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We study the incubation effect during laser ablation of stainless steel with ultrashort pulses to boost the material removal efficiency at high repetition rates. The multi-shot ablation threshold fluence has been estimated for two pulse durations, 650-fs and 10-ps, in a range of repetition rates from 50kHz to 1 MHz. Our results show that the threshold fluence decreases with the number of laser pulses N due to damage accumulation mechanisms, as expected. Moreover, approaching the MHz regime, the onset of heat accumulation enhances the incubation effect, which is in turn lower for shorter pulses at repetition rates below 600 kHz. A saturation of the threshold fluence value is shown to occur for a significantly high number of pulses, and well fitted by a modified incubation model.
© 2014 Optical Society of America
1. Introduction
Ultrafast laser micromachining enables fabrication of precise microstructures through laser ablation of a large variety of materials with unique capabilities in terms of flexibility, accuracy and reliability. Literature already reports several studies aiming to boost the laser ablation efficiency by optimizing the laser process parameters according to the machined material [1–4]. Thanks to the recent development of fs and ps laser sources operating at repetition rates of hundreds kHz up to a few MHz and delivering several tens of watts of average power, thus allowing a scaling of productivity, this technology is expected in the next few years to be established on an industrial scale in a growing number of applications. However, several studies have revealed that by increasing the repetition rate, plasma and/or particle shielding may limit the laser ablation efficiency, while heat accumulation causes melting thus preventing the high level of precision achievable at lower repetition rates [2,5]. Using femtosecond pulses instead of picosecond ones may contribute to prevent heat accumulation at high repetition rates, mainly in case of metals with a relatively low thermal conductivity like stainless steel [6]. Nevertheless, as soon as the pulse energy is increased, melting cannot be avoided in multi-pulses femtosecond laser ablation processes.
Therefore, a mere increase of the pulse energy or of the repetition rate does not yield to scale the productivity still keeping a high level of precision of the micro-structuring. Several processing strategies can be proposed aiming at exploiting the full average power available with the new generation of ultrafast laser sources. One approach consists in working with bursts of pulses. For many materials it has been experimentally observed that by irradiating their surfaces with bursts of N consecutive fs- or ps- pulses, their ablation threshold, defined as the minimum laser fluence to start the ablation process, is lowered [4,7–11]. This effect, known as incubation, was firstly observed during laser ablation of metals with nanosecond pulses [9]. The decrease of the ablation threshold with multi-pulse irradiation was ascribed to the accumulation of laser-induced chemical and/or structural changes of the material and/or plastic deformation of the surface [7,9]. The physical mechanisms underlying incubation are still debated among the scientific community. Neuenschwander et al. [12] carried out X-ray diffraction analyses on single crystal iron and copper targets irradiated by multiple femtosecond laser pulses and they did not found any significant increase of local defects in the crystal lattice, thus excluding an explanation of the incubation mechanism based on thermal stress-strain energy storage and/or plastic deformations accumulation, at least in the fs ablation regime. Therefore, the most likely hypothesis on the origin of incubation is an increase of surface roughness after multi-shot irradiation, due to ripples formation or accumulation of surface defects. Such defects, generated by the first impinging pulses, facilitate absorption of the next coming laser pulses, thus enhancing ablation and material removal [12].
A power law equation, defining an incubation coefficient S, was originally proposed to describe the dependence of multi-shot ablation threshold Φth,N of metals on the number N of impinging pulses and the threshold fluence with a single pulse Φth,1 [8,9]. Incubation is absent for S = 1 and the ablation threshold stays constant irrespective of the number of pulses. As far as the damage accumulation mechanism reduces the ablation threshold, the incubation coefficient gets lower [7–11]. This model implies that Φth,N should approach zero as far as the number of laser pulses is significantly high, which clearly does not find a physical interpretation. Similarly to what already observed in case of dielectrics [13], Neuenschwander et al. [12] proposed for multi-shot laser ablation of metals a slightly different model that introduces a constant threshold fluence Φth,∞ in case of bursts with an infinite number of pulses.
