Metal nanoparticle inks are promising to fabricate conductors for low-cost, printed electronics. Low electrical resistivity can be achieved by nanoparticle sintering. The thermal properties of metal nanoparticle thin films have not been studied extensively but can yield great insights for the optimization of the sintering conditions. For example, in laser sintering, monitoring the changes in thermal conductivity over different stages of the process can help estimating the local temperature as well as the electrical conductivity of the film. In this work, we use frequency-domain thermoreflectance to measure these properties in a silver nanoparticle thin film thermally sintered ex-situ. The film is fabricated by spin coating a commercial printable silver ink with monodispersed 35 nm silver nanoparticles surrounded by a ligand. Using frequencydomain thermoreflectance (FDTR), we measured the thermal conductivity of the thin film by modulating the heat flux over a wide range of frequencies up to 50 MHz. An increase of thermal conductivity with increasing sintering temperature is observed up to a sintering temperature of 155°C, as measured by thermoreflectance or inferred through theWiedemann- Franz Law based on electrical conductivity measurements. The results are corroborated with material characterization by Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). The thermal and electrical properties are correlated throughout the different stages of sintering. For unsintered films, thermoreflectance gives more accurate values of thermal conductivity because it measures thermal conduction by both electrons and phonons. The Wiedemann- Franz Law underestimates the thermal conductivity by 50% in the unsintered case, which is problematic for modeling and optimization of the sintering process. In the sintered state, thermoreflectance and electrical conductivity measurements are in good agreement, as the contribution to heat transport is dominated by electrons. The thermoreflectance metrology can be used as a non-contact method to determine film conductivity, both thermal and electrical, during manufacturing processes involving nanoparticle inks.
Gravure printing is a very promising method for printed electronics because it combines high throughput with high resolution. Recently, printed lines with 2μm resolution have been demonstrated at printing speeds on the order of 1m/s. Here we build on these results to study how more complex patterns can be printed that will ultimately lead to printed circuits. We study how the drag-out effect leads to proximity effects in gravure when multiple lines are printed close to each other. Drag-out occurs as the doctor blade passes over the roll surface to remove excess ink from the land areas in between the cells that make up the pattern. In addition to this desirable removal of excess ink, some ink from the cells also wicks up the doctor blade and is removed from the cells. This ink is subsequently deposited on the land area behind the cells leading to characteristic drag-out tails. If multiple lines, oriented perpendicular to the print direction, are printed close to each other, the ink that has wicked up the doctor blade from the first line will affect the drag-out process for subsequent lines. Here we show how this effect can be used to enhance print quality of lines as well as how it can deteriorate print quality. Important variables that will determine the regime for printing optimization are ink viscosity, printing speed, cell size, cell spacing and relative placement of lines. Considering these factors carefully allows one to determine design rules for optimal printing results.
High-resolution features are key to achieve high performance printed electronics devices such as transistors. Gravure printing is very promising to achieve high resolution in combination with high printing speeds on the order of 1m/s. High-speed gravure has recently been shown to print high resolution features down to linewidths and spacing of 2μm. Whilst this was a tremendous improvement over previous reports, these results had been obtained using silicon printing plates. These silicon printing plates are fabricated using microfabrication techniques which offer several advantages over traditional metal gravure cylinders where the features are defined by techniques such as stylus engraving, laser engraving or etching. This offers much greater precision and design freedom in terms of feature size, surface roughness, cell placement and cell shape. However, rigid silicon printing plates cannot be used in a roll-to-roll printing process that would truly enable low-cost printed electronics. Here we demonstrate for the first time a gravure printing roll that combines the precision of silicon printing plates with the form factor of a metal cylinder. The fabrication process starts with a silicon master whose pattern is replicated by polymer molding. The actual metal printing plate is then built up on the polymer negative of the pattern by a combination of electroless and electroplating. After separation of the polymer and the metal, the metal printing plate can be mounted on a magnetic roll for printing. Printing of highly scaled 2μm features is demonstrated. Different metal surfaces were explored to optimize printing performance and wear during printing.
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