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Low-fire dielectric compositions with permittivity 20–60 for LTCC applications
Materials Chemistry and Physics 88 (2004) 308–312
Low-fire dielectric compositions with permittivity 20–60 for LTCC applications Jae-Hwan Park∗ , Young-Jin Choi, Jeong-Hyun Park, Jae-Gwan Park Multifunctional Ceramics Research Center, Materials Science and Engineering Division, Korea Institute of Science and Technology, 39-1, Howolkok, Sungbukku, Seoul 136-791, South Korea Received 5 February 2004; received in revised form 29 June 2004; accepted 12 July 2004
Abstract Middle-permittivity dielectric compositions are necessary to realize various distributed circuit components in Low temperature co-fired ceramics (LTCC) structures. In this study, we are reporting some dielectric compositions the permittivity range of which are in the range of 20–60. Various borosilicate-based glass compositions were added to two kinds of middle-k commercial dielectric compositions to get the densification temperatures of lower than 875 ◦ C. The effects of the composition and the amount of borosilicate-based glass systems on the densification and the microwave properties were examined. © 2004 Elsevier B.V. All rights reserved. Keywords: Middle-permittivity; BaTi4 O9 ; BaO–Nd2 O3 –TiO2 ; LTCC; Glass frit
1. Introduction Low temperature co-fired ceramics (LTCC) technology becomes crucial in the development of various modules and substrates in electronic packaging, especially in wireless and microwave applications. Generally, LTCC multilayer structures have been used as 3D wiring circuit boards to date, using low-permittivity dielectric compositions. To realize highly integrated and functional LTCC modules, however, it is important to integrate passive components in multilayer LTCC structure [1,2]. Considering the RF frequency range which is used in current telecommunication systems (1–30 GHz) and the desirable chip sizes in the current technologies (2–10 mm), the dielectric compositions the permittivity range of which is 20–100 are most appropriate for realizing strip or microstrip resonator structures in LTCC multilayer structures.
∗
Corresponding author. Tel.: +82 2 958 5510; fax: +82 2 958 5509. E-mail address: [email protected] (J.-H. Park).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.07.016
Several middle-permittivity dielectric compositions, e.g. the sillenite compound Bi12 MO20−␦ , have been reported to exhibit densification at less than 900 ◦ C. However, it is more general and easy method to add glass frits into the microwave dielectric compositions to realize good electrical properties together with acceptable densification at the low temperature range of less than 875 ◦ C. Several microwave dielectric compositions including (Zr,Sn)TiO4 [3], BaO–TiO2 –WO3 [4], and BaO–Ln2 O3 –TiO2 (Ln:La,Sm,Nd) systems [5,6] have been studied for the development of the middle-k dielectric compositions by using glass frits for low temperature firing. However, the temperature of densification was generally higher than 950 ◦ C, which is still high for co-firing of Ag inner electrodes. In this study, we are to develop LTCC compositions which have a permittivity range of 20–60 and can be sintered at 875 ◦ C by adding borosilicate-based glass compositions. Two kinds of middle-k commercial dielectrics were used as base dielectric materials. The effects of glass compositions and the amount of added glass affecting the densification and the microwave properties of the ceramics were examined and discussed.
