Development of high-permittivity glasses for microwave LTCC tapes

Abstract

One of today's leading packaging concepts in microelectronics and microsystems is the technology of Low Temperature Cofiring Ceramics (LTCC). It is based on ceramic tapes that sinter at low temperatures (< 950 °C) and are capable of fine-line wiring by use of high-conductivity metals, preferably silver ( ϑm = 961 °C). To lower sintering temperatures of crystalline powders, glass powder is added. Depending on the amount of glass added, two mechanisms of sintering are possible: reactive or nonreactive liquid phase sintering. Reactive sintering requires small amounts (< 10 vol.%) of a very low softening glass (ϑsoft < 400°C). If the glass amount is increased to more than 50 vol.%, nonreactive sintering occurs in the glass softening range. According to the microstructure observed for each case, the respective types of materials are called Glass Bonded Ceramics (GBC) and Glass Ceramic Composites (GCC). Integration of microwave filters in LTCC multilayers requires inner dielectric layers with medium, temperature-stable permittivities (εr = 30 to 60) and low dielectric loss (tan δ < 2 ∙ 10^-3). Up to now, appropriate materials have been realized only by GBC sintering at 900 °C and above. Hence, pure silver metallization is not possible. To overcome this limitation, GCC are a promising way. Therefore, crystalline substances possessing εr of about 100 and low dielectric loss - for example Ba(La, Nd)2Ti4O12 - were combined with special high-permittivity glasses. A development of such glasses is presented in this work. Addition of oxides of easily polarizable elements (e. g. Nb2O5, La2O3 , TiO2 ) effects an increase in permittivity of selected basis compositions from the systems BaO-Al2O3-B2O3-SiO2 (εr up to 15) and La2O3-Ba2O3-TiO2 (εr up to 20). GCC mixtures made of 65 vol.% of these glasses and 35 vol.% BaLa2Ti4O12 showing microwave properties and sintering temperatures below 900 °C are introduced.