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Optimization of ultrafast laser parameters for 3D micromachining of fused silica
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Optimization of ultrafast laser parameters for 3D micromachining of fused silica ⁎
Yusuf Dogana,b, , Christi K. Madsena a b
Electrical and Computer Engineering, Texas A&M University, College Station, TX, USA Electrical and Electronic Engineering, Gumushane University, Gumushane, Turkey
H I GH L IG H T S
laser parameters after etching were found for high quality optical devices. • Optimal of 21.8 nm for 1 mm area in silica was achieved. • A1.25smoothness m/s scan rate for a system operating with a 2 MHz repetition rate was proved. • Complex writing involving surfaces at different orientations was simplified. • 2
A R T I C LE I N FO
A B S T R A C T
Keywords: Ultrafast laser Direct writing Chemical etching Selectivity Roughness 3D microfabrication
We present an optimization study on laser parameters for 3D micromachining of fused silica to achieve critical goals for practical applications including high surface quality, high volume production, and complex surfaces by ultrafast laser direct writing assisted chemical etching. We conducted experiments on laser pulse width of 300 fs and 1 ps, pulse energy ranging from 0.1 µJ to 1.6 µJ, three different polarizations (circular, parallel and perpendicular) and number of overlapped pulses from 3 to 10,000 at 1030 nm with up to 2 MHz repetition rate to investigate their effect on nanogratings and one dimensional (1D) channel and two dimensional (2D) planar surface selective etching on 1 mm thick fused silica. In one configuration, we achieved 21.8 nm RMS surface roughness with 80 µm Gaussian filtering and in another configuration, we estimated the maximum writing speed to be 1.25 m/s for given 2 MHz repetition rate with less than 400 nm filtered root mean square (RMS) surface roughness at a 1 mm2 area which covers the thickness of the glass.
1. Introduction Direct writing in transparent materials by femtosecond laser is a promising technique enabling complex three-dimensional precise micro fabrication. This technique, known also as femtosecond laser micromachining, can be summarized as non-linear modification in glass and it has been a center of interest and studied by many regarding nanograting formation [1,2] and refractive index change[3,4]. It is proved to be ideal for rapid and easy production of passive [5–7] and active micro devices [8–11] with some other applications [12–15]. The greatest advantage of the direct writing technique over the conventional methods of glass cutting such as diamond cutting [16], score and break cutting [16] or the other non-conventional methods such as hot air jet cutting [17] and waterjet cutting[18] is that it offers the possibility of directly writing complex 3D structures while the other methods only allow for planar geometries. ⁎
Ultrafast laser irradiation followed by chemical etching (Fig. 1) in glass is a popular, simple and maskless process for fabricating 3D structures with a great potential for glass micromachining [13,19–22]. Laser-assisted wet etching is a selective method since the laser affected zone is selectively etched while the rest of the sample is barely etched. Moreover; it enables high selectivity with an aspect ratio of 200:1 for microfluidic channels [23], a controllable etch rate [24] on both planar and non-planar substrates, high contrast nanograting [25], easeful processing environment [24]. There are two well-known etchants, and one is HF and the other one is KOH. While HF acid has high etching rate and low selectivity, KOH solution has low etching rate, high selectivity and it is less toxic and easier to handle regarding safety perspective [25–27]. Selectivity, the ratio of etching rates of modified region vs non-modified region, is the key factor to understand the etching efficiency. High selectivity and surface quality are strongly related to the arrangement of nanogratings
Corresponding author at: Electrical and Electronic Engineering, Gumushane University, Gumushane, Turkey. E-mail address: [email protected] (Y. Dogan).
https://doi.org/10.1016/j.optlastec.2019.105933 Received 21 June 2019; Received in revised form 23 September 2019; Accepted 26 October 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Yusuf Dogan and Christi K. Madsen, Optics and Laser Technology, https://doi.org/10.1016/j.optlastec.2019.105933
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Fig. 1. Femtosecond laser irradiation followed by chemical etching (FLICE).
for cross sectional analysis. The diced surface is then finely polished by using lapping machine. The polished sample is placed into 0.5% HF for 2 min, afterwards to get better imaging of nanograting regions and investigation of the modified zone under scanning electron microscope (SEM) as flash etching clears the modified regions and allows for high contrast images. 2 nm of Platinum coating is used to eliminate the charging effects of SEM. Periodic nanograting formation perpendicular to the writing direction is observed as it is shown in Fig. 3.
that form within the inscribed region for a specific window of parameters. This study investigates the influence of ultrafast laser parameters on nanogratings, 1D and 2D etching selectivity aiming to accomplish the optimum surface quality, the highest laser manufacturing speed and complex writings. Laser parameters of pulse width, pulse energy, polarization and number of overlapped pulses were tested. 1D etching has been addressed in some previous studies [20,27–30], however they do not offer an overall study of the laser parameter tradeoffs or a detailed analysis of the 2D etching selectivity. Recently, 2D etching has been studied by Ross et al. [31], which mainly focuses on selectivity and has no data for surface quality measurements. In our study, parameters were evaluated in combinations and many possibilities were screened. We found that minimum surface roughness high volume production or complex surfaces can be achieved depending on the choice of three different sets of laser parameters. These achievements can lead to great improvements in the performance of optical components and applications like glass cutting, engraving and building 3D structures [32].
2.1. Laser parameters effect on nanogratings The laser parameters such as polarization, writing speed or corresponding number of overlapped pulses and pulse energy have a great impact on nanograting formation which is the key factor for the 1D and 2D etching process [26,33]. As indicated in several studies, etching rate is not independent from nanograting directions.[26,34] The difference in the directions of nanogratings in the perpendicular and parallel polarization according to the writing direction is clearly visible in Fig. 4(b) and (c). Polarization and nanogratings are perpendicular to each other. While the perpendicular polarization has nanogratings towards the image plane, the parallel polarization has parallel nanogratings to the image plane. Fig. 4(b) and (c) give us a clue to make the interpretation that the direction of nanogratings for circular polarization should be a combination of the nanogratings formed by other linear polarized beams. One of the key parameters for nanograting formation is the writing speed. Considering the limitation of nanopositioning stage to 200 mm/s of maximum writing speed and the required time and distance for acceleration, the easy and accurate adjustment of pulse repetition rate leads us to understand the writing speed effect on nanogratings via the number of overlapped pulses, which decreases if the writing speed increases for a given pulse repetition rate. The writing speed and PRR are adjusted to control the desired N for the experiments. The number of overlapped pulses can be calculated with the equation below:
2. Experimental setup & procedures High energy femtosecond laser machining system includes Uranus (PolarOnyx) and Satsuma (Amplitude Systems) ytterbium-doped fiber lasers, Aerotech high precision positioning stages and automation control system. The laser system generates from 300 fs to 4 ps pulses at 1030 nm with up to 2 MHz repetition rate and delivers maximum 20 µJ pulse energy with beam propagation ratio of < 1.2. The beam is focused by high power micro focusing objective (Thorlabs 0.4NA, 20×). The process is controlled by nano positioning stages on all three axes. The schematic of the ultrafast laser micromachining system is represented in Fig. 2. In the experimental configuration, an isolator (Electro-Optics Technology Inc.) is used to minimize the back reflection to the laser, and the polarization before and after the acoustic modulator is controlled with Thorlabs wave plates (WP). Gooch & Housego acousto-optic modulator (AOM) is integrated to control the power of the beam by adjusting the intensity of the beam orders. The delay generator (Highland Technology Inc.) helps changing the pulse repetition rate, and Altechna tunable beam expander alters the beam diameter. This laser system enables us a broader perspective and ability to optimize tests by manipulating laser parameters with a great variety of choices. The focal spot diameter (1/e2) is theoretically estimated around 1.9 µm under the given condition. Direct writing experiments are performed 300 μm beneath the top surface of 1 mm thick 1 in. square Corning 7980 fused silica plate with 50 μm separated varying parameters of raster scans. The laser modified sample is diced through the perpendicular direction to writing direction
N = PRR ∗ d/v where N is number of overlapped pulses, PRR is pulse repetition rate, d is spot size diameter and v is writing speed. An increase in N causes a greater number of nanogratings and larger lateral and vertical modified spot sizes while it lessens the period of the nanogratings as seen in Fig. 5. The deformations through the left side of nanogratings result from the lapping process. The affected zone dimensions corresponding to the maximum values are measured under SEM, and the average grating period is calculated by dividing the affected zone length to the number of nanogratings. When N increases, the number of nanogratings is increased from a few nanogratings to 2
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Fig. 2. Schematic of fs laser system (Uranus).
