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Abstract
This paper reports a mechanism to suppress nanoparticle (NP) generation during femtosecond laser processing of 64FeNi alloy (Invar) to realize high precision fine metal masks. Nanoparticle redeposition during processing can reduce precision and ablation efficiency. Since Gaussian laser beams have spatially distributed fluence, NP types can vary even within a laser spot. Surface areas irradiated by the beam center with high peak fluence can be decomposed into vapor and liquid droplets by phase explosion; whereas positions irradiated by the beam edge, where fluence is close to ablation threshold, can be decomposed by stress confinement under the surface, known as spallation. Spallation characteristics were verified from target surfaces covered with exfoliation and fragments. It occurred above a certain number of pulses, indicating a significant incubation effect. Spallation induced NPs, i.e., agglomerated fragments, distort micro-hole size and shape, but were effectively suppressed by increasing repetition rate, due to increased surface temperature, i.e., heat accumulation. Suppression also occurred from direct sample heating using a hot plate. Thus, thermal energy can relax stress confinement and inhibit spallation induced NPs. Numerical simulation for heat accumulation also confirmed that suppression arises from thermal effects. Increasing repetition rate also helped to increase productivity.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Femtosecond (fs) lasers are promising tools to realize precise micromachining on various materials, providing thermal damage free processing by selecting proper laser parameters [1,2], particularly for thin films [3–5] which can be easily deformed by even small thermal loads. Therefore, previous studies focused on controlling heat effects during fine metal mask (FMM) fabrication on thin 64FeNi alloy (Invar) sheets, enabling mass production of high precision FMMs with high throughput [2,6].
Another critical issue for microstructure accuracy during laser material processing is ablated material redeposition. Target interactions with fs pulses produce nanoparticles (NPs) [7,8], with NP characteristics depending on laser parameters, such as fluence (F) [9] and repetition rate (frep) [10]. By-products can adhere to processed surfaces, distorting microstructure size and shape and reducing ablation efficiency. Although liquid assisted laser processing can address redeposition problems [11,12], laser-matter interaction dynamics can be affected during processing due to changing liquid concentration, and it is very difficult to handle thin films under liquid. Therefore, controlling NP formation in air is essential for effective laser material processing.
Material decomposition mechanisms by fs pulses have been studied by molecular dynamic (MD) simulations [13–18], identifying two regimes depending on F. Solids irradiated near their ablation threshold fluence (Fth) can be decomposed by laser induced stress confinement just under the target surface, i.e., spallation [15–17]. As F increases, the surface layer tends to become superheated and decomposes into a mixture of vapor and liquid droplets, called phase explosion [15–17]. They can appear on a spot at the same time because laser beams generally have spatially distributed intensity. Thus, phase explosion and spallation can arise at the beam center and edge, respectively, when a Gaussian beam with F > Fth irradiates a target.
This paper proposes a method to suppress spallation induced NPs generated at Gaussian beam edges for F ≈ Fth. Spallation characteristics were verified by inspecting Invar surfaces covered with exfoliation and fragments. In particular, spallation induced NPs were formed above a certain number of pulses, N, suggesting that they are caused by an incubation effect [19]. Suppression was realized by increasing frep due to heat accumulation [20–22], which can relax stress confinement. Thermal effects on suppression were confirmed by direct heating on a hot plate. NP production for frep = 500 kHz at room temperature was similar to that for frep = 50 kHz at 600 K. The temperature (600 K) corresponds to the residual heat at frep = 500 kHz, which can be estimated by numerical simulation. Thus, high precision FMMs were successfully realized by applying megahertz frep pulses, which also enabled higher throughput processing due to the increased frep.
