Ranfeld, Constanze (2015)
Wet etching of printed silver layers using an etch resist structured by flexography.
Technische Universität Darmstadt
Dissertation, Erstveröffentlichung

Kurzbeschreibung (Abstract)

Flexographic printing is a common technique in the production of printed electronics. Its flexible printing plates offer many advantages, especially regarding printing on rigid and fragile substrates. Nevertheless, one major drawback is the so-called halo effect where ink is squeezed at the edges of the raised elements of the printing plate during transfer onto the substrate. The result is a halo-like structure surrounding the desired layout with a zone depleted of ink in-between. The approach of this study is to utilize the halo effect in flexographic printing for the structuring of a resist layer in wet etching processes of silver layers with the goal of integrating this process into printed electronics production. For this, three different aspects need to be considered. The first aspect is the influence of post-processing on printed silver layers. I investigated the effect of several parameters of photonic curing on the etching rate and on the sheet resistance of printed silver nanoparticle layers. For the latter, I utilized design of experiments (DoE). The second aspect is the impact of printing parameters on the behavior of printed lines regarding the halo effect in flexography. I investigated the halo effect of printed line structures on an IGT F1 printability tester, also utilizing DoE. With this method, I structured a resist layer of poly(methyl methacrylate) (PMMA) for wet etching processes. The third aspect is the transferral of the layout of the printed etch resist to the silver layer using wet etching. For these trials I used nitric acid as etchant. The results of these investigations are the following: The printed silver layers were homogenous with a roughness of few ten nanometers. As expected, photonic curing does increase the conductivity. The extend of the increase depends on the energy density the sample is exposed to. I found a method to evaluate the isotropy of the electrical properties of printed silver layers using the correction factor in van der Pauw measurements. When printing line shaped elements the halo effect needs to be considered. Nearly all printed samples, regardless the printing parameters, show a more or less pronounced halo effect. Here, the most influential parameter is the pick-up volume: A high pick-up volume results in wide halos around a printed line with a distinct void in-between. The second largest impact has the printing force, whereas the influence of the printing speed is almost negligible. It was not possible to determine a relation between degree of post-processing, i.e. energy density during photonic curing, and etching rate. The measurements were inconclusive. Therefore, I used a different dilution of the etchant to transfer the layout of the printed resist structures into the silver layer. Using the halo effect in flexographic printing for structuring the resist layer, we can obtain line widths smaller than those attainable by directly using flexographic printing. To achieve this, anilox rollers with low pick-up volume are needed. Nevertheless, those lines are hardly reproducible. Their line quality needs further improvement regarding edge sharpness, bulging and interruptions.

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