A multi-disciplinary European team of scientists, jointly led by the University of Groningen (The Netherlands) and Philips Research, has successfully fabricated integrated circuits by immersing a pre-patterned substrate into a solution containing molecules of organic semiconductor material. This is the first time that the spontaneous assembly of organic molecules into dense monolayers on a substrate has successfully been demonstrated to create working integrated circuits.
The logic circuit produced by the team to demonstrate the technology takes the form of a 15-bit code generator in which hundreds of self-assembled monolayer transistors are simultaneously addressed. The team’s findings will be published in Nature on 16 October.
For more than half a century, scientists have been fascinated by the elegance with which nature is able to self-assemble molecules into complex structures. Biologists and chemists have managed to mimic these processes, particularly in the pharmaceutical and plastics industries, in order to produce synthetic macromolecules. In the field of electronics however, the advances have been slow. The fabrication of a single transistor with a self-assembled monolayer of active semiconductor material, let alone its integration into a working circuit, has always been seen as a daunting challenge. The European team has now demonstrated that self-assembly is feasible and can be used to make organic integrated circuits.
To do so, it employed a ‘bottom-up’ fabrication method. This involved designing liquid-crystal organic semiconductor molecules that have an ‘anchor group’ attached to them via a spacer at one end. When a suitably prepared substrate is immersed into a solution of these molecules, the anchor groups cause them to self-assemble into a monolayer on the surface of the substrate. Critical to obtaining the required semiconducting properties in this layer are the packing density and order of the self-assembled molecules – two requirements that the team has successfully been able to achieve.
To fabricate transistors, the substrate is pre-patterned with electrodes and treated so that the mono-layer forms over selected areas. The performance of the transistors produced is comparable to that of state-of-the-art organic transistors. In addition, the parameter spread between transistors is negligible, allowing them to be combined into integrated circuits comprising over 300 self-assembled transistors. Their functionality was demonstrated by producing a 15-bit code generator comprising a clock circuit and memory array that outputs a pre-programmed code.
The ultimate production technology
Self-assembly is considered the ultimate production technology for organic IC manufacture because it eliminates many of the process steps associated with conventional organic transistor fabrication. Application of the technology is foreseen in organic integrated circuits for contactless identification tags and drivers for flexible active-matrix displays. The team is now focusing on monolayer transistors for use in high-speed gas sensors, because the single-molecule thickness of their semiconductor layer makes them very sensitive to the presence of certain gases.
Nature publication: Bottom-up organic integrated circuits, Edsger C. P. Smits1,2,3, Simon G. J. Mathijssen2,4, Paul A. van Hal2, Sepas Setayesh2, Thomas C. T. Geuns2, Kees A. H. A. Mutsaers2, Eugenio Cantatore5, Harry J. Wondergem2, Oliver Werzer6, Roland Resel6, Martijn Kemerink4, Stephan Kirchmeyer8, Aziz M. Muzafarov7, Sergei A. Ponomarenko7, Bert de Boer1, Paul W. M. Blom1 & Dago M. de Leeuw1,2
1 Molecular Electronics, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands. 2 Philips Research Laboratories, High Tech Campus 4, 5656 AE Eindhoven, The Netherlands. 3 Dutch Polymer Institute, PO Box 902, 5600 AX Eindhoven, The Netherlands.4 Department of Applied Physics, 5 Mixed-Signal Microelectronics Group, Department of Electrical Engineering, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands. 6 Institute of Solid State Physics, Graz University of Technology, Petersgasse 16A, 8010 Graz, Austria. 7 Enikolopov Institute of Synthetic Polymer Materials, Russian Academy of Sciences, Profsoyuznaya 70, 117393 Moscow, Russia. 8 H. C. Starck GmbH, Chemiepark Leverkusen, Building B202, 51368 Leverkusen, Germany.
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