In this work we studied the multiple shot laser ablation of stainless steel targets with femtosecond and picosecond pulses at repetition rates from 50 kHz up to 1 MHz. The influence of the repetition rate and pulse duration on the incubation mechanisms responsible for the reduction of the ablation threshold has been investigated. In particular, we have analyzed the behavior of the threshold fluence in case of bursts with a significantly high number of pulses N. We have also evaluated potential contribution of heat cumulative processes to the incubation mechanism at significantly high repetition rates. In fact, as far as the repetition rate approaches the MHz regime, the very short time interval between subsequent pulses prevents heat dissipation into the bulk material through thermal conduction, thus causing substrate heating that has been already demonstrated to significantly reduce the ablation threshold in industrial silicon processing using infrared ultrashort laser pulses [14]. We have investigated if, in case of multi-shot laser ablation at high repetition rates, the local temperature rise caused by the first pulses could enhance absorption of the following ones, owing to the temperature dependence of the linear absorption coefficient, thus lowering the ablation threshold. Previous studies focused on the incubation effect at repetition rates ranging from 100 Hz [7] up to several kHz [4]. At such a low repetition rates heat accumulation can be neglected.
2. Experimental setup
Ablation experiments were performed using a fiber laser amplifier from Active Fiber Systems GmbH based on the chirped pulse amplification technique (CPA), delivering an almost diffraction limited beam (M2 ~1.25) at a wavelength of 1030 nm. The pulse width is tunable in the range from 650 fs to 20 ps, with repetition rates programmable from 50 kHz to 10 MHz. The maximum pulse energy is limited to 100 μJ at lower repetition rates, while above 500 kHz, the maximum available average power is 50 W. An external Acousto-Optic Modulator (AOM) allows the generation of bursts of any desired number of pulses. The system also included an autocorrelator where a small fraction of the beam was sent in order to check the actual pulse duration. Figure 1 shows the schematic layout of the experimental setup.
Fig. 1 Schematic layout of experimental setup. AOM: acousto-optic modulator. QWP: quarter-wave-plate.
The linearly polarized beam exiting from the laser source was first converted to circular polarization by means of a quarter-wave-plate in order to prevent anisotropic absorption inside the ablation craters and then expanded to a diameter of about 10 mm, before being guided to the entrance aperture of a computer interfaced galvoscanner equipped with a 100-mm-focal-length F-Theta lens. The targets were 2-mm-thick stainless steel (AISI 304) plates carefully cleaned with acetone. The sample surfaces were placed in the focal plane of the F-Theta lens and then irradiated with selected number of pulses N (from N = 2 to N = 250000), at different pulse energies Ep from 1 μJ to 50 μJ and repetition rates fR ranging from 50 kHz up to 1 MHz. Ablation experiments were carried out in ambient air without any gas shielding and replicated for two different pulse durations tp: 650 fs and 10 ps.
Table 1 summarizes the explored levels of working parameters. For each combination of process parameters several replica of the ablation process were produced. The morphology of the obtained craters was analyzed with a scanning electron microscope and the corresponding diameters were accurately measured. Thus, the crater diameter average value and the associated variance were estimated for each different set of operating conditions.
Table 1. Operating parameter
3. Results and discussion
3.1 Multi-shot threshold fluence
Starting with the measurement of the craters diameters, we estimated the focused laser spot size on the target surface w and the empirical multi-shot ablation threshold fluence Φth,N for each number of pulses applied N, using the method proposed by Liu [15] which supposes a Gaussian profile for the spatial energy distribution of our laser beam. This method, formerly developed to calculate the single-shot ablation threshold, has been chosen to estimate the empirical multi-shot ablation threshold because it is particularly advantageous and simple, since it does not require a full characterization of the laser beam and it allows estimating the laser spot size on the sample surface regardless the specific optical arrangement. Being Φth,N the laser ablation threshold with bursts of N consecutive laser pulses, the squared diameter D2 of the corresponding ablation crater is related to the peak laser fluence Φ0 by the following equation:
where Φ0 is defined as:and Ep is the incident laser pulse energy.By combining Eqs. (1) and (2) it is possible to relate the crater diameters, directly measured onto the sample surfaces, to the applied laser pulse energy. The linear fitting of the squared diameters D2 versus the logarithm of the pulse energy Ep allows determining the laser spot radius w and the multi-shot threshold fluence Φth,N. This procedure has been applied to calculate the empirical multi-shot ablation threshold fluence for each number of applied laser pulses N, at different repetition rates and pulse durations.
