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Abstract
Bursts of femtosecond laser pulses were used to record internal modifications inside fused silica for selective chemical etching. Two-pulse bursts with a variable energy ratio between those pulses at a fixed inter-pulse duration of 14.5 ns were applied for the first time. The selective chemical etching rate of the laser-modified material with the burst of two pulses was compared to the single-pulse regime when etching in HF and KOH etchants. The advantage of the burst-mode processing was demonstrated when etching was performed in the KOH solution. More regular nanogratings were formed, and the etching initiation was more stable when burst pulses were applied for fused silica modification. The vertical planar structures were obtained using the two-pulse bursts with an energy ratio of 1:2, increasing the etching rate by more than 35% compared to the single-pulse processing. The highest ever reported selectivity of 1:2000 was demonstrated by introducing the two-pulse burst mode.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Modification of transparent materials with ultrashort laser pulses is one of the most discussed topics in the last two decades [1,2]. Glass cutting, welding, volume modifications and microstructuring for the photonics industry are the main applications that lead forward this field [3–10]. Fused silica and sapphire are commonly used transparent materials with demanding optical and physical properties. Moreover, they are extensively introduced for consumer electronics production [11], such as smartphones and optoelectronics.
Direct laser processing of transparent materials via ablation involves only surface processing up to 2.5D configuration. The femtosecond-laser induced selective etching (FLISE) is a key technology to expand the application field to 3D volume processing. Marcinkevičius et al. [12] initially demonstrated that the internal modifications written with femtosecond laser pulses in fused silica could be selectively removed. It was later explained that selective etching appears due to the formation of nanogratings perpendicular to the laser polarisation [13]. The FLISE technology broadly expanded the laser application field from micro and nanostructuring to fabrication of embedded micromechanical [14], microfluidic, optofluidic [9,15] or micro-optical [7,16] devices. Despite the mentioned advantages, some drawbacks limit the use of the technology for mass production: low processing and etching speed, thermal accumulations due to the high repetition rate [17] and distortion of local material modification confined within the focal volume due to aberrations [18]. A few approaches were proposed to increase the processing throughput, such as double-pulse fabrication, reducing heat accumulation and enhancing the etching rate [19–21].
The use of bursts of pulses already has shown promising processing results in various materials. It was possible to enhance the ablation removal rate by ∼ 20% using 3 pulses in a burst [22]. More efficient BK7 glass drilling with a burst of pulses was achieved, demonstrating crack-free and high aspect ratio holes [23]. Welding of fused silica with optimised temporal separation of pulses in a burst showed a significantly higher breaking resistance of bonded glass [24]. More freedom exists in energy deposition by utilising the burst regime for glass cutting, offering better cutting process control [5]. However, the burst mode still was not demonstrated for the FLISE method in transparent materials. Recent works indicate that energy deposition differs significantly when burst pulses are applied for transparent material processing. Due to lower energies in sub-pulses, no heat accumulation was observed up to 1 MHz repetition rate for 2 pulses in a burst processing [25]. In the soda-lime glass, almost no difference in the heat-affected zone (HAZ) formation was observed for 2-5 burst pulses at a 500 kHz repetition rate. It was also demonstrated that each pulse in the burst negatively influences the absorption efficiency at the nanosecond time scale [26]. For a higher number of pulses in the burst, the heat accumulation starts to be substantial. When 5 pulses in the burst are used, the softening temperature is achieved much faster than in the single-pulse case, and the heating and cooling dynamics are entirely different. It was also shown that the irradiated area has not yet returned to room temperature for a burst of 2 pulses, and some intermediate cooling dynamics occur.
