Fine-line structuring of microwave components on LTCC substrates

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 advantage of high frequencies with guided wavelengths of the order of millimeters, a precision of lines and spaces better than the resolution of 50 μm available with standard patterning methods is crucial. Examples of relevant LTCC-modules are filters, couplers, and resonators, e.g., for the Ka-band frequency range (1722 GHz satellite downlink). Our recent work in the project KERAMIS (ceramic microwave circuits for satellite applications) aims for the verification of the LTCC-modules in space. The manufacturing of these modules is based on screen printing and, therefore, the structural precision is limited to approximately 10 μm. Microwave designs with lines and spaces of 50 μm hence result in geometrical differences of up to 20%. This is the major drawback of using narrow linewidths. To overcome this drawback, several techniques can be applied. One can use special trampoline screens or special mesh coatings. Another method to achieve higher resolutions with good repeatability is the use of imageable pastes such as Fodel®. Furthermore, etching of thick film conductors and thin films on LTCC can be used. This paper describes yet another promising approach, namely the investigation of a fine-line structuring process in combination with thin-film and thick-film technologies. The new process replaces the commonly used expensive sputtered layers required for thin-film structuring by standard LTCC-technology. The initial layer is replaced by a screen printed metallo-organic (resinate) paste, a noble metal compound (e.g. gold or silver) that is 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 20 μm for lines and spaces. The benefits for the design of microwave device applications are obvious given an improved resolution and parameter spread and an accordingly reduced variation of the resulting structures. Microwave filters are in preparation to demonstrate the advantages of this new structuring method on a quantitative level.