濾波器為通訊系統接收端的重要元件之一，近年來隨著微波通訊的蓬勃發展，對元件的特性及尺寸微小化的要求越趨嚴格。而低溫共燒陶瓷(LTCC, Low Temperature Co-Fired Ceramic) 技術因具有高密度、多層結構及可將被動元件內埋等多項優點而被廣泛應用於電子產業，特別是以無線通訊元件及模組為主的相關應用。
本論文主要在於利用低溫共燒陶瓷材料及製程技術為基礎，設計及實現低損耗、小尺寸的帶通濾波器，並嘗試以複合式的介電材料共燒方式，來實現半集總式帶通濾波器。採用複合式的介電材料共燒方式作為設計的基礎，其設計的概念主要著眼於，當單純使用低介電常數材料為基材時元件體積過大，以及當單純使用高介電常數材料為基材時元件特性不佳的缺點，而以複合式的介電材料共燒方式來實現濾波器，既可利用高介電常數材料來實現電容以期縮小元件體件，同時搭配低介電常數材料來實現傳輸線的部分，藉以提高品質因子﹙quality factor﹚，藉由兩種介電材料優點的結合，而達到縮小元件體積、同時還能維持良好的元件特性。
因此本論文的研究主要區分為三個主軸，首先對於所選用的商業化低介電常數LTCC材料 (其介電常數為7.8) 做研究，因LTCC的材料系統是所謂的玻璃陶瓷系統 (glass-ceramic)，其在燒結過程中玻璃相會因結晶而逐漸減少，而燒結後系統的結晶相與玻璃相的種類及其相對量會影響基材的介電特性，因此有必要在設計濾波器之前，對於所選用的LTCC材料在燒結過程中，其結晶相的演化過程及其對介電特性的影響先行了解。
第二部分則是對於複合介電材料中用以實現電容的高介電常數材料 (Zn,Mg)TiO3 (其介電常數為27) 做研究。(Zn,Mg)TiO3 具有良好的微波介電特性，然而為了能與LTCC材料共燒，分別添加3ZnO-B2O3玻璃和Bi2O3以降低其燒結溫度。同時為了了解 (Zn,Mg)TiO3 作為電容設計基材的可行性，研究中以 (Zn,Mg)TiO3 為基材做成積層陶瓷電容器 (MLCC, Multilayer Ceramic Capacitor)，並使用不同比例的銀鈀合金作為電容的內電極來進行特性分析，其中針對銀的擴散 (silver migration) 導致MLCC信賴度 (reliability) 變差的現象有深入的討論。
第三部分則主要著眼於 2.4 GHz 與 5.2 GHz 帶通濾波器的設計及實作。結果顯示，利用短路耦合傳輸線、並聯諧振腔與輸入輸出間的回授電容效應可實現具有高衰減速度與止帶抑制之二階三零點濾波器，在其中心頻率 5.15~5.35 GHz 的插入損耗僅為 2.0 dB，濾波器尺寸也只有 2.5 mm x 2.0 mm x 1.0 mm。此外，以複合式的介電材料共燒實現半集總式帶通濾波器，實作的結果顯示，利用複合式的介電材料共燒所實現的濾波器在通帶 2.4 GHz 頻率點輸入損耗約為 2 dB，衰減量在 1.9 GHz 與 5.0 GHz 分別為 42 dB 及 35 dB，元件尺寸則為 2.0 mm x 1.2 mm，此結果驗證使用複合式的介電材料共燒來實現半集總式帶通濾波器的確可同時達到低損耗特性與縮小元件尺寸訴求。

Microwave filters are one of the most important components in receivers, they are used to select or confine the RF/microwave signals within assigned spectral limits. Recently, in the wireless and mobile communication system, trends toward miniaturization, higher performance, and lower electric power consumption have become increasingly prominent. Low temperature co-fired ceramics (LTCC) have become an attractive technology for electronic components and substrates because it possesses several advantages, such as a low dielectric constant, low dielectric losses, high thermal conductivity, high mechanical strength, and the ability of integrating passive elements into the substrate. These advantages provide a promising way to realize high quality and compact microwave components.
