Influence of the pulse duration at the laser processing of nitride ceramics

This paper presents results on laser ablation of AlN and Si3N4 ceramics by laser pulses with different duration. Three types of laser systems, a Nd:YAG one, operated at wavelength of 1064 nm and pulse duration of 15 ns, a Nd:YAG, operated at wavelength of 1064 nm and pulse duration of 10 ps, and a femtosecond laser system, operated at 800 nm, with a pulse duration of 75 fs, are used for experiments. Details on the ablation efficiency, surface morphology, and the chemical composition of the irradiated zones as a function of the pulse duration are given and discussed. It is demonstrated that the ablation rate (ablation depth per pulse) is highest for processing with nanosecond pulses and it is lowest for the femtosecond regime. The laser ablation results in significant change of the surface morphology, as its characteristics are influenced by the pulse duration. For all pulse durations conditions for formation of ripples structures are found. The ablation process is realized by decomposition of the ceramics and the composition of the remaining surface layer is governed by oxidation and carbonization.


Introduction
In recent years, laser technologies have established themselves as one of the main ones in the processing of various materials [1,2].The current level of development in the field of laser systems allows operation from continuous mode generation, to the use of pulses with a duration of tens of femtoseconds.The ability of high spatial and temporal energy confinement that the application of lasers offers, benefits efficient processing of wide variety of materials, especially where the other commercial technologies are not effective [3].The formation of high temperatures, temperature gradients and pressures that accompany laser processing, allows effective impact on materials with high hardness, thermal conductivity and brittleness.Varying the laser parameters enables the processing of materials with a spatial resolution of the order of the wavelength used, effectively limiting the width of the modified zone around the processed one [3].In addition to influencing the quantitative characteristics of the processed area, the appropriate choice of experimental parameters, when using laser processing, enables effective control of the phase changes that occur in the processed material.This gives the opportunity for the implementation of various modification mechanisms -from annealing to ablation, in which the morphology, composition and structure of the material in the affected area can be changed effectively and in a controlled manner.One of the key parameters that determines the result of the impact of laser radiation in this qualitative aspect, is the duration of laser pulses.It is shown [4] that the use of pulses with a duration of milliseconds to nanoseconds mainly leads to the realization of thermal mechanisms of modification of the processed material, where the effects of melting and the dynamics of the melted material determine the characteristics of the processed zone.The use of pico-and femtosecond pulses, leads to a limitation of the thermal effects, as well as, to a realization of various modification mechanisms, such as phase explosion, spallation, spinodal decomposition, that are related to the faster deposition of the energy into the material with respect to the relaxation mechanisms [5].Thus, the appropriate choice of this parameter can enable precise and efficient processing of materials that are difficult to process by mechanical methods, such as diamonds, ceramics, and glasses [6].An example of such materials are nitride ceramics.They express a high hardness, resistance to most of the chemical etches, composition and structure stability to temperatures over 2 000 K [7].These ceramics also have excellent electrical insulator properties and high thermal conductivity.These properties determine the use of nitride ceramics in various industries, such as abrasive and cutting materials, elements in engines, components in high-temperature technologies, substrates in electronic systems to conduct the emitted heat [8].The practical use of nitrite ceramics requires precise processing, which usually includes cutting, drilling, scribing.In this field, high efficiency in the application of laser systems is demonstrated [9].However, there are still challenges related to heat-affected zone reduction, crack formation, and compositional changes during processing.To solve the problems, different laser systems, operated at a broad range of processing parameters are used for nitride ceramics processing [10][11][12].Due to the complexity of the processes involved in the interaction of laser radiation with ceramics and the subsequent phase changes, the detailed description of the mechanisms of laser processing has not yet been given, and the role of some experimental parameters has not been investigated.This makes it difficult to develop an efficient technology for precision machining of nitride ceramics.
In this work we present results on laser ablation of AlN and Si3N4 ceramics with laser pulses with nanosecond, picosecond and femtosecond pulse duration.The ablation efficiency, the characteristics of the processed area morphology, and the composition changes in a comparative way are presented.The reported results give new data that can be used in the development of laser processing technology for ceramic machining.

Experimental
The samples used for ablation experiment are AlN and Si3N4 plates (CeramTec), with thickness of about 0.6 and 0.3 mm, respectively.Three laser systems are applied for processing, namely ns and ps Nd:YAG lasers, and fs Ti:sapphire one.The parameters of the laser systems are given in table 1.The ablation experiments are performed in air, as the laser radiation is focused on the sample surface by lens with a focal length of 20 