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Abstract
This paper investigated the effects of femtosecond laser beam polarization on ablation efficiency and microstructure symmetricity for 64FeNi alloy (Invar) sheet processing to fabricate fine metal masks. It was found that the ablation efficiency for linear polarization was approximately 15% higher than that for circular polarization due to electric field enhancement induced by low-spatial-frequency laser-induced periodic surface structures (LIPSS). The hole size and sidewall taper angles for the microstructures generated by linear polarization were asymmetric, whereas those generated by circular polarization were symmetric due to non-oriented LIPSS. The asymmetric and symmetric three-dimensional microstructure profiles, measured by using a confocal laser scanning microscope, were verified by employing an analytical model that was derived using the total input fluence and the ablation rates for linear and circular polarizations, respectively.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Femtosecond (fs) laser-matter interactions have been extensively studied for many potential industrial applications, such as for producing fine metal masks (FMMs) [1], for the fabrication of colored [2,3] and superhydrophobic surfaces [4,5]. Laser parameters, such as fluence, pulse repetition rate, and the number of pulses should be appropriately controlled to realize thermal damage-free [6] and functional surfaces [2–5]. Processing methods for fabricating high precision FMMs were developed by suppressing surface thermal damage [6] and nanoparticle generation [7].
Laser processing results can also be affected by polarization. Multiphoton ionization in dielectrics depends on polarization [8]. Electric field for a tight focusing spot at a focal point can be enhanced along the polarization direction [9]. Hot electrons in plasma generated by laser ablation can be coupled to pulse electric field [10]. These polarization effects can induce asymmetric laser ablation.
The gratings on material surfaces can affect laser-matter interactions [11]. Orientation for laser-induced periodic surface structures (LIPSS) is determined by the polarization direction [12]. Low-spatial-frequency LIPSS (LSFL) on metal surfaces can act as gratings to diffract light. Thus, various color markings can be obtained by rotating the polarization direction [13,14]. The ablation line width produced through laser scanning can also be modulated by rotating polarization [15]. The orientation of LSFL is perpendicular to the polarization direction [12], and the line width lengthens along the direction perpendicular to LSFL due to electric field enhancement [16]. Thus, the line width attains the maximum value when the polarization direction is perpendicular to the scanning direction [15].
This study investigated the effects of polarization on ablation efficiency and microstructure symmetricity. It was revealed that the ablation efficiency for linear polarization was approximately 15% higher than that for circular polarization due to electric field enhancement induced by LSFL [16], i.e., grating effects. Asymmetric microstructures were fabricated by linear polarization due to improvements in ablation rate along the polarization direction, whereas symmetric results were obtained by circular polarization because the LSFL had no specific orientation, i.e., dot-like patterns were exhibited [6,17]. The three-dimensional microstructure profiles (lateral and depth positions) could be estimated by employing an analytical model.
2. Experimental setup
An infrared fs laser (Monaco, Coherent), with an average pulse duration of 300 fs and maximum output power of 60 W at 1035 nm wavelength, was used for experiments. Thin Invar sheets (20–50 µm) were fixed on an electrostatic chuck mounted on an XYZ translation stage. Gaussian laser beams were directed and focused onto the targets by a galvanometer scanner (hurrySCAN 14, Scanlab) and f-theta lens with a focal length of 100 mm. A Liu plot [18] was used for estimating the 1/e2 Gaussian beam radius (ω0). The peak fluence was calculated by the formula F0 = 2Ep / (πω02), where Ep is the pulse energy. The original linear polarization beam was converted to circular polarization using a quarter-wave plate. The ablation depth and volume were measured using a confocal laser (404 nm wavelength) scanning microscope (VK-X1100, Keyence) to estimate the ablation rate and efficiency, respectively. The surface morphologies were inspected through scanning electron microscopy (SEM).
3. Results and discussions
3.1 Ablation efficiency
The effects of polarization on ablation efficiency were investigated by measuring the 500 × 500 µm2 crater depth (z). Invar sheets (50 µm) were ablated in the low-fluence regime (F0 < 1.5 J/cm2), with the repetition rate of frep = 200 kHz to avoid a high effective penetration depth and heat accumulation, leading to burr formation [6]. Figure 1(a) shows the scanning details to fabricate craters and the image of an optical microscope. A square layer was scanned by multiple lines (arrows), with the hatch spacing dh and the number of repetitions Nscan. Each hatch line comprised many overlapping laser beams separated by the pulse spacing dp = v/frep, where v is the scanning speed. Table 1 lists all the relevant processing parameters. Figure 1(b) depicts a three-dimensional profile of the crater, whose depth was obtained from the mean value of six cross sections (Fig. 1(b), dotted lines). Figure 1(c) shows a depth of ≈ 25.5 µm measured along the crater’s cross section, fabricated at F0 = 0.36 J/cm2 and Nscan = 20 passes, through circular polarization.