Fig. 7. When the pulse energy is changed from 0.2 µJ to 0.8 µJ, the number of nanograting increases roughly from 5 to 15 while the nanograting period stays almost the same. Shimotsuma argues that the period is changed from 180 nm to 320 nm when the laser pulse energy is increased from 1 µJ to 2.8 µJ [36]. This data do not match our experiments. On the other hand, according to Sun, the nanograting period keeps constant with the change in laser pulse energy from 170 nJ to 870 nJ, which has a similar trend with our results [41]. Bhardwaj et al, likewise, states that the spacing is independent of pulse energy [2]. All of these differences in nanogratings such as directions of gratings, spacing of grating, number of gratings and void formation has great influence on selective etching in 1D and 2D plane. The etching effect will be studied in the following chapters.
around 20 nanogratings and the period of nanogratings is decreased from around 500 nm to nearly 150 nm as shown in Fig. 6. Nanograting period is expected to be around 355 nm (??/2n) [35], which stays in the range of our above mentioned findings. According to Shimotsuma, the nanogratings period is decreased from 240 nm to 140 nm as the laser pulse repetition rate is increased from 50 kHz to 800 kHz, which closely matches our results [36]. We should note that the change in these values is not linear and it is more significant when N is smaller than 1000. The size of the modified region at beam spot is related to N as well. The lateral and vertical dimensions of the modified zone stay nearly the same once N is greater than 333, since the affected zone reaches the beam waist diameter, and laser pulse fluence decrease dramatically as the area gets larger. The refractive index change regime is not seen in our experiments as laser pulse duration is greater than 300 fs which is measured by Mini TPA (APE) autocorrelator and this regime is accessible with the pulse duration smaller than 200 fs [35]. Nanograting and micro explosion regimes are the main focus in our study. Pulse energy has a crucial effect on laser irradiation regimes. The formation of the nanogratings are observed beyond the threshold pulse energy [35,37,38]. The minimum peak power intensity for nanograting formation is estimated around 7E+12 W/cm2 which closely matches with some other studies [39,40]. The number of nanogratings increases and the axial and lateral dimensions of modified region gets larger as the pulse energy gets higher [35]. Micro explosions occur when the pulse energy is higher than it is required for nanograting formation. When the peak power intensity exceeds 3E+13 W/cm2, void generation is started [35,38]. The modification of nanograting formation and voids can be seen in
2.2. Laser parameters effect on selective etching Our first goal was to optimize all the laser parameters to achieve the highest selectivity for 2D plane etching to demonstrate the effect of selectivity on the surface quality. It was expected for 2D plane etching that low selectivity would generate a form with high spatial wavelength leading high surface roughness on the surface. as the non-modified region gets etched more due to longer etching time similar to 1D channel etching, where at low selectivity the channel has angled side walls with high cone angle [22]. It was essential to make some selectivity studies on 1D channel etching before for a better insight and an opportunity to make comparisons. Direct writing methods were applied on a silica plate with 50 µm spaced raster scanned lines 300 µm beneath
Fig. 3. Cross section SEM images: perpendicular polarization with 0.4 μJ, 300 fs and N = 3333 (a) at low magnification (8 K zoom & 1 μm scale) and (b) at high magnification (40 K zoom & 100 nm scale). E, S and k represent the polarization plane, the scanning direction and the light propagation direction, respectively. 3
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Fig. 4. Cross section SEM images corresponding to (a) circular, (b) perpendicular and (c) parallel polarization with 300 fs, 0.8 µJ and N = 3333.
power enable the grating growth more uniformly while higher peak powers may introduce localized degradations to the grating structure. Setting the pulse duration to 1000 fs, the same experiment was conducted under a variety of polarization conditions. In all three polarizations the selectivity reached up to 500 in some given conditions as seen in Fig. 10. In parallel polarization, at the low writing speed or at high overlapped pulses, the channel did not etch nor give any selectivity. The selectivity at circular and perpendicular polarizations were quite similar, except that perpendicular polarization gave a much wider range of both writing speed and pulse energy. In overall, there are many sets of conditions that have high selectivity and could be a good candidate for 2D plane surface etching studies.
the top surface under the same conditions. Later, the silica plate was diced with a precise dicing saw machine. Thus, we obtained many identical irradiated silica pieces for etching selectivity studies. 5–55% (w/w) KOH solution at 85 °C with 75 rpm stirring speed was chosen to be used for our optimization experiments. KOH was preferred over HF for its advantages like higher selectivity, less toxicity and easier handling. 1D channel etching of the modified and non-modified region was measured in an hourly fashion under microscope. As the concentration of solution increased, the etch rates of bare silica increased almost linearly through 45% KOH solution, and the solution got saturated above that percentage. However, as seen in Fig. 8, it was observed that the selectivity decreased when the etchant went deeper through the modified channel, since the penetration of the etchant and diffusion of by-products got harder. This challenge had been addressed by some studies of one-dimensional channel etching as the channel dimensions vary from the inlet to inside of the material. In our study, the etched length of the channels was analysed by optical contrast measurements and the best selectivity over time was observed with 5% KOH solution, which was accordingly chosen to be used in the further experiments. Pulse width effect on selective etching for 1D channel was studied by testing various conditions and combinations of pulse width, pulse energy and overlapped number of pulses. It was found that the selectivity of the modified channel with pulse width of 1000 fs was much higher than 300 fs pulse width as shown in Fig. 9. As it is claimed by Herman, the pulse duration over 800 fs gives more stable nanograting formation [27]. The laser pulse of 1000 fs yielded higher selectivity with a broader pulse energy and overlapped number of pulses range. The reason could be that the longer pulse duration and lower peak
3. 2D plane etching, optimization and results Following the 1D channel optimization, the 2D plane etching optimization was pursued for analyzing the surface quality further. Etching in 2D plane refers that the laser modified region is not a line but a modified area, namely a combination of 1D modified channels. The writing started at the bottom of the sample and moved up step by step with z increments. To understand the 2D writings and etching, simple rectangular shapes (1 mm × 10 mm) were written on 1 mm thick Corning 7980 high purity fused silica with various laser parameters to analyze the surface quality and compare the parameters as shown in Fig. 11. The samples were cleaned carefully with deionized (DI) water for 10 min, methanol for 10 min, isopropyl alcohol (IPA) for 10 min and DI water, respectively, prior to initiating the examination process. To evaluate the scanned surface shape and quality, white light
Fig. 5. Effect of the number of overlapped pulses (a) N = 3333, (b) N = 1000, (c) N = 333, (d) N = 100, (e) N = 40 for perpendicularly polarized light with 300 fs and 0.8 µJ. 4
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Fig. 6. Effect of (a) N to nanograting numbers and periods, and (b) the lateral and vertical dimensions of laser modified region for given N.
