A novel Low Temperature Co-firing Ceramic (LTCC) material for telecommunication devices

Heli Jantunen

Department of Electrical Engineering and Infotech Oulu, University of Oulu

Abstract

The thesis describes the development of a novel LTCC material system for RF and microwave telecommunication purposes.

The work has been divided into three parts. In the first section, the compositional and firing properties of this novel LTCC dielectric have been studied as well as its thermomechanical and dielectric properties. The second section describes the multilayer component preparation procedure for the ceramic material including tape casting and lamination parameters and the selection of the conductor paste. In the last section, the novel LTCC material system has been used to demonstrate its properties in RF multilayer resonators and a bandpass filter.

The dielectric material for the novel LTCC system was prepared using magnesium calcium titanate ceramic, the firing temperature of which was decreased to 900°C by the addition of a mixture of zinc oxide, silicon oxide and boron oxide. The powder was made without any prior glass preparation, which is an important process advantage of this composition. The fired microstructure was totally crystalline with high density (3.7 Mg m-3) and low porosity (0.5 %). The mechanical properties were virtually identical to the values of the commercial LTCCs, but the higher thermal expansivity makes it most compatible with alumina substrates. The dielectric values were also good. The permittivity was 8.5 and the dissipation factor (0.9·10-3 at 8 GHz) less than that of the commercial LTCCs. Furthermore, the temperature coefficient of the resonance frequency was demonstrated to be adjustable between the range of +8.8 ... -62 ppm/K with a simple compositional variation of titanium oxide.

The slurry for the tape casting was prepared using poly(vinyl butyral) -base organic additives and the 110 μm thick tapes had a smooth surface (RA < 0.5 μm). The multilayer components were prepared using 20 MPa lamination pressure, 90°C temperature and 1 h dwell time. The most suitable conductor paste for this composition was found to be commercial silver paste (duPont 6160), which produced satisfactory inner and outer conductor patterns for multilayer components. Finally, resonators with a resonant frequency range of 1.7 ... 3.7 GHz were prepared together with a bandpass filter suitable for the next generation of telecommunication devices. This demonstration showed the potential of the developed novel LTCC material system at high RF frequencies.

Dedication

Aikaa myöten
ajan tajun aistimilla,
alkaa ympäristö yllättää,
aivot ymmärtää
erilaista elomuotoa
ekologista elinkaarta,
tulee tutuksi tulevaisuus
ja haluaa hallita
hiukkasen

Omistettu hyvälle ystävälleni Helille väitöspäivänä, kiitoksena lukuisista ja monimuotoisista keskusteluista matkan varrella

Kaisa Kerätär

Table of Contents
Acknowledgements
List of abbreviations and symbols
List of original papers
1. Introduction
1.1. The LTCC material system for RF and microwave components
1.2. Objective and outline of this thesis
2. Development of a LTCC dielectric material
2.1. Basis of the dielectric materials for the LTCC process
2.2. Test sample preparation and measurements
2.3. Investigated compositions and the results
3. Material development for the multilayer process
3.1. Demands of the LTCC material systems
3.2. Determination of the process parameters
4. Properties of the multilayer RF components
5. Conclusions
References
List of Tables
1. Firing profile for duPont’s 943 LTCC material system.
2. Dielectric properties of commercial LTCC dielectrics.
3. Thermomechanical properties of commercial LTCCs, glasses, glass-ceramics and pure alumina.
4. Firing profile for the novel LTCC dielectric.
5. Measured dielectric and thermomechanical properties of the developed microwave LTCC ceramic. [Papers II, III]
6. Composition of the slurry for tape casting. [Paper VI]
7. Tape properties and process parameters for the novel LTCC and duPont’s 943 material systems.
List of Figures
1-1. The stages of development used for a novel LTCC material system.
2-1. Typical firing profile for LTCC components.
2-2. Two different preparation routes for the novel LTCC dielectric. [Paper II]
2-3. Measurement system of dielectric properties with Hakki-Coleman configuration. [Dube et al. 1997]
2-4. Measurement systems for coaxial resonators. [Kemppinen & Leppävuori 1986]
2-5. Shrinkage behaviour of the ZSB(g)/MMT-20 and ZSB(r)/MMT-20 compositions. [Paper I, II]
2-6. FESEM/BE images of sintered (900°C, 80 min) a) ZSB(g)/MMT-20 and b) ZSB(r)/MMT-20 mixtures. [Paper II]
2-7. XRD patterns of the ZSB(g)/MMT-20 and ZSB(r)/MMT-20 mixtures after firing at 900°C for 80 min. [Paper II]
3-1. General production process of cast ceramic tapes and tape-based multilayer components. [Cahn et al. 1996, chap. 7]
3-2. SEM/BE image of the tape cast and sintered dielectric material. [Paper VI]
3-3. Typical cross-section view of the inner conductive line between dielectric layers.
4-1. Resonators prepared for the Q-factor measurements. a) straight and b) U-shaped λ /2 resonators and c) straight λ /4 resonators. [Papers V, VI]
4-2. Simulated (solid line) Q-factors and measured Q-factors as a function of resonance frequency for straight-line shape λ /2 resonator (▴) and U-shape λ /2 resonator (l) resonators made of the novel LTCC material system and for a resonator made of 943 material (○). [Papers V, VI]
4-3. The equivalent circuit diagram of bandpass filter using the novel LTCC material ­system. [Paper VI]
4-4. 3-dimensional structures of the 2-pole bandpass filter made of the novel LTCC material system. [Paper VI]
4-5. The simulated (a) and measured (b) frequency response of the prepared filter (… S11, — S21) indicating desired bandwidths and attenuation (—). [Paper VI]
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