Development of novel temperature-stable Al₂O₃–TiO₂-based dielectric ceramics featuring superior thermal conductivity for LTCC applications

Abstract

A new composition was developed using sintering to improve the dielectric properties of low-temperature co-fired alumina (LTCA) containing CuO–TiO₂–Nb₂O₅–Ag₂O. By substituting some alumina with rutile TiO₂, the second-phase compound could be changed from AgNbO₃ to the rutile phase. Further, low-temperature sintering at temperatures below 960 °C was possible, suppressing Al₂TiO₅ formation during firing. The dielectric characteristics, particularly the temperature coefficient of the resonant frequency (τ_f) and Q × f values, were improved without significantly reducing the sinterability and thermal conductivity. The dielectric properties of the developed 88Al₂O₃–12TiO₂-based ceramic were ε_r: 14.7, τ_f: +0.8 ppm/K, and Q × f: 13,383 GHz (at ∼10 GHz) at a firing temperature of 940 °C. The thermal conductivity was 18.5 W/mK, which is the highest value for reported temperature-stable low-temperature co-fired ceramics (LTCCs). These results provide one of the key technologies for the practical application of LTCCs with superior thermal conductivities.

Introduction

Alumina (Al₂O₃) exhibits high thermal conductivity, high mechanical strength, and good electrical characteristics; therefore, it is widely used for fabricating electronic components such as wiring substrates and integrated circuits [[1], [2], [3], [4], [5]]. However, because the sintering temperature of alumina is relatively high (approximately 1500 °C), co-fired conductor materials (such as W or Mo) must possess higher melting points to ensure high electrical resistances. Ceramics that can be co-fired with low-resistance conductor materials such as Ag (melting point, 961 °C) and Cu (melting point, 1084 °C) are called low-temperature co-fired ceramics (LTCCs) [[6], [7], [8], [9]]. As LTCCs enable the fabrication of built-in capacitors, inductors, and low-loss transmission lines, they are widely used for producing high-frequency modules. However, conventional LTCCs generally contain large amounts of glass (approximately 50 wt%) to ensure low sintering temperatures. Thus, most of these materials exhibit low thermal conductivities (2–7 W/mK) and poor mechanical strengths (150–250 MPa).

As a first step toward the production of LTCCs with high thermal conductivities and physical strengths comparable to those of alumina, many sintering additives that enable low-temperature sintering at atmospheric pressure when admixed in small quantities have been developed [[10], [11], [12], [13], [14]]. Particularly, the sintering of additives based on Cu₂O–TiO₂ [10], CuO–TiO₂ [[11], [12], [13]], and MnO–TiO₂ [10,14] have been studied. However, none of these binary composition additives exhibited satisfactory sintering performances at temperatures below 1000 °C. Nevertheless, the use of sintering temperatures in the range of 925–1000 °C combined with the addition of 5–10 wt% sintering additives [CuO–Nb₂O₅ or CuO–TiO₂–Nb₂O₅ (ternary type)] to Al₂O₃ creates dense alumina [[15], [16], [17]]. These studies have also shown that the best sintering performance is obtained when the ternary component CuO–TiO₂–NbO_2.5, with a 4:1:4 M ratio, is used [15]. Furthermore, when using 5 wt% quaternary sintering additives, the sintering temperature can be decreased to less than 900 °C. Shigeno et al. added Ag₂O to the ternary sintering additive CuO–TiO₂–Nb₂O₅ [18]. The resultant sintered materials exhibited a thermal conductivity (κ) of 18 W/mK, which is much higher than those of conventional LTCCs. Moreover, increasing the Ag₂O content in the sintering additive makes it possible to co-fire these materials with Ag-electrodes at temperatures less than 900 °C [19]. This suggests that given their high thermal conductivities, the afforded materials can be used as LTCC modules. These novel sintered compounds are called low-temperature co-fired alumina (LTCA) materials.

One of the properties required for microwave dielectric materials is a temperature coefficient of resonant frequency (τ_f) that approaches zero. However, owing to the large negative τ_f value of alumina (−60 ppm/K), it is presumed that LTCA materials have large τ_f values; hence, these materials must be improved. The objective of this research was to bring τ_f close to zero while simultaneously securing the sinterability and superior thermal conductivity of the LTCA material. We focused on rutile-phase titania (TiO₂), which is effective for the low-temperature sintering of alumina and has an extremely large positive τ_f value of +450 ppm/K. The addition of TiO₂ to an LTCC material containing glass as the main component, to attain a τ_f value that approaches zero, has been reported [20]. Several studies in which TiO₂ was added to alumina have also been reported [[21], [22], [23]]; however, high-temperature sintering at approximately 1400 ℃ was necessary. The TiO₂–Al₂O₃ phase diagram [21] (Fig. 1) reveals that TiO₂ reacts with Al₂O₃ at temperatures ≥1150 ℃ as follows:

$${Al}_2{O}_3+Ti{O}_2\to {Al}_{2}Ti{O}_5$$

The afforded Al₂TiO₅ has a τ_f value of +79 ppm/K [24], which impedes τ_f from approaching zero. This has led to studies on post-heat treatment [21] and the search for suitable additives such as MnO [22] and MnCO₃ [23]. However, when using low-temperature sintered alumina with the additives developed by the authors [19], it is unlikely that this problem will arise owing to the necessary low firing temperature. Further, as TiO₂ is one of the low-temperature sintering additive components of alumina that we developed, we predicted that its addition would not significantly deteriorate the sinterability of the material. Therefore, in this study, we investigated the influence of substituting some of the alumina with TiO₂, on the sinterability and the thermal and dielectric properties of alumina comprising 4 wt% CuO–TiO₂–Nb₂O₅–Ag₂O additive, for LTCC applications.