The main advantage of FLISE technique is the ability to record a real three-dimensional structure embedded in bulk of silica glass. There are plenty different applications, where the high etching rate and selectivity are required. As for example micro hole array that are usually fabricated with mechanical drilling, electrochemical drilling [27] or electric discharge machining (EDM) [28]. The holes fabricated in conventional techniques are limited in diameter up to 30 µm and hole width to depth ratio up to 1:10. The FLISE technology allows to record a microhole array or microfluidic channels in more flexible way [29,30]. However, limited selectivity in etching leads to tapered shape channels. Therefore, the etching selectivity improvement is highly required to get predictable geometry of the FLISE fabricated features. High etching selectivity allows precisely control geometry of complex 3D high-density gas capillary nozzles used for laser wakefield acceleration [31,32]. The nozzles should be fabricated from the material resistant to harsh operational conditions. Therefore, fused silica is almost the best candidate allowing flexible manufacturing. The nozzles are usually of a complicated geometry starting from the high aspect ratio micro-hole and ending with the complex 3D embedded geometry, limiting the use of other manufacturing techniques.
In this work, we demonstrate the use of the two-pulse burst for the fused silica modification and selective chemical etching for the first time. The dependences of the fused silica etching rate on the energy ratio between the sub-pulses and variable translation, energy and pulse duration parameters were revealed. We try to answer a general question: what is better to use for FLISE – a high repetition rate or lower repetition rate with pulses split into bursts.
2. Materials and methods
The experimental setup is shown in Fig. 1. The Yb:KGW femtosecond laser (Pharos, Light Conversion) generated pulses with the 1030 nm wavelength, 500 kHz repetition rate and pulse duration variable from 290 fs to 10 ps. The beam was translated through the attenuator and half-wavelength phase plate to set the polarisation perpendicular to the scanning direction. A pair of lenses was used to reduce the beam size and fit into the entrance aperture of the focusing objective (100x, 0.5 NA, Mplan Apo NIR, Mitutoyo). A CCD camera and 20x microscope objective (Olympus, UPlanFLN, 20x) were integrated for sample observation during the processing. The sample was mounted on two-axis gimbal mounts (GM200/M, Thorlabs) to align the sample plane perpendicular to the laser beam. The sample was translated in the XY directions by the high-resolution positioning stages (ANT-130, Aerotech).
Fig. 1. Experimental schema of laser setup for burst pulses fabrication. Enlarged pictures show the available pulse sequence regimes. The train of burst pulses was generated by changing the cavity dumping time.
The burst of two pulses was generated by the laser head. Energies of the pulses in a burst were adjusted by tuning a cavity dumping time (CDT) that controls the temporal operation of a regenerative amplifier by extracting a pulse from an oscillator after one roundtrip. Typically, the cavity dumping time is adjusted to open the Pockels cell in the regenerative amplifier exactly when only a single pulse is generated. However, as the oscillator frequency is ∼ 68.9 MHz, it was possible to tune the cavity dumping time to have a burst of pulses separated by 14.5 ns, i.e., a single optical cycle in the oscillator. By fine adjusting the cavity dumping time, it was possible to get a different energy ratio between two pulses per burst (PPB). As shown in Fig. 1, three different cases were available: 1) 2 PPB with 1:1 energy ratio; 2) 2:1 energy ratio; 3) 1:2 energy ratio.
The commercially available fused silica samples (20 × 15 × 2 mm3, JGS1) were used. Both top and bottom surfaces were polished to optical quality. The samples were sliced into smaller pieces of 20 × 2x2 mm3 size. After laser processing, the samples were rinsed into two different etchant solutions: 1) 5-10% HF, heated up to 40°C and 2) 10M (mol/L) KOH, heated up to 80°C. The etching was performed in an ultrasonic batch 1 hour for 5% HF, 0.5 hours for 10% HF and 2-4 hours for 10 M KOH. The etching parameters for different materials are summarized in Table 1. After etching, the samples were cleaned for 5 min in deionised water.
Table 1. Parameters used for chemical etching of the modifications processed by laser radiation
To perform the fused silica etching rate and nanograting characterisation experiments, the modifications matrix was recorded at a constant focusing depth of 100 µm. Various pulse energy values, pulse density, pulse duration, and spacing between vertical channels were applied. The used processing parameters are shown in Table 2. The repetition rate in all experiments was constant and set to 500 kHz. Three lines in each group were recorded under the same conditions (separated by a 20 µm distance), and measured data of the channel geometry were averaged. When the burst regime was used, the total pulse energy of two pulses was measured (the energy for a separate sub-pulse was twice lower than the single pulse). To make the etchant access to the laser-modified area (Fig. 2(a)), the etchant penetration plane was recorded in the middle of the tracks. Vertically stacked modifications were raster-scanned, changing the focus position towards the sample top surface. For each set of experiments, the etchant penetration plane was recorded with a constant set of parameters. Samples were duplicated with the same set of parameters to perform similar etching experiments in various etchant solutions (Table 1). The reference samples were prepared by inscribing the modification using the single-pulse regime with already mentioned laser parameters (Table 2).