One of the scopes of our study was to evaluate the contribution of heat accumulation, especially at relatively high repetition rates, to enhance the incubation mechanism when the ablation process is still at an early stage. In fact, we investigated working conditions in which a significant advantage can be found due to the decrease of the ablation threshold, as long as melting is prevented. For this reason we excluded from the linear fits of the experimental data the craters affected by prominent melting of the ablation surface, since the high level of precision and accuracy of laser ablation with ultra-short pulses are lost when melting occurs. Figure 2 shows some representative SEM images of ablated craters produced with bursts of different numbers of pulses and pulse energies, at 50 kHz of repetition rate. Here, it can be noticed that while at 1 μJ of pulse energy and N = 25 and N = 250 [Fig. 2(a),(b)], the ablated areas are characterized by rippled structures thus indicating that ablation is mainly characterized by spallation of a very thin molten layer [16], at a higher pulse energy of 30 μJ and N = 25 [Fig. 2(c)] the crater is affected by a thicker layer of molten material in its central part. Therefore this crater has been excluded from the computation of the multi-shot ablation threshold for N = 25.
Fig. 2 SEM images of craters ablated with bursts of N pulses at 50 kHz repetition rate, 650-fs pulse width and (a) N = 25 and Ep = 1 μJ, (b) N = 250 and Ep = 1 μJ, (c) N = 25 and Ep = 30 μJ.
Figure 3 shows, as representative of all the examined process parameters combinations, the plots of the experimental data to determine the multi-shot ablation threshold Φth,N at 1MHz of repetition rate for N = 5, 50, 500, 25000, and two different pulse widths: 650-fs and 10-ps. It is clearly evident that the crater diameters increase either by increasing the pulse energy or the number of pulses. Hence, our results are consistent with the theory of damage accumulation in multi-shot irradiation [9]. Furthermore, it can be noticed that the slope of the linear fitting becomes steeper when the pulse energy exceeds a typical value of 10-15μJ. The slope change is more evident for N > 500 incident laser pulses. This behavior has been already observed in previous analogous experiments [4,7] and was ascribed to a transition from a “gentle” to a “strong” ablation regime. The change of ablation mechanism is confirmed by SEM analyses of the craters morphologies. The insets in Fig. 3 report images of the ablated craters produced for a high number of laser shots (N = 25000) with the shortest pulse duration of 650-fs and two different pulse energies. At such a high repetition rate of 1 MHz, even when using femtosecond pulses, 10 μJ of pulse energy were enough to establish a “strong” and thermal ablation regime. Here, the crater morphology changed exhibiting smoother inner surfaces due to the formation of a resolidified melt layer originated by heat accumulation. A rim of molten material can be noticed also on the crater edges. Such a transition to a thermal ablation regime starts at lower pulse energy when the repetition rate is increased. Therefore, as far as the repetition rate approached the MHz regime, only the craters obtained with a pulse energy lower than 10 μJ were considered for the threshold calculation.
Fig. 3 The squared diameter of the ablated craters in stainless steel (AISI 304) versus pulse energy for N = 5, 50, 500, 25.000 at 1MHz and pulse width of (a) 650fs and (b) 10ps. Insets show SEM images of two craters exhibiting different morphology and produced at N = 25.000 shots, 650fs pulse width, 1MHz repetition rate and pulse energies of 1µJ and 10µJ, respectively.