2. Experimental setup
We used an infrared fs laser (Monaco, Coherent) with 300 fs average pulse duration and 40 W maximum output power at 1035 nm for the experiments. Pulse repetition rate could be tuned up to 50 MHz with 0.8 μJ pulse energy. Thin Invar sheets (20 μm) were placed and fixed on an electrostatic chuck mounted on an xyz translation stage. A Gaussian laser beam was directed and focused onto the targets by a galvanometer scanner (hurrySCAN 14, SCANLAB) and f-theta lens with 100 mm focal length. Focused beam diameter was estimated by linear slope [23]
where D is crater diameter, measured by scanning electron microscopy (SEM); ω0 is 1/e2 Gaussian beam radius; Ep is pulse energy, and Eth is threshold pulse energy. Beam size on the targets was tuned for processing using a beam expander, and fluences were calculated as peak, i.e., at the beam center. We used both linearly polarized (original polarization state) and circularly polarized (converted by a quarter-wave plate) beams. To verify surface temperature influence on NP generation, samples were heated by a hot plate to 320 °C (∼ 600 K) during stationary irradiation.3. Results and discussions
3.1 Repetition rate influence on microstructure quality
Figure 1 shows typical SEM images for FMMs fabricated at frep = 50 kHz, 100 kHz, 200 kHz, 500 kHz, 1 MHz, and 2 MHz. Figure 1(f) shows scanning details to realize the microstructures. Each FMM hole was patterned by repetitively scanning a layer (dashed line) containing multiple parallel hatching lines with scan directions (arrows) and line spacing dh. The number of repeats for each hole is Nscan. Pulse spacing dp = v/frep, where v is scanning speed. We fixed dp = 0.5 μm for the various frep by changing v appropriately. Thus, we could focus on frep influences on processing quality. Table 1 shows all relevant processing parameters. Micro-hole arrays were realized by hole-by-hole fabrication. Since time intervals between successive pulses were long enough to prevent melting layers [2], metal foil deformation did not occur.
Fig. 1. SEM images for typical fine metal masks (FMMs) fabricated at different repetition rates frep = (a) 50 kHz, (b) 100 kHz, (c) 200 kHz, (d) 500 kHz, (e) 1 MHz, and (f) 2 MHz. Dashed line, solid circles, and arrows in (f) indicate scanning layer, laser spots, and hatching lines with directions, respectively. Pulse spacing, hatching spacing, and fluence were fixed at dp = 0.5 μm, dh = 1 μm, and F = 0.35 J/cm2, respectively, for all cases.
Table 1. Experimental conditions for fabricating fine metal masks.a
Precise laser micromachining can be negatively affected by NP generation and redeposition. Figure 1 shows processing quality dependence on frep. Processing quality gradually improved as frep increases in terms of realizing the intended design (40 μm square) and uniformity because NP generation was suppressed by surface heat accumulation due to increased frep. Thus, we simultaneously improved precision and scanning speed. Details of the heat effect are discussed below.
3.2 Nanoparticles induced by incubation
We investigated target surfaces under stationary laser irradiation to better understand NP generation and redeposition during FMM fabrication. Figures 2(a)–(c) show typical SEM images for target surfaces irradiated with N = 10, 20, and 40 shots at fixed frep = 50 kHz and F = 0.35 J/cm2, respectively. Crater diameter increases with increasing N due to reduced Fth with increasing N, called incubation [19]. Figure 3(a) confirms the effect, exhibiting linear relationship between D2 and log(F). Repetition rate was fixed at frep = 50 kHz to exclude heat accumulation [2]. Figure 3(a) inset shows a typical SEM image for craters created under F = 0.62 J/cm2 and N = 100 shots.
Invar incubation behavior was investigated using circularly polarized pulses, which allows us to estimate crater diameters with high circularity; whereas craters irradiated with linearly polarized beams tend to elongate along the polarization direction depending on F [24]. Figure 3(a) shows ablation thresholds for each N (horizontal axis intercept), Fth(N), and Fig. 3(b) shows Fth(N) with respect to N. The inset table lists derived Fth(N), which are well described by the incubation model [25]
where Fth(1) ≈ 0.25 J/cm2 is the single pulse threshold and S ≈ 0.76 is an incubation coefficient.Fig. 2. SEM images for craters generated by stationary irradiation for number of pulses N = (a) 10, (b) 20, and (c) 40 shots; (d)–(f) show enlarged surface morphologies from the marked squares in (a)–(c), respectively. Fluence and repetition rate were fixed at F = 0.35 J/cm2 and frep = 50 kHz, respectively. Metal layer spallation characteristics occur in (e), with exfoliation and fragments identified. Spherical nanoparticles (NPs) with diameter ≈ 20–150 nm are deposited over the surfaces.
