Fineline structuring on LTCC-substrates for 60 GHz line coupled filters

Low Temperature Co-fired Ceramics (LTCC) are used in a wide range of applications. They provide true three- dimensional integration, hermetical sealing, hybrid integration and good microwave properties at moderate costs. In order to take advantage of millimetre wave frequencies a resolution of lines and spaces of less than the screen printable 50 μm is desired. For frequencies in the 60 GHz range integrated coupled line filters require coupling gaps as small as 30 μm. However, the screen printing related accuracy results in dimensional tolerances of line widths or spaces of up to 20%. This paper describes the technological development of a fineline structuring process based on resinate thin films using electroplating on LTCC. The initial layer is applied by screen printing of a metal-organic (resinate) paste, a noble metal compound (e.g. gold and silver) dissolved in organic oils. The film thickness after firing is typically less than 1 μm. This layer can be used to define the structures using photolithography and electroplating followed by an etching process. The etching processes show a promising resolution of 20 μm lines and spaces with tolerances defined by the tolerances of the photoresist (typically below ± 1 μm) and the substrate flatness. Benefits for the design of microwave and millimetre wave applications are obvious due to the higher resolution and reduced variation of the final structures. A 60 GHz line coupled band-pass filter is used to demostrate process stability and electrical performance of this new structuring method.