Simulation Model for Direct Laser Writing of Metallic Microstructures Composed of Silver Nanoparticles

Three-dimensional metallic microstructures find applications as stents in medicine, as ultrabroadband antennas in communications, in micromechanical parts, or as structures of more fundamental interest in photonics like metamaterials. Direct metal printing of such structures using three-dimensional (3D) laser lithography is a promising approach, which enables the fabrication of 3D structures with sub-micron-sized features. Yet, this fabrication technique is not extensively applied, as fabrication speed, surface quality, and stability of the resulting structures are limited so far. To identify the limiting factors, we investigate the influence of light-particle interactions and varying scanning speed on heat generation and particle deposition in direct laser writing of silver. We introduce a theoretical model which captures diffusion of particles and heat as well as the fluid dynamics of the photoresist. Chemical reactions are excluded from the model, but particle production is calibrated using experimental data. We find that optical forces generally surmount those due to convection of the photoresist. Simulations predict overheating of the photoresist at laser powers similar to those found in experiments. The thermal sensitivity of the system is essentially determined by the largest particles present in the laser focus. Our results suggest that to improve nanoparticle deposition and to achieve higher writing speeds in metal direct laser writing, strong optical trapping of the emerging particles is desirable. Furthermore, precise control of the particle size reduces the risk of spontaneous overheating.