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Table 1 Properties of lithium–borosilicate glass system Glass code
G01 G03 G05 G11 G12 G16 G21 G22 G24 G26
Composition (mol%) Li2 O
B2 O3
SiO2
CaO
Al2 O3
51.30 35.14 50.00 56.92 28.00 25.00 28.00 52.45 44.30 36.15
36.53 31.66 40.24 37.59 27.00 30.00 27.00 31.06 29.71 28.35
12.17 33.20 9.76 5.49 30.00 33.00 27.00 11.99 16.99 22.00
– – – – 5.00 5.00 8.00 2.00 4.00 6.00
– – – – 10.00 7.00 10.00 2.50 5.00 7.50
Density (g cm−3 )
k (@1 MHz)
tan δ (%) (@1 MHz)
2.38 2.34 2.40 2.23 2.36 2.42 2.45 2.31 2.32 2.38
7.71 6.44 7.58 8.15 8.12 8.61 8.31 8.76 8.52 8.42
0.40 0.36 0.45 0.57 0.25 0.24 0.27 0.42 0.37 0.36
τ f (ppm ◦ C−1 ) −92 −96 −86 −89 −223 −180 −143 −104 −77 −88
2. Experimental
3. Results and discussion
Two kinds of middle-k commercial dielectrics, MWF-38 (Hayashi Chemical Industry Co. Ltd., Japan) and MBRT90 (Fuji Titanium Industry Co. Ltd., Japan) ceramics, were used as base dielectric materials. Without the addition of glass, the MWF-38 ceramic can be sintered at 1360 ◦ C and exhibits a relative density of more than 98% and good dielectric properties: a dielectric constant (k) of 38.6, a quality factor Q × f of 44 500 GHz, and a temperature coefficient of resonant (τ f ) of +1.3 ppm ◦ C−1 . The MBRT-90 ceramic exhibits, when sintered at 1300 ◦ C for 2 h, k = 90, Q × f = 6100 GHz, τ f = +6 ppm ◦ C−1 . To lower the temperature range of densification of these base dielectrics, a series of borosilicate glass systems were designed. Borosilicate glass systems have been used as the frits for electrical applications. Based on borosilicate systems, we formulated some Li2 O, CaO, and Al2 O3 for optimum sintering properties and electrical properties. From the preliminary studies on the physical and electrical properties of various borosilicate glass frits, a series of compositions listed in Table 1 was found to exhibit both a low glass transition temperature (Tg ) below 500 ◦ C and acceptable electrical properties. Raw materials (>99.9%, High Purity Chem., Japan), as formulated in Table 1, were weighed and mixed by dry ball-milling for 12 h. The mixture was then heated to 1100 ◦ C in Pt crucible to form glass melt. The melt was quenched and crushed by mortar and wet ball-milling for 24 h to form fine glass frits. The middle-k dielectric materials and 5–25 wt% of glass frits were mixed by ball-milling for 24 h. The slurry was dried at 90 ◦ C for several hours and granulated with a 3 wt% of poly-vinyl alcohol (PVA) solution. The granulated powder was then pressed into disk shapes and sintered in the temperature range of 800–950 ◦ C. The dielectric constant, microwave quality factor, and temperature coefficient of the resonant frequency were measured by the parallel plate resonator method and the cavity resonator method with network analyzer (8720C, HP, USA) [7].
3.1. Properties of the glass frits
Tg (◦ C)
Ts (◦ C)
403 488 410 379 456 470 450 373 409 444
422 513 433 398 484 520 470 389 427 464
Table 1 shows the physical and the electrical properties of the borosilicate glass system used in this study. The densities of the glass compositions are in the range of 2.3–2.5 g cm−3 and the dielectric constants are 6.4–8.8. The glass transition temperatures (Tg ) are in the range of 370–470 ◦ C. As shown in the table, the magnitude of dielectric loss increases and the Tg decreases with increasing Li2 O and/or decreasing SiO2 content in the glass composition. The compositions with higher Li2 O and/or lower SiO2 content, such as G11 and G22, exhibit lower Tg of less than 400 ◦ C and higher dielectric loss of more than 0.4. When the glass compositions were modified with CaO and Al2 O3 , the dielectric loss rather decreased while the dielectric constant increased, still maintaining the temperature range of Tg . 3.2. Sintering behaviors and microwave dielectric properties of the LTCC systems The effect of glass frits on the sintering behavior and electrical properties of the MWF-38 and the MBRT-90 dielectric composition were examined. Fig. 1 shows the effects of G22 glass contents and sintering temperature on the density of MWF-38 ceramics. As the added amount of G22 glass frit increases, the relative density increases. When 15 wt% of G22 frit was added, the significant densification was achieved at 875 ◦ C. Though not presented, the linear shrinkage of the samples ranged from 17 to 18% in the samples showing the relative density higher than 95%. Fig. 2 shows scanning electron micrography (SEM) images of the polished surface of a specimen sintered at 875 ◦ C with 15 wt% of the G22 glass frit. The microstructures show the densification by the sufficient liquid phase (dark area in the image) during firing resulting from the softened glass frit. Fig. 3 shows the effect of glass contents on microwave dielectric properties of MWF-38 ceramics. As the added amount of G22 frit increases, the magnitude of k and Q × f
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Fig. 1. Effects of G22 glass contents and sintering temperature on the relative density of MWF-38 ceramics.