Fig. 3. (a) Fluence (F) relationship to crater diameter (D2) with respect to number of pulses (N), inset shows a typical SEM image for craters irradiated at F = 0.62 J/cm2 and N = 100 shots. (b) Threshold fluences with respect to N, Fth (N), estimated from horizontal axis intercept in (a), inset shows derived Fth(N) values.
As discussed above, incubation for multiple shots reduces the surface damage threshold, which can induce crater periphery decomposition. This may not occur under low N due to low F in the Gaussian distribution tail (compare surface morphologies between Figs. 2(d) and (e), or (d) and (f)). Although Invar surfaces were not decomposed for N = 10 (Fig. 2(d)), significant decomposition occurred for N > 20 (Figs. 2(e) and (f)) due to incubation.
Figure 4(a) shows the laser irradiated crater for N = 40, frep = 50 kHz, and F = 0.35 J/cm2 divided by surface morphology to more clearly examine crater edge decomposition. The central part of the crater (region A) was ablated by the high fluence central Gaussian beam region and was covered in laser induced periodic structures (LIPSS) with orientation perpendicular to the linear polarization. Average LIPSS period Λ ≈ 1 μm, which is close to the laser wavelength (λ = 1035 nm), and is referred to as low spatial frequency LIPSS [26]. High spatial frequency LIPSS (HSFL) occurred near the region A boundary, where applied F ≈ Fth [26]. Figure 5(a) confirms HSFL at higher magnification, oriented parallel to the polarization with Λ ≈ 100 nm.
Fig. 4. (a) SEM image for the crater formed by stationary irradiation for N = 40 pulses at repetition rate frep = 50 kHz and peak fluence F = 0.35 J/cm2. Region A is laser induced periodic structures (LIPSS), B is spallation induced decomposition, and C is undecomposed. (b) Gaussian laser beam radial distribution. Threshold fluences Fth-spallation(N = 40) and Fth-LIPSS(N = 40) for spallation and LIPSS formation at N = 40, respectively. Thresholds depend on N, i.e., incubation.
Fig. 5. SEM images for crater peripheries generated by stationary irradiation for N = 40 pulses, fixed fluence F = 0.35 J/cm2, and repetition rates frep = (a) 50 kHz, (b) 100 kHz, (c) 200 kHz, (d) 500 kHz, (e) 1 MHz, and (f) 2 MHz. Lower images in each pair indicate enlarged areas, e.g., rectangular region in (a). High spatial frequency laser induced periodic structures (HSFL) occur at crater edges, e.g., the arrow in (a).
Molecular dynamic simulations for nickel [15], chromium [16], and aluminum [17] decomposition mechanisms under ultrashort pulses confirm that the threshold fluences for phase explosion ≈ 2–3 times those for spallation. The spallation threshold can be estimated by ablation experiments, as shown in Fig. 3. Since spallation is induced by incubation, the threshold can be expressed with respect to N, where Fth-spallation(N = 40) ≈ 0.05 J/cm2 experimentally from the surface morphology (Fig. 4). The threshold for generating LIPSS at N = 40, Fth-LIPSS(N = 40) ≈ 0.1 J/cm2, is consistent with the incubation model (Eq. (3)). Since we extracted Fth(1) and S for Invar with various N by measuring crater diameters (region A), the incubation model represents the threshold for generating LIPSS well.
Therefore, the ablation regime near F = 0.35 J/cm2 is assumed to be phase explosion since Fth-spallation(N = 40) ≈ 0.05 J/cm2. Spherical NPs spread over the surface close to the craters support phase explosion (Fig. 4(a)). Enlarged views of the spherical NP surface morphologies are also presented in Figs. 2(d)–(f). Spherical NPs can be formed in the phase explosion regime since overheated melting layers are separated and ejected from target surfaces with minimized surface free energy [27].
Figure 4(a) shows that region B is clearly distinguishable from region C where the Invar surface is undecomposed and covered with spherical NPs generated from region A. Figure 2(f) shows an enlarged region B image. Different NP types occur on the bare Invar surface. The surface is mainly covered with agglomerated fragments and spherical NPs with and without fragment coverings. Differences between the two NP types (spherical NPs with and without fragment coverings) could be due to multiple shots. Spherical NPs created in region A for some N fall onto region B. Fragments, generated from region B by the following pulse, can adhere to the spherical NPs. Spherical NPs not covered by fragments may have been generated by the final pulse.
