  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
laser lithography is a promising approach, which is not extensively applied
yet, as fabrication speed, surface quality, and stability of the resulting
structures are limited so far. In order to identify the limiting factors, we
investigate the influence of light-particle interactions and varying scan 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 photo-resist. 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 photo-resist. Simulations predict overheating of the
photo-resist 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 particle
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.
