4/ncc (no.130). In the present paper, we report the microwave dielectric properties of the SCS ceramic and its low temperature sintering for the first time. 2. Experimental Stochiometric amounts of high purity powders of SrCO3, CuO and SiO2 (99.9%, Sigma Aldrich) were weighed and ball milled for 24 h in distilled water medium. The dried powders were calcined at 950 °C for 18 h. Different wt.% of lithium magnesium zinc borosilicate (LMZBS) glass were added to the fine powder of calcined SCS. Cylindrical pucks of various dimensions and square sheets of dimensions (50 mm × 50 mm × 1.8 mm) were made for the characterization in radio and microwave frequencies respectively. The sintering was carried out at temperatures in the range 800 °C– 1125 °C for 6 h by muffling the samples with the powder of the same composition to restrict the escape of copper. The sintered and powdered samples were used to analyze the crystal structure and phase purity by the X-ray diffraction (XRD) method using CuKα radiation (Philips X'pert PRO MPD XRD; Philips, Eindhoven, Netherlands). The microstructure of the samples was recorded using a scanning electron microscope (SEM) (JEOL-SEM 560lv, Tokyo, Japan). The sintered densities of the specimens were measured by the Archimedes method. The dielectric properties at 1 MHz were measured using a LCR meter (Hioki 3532-50 LCR HiTESTER, Nagano, Japan). The microwave dielectric measurements were performed by the split post dielectric resonator (SPDR) method [3] using a Vector Network Analyzer (Agilent Technologies, Model No.8753 ET Inc., Palo
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Fig. 1. X-ray diffraction patterns of (a) SrCuSi4O10 (SCS) sintered at 1100 °C/6 h and (b) SCS + 5 wt.% LMZBS glass + 20 wt.% Ag sintered at 900 °C/6 h.
Alto, CA). The coefficient of linear thermal expansion was measured by a Dilatometer (DIL 402 PC, NETZSCH, Germany) from 30 to 600 °C. 3. Results and discussion Fig. 1(a) shows the X-ray diffraction pattern of the SCS sintered at 1100 °C/6 h. The powder diffraction pattern was indexed based on the ICDD file no. 81-1239 and no additional peaks were observed. The lattice parameters a = 7.368 Å and c = 15.588 Å were calculated from the XRD pattern by graphical extrapolation using the Nelson–Riley function [7] and are in good agreement with an earlier report [6]. Fig. 2 (a) shows the microstructure of the SCS ceramic sintered at 1100 °C/ 6 h. The presence of small grains with a porous structure indicates poor densification (88%) of the SCS ceramic. The XRD pattern of the SCS with 5 wt.% LMZBS glass mixed with 20 wt.% silver(b250 μm) sintered at 900 °C/6 h is shown in Fig. 1(b). It can be observed that no chemical reaction has taken place between the SCS–glass composite and silver. Fig. 2(b) shows the microstructure of the 20 wt.% silver added SCS with 5 wt.% LMZBS glass. It can be seen that the composite shows a relatively dense microstructure with an increase in the grain size. The unreacted silver particles can be clearly seen from the micrograph. Table 1 gives the relative density and dielectric properties of the SCS ceramic with different wt.% of LMZBS glass at their optimized sintering temperatures. The high processing temperature (1100 °C) and poor densification (88%) limit the applications of the pure SCS in LTCC based devices. The multicomponent boron rich glasses are effective in reducing the sintering temperature due to the low glass transition temperature of B2O3 [8]. Hence we have selected LMZBS glass to lower the sintering temperature well below the melting point
of silver (960 °C). It is clear from Table 1 that the sintering temperature of the SCS ceramic decreased from 1100 °C/6 h to 900 °C/6 h with the addition of 5 wt.% LMZBS glass. The addition of 1 wt.% LMZBS glass increased the relative density to 98.0%. The increase in densification was due to the elimination of pores by the glassy liquid phase which enhanced material transport [8]. The relative density of the SCS with 3 wt.% LMZBS glass was found to be 97.0% and it decreased to 95.2% for 5 wt.% LMZBS glass. The formation of pores by the evaporation of excess glass components may be the reason for reduction in the relative density for higher wt.% glass fluxing [8]. The dielectric properties are interrelated with the relative density of ceramic–glass composites [4]. The dielectric properties at 1 MHz vary in a manner similar to that of the relative density. The SCS ceramic sintered at 1100 °C showed εr = 5.05 and tan δ = 9.6 × 10−4. The εr increased to 6.01 with the addition of 1 wt.% LMZBS glass. The improvement in the εr may be due to enhancement in the relative density and also due to the higher relative permittivity of LMZBS glass (εr = 6.9) [9] compared to the pure SCS ceramic. It can be seen that subsequent addition of LMZBS glass reduced the relative permittivity. The porosity corrected εr values (εrcorr.) of the SCS–glass composites were calculated using the equation derived by Penn et al. [10]. The εrcorr. values of all the SCS–glass composites were found to be relatively higher than that of the pure SCS, due to the relatively large εr value of LMZBS glass at 1 MHz. The tan δ of the SCS at 1 MHz decreased from 9.6 × 10−4 to 8.1 × 10−4 with the addition of 1 wt.