Laser Thermal Processing of Nanoparticle Inks on Flexible Substrates for Biological Sensing

Most current flexible electronics suffer from drawbacks that limit their application in biological systems. Devices are typically incompatible with in-vivo applications, degrade rapidly in aqueous environments, and have low carrier mobility. A critical need remains for stable, bio-compatible, integrated, and higher performance flexible electronics. This research explores zinc oxide nanoparticle suspensions as a system to investigate novel deposition methods and transient thermal annealing techniques adapted for low-temperature substrates with biological sensing and signaling applications in mind.

Zinc oxide nanoparticles were sintered into densified, polycrystalline thin films using a transient high-temperature laser sintering process which enabled TFT fabrication on low-temperature substrates. Two deposition methods, spin casting and nanoparticle electrophoresis, were used to create green films on rigid and flexible substrates. A two-step sintering process was employed to improve performance with minimal film and substrate damage. The first annealing stage heated the film to moderate temperature for milliseconds using a continuous wave (CW) CO2 laser. The extended time at low temperature enabled complete evaporation of remaining solvent, decomposition organic ligands, and improvement in the film integrity for the higher temperature sintering anneal. The second stage utilizes pulsed excimer laser to transient temperatures approaching 1200-1300 C for approximately 35 ns duration. Because solvents and stabilizing ligands had been removed, material loss and film damage were minimized while sintering.