Fig. 2. Schematic illustration of the experiments: (a) inscribing and polishing of the modifications formed with bursts of pulses for the etching rate and nanogratings observation by a scanning electron microscope (SEM); (b) inscribing of the vertical planar structures for the 2D etching rate investigation from two sides.
Table 2. Parameters used for the laser writing of internal modifications in fused silica.
Vertical planar modifications (vertical stack of laser written lines) were processed to investigate the etching rate from the sample top and bottom surfaces in the 2D plane. The vertical structures (Fig. 2(b)) were recorded by a raster scanning the single line trajectory laterally moving through the laser focus with a constant vertical Z step, starting beneath the bottom sample surface and ending on the sample top surface.
The samples were side polished in the ZY plane to recover a smooth, transparent surface for the high-contrast SEM measurements and nanograting observation.
3. Results and discussion
3.1 Etching rate comparison in the different etching media
3.1.1 Etching in HF
The first processed samples were etched in HF acid. Two sets of the samples were prepared to etch with different concentrations of HF acid: 5% and 10%. The quick observation revealed two times higher etching rate in 10% HF acid. The results are shown in Fig. 3. The achieved etching rate of samples modified using 500 kHz and two-pulse bursts is similar to [33], where a 1 MHz single-pulse case was discussed. However, the etching rate in the samples prepared at 1 MHz drops down faster when the pulse energy is increased.
Fig. 3. The comparison of the fused silica etching rates at various concentrations of HF acid by using different regimes of burst pulse: a) 1-hour etching in 5% HF versus the total burst energy, pulse duration 290 fs; b) 0.5-hour etching in 10% HF, the total burst pulse energy is considered, 290 fs pulse duration; c) 1-hour etching in 5% HF, the total burst pulse energy is considered, 1 ps pulse duration; d) 0.5-hour etching in 10% HF, the total burst pulse energy is considered, 1 ps pulse duration; The pulse density was set to 100 ppµ. The error bar represents the standard deviation.
The etching tendencies for 5% and 10% HF acid were similar. As mentioned before, the total pulse energy for both pulses was measured. For the single-pulse fabrication and 290 fs pulse duration, the maximum etching rate was obtained at ∼ 200-350 nJ pulse energy. The fast drop of the etching rate was observed in the samples prepared using higher energies. The etching rate drop (ΔR/ΔE) was faster using 5% HF comparing to etching in 10% HF and was consequently ∼1 µm/h/nJ and ∼0.75 µm/h/nJ, indicating less aggressive etchant diffusion to the laser-modified material and lower etching of the pristine fused silica material.
For burst pulses with 2PPB 2:1 regime, the etching rate drop started at the total pulse energy of∼ 500 nJ (the first pulse energy ∼ 335 and 165 nJ for the second pulse). The first pulse energy of the 2PPB 2:1 and 2PPB 1:1 corresponded well to the energy level of the single-pulse processing where the etching drop started. For 2PPB 1:1, the etching rate drop versus pulse energy increase was significantly lower than for the single-pulse and 2PPB 2:1 cases. The faster etching rate drop could be related to heat accumulation at higher first pulse energy values approaching the threshold of Type III modifications. Non-uniform nanograting formation was obtained in this case, negatively affecting the etching rate. On the other hand, the burst pulses with equal pulse energy could facilitate the formation of regular nanogratings even at higher pulse energies. That could be related to the enhanced absorption of the laser pulses separated temporarily in the nanosecond range. Similar effects were linked to retardance enhancement while processing with a train of three pulses in a burst [34].