decreases due to the low k and Q × f of the glass frits. The increase of k with increasing the sintering temperature could be ascribed to the better density obtained at higher temperatures. The fact that the magnitude of Q × f rather decreased with an increase in the sintering temperature could be related to the formation of second phases generated from the reaction between dielectric composition and glass frits. Though not presented, second phases, which include BaTi5 O11 , Ba2 Ti9 O20 , and Ba4 Ti13 O30 phases were identified by XRD analysis at temperatures higher than 900o C due to the reaction between BaTiO4 and glass frits. From the literature, Q × f values of the BaTi5 O11 , Ba2 Ti9 O20 , and Ba4 Ti13 O30 were considerably lower than BaTi4 O9 phase [9–10]. The decrease of τ f with increasing glass frits can be ascribed to the negative τ f of glass frits. Typically, the glass compositions in this study exhibit the τ f of −70 to −220 ppm ◦ C−1 as listed in Table 1. More importantly, with 15 wt% of the G22 glass frit, the sample exhibited 97% of relative density, k = 26, Q × f = 10,000 GHz, and τ f = −4 ppm ◦ C−1 at the sintering temperature of 875 ◦ C.
Fig. 3. Effect of G22 glass addition on the microwave dielectric properties: (a) k, (b) Qf , and (c) τ f of MBRT-90 ceramics at selected sintering temperatures.
Fig. 2. A SEM image of the polished surface of the MWF-38 ceramics sintered at 875 ◦ C with 15 wt% of G22 glass frit.
The glass frit compositions in this study also could be applied to MBRT-90 dielectric compositions. Fig. 4 shows the sintering behaviors of MBRT-90 ceramics with a G22 glass frit at a temperature range of 800–900 ◦ C for 2 h. The relative density increases with the glass addition. When sintered at 875 ◦ C, a relative density of more than 98% obtained regardless of the amount of glass frit. The temperature of densification decreases from 1300 to 875 ◦ C with the glass addition. At the temperatures higher than 900 ◦ C, the magnitude of relative density reaches nearly 100% regardless of the content of glass frit. This experimental result explains that
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Fig. 4. Effects of G22 glass contents and sintering temperature on the relative density of MBRT-90 ceramics.
the optimal amount of glass frits should be adjusted with a variation in sintering temperatures. Fig. 5 shows the polished surface of a sample sintered at 875 ◦ C with 13 wt% of MBRT-90 ceramics with G22 glass frit. Glass phases are evenly distributed among the ceramic fillers. The fact that there are no open pores in the sample is consistent with the high relative density of more than 98% as shown in Fig. 4. Fig. 6 shows the microwave dielectric properties of MBRT-90 ceramics with a variation in the firing temperature and the amount of G22 glass frits. On the whole, the decreases in the k and Q × f were observed according to the addition of glass frits. The τ f , however, increases when the amount of G22 frit increases over 13 wt%. This is attributed to the decomposition of middle-k dielectric filler due to the addition of excessive glass. From XRD analysis, (Li,Nd)TiO3 phases are confirmed due to the reaction between Li2 O in the frit and dielectric composition. (Li,Nd)TiO3 phases have been reported to have very high τ f of more than 1000 ppm ◦ C−1 [8]. Fig. 6. Effect of G22 glass addition to MBRT-90 ceramics on the microwave dielectric properties: (a) k, (b) Qf , and (c) τ f with selected firing temperature.