and from 16 to 1000 kHz, respectively, to adjust the corresponding N. The z increment is adjusted to 5 μm in air which corresponded to 7.5 μm depth in silica plate, and the depth of focus is theoretically estimated around 6 μm. The pulse energy and z increment were studied after optimizing the polarization and number of overlapped pulses. As shown in Fig. 13, in parallel polarization, the minimum surface roughness was achieved at N = 40. While N < 100 or high writing speed gives relatively acceptable surface quality, the surface gets worse when N > 100, as the writing speed is decreasing. The surface does not etch wholly and the samples could not be released as a result when N > 333 as the selectivity is so low that the etchant does not reach to the center of the sample and not release. Interpreted from the 1D selectivity experiment, no selectivity is observed at 0.4 μJ after N > 100 in Fig. 10a, and the 2D experimental data give a great match to 1D experiments. The circular polarization has a similar trend with parallel polarization. The best surface quality is observed at N = 40 while the worst one is observed at N = 10000 which corresponds to the lowest writing speed. As the micro roughness stays between 300 nm and 500 nm, the form effect can be seen at lower speeds as the total roughness gets higher. In perpendicular polarization, the surface gets better when N increases, unlikely to the other two polarizations. The surface roughness change gets minimal for N > 1000. According to the RMS roughness data, the best surface was achieved at N = 3333. Evidently, the beam perpendicularly polarized to writing direction resulted in the best surface quality in general, especially with high number of overlapped pulses. While the minimum surface roughness for high speed is observed at parallel polarization, for low speed the perpendicular polarization gives better surface quality. For the parallel
interferometry (WLI) images taken by Profilm3D (Filmetrics) which have an area a little bigger than 1 mm2 covering the thickness of the silica plate were used. Surface texture is composed of various ranges of spatial wavelength which are typically referred to as micro roughness, waviness and form. Form has higher spatial wavelengths while micro roughness has lower spatial wavelengths and waviness has wavelengths in between [42]. To distinguish the micro roughness values from the surface texture, Gaussian low pass filtering is one of the well-known and practical solutions that form and waviness are substantially filtered out. For nearly 1 mm2 scanned surface area with 512x512 sampling, 80 µm cut off wavelength is chosen to distinguish the micro roughness from the form and waviness according to the recommended cut off wavelengths by ISO 4288-1996. We demonstrated and compared both total surface roughness without any filtering (Sq) and micro roughness (Sq_gf) data to understand the micro roughness and form of the profiles. As with the 1D selectivity experiments, polarization is one of the key factors effective on surface quality. Some of the optical profile images obtained from different polarization applied surfaces are shown in Fig. 12. In general, perpendicular polarization provides the smallest roughness, while parallel polarization gives the highest roughness due to having form on the surface. Circular polarization, on the other hand, yields the highest micro roughness. The polarization effect with various conditions will be presented later. The polarization and writing speed optimization experiment was conducted with pulse energy of 0.4 μJ as it has the best selectivity for all three polarizations with a wide range of N from 3 to 10,000 as seen in Fig. 10. The writing speed and PRR are changed from 0.2 to 10 mm/s
Fig. 7. SEM images of modified spot with various pulse energies (a) 0.2 µJ, (b) 0.4 µJ, (c) 0.8 µJ and (d) 1.6 µJ by perpendicularly polarized beam which has a pulse width of 1000 fs with 1000 overlapped number of pulses. 5
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Fig. 8. (a) Microscope image of one-dimensional etched channel and (b) normalized 1D selectivity over time with various KOH etchants.
Fig. 9. Pulse width effect at various conditions on 1D channel selectivity at perpendicular polarization with various writing speeds (0.1–10 mm/s) and PRR (16–500 kHz) for (a) 300 fs and (b) 1000 fs.
Fig. 10. Polarization effect on selectivity under various channel writing conditions; (a) parallel, (b) perpendicular and (c) circular polarization at 1000 fs.
with z increments and pulse energy is presented with both RMS roughness data in Fig. 14. Z stage increments have an undeniable impact on the surface quality. 2.5, 5, 10 and 15 μm increments in air were applied to the surfaces with different pulse energies, and surface RMS roughness and Gaussian filtered roughness were analyzed. Lowering the z increment normally reduces the high surface roughness; however, it could deform the surface if the increment size reduces more.
polarization, the 2D plane etching test matches 1D selectivity data in Fig. 10, since both data show that no etching occurs at low speed direct writing. For other two polarizations, while number of overlapped pulses or writing speed have nearly the same selectivity at 1D selectivity test, a clear difference is observed in 2D plane etching studies. The z increment and pulse energy are strongly related with each other and they should be analyzed together. Surface quality change 6
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Fig. 11. Laser irradiation and released material from 1 mm fused silica substrate after chemical etching.
Fig. 12. White light interferometry images and profiles of the surfaces which are written at 1 mm/s, 0.4 μJ, 1000 fs, N = 1000 and z increment of 10 μm in air with (a) parallel, (b) perpendicular and (c) circular polarizations.
Fig. 13. Surface roughness over overlapped number of pulses with 3 different polarizations: (a) parallel, (b) perpendicular and (c) circular at 0.4 μJ, 1000 fs with z increment of 5 μm in air.
4. Conclusion
We determined three configurations to optimize the scanned surface for different purposes: high manufacturing speed, complex writing and high surface quality. The laser parameters should be optimized considering the requirements the application like speed, complex writing and surface quality. Therefore, configurations can be adjusted accordingly as shown in Table 1.
Ultrafast laser parameters are evaluated for surface quality and speed by conducting successive tests for polarization, pulse width, pulse energy and number of overlapped pulses. The effect of these parameters on nanogratings and 1D and 2D selective etching is studied. Gaussian filtered surface roughness of 21.8 nm for 1 mm2 area is demonstrated for the cases where surface quality is the priority. This is the lowest 7
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Fig. 14. The laser writing conditions for best surface quality. Table 1 Some of the optimized configurations for the application needs for high manufacturing speed, complex writing and high surface quality. Parameters
Configuration 1 (Speed)
Pulse Width Beam Polarization Theoretical Spot Size (1/e2) Pulse Energy PRR Writing Speed (v) Number of Pulses N Writing Speed at 2 MHz Z inc. in Air Etching Specs Surface RMS Sq. (1 × 1 mm2)
1000 fs Parallel 1.9 μm 0.4 μJ 16 kHz 10 mm/s 3.2 1.25 m/s 5 μm 12–24 hr at 5% KOH, 85 °C with 75 rpm Sq = 477 nm Sq_gf = 395 nm
Configuration 2 (Complex Writing)
Configuration 3 (Surface Quality)
Circular
Perpendicular
200 kHz 10 mm/s 40 100 mm/s
1000 kHz 0.6 mm/s 3333 1.2 mm/s
Sq = 388 nm Sq_gf = 341 nm
Sq = 63.4 nm Sq_gf = 21.8 nm
Surface 3D Profiles (1 × 1mm2)
Surface 2D Profiles (nm)
surface roughness for femtosecond laser processing at the given area in the literature so far. A maximum writing speed of 1.25 m/s is estimated at 2 MHz repetition rate with about three overlapped pulses and 395 nm Gaussian filtered RMS surface roughness, which is very promising for high-volume production. Gaussian filtered RMS surface roughness of 341 nm is reached at 100 mm/s writing speed in circular polarization, which can be utilized if complex writing is targeted without needing to change the polarization. Overall, these optimized parameters can guide the work in evaluating practical fabrication tradeoffs.
