CaF2-AlF3-SiO2 glass-ceramic with low dielectric constant for LTCC application

Article abstract of DOI:10.1016/j.jallcom.2009.09.031

Low temperature co-fired ceramic (LTCC) materials based on CaF₂-AlF₃-SiO₂ oxyfluoride glass-ceramics were prepared. The sintering behaviors, phase transition, microstructures, and the dielectric properties of the glass-cer

Full text of DOI:10.1016/j.jallcom.2009.09.031

Journal of Alloys and Compounds 490 (2010) 204–207
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
CaF2–AlF3–SiO2 glass-ceramic with low dielectric constant for LTCC application
Rui Wang, Ji Zhou∗, Bo Li, Longtu Li
State Key Lab of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 June 2009
Received in revised form 6 September 2009
Accepted 7 September 2009
Available online 15 September 2009
Low temperature co-fired ceramic (LTCC) materials based on CaF2–AlF3–SiO2 oxyfluoride glass-ceramics
were prepared. The sintering behaviors, phase transition, microstructures, and the dielectric properties
of the glass-ceramics, as well as their compatibility with Ag electrode were investigated systematically. It
was revealed that the oxyfluoride glass-ceramics could be sintered at 800–825 ◦C. Moreover, as-prepared
glass-ceramics exhibited low dielectric constant (5.93–4.93), low dielectric loss (<1 × 10−3), sufficient
mechanical strength (>150 MPa), proper thermal expansion coefficient (8.4–6.0 ppm/◦C) and good sin-
tering compatibility with silver electrode, which provided promising candidates for LTCC application.
Keywords:
LTCC
Glass-ceramic
Substrate
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
of bulk materials [5,11,12]. In this case, the transition temperature
(Tg) of glass plays an important role in the reduction of sinter-
In recent years, electronic devices with small size, high perfor-
mance, low cost and easy fabrication are in great demand to meet
the rapid development of information technology. Among various
fabrication methods of electronic devices, low temperature co-fired
ceramic (LTCC) technology has become an attractive manufactur-
ing platform with high speed and good functionality for compact,
light-weight and integrated electronic components, modules, sub-
strates and devices [1–4]. In this technology, passive components
(such as capacitors and inductors) can be embedded into substrates,
and co-fired with high-conductive metals (such as silver and cop-
per) below 900 ◦C. Therefore, the properties of LTCC substrates are
of great importance to the performance of products.
In order to obtain substrates with good performance for high
speed transmission of signals, dielectric ceramics with both low
dielectric constant (better less than 5) and low sintering tem-
perature are highly required. In order to lower the sintering
temperature, sintering aids are necessary [5–7]. However, most of
the sintering aids such as Bi2O3 and Li2O will lead to an increase
in dielectric constant or dielectric loss, while for the sintering aids
with low dielectric constant, B2O3, for example, large amount is
often needed to attain the low dielectric constant. The large con-
tent of B2O3 might cause the porosity increasing and damage the
mechanical strength of system [6–9]. An alternative way to reduce
the sintering temperature of dielectrics is to use crystallized glass or
glass fillers [3–5,10,11]. During heating process, the glass melts and
fills the capillary pore channels, which improves the densification
ing temperature. However, Tg of current crystallized glass or glass
fillers, such as borosilicate glass and aluminosilicate glass, is usually
at a relative high level (about 700–800 ◦C) [1,7,10–15]. Even though
the BaO–B2O3–SiO2 system with large B2O3 content could exhibit a
Tg of 619 ◦C, the microstructure will be destroyed and the dielectric
loss increases due to the large amount of B2O3 [14]. Therefore, it
is a key point to search for new glass or glass-ceramic systems for
LTCC substrate materials with low sintering temperature and low
dielectric constant simultaneously.
Recently, oxyfluoride glass and glass-ceramics have attracted
more and more attentions. Compared with other kinds of glass,
oxyfluoride glass softens at a lower temperature due to the increase
in the number of non-bridging bonds, which results from the sub-
stitution of one Si–O bond by two Si–F bonds during the process of
fluoride ions into glass network [16,17]. In addition, it is reported
that because of its large electro-negativity, the incorporation of flu-
oride ions will also lead to a decrease in the electronic polarizability
and the ionic polarizability of dielectrics, which helps to achieve a
much lower dielectric constant [18,19]. Therefore, oxyfluoride glass
and glass-ceramics are promising for LTCC substrates with both low
sintering temperature and low dielectric constant.
In this paper, LTCC materials based on CaF2–AlF3–SiO2 oxyfluo-
ride glass-ceramics were prepared. The sintering behaviors, phase
transition, microstructures and dielectric properties of the as-
prepared materials, as well as their compatibility with Ag electrode,
were investigated.
2. Experimental procedures
∗
Corresponding author. Tel.: +86 010 62772975; fax: +86 010 62772975.
E-mail address: zhouji@mail.tsinghua.edu.cn (J. Zhou).
Reagent-grade raw materials of AlF3·3H2O, CaF2 and SiO2 with purities higher
than 99 wt.% were used to prepare the starting glass powders. The compositions
0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2009.09.031