The experiments were continued with various pulse durations. The results show similar etching rates, except that the energy window for the 1 ps pulse duration was ∼ 70% broader. A negligible etching rate dependence on the pulse energy of burst pulses was observed in the tested energy range. Usually, the material etching is characterised by the etching selectivity parameter S. It is expressed as the etching rate ratio of the laser-modified and pristine materials as
where R is the etching rate of the modified material and ε is the etching rate of the pristine material. From the experimental data described in [33], the etching rate of the pristine fused silica in 5% HF is ∼ 4.5 µm/h, which is close to the experimental data presented by Ross et al. [35] (∼5.8 µm/h). The measured pristine fused silica etching rate in 10% HF was ∼10.3 µm/h. Our experiments shows that the maximum etching selectivity in 5% HF acid for samples inscribed by two-pulse burst with the energy ratio of 2:1 and using the 1 ps pulse duration was ∼ 1:79. The etching selectivity was increased further to 1:86 when 5 ps pulse duration was applied. The etching selectivity was similar when the pulse duration was 290 fs. For etching in 10% HF acid, the maximum selectivity was achieved for 2PPB 2:1 and 1 ps pulse duration ∼ 1:93. Generally, the etching results in HF acid were comparable either for the single-pulse or two-pulse burst regimes. The only slight advantage of burst pulses could be observed since the high etching rate could be maintained in a broader range of the total pulse energy values than single pulse processing. That could be related to lower energy of sub-pulses in a burst mode and more efficient energy confinement.3.1.2 Etching in KOH
The samples prepared with the same laser processing parameters were rinsed to the 10M KOH etchant. The etching rate dependence on the pulse duration at different burst regimes is shown in Fig. 4.
Fig. 4. The comparison of the fused silica etching rates in 10 M KOH in samples prepared at different pulse durations using different 2PPB regimes. The used pulse density was 100 ppµ. The error bar indicates the standard deviation.
The samples prepared using the 290 fs pulse duration demonstrated the lowest etching rate comparing to longer pulse durations. The etching of the samples irradiated with a single-pulse was initiated only when the 5 ps pulse duration was applied. This result agrees well with the research reported by Hermans et al. [36], where selective etching window was not found at 300 fs pulse duration and processing speed of 50 mm/s. While applying the burst mode in our experiments, the total pulse energy window for the maximum etching rate at 290 fs was 200–400 nJ. In contrast, for 1 ps and 5 ps, the optimal burst energy window was broader: from 250 nJ to 650 nJ (the energy presented on the graphs is the total energy of two pulses in a burst). For 290 fs 2PPB 2:1 processing (e.g., for the total 500 nJ pulse energy, the 1st pulse energy was ∼ 330 nJ and 2nd pulse energy ∼ 170 nJ), the maximum etching rate was observed in a narrow energy window 200-400 nJ. That could be related to the enhanced heat accumulation at the highest intensity, where better absorption of the second pulse is expected. Such a thermal regime is responsible for destroying the nanogratings, confirmed by the etching rate drop. The KOH etching rate drop for 2PPB 2:1 was 1.7 µm/h/nJ and faster than the etching in HF. Such distinction is due to lower etching rate of pristine fused silica and different chemical reactions in KOH where the OH- ions react with the silicon-richer material in the nanogratings [37], complicating the etchant diffusion deep into the channel.
The etching selectivity in KOH was an order of magnitude higher. That comes from the fact that the pristine material etching in KOH is slower compared to the HF. Kiyama et al. reported the etching rate of 0.25 µm/h for pristine fused silica in 10 M KOH solution at 80°C [37]. Therefore, higher aspect ratio channels can be etched. The maximum obtained etching rate was for 1-5 ps pulse duration and was ∼ 275-290 µm/h, following the etching selectivity of 1:1100-1:1160, respectively. The obtained selectivity values are comparable to the results presented by Hermans et al. [36]. In their work, maximal selectivity of 1:1400 at 200 mm/s scan speed was obtained and decreased to ∼ 1:1100 when the speed was reduced to 50 mm/s. Following this tendency, we can speculate that at even lower processing speeds, the etching selectivity could be < 1:1000. At the same time, we obtained 1:1100 etching selectivity at a significantly lower processing speed of 5 mm/s. Therefore, the application of burst pulses demonstrates the significant improvement of the processing window, especially at lower processing speeds. Unfortunately, extending the processing speed up to 200 mm/s was not possible in our processing setup. However, we can speculate that faster movement would allow us to reach higher etching selectivity. Therefore, the 2PPB 2:1 regime could exceed the 1:1400 etching selectivity. Considering the variation of the pristine material etching rate in KOH from 0.21 µm/h to 0.38 µm/h [29–31], the comparison of the results may not be straightforward.
