Silicon, the conventional semiconductor used to build up electronics, is processed at very high temperature – over one thousand Celsius degrees - and it is difficult to recycle. With the aim of facilitating the sustainable and competitive manufacture of new displays with low-cost technology, scientists of the European project MULTIFLEXIOXIDES have developed new ceramic thin films containing amorphous or nano-structured oxides processed at low temperature to be used as transparent materials, conducting, semiconducting or insulating components in rigid and flexible electronic devices.
“The challenge is that we use zinc oxide, which is a good-behaviour ceramic material, with gallium and indium oxides and mix them within a ceramic thin film to make new materials with electronic performances as good as silicon, something that was never tried before” says Rodrigo Martins, director of CEMOP (Center of Excellence in Microelectronics Optoelectronics and Processes) at the Portuguese University-Enterprise association UNINOVA (Institute for the Development of New Technologies), president of the European Materials Research Society and coordinator of the MULTIFLEXIOXIDES project.
“These oxides have such excellent performances because they are made at nanoscale. When the zinc oxide is thick you cannot use it for electronics, but when you make it very thin at nanoscale level the properties change dramatically and you can obtain a material so plastic, so elastic that it can go from a conductive material to a semiconductor or dielectric material using the same process”.
The groups involved in the MULTIFLEXIOXIDES project are known as the inventors of this technology at room temperature. Groups at the Tokyo Institute of Technology, Japan, and at Oregon State University, USA, have also developed oxide transistors but at temperatures higher than 600°C. “As we can process the oxide semiconductors at room temperature, we can deposit the material on very cheap substrates like plastic or paper, that can be re-used and are 100% recyclable, because I am only using nanometers of the material such as paper, which also contains oxides”.
“In my opinion, depositing oxides on flexible substrates and polymers is the most innovative aspect of this project, because oxides provide enhanced stability are low-cost with respect to conventional semiconductors such as silicon or germanium and can be deposited more easily”, says Alberto Calleja, an external expert not participating in this project. He is a Ramón y Cajal post-doctoral researcher of the ‘Superconductor materials and large scale nanostructures’ group at ICMAB (Institut de Ciència de Materials de Barcelona- CSIC). “Making electronic circuits on flexible substrates is not something new; what indeed is new is the fact of putting oxides as a circuit”.
But, how is the oxide deposited on the polymers? “Via a physical route, where we transfer, under vacuum conditions (particles travel very fast!) atoms from a ceramic target to a substrate where the film is deposited by using an inert gas whose atoms have been energetically excited (rf magnetron sputtering technique) and via a chemical one that involves the use of a chemical solution and of a normal inkjet printer! In this way we can write directly the circuits, avoiding the need of using lithographic masks and the required patterning procedures, which typically represent more than 66% of the total cost in conventional microelectronics, as it is the case of processing silicon devices”, Martins explains.
On the possible toxicity of the multi-component oxide Martins stresses that “zinc is a quite friendly element, it is even used in the food industry and in cosmetic products and it is not toxic either for the people or for the environment. Since the material we use is not harmful at macro-scale it should not be at nanoscale, but this is something that is being studied now”. No nanoparticles nor nanotubes are involved in this technology. "As the material at nano-level is processed under solid state conditions and the processed film is amorphous, we also promote the ‘sequestration’ of some metals within the device structure. Nevertheless, as we are speaking of devices with thicknesses in the range of some nanometers, it is not clear how such sizes, when disintegrated, may affect the human body or the environment”, he admits.
Calleja points out that the fact that a material is not harmful at macro-scale does not imply that it is also at nano-scale, since there are reasons to think that if something is nanometric it can easily go through an organism, dissolve easily within it and therefore enter the tissues and be harmful to the human beings. “An excess of zinc oxide can produce toxic effects and cause harm to the environment when dissolved. Especially when a material is low-cost it is much harder to recycle its whole volume, something that we see happening everyday with plastic bags for example”.
On the possible applications of this technology Calleja is convinced that everything that is done with electronics could be done this way, while Martins thinks of GPS, paper batteries, memories transparent displays, transparent solar cells and antennas.
A number of prototypes using the technology developed during the project have been completed by companies such as HP and Fiat. Apart from that, thanks to the existing cooperation between UNL (New University of Lisbon) and Samsung, with which the research group already has a patent for its transparent displays, the latter has also developed applied prototypes. The use of paper as a dielectric is subject to a spin-off. The research group made the first European display of around 10cm x 10cm using this method. The production cost of displays using oxides, according to Martins, would be less than one third of those using silicon, while in the case of using inkjet they would be reduced to one seventh of those spent in today’s technology.