Effect of sintering conditions on the thermal properties of printable metal nanoparticle ink studied by thermoreflectance

Abstract

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.