Due to the high intensity at 290 fs pulse duration the more effective energy confinement was expected for 2PPB with the 1:2 case. Despite the lower 1st pulse energy, the intensity was sufficient to induce efficient multi-photon ionisation and, afterwards, the absorption of the second pulse with higher energy. For 2PPB 2:1 and 2PPB 1:1, the first pulse energy was higher than the modification threshold. That triggered faster increase of the etching rate to its maximum value but, in the same way, also narrowed the processing window. This effect occurred due to the quicker rise of the first pulse energy, causing the micro-voids formation. The comparison of the results shows that the first pulse energy should be ∼330-350 nJ to induce more efficient multi-photon absorption and, consequently, energy confinement of the second pulse.
From the measured data, it can be distinguished that the highest etching rate was achieved in the samples prepared using the 1 ps and 5 ps pulse duration and 2PPB 2:1. Comparing to the single-pulse processing, the etching rate increased by ∼ 27% and ∼ 38% for the 1 ps and 5 ps, respectively. Similar behaviour was observed by ablating the fused silica with burst pulses in [38]. The enhanced ablation was demonstrated for decreasing burst envelope (i.e., first pulse higher energy and decreasing energy for consecutive pulses in a burst). For 1 ps and 5 ps pulse duration, the pulse intensity was lower at the same pulse energy than in the case of the 290 fs pulse duration. That can initiate the optimal conditions for the most efficient second pulse absorption in the case of 2PPB with equal energies and the 2:1 energy ratio. The etching rate drop for 2PPB 1:2 was caused by insufficient energy of the 1st pulse to be efficiently coupled to the material. Consequently, the reduced efficiency of the multi-photon absorption took place. That can be observed better even at a longer pulse duration of 5 ps, where the etching rate was low, but its value was almost constant, increasing the pulse energy. It was found that for 500-2000 ppµ, the maximum etching rate was comparable to the 100 ppµ case (not shown in the graph). It means that better process efficiency can be achieved at a faster scanning speed. The main experimental results using different etching agents are summarised in Table 3.
Table 3. Comparison of the etching results obtained using 2PPB 2:1 pulses
An in-depth investigation of the modified areas was performed to understand the etching rate difference in samples prepared at different burst regimes. Modified samples were side-polished and etched for 1 min in 5% HF to reveal the nanograting morphology observed by SEM. The optical microscope and SEM pictures of the side-polished samples prepared at the different burst regimes are shown in Fig. 5.
Fig. 5. The cross-section of the modifications inscribed in fused silica with different pulse regimes: single pulse, 2PPB with equal pulse energies and 2PPB with 1st pulse energy higher. a) Optical microscope pictures of the modifications inscribed at the 300 µm depth with 500 nJ energy and the 1500 µm depth with 900 nJ, the pulse duration 290 fs; b) Optical microscope pictures of the modifications inscribed at the 300 µm depth with 500 nJ energy and the 1500 µm depth with 900 nJ, the pulse duration 1 ps; c) SEM images of the modifications recorded at the 300 µm depth and 500 nJ pulse energy and pulse duration of 1 ps.
The longest radial and axial modifications were observed in all cases for the single-pulse fabrication and the shortest ones at the same pulse duration in the case of 2PPB 1:1. That can be related to a two-times lower pulse energy compared to the single pulse regime and increased transmission in the time scale of the ns range. The visual inspection does not show any signs of heat accumulation for the 2PPB case. That agrees well with the results demonstrated by Gattass et al. [25], where it was revealed that the heat accumulation significantly raises starting from 5 pulses in a burst.
