Glass and Glass/LTCC Interposers as Heterogenous Integration Platform

Glass as substrate or interposer material is gaining more and more interest due to its thin film compatibility and thus high-density integration capability. Multilayer stacks for processor packaging applications can be achieved by laminating Ajinomoto build-up film (ABF) on glass substrates. Some glasses exhibit very frequency stable dielectric properties and low losses which makes glass substrates also favorable for microwave applications. Moreover, in contrast to silicon as interposer material, glass materials are cheaper, they can be processed in large panel format, they do not require deposition of insulating dielectric layers and they are available with different temperature coefficients of expansion (TCE). The latter was the pre-requisite to combine glasses with TCE-matching Low Temperature Cofired Ceramic (LTCC) materials to achieve an inorganic compound substrate offering both multilayer and thin-film capability in one substrate. In addition, the glass side of the interposer might be used for integrating photonic elements and components for mixed electronic/photonic applications. Green LTCC tapes are processed in the standard sequence up to the lamination step. In parallel, holes for vias are laser drilled in the glass and filled with thick-film conductor paste. The LTCC stack is then laminated onto the glass substrate and the laminate is fired according to sintering conditions of the LTCC. During firing, the sintered LTCC establishes a strong bond to the glass without the need of an extra bonding layer. This multilayer Tape-on-Substrate approach results in zero lateral shrinkage of the LTCC and therefore high lateral dimensional accuracy. After cleaning, the glass side can be used for semi-additive or additive conductor structuring. Thick-film printing on glass as well as printing of metal-organic pastes as basis for electroplating are demonstrated. This paper summarizes the achieved results on process development and interposer substrate evaluation comprising assembly technologies and high-frequency properties. Key words: Glass-Interposer, LTCC-substrate, Thick- and thin film deposition, photonic components