Especially, with 13 wt% of the G22 glass frit, the sample exhibits 98% of relative density, k = 53, Q × f = 2400 GHz, and τ f = +18 ppm ◦ C−1 at the sintering temperature of 875 ◦ C. As this composition exhibits reasonable electrical properties and τ f , this also could be applied to the substrate of various microwave resonators. 3.3. Effects of glass composition on the sintering behaviors and microwave dielectric properties
Fig. 5. A SEM image of the polished surface of the MBRT-90 ceramics sintered at 875 ◦ C with 13% of G22 glass frit.
Table 2 shows the properties of MWF-38 and MBRT90 sintered with several lithium–borosilicate glass compositions. As the Li2 O content increases and SiO2 decreases in the frit compositions, the optimum percentage of frit
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Table 2 Properties of MWF-38 and MBRT-90 sintered with lithium–borosilicate glass system Ceramic composition
Glass composition
Type
Content (wt%)
Type
Content (wt%)
MWF-38 MWF-38 MWF-38 MBRT-90 MBRT-90 MBRT-90
90 70 85 90 80 90
G11 G16 G22 G11 G16 G22
10 30 15 10 20 10
Firing temperature (◦ C)
Relative density (%)
Dielectric constant (k)
Q × f (GHz)
τ f (ppm ◦ C−1 )
875 875 875 875 875 875
99.81 94.38 98.20 98.41 92.16 99.86
30.2 17.7 26.0 55.3 31.9 61.6
9500 3700 10200 2500 2200 2500
+3 −15 −4 +26 +20 +18
additions decreases. For example, in the case of a G16 composition with 25 wt% of Li2 O, it should be used by 20–30 wt% in the base dielectric compositions to get acceptable densification at 875 ◦ C. G22 and G11, however, should be used by less than 20 wt%. This could be related to the lower Tg of the frits as the Li2 O content increases and SiO2 decreases in the composition. While using frits with higher Li2 O contents and lower SiO2 contents (G11, G22), better relative densities and electrical properties were obtained. Furthermore, the electrical properties rather increased by designing additional CaO and Al2 O3 in the glass compositions.
sample was sintered at 875 ◦ C with 13 wt% of glass, the dielectric properties of k = 53 and Q × f = 2400 were obtained. These compositions could be applied to various microwave resonators in highly functional and integrated LTCC modules.
Acknowledgement This research was supported in part by a grant from the Center for Advanced Materials Processing (CAMP) of the 21st Century Frontier R&D Program funded by the Ministry of Science and Technology, Republic of Korea.
4. Conclusion In this study, two kinds of dielectric compositions which have rather high permittivity over 20 were studied. The effects of the addition of borosilicate-based glass addition on the densification and the microwave properties of commercial MWF-38, and the MBRT-90 dielectric ceramics system were studied at the sintering temperature of 875 ◦ C. When 10 wt% of the G22 glass frit was added to the MWF-38 ceramics, a relative density higher than 98% was reached at the sintering temperature of 875 ◦ C. When sintered at 875 ◦ C, the microwave dielectric properties of k = 32 and Q × f = 9000 GHz were obtained. When 10–15 wt% of the G22 glass frit was added to the MBRT-90 dielectric ceramics, the sintering temperature decreased from 1300 ◦ C to below than 875 ◦ C and a relative density of more than 97% was obtained. When the
References [1] R.R. Tummala, J. Am. Ceram. Soc. 74 (5) (1939) 895–908. [2] H.T. Sawhill, Ceram. Trans. 26 (1987) 307–319. [3] T. Takada, S.F. Wang, S. Yoshikawa, S.-J. Jang, R.E. Newnham, J. Am. Ceram. Soc. 77 (9) (1994) 2485–2488. [4] T. Takada, S.F. Wang, S. Yoshikawa, S.-J. Jang, R.E. Newnham, J. Am. Ceram. Soc. 77 (7) (1994) 1909–1916. [5] W. Wersing, Electronic Ceramics, Elsevier Science, London, 1991. [6] S. Gabrscek, D. Kola, J. Mater. Sci. Lett. 1 (1982) 37–38. [7] B.W. Hakki, P.D. Coleman, IRE Microwave Theor. Tech. MTT-8 (1960) 402–410. [8] A. Silva, F. Azough, R. Freer, C. Leach, J. Eur. Ceram. Soc. 20 (2000) 2727–2734. [9] H.M. O’Byran, J. Thomson, J. Am. Ceram. Soc. 58 (1975) 454. [10] T. Jaakola, A. Uusimaki, S. Leppavuori, Int. J. High Technol. Ceram. 2 (1980) 195.
