# Laser-Assisted Etching of EagleXG Glass by Irradiation at Low Pulse-Repetition Rate
## 3 Experimental Results  
### Analysis of the Irradiation Parameters  
In our previous work [23], we found good working conditions for laser-assisted etching of EagleXG glass at 1 MHz repetition rate and scan speeds >50 mm/s, while lower speeds were ineffective in achieving favourable etching selectivity. We note that, at that repetition rate of the laser pulses, relevant thermal accumulation effects are present in the irradiation process, which affect the material modification [14]. Here, we aim to assess the existence of a suitable range of irradiation parameters, which provides good selectivity while keeping the scan speed in the order of a few millimetres per second. Hence, we attempt the investigation of a totally different irradiation regime, for which the temporal separation between two subsequent pulses is sufficiently large to avoid heat build-up in the material. In detail, as the threshold for the onset of thermal accumulation effects in similar glasses is typically placed around 200 kHz repetition rate [14], we select the much lower value of 25 kHz. As a preliminary characterisation of the laser-assisted etching process at 25 kHz, a limited set of straight tracks was written in the substrate, with a coarse-grained sampling of the available pulse energies, and values for \(v_{t}\) between 0.5 mm/s and 4 mm/s. In addition, each track was written with two laser polarisations, orthogonal and parallel to the translation direction. Indeed, in laser-assisted etching processes, a marked dependence of the etching selectivity on the polarisation direction of the irradiation laser beam is often observed. After the fabrication, the sample was cut in two pieces, transversally with respect to the tracks, and lateral facets were polished to optical quality. The two resulting samples were then etched, one in HF and the other in KOH, as detailed in Section 2. Figure 1a gives a schematic representation of the laser-assisted etching process we adopt in our experiments. As the etchant preferably attacks the laser-irradiated tracks, hollow cones are excavated along these tracks, starting from the lateral facet of the sample exposed to the acid, as shown in Figure 1b. The aspect ratio (A.R.) of the cones, defined as the ratio of the length over the basis radius, is known to be approximately equal to the ratio of the etching rate of the irradiated track over the etching rate of the unmodified substrate [23] and it directly indicates the etching selectivity. Thus, after etching, the samples were characterized with the optical microscope, measuring the dimensions of the etched cones and calculating the A.R.s; results are reported in Figure 2.  
As a matter of fact, to finely characterise the etching dynamics in the HF solution, each sample was etched in four successive steps of 5 min each, measuring the A.R. of the etched cones after each step. In fact, the changing equilibrium between chemical reaction and diffusion processes, inside the etched cone, can cause a decrease in the advancement rate of the tip of the cone, as it becomes longer [4; 23]. This is typically observed as a sort of saturation of the length of the etched cone, and a decrease in the measured A.R., as the etching time increases. To calculate a reliable estimation of the etching selectivity in optimal conditions, it is appropriate to measure the A.R. of the cones at the shortest possible time, for which both the length and the radius of the cones can be measured accurately. In our experiments, this condition is met at 10 min, the etched cones still being too small after the first 5-min step. On the other hand, in the case of the KOH etching, which is a much slower process, we did not observe relevant saturation effects; the results plotted in Figure 2 refer to an etching time of 6 h.  
Little to no selectivity was observed for the lowest pulse energy (\(E_{p}=0.84\) uJ), for both etchants, and independently of the adopted scan speed and polarisation.