The boost of the etching rate for the 2PPB 2:1 case is related to an enhanced second pulse absorption and, therefore, more efficient energy coupling. As demonstrated in [39], the maximum absorption occur for the 2 pulses in a burst with a time separation of ∼ 10 ps and decreases by increasing the time separation between sub-pulses. However, at the time scale of 14.5 ns, it was still higher compared to the single-pulse processing. The generated free electrons are captured to the localized energy states creating self-trapped excitons (STE). The STE lifetime ranges from hundreds of picoseconds up to a few nanoseconds. The second pulse interacts with STE transferring electrons to the conduction band more efficient than direct multi-photon ionisation from the valence band. The second pulse is absorbed more uniformly and effectively, leading to an enhanced etching rate [40]. The selective chemical etching is strictly related to quality and uniformity of the induced nanogratings.
Figure 6(a) shows the dependence of the modification radial size on the pulse energy for different pulse regimes. As already discussed, the pulse energy dependence revealed that the axial length is maximal for the single-pulse regime and raises linearly with the pulse energy increase. Therefore, the radial size of the modification made by 2PPB is lower compared to the radial dimension of the modification made in the single-pulse regime. That is in good agreement with [39]. In addition, it was noted that the heat-affected zone for pulses with time separation > 10 ns decreases and is lower than in the case of the single-pulse regime. For example, the red dashed lines show the case when the radial size of the modification (∼ 4 µm) for 2PPB 2:1 (700 nJ) is equal to the radial extent of the modification fabricated in the single-pulse regime ∼ 400 nJ. In the case of 2PPB, the 1st pulse energy was ∼ 460 nJ and the second pulse ∼ 240 nJ. Thus, the energy of the 1st pulse almost corresponded to the energy of the single-pulse regime, and the second pulse did not contribute significantly to the size of nanogratings.
Fig. 6. Investigation of the modifications inscribed in fused silica at the 300 µm depth. a) Dependence of the modification radial size on the pulse energy for different pulse regimes and pulse durations; b) dependence of the nanograting period on the pulse energy for various pulse regimes. The insets show SEM images of the nanogratings at defined pulse energies for three different pulse burst regimes.
The nanograting period was in the range of ∼ 300-450 nm. The period for 2PPB 1:1 and 2PPB 2:1 regimes have a similar tendency depending on the pulse energy. However, the nanogratings period was almost constant ∼ 400 nm for the single-pulse regime, slightly increasing at the 700 nJ pulse energy. The nanograting period for the single-pulse regime was close to the middle between ∼ λ/2 and λ/2n values. For 2PPB 2:1, the nanograting period tends to decrease from ∼ λ/2 at lower pulse energies to λ/2n at higher pulse energies (>600 nJ). The SEM pictures in Fig. 5(c) show that the nanogratings fabricated with the single pulse are fragmented. In contrast, the nanogratings processed with burst pulses demonstrate a more uniform morphology, which is critical for easier etchant penetration and faster etching. Furthermore, we can distinguish the correlation between the nanograting period and the etching rate. The results in Fig. 4 show that the maximum etching rate was achieved when nanogratings with the largest period were formed (at 400-500 nJ). The rate dropped down when the nanograting period decreased at higher pulse energies. For the single-pulse processing at higher pulse energies (>500 nJ), the nanogratings were chaotic and not regular, evidencing the formation of Type III modification.
3.2 Etching of vertical structures
The 1D etching experiments are usually performed to quickly compare the different parameters and materials for the quantitative analysis. However, advanced experiments on complex geometries should be performed to move further to the real 3D fabrication. For that reason, the vertical structures (or 2D planar structures) were inscribed according to the experiment schematic illustrated in Fig. 2(b). Multiple horizontal lines with a vertical Z step were stacked from the sample bottom to the sample top. The Z step was constant during the recording of a single vertical structure. The vertical structures with 4 different Z steps 5, 7.5, 10 and 15 µm were inscribed. Two approaches: constant pulse energy and energy gradient along the vertical direction, were used. The energy gradient (different pulses energies at different Z positions) was applied to overcome the aberration induced energy drop by setting the maximal energy at the sample bottom and gradually lowering the pulse energy while moving towards the top surface. The intermediate pulse energies were changed linearly from the maximum to the minimum. The etching depth was measured from the sample top and bottom. The etching rate results of the vertical channels from the sample top are shown in Fig. 7.
Fig. 7. Etching rate dependence on the pulse energy for vertical structures etched from the top surface in 10M KOH for 2 hours. The modifications were inscribed through the whole sample thickness. Dark areas from the top and bottom show the etched material. a) 290 fs pulse duration; b) 1 ps pulse duration; c) 3 ps pulse duration. The z step distance was set to 5 µm. The colour border of the microscope pictures d-f corresponds to the curve colour on the graphs. The error bar shows the standard deviation from the averaged value.
The experiments were done for 4 different cases: 1) the single-pulse processing as a reference; 2) 2PPB with 1:1 energy ratio; 3) 2PPB with 2:1 energy ratio; 4) 2PPB with 1:2 energy ratio. For 290 fs pulse duration, the maximal vertical etching rate of ∼ 300 µm/h was achieved. However, the etching was initiated only from the channel right side (instead of whole widths of the vertical structure) and cannot be qualitatively compared with the other results. Therefore, it should be considered as an only eye guide. (Figure 7(d) top left picture). Furthermore, the etching initiation for the single-pulse regime and 290 fs pulse duration was unpredictable due to uneven etching from both channel sides.
In contrast, the etching was stable and easily predictable for the 2PPB case with different energy ratios (the channel was evenly etched from surface to volume). The etching rate peak of ∼ 260 µm/h (corresponding etching selectivity S∼ 1:1240) for 2PPB 1:1 was observed when the 500 nJ total pulse energy was applied. This regime has an advantage against the other 2PPB regimes as the etching from the bottom was initiated already at the 700 nJ pulse energy, while for 2PPB 1:2 regime, the etching from the bottom side did not appear. For 2PPB 2:1 regime, the etching from the bottom appeared only at the 1400 nJ pulse energy.
Starting from 1 ps pulse duration (Fig. 7(b) and (e)), the etching rate from the top for the single-pulse regime and 2PPB with a 2:1 regime was comparable. The etching performance from the bottom provided a better result for the single-pulse case due to a higher pulse energy. Comparing the etching with the 2PPB regime with non-equal pulse energies, we can distinguish the highest obtained etching rate for 2PPB 1:2 case, which is different from observations in single horizontal channels (Fig. 4). The enhancement was ∼ 35% compared to the single-pulse regime, and the vertical etching rate from the top was > 500 µm/h (S ∼1:2000), which was two times higher than the horizontal channels etching. That is the highest value of the 2D channel etching selectivity. Recently reported etching selectivity for vertical XZ planes was up to ∼ 1:1000 [41] when the single pulses with 700 fs pulse duration were applied.
At 3 ps pulse duration, the highest etching rate was obtained for 2PPB 2:1 regime, that is different than for 1 ps pulse duration, revealing the possible influence of the pulse duration. The single pulse processing had only slightly lower etching performance and raised more for higher pulse energies. At 3 ps pulse duration, the pulse intensity is lower. Therefore, the pulse energy required to initiate the nanogratings formation is higher. Li et al. demonstrated that for the longer pulse durations, the interconnected nanogratings are formed, which facilitates the etchant penetration to the microchannel causing the faster etching independent of the polarisation applied [42]. When comparing different z spacing, we observed the drop of the etching rate, and the maximum etching was achieved for 2PPB with 1st pulse higher energy. The optimal processing window was obtained for the 7.5 µm z spacing as the etching rate for this separation was comparable to the 5 µm. However, the processing speed can be increased by ∼ 30%.
