Thermoreflectance for Contactless Sintering Characterization: From Metal Nanoparticles to Stretchable Conductors

Sintering metal nanoparticles is a crucial step to achieve printed conductors. It is important to characterize and monitor nanoparticle sintering for process optimization and control. Here, we demonstrate that frequency-domain thermoreflectance (FDTR), an optical pump-probe technique, can be used for non-contact, non-destructive process monitoring that is compatible with high-throughput printed electronics manufacturing, unlike traditional electrical resistance measurements. The thermal conductivity measured from FDTR agrees well with thermal conductivity calculated using Wiedemann-Franz law from electrical conductivity measurements. Measurement time is reduced to 12 s by choosing a small number of measurement frequencies instead of a full frequency sweep and measuring them simultaneously. A Monte Carlo simulation was performed to predict the possibility of further reducing measurement time. Understanding of the sintering process allows tailoring of materials properties as demonstrated here to create a novel stretchable conductor. Differently sintered layers are combined to achieve a desirable stretchability-conductivity profile.