R. Wang et al. / Journal of Alloys and Compounds 490 (2010) 204–207
205
Table 1
mixed together and ground in absolute ethanol for 24 h in ball mills. After being
dried completely, the powders were melted at 1250–1350 ◦C for 2 h in a covered
alumina crucible and then quenched in cold water to form glass cullet. The glass
cullet was ground and then milled to 1–2 ␮m by ball-milling. After dried completely,
the powders were mixed with a small amount of polyvinyl alcohol (PVA, 10 wt.%)
as organic binder and pressed uniaxially under a pressure of 80 MPa to form green
compacts, which were then sintered at 750–850 ◦C for 2 h. For compatibility test
with silver electrode, silver paste was coated on the green compacts and co-fired at
a certain temperature for 2 h.
The compositions of the starting oxyfluoride glass (wt.%).
Sample no.
AlF3
CaF2
SiO2
S1
S2
S3
36
30
35
24
20
15
40
50
50
The characteristic temperatures of the glass powders were determined using
a thermal analyzer (NETZSCH STA 409 PC/PG). The densities of the sintered bod-
ies were measured by Archimedes’ method. The crystalline phases of the samples
were determined by X-ray diffraction (XRD, Rigaku X-ray diffractometer), and the
microstructure observation was performed by scanning electron microscopy (SEM,
FEI QUANTA 200F). The dielectric properties of the sintered samples were measured
at 1 GHz using Agilent 4991A RF impedance/material analyzer after the coating of
silver electrodes. The flexural strength and average linear thermal expansion coeffi-
cient (TCE) were measured via an electronic universal testing machine (DWD 3020)
and a thermal mechanical analyzer (TMA, NETZSCH DIL 402 PC), respectively.
are listed in Table 1. In S1 and S2, the mass ratio between AlF3 and CaF2 was kept
constant as 3:2 and the SiO2 content increased from 40 to 50 wt.%. While in S2 and
S3, the amount of CaF2 was reduced from 20 to 15 wt.% and the amount of SiO2 was
constant.
Before the raw materials mixed, AlF3·3H2O was baked firstly at 550 ◦C for 1 h in
a NH4F protective atmosphere to prepare anhydrous AlF3. The raw materials were
Fig. 1. DSC curves of the oxyfluoride glass-ceramics in air with a heating rate of
10 ◦C/min.
Fig. 3. XRD patterns of the samples sintered at different temperatures. (a) The start-
ing glass and glass-ceramics sintered at their maximal densification temperatures
for 2 h and (b) the samples of S1 sintered at different temperatures for 2 h.
Fig. 2. (a) Shrinkages and (b) densities of the samples sintered at different temper-
atures.

206
R. Wang et al. / Journal of Alloys and Compounds 490 (2010) 204–207
3. Results and discussion
(anorthite) and CaF2. The XRD patterns in Fig. 3(b) illustrate the
phase transition of S1 at different sintering temperatures. It can
be found that when sintered at 750 ◦C, slight amount of CaF2 (as
marked with arrows) precipitates firstly, and as the sintering tem-
perature increases to 800 ◦C, the diffraction peaks corresponding
to anorthite phase appear. Moreover, the peak intensity at about
2ꢀ = 28◦ is enhanced with the increase in sintering temperature.
Combined the XRD results with the DSC curve of S1 in Fig. 1, it can
be concluded that the exothermic peak at 750 ◦C is correspond-
ing to the crystallization of CaF2, while the other one at 805 ◦C is
corresponding to the crystallization of anorthite.
The SEM images of fractured surfaces of the samples are shown
in Fig. 4. It reveals that all the samples are densified effectively at
their maximal densification temperatures (see Fig. 4(a–c)), and the
surfaces are covered with large amount of glassy phase. However,
there are still small amount of pores left in the samples. That is
because the gases, which result from the decomposition of organic
binder and the partial release of fluoride, are wrapped by the soft-
ened glassy phase and cannot be released efficiently. From S1 to
S3, the porosity increases and the pores become small and disper-
sive. When the sintering temperature rises to 850 ◦C, the samples
become more porous than those sintered at low temperatures, and
some pores even connect together.
Table 2 lists a summary of the properties of the samples at
their maximum densification temperatures. Because of the large
electro-negativity of fluoride ions, the dielectric constants (εr) of
the oxyfluoride glass-ceramics are less than 6. Moreover, all the
samples exhibit low dielectric losses (tan ı) less than 0.001. From S1
to S3, the decreasing intensities of the exothermic peaks in the DSC
curves in Fig. 1 indicate a decreasing amount of crystalline phases
which precipitate during the heat treatment. As the amount of the
crystalline phases (CaF2 and anorthite) with high εr reduces from
S1 to S3, the εr of the samples decrease from 5.93 to 4.83. In addi-
tion, crystalline CaF2 exhibits a large TCE value (∼18 ppm/◦C) [20],
and the TCE values of glassy phase and polycrystalline anorthite
are relatively low. As the content of CaF2 decreases from S1 to S3,
Fig. 1 shows the result of differential scanning calorimetry (DSC)
measurement of the glass powders in air up to 1000 ◦C with a heat-
ing rate of 10 ◦C/min. Tg of this glass system is identified as about
550 ◦C from the base line changing in the DSC traces. Compared
with the common borosilicate glass and aluminosilicate glass for
LTCCs [1,7,10–15], the Tg of present oxyfluoride glass is much lower,
which is mainly due to the increase in non-bridging bonds caused
by the fact that fluoride ions penetrate into the glass network and
one Si–O bond is replaced by two Si–F bonds [16,17]. In addition, as
marked with arrows in the DSC trace of S1, two exothermic peaks
appear which is caused by the crystallization and can be identi-
fied as about 750 and 805 ◦C, respectively. It can be found that the
exothermic peak at 750 ◦C is not as sharp as the one at 805 ◦C, which
indicates that the crystallization corresponding to 750 ◦C occurs in
a wide temperature range. Moreover, only a broad exothermic peak
can be observed in the DSC trace of S2, while for S3 there is even no
obvious exothermic peak appeared in the DSC trace. The decrease
in the number of exothermic peaks in DSC traces of the samples
confirms to the decrease in the amount of crystalline phases from
S1 to S3.
Fig. 2 shows the shrinkage behaviors and bulk densities of the
glass-ceramics as a function of sintering temperature. As the tem-
perature increases from 750 to 850 ◦C, both the liner shrinkages and
the bulk densities of three compositions increase and reach their
maximal values at 800 ◦C for S1 and S2, and 825 ◦C for S3. From S1
to S3, the shrinkage of the samples varies from 15 to 17%, while the
densities decrease from 2.75 to 2.59 g/cm3.
Fig. 3(a) shows the XRD results of the starting glass and the sam-
ples sintered at their maximum densification temperatures (800 ◦C
for S1, 800 ◦C for S2 and 825 ◦C for S3). It is revealed that the starting
glass powders are basically composed of amorphous glassy phases
and there are little crystalline phases existed. However, after sin-
tered at their maximum densification temperatures for 2 h, all the
samples are mainly composed of crystalline phases of CaAl2Si2O8
Fig. 4. SEM micrographs of the samples. (a) S1 800 ◦C, 2 h; (b) S2 800 ◦C, 2 h; (c) S3 825 ◦C, 2 h; (d) S1 850 ◦C, 2 h; (e) S2 850 ◦C, 2 h and (f) S3 850 ◦C, 2 h.

