Fabrication and analytical evaluation of three-dimensional microsolenoids achieved in fused silica by femtosecond-laser-based microsolidifying process

Abstract

In this reported work, a femtosecond-laser-based microsolidifying method was employed to fabricate three-dimensional (3D) microsolenoids by injecting liquid metal into helicalmicrochannels in fused silica and solidifying the liquid metal, and a proposed finite element method-3D-vector simulation approach was used to analyse the structure influence. An improved femtosecond laser irradiation followed by chemical etching technology was used to fabricate the complex 3D microchannels, in which an optimal laser power compensation strategy of tuning laser power from 7 to 12 mW was involved. Asilanisation process was carried out before the injection process to facilitate the injection of gallium. 3D numerical simulations of the microsolenoids were carried out by an electromagnetic-coupled analysis method; and a simulation-based co-energy calculation method was used to evaluate the inductance of the 3D microsolenoids. 3D microsolenoids of optimised configurations were then achieved according to the analytical results.1. Introduction: Three-dimensional (3D) microsolenoids have always been critical for miniaturisation of large-scale functional systems, including integrated circuits (ICs), microfluidic systems and microelectromechanical systems (MEMS) [1][2][3][4][5]. Although many efforts have been directed towards 3D microfabrication, it is still a technical challenge to achieve complex 3D microsolenoids [4][5][6][7]. The reported microsolenoids were mainly based on the lithographic process which can only provide planar microcoils or microsolenoids of a multiplayer structure, not to mention the tedious fabricating process and expensive cost.At the same time, analytical evaluation of microsolenoids has mostly concentrated on the 2D planar spirals. Nowadays, the performance prediction of 3D microsolenoids mainly depends on the empirical formula or scalar analysis by the finite element method (FEM) [8][9][10][11][12]. These methods have limitations to the precise calculation or prediction of the properties of microscale solenoids owing to their lack of considering the unique geometry configurations of 3D microsolenoids, such as a greater ratio of helix pitch to radius etc. However, the reported 3D microsolenoids, which have been applied in on-chip nuclear magnetic resonance (NMR) and other microsystems [1][2][3][4][5], mostly consist of a sparse structure with a large ratio of helix pitch to coil radius; and the influence of the wire diameter cannot be neglected.With the development of femtosecond laser technology, a femtosecond laser irradiation followed by chemical etching (FLICE) technology has been utilised to fabricate complex 3D microchannels in fused silica [13][14][15][16]. In this Letter, a femtosecond-laser-based microsolidifying process is proposed to fabricate 3D cylindrical microsolenoids by combining the FLICE technology and liquid-metalinjection technology. Meanwhile, 3D microsolenoids of different geometrical parameters were modelled and numerically simulated using a FEM-3D-vector analysis method. The simulati...