Phys. Rev. Lett. 96 (2006) 057404. [3] V.R. Bhardwaj, E. Simova, P.B. Corkum, D.M. Rayner, C. Hnatovsky, R.S. Taylor, B. Schreder, M. Kluge, J. Zimmer, Femtosecond laser-induced refractive index modification in multicomponent glasses, J. Appl. Phys. 97 (2005) 083102. [4] M. Lancry, B. Poumellec, A. Chahid-Erraji, M. Beresna, P.G. Kazansky, Dependence of the femtosecond laser refractive index change thresholds on the chemical composition of doped-silica glasses, Opt. Mater. Express 1 (2011) 711–723. [5] K. Miura, J. Qiu, H. Inouye, T. Mitsuyu, K. Hirao, Photowritten optical waveguides in various glasses with ultrashort pulse laser, Appl. Phys. Lett. 71 (1997) 3329–3331. [6] K. Hirao, K. Miura, Writing waveguides and gratings in silica and related materials by a femtosecond laser, J. Non-Cryst. Solids 239 (1998) 91–95. [7] R.R. Thomson, R.J. Harris, T.A. Birks, G. Brown, J. Allington-Smith, J. BlandHawthorn, Ultrafast laser inscription of a 121-waveguide fan-out for astrophotonics, Opt. Lett. 37 (2012) 2331–2333. [8] Y. Sikorski, A.A. Said, P. Bado, R. Maynard, C. Florea, K.A. Winick, Optical waveguide amplifier in Nd-doped glass written with near-IR femtosecond laser pulses, Electron. Lett. 36 (2000) 226–227. [9] A. Ródenas, L.M. Maestro, M.O. Ramírez, G.A. Torchia, L. Roso, F. Chen, D. Jaque, Anisotropic lattice changes in femtosecond laser inscribed Nd3+:MgO:LiNbO3 optical waveguides, J. Appl. Phys. 106 (2009) 013110. [10] Y. Ren, G. Brown, A. Ródenas, S. Beecher, F. Chen, A.K. Kar, Mid-infrared waveguide lasers in rare-earth-doped YAG, Opt. Lett. 37 (2012) 3339–3341. [11] W. Yang, P.G. Kazansky, Y.P. Svirko, Non-reciprocal ultrafast laser writing, Nat. Photon. 2 (2008) 99. [12] R.S. Taylor, C. Hnatovsky, E. Simova, D.M. Rayner, V.R. Bhardwaj, P.B. Corkum,
Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.optlastec.2019.105933. References [1] F. Liang, R. Vallée, S.L. Chin, Mechanism of nanograting formation on the surface of fused silica, Opt. Express 20 (2012) 4389–4396. [2] V.R. Bhardwaj, E. Simova, P.P. Rajeev, C. Hnatovsky, R.S. Taylor, D.M. Rayner, P.B. Corkum, Optically produced arrays of planar nanostructures inside fused silica,
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[13]
[14] [15] [16] [17]
[18] [19]
[20]
[21] [22]
[23]
[24]
[25]
[26]
[27]
[28] R. Drevinskas, M. Gecevičius, M. Beresna, Y. Bellouard, P.G. Kazansky, Tailored surface birefringence by femtosecond laser assisted wet etching, Opt. Express 23 (2015) 1428–1437. [29] J. Gottmann, M. Hermans, N. Repiev, J. Ortmann, Selective laser-induced etching of 3D precision quartz glass components for microfluidic applications—up-scaling of complexity and speed, Micromachines 8 (2017) 110. [30] X. Yu, Y. Liao, F. He, B. Zeng, Y. Cheng, Z. Xu, K. Sugioka, K. Midorikawa, Tuning etch selectivity of fused silica irradiated by femtosecond laser pulses by controlling polarization of the writing pulses, J. Appl. Phys. 109 (2011) 053114. [31] C.A. Ross, D.G. MacLachlan, D. Choudhury, R.R. Thomson, Optimisation of ultrafast laser assisted etching in fused silica, Opt. Express 26 (2018) 24343–24356. [32] Y. Dogan, M. Morrison, C. Hu, R. Atkins, M. Solmaz, C.K. Madsen, Fabrication of advanced glass light pipes for solar concentrators, SPIE Optifab, SPIE2017, (2017) pp. 6. [33] S. LoTurco, R. Osellame, R. Ramponi, K.C. Vishnubhatla, Hybrid chemical etching of femtosecond laser irradiated structures for engineered microfluidic devices, J. Micromech. Microeng. 23 (2013) 085002. [34] C. Hnatovsky, R.S. Taylor, E. Simova, V.R. Bhardwaj, D.M. Rayner, P.B. Corkum, Polarization-selective etching in femtosecond laser-assisted microfluidic channel fabrication in fused silica, Opt. Lett. 30 (2005) 1867–1869. [35] R. Taylor, C. Hnatovsky, E. Simova, Applications of femtosecond laser induced selforganized planar nanocracks inside fused silica glass, Laser Photon. Rev. 2 (2008) 26–46. [36] Y. Shimotsuma, P.G. Kazansky, J. Qiu, K. Hirao, Self-organized nanogratings in glass irradiated by ultrashort light pulses, Phys. Rev. Lett. 91 (2003) 247405. [37] F. Liang, Q. Sun, D. Gingras, R. Vallée, S.L. Chin, The transition from smooth modification to nanograting in fused silica, Appl. Phys. Lett. 96 (2010) 101903. [38] C. Hnatovsky, R.S. Taylor, P.P. Rajeev, E. Simova, V.R. Bhardwaj, D.M. Rayner, P.B. Corkum, Pulse duration dependence of femtosecond-laser-fabricated nanogratings in fused silica, Appl. Phys. Lett. 87 (2005) 014104. [39] P. Wang, W. Chu, W. Li, Y. Tan, F. Liu, M. Wang, J. Qi, J. Lin, F. Zhang, Z. Wang, Y. Cheng, Three-dimensional laser printing of macro-scale glass objects at a microscale resolution, Micromachines 10 (2019) 565. [40] Y. Dai, A. Patel, J. Song, M. Beresna, P.G. Kazansky, Void-nanograting transition by ultrashort laser pulse irradiation in silica glass, Opt. Express 24 (2016) 19344–19353. [41] Q. Sun, F. Liang, R. Vallée, S.L. Chin, Nanograting formation on the surface of silica glass by scanning focused femtosecond laser pulses, Opt. Lett. 33 (2008) 2713–2715. [42] J. Raja, B. Muralikrishnan, S. Fu, Recent advances in separation of roughness, waviness and form, Precis. Eng. 26 (2002) 222–235.
Femtosecond laser fabrication of nanostructures in silica glass, Opt. Lett. 28 (2003) 1043–1045. Y. Bellouard, A. Said, M. Dugan, P. Bado, Fabrication of high-aspect ratio, microfluidic channels and tunnels using femtosecond laser pulses and chemical etching, Opt. Express 12 (2004) 2120–2129. K. Sugioka, Y. Cheng, Femtosecond laser three-dimensional micro- and nanofabrication, Appl. Phys. Rev. 1 (2014) 041303. D. Choudhury, J.R. Macdonald, A.K. Kar, Ultrafast laser inscription: perspectives on future integrated applications, Laser Photon. Rev. 8 (2014) 827–846. S. Nisar, L. Li, M.A. Sheikh, Laser glass cutting techniques—A review, J. Laser Appl. 25 (2013) 042010. E.S. Prakash, K. Sadashivappa, V. Joseph, M. Singaperumal, Nonconventional cutting of plate glass using hot air jet: experimental studies, Mechatronics 11 (2001) 595–615. A.A. Khan, M.M. Haque, Performance of different abrasive materials during abrasive water jet machining of glass, J. Mater. Process. Technol. 191 (2007) 404–407. A. Marcinkevičius, S. Juodkazis, M. Watanabe, M. Miwa, S. Matsuo, H. Misawa, J. Nishii, Femtosecond laser-assisted three-dimensional microfabrication in silica, Opt. Lett. 26 (2001) 277–279. C. Hnatovsky, R.S. Taylor, E. Simova, P.P. Rajeev, D.M. Rayner, V.R. Bhardwaj, P.B. Corkum, Fabrication of microchannels in glass using focused femtosecond laser radiation and selective chemical etching, Appl. Phys. A 84 (2006) 47–61. K. Sugioka, Y. Cheng, Ultrafast lasers—reliable tools for advanced materials processing, Light: Sci. Appl. 3 (2014) e149. K.C. Vishnubhatla, N. Bellini, R. Ramponi, G. Cerullo, R. Osellame, Shape control of microchannels fabricated in fused silica by femtosecond laser irradiation and chemical etching, Opt. Express 17 (2009) 8685–8695. F. He, Y. Liao, J. Lin, J. Song, L. Qiao, Y. Cheng, K. Sugioka, Femtosecond laser fabrication of monolithically integrated microfluidic sensors in glass, Sensors 14 (2014) 19402. F. Chen, H. Liu, Q. Yang, X. Wang, C. Hou, H. Bian, W. Liang, J. Si, X. Hou, Maskless fabrication of concave microlens arrays on silica glasses by a femtosecond-laserenhanced local wet etching method, Opt. Express 18 (2010) 20334–20343. R. Drevinskas, M. Gecevičius, M. Beresna, Y. Bellouard, P.G. Kazansky, Nanotexturing of glass surface by ultrafast laser assisted wet etching, CLEO: 2014, Optical Society of America, San Jose, California, 2014 pp. AM1L.2. S. Kiyama, S. Matsuo, S. Hashimoto, Y. Morihira, Examination of etching agent and etching mechanism on femotosecond laser microfabrication of channels inside vitreous silica substrates, J. Phys. Chem. C 113 (2009) 11560–11566. M. Hermans, J. Gottmann, F. Riedel, Selective, laser-induced etching of fused silica at high scan-speeds using KOH, J. Laser Micro/Nanoeng. 9 (2014) 126–131.