The most probable fragment generation mechanism in region B is spallation (or photomechanical ablation), which arises from confining laser induced stress in the surface region [15–17]. Figure 2(e) shows a better view to help understand spallation characteristics for surfaces irradiated at N = 20, exhibiting initial fractures in the top layers. Mean fragment size ≈ 20 nm, much smaller than spherical NPs (20–150 nm). Exfoliation, another spallation feature, shows relatively large surface area separation from the bare Invar surface. Figure 2(f) shows the fragments are agglomerated as N > 20.
3.3 Heat accumulation influence on spallation induced nanoparticles
Figure 5 shows SEM images for crater rims, i.e., region B in Fig. 4(a), generated by stationary irradiation for N = 40 shots with frep = 50, 100, 200, 500, 1000, and 2000 kHz, respectively, and fixed F = 0.35 J/cm2. NP surface morphologies are shown more clearly in the enlarged SEM images below each figure. Spallation induced NPs, i.e., agglomerated fragments, disappear as frep increases. Suppression can be explained by heat accumulation. Surface temperature increases by heat accumulation when repetitive laser pulses irradiate a target successively and time interval between successive pulses, i.e., 1/frep, is shorter than dissipation time for residual heat on the materials [20–22].
Time (t) evolution for surface temperature by irradiating a train of N pulses can be expressed as [2]
Figure 6(a) shows temporal evolutions for surface temperature in region B at frep = 500 kHz, 1 MHz, and 2 MHz. Figure 6(b) shows an enlarged plot from the marked circle in Fig. 6(a) to clarify heat accumulation dependence on frep. Figure 6(c) shows residual heat, defined as thermal energy left in the target by each repetitive pulse [20,21], using the temperatures immediately before pulse arrival, e.g., the arrow in Fig. 6(b). Residual heat increases with increasing frep due to heat accumulation. Table in Fig. 6(c) shows extracted surface temperatures from the curves. The simple model explains well heat accumulation when beam size (ω0 ≈ 40 μm) > effective penetration depth (α-1 ≈ 30 μm), i.e., heat flow is assumed to be one-dimensional process [2]. The model may need to be modified for ω0 ≤ α-1 [20].
Fig. 6. (a) Temporal evolution for surface temperature by heat accumulation at repetition rates frep = 500 kHz, 1 MHz, and 2 MHz. (b) Highlighted plot from the marked circle in (a). (c) Lower envelops for (a), i.e., residual heat, with respect to number of pulses (N). Inset shows extracted temperatures from the curves.
We investigated crater rims generated on heated Invar by a hot plate to prove heat accumulation effects on spallation suppression. Figure 2 shows that spallation induced NPs are generated for approximately N > 20. Therefore, samples should be heated to temperatures corresponding to N > 20 from the numerical model to verify heat effects on suppression. Heat effects from repetitive pulses in the MHz range are difficult to demonstrate due to typical hot plate temperature limitations, e.g., targets would need to reach approximately 1000 K to verify heat effects equivalent to frep = 2 MHz (Fig. 6(c), inset), whereas the kHz range was verified from the maximum achievable temperature (≈ 600 K). Figure 7 and 5(a) (with and without heating at frep = 50 kHz and N = 40) confirm considerable NP spallation suppression due to the heat effect. Heating temperature 600 K corresponds to residual heat estimated by the numerical model for frep = 500 kHz and N = 20 (Fig. 6(c), inset). The surface morphology similarity between Fig. 5(d) (without heating at frep = 500 kHz) and Fig. 7 (with heating at frep = 50 kHz) also prove the heat effect on suppression.
Fig. 7. SEM image for typical crater periphery generated by stationary irradiation on a heated target at temperature T = 600 K, with repetition rate frep = 50 kHz, fluence F = 0.35 J/cm2, and number of pulses N = 40 shots.
