| Summary: | 博士 === 國立臺北科技大學 === 工程科技研究所 === 96 === Among a variety of LTCC glass-ceramic systems available, calcium boron silicon system (CaO-SiO2-B2O3) is often used in microwave electronic components and microelectronic packaging, because of its low cost, low sintering temperature and low dielectric constant, etc. It has become a potential advantage of the glass-ceramic to use calcium with boron and silicon substrates in the application of LTCC. However, there are still many problems to be overcome, including the too narrow range of processing, the very sensitive organization of oxygen, and the electrodes being burned and buried within due to the instability and mismatch with other components. So in response to practical applications of LTCC, there is a need for in-depth understanding of calcium silicate boron glass-ceramic, including the composition of glass-ceramic, firing conditions, the micro-structure and the physical nature of the relevance that could be used to enhance the process stability.
The purpose of this study is to understand the glass-ceramic CBS (CaO-SiO2-B2O3 system) in terms of its composition, processing etc. in addition to its physical nature of relevance with the same composition of the glass. The high and low softening points of glass were modified through the addition of filling materials and their sintering behavior and resistance were also explored. Finally, this study optimizes an appropriate making of the actual glass-ceramic chip antenna and verifies its feasibility of application in microwave antennas.
The results showed that: CBS glass composition proportionally matches with its thermal properties and the dielectric nature. Most of the glass softening points are from 672 to 723oC. The high CaO glass with high dielectric constant trend (7-8), gets sintered after the main crystallization phase of CaSiO3. The high SiO2 glass has an advantage (4-5) of low dielectric constant but B2O3 content has to be raised in order to reduce the sintering temperature. The need to add a filler to modify or prevent sintering, has led to quartz (Cristobalite) generation as well. The glass-ceramic with formula 50.1CaO-7.3B2O3-42.6SiO2 (CBS-10) is a low-temperature sinterable glass-ceramic substrate. The reasons for obtaining a dense ceramic with a sintering temperature below 850 oC, at 4.3 GHz are (i) its dielectric constant which was 6, (ii) dielectric loss that was 0.0014 and hence it is applicable to the use of microwave field. CBS-9 has a low dielectric constant (3.8), a low dielectric loss (0.0017), a thermal conductivity of 1.1W/mK, a resistance value of 25.8 × 1011 Ω, a coefficient of thermal expansion of 3.2 ppm /°C, but because of its dense sintering temperature of 1035oC, an appropriate filler should be added that can change or modify its features, and therefore will have good characteristics in microwave applications.
High Low softening point CBS-950 composite glass is designed by using a high-temperature glass 10.5CaO-22.2B2O3-67.3SiO2 (CBS-9) as a substrate and a low-temperature glass 45CaO-31.7B2O3-23.3SiO2 (CBS-4) as a sintering aid to lower the temperature (800 ~ 900 oC) of sintering across the high-density and high strength of the substrate. Selection of the glass design makes use of high and low softening point, in the same series of glass systems, to avoid the filling material’s reaction with the glass. Under the same conditions of sintering, a glass-ceramic of composition 27.8 CaO-27B2O3-45.3SiO2 (CBS-950) and a single glass of same composition 27.8 CaO-27B2O3-45.3SiO2 (CBS-11) are examined. The main difference of the glass-ceramic CBS-950 with that of pure amorphous glass CBS-11 is that the CBS- 11 is a pure non-crystal glass and as the sintering temperature is gradually increased the crystallization increases resulting in densely sintered material with the formation of CaB2O4 and SiO2 (Tridymite) phases. There is a reduction of 25oC sintering temperature compared to the sintering temperature of commercial LTCC glass substrate materials. In the CBS-950 composite glass sintering process, the reaction between CaSiO3 and SiO2 (Cristobalite) is involved and their sintering has broad temperature range of contraction, with final contraction rate of 18.8 %. This is conducive to the improvement of the process window, similar to the overall dielectric properties which showed consistent results, in addition to sintering of dense glass after crystallization of the main phase from CaB2O4 and SiO2 (Tridymite).
To explore further we added fillers, wetting agents, and nuclear agents into the composite glass CBS-950. The impact of additive Li2CO3 (1wt%) is that densification temperature has significantly reduced from 875 oC to 800 oC by promoting the growth and function of CaB2O4 and quartz crystals. Fillers like Al2O3 (greater than 30 wt%) can inhibit the Li4B2O5 formation and impede the crystallization of CaB2O4 phase and quartz crystal growth. However it regulates the process in the well-off role, and does not involve in the merit of chemical reactions. Addition of TiO2 (about 0.3 wt%) will inhibit the formation of cristobalite but keeps the quartz phase and CaB2O4 phase stable.
CBS-10 was used to make miniatured unipolar chip antenna. It consists of a meander electrode wire and a capacitive loading principle is used in its design by applying 1/4 wavelength. To reduce the resulting antenna size, the central frequency of antenna design is kept at 1.575 GHz, with its bandwidth of about 100 MHz. The actual size of the chip antenna is 10 mm × 10mm × 2 mm and by using the frequency of 1.575 GHz, the Omni-direction of the field pattern makes the antenna standing wave ratio (VSWR) to be 1.5. For practical applications, the standing wave ratio should be less than 2 and have a good point of omni-direction, to gain the change of 2.058 ~ 3.154 dBi. In reality, the reflection loss of antennas is less than -12 dB, which conforms with the commercial antennas that are smaller than characteristic -10 dB, and therefore applicable to miniatured communications products.
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