Fig. 1. (a) Optical microscope image (b) Three-dimensional profile inspected using a confocal laser scanning microscope for a typical 500 × 500 µm2 crater fabricated by the scanning process through circular polarization. (c) Crater depth (≈ 25.5 µm) measured along the dotted line in (b). The fluence, repetition rate, scanning speed, hatch spacing, and the number of repetitions for fabricating craters were F0 = 0.36 J/cm2, frep = 200 kHz, v = 0.2 m/s, dh = 5 µm, and Nscan = 20, respectively. The pulse spacing was dp = v/frep = 1 µm. The circles and arrows in (a) represent the laser beams and hatch lines, respectively.
Table 1. Laser processing parameters for fabricating craters and micro-holes obtained through the scanning process.a
Figure 2 shows z formed by circular polarization with respect to Nscan. The linear slopes for each F0 indicate the linearity of the ablation depth in the fluence range. Figure 3(a) shows the volume ablation rate dV (ablated volume per pulse) calculated by following the procedure outlined by [19]
Figure 3(b) shows the ablation efficiency (volume ablation rate dV per pulse energy applied dEp) calculated using the formula [19]
Fig. 2. Number of repetitions (Nscan) and its relationship with the crater depth (z) with respect to fluence F0 = (a) 0.24–0.60 J/cm2 and (b) 0.66–1.15 J/cm2. Each dotted line, for different F0 values, indicates a linear trend. The craters, to measure the depth, were fabricated by circular polarization.
Fig. 3. Relationship between fluence (F0) and (a) volume ablation rate and (b) ablation efficiency with respect to the polarization (pol.) directions. The volume ablation rate and ablation efficiency for circular polarization were lower than those for linear polarization.
The ablation efficiency depended on polarization. The maximum ablation efficiency at F0 = 0.78 J/cm2 for linear and circular polarization were ≈ 0.31 µm3/µJ and 0.27 µm3/µJ, respectively. The improvement in ablation efficiency (0.04 / 0.27 × 100 ≈ 15%) by linear polarization could be explained by electric field enhancement due to LSFL [16]. The well-aligned surface ripples formed initially by linear polarization, i.e., LSFL with an orientation perpendicular to the polarization direction, can enhance incident light (electric field) with an orientation perpendicular to the ripple direction, resulting in asymmetric ablation [15,16]. The scanning direction (Fig. 1(a)) is parallel (perpendicular) to the horizontal (vertical) linear polarization direction. Thus, Fig. 3 demonstrates that the scanning direction with respect to the linear polarization directions did not influence ablation efficiency.
We also estimated the ablation efficiency using the ablation volume (V) for the craters fabricated by stationary irradiation to confirm the influence of polarization on the ablation efficiency. Figure 4(a) shows the linear relationship between V and the number of stationary pulses (Nsta). Figures 4(b) and (c) represent typical three-dimensional images and cross-sectional profiles measured along the crater centers fabricated at F0 = 0.78 J/cm2 and Nsta = 180 shots, with vertical linear and circular polarization, respectively. The oval craters were fabricated by linear polarization, whereas the round craters were fabricated by circular polarization. The oval major and minor axis lengths were about 44 µm and 40 µm (Fig. 4(b)), respectively, and the circular crater diameter was 40 µm (Fig. 4(c)). The ablation depth (L) and volume for linear polarization (L ≈ 7.1 µm and V ≈ 3440 µm3) were greater than those for circular polarization (L ≈ 6.3 µm and V ≈ 3040 µm3).
Fig. 4. (a) Relationship between the number of stationary pulses (Nsta) and the ablated volume (V) with respect to fluence (F0). The craters fabricated by stationary irradiation with circular polarization (pol.) were measured by a confocal laser scanning microscope. (b) and (c) show the three-dimensional morphologies and depth profiles measured along the dotted lines for the craters fabricated at F0 = 0.78 J/cm2 and Nsta = 180 with vertical linear polarization and circular polarization, respectively. (d) Relationship between F0 and ablation efficiency with respect to the polarization directions and processing methods.