3.4 Nanoparticles for flat-topped multi-Gaussian beams
A single crater was fabricated and observed to understand NP generation level on hole arrays. Figure 8(a) shows a typical SEM image for craters fabricated by scanning with Nscan = 1 at frep = 50 kHz (Table 1 shows the other processing parameters). Surface morphologies for crater rims generated by scanning were similar to those from stationary irradiation. Figure 8(b) confirms agglomerated fragments in LIPSS vicinity. Microstructures can be fabricated by scanning multiple hatching lines (Fig. 8(a), arrows). A hatching line comprises many superposed laser beams separated by dp. Figure 8(c) shows the resulting flat-topped multi-Gaussian function, i.e., sum over Gaussian beams [29]. Generally, the function sides determine hole sidewall taper angles due to the energy distribution. Therefore, spallation induced NPs can be generated and piled up on processed areas near tapered sidewalls, due to lower fluences. Figure 8(d) shows an SEM image for craters fabricated with Nscan = 4. The inset confirms that the amount of generated and agglomerated NPs on the surface area increases with Nscan. Consequently, increasing NP generation and redeposition with completing the micro-hole arrays, can reduce processing quality and efficiency (see Fig. 1(a)). Although flat-topped multi-Gaussian beams with time-variant energy distribution are different from top-hat beams transformed by beam shaping lens, their processing results can be similar because the repetition rate range frep = 50 kHz–2 MHz for FMM fabrication is the thermal damage free processing regime [2].
Fig. 8. SEM images for (a) crater patterned at repetition rate frep = 50 kHz with single multi-pass scanning (Nscan) and (b) enlarged surface morphology irradiated by flat-topped multi-Gaussian beam edges. (c) Fluence distribution for a flat-topped multi-Gaussian function by superposing Gaussian functions separated by pulse spacing (dp). (d) SEM image for a crater patterned at frep = 50 kHz with Nscan = 4. Inset shows enlarged surface morphology for the tapered crater sidewall. Fixed hatching line spacing dh = 1 μm, dp = 0.5 μm, fluence F = 0.35 J/cm2, and scanning speed v = 25 mm/s.
Figure 9 shows that high precision FMMs with various designs were realized using the processing strategy. Laser processing precision and efficiency were dramatically improved because spallation induced NPs were suppressed as frep increases. Pattern size processed at frep = 2 MHz was larger than that processed at frep = 50 kHz. Surface temperature should be limited to maintain thermal damage free processing as an important consideration for increasing frep to suppress NPs. Processing quality can be significantly degraded when residual heat exceeds a melting point by increasing frep due to burr formation from significant surface melting and resolidification [2].
Fig. 9. SEM images for three different fabricated fine metal masks at repetition rates frep = (a)–(c) 50 kHz and (d)–(f) 2 MHz. Intended shapes (a) and (d) circles; (b) and (e) diamonds; (c) and (f) honeycombs. Table 1 shows the other processing parameters. All processes were conducted at room temperature.
Throughput could be improved at least more than 2 times by increasing v with frep, and the whole processing time for a square hole was about hundreds of milliseconds at v = 1000 mm/s. Although scanning speed v could increase in proportional to frep increase up to 40 times (Table 1, v = 25–1000 mm/s), total processing time (throughput) did not increase up to the maximum value due to other scanning parameters, such as delay time between hatching lines.
4. Conclusions
Nanoparticles, i.e., agglomerated fragments, were generated on surface areas decomposed by F ≈ Fth. They began to be formed over a certain number of pulses N, indicating that incubation induces target decomposition. Spallation, i.e., incubation induced fragments, was effectively suppressed by increasing frep due to heat accumulation. Suppression also occurred for direct sample heating using a hot plate. Comparing surface morphologies generated by stationary irradiation at high frep without sample heating and low frep with heating confirmed that decomposition, driven by stress confinement, can be relaxed by thermal energy. Precise FMMs were fabricated with high throughput by applying MHz repetition rate pulses. The processing strategy significantly improved laser processing precision and efficiency, greatly broadening potential industrial applications for ultrashort pulse lasers. Nanoparticle engineering can contribute to deeper understandings for fabricating functional surfaces, such as colored [10,30] and superhydrophobic [31,32] surfaces.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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