Low-fire dielectric compositions with permittivity 20–60 for LTCC applications Jae-Hwan Park∗ , Young-Jin Choi, Jeong-Hyun Park, Jae-Gwan Park Multifunctional Ceramics Research Center, Materials Science and Engineering Division, Korea Institute of Science and Technology, 39-1, Howolkok, Sungbukku, Seoul 136-791, South Korea Received 5 February 2004; received in revised form 29 June 2004; accepted 12 July 2004
Abstract Middle-permittivity dielectric compositions are necessary to realize various distributed circuit components in Low temperature co-fired ceramics (LTCC) structures. In this study, we are reporting some dielectric compositions the permittivity range of which are in the range of 20–60. Various borosilicate-based glass compositions were added to two kinds of middle-k commercial dielectric compositions to get the densification temperatures of lower than 875 ◦ C. The effects of the composition and the amount of borosilicate-based glass systems on the densification and the microwave properties were examined. © 2004 Elsevier B.V. All rights reserved. Keywords: Middle-permittivity; BaTi4 O9 ; BaO–Nd2 O3 –TiO2 ; LTCC; Glass frit
1. Introduction Low temperature co-fired ceramics (LTCC) technology becomes crucial in the development of various modules and substrates in electronic packaging, especially in wireless and microwave applications. Generally, LTCC multilayer structures have been used as 3D wiring circuit boards to date, using low-permittivity dielectric compositions. To realize highly integrated and functional LTCC modules, however, it is important to integrate passive components in multilayer LTCC structure [1,2]. Considering the RF frequency range which is used in current telecommunication systems (1–30 GHz) and the desirable chip sizes in the current technologies (2–10 mm), the dielectric compositions the permittivity range of which is 20–100 are most appropriate for realizing strip or microstrip resonator structures in LTCC multilayer structures.
∗
Corresponding author. Tel.: +82 2 958 5510; fax: +82 2 958 5509. E-mail address: [email protected] (J.-H. Park).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.07.016
Several middle-permittivity dielectric compositions, e.g. the sillenite compound Bi12 MO20−␦ , have been reported to exhibit densification at less than 900 ◦ C. However, it is more general and easy method to add glass frits into the microwave dielectric compositions to realize good electrical properties together with acceptable densification at the low temperature range of less than 875 ◦ C. Several microwave dielectric compositions including (Zr,Sn)TiO4 [3], BaO–TiO2 –WO3 [4], and BaO–Ln2 O3 –TiO2 (Ln:La,Sm,Nd) systems [5,6] have been studied for the development of the middle-k dielectric compositions by using glass frits for low temperature firing. However, the temperature of densification was generally higher than 950 ◦ C, which is still high for co-firing of Ag inner electrodes. In this study, we are to develop LTCC compositions which have a permittivity range of 20–60 and can be sintered at 875 ◦ C by adding borosilicate-based glass compositions. Two kinds of middle-k commercial dielectrics were used as base dielectric materials. The effects of glass compositions and the amount of added glass affecting the densification and the microwave properties of the ceramics were examined and discussed.
