The attraction of printing technology for the fabrication of electronics mainly results from the possibility of preparing stacks of micro-structured layers (and thereby thin-film devices) in a much simpler and cost-effective way compared to conventional electronics. Also, the ability to implement new or improved functionalities (e.g. mechanical flexibility) plays a role. The selection of the printing method used is determined by requirements concerning printed layers, by the properties of printed materials as well as economic and technical considerations of the final printed products. Printing technologies divide between sheet-based and
roll-to-roll-based approaches. Sheet-based
inkjet and
screen printing are best for low-volume, high-precision work.
Gravure,
offset and
flexographic printing are more common for high-volume production, such as solar cells, reaching 10,000 square meters per hour (m2/h). and organic conductors (the latter also for dielectrics),
gravure printing is especially suitable for quality-sensitive layers like organic semiconductors and semiconductor/dielectric-interfaces in transistors, due to high layer quality. and organic conductors. Organic
field-effect transistors and
integrated circuits can be prepared completely by means of mass-printing methods. However, inkjets offer lower throughput of around 100 m2/h and lower resolution (ca. 50 μm). Inkjet printing is preferable for organic semiconductors in
organic field-effect transistors (OFETs) and
organic light-emitting diodes (OLEDs), but also OFETs completely prepared by this method have been demonstrated. Frontplanes and
backplanes of OLED-displays, integrated circuits,
organic photovoltaic cells (OPVCs) and other devices can be prepared with inkjets.
Screen printing Screen printing is appropriate for fabricating electrics and electronics due to its ability to produce patterned, thick layers from paste-like materials. This method can produce conducting lines from inorganic materials (e.g. for circuit boards and antennas), but also insulating and passivating layers, whereby layer thickness is more important than high resolution. Its 50 m2/h throughput and 100 μm resolution are similar to inkjets. but also organic semiconductors, e.g. for OPVCs, and even complete
OFETs is another material deposition technology for printed electronics. The Aerosol Jet process begins with
atomization of an ink, via ultrasonic or pneumatic means, producing droplets on the order of one to two micrometers in diameter. The droplets then flow through a virtual impactor which deflects the droplets having lower momentum away from the stream. This step helps maintaining a tight droplet size distribution. The droplets are entrained in a gas stream and delivered to the print head. Here, an annular flow of clean gas is introduced around the aerosol stream to focus the droplets into a tightly collimated beam of material. The combined gas streams exit the print head through a converging nozzle that compresses the aerosol stream to a diameter as small as 10 μm. The jet of droplets exits the print head at high velocity (~50 meters/second) and impinges upon the substrate. Electrical interconnects, passive and active components are formed by moving the print head, equipped with a mechanical stop/start shutter, relative to the substrate. The resulting patterns can have features ranging from 10 μm wide, with layer thicknesses from tens of nanometers to >10 μm. A wide nozzle print head enables efficient patterning of millimeter size electronic features and surface coating applications. All printing occurs without the use of vacuum or pressure chambers. The high exit velocity of the jet enables a relatively large separation between the print head and the substrate, typically 2–5 mm. The droplets remain tightly focused over this distance, resulting in the ability to print conformal patterns over three dimensional substrates. Despite the high velocity, the printing process is gentle; substrate damage does not occur and there is generally minimal splatter or overspray from the droplets. Once patterning is complete, the printed ink typically requires post treatment to attain final electrical and mechanical properties. Post-treatment is driven more by the specific ink and substrate combination than by the printing process. A wide range of materials has been successfully deposited with the Aerosol Jet process, including diluted thick film pastes, conducting polymer inks, thermosetting polymers such as UV-curable epoxies, and solvent-based
polymers like polyurethane and polyimide, and biologic materials. Recently, printing paper was proposed to be used as the substrate of the printing. Highly conductive (close to bulk copper) and high-resolution traces can be printed on foldable and available office printing papers, with 80°Celsius curing temperature and 40 minutes of curing time.
Evaporation printing Evaporation printing uses a combination of high precision screen printing with material vaporization to print features to 5
μm. This method uses techniques such as thermal, e-beam, sputter and other traditional production technologies to deposit materials through a high precision shadow mask (or stencil) that is registered to the substrate to better than 1 μm. By layering different mask designs and/or adjusting materials, reliable, cost-effective circuits can be built additively, without the use of photo-lithography.
Other methods Other methods with similarities to printing, among them
microcontact printing and
nano-imprint lithography are of interest. Here, μm- and nm-sized layers, respectively, are prepared by methods similar to stamping with soft and hard forms, respectively. Often the actual structures are prepared subtractively, e.g. by deposition of etch masks or by lift-off processes. For example, electrodes for OFETs can be prepared. Sporadically
pad printing is used in a similar manner. Occasionally so-called transfer methods, where solid layers are transferred from a carrier to the substrate, are considered printed electronics.
Electrophotography is currently not used in printed electronics. ==Materials==