R. Wang et al. / Journal of Alloys and Compounds 490 (2010) 204–207
207
Table 2
Overview of the properties of the samples.
Sample no.
Sintering temperature (◦C)
Density (g cm−3
)
␧
(1 GHz)
tan ı (×10−3) (1 GHz)
TCE (ppm/◦C) (20–200 ◦C)
Flexural strength
(3 point bend) (MPa)
r
S1
S2
S3
800
800
825
2.75
2.66
2.59
5.93
5.72
4.83
<1
8.4
7.5
6.0
200
164
150
the XRD patterns which indicates that the glass-ceramic compos-
ite does not react with the Ag conductor. Fig. 5(b) shows the SEM
image of S2 co-fired with silver paste at 800 ◦C for 2 h. No cracks
appear and the interface between LTCC material and electrode is
clear. It is revealed that the sample has a good compatibility with
silver paste. Moreover, the interface between the glass-ceramic and
the Ag electrode matches well and gets good adherence.
4. Conclusions
The CaF2–AlF3–SiO2 ternary oxyfluoride glass-ceramics with
low dielectric constant and low sintering temperature were pre-
pared. The sintering behavior, microstructures, phase compositions
and dielectric properties were studied. These glass-ceramics can
be sintered at a low temperature 800–825 ◦C, and as the compo-
sition changed from S1 to S3, the shrinkage increased from 15
to 17%. Moreover, the dielectric constant decreased from 5.93 to
4.83, and TCE value is reduced from 8.4 to 6.0 ppm/◦C. The oxyflu-
oride glass-ceramics, with low dielectric losses (<0.001), enough
mechanical strength and good co-fired compatibilities with Ag
electrode, should be promising candidates of LTCC dielectric mate-
rial for electronic substrates or passive integration.
Acknowledgments
This work was supported by National Science Foundation
of China under grants of 50425204, 50572043, 10774087 and
50621201, and Ministry of Education of China through Seeding
Foundation of Major Projects.
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the amount of precipitated CaF2 in the samples decreases corre-
spondingly. Consequently, the TCE values of the samples reduced
from 8.4 to 6.0 ppm/◦C (see Table 2). The flexural strength also
decreases from 200 to 150 MPa, which is because of the variation of
microstructure with the changing of the composition. As shown in
Fig. 4(a–c), more dispersive small pores are left from S1 to S3, and
even some pores connect together in S3. As a result, the flexural
strength decreases with the damage of the microstructures. But in
general, all the three samples exhibit proper TCE values which are
close to that of GaAs chips (5.5 ppm/◦C) and relatively high strength
which is enough for LTCC substrates.
Fig. 5(a) shows the XRD result of S2 co-fired with 10 wt.% sil-
ver powders at 800 ◦C for 2 h. There is no new phase appeared in