J.-H. Park et al. / Materials Chemistry and Physics 88 (2004) 308–312
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Table 1 Properties of lithium–borosilicate glass system Glass code
G01 G03 G05 G11 G12 G16 G21 G22 G24 G26
Composition (mol%) Li2 O
B2 O3
SiO2
CaO
Al2 O3
51.30 35.14 50.00 56.92 28.00 25.00 28.00 52.45 44.30 36.15
36.53 31.66 40.24 37.59 27.00 30.00 27.00 31.06 29.71 28.35
12.17 33.20 9.76 5.49 30.00 33.00 27.00 11.99 16.99 22.00
– – – – 5.00 5.00 8.00 2.00 4.00 6.00
– – – – 10.00 7.00 10.00 2.50 5.00 7.50
Density (g cm−3 )
k (@1 MHz)
tan δ (%) (@1 MHz)
2.38 2.34 2.40 2.23 2.36 2.42 2.45 2.31 2.32 2.38
7.71 6.44 7.58 8.15 8.12 8.61 8.31 8.76 8.52 8.42
0.40 0.36 0.45 0.57 0.25 0.24 0.27 0.42 0.37 0.36
τ f (ppm ◦ C−1 ) −92 −96 −86 −89 −223 −180 −143 −104 −77 −88
2. Experimental
3. Results and discussion
Two kinds of middle-k commercial dielectrics, MWF-38 (Hayashi Chemical Industry Co. Ltd., Japan) and MBRT90 (Fuji Titanium Industry Co. Ltd., Japan) ceramics, were used as base dielectric materials. Without the addition of glass, the MWF-38 ceramic can be sintered at 1360 ◦ C and exhibits a relative density of more than 98% and good dielectric properties: a dielectric constant (k) of 38.6, a quality factor Q × f of 44 500 GHz, and a temperature coefficient of resonant (τ f ) of +1.3 ppm ◦ C−1 . The MBRT-90 ceramic exhibits, when sintered at 1300 ◦ C for 2 h, k = 90, Q × f = 6100 GHz, τ f = +6 ppm ◦ C−1 . To lower the temperature range of densification of these base dielectrics, a series of borosilicate glass systems were designed. Borosilicate glass systems have been used as the frits for electrical applications. Based on borosilicate systems, we formulated some Li2 O, CaO, and Al2 O3 for optimum sintering properties and electrical properties. From the preliminary studies on the physical and electrical properties of various borosilicate glass frits, a series of compositions listed in Table 1 was found to exhibit both a low glass transition temperature (Tg ) below 500 ◦ C and acceptable electrical properties. Raw materials (>99.9%, High Purity Chem., Japan), as formulated in Table 1, were weighed and mixed by dry ball-milling for 12 h. The mixture was then heated to 1100 ◦ C in Pt crucible to form glass melt. The melt was quenched and crushed by mortar and wet ball-milling for 24 h to form fine glass frits. The middle-k dielectric materials and 5–25 wt% of glass frits were mixed by ball-milling for 24 h. The slurry was dried at 90 ◦ C for several hours and granulated with a 3 wt% of poly-vinyl alcohol (PVA) solution. The granulated powder was then pressed into disk shapes and sintered in the temperature range of 800–950 ◦ C. The dielectric constant, microwave quality factor, and temperature coefficient of the resonant frequency were measured by the parallel plate resonator method and the cavity resonator method with network analyzer (8720C, HP, USA) [7].
