The requirements for novel additive manufacturing vary from application to application, but generally include high resolution, simplicity, low cost, and short processing time. Typically in such printing methods the printed material is in the liquid-phase as a “metal ink”, an ink consisting of a dispersion of nanoparticles, and printing is carried out by adapting methods from the graphic art industry such as screen printing, inkjet printing, flexography and gravure. There are several known limitations associated with current digital printing methods. For one, there is a rather limited range of metals that can be made into printable materials as inks or pastes, due to increased reactivity in the nano-size. Limited resolution and limitations on pattern geometry are typically dictated by wetting properties and droplet volume. Finally there is a thermal post-treatment step, which is almost always essential, to sinter the metal particles and render the print track conductive. This last step often limits the type of substrates that can be used or otherwise impairs the conductivity of the printed line.
Optically-controlled digital electro-deposition (ODE) has the advantage of being able to solve, under certain conditions, most of the problems related to digital printing. Indeed, the three steps –deposition, patterning and thickening– are all performed in only one step. Moreover, no post-treatment is required to make the printed track conductive. The method is fast, simple and cheap, and also allows for high resolutions. In principle, every material that can be electro-deposited can be digitally deposited with ODE, opening the doors to a large range of materials. In addition, the preparation of the chemical solution is trivial and does not require a complex process as the synthesis of metal inks with nano-particles does. The requirements are to provide a micro-fluidics channel where both sides are coated by transparent conductive layers that are used as electrodes and a photoconductive semiconductor layer on the illuminated side.
The ODE method uses the particular property of materials called photo-conductivity. The conductivity of certain semiconductors increases by orders of magnitude when illuminated by light, due to the photo-generation of charge carriers in the material. Thus, electro-deposition of the metal ion occurs only onto the illuminated area of the semiconductor. This selectivity enables the electro-deposition method to be digital: the shape of the deposited metallic structure is controlled by the spatial modulation of the light, itself generated with a digital system composed of a digitally-controlled projector and an imaging lens.
This article by Liu and co-authors shows the results of silver micro-structures printed on a glass substrate, which can be used – in the example application shown in the paper – for the fabrication of the nano-wires of a copper oxide transistor. However, the impact and possible applications of the method are clearly farther reaching. The technique could be applied to other material solutions and other substrates. ODE could be applicable to the fabrication of micro-devices on flexible foils, including the fabrication of flexible displays. For what regards the material type, ODE has the advantage of not using nanoparticles, thus avoiding their increased reactivity, so that noble metals could be replaced by lower-cost materials such as copper. Moreover, this method could possibly enable multi-layer stack fabrication with different metals by replacing the chemical formulation during the process. For example, one of the most important open problems in the metallization of low cost solar cells is to replace the silver by copper to reduce the fabrication cost. However, copper cannot be directly in contact with the celldue to the fast migration of copper atoms into silicon, so it is necessary to add a barrier layer like nickel or titanium before the metallization. ODE could possibly solve this issue by first depositing a thin layer of nickel followed by a thick layer of copper on top. This technique will certainly enhance the capabilities of digital printing and provide new opportunities in digital micro-fabrication.
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