% LMZBS glass. The enhanced densification, which resulted in the reduction of the grain boundary (2-dimensional defect) density, or entrapped moisture (3-dimensional defect) may be the reason for reduction in the dielectric loss. The tan δ increases with addition of higher wt.% of LMZBS glass and it may be due to the relatively large dielectric loss of the glass [3]. The electrical resistivity of the SCS–glass composites at 1 MHz is also given in the Table 1. The relative permittivity and dielectric loss at about 5 GHz (4.9 to 5.1 GHz) for different wt.% LMZBS glass are given in the Table 1. The relative permittivity at microwave frequencies is lower than that obtained at 1 MHz. The reduction in εr can be attributed due to the absence of interfacial and dipolar polarizations at microwave frequencies which are predominant at 1 MHz [3]. The SCS ceramic sintered at 1100 °C/6 h showed εr = 4.0 and tan δ = 1.1 × 10−3 at 5 GHz. The addition of 1 wt.% LMZBS glass increased the εr to 5.3 and tan δ reduced to 8.8 × 10−4 at 5 GHz whereas the SCS mixed with 5 wt. % LMZBS glass sintered at 900 °C/6 h showed εr = 5.0 and tan δ = 1.9 × 10−3 at 5 GHz. The porosity corrected εr values of the SCS– glass composites were also calculated [10] and are given in Table 1. The SCS–LMZBS glass composites developed in the present study with inexpensive raw materials have low bulk density (b3.5 g/cm3). Hence these composites are advantageous in terms of their dielectric properties, light weight nature and the low cost of production. Table 1 also gives the coefficient of linear thermal expansion (CTE) of the SCS ceramics mixed with different wt.% LMZBS glass measured in the temperature range 30–600 °C. The CTE of the pure SCS was found to be −2.3 ppm/°C. It can be observed that with the addition of
Fig. 2. SEM images of (a) SrCuSi4O10 (SCS) sintered at 1100 °C/6 h and (b) SCS + 5 wt.% LMZBS glass + 20 wt.% Ag sintered at 900 °C/6 h.
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K.M. Manu et al. / Materials Letters 65 (2011) 565–567 Table 1 The sintering temperature, relative density, dielectric properties and the coefficient of linear thermal expansion of SCS ceramics with addition of different wt.% LMZBS glass. At 1 MHz
At 4.9 to 5.1 GHz
wt.% of LMZBS glass
Sintering temperature Relative εr (°C) density
εrcorr. (porosity corrected)
tan δ
Electrical resistivity (Ω cm)
0 1 3 5
1100/6 h 1075/6 h 975/6 h 900/6 h
6.10 6.20 6.23 6.32
9.6 × 10−4 8.1 × 10−4 9.0 × 10−4 1.0 × 10−3
2.2 × 108 5.3 × 106 4.7 × 106 7.1 × 106
87.8 98.0 97.0 95.2
5.05 6.01 5.95 5.90
r
4.0 5.3 5.1 5.0
εrcorr. (porosity corrected)
tan δ
CTE (ppm/°C)
4.9 5.4 5.3 5.4
1.1 × 10−3 8.8 × 10−4 1.0 × 10−3 1.9 × 10−3
−2.3 −3.0 −0.7 −1.4
added composites show relatively small deviation compared to the pure SCS ceramic in this range. 4. Conclusions SCS ceramics were prepared by the conventional solid-state ceramic route. The SCS sintered at 1100 °C /6 h showed εr = 4.0 and tan δ = 1.1 × 10−3 at 5 GHz. The sintering temperature of the SCS was lowered to 900 °C by the addition of 5 wt.% LMZBS glass. The composite showed good microwave dielectric properties (εr = 5.0 and tan δ = 1.9 × 10−3 at 5 GHz) and nearly zero CTE (−1.4 ppm/°C). The composite was found to be chemically compatible with silver. The observed properties indicate that the SCS with a 5 wt.% LMZBS glass composite can be a possible candidate for LTCC substrate applications. Acknowledgement Authors are grateful to the Department of Science and Technology, Government of India for the financial assistance.
Fig. 3. Variation of relative permittivity with temperature at 1 MHz for (a) SrCuSi4O10 (SCS) sintered at1100 °C/6 h, (b) SCS + 5 wt.% LMZBS glass sintered at 900 °C/6 h, (c) SCS + 3 wt.% LMZBS glass sintered at 975 °C/6 h and (d) SCS + 1 wt.% LMZBS glass sintered at 1075 °C/6 h.
References
5 wt.% LMZBS glass, the CTE of the SCS reached to a nearly zero value which is an added advantage. Knight et al. already reported the negative volume thermal expansion behavior of Ba0.5Sr0.5CuSi4O10 up to 100 K [11]. The unusual behavior of the CTE for the SCS–glass composites in the present study may be due to the transverse thermal motion of oxygen in the Si–O–Si linkage [12] or due to the tilting of the SiO4 tetrahedra and further work is required in this direction. Fig. 3 shows variation of relative permittivity at 1 MHz with the temperature for the SCS ceramic mixed with different wt.% of LMZBS glass. A peak in the temperature range 14 °C–21 °C is observed for all the samples. The reason for this anomalous behavior may be due to the tilting of SiO4 tetrahedra and needs further investigation. The glass
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