The multi-shot threshold fluence was thus determined for each number of incident laser pulses, pulse duration and repetition rate. Figure 4 represents the multi-shot threshold fluence for N = 2500 as a function of the repetition rate for two pulse durations. In the range from 50 kHz to 200-400 kHz an increase of the ablation threshold is observed for both pulse widths. For such a relatively high number of incident pulses (N = 2500), this behavior can be ascribed to shading effects such as particle shielding according to which particles ablated and ejected by the first impinging laser pulses do not fade away within two subsequent pulses [5,6]. As a consequence, they reside on top of the irradiated area and interfere with the next coming pulses, thus scattering, absorbing or reflecting part of their energy. In this way, the ablation threshold is increased.
Fig. 4 Multi-shot threshold fluence as a function of repetition rate at 2500 incident pulses in stainless steel (AISI 304) sample for pulse width of 650fs (square solid) and 10 ps (solid circle)
For repetition rates higher than 400kHz a decrease of the multi-shot ablation threshold is found for both pulse duration, except for the experimental point at 800 kHz and 650-fs which slightly deviates from this trend. The lowering of the ablation threshold can be ascribed to the heating of the substrate caused by heat accumulation which clearly overbalances particle shielding at higher repetition rates. An analogous reduction of the ablation threshold was observed on silicon by Thorstensen et al. [14] when the substrate temperature was increased.
3.2 Incubation models
Figure 5 shows the multi-shot threshold fluences as a function of the number of applied laser pulses, at 50kHz and 1MHz repetition rates and for the two different pulse widths, respectively. The ablation threshold decreases with the number of incident pulses, as expected. There is no apparent influence of the pulse duration on this trend for all the repetition rates. For a significantly high number of pulses (e.g. N > 25000) the ablation threshold is up to four times lower than irradiation with a few shots. This behavior is consistent with the incubation model proposed by Jee et al. [9]:
Fig. 5 Multi-shot threshold fluence versus the number of applied laser pulses in stainless steel (AISI 304) sample at two pulse widths of 650fs and 10ps at (a) 50kHz; (b) 1MHz.
Equation (3) relates the multi-shot threshold fluence Φth,N to the single pulse ablation threshold Φth,1 through a power law that introduces the so-called incubation coefficient S. Previous studies have shown that this coefficient is material dependent. In case of multi-shot laser ablation of metals at relatively low repetition rates (< 100 kHz), typical values of S were found in the range between 0.8 and 0.9 [4,7,11].
From the graphs shown in Fig. 5, it is worth noting that for N > 1000, a saturation of the incubation effect occurs and the multi-shot threshold fluence does not decrease by further increasing the number of laser shots. This result is not coherent with the incubation model described by Eq. (3).
Therefore, we employed a modified incubation model introducing a constant threshold fluence Φth,∞ in case of bursts with an infinite number of pulses:
Here, ΔΦth,1 represents the complementary value to be added to the offset Φth,∞ to obtain the single-shot ablation threshold, while S* indicates a modified incubation coefficient. The idea of a constant offset for the multi-shot threshold fluence for a relatively high number of pulses was originally introduced in case of laser ablation of dielectrics [13] and successively proposed also for metals [12]. In Fig. 6 the measured multi-shot threshold fluences are represented as a function of the number of applied laser pulses for 100 kHz and 1 MHz repetition rates and two different pulse durations. It can clearly be seen that the modified incubation model introduced by Eq. (4) better fits the experimental data compared to the standard model of Eq. (3), for all the investigated operating conditions. In particular, the modified model is more accurate to fit the experimental data for lower repetition rates and shorter pulse durations.
Fig. 6 Fits of the experimentally measured multi-shot threshold fluence versus the number of applied laser pulses with the standard incubation models of Eq. (3) (dashed line) and modified model of Eq. (4) (solid line): (a) 100 kHz repetition rate, 650 fs pulse duration; (b) 100 kHz, 10 ps; (c) 1 MHz, 650 fs; (d) 1 MHz, 10 ps.