Figure 4(d) shows the ablation efficiency for stationary irradiation calculated using the formula
The maximum ablation efficiency with stationary irradiation at F0 = 0.78 J/cm2 for linear and circular polarization were ≈ 0.30 µm3/µJ and 0.26 µm3/µJ, respectively. Thus, an increase in ablation efficiency by linear polarization (0.04 / 0.26 × 100 ≈ 15%) was confirmed by stationary irradiation.
The ablation efficiency for scanning (ηscan) was greater than that for stationary irradiation (ηsta) (Fig. 4(d)). The maximum increase for scanning was ≈ 8% ((ηscan - ηsta) / ηsta × 100), which may be explained by the ablated area geometry [20]. The crater sidewalls formed by stationary multi-pulses can reflect part of the energy of the following laser pulses. Additionally, the interaction area for stationary irradiation is larger than that for scanning due to the closed sidewall structures. Thus, effective absorbed energy density (pulse energy/area) for stationary processing can be smaller than that for scanning.
Figure 5 shows the SEM images for the craters fabricated by stationary irradiation at F0 = 0.78 J/cm2 and Nsta = 35 shots with different polarization. Figure 5(a) shows a crater with high roundness irradiated by circular polarization, with the round crater diameter of about 35 µm. The difference in crater size between Nsta = 35 and 180 shots (Fig. 4(c), ≈ 40 µm) is due to incubation [7]. Figure 5(d) shows dot-like nanoscale patterns formed by circular polarization because it has no preferential electric field directions [17]. Figures 5(b) and (c) show oval craters irradiated by horizontal and vertical linear polarization, respectively. The craters lengthened along the orientation perpendicular (parallel) to the ripple (polarization) direction (Figs. 5(e) and (f)). The oval major and minor axis lengths were about 41 µm and 35 µm, respectively.
Fig. 5. SEM images for craters fabricated by stationary irradiation with (a) circular, (b) horizontal linear, and (c) vertical linear polarization. (d)–(f) Enlarged surface morphologies from the marked squares in (a)–(c), respectively. The double-headed arrows in (b) and (c) indicate the horizontal and vertical polarization directions, respectively. The fluence and number of pulses for stationary irradiation were fixed at F0 = 0.78 J/cm2 and Nsta = 35 shots, respectively.
We compared crater symmetricity generated by linear polarization with two different frep. Figures 6(a) and (b) show the SEM images for the craters fabricated by horizontal linear polarization at F0 = 0.27 J/cm2 and Nsta = 20 shots. Figure 6(a) shows a typical surface morphology for the craters covered with LIPSS fabricated at the thermal damage free processing regime with frep = 500 kHz [6]. The marked area by the dashed rectangle represents protrusions formed on the crater periphery, which can demonstrate electric field enhancement along the direction perpendicular to LSFL [16,21]. The protrusions were not observed on the craters fabricated at the higher F0 = 0.78 J/cm2 (Figs. 5(b) and (c)) because the protrusion vicinities were ablated by the high fluence. Figure 6(b) shows a typical surface morphology for the craters without LIPSS formed at frep = 50 MHz. Prominent melting layers on the surface were formed at the heat accumulation regime with the high frep [6], which induced the grating free surfaces. Although the two craters were fabricated by horizontal linear polarization with the same laser parameters, only the crater with the surface gratings lengthened along the orientation perpendicular to the gratings (Fig. 6(a)), whereas the shape of the crater without the surface gratings was symmetric even with irradiating linear polarization pulses (Fig. 6(b)). Thus, these results can confirm the effect of the grating induced electric field enhancement on microstructure symmetricity.
Fig. 6. SEM images for craters fabricated by stationary irradiation with horizontal linear polarization at the repetition rates frep = (a) 500 kHz and (b) 50 MHz. The double-headed arrows in (a) and (b) indicate the horizontal linear polarization. The dashed rectangular area in (a) indicates protrusions generated on the crater edge. The fluence, number of pulses were fixed at F0 = 0.27 J/cm2 and Nsta = 20 shots, respectively. Noticeable surface melting and resolidification layers were observed on the crater rims fabricated at frep = 50 MHz (heat accumulation regime), as shown in (b).
