# Laser-Assisted Etching of EagleXG Glass by Irradiation at Low Pulse-Repetition Rate
## 4 Conclusions  
We have demonstrated a new regime of laser-assisted etching in EagleXG glass substrate, with both HF and KOH etchants. Differently from previous work [23], where only high scan speeds for irradiation were found effective to induce etching selectivity, here we show that good working conditions can be achieved for about 1 mm/s scan speed at 25 kHz repetition rate. Best selectivity is observed with KOH etchant, reaching values above 80.  
A low scan speed facilitates the irradiation of paths with tight curvatures, as required, for example, in the fabrication of microfluidic structures and to achieve precise control on the shape of the etched volumes [24].  
We note that the two etchants provide different advantages and drawbacks, and one or the other may be adopted, depending on the application. For instance, KOH is suitable to realize hollow structures with high A.R. On the other hand, removal of large volumes of material may benefit from the lower selectivity and faster etching dynamics of HF.  
We observe a polarization dependence of the etching selectivity, particularly pronounced in the case of KOH etchant. Nonetheless, nanogratings, responsible for polarization-dependent selectivity in fused silica [4], were not apparent in our experiments, and the etching process seems to be related to a point-like microscopic feature, akin to the one observed previously at 1 MHz repetition rate [23]. Additional studies are required on the microscopic origin of the laser-enhanced etching sensitivity in this material, which could explain the physical nature of the black dot in the cross-section, its authentic role in the etching process, and the dependence on the laser polarization.  
We believe that this work opens the door to the effective use of EagleXG glass for optofluidic devices, fabricated by femtosecond laser writing followed by wet etching, as the low-repetition rate irradiation regime demonstrated here highly enhances the versatility of the process. In fact, in this regime, cumulative thermal effects are negligible, which means that scan speed and repetition rate can be adjusted together, keeping the same etching selectivity. In particular, the speed could be further lowered for inscribing details with higher accuracy, and set back to higher values to trace longer straight paths.  
Author Contributions:[PERSON] and [PERSON] carried out the experimental activity. [PERSON], [PERSON] and A.C. conceived the work and supervised its development. All authors contributed to data analysis and to writing the paper. All authors have read and agreed to the published version of the manuscript.  
A.C. acknowledges funding by the Italian Ministry of University and Research (PRIN 2017 programme, QUSHIP project--id. 2017 SRNBRK). [PERSON] acknowledges funding from EU under the European Union's Horizon2020 FET Open programme (PROCHIP project--grant agreement no.801336). [PERSON] acknowledges funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (project CAPABLE--Grant agreement No. 742745).  
Not applicable.  
Raw data represented in the graphs of this manuscript are available from the corresponding author, upon reasonable request.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.