Fabrication of Flexible Multilayer Composite Capacitors Using Inkjet Printing

Abstract

This paper shows a straightforward method for printing multilayer composite capacitors with three dielectric layers on flexible substrates. As known from multilayer ceramic chip capacitors (MLCCs), it is possible to create a parallel connection of the layers without enlarging the needed area. Hence, the overall capacitance is increased, as the capacitances of the single dielectric layers add up. To realize printed capacitors, a special ceramic/polymer composite ink is used. The ink consists of surface-modified Ba_0.6Sr_0.4TiO₃ (BST), a polymeric crosslinking agent and a thermal initiator, which allows an immediate polymerization of the ink, leading to very homogenous layers. The dielectric behavior of the capacitors is examined for each completed dielectric layer (via impedance spectroscopy) so that the changes with every following layer can be analyzed. It is demonstrated that the concept works, and capacitors with up to 3420 pF were realized (permittivity of ~40). However, it was also shown that the biggest challenge is the printing of the needed silver electrodes. They show a strong coffee stain effect, leading to thicker edge areas, which are difficult to overprint. Only with the help of printed supporting structures was it possible to lower the failure rate when printing thin dielectric layers.

Keywords: ceramic/polymer composites; inkjet printing; multilayer; printed capacitors.

Figure 1

Microscope image (left) and schematic diagram (right) of a multilayer capacitor with three dielectric layers (yellow) and four Ag-electrodes (green). (1) First electrode; (2) First dielectric layer; (3a) Second electrode; (3b) Supporting structure; (4) Second dielectric layer; (5a) Third electrode; (5b) Supporting structure; (6) Third dielectric layer; (7) Fourth electrode.

Figure 2

(a) Schematic representation of a printed composite layer on a silver electrode, which has a thick edge area. The dashed area of the composite layer represents the thinnest part, which is susceptible to cracking; (b) 3D-topography of a printed silver electrode on PET, with the intended thick edge area.

Figure 3

3D-topography of a printed multilayer capacitor, with three parallel-connected dielectric layers (each printed with a drop spacing of p = 90 µm), printed as shown in Figure 1.

Figure 4

(a) Increasing capacitances of the multilayer capacitors with every additional layer, printed with a drop spacing of each 70 µm; (b) SEM image of the corresponding three-layered capacitor, with the same labeling as in Figure 1. (1) First electrode; (2) First dielectric layer; (3a) Second electrode; (3b) Supporting structure; (4) Second dielectric layer; (5a) Third electrode; (5b) Supporting structure; (6) Third dielectric layer; (7) Fourth electrode.

Figure 5

(a) Microscope image in which the dashed line shows the cut made to prepare the SEM images. The rectangle represents the area shown in the following cross-sectional SEM images, and the arrow shows the viewing direction; (b–d) SEM images of the area shown in (a) with the same labeling as in Figure 1. (1) First electrode; (2) First dielectric layer; (3a) Second electrode; (3b) Supporting structure; (4) Second dielectric layer; (5a) Third electrode; (5b) Supporting structure; (6) Third dielectric layer; (7) Fourth electrode.

Figure 6

(a) Increasing capacitances of the multilayer capacitors with every additional layer, printed with drop spacings of 70-80-80 µm; (b) SEM image of the corresponding three-layered capacitor, with the same labeling as in Figure 1. (1) First electrode; (2) First dielectric layer; (3a) Second electrode; (3b) Supporting structure; (4) Second dielectric layer; (5a) Third electrode; (5b) Supporting structure; (6) Third dielectric layer; (7) Fourth electrode.