3.1. Properties of the glass frits
Tg (◦ C)
Ts (◦ C)
403 488 410 379 456 470 450 373 409 444
422 513 433 398 484 520 470 389 427 464
Table 1 shows the physical and the electrical properties of the borosilicate glass system used in this study. The densities of the glass compositions are in the range of 2.3–2.5 g cm−3 and the dielectric constants are 6.4–8.8. The glass transition temperatures (Tg ) are in the range of 370–470 ◦ C. As shown in the table, the magnitude of dielectric loss increases and the Tg decreases with increasing Li2 O and/or decreasing SiO2 content in the glass composition. The compositions with higher Li2 O and/or lower SiO2 content, such as G11 and G22, exhibit lower Tg of less than 400 ◦ C and higher dielectric loss of more than 0.4. When the glass compositions were modified with CaO and Al2 O3 , the dielectric loss rather decreased while the dielectric constant increased, still maintaining the temperature range of Tg . 3.2. Sintering behaviors and microwave dielectric properties of the LTCC systems The effect of glass frits on the sintering behavior and electrical properties of the MWF-38 and the MBRT-90 dielectric composition were examined. Fig. 1 shows the effects of G22 glass contents and sintering temperature on the density of MWF-38 ceramics. As the added amount of G22 glass frit increases, the relative density increases. When 15 wt% of G22 frit was added, the significant densification was achieved at 875 ◦ C. Though not presented, the linear shrinkage of the samples ranged from 17 to 18% in the samples showing the relative density higher than 95%. Fig. 2 shows scanning electron micrography (SEM) images of the polished surface of a specimen sintered at 875 ◦ C with 15 wt% of the G22 glass frit. The microstructures show the densification by the sufficient liquid phase (dark area in the image) during firing resulting from the softened glass frit. Fig. 3 shows the effect of glass contents on microwave dielectric properties of MWF-38 ceramics. As the added amount of G22 frit increases, the magnitude of k and Q × f
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Fig. 1. Effects of G22 glass contents and sintering temperature on the relative density of MWF-38 ceramics.
decreases due to the low k and Q × f of the glass frits. The increase of k with increasing the sintering temperature could be ascribed to the better density obtained at higher temperatures. The fact that the magnitude of Q × f rather decreased with an increase in the sintering temperature could be related to the formation of second phases generated from the reaction between dielectric composition and glass frits. Though not presented, second phases, which include BaTi5 O11 , Ba2 Ti9 O20 , and Ba4 Ti13 O30 phases were identified by XRD analysis at temperatures higher than 900o C due to the reaction between BaTiO4 and glass frits. From the literature, Q × f values of the BaTi5 O11 , Ba2 Ti9 O20 , and Ba4 Ti13 O30 were considerably lower than BaTi4 O9 phase [9–10]. The decrease of τ f with increasing glass frits can be ascribed to the negative τ f of glass frits. Typically, the glass compositions in this study exhibit the τ f of −70 to −220 ppm ◦ C−1 as listed in Table 1. More importantly, with 15 wt% of the G22 glass frit, the sample exhibited 97% of relative density, k = 26, Q × f = 10,000 GHz, and τ f = −4 ppm ◦ C−1 at the sintering temperature of 875 ◦ C.
Fig. 3. Effect of G22 glass addition on the microwave dielectric properties: (a) k, (b) Qf , and (c) τ f of MBRT-90 ceramics at selected sintering temperatures.
Fig. 2. A SEM image of the polished surface of the MWF-38 ceramics sintered at 875 ◦ C with 15 wt% of G22 glass frit.
The glass frit compositions in this study also could be applied to MBRT-90 dielectric compositions. Fig. 4 shows the sintering behaviors of MBRT-90 ceramics with a G22 glass frit at a temperature range of 800–900 ◦ C for 2 h. The relative density increases with the glass addition. When sintered at 875 ◦ C, a relative density of more than 98% obtained regardless of the amount of glass frit. The temperature of densification decreases from 1300 to 875 ◦ C with the glass addition. At the temperatures higher than 900 ◦ C, the magnitude of relative density reaches nearly 100% regardless of the content of glass frit. This experimental result explains that
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Fig. 4. Effects of G22 glass contents and sintering temperature on the relative density of MBRT-90 ceramics.
