LTCC glass-ceramic composites for microwave application

Abstract

Advanced materials of rare earth derived glass–and reactive bonded glass–ceramic composites are exceptionally interesting for IC packaging, radar, antennas and wireless technologies for the next generation of miniature electronic devices. Glass–ceramic composites in the system 1:1:4 BaO–Nd₂O₃–TiO₂ and modified rare earth glasses based on boron oxide for passive integration in LTCC demonstrate excellent dielectric properties in the middle permittivity ε range of 20–70 with high quality factor Q and low temperature coefficient TCf at microwave frequencies. Depending on the glass–ceramic system, concentration, significant processing parameters e.g. powder preparation techniques and sintering of dense composite at temperatures <900°C were achieved. Dielectric properties were studied by a cavity resonator method at frequencies 1–6 GHz in correlation of the crystalline microstructure. This work was supported by tridimensional modeling systems to estimate dielectric behavior of multiphase glass–ceramic composites.

Introduction

Low-temperature-cofired ceramics (LTCC) for microwave application represent a key position in the development of future electronic products in a high frequency application for IC packaging radar, antennas and wireless technologies. The integration of passive components in LTCC is, therefore, particularly interesting in multi-layers technology. Integration of passive devices in wireless application corresponds to the trend of mobilization and miniaturization with high electrical performance using conductive electrode materials such as gold, silver and copper. The melting temperature of silver electrodes in a multi-layer device limits the sintering temperature to 900°C. Material systems of BaO–Re₂O₃–TiO₂ (Re=La, Sm, Nd, Eu) in 1:1:4 composite are especially suited for the development of dielectric materials1, 2 which are characterized by excellent dielectric constant ε, high quality factor Q as inverse of dielectric loss Q=(tan δ)⁻¹ and low temperature coefficient of frequency TCf characteristics.

Exceptional dielectric characteristics of 1:1:4 composite are achieved at sintering temperature >1300°C and, therefore, contrary compared to the demanded temperatures of 900°C in the case of LTCC technology with Ag-electrodes. Development trends of LTCC materials are glass–ceramics systems, including low softening glasses and high sintering microwave ceramic. Typical glass systems are borosilicate, lead borosilicate or earth alkali CaO–B₂O₃ SiO₂ glass which crystallize at optimized processing parameters.³ The requirement catalogue for glass–ceramic composites is: densification temperature <900°C, combined with dielectric characteristics ε 20…70, Q 1000 and TCf approximately 0 ppm/°C of composite. Promising candidates of middle ε 20…30 composite are glass–ceramic by restricting on lead-free glasses especially rare earth derived microwave glasses La–B–Ti–O (LBT).⁴ An alternative route for high ε materials 60–80 are low viscosity reactive glasses in a B–Bi–Zn–O (BBSZ) system. BaNd₂Ti₄O₁₂ microwave ceramic filler was added with earth alkali ZnO less 1 wt.% to optimized sintering and dielectric properties in adjustment of glasses.