A novel printing technique for highly integrated organic devices
Introduction
Organic materials have been extremely successful in the field of light emitting devices, organic solar cells, organic field-effect transistors and molecular memories. The strength of organic materials is based on the diversity of possible materials, i.e. depending on the application the material can be optimized by organic synthesis. Furthermore, considering that organic layers can be deposited from the liquid phase by spin-coating or printing, organic electronics is potentially low-cost.
However, the main drawback of organic materials is the lack of an appropriate technology for high-density integration. Most organic layers are incompatible with conventional lithography, since solvents used in different process steps of lithography damage or at least degrade the molecular layer. Recently, two methods to minimize the exposure of the organic layers to solvents were proposed [1], [2]. Both methods rely on protective layers on top of the organic stack (e.g. Ag [1] or parylene [2]) that encapsulate the organic stack. All lithographic steps such as spin-coating, exposure and development of the resist are performed on top of this encapsulation layer and only during the last step the resist pattern is transferred into the molecular layer by dry etching.
In addition to these two approaches, Katsia et al. reported on an alternative method to structure molecular layers by UV-lithography [3]. Via-holes are prepared in a passive layer and filled with semiconducting polymers. These via-holes are reported to protect the polymeric layers from damage during the lithography process.
However, all these methods do not fully protect the molecular layer from either reactive gases during pattern transfer [1], [2], solvents during lift-off [3], damages during mechanically peeling off the parylene layer [2] or elevated processing temperatures. To avoid any damage to the molecular stack, a purely additive method such as a printing technique has to be employed.
The technology of printing and its applications have received considerable interest during the last years [4], [5]. Several printing techniques have been developed that are used e.g. for lithography (nano-imprint lithography [6], [7]), for structuring molecular layers (micro-contact printing [8], [9], [10], [11], [12], [13]) and for printing of low-cost electronic devices (inkjet printing and screen printing) [14], [15], [16], [17].
A special class of printing methods has been developed to transfer structured metallic electrodes onto a predefined substrate, e.g. an organic layer. One of these methods, nanotransfer printing (nTP) [18], [19], has been used to define electrodes on thin organic films to form an organic transistor [18], to print electrodes onto molecular monolayers [20] or to form three-dimensional structures [21].
The nTP process utilises a stamp with raised features on top for pattern definition. This stamp is coated with a metal layer and pressed against the target substrate, so that the metal film is transferred onto the substrate at the raised parts of the stamp only. nTP relies on adhesion of the printed metal to the second surface. This adhesion can be increased if the top electrode is printed onto surfaces that possess a terminal group, which binds strongly to the metal.
Besides nTP a printing technique called polymer assisted lift-off (PALO) printing has been developed recently [22]. As for its predecessor, the lift-off/float-on technique [23], a metallic top electrode is deposited and patterned on a solid substrate. The electrodes are then covered by a thin layer of poly(methylmethacrylate) (PMMA) and are subsequently released from the substrate by immersing them successively into potassium hydroxide solution, acetic acid solution and water. Finally, the resulting PMMA foil with the electrode structures underneath remains floating on the water surface. A target substrate can then be immersed into the water and the top electrode can be floated onto this substrate. However, this stamping or printing step has to take place in water and is therefore not applicable for most organic devices.
In this article, we report on a new printing technique that is especially optimized for organic electronics. It does not heavily rely on interfacial reactions such as the nTP method and can be performed without the exposure of the molecular layer to solvents. Damages to the molecular layers, either due to baking at high temperatures associated with lithography or due to the deposition of the top electrodes by conventional techniques such as thermal evaporation or sputtering [24], [25], [26], are avoided. Furthermore, by combining our printing method with nano-imprint lithography small feature sizes (below 100 nm) and thus high integration densities are possible.
Section snippets
Experimental details
The stamps used in this study are prepared on sacrificial substrates such as silicon wafers coated with native SiO2. A water-soluble layer of poly(acrylic acid) (PAA) (Polysciences, MW 9000) is deposited onto a native oxide silicon wafer by spin-coating (at 4000 rpm). To remove remaining water in the PAA film, the sample is baked at 150 °C for 2 min. For optical lithography the positive tone resist AZ5214 (MicroChemicals GmbH, Germany) is employed. After development of the optical resist (35 s in
Process flow
The process flow of the stamping process is shown in Fig. 1. The process is divided in several steps: preparation of the stamp on a sacrificial silicon substrate, removal of the stamp from the substrate and stamping onto the molecular layer.
To be able to release the stamp from the substrate later on, a water-soluble sacrificial layer is deposited onto the silicon substrate in the first step of the process flow (Fig. 1a). Possible candidates of water-soluble layers are inorganic salts [27], CaO
Conclusion
The printing technique presented here is capable of printing very small feature sizes onto a wide variety of substrates. It allows depositing metallic electrodes onto substrates that are otherwise not compatible with lithography or conventional physical vapour deposition methods such as organic layers.
The printing technique is completely additive, i.e. the whole top-electrode structure is prepared on a sacrificial substrate. The complete stamp is printed onto the molecular layer only during the
Acknowledgements
We would like to thank William Ford and Florian von Wrochem for their helpful comments and ideas and Boris Vratzov (Nanotechnology and Devices) for preparing the nano-imprint samples.
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