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

Abstract

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.

Direct Laser Writing: From Materials Synthesis and Conversion to Electronic Device Processing

“…Reproduced with permission. [ 158 ] Copyright 2021, American Chemical Society. b) Effect of focal point positioning on the nature of laser‐synthesized structures and c) representation of Marangoni flow and renovation of precursors over the irradiation path.…”

“…[ 87,115,164 ] Sodium citrate has been the main resource used in laser photon or thermal‐induced reduction of silver ions. [ 158,166 ] Another reported mechanism for the formation of metallic films involves the use of semiconductor NPs as reducing agents. [ 147,173,174 ] Upon laser exposure, the electrons in the valence band are promoted to the semiconductor's conduction band, promoting the reduction of metal ions at their surface.…”

“…In these cases, TPA phenomenon translates to two-photon reduction (TPR) mechanisms, [118,156] in opposition to other mechanisms outside the scope of this review, such as two-photon polymerization. This process can either directly affect the precursor metal ions, [98,157] or occur through a photoinitiator or other excitable component within a precursor formulation, that transfers electrons following photon-induced excitation, acting as photoreducing agents (e.g., photoresists, [158] sodium citrate, [159] semiconducting structure). [160] Commonly, TPR also requires ultrashort pulse laser systems, most commonly femtosecond lasers, capable of providing the high photon flux stimulus needed to initiate photoreduction.…”

“…[124] However, other formulations using gelatine matrices have also been employed. [162,163] Several metallic salts have been used for this purpose, including metal nitrates, chlorides and acetates, such as AgNO 3 , [42,147,[163][164][165][166][167] AgClO 4 , [158,162] Cu(NO 3 ) 2 , [82,103,115,160,164,[168][169][170][171] Cu(CH 3 COO) 2 , [172] HAu-Cl 4 , [40,147,173,174] PtCl 4 , [42] and NiCl 2 . [147] Regarding reducing agents, polyvinylpyrrolidone (PVP) is a common choice, whose decomposition produces formic acid, able to reduce metal ion species.…”

Homogenization based heating control for moist paperboard with evaporation on the pore surface

“…Computations of the effective heat conduction of a package of balls from PhD‐Thesis of Thomas Palmer (Fraunhofer ITWM) [20] ($$A_{contact}$$ is the contact surface).…”