Precision Glass Microstructuring using Femtosecond Laser Induced Chemical Etching – Mr Wenhe Feng
Ultrafast laser irradiation is capable of substantially increasing the chemical activity of fused silica glass so that fabrication of devices can be realised by applying a wet etching process after their geometries being defined by the laser scribing within the glass bulk. Compared with techniques such as direct laser ablation, e-beam lithography or focused ion beam micromachining, this technique gives the capability for rapid interior micromachining on glass substrates. KOH water solution has been found to be an effective etchant compared with conventional HF etchant. It does not attack unmodified glass, as a result the etched tunnels’ aspect ratio is advantageous over those treated with HF; moreover, KOH is much less toxic. Functional devices such as microfluidic reactors, micro-lens arrays and micro-actuators have already been made in laboratories for the proof of concept.
The focus of this project is to clarify the etch mechanism, to investigate the process recipe, to establish a design rule in 3D, and finally to develop a process allowing an arbitrary shape to be fabricated in-bulk with optimised precision.
- Single tunnels were made by a two-step FLICE process. Processing windows found for the two lasers were rather broad to enable selective etching based on four laser parameters namely pulse duration, pulse energy, repetition rate and writing speed, and two etching parameters, namely etchant concentration and etching temperature.
- A versatile FLICE fabrication route of arbitrary 3D structures were established via either a 2.5D manual approach or a 3D CAD/CAM laser patterning process. Design rules involving the laser exposure parameters and LAZ spacings were also established.
- Prototype microfluidic chips were designed and made with FLICE to which commercial microfluidic tubing systems could be conveniently connected so that injection and withdrawn of liquid were simplified.
- The VP-SEM confirmed the existence of nano-porosity contained within the LAZ, and Raman spectra of the LAZs revealed an increase of less-membered rings in the silica network that was in favour of the enhanced FLICE etch rate as well as strong fluorescence as a proof of heat-induced defects. The findings added up to a conclusion that an enhancement of etch rate in the LAZs depended not only the chemical modification to the silica network but was also influenced by the heat induced by the laser irradiation.
- Microfluidic devices for liquid mixing, reaction or droplet generation
- Opto-fluidic chips for biological or chemical analysis applications
- Micro-mechanic devices for actuating or sensing
Feng, W., Sparkes, M. and O’Neill, B. (2014). Precision glass microstructure using femtosecond laser induced chemical etching (FLICE), EPSRC Centre in Ultra Precision Mid-Term Review, 20 May 2014, Cranfield University.
Feng, W. (2015). Precision Glass Microstructuring using Femtosecond Laser Induced Chemical Etching, PhD project video produced for an informal Centre competition in 2015.