9
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Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
Optimization of ultrafast laser parameters for 3D micromachining of fused silica ⁎
Yusuf Dogana,b, , Christi K. Madsena a b
Electrical and Computer Engineering, Texas A&M University, College Station, TX, USA Electrical and Electronic Engineering, Gumushane University, Gumushane, Turkey
H I GH L IG H T S
laser parameters after etching were found for high quality optical devices. • Optimal of 21.8 nm for 1 mm area in silica was achieved. • A1.25smoothness m/s scan rate for a system operating with a 2 MHz repetition rate was proved. • Complex writing involving surfaces at different orientations was simplified. • 2
A R T I C LE I N FO
A B S T R A C T
Keywords: Ultrafast laser Direct writing Chemical etching Selectivity Roughness 3D microfabrication
We present an optimization study on laser parameters for 3D micromachining of fused silica to achieve critical goals for practical applications including high surface quality, high volume production, and complex surfaces by ultrafast laser direct writing assisted chemical etching. We conducted experiments on laser pulse width of 300 fs and 1 ps, pulse energy ranging from 0.1 µJ to 1.6 µJ, three different polarizations (circular, parallel and perpendicular) and number of overlapped pulses from 3 to 10,000 at 1030 nm with up to 2 MHz repetition rate to investigate their effect on nanogratings and one dimensional (1D) channel and two dimensional (2D) planar surface selective etching on 1 mm thick fused silica. In one configuration, we achieved 21.8 nm RMS surface roughness with 80 µm Gaussian filtering and in another configuration, we estimated the maximum writing speed to be 1.25 m/s for given 2 MHz repetition rate with less than 400 nm filtered root mean square (RMS) surface roughness at a 1 mm2 area which covers the thickness of the glass.
1. Introduction Direct writing in transparent materials by femtosecond laser is a promising technique enabling complex three-dimensional precise micro fabrication. This technique, known also as femtosecond laser micromachining, can be summarized as non-linear modification in glass and it has been a center of interest and studied by many regarding nanograting formation [1,2] and refractive index change[3,4]. It is proved to be ideal for rapid and easy production of passive [5–7] and active micro devices [8–11] with some other applications [12–15]. The greatest advantage of the direct writing technique over the conventional methods of glass cutting such as diamond cutting [16], score and break cutting [16] or the other non-conventional methods such as hot air jet cutting [17] and waterjet cutting[18] is that it offers the possibility of directly writing complex 3D structures while the other methods only allow for planar geometries. ⁎
Ultrafast laser irradiation followed by chemical etching (Fig. 1) in glass is a popular, simple and maskless process for fabricating 3D structures with a great potential for glass micromachining [13,19–22]. Laser-assisted wet etching is a selective method since the laser affected zone is selectively etched while the rest of the sample is barely etched. Moreover; it enables high selectivity with an aspect ratio of 200:1 for microfluidic channels [23], a controllable etch rate [24] on both planar and non-planar substrates, high contrast nanograting [25], easeful processing environment [24]. There are two well-known etchants, and one is HF and the other one is KOH. While HF acid has high etching rate and low selectivity, KOH solution has low etching rate, high selectivity and it is less toxic and easier to handle regarding safety perspective [25–27]. Selectivity, the ratio of etching rates of modified region vs non-modified region, is the key factor to understand the etching efficiency. High selectivity and surface quality are strongly related to the arrangement of nanogratings
Corresponding author at: Electrical and Electronic Engineering, Gumushane University, Gumushane, Turkey. E-mail address: [email protected] (Y. Dogan).
https://doi.org/10.1016/j.optlastec.2019.105933 Received 21 June 2019; Received in revised form 23 September 2019; Accepted 26 October 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Yusuf Dogan and Christi K. Madsen, Optics and Laser Technology, https://doi.org/10.1016/j.optlastec.2019.105933
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Fig. 1. Femtosecond laser irradiation followed by chemical etching (FLICE).
for cross sectional analysis. The diced surface is then finely polished by using lapping machine. The polished sample is placed into 0.5% HF for 2 min, afterwards to get better imaging of nanograting regions and investigation of the modified zone under scanning electron microscope (SEM) as flash etching clears the modified regions and allows for high contrast images. 2 nm of Platinum coating is used to eliminate the charging effects of SEM. Periodic nanograting formation perpendicular to the writing direction is observed as it is shown in Fig. 3.
that form within the inscribed region for a specific window of parameters. This study investigates the influence of ultrafast laser parameters on nanogratings, 1D and 2D etching selectivity aiming to accomplish the optimum surface quality, the highest laser manufacturing speed and complex writings. Laser parameters of pulse width, pulse energy, polarization and number of overlapped pulses were tested. 1D etching has been addressed in some previous studies [20,27–30], however they do not offer an overall study of the laser parameter tradeoffs or a detailed analysis of the 2D etching selectivity. Recently, 2D etching has been studied by Ross et al. [31], which mainly focuses on selectivity and has no data for surface quality measurements. In our study, parameters were evaluated in combinations and many possibilities were screened. We found that minimum surface roughness high volume production or complex surfaces can be achieved depending on the choice of three different sets of laser parameters. These achievements can lead to great improvements in the performance of optical components and applications like glass cutting, engraving and building 3D structures [32].
2.1. Laser parameters effect on nanogratings The laser parameters such as polarization, writing speed or corresponding number of overlapped pulses and pulse energy have a great impact on nanograting formation which is the key factor for the 1D and 2D etching process [26,33]. As indicated in several studies, etching rate is not independent from nanograting directions.[26,34] The difference in the directions of nanogratings in the perpendicular and parallel polarization according to the writing direction is clearly visible in Fig. 4(b) and (c). Polarization and nanogratings are perpendicular to each other. While the perpendicular polarization has nanogratings towards the image plane, the parallel polarization has parallel nanogratings to the image plane. Fig. 4(b) and (c) give us a clue to make the interpretation that the direction of nanogratings for circular polarization should be a combination of the nanogratings formed by other linear polarized beams. One of the key parameters for nanograting formation is the writing speed. Considering the limitation of nanopositioning stage to 200 mm/s of maximum writing speed and the required time and distance for acceleration, the easy and accurate adjustment of pulse repetition rate leads us to understand the writing speed effect on nanogratings via the number of overlapped pulses, which decreases if the writing speed increases for a given pulse repetition rate. The writing speed and PRR are adjusted to control the desired N for the experiments. The number of overlapped pulses can be calculated with the equation below:
2. Experimental setup & procedures High energy femtosecond laser machining system includes Uranus (PolarOnyx) and Satsuma (Amplitude Systems) ytterbium-doped fiber lasers, Aerotech high precision positioning stages and automation control system. The laser system generates from 300 fs to 4 ps pulses at 1030 nm with up to 2 MHz repetition rate and delivers maximum 20 µJ pulse energy with beam propagation ratio of < 1.2. The beam is focused by high power micro focusing objective (Thorlabs 0.4NA, 20×). The process is controlled by nano positioning stages on all three axes. The schematic of the ultrafast laser micromachining system is represented in Fig. 2. In the experimental configuration, an isolator (Electro-Optics Technology Inc.) is used to minimize the back reflection to the laser, and the polarization before and after the acoustic modulator is controlled with Thorlabs wave plates (WP). Gooch & Housego acousto-optic modulator (AOM) is integrated to control the power of the beam by adjusting the intensity of the beam orders. The delay generator (Highland Technology Inc.) helps changing the pulse repetition rate, and Altechna tunable beam expander alters the beam diameter. This laser system enables us a broader perspective and ability to optimize tests by manipulating laser parameters with a great variety of choices. The focal spot diameter (1/e2) is theoretically estimated around 1.9 µm under the given condition. Direct writing experiments are performed 300 μm beneath the top surface of 1 mm thick 1 in. square Corning 7980 fused silica plate with 50 μm separated varying parameters of raster scans. The laser modified sample is diced through the perpendicular direction to writing direction
N = PRR ∗ d/v where N is number of overlapped pulses, PRR is pulse repetition rate, d is spot size diameter and v is writing speed. An increase in N causes a greater number of nanogratings and larger lateral and vertical modified spot sizes while it lessens the period of the nanogratings as seen in Fig. 5. The deformations through the left side of nanogratings result from the lapping process. The affected zone dimensions corresponding to the maximum values are measured under SEM, and the average grating period is calculated by dividing the affected zone length to the number of nanogratings. When N increases, the number of nanogratings is increased from a few nanogratings to 2
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Fig. 2. Schematic of fs laser system (Uranus).
