Fine-Line Structuring of Microwave Components on LTCC Substrates

Abstract

Low temperature co-fired ceramics (LTCC) are used in a wide range of RF and microwave applications. The ceramic multilayer technology provides a truly three-dimensional circuit technology, hermetical sealing, hybrid integration, and favorable microwave properties at moderate costs. In order to take full advantage of millimeter wave frequencies, resolutions of lines and spaces below the typical, screen printable 50 μm are desired. This paper describes the technological development of a fine-line structuring process based on resinate thin films and electroplating on LTCC. A combination of thin-film technology and thick-film technology leads to a novel resinate technology. This new process replaces the commonly used expensive sputtered layers, usually required for thin-film structuring combined with standard LTCC technology. The initial layer is replaced by a screen-printed metallo-organic paste (resinate paste). Resinate pastes consist of metallo-organic noble metal compounds (e.g. gold and silver), which are dissolved in organic suspensions. The film thickness after firing is typically below 1 μm. This layer can be used to define structures with high precision using photolithography and electroplating followed by an etching process. The etching processes investigated here showed promising resolutions of 25 μm for lines and spaces. But wet-chemical processes can be applied to sintered LTCC surfaces only. Buried fine structures may be achieved with tape-on-substrate (ToS) technology which allows for laminating and subsequent sintering of structured greensheets onto sintered ceramics. The benefits of this technology for microwave applications are obvious, given an improved resolution and parameter spread, and an accordingly reduced variation of the resulting structures.