Microwave flash sintering of inkjet-printed silver tracks on polymer substrates.

Within the last few decades inkjet printing has grown into a mature noncontact patterning method, since it can produce large-area patterns with high resolution at relatively high speeds while using only small amounts of functional materials. The main fields of interest where inkjet printing can be applied include the manufacturing of radiofrequency identification (RFID) tags, organic thin-film transistors (OTFTs), and electrochromic devices (ECDs), and are focused on the future of plastic electronics. In view of these applications on polymer foils, micrometersized conductive features on flexible substrates are essential. To fabricate conductive features onto polymer substrates, solutionprocessable materials are often used. The most frequently used are dispersions of silver nanoparticles in an organic solvent. Inks of silver nanoparticle dispersions are relatively easy to prepare and, moreover, silver has the lowest resistivity of all metals (1.59mV cm). After printing and evaporation of the solvent, the particles require a thermal-processing step to render the features conductive by removing the organic binder that is present around the nanoparticles. In nonpolar solvents, long alkyl chains with a polar head, like thiols or carboxylic acids, are usually used to stabilize the nanoparticles. Steric stabilization of these particles in nonpolar solvents substantially screens van der Waals attractions and introduces steep steric repulsion between the particles at contact, which avoids agglomeration. In addition, organic binders are often added to the ink to assure not only mechanical integrity and adhesion to the substrate, but also to promote the printability of the ink. Nanoparticles with a diameter below 50 nmhave a significantly reduced sintering temperature, typically between 160 and 300 8C, which is well below the melting temperature of the bulk material (Tm1⁄4 963 8C). Despite these low sintering temperatures conventional heating methods are still not compatible with common polymer foils, such as polycarbonate (PC) and polyethylene terephthalate (PET), due to their low glass-transition temperatures (Tg). In fact, only the expensive high-performance polymers, like polytetrafluoroethylene (PTFE), poly(ether ether ketone) (PEEK), and polyimide (PI) can be used at these temperatures. This represents, however, a significant drawback for the implementation in a large-area production of plastic electronics, being unfavorable in terms of costs. Furthermore, the long sintering time of 60min or more that is generally required to create conductive features also obstructs industrial implementation. Therefore, other techniques have to be used in order to facilitate fast and selective heating of materials. One selective technique for nanoparticle sintering that has been described in literature is based on an argon-ion laser beam that follows the as-printed feature and selectively sinters the central region. Features with a line width smaller than 10mm have been created with this technique. However, the large overall thermal energy impact together with the low writing speed of 0.2mm s 1 of the translational stage are limiting factors. A faster alternative to selectively heat silver nanoparticles is to use microwave radiation. Ceramics and other dielectric materials can be heated by microwaves due to dielectric losses that are caused by dipole polarization. Under ambient conditions, however, metals behave as reflectors for microwave radiation, because of their small skin depth, which is defined as the distance at which the incident power is reduced to half of its initial value. The small skin depth results from the high conductance s and the high dielectric loss factor e00 together with a small capacitance. When instead of bulk material, the metal consists of particles and/or is heated to at least 400 8C, the materials absorbs microwave radiation to a greater extent. It is believed that the conductive particle interaction with microwave radiation, i.e., inductive coupling, is mainly based on Maxwell–Wagner polarization, which results from the accumulation of charge at the materials interfaces, electric conduction, and eddy currents. However, the main reasons for successful heating of metallic particles through microwave radiation are not yet fully understood. In contrast to the relatively strongmicrowave absorption by the conductive particles, the polarization of dipoles in thermoplastic polymers below the Tg is limited, which makes the polymer foil’s skin depth almost infinite, hence transparent, to microwave radiation. Therefore, only the conductive particles absorb the microwaves and can be sintered selectively. Recently, it has been shown that it is possible to create conductive printed features with microwave radiation within 3–4min. The resulting conductivity, however, is only approximately 5% of the bulk silver value. In this contribution, we present a study on antenna-supported microwave sintering of conducted features on polymer foils.