Fig. 7. When the pulse energy is changed from 0.2 µJ to 0.8 µJ, the number of nanograting increases roughly from 5 to 15 while the nanograting period stays almost the same. Shimotsuma argues that the period is changed from 180 nm to 320 nm when the laser pulse energy is increased from 1 µJ to 2.8 µJ [36]. This data do not match our experiments. On the other hand, according to Sun, the nanograting period keeps constant with the change in laser pulse energy from 170 nJ to 870 nJ, which has a similar trend with our results [41]. Bhardwaj et al, likewise, states that the spacing is independent of pulse energy [2]. All of these differences in nanogratings such as directions of gratings, spacing of grating, number of gratings and void formation has great influence on selective etching in 1D and 2D plane. The etching effect will be studied in the following chapters.
around 20 nanogratings and the period of nanogratings is decreased from around 500 nm to nearly 150 nm as shown in Fig. 6. Nanograting period is expected to be around 355 nm (??/2n) [35], which stays in the range of our above mentioned findings. According to Shimotsuma, the nanogratings period is decreased from 240 nm to 140 nm as the laser pulse repetition rate is increased from 50 kHz to 800 kHz, which closely matches our results [36]. We should note that the change in these values is not linear and it is more significant when N is smaller than 1000. The size of the modified region at beam spot is related to N as well. The lateral and vertical dimensions of the modified zone stay nearly the same once N is greater than 333, since the affected zone reaches the beam waist diameter, and laser pulse fluence decrease dramatically as the area gets larger. The refractive index change regime is not seen in our experiments as laser pulse duration is greater than 300 fs which is measured by Mini TPA (APE) autocorrelator and this regime is accessible with the pulse duration smaller than 200 fs [35]. Nanograting and micro explosion regimes are the main focus in our study. Pulse energy has a crucial effect on laser irradiation regimes. The formation of the nanogratings are observed beyond the threshold pulse energy [35,37,38]. The minimum peak power intensity for nanograting formation is estimated around 7E+12 W/cm2 which closely matches with some other studies [39,40]. The number of nanogratings increases and the axial and lateral dimensions of modified region gets larger as the pulse energy gets higher [35]. Micro explosions occur when the pulse energy is higher than it is required for nanograting formation. When the peak power intensity exceeds 3E+13 W/cm2, void generation is started [35,38]. The modification of nanograting formation and voids can be seen in
2.2. Laser parameters effect on selective etching Our first goal was to optimize all the laser parameters to achieve the highest selectivity for 2D plane etching to demonstrate the effect of selectivity on the surface quality. It was expected for 2D plane etching that low selectivity would generate a form with high spatial wavelength leading high surface roughness on the surface. as the non-modified region gets etched more due to longer etching time similar to 1D channel etching, where at low selectivity the channel has angled side walls with high cone angle [22]. It was essential to make some selectivity studies on 1D channel etching before for a better insight and an opportunity to make comparisons. Direct writing methods were applied on a silica plate with 50 µm spaced raster scanned lines 300 µm beneath
Fig. 3. Cross section SEM images: perpendicular polarization with 0.4 μJ, 300 fs and N = 3333 (a) at low magnification (8 K zoom & 1 μm scale) and (b) at high magnification (40 K zoom & 100 nm scale). E, S and k represent the polarization plane, the scanning direction and the light propagation direction, respectively. 3
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Fig. 4. Cross section SEM images corresponding to (a) circular, (b) perpendicular and (c) parallel polarization with 300 fs, 0.8 µJ and N = 3333.
power enable the grating growth more uniformly while higher peak powers may introduce localized degradations to the grating structure. Setting the pulse duration to 1000 fs, the same experiment was conducted under a variety of polarization conditions. In all three polarizations the selectivity reached up to 500 in some given conditions as seen in Fig. 10. In parallel polarization, at the low writing speed or at high overlapped pulses, the channel did not etch nor give any selectivity. The selectivity at circular and perpendicular polarizations were quite similar, except that perpendicular polarization gave a much wider range of both writing speed and pulse energy. In overall, there are many sets of conditions that have high selectivity and could be a good candidate for 2D plane surface etching studies.
the top surface under the same conditions. Later, the silica plate was diced with a precise dicing saw machine. Thus, we obtained many identical irradiated silica pieces for etching selectivity studies. 5–55% (w/w) KOH solution at 85 °C with 75 rpm stirring speed was chosen to be used for our optimization experiments. KOH was preferred over HF for its advantages like higher selectivity, less toxicity and easier handling. 1D channel etching of the modified and non-modified region was measured in an hourly fashion under microscope. As the concentration of solution increased, the etch rates of bare silica increased almost linearly through 45% KOH solution, and the solution got saturated above that percentage. However, as seen in Fig. 8, it was observed that the selectivity decreased when the etchant went deeper through the modified channel, since the penetration of the etchant and diffusion of by-products got harder. This challenge had been addressed by some studies of one-dimensional channel etching as the channel dimensions vary from the inlet to inside of the material. In our study, the etched length of the channels was analysed by optical contrast measurements and the best selectivity over time was observed with 5% KOH solution, which was accordingly chosen to be used in the further experiments. Pulse width effect on selective etching for 1D channel was studied by testing various conditions and combinations of pulse width, pulse energy and overlapped number of pulses. It was found that the selectivity of the modified channel with pulse width of 1000 fs was much higher than 300 fs pulse width as shown in Fig. 9. As it is claimed by Herman, the pulse duration over 800 fs gives more stable nanograting formation [27]. The laser pulse of 1000 fs yielded higher selectivity with a broader pulse energy and overlapped number of pulses range. The reason could be that the longer pulse duration and lower peak
3. 2D plane etching, optimization and results Following the 1D channel optimization, the 2D plane etching optimization was pursued for analyzing the surface quality further. Etching in 2D plane refers that the laser modified region is not a line but a modified area, namely a combination of 1D modified channels. The writing started at the bottom of the sample and moved up step by step with z increments. To understand the 2D writings and etching, simple rectangular shapes (1 mm × 10 mm) were written on 1 mm thick Corning 7980 high purity fused silica with various laser parameters to analyze the surface quality and compare the parameters as shown in Fig. 11. The samples were cleaned carefully with deionized (DI) water for 10 min, methanol for 10 min, isopropyl alcohol (IPA) for 10 min and DI water, respectively, prior to initiating the examination process. To evaluate the scanned surface shape and quality, white light
Fig. 5. Effect of the number of overlapped pulses (a) N = 3333, (b) N = 1000, (c) N = 333, (d) N = 100, (e) N = 40 for perpendicularly polarized light with 300 fs and 0.8 µJ. 4
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Fig. 6. Effect of (a) N to nanograting numbers and periods, and (b) the lateral and vertical dimensions of laser modified region for given N.
