M. Mikolajek, J.R. Binder
Karlsruhe Institute of Technology (KIT),
Keywords: inkjet printing, dielectrics, ceramic, polymers, composites
Summary:Inkjet printing offers a high flexibility and therefore is increasingly gaining importance for the selective deposition of functional materials, for example in printed electronics. However, till today either particle based inks or inks based on organic molecules are used. Especially in the field of dielectrics, there is always a compromise between the permittivity and the processability of a material. Ferroelectric ceramic particles offer high values for κ, but also require high temperature sintering, which limits their field of application. In contrast, organic polymers are known for high mechanical flexibility and good processability but low permittivity. This gab is now closed by our approach to utilise a ceramic/polymer composite ink for a one step fabrication via inkjet printing. A printable barium strontium titanate (BST) ink for the fabrication of tunable microwave dielectrics was developed by our institute. Based on those experiences, especially the prevention of the coffee stain effect to achieve homogeneous printed films, we developed a composite ink system. Therefore, we use BST as the high-κ filler and poly(methyl methacrylate) (PMMA) as a matrix. Our developed ink allows process temperatures below 150 °C, an important requirement for printing on flexible substrates, like PET. We are able to fabricate homogeneous composite thick films consisting of up to 75 vol.% ceramics. This allows us to obtain values for κ up to 20 times higher than for pure PMMA (~3). Such composite inks connected with the flexibility of the inkjet printing process offer new possibilities to fabricate and integrate dielectric thick films with a much higher dielectric constant into printed devices. This presentation covers the preparation route for the developed BST/PMMA ink, starting with the fabrication of a highly stable BST dispersion. The requirements of the inkjet printing process will shortly be discussed in connection with the composite ink. Furthermore the drying behavior and the main influence factors will be displayed. Afterwards, the fabrication of fully inkjet printed capacitors is shown, as well as the characterization of their dielectric properties. In the end, it will be discussed how to optimize the quality of the printed devices and how to achieve even higher values for κ.