3.3 Incubation coefficient and threshold fluence for an infinite number of pulses
We have investigated the dependence of the incubation coefficient S, defined by the original model, and the threshold fluence for an infinite number of pulses Φth,∞ from the repetition rate and the pulse duration. As shown in Fig. 7, below 600 kHz of repetition rate, the incubation coefficient stays almost constant for both pulse widths, even though the S-value is lower in case of picosecond pulses with respect to femtosecond ones. This indicates that incubation mechanisms are facilitated when using longer pulse durations. As far as the repetition rate is increased above 600 kHz, the damage accumulation mechanism is even more pronounced and a significant drop of the incubation coefficient was observed for both pulse durations, even though for 10-ps pulses the decrease of S takes place at lower repetition rates. Above 800 kHz, when the heat accumulation effect is supposed to be predominant, similar values for S were found for 10-ps and 650-fs pulses.
Fig. 7 Incubation coefficient S as a function of the repetition rate for multi-pulse laser ablation of stainless steel (AISI304) targets with 650-fs and 10-ps laser pulse width.
Figure 8 reports the constant threshold fluence Φth,∞ values for an infinite number of pulses, estimated starting with the same set of experimental data and employing the alternative incubation model of Eq. (4). It is now this term that is clearly related to the incubation mechanism because it significantly decreases with increasing repetition rates both for picosecond and femtosecond pulses. The modified incubation coefficient S* does not show a clear trend with respect to the repetition rate, probably because in the alternative incubation model the physical information on the ablation threshold reduction with multi-shot irradiation is already contained in the constant offset value Φth,∞.
Fig. 8 Multi-shot threshold fluence with an infinite number of pulses as a function of the repetition rate measured during laser ablation of stainless steel (AISI304) targets with (a) 650-fs and (b) 10-ps pulse widths.
Although a detailed investigation of the microscopic cause of incubation effect during multi-shot laser ablation of metals was beyond the scope of this work, our results suggest that in case of very high repetition rates heat accumulation plays an important role in reducing the multi-shot ablation threshold.
4. Conclusions
We performed a systematic study of the incubation effect during multi-shot laser ablation of steel targets, by irradiating the sample surfaces with bursts of a variable number of ultrashort pulses (from N = 2 to N = 250000). The dependence of the multi-shot ablation threshold fluence on the repetition rate, from 50 kHz to 1 MHz, and the pulse duration (650-fs versus 10-ps), was investigated.
Our data confirm that for a growing number of incident laser pulses, incubation is responsible for a decrease of the multi-shot threshold fluence, probably due to a damage accumulation mechanism. This behavior can be described by a power law equation defining the so-called incubation coefficient S, which was estimated for different repetition rates and pulse durations. We found that below 600 kHz of repetition rate, the incubation coefficient stays almost constant for both pulse widths, even though incubation is higher in case of picosecond pulses compared to femtosecond ones. When approaching the MHz regime, the decrease of the time interval between consecutive pulses does not allow an efficient heat dissipation into the bulk material thus causing a temperature rise of the irradiated surface. This heat accumulation mechanism facilitates incubation, so that a decrease of the S-value and, consequently, of the multi-shot ablation threshold fluence was observed, regardless of the pulse width.
However, this incubation model implies that for a significantly high number of pulses, the multi-shot threshold fluence should approach zero. We rather observed that with increasing number of pulses, the threshold fluence saturates to a constant value Φth,∞. Therefore, we propose a revised incubation model that takes into account this constant term to fit our experimental results. By studying the dependence of Φth,∞ on the repetition rate, we found that it considerably decreases at higher repetition rate both for picoseconds and femtosecond pulses. This result confirm the idea that heat accumulation plays an important role on multi-shot ablation threshold reduction.
Our results are very promising from an application point of view because a reduction of the ablation threshold by multi-shot irradiation might be useful to increase the material removal rate during laser micromachining processes, thus exploiting the potential of the novel generation of ultrafast high repetition rates and high average power laser sources.
Acknowledgments
The authors gratefully acknowledge the Apulian Region and the Italian Ministry of Education, University and Research for having supported this research activity within the project TRASFORMA Laboratory Network cod. 28, and project PON02_00576_3333604 “INNOVHEAD” related with the Apulian Technological District on Mechatronics – MEDIS.
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