Repetitive ultramicrotome trimming and SEM imaging for characterizing printed multilayer structures

Abstract

Ultramicrotomy is a well-established technique that has been applied in biology and medical research to produce thin sections or a blockface of an embedded sample for microscopy. Recently, this technique has also been applied in materials science or micro- and nanotechnology as a sample preparation method for subsequent characterization. In this work, an application of ultramicrotomy for the cross-section preparation of an inkjet-printed multilayer structure is demonstrated. The investigated device is a capacitor consisting of three layers. The top and bottom electrodes are printed with silver nanoparticle ink and the dielectric layer with a ceramic nanoparticle/polymer ink. A 3D profilometer is initially used to study the surface morphology of the printed multilayer. The measurements show that both electrodes exhibit a coffee-ring effect, which results in an inhomogeneous layer structure of the device. To obtain precise 3D information on the multilayer, cross-sections must be prepared. Argon ion beam milling is the current gold standard to produce a single cross-section in good quality, however, the cross-section position within the multilayer volume is poorly defined. Moreover, the milling process requires a significant investment of time and resources. Herein, we develop an efficient method to realize repetitive cross-section preparation at well-defined positions in the multilayer volume. Repetitive cross-sections are exposed by trimming with an ultramicrotome (UM) and this blockface is subsequently transferred into a scanning electron microscope (SEM) for imaging. A combination of custom-modified UM and SEM specimen holders allows repeated transfer of the clamped multilayer sample between instruments without damage and with high positioning accuracy. This novel approach enhances the combination of an established ultramicrotome and a SEM for multilayer sample volume investigation. Thus, a comprehensive understanding of printed multilayer structures can be gained, to derive insights for optimization of device architecture and printing process.

Keywords: Inkjet printing; Multilayer; SEM imaging; Ultramicrotomy.

Fig. 1

The printed capacitors: (a) a 3D schematic of the printed multilayer structure with six contact pads, three conductive lines and a 3 x 3 mm square dielectric layer; (b) top view captured with an optical microscope of the printed multilayer structure; (c) enlarged regions of the two capacitors at the intersection of the top and bottom electrodes. Scale bars: (b) $500\mu \text{m}$, (c) $200\mu \text{m}$.

Fig. 2

Surface profile analysis of the printed structure: (a) the regions marked in red are investigated for surface quality. Scale bar: $500\mathrm{\mu }\text{m}$; (b) overview of the scanned sample surface region as marked with a square in (a); (c) the surface roughness determined by white light interferometry of the marked region on the dielectric layer is $0.12\mathrm{\mu }\text{m}$; (d) surface topography of a section in the bottom electrode; (e) all available profiles across (as indicated by the grey bar) the bottom electrode are integrated in a diagram using the MountainsLab software; (f) the average profile across the bottom electrode is calculated by MountainsLab. The regions marked in orange are used to determine the electrode thickness. The red horizontal line across the upper orange region represents the average height of the marked region on the electrode surface. The distance between this red line and the substrate surface (the average height of the two lower orange regions) represents the average thickness of the bottom electrode. The thickness measured is $0.26\mathrm{\mu }\text{m}$. In (e, f) different scaling of the y axis.

Fig. 3

Utilizing an ultramicrotome to produce cross-sections of the printed multilayer structure for subsequent SEM imaging: (a) cross-sections prepared by Ar-ion beam milling and ultramicrotome trimming viewed in an SEM, scale bar: $1\mathrm{\mu }\text{m}$; (b) a non-magnetic flat sample holder for UM (left), the tip of the clamping jaws is trimmed (right) to create a flat opening, and the central borehole of the SEM specimen holder is enlarged to 9.85 mm (center) to mount the UM flat sample holder, diameter of SEM specimen holder, 46 mm; (c) the sample is fixed in the UM holder for three times repetitive trimming and SEM imaging, scale bar in top view: 1 mm, in images of sample trimming: $250\mathrm{\mu }\text{m}$.

Fig. 4

Illustration of the repetitive UM trimming and SEM imaging process with correlation to the 3D profilometer data: (a) an overview of the scanned surface of the printed dielectric layer and the top electrode; (b) a 3D illustration derived from profilometer measurement data. The yellow dashed lines represent the approximate trimming positions; (c) illustration of the approximate trimming positions in the top view; (d) SEM images are taken in the center of the obtained cross-sections after each trimming; (e) six images are taken along the length of a capacitor from the cross-section of the second trimming. Scale bar: (c) $200\mathrm{\mu }\text{m}$, (d, e) 1 $\mathrm{\mu }\text{m}$.