the optimal amount of glass frits should be adjusted with a variation in sintering temperatures. Fig. 5 shows the polished surface of a sample sintered at 875 ◦ C with 13 wt% of MBRT-90 ceramics with G22 glass frit. Glass phases are evenly distributed among the ceramic fillers. The fact that there are no open pores in the sample is consistent with the high relative density of more than 98% as shown in Fig. 4. Fig. 6 shows the microwave dielectric properties of MBRT-90 ceramics with a variation in the firing temperature and the amount of G22 glass frits. On the whole, the decreases in the k and Q × f were observed according to the addition of glass frits. The τ f , however, increases when the amount of G22 frit increases over 13 wt%. This is attributed to the decomposition of middle-k dielectric filler due to the addition of excessive glass. From XRD analysis, (Li,Nd)TiO3 phases are confirmed due to the reaction between Li2 O in the frit and dielectric composition. (Li,Nd)TiO3 phases have been reported to have very high τ f of more than 1000 ppm ◦ C−1 [8]. Fig. 6. Effect of G22 glass addition to MBRT-90 ceramics on the microwave dielectric properties: (a) k, (b) Qf , and (c) τ f with selected firing temperature.
Especially, with 13 wt% of the G22 glass frit, the sample exhibits 98% of relative density, k = 53, Q × f = 2400 GHz, and τ f = +18 ppm ◦ C−1 at the sintering temperature of 875 ◦ C. As this composition exhibits reasonable electrical properties and τ f , this also could be applied to the substrate of various microwave resonators. 3.3. Effects of glass composition on the sintering behaviors and microwave dielectric properties
Fig. 5. A SEM image of the polished surface of the MBRT-90 ceramics sintered at 875 ◦ C with 13% of G22 glass frit.
Table 2 shows the properties of MWF-38 and MBRT90 sintered with several lithium–borosilicate glass compositions. As the Li2 O content increases and SiO2 decreases in the frit compositions, the optimum percentage of frit
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Table 2 Properties of MWF-38 and MBRT-90 sintered with lithium–borosilicate glass system Ceramic composition
Glass composition
Type
Content (wt%)
Type
Content (wt%)
MWF-38 MWF-38 MWF-38 MBRT-90 MBRT-90 MBRT-90
90 70 85 90 80 90
G11 G16 G22 G11 G16 G22
10 30 15 10 20 10
Firing temperature (◦ C)
Relative density (%)
Dielectric constant (k)
Q × f (GHz)
τ f (ppm ◦ C−1 )
875 875 875 875 875 875
99.81 94.38 98.20 98.41 92.16 99.86
30.2 17.7 26.0 55.3 31.9 61.6
9500 3700 10200 2500 2200 2500
+3 −15 −4 +26 +20 +18
additions decreases. For example, in the case of a G16 composition with 25 wt% of Li2 O, it should be used by 20–30 wt% in the base dielectric compositions to get acceptable densification at 875 ◦ C. G22 and G11, however, should be used by less than 20 wt%. This could be related to the lower Tg of the frits as the Li2 O content increases and SiO2 decreases in the composition. While using frits with higher Li2 O contents and lower SiO2 contents (G11, G22), better relative densities and electrical properties were obtained. Furthermore, the electrical properties rather increased by designing additional CaO and Al2 O3 in the glass compositions.
sample was sintered at 875 ◦ C with 13 wt% of glass, the dielectric properties of k = 53 and Q × f = 2400 were obtained. These compositions could be applied to various microwave resonators in highly functional and integrated LTCC modules.
Acknowledgement This research was supported in part by a grant from the Center for Advanced Materials Processing (CAMP) of the 21st Century Frontier R&D Program funded by the Ministry of Science and Technology, Republic of Korea.
4. Conclusion In this study, two kinds of dielectric compositions which have rather high permittivity over 20 were studied. The effects of the addition of borosilicate-based glass addition on the densification and the microwave properties of commercial MWF-38, and the MBRT-90 dielectric ceramics system were studied at the sintering temperature of 875 ◦ C. When 10 wt% of the G22 glass frit was added to the MWF-38 ceramics, a relative density higher than 98% was reached at the sintering temperature of 875 ◦ C. When sintered at 875 ◦ C, the microwave dielectric properties of k = 32 and Q × f = 9000 GHz were obtained. When 10–15 wt% of the G22 glass frit was added to the MBRT-90 dielectric ceramics, the sintering temperature decreased from 1300 ◦ C to below than 875 ◦ C and a relative density of more than 97% was obtained. When the
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