In this dissertation, we describe a systematic design and analysis procedure towards the successful implementation of three-dimensional LTCC multilayer bandpass filters at microwave frequencies. The work has been divided into three major parts. In the first part, the phase evolution and dielectric properties of the commercial LTCC dielectric have been studied. As LTCC is a glass-ceramic composite, after firing the glass amount is reduced by crystallization. The amount and type of crystalline and amorphous phases in the fired LTCC substrate determine the final properties. Therefore, the phase evolution and dielectric properties at various firing temperatures of the commercial BaAl2Si2O8-based LTCC material, which was used for microwave filter design in the dissertation were studied. The results show that there are three crystalline phases, Al2O3, TiO2, and Zn2SiO4 existing in the as-received powder. As the firing temperature increased, Al2O3, TiO2, and Zn2SiO4 gradually vanished and a new crystalline phase BaAl2Si2O8 was formed from 850oC and its quantity increased with the firing temperatures. The degree of crystallinity of the as-received powder was around 53 wt% and gradually increased, reaching a maximum of ~96 wt% at 900oC. As the firing temperature increased, the dielectric constant of the fired specimens decreased, but the quality factor increased. The decrease in dielectric constant is attributed to the porous microstructure and the increase of the quality factor is attributed to the increasing degree of crystallinity at high temperatures. However, the porous microstructure deteriorates the mechanical strength of the fired specimens. So a trade-off between the dielectric properties and the mechanical strength should be made.
The second part describes the characteristics of (Zn,Mg)TiO3 dielectric, which has a dielectric constant of 27 and was used as the dielectric material for embedded capacitor in the microwave filter design to minimize the component size. To co-fire (Zn,Mg)TiO3 dielectric with LTCC material, 3ZnO-B2O3 glass and Bi2O3 were used as sintering aid to lower the sintering temperature of (Zn,Mg)TiO3 material, respectively. MLCC (Multilayer Ceramic Capacitor) was prepared with (Zn,Mg)TiO3 dielectric and electrodes in various silver-palladium ratios. The diffusion of silver into the dielectric will cause a serious problem in long-term reliability of MLCC, which uses Ag or Ag/Pd as the inner electrode. The electrochemical migration of silver can be significantly reduced in the presence of 15-40% palladium. However, the high ratio of palladium in silver-palladium alloy will cause high conductor loss, which makes the component inappropriate to be used in microwave frequencies. Low palladium content Ag/Pd alloys of Ag/Pd = 99/01, 97/03, 95/05, and 90/10 were used as the inner electrode in (Zn,Mg)TiO3 MLCC to understand if low palladium content (£ 10%) in Ag/Pd electrode is still able to stabilize the silver migration to avoid the degradation of insulation resistance in (Zn,Mg)TiO3 MLCC when sinter at 925oC for 2h. The results show that the lifetime of (Zn,Mg)TiO3 MLCC is inversely proportional to the silver content in Ag/Pd inner electrode. The diffusion of silver into (Zn,Mg)TiO3 dielectric during sintering and the migration of silver during reliability test at 140oC/200 V are both responsible for the short lifetime of (Zn,Mg)TiO3 MLCC. In terms of using Bi2O3 as the sintering aid, a Bi,Sb-rich secondary phase exists at the triple junction and also a trace of silver is detected in Bi,Sb-rich phase only when Ag/Pd = 99/01 alloy was used as the inner electrode in (Zn,Mg)TiO3 MLCC. The degradation of insulation resistance in (Zn,Mg)TiO3 MLCC is attributed to the excessive formation of oxygen vacancies from the substitution of Ag1+ to Bi3+ or Sb5+. The excessive oxygen vacancies and decreasing Ag-Pd bond strength in (Zn,Mg)TiO3 MLCC with high silver content Ag/Pd electrodes lower the activation energy of (Zn,Mg)TiO3 dielectric, as a result, the silver migration is enhanced.
In the last section, a 5.25 GHz LTCC bandpass filter and a 2.4 GHz semi-lump bandpass filter are implemented with BaAl2Si2O8-based LTCC substrate and the hybrid dielectrics of BaAl2Si2O8-based LTCC and (Zn,Mg)TiO3 dielectric. The designed compact combline LTCC filter possesses three attenuation poles, including two in the lower stopband and one in the higher stopband. The insertion loss at passband is less than 2 dB and the out-of-band rejection is very shape. Besides, the designed semi-lump filter was implemented by using LTCC technology with different dielectrics. In which, when the multilayer architecture of the 2.4 GHz semi-lump bandpass filter was implemented by hybrid dielectrics, that is to realize the striplines (inductors) on BaAl2Si2O8-based LTCC dielectric (k = 7.8) to enhance the quality factor of resonators and to realize the capacitors on relatively high-k (Zn,Mg)TiO3 dielectric to decrease the component size, a compact and superior filter can be made. The results show that the designed semi-lump filter demonstrates a compact size of 2.0 mm ´ 1.2 mm and superior characteristics of 2 dB insertion loss at 2.4 GHz and 42 dB and 35 dB attenuation at 1.9 GHz and 5 GHz, respectively, which is better than that of filters implemented by single dielectric, such as BaAl2Si2O8-based LTCC or (Zn,Mg)TiO3 dielectric.

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