High-performance structures with photo-imageable pastes for mmWave applications

The entry into the age of 'big data' is driving new applications in the areas of mobile communication (5G, 6G), (autonomous) mobility and the Internet of Things (smart cities, wearables, object tracking, smart grids, video security). These require ever higher transmission bandwidths with low latencies, which results in ever higher operating frequencies, in some cases >100 GHz. This increasingly higher operating frequencies require signal transmission lines with increased structure resolution and, due to increasing transmission losses, high geometric quality, and production reproducibility.
Due to particularly favorable dielectric properties, reliability and suitable technological features (e.g., functional 3D integration), the ceramic LTCC technology (Low Temperature Co-fired Ceramics) is already frequently used for the manufacturing of reliable and high-performance high-frequency assemblies. The screen-printing technology previously used in LTCC technology for the deposition and structuring of functional thick films can no longer meet these requirements for high-frequency components. For example, structural resolutions in industrial applications reached 75 µm (line/space) with limited geometry quality. So, high-performance technologies are needed to produce geometrically high-resolution structures for next generation of high-frequency circuits. For this the Fraunhofer IKTS has developed since 2019 thick-film pastes that are photo-imageable (PI) using UV light. This technology allows structural geometries less than 20 µm line-space with a very high accuracy of the edge sharpness. The PI technology is based on a UV-curable binder system into which the functional paste components (metals, alloys, oxides, glasses, additives) are mixed. The pastes produced in this way are printed onto substrates using screen printing, whereby a major advantage is that no yellow room is required. The required functional elements are then created by UV exposure, i.e. selective curing of the photosensitive binder. Exposure can be carried out using photo masks or direct writing using a UV laser. This new feature by laser contributes to the digitalization of this process and to a further improvement in structural accuracy. Areas that have not been exposed are then removed by spray development. Immediately after the development process, the samples are sintered in a standard thick film regime to produce the final functional layer properties. The two additional steps needs only between 5 s and 20 s each and they can be easily integrated into the production process line. The main benefit is next to the small line resolutions, that no yellow room is necessary with the current developed pastes.
The essential innovative approach of the photo-imageable pastes lies in the possibility of generating high structure resolutions with the necessary surface quality for the cosintering process in LTCC and thus transferring the LTCC technology to mass production for operating frequencies above 100 GHz. The advantages of thin and thick film technology will be combined using existing material systems and processes already established in the industry; the pastes and processes will be adapted to commercially available LTCC or other substrate types, which will increase market acceptance.
The development focus for current PI pastes was on metallizations, such as silver and gold as conductor and contacting systems for sintering temperatures up to 1000 °C, platinum as a heater or conductive paste for sintering temperatures up to 1500 °C, and copper to reduce material costs. Furthermore, PI resistor and dielectric pastes were developed to meet the demand for further functional pastes for the PI process for the making of high-frequency circuits.
The present work is intended to give an overview of the current developments in PI pastes at the IKTS and should give a comparison of the PI technologies. On the one hand side the masked based PI process, which is suitable for mass production and on the other hand side, the laser direct imaging (LDI) process, which offers the possibility of prototype manufacturing.