and from 16 to 1000 kHz, respectively, to adjust the corresponding N. The z increment is adjusted to 5 μm in air which corresponded to 7.5 μm depth in silica plate, and the depth of focus is theoretically estimated around 6 μm. The pulse energy and z increment were studied after optimizing the polarization and number of overlapped pulses. As shown in Fig. 13, in parallel polarization, the minimum surface roughness was achieved at N = 40. While N < 100 or high writing speed gives relatively acceptable surface quality, the surface gets worse when N > 100, as the writing speed is decreasing. The surface does not etch wholly and the samples could not be released as a result when N > 333 as the selectivity is so low that the etchant does not reach to the center of the sample and not release. Interpreted from the 1D selectivity experiment, no selectivity is observed at 0.4 μJ after N > 100 in Fig. 10a, and the 2D experimental data give a great match to 1D experiments. The circular polarization has a similar trend with parallel polarization. The best surface quality is observed at N = 40 while the worst one is observed at N = 10000 which corresponds to the lowest writing speed. As the micro roughness stays between 300 nm and 500 nm, the form effect can be seen at lower speeds as the total roughness gets higher. In perpendicular polarization, the surface gets better when N increases, unlikely to the other two polarizations. The surface roughness change gets minimal for N > 1000. According to the RMS roughness data, the best surface was achieved at N = 3333. Evidently, the beam perpendicularly polarized to writing direction resulted in the best surface quality in general, especially with high number of overlapped pulses. While the minimum surface roughness for high speed is observed at parallel polarization, for low speed the perpendicular polarization gives better surface quality. For the parallel
interferometry (WLI) images taken by Profilm3D (Filmetrics) which have an area a little bigger than 1 mm2 covering the thickness of the silica plate were used. Surface texture is composed of various ranges of spatial wavelength which are typically referred to as micro roughness, waviness and form. Form has higher spatial wavelengths while micro roughness has lower spatial wavelengths and waviness has wavelengths in between [42]. To distinguish the micro roughness values from the surface texture, Gaussian low pass filtering is one of the well-known and practical solutions that form and waviness are substantially filtered out. For nearly 1 mm2 scanned surface area with 512x512 sampling, 80 µm cut off wavelength is chosen to distinguish the micro roughness from the form and waviness according to the recommended cut off wavelengths by ISO 4288-1996. We demonstrated and compared both total surface roughness without any filtering (Sq) and micro roughness (Sq_gf) data to understand the micro roughness and form of the profiles. As with the 1D selectivity experiments, polarization is one of the key factors effective on surface quality. Some of the optical profile images obtained from different polarization applied surfaces are shown in Fig. 12. In general, perpendicular polarization provides the smallest roughness, while parallel polarization gives the highest roughness due to having form on the surface. Circular polarization, on the other hand, yields the highest micro roughness. The polarization effect with various conditions will be presented later. The polarization and writing speed optimization experiment was conducted with pulse energy of 0.4 μJ as it has the best selectivity for all three polarizations with a wide range of N from 3 to 10,000 as seen in Fig. 10. The writing speed and PRR are changed from 0.2 to 10 mm/s
Fig. 7. SEM images of modified spot with various pulse energies (a) 0.2 µJ, (b) 0.4 µJ, (c) 0.8 µJ and (d) 1.6 µJ by perpendicularly polarized beam which has a pulse width of 1000 fs with 1000 overlapped number of pulses. 5
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Fig. 8. (a) Microscope image of one-dimensional etched channel and (b) normalized 1D selectivity over time with various KOH etchants.
Fig. 9. Pulse width effect at various conditions on 1D channel selectivity at perpendicular polarization with various writing speeds (0.1–10 mm/s) and PRR (16–500 kHz) for (a) 300 fs and (b) 1000 fs.
Fig. 10. Polarization effect on selectivity under various channel writing conditions; (a) parallel, (b) perpendicular and (c) circular polarization at 1000 fs.
with z increments and pulse energy is presented with both RMS roughness data in Fig. 14. Z stage increments have an undeniable impact on the surface quality. 2.5, 5, 10 and 15 μm increments in air were applied to the surfaces with different pulse energies, and surface RMS roughness and Gaussian filtered roughness were analyzed. Lowering the z increment normally reduces the high surface roughness; however, it could deform the surface if the increment size reduces more.
polarization, the 2D plane etching test matches 1D selectivity data in Fig. 10, since both data show that no etching occurs at low speed direct writing. For other two polarizations, while number of overlapped pulses or writing speed have nearly the same selectivity at 1D selectivity test, a clear difference is observed in 2D plane etching studies. The z increment and pulse energy are strongly related with each other and they should be analyzed together. Surface quality change 6
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Fig. 11. Laser irradiation and released material from 1 mm fused silica substrate after chemical etching.
Fig. 12. White light interferometry images and profiles of the surfaces which are written at 1 mm/s, 0.4 μJ, 1000 fs, N = 1000 and z increment of 10 μm in air with (a) parallel, (b) perpendicular and (c) circular polarizations.
Fig. 13. Surface roughness over overlapped number of pulses with 3 different polarizations: (a) parallel, (b) perpendicular and (c) circular at 0.4 μJ, 1000 fs with z increment of 5 μm in air.
4. Conclusion
We determined three configurations to optimize the scanned surface for different purposes: high manufacturing speed, complex writing and high surface quality. The laser parameters should be optimized considering the requirements the application like speed, complex writing and surface quality. Therefore, configurations can be adjusted accordingly as shown in Table 1.
Ultrafast laser parameters are evaluated for surface quality and speed by conducting successive tests for polarization, pulse width, pulse energy and number of overlapped pulses. The effect of these parameters on nanogratings and 1D and 2D selective etching is studied. Gaussian filtered surface roughness of 21.8 nm for 1 mm2 area is demonstrated for the cases where surface quality is the priority. This is the lowest 7
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Fig. 14. The laser writing conditions for best surface quality. Table 1 Some of the optimized configurations for the application needs for high manufacturing speed, complex writing and high surface quality. Parameters
Configuration 1 (Speed)
Pulse Width Beam Polarization Theoretical Spot Size (1/e2) Pulse Energy PRR Writing Speed (v) Number of Pulses N Writing Speed at 2 MHz Z inc. in Air Etching Specs Surface RMS Sq. (1 × 1 mm2)
1000 fs Parallel 1.9 μm 0.4 μJ 16 kHz 10 mm/s 3.2 1.25 m/s 5 μm 12–24 hr at 5% KOH, 85 °C with 75 rpm Sq = 477 nm Sq_gf = 395 nm
Configuration 2 (Complex Writing)
Configuration 3 (Surface Quality)
Circular
Perpendicular
200 kHz 10 mm/s 40 100 mm/s
1000 kHz 0.6 mm/s 3333 1.2 mm/s
Sq = 388 nm Sq_gf = 341 nm
Sq = 63.4 nm Sq_gf = 21.8 nm
Surface 3D Profiles (1 × 1mm2)
Surface 2D Profiles (nm)
surface roughness for femtosecond laser processing at the given area in the literature so far. A maximum writing speed of 1.25 m/s is estimated at 2 MHz repetition rate with about three overlapped pulses and 395 nm Gaussian filtered RMS surface roughness, which is very promising for high-volume production. Gaussian filtered RMS surface roughness of 341 nm is reached at 100 mm/s writing speed in circular polarization, which can be utilized if complex writing is targeted without needing to change the polarization. Overall, these optimized parameters can guide the work in evaluating practical fabrication tradeoffs.