3.2 Microstructure symmetricity
Figure 7 shows illustrations of symmetrical marking objects (square, diamond, and circle) for fabricating typical FMM patterns and the laser processing results. When the laser scanning length = l (Fig. 7(a)), the micro-hole entrance lengths are wx, y ≈ l + 2ω0 + Δx, y (Fig. 7(b)), where Δx, y are the ablation lengths that increase due to electric field enhancement. The micro-hole exit lengths lx, y and the sidewall taper angles φx, y were determined by the laser processing parameters and the sample thickness t (Fig. 7(c)). The details of lx, y and φx, y are discussed below.
Fig. 7. Illustrations for (a) square, diamond, and circle laser-marking objects with the length l; (b) their processing results (top views) with the micro-hole entrance lengths wx, y ≈ l + 2ω0 + Δx, y and the micro-hole exit lengths lx, y in the x and y directions, respectively, where 2ω0 is the Gaussian beam diameter and Δx, y represent the increase in ablation lengths. (c) Hole cross-sectional view marked by the dotted lines in (b), where φx, y are the sidewall taper angles in the x and y directions, respectively, and t is the sample thickness.
Figure 8 shows the fabricated micro-holes for the marking objects with l = 120 µm on t = 50 µm Invar sheets at F0 = 0.78 J/cm2. The symmetrical micro-holes were fabricated by circular polarization at Nscan = 16 with wx, y ≈ 155 µm (2ω0 ≈ 35 µm, Δx, y = 0 µm) and lx, y ≈ 100 µm, whereas the asymmetric micro-holes were fabricated by linear polarization at Nscan = 14 with wx ≈ 160 µm (Δx ≈ 5 µm), wy ≈ 155 µm, lx ≈ 100 µm, and ly ≈ 87 µm for horizontal linear polarization (wx ≈ 155 µm, wy ≈ 160 µm (Δy ≈ 5 µm), lx ≈ 87 µm, and ly ≈ 100 µm for vertical linear polarization). The increase in ablation lengths by linear polarization was confirmed through the stationary irradiation experiments (Figs. 5(b) or (c)).
Fig. 8. SEM images to demonstrate the effects of polarization (pol.) on micro-hole symmetricity for square, diamond, and circle marking objects. The symmetric micro-holes were fabricated by circular polarization, whereas the asymmetric ones were fabricated by linear polarization, where fluence F0 = 0.78 J/cm2, and the number of repetitions Nscan = 16 and 14 for circular and linear polarization, respectively. The micro-hole size lengthened along the linear polarization directions (horizontal and vertical directions).
3.3 Pattern generation model
The micro-hole profiles created by laser scanning were estimated by a model to predict lx, y and φx, y. The two-dimensional Gaussian fluence distribution is represented as follows:
The details for the derivation of Ftotal are presented in Supplement 1.
Figure 9 shows the ablation rate for stationary irradiation ΔL estimated by L/Nsta, exhibiting a linear relationship of ΔL = α-1ln(F0/Fth) [22], where α-1 is the effective penetration depth and Fth is the threshold fluence. The ablation depth (L) to obtain ΔL was measured at the crater center (maximum depth), with a confocal laser scanning microscope (Fig. 4(b) and (c)). The effective penetration depths (Fig. 9, linear slopes) for linear and circular polarization were ≈ 24 nm and 20 nm, respectively. The ablation enhancement for linear polarization was confirmed by the increases in ΔL and α-1.
Fig. 9. Relationships between fluence (F0) and ablation rate for stationary irradiation (ΔL) with respect to linear and circular polarization (pol.).
We estimated the micro-hole depth profiles z(x) using Eq. (6) with – ΔL × (Ftotal / F0)
Figure 10 shows the experimental values and those calculated by Eq. (8) using ΔL ≈ 36 nm (Fig. 9) and D ≈ 35 µm (Fig. 5(a)) for circular polarization, confirming a sound agreement between the values.
Fig. 10. Experimental values and those calculated by Eq. (8) for crater depth profiles z(x) through circular polarization with the number of repetitions Nscan = 1, 4, 8, and 12. The inset shows a typical three-dimensional image of a crater with Nscan = 12, inspected by a confocal laser scanning microscope. The craters were measured along the lateral position, e.g., the dotted line in the inset. The fluence, beam radius, ablation rate for circular polarization, crater diameter for circular polarization, pulse spacing, and hatch spacing were fixed at F0 = 0.78 J/cm2, ω0 ≈ 17.5 µm, ΔL ≈ 36 nm, D ≈ 35 µm, dp = 1 µm, and dh = 5 µm, respectively.
