ISSUE 115 MARCH 2025 LASER ETCHING


SELECTIVE LASER ETCHING:


TOWARDS INDUSTRIAL APPLICATION CALUM ROSS Ultrafast laser inscription (ULI) is a


high-precision, flexible laser fabrication technique notable for in-volume processing of transparent materials such as fused silica. ULI is widely applicable; depending on the irradiation conditions, direct refractive index modification, enhanced chemical etching, material welding and ablation are all possible and explored widely in the literature. Crucially, ULI relies on non-linear photon absorption processes driven by focused ultrashort laser pulses, and therefore, the material modification is tightly confined to the laser focus inside the material while the surrounding material is unaffected.


Selective laser etching


ULI can be combined with subsequent wet chemical etching (often referred to as selective laser etching (SLE)) to establish a fully three- dimensional glass microfabrication technique, whereby 3D structures are first formed in the glass by translating the substrate around the laser focus and then selectively removed by chemical etching (see Figure 1). The key to high- quality SLE is the etching selectivity, which is the rate at which the laser-modified glass is removed compared to the pristine glass while etching. When this phenomenon was first discovered, the etching selectivity was limited to a ratio below 100, restricting the technique’s usefulness to specific research applications. Nowadays, however, an etching selectivity upwards of 1500 is achievable, vastly broadening the application space and potential for industry-scale manufacturing.


Several technological advances have led to the high etching selectivity available today. Potassium and sodium hydroxide have replaced hydrofluoric acid for chemical etching, with an increase in selectivity and improved user safety. The pulse quality delivered by bulk solid-state lasers is typically better than fibre lasers, enabling better control of the light-material interaction and leading to lower minimum pulse energies required to achieve high selectivity. A better understanding of the ideal irradiation conditions, notably the lateral pulse density [1], has enabled effective isotropic etching irrespective of the laser polarisation orientation. The ideal lateral pulse density is approximately 4 – 10 pulses per micron, which favours high-speed laser writing as an added benefit.


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THE LASER USER


Figure 1: During SLE, a 3D structure is inscribed into glass using ultrashort laser pulses and subsequently dissolved by chemical etching.


SLE is widely applicable and is often used to fabricate micro-optics, microfluidics and optical interconnects. Like many laser-based fabrication techniques, SLE is well-suited to rapid prototyping but is equally suited to high-value, bespoke component production. Freeform silica micro-optics is one area of particular interest. Unfortunately, the surface roughness achieved by SLE is typically > 100 nm which is too rough for many optical applications. However, several thermal post-processing techniques can be


applied, including precision flame brushing, CO2 laser polishing, and bulk annealing, to reduce the roughness to below 5 nm.


The ability to fabricate several features on a single chip means that complex, miniaturised assemblies can be reduced to passively aligned monolithic components, greatly simplifying costly micro-assembly processes. For example, it is possible to fabricate monolithic, multi- element freeform lenses with a built-in optical fibre alignment tunnel used to passively couple the lens to an optical fibre. With this approach, passive fibre alignment to a precision of < 1 μm is readily achievable.


SLE in manufacturing


The case for SLE in industrial manufacturing is compelling: modern ultrashort pulsed lasers are robust and easy to install and use. The chemical etching process is inexpensive and performed in a standard fume cupboard using non-hazardous chemicals, namely sodium hydroxide in dilute solution. One remaining concern is long laser inscription times. Linear translation stages can provide < 100 nm positioning precision, but only at few mm/s speeds when writing small features. On the other hand, galvanometers offer fast beam translation but often trade off the quality or high numerical aperture of the focused laser beam.


Figure 2: The cross-section of channels written and etched in fused silica with increasing conical phase applied to the laser beam.


Beam shaping, using adaptive optics, is a promising solution. Beam shaping distributes the laser power in the focus to suit a particular writing application. A common example is a Bessel beam, which has an elongated focal region and can modify long channels in a single exposure, or large planes in a single pass of the laser. A Bessel beam can be obtained simply by passing the beam through an axicon lens. However, a more flexible method is to use adaptive optics, such as a spatial light modulator (SLM), which enables beam shaping on the fly. In our recent work, we demonstrated a vortex beam for laser inscription, produced by imparting a conical phase front across the beam using an SLM, with tuneable elongation (See Figure 2) [2]. Beam shaping can significantly reduce laser inscription time. It can also be used to compensate for aberrations introduced when focusing the laser deep within a material. By combining beam shaping with aberration compensation, it is possible to fabricate cm-scale components, with micron-scale feature resolution.


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