As shown in Fig. 8 the etching from the top and bottom surfaces were initiated differently. The etching from the bottom surface was induced at higher pulse energies and, in all cases, had a lower etching rate comparing to the etching from the top surface. This effect was caused by the spherical aberrations that scatter the laser radiation, and the effective intensity at the deep focusing was significantly reduced [18]. Our goal was to achieve an equal etching rate from the top and bottom surfaces. Therefore, the vertical structures were inscribed, decreasing the pulse energy with shifting focus from the bottom to the top of the sample. The top energy was always constant and set to 500 nJ (corresponding to the maximal etching rate from the surface). The bottom energy was set higher to compensate for the effect of aberrations and induce faster etching. The intermediate energies were linearly changed from the higher value to the lower one. We tried to find the top to bottom etching rate ratio approaching 1.
Fig. 8. The vertical structures fabricated in the 2 mm depth sample with the gradient energy change. The top energy was set to a constant pulse energy of 500 nJ, and the bottom energy was changed from 500 nJ to 1400 nJ. a) Microscope picture of vertical structures with different side lengths etched in 10M KOH for 2 hours; b) The bottom/top etching ratio dependence on bottom pulse energy. c) The microchannel of 2 mm length and 20 µm in diameter, etched through after 4 hours etching.
For this reason, we inscribed a square vertical structure of square shape with various side lengths from 15 µm to 120 µm. From the data in Fig. 8(b), we found that the top to bottom etching rate ratio approaches 1 when the bottom energy is set to ∼ 900-1200 nJ and the top energy was fixed at 500 nJ. Application of energy gradient and 2PPB regime was involved to improve the etching rate and make an equal etching from the sample top and bottom. We were able to inscribe and etch the smallest symmetrical circular structure of 20 µm in diameter after 4 hours of etching in 2 mm length fused silica by choosing the optimal process parameters. That corresponds to the 1:100 width to depth ratio. The results are promising because they demonstrate the high etching selectivity and can minimize the tapering of the etched structures involving the even etching from the sample top and bottom. That is very important for fabricating the microhole array with a high aspect ratio for filtering purposes or microfluidic channels, or difficult 3D structures allowing to reach very precise geometry.
4. Conclusions
In conclusion, we have demonstrated the advantage of the burst-mode processing for laser-induced selective chemical etching of fused silica. The ∼38% increase in the etching rate for the burst-mode processing of horizontal channels was obtained with burst pulses divided by the energy ratio of 2:1 compared to the single-pulse processing at 5 ps pulse duration. In the case of vertical 2D planes, the ∼ 35% etching rate enhancement was achieved when burst pulses at 1 ps pulse duration were applied. The effect is related to the enhanced absorbance of the second pulse and more efficient energy coupling, initiating a shift of the modification threshold by the first pulse. The second pulse was more effectively absorbed due to the self-trapped excitons. Due to the enhanced absorption of the second pulse at a lower energy level, regular nanogratings were formed inside the material. A more stable etching initiation occurred for the burst-mode processing than a single pulse. The investigation of the modifications did not show any signs of heat accumulation for 2PPB comparing to the single-pulse regime when the moderately low pulse energy was applied.
The etching rate correlates with the nanograting period. It is suggested that the processing with a longer laser wavelength can induce even a higher etching rate, which needs further investigation.
Using two pulses in a burst, the maximum etching rate for single-line horizontal channels was ∼ 280 µm/h and ∼ 500 µm/h for the vertical planar structures when the etching in KOH was done. The processing of the vertical structures was made with a 100 ppµ pulse density to get the optimal processing speed and achieve the predictable etching. The application of burst pulses demonstrated the etching selectivity of 1:2000, which is the highest reported value to our knowledge.
Finally, it was shown that applying the energy gradient for the vertical structures makes it possible to obtain equal etching from the sample top and bottom sides, allowing to fabricate symmetrical high-aspect-ratio structures. The repetition rate of 500 kHz and 2PPB regime provided a better performance comparing the 1 MHz processing due to the higher absorption at lower repetition rates causing better energy confinement and more stable and pronounced etching at higher processing speeds.
Funding
Lietuvos Mokslo Taryba (01.2.2-LMT-K-718-01-0003).
Acknowledgments
This project has received funding from European Regional Development Fund (project No 01.2.2-LMT-K-718-01-0003) under a grant agreement with the Research Council of Lithuania (LMTLT).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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