Equation (8) can be expressed as
Fig. 11. Three-dimensional microstructure images fabricated by circular polarization for (a) crater at the number of repetitions Nscan = 13 and (b)–(d) through-holes at Nscan = 14–16, respectively. (e) Micro-hole depth profiles z(x) at Nscan = 16 measured along the lateral position x ((d), dotted line) by a confocal laser scanning microscope and calculated using Eq. (8), where the micro-hole size lx ≈ 100 µm at z(x) = -50 µm. (f) Experimental and calculated values using Eq. (12) of the maximum linear slopes at x = x0 = 50 µm with Nscan = 16 for estimating micro-hole taper angle φx. (g) Experimental and calculated values using Eq. (12) φx with respect to Nscan. The fluence, beam radius, ablation rate for circular polarization, crater diameter for circular polarization, pulse spacing, and hatch spacing were fixed at F0 = 0.78 J/cm2, ω0 ≈ 17.5 µm, ΔL ≈ 36 nm, D ≈ 35 µm, dp = 1 µm, and dh = 5 µm, respectively.
Figure 11(f) shows the experimental and calculated sidewall taper angles φx for FMMs defined as the maximum microstructure slope, which can be estimated by drawing tangential lines at x = x0 or x0 + l. The taper angles for calculation can be obtained by the derivative of Eq. (8)
Equation (11) can be approximated as
The inset of Fig. 12 shows a three-dimensional image of a crater fabricated by linear polarization at F0 = 0.78 J/cm2 with Nscan = 10. The crater’s size lengthened along the polarization direction (y) by ≈ 11% ((94 - 85) / 85 × 100). Figure 12 shows the asymmetric crater depth profiles, which were demonstrated by Eq. (8), with ΔL ≈ 36 nm and 41 nm for the x and y directions, respectively. The ablation rates ΔL for the x and y directions correspond to those for circular and linear polarization, respectively (Fig. 9). The measured taper angles for the x and y directions were ≈ 55° and 61°, respectively. These values were in sound agreement with the values calculated using Eq. (12): ≈ 58° and 61° for the x and y directions, respectively, demonstrating an increase in the taper angle along the polarization (y) direction due to ablation rate enhancement. The difference between the measured and calculated bottom lines of the crater for the x direction (Fig. 12, the gray solid line and green dash-dotted line, respectively) was due to an increase in ablation depth (≈ 5 µm) in the y (polarization) direction.
Fig. 12. Experimental and calculated values using Eq. (8) of the crater depth profiles for linear polarization with the number of repetitions Nscan = 10. The inset shows a typical crater’s three-dimensional image inspected using a confocal laser scanning microscope. The crater lengths measured along the lateral positions x and y (polarization direction) were ≈ 85 µm and 94 µm, respectively. The asymmetrical depth profiles were calculated by different ablation rates ΔL based on the directions, i.e., ΔL for the x and y directions were 36 nm and 41 nm, respectively. The fluence, beam radius, crater diameter for circular polarization, pulse spacing, and hatch spacing were fixed at F0 = 0.78 J/cm2, ω0 ≈ 17.5 µm, D ≈ 35 µm, dp = 1 µm, and dh = 5 µm, respectively.
4. Conclusions
Ablation efficiency for linear polarization increased by ≈ 15% compared to that for circular polarization due to electrical field enhancement induced by LSFL, leading to asymmetric hole size and sidewall taper angles, whereas symmetric microstructures were fabricated by circular polarization due to non-oriented LSFL. The microstructure profiles could be estimated from the total input fluence multiplied by the ablation rate. The asymmetric structures that lengthened along the polarization direction were calculated by the enhanced ablation rate for linear polarization. Three-dimensional microstructure profile estimation with respect to the laser processing parameters, can help in predicting fs laser processing outcomes and provide an effective processing strategy. The pattern generation model could be applicable to all kinds of materials including semiconductors and dielectrics as well as metals because fs pulses can ablate materials by the cold process indicating the linearity of ablation. Polarization is a key parameter to control symmetricity for microstructures. Circular polarization can be favored for fabricating symmetric microstructures, e.g., FMMs, whereas linear polarization can be favored for processing asymmetric shapes and requiring high ablation efficiency, e.g., laser cutting. The comprehension for the surface grating effects on processing results can facilitate designing optical devices fabricated on non-metallic materials with gratings [24,25].
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.
Supplemental document
See Supplement 1 for supporting content.
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