Phys. Rev. Lett. 96 (2006) 057404. [3] V.R. Bhardwaj, E. Simova, P.B. Corkum, D.M. Rayner, C. Hnatovsky, R.S. Taylor, B. Schreder, M. Kluge, J. Zimmer, Femtosecond laser-induced refractive index modification in multicomponent glasses, J. Appl. Phys. 97 (2005) 083102. [4] M. Lancry, B. Poumellec, A. Chahid-Erraji, M. Beresna, P.G. Kazansky, Dependence of the femtosecond laser refractive index change thresholds on the chemical composition of doped-silica glasses, Opt. Mater. Express 1 (2011) 711–723. [5] K. Miura, J. Qiu, H. Inouye, T. Mitsuyu, K. Hirao, Photowritten optical waveguides in various glasses with ultrashort pulse laser, Appl. Phys. Lett. 71 (1997) 3329–3331. [6] K. Hirao, K. Miura, Writing waveguides and gratings in silica and related materials by a femtosecond laser, J. Non-Cryst. Solids 239 (1998) 91–95. [7] R.R. Thomson, R.J. Harris, T.A. Birks, G. Brown, J. Allington-Smith, J. BlandHawthorn, Ultrafast laser inscription of a 121-waveguide fan-out for astrophotonics, Opt. Lett. 37 (2012) 2331–2333. [8] Y. Sikorski, A.A. Said, P. Bado, R. Maynard, C. Florea, K.A. Winick, Optical waveguide amplifier in Nd-doped glass written with near-IR femtosecond laser pulses, Electron. Lett. 36 (2000) 226–227. [9] A. Ródenas, L.M. Maestro, M.O. Ramírez, G.A. Torchia, L. Roso, F. Chen, D. Jaque, Anisotropic lattice changes in femtosecond laser inscribed Nd3+:MgO:LiNbO3 optical waveguides, J. Appl. Phys. 106 (2009) 013110. [10] Y. Ren, G. Brown, A. Ródenas, S. Beecher, F. Chen, A.K. Kar, Mid-infrared waveguide lasers in rare-earth-doped YAG, Opt. Lett. 37 (2012) 3339–3341. [11] W. Yang, P.G. Kazansky, Y.P. Svirko, Non-reciprocal ultrafast laser writing, Nat. Photon. 2 (2008) 99. [12] R.S. Taylor, C. Hnatovsky, E. Simova, D.M. Rayner, V.R. Bhardwaj, P.B. Corkum,
Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.optlastec.2019.105933. References [1] F. Liang, R. Vallée, S.L. Chin, Mechanism of nanograting formation on the surface of fused silica, Opt. Express 20 (2012) 4389–4396. [2] V.R. Bhardwaj, E. Simova, P.P. Rajeev, C. Hnatovsky, R.S. Taylor, D.M. Rayner, P.B. Corkum, Optically produced arrays of planar nanostructures inside fused silica,
8
Optics and Laser Technology xxx (xxxx) xxxx
Y. Dogan and C.K. Madsen
[13]
[14] [15] [16] [17]
[18] [19]
[20]
[21] [22]
[23]
[24]
[25]
[26]
[27]
[28] R. Drevinskas, M. Gecevičius, M. Beresna, Y. Bellouard, P.G. Kazansky, Tailored surface birefringence by femtosecond laser assisted wet etching, Opt. Express 23 (2015) 1428–1437. [29] J. Gottmann, M. Hermans, N. Repiev, J. Ortmann, Selective laser-induced etching of 3D precision quartz glass components for microfluidic applications—up-scaling of complexity and speed, Micromachines 8 (2017) 110. [30] X. Yu, Y. Liao, F. He, B. Zeng, Y. Cheng, Z. Xu, K. Sugioka, K. Midorikawa, Tuning etch selectivity of fused silica irradiated by femtosecond laser pulses by controlling polarization of the writing pulses, J. Appl. Phys. 109 (2011) 053114. [31] C.A. Ross, D.G. MacLachlan, D. Choudhury, R.R. Thomson, Optimisation of ultrafast laser assisted etching in fused silica, Opt. Express 26 (2018) 24343–24356. [32] Y. Dogan, M. Morrison, C. Hu, R. Atkins, M. Solmaz, C.K. Madsen, Fabrication of advanced glass light pipes for solar concentrators, SPIE Optifab, SPIE2017, (2017) pp. 6. [33] S. LoTurco, R. Osellame, R. Ramponi, K.C. Vishnubhatla, Hybrid chemical etching of femtosecond laser irradiated structures for engineered microfluidic devices, J. Micromech. Microeng. 23 (2013) 085002. [34] C. Hnatovsky, R.S. Taylor, E. Simova, V.R. Bhardwaj, D.M. Rayner, P.B. Corkum, Polarization-selective etching in femtosecond laser-assisted microfluidic channel fabrication in fused silica, Opt. Lett. 30 (2005) 1867–1869. [35] R. Taylor, C. Hnatovsky, E. Simova, Applications of femtosecond laser induced selforganized planar nanocracks inside fused silica glass, Laser Photon. Rev. 2 (2008) 26–46. [36] Y. Shimotsuma, P.G. Kazansky, J. Qiu, K. Hirao, Self-organized nanogratings in glass irradiated by ultrashort light pulses, Phys. Rev. Lett. 91 (2003) 247405. [37] F. Liang, Q. Sun, D. Gingras, R. Vallée, S.L. Chin, The transition from smooth modification to nanograting in fused silica, Appl. Phys. Lett. 96 (2010) 101903. [38] C. Hnatovsky, R.S. Taylor, P.P. Rajeev, E. Simova, V.R. Bhardwaj, D.M. Rayner, P.B. Corkum, Pulse duration dependence of femtosecond-laser-fabricated nanogratings in fused silica, Appl. Phys. Lett. 87 (2005) 014104. [39] P. Wang, W. Chu, W. Li, Y. Tan, F. Liu, M. Wang, J. Qi, J. Lin, F. Zhang, Z. Wang, Y. Cheng, Three-dimensional laser printing of macro-scale glass objects at a microscale resolution, Micromachines 10 (2019) 565. [40] Y. Dai, A. Patel, J. Song, M. Beresna, P.G. Kazansky, Void-nanograting transition by ultrashort laser pulse irradiation in silica glass, Opt. Express 24 (2016) 19344–19353. [41] Q. Sun, F. Liang, R. Vallée, S.L. Chin, Nanograting formation on the surface of silica glass by scanning focused femtosecond laser pulses, Opt. Lett. 33 (2008) 2713–2715. [42] J. Raja, B. Muralikrishnan, S. Fu, Recent advances in separation of roughness, waviness and form, Precis. Eng. 26 (2002) 222–235.
Femtosecond laser fabrication of nanostructures in silica glass, Opt. Lett. 28 (2003) 1043–1045. Y. Bellouard, A. Said, M. Dugan, P. Bado, Fabrication of high-aspect ratio, microfluidic channels and tunnels using femtosecond laser pulses and chemical etching, Opt. Express 12 (2004) 2120–2129. K. Sugioka, Y. Cheng, Femtosecond laser three-dimensional micro- and nanofabrication, Appl. Phys. Rev. 1 (2014) 041303. D. Choudhury, J.R. Macdonald, A.K. Kar, Ultrafast laser inscription: perspectives on future integrated applications, Laser Photon. Rev. 8 (2014) 827–846. S. Nisar, L. Li, M.A. Sheikh, Laser glass cutting techniques—A review, J. Laser Appl. 25 (2013) 042010. E.S. Prakash, K. Sadashivappa, V. Joseph, M. Singaperumal, Nonconventional cutting of plate glass using hot air jet: experimental studies, Mechatronics 11 (2001) 595–615. A.A. Khan, M.M. Haque, Performance of different abrasive materials during abrasive water jet machining of glass, J. Mater. Process. Technol. 191 (2007) 404–407. A. Marcinkevičius, S. Juodkazis, M. Watanabe, M. Miwa, S. Matsuo, H. Misawa, J. Nishii, Femtosecond laser-assisted three-dimensional microfabrication in silica, Opt. Lett. 26 (2001) 277–279. C. Hnatovsky, R.S. Taylor, E. Simova, P.P. Rajeev, D.M. Rayner, V.R. Bhardwaj, P.B. Corkum, Fabrication of microchannels in glass using focused femtosecond laser radiation and selective chemical etching, Appl. Phys. A 84 (2006) 47–61. K. Sugioka, Y. Cheng, Ultrafast lasers—reliable tools for advanced materials processing, Light: Sci. Appl. 3 (2014) e149. K.C. Vishnubhatla, N. Bellini, R. Ramponi, G. Cerullo, R. Osellame, Shape control of microchannels fabricated in fused silica by femtosecond laser irradiation and chemical etching, Opt. Express 17 (2009) 8685–8695. F. He, Y. Liao, J. Lin, J. Song, L. Qiao, Y. Cheng, K. Sugioka, Femtosecond laser fabrication of monolithically integrated microfluidic sensors in glass, Sensors 14 (2014) 19402. F. Chen, H. Liu, Q. Yang, X. Wang, C. Hou, H. Bian, W. Liang, J. Si, X. Hou, Maskless fabrication of concave microlens arrays on silica glasses by a femtosecond-laserenhanced local wet etching method, Opt. Express 18 (2010) 20334–20343. R. Drevinskas, M. Gecevičius, M. Beresna, Y. Bellouard, P.G. Kazansky, Nanotexturing of glass surface by ultrafast laser assisted wet etching, CLEO: 2014, Optical Society of America, San Jose, California, 2014 pp. AM1L.2. S. Kiyama, S. Matsuo, S. Hashimoto, Y. Morihira, Examination of etching agent and etching mechanism on femotosecond laser microfabrication of channels inside vitreous silica substrates, J. Phys. Chem. C 113 (2009) 11560–11566. M. Hermans, J. Gottmann, F. Riedel, Selective, laser-induced etching of fused silica at high scan-speeds using KOH, J. Laser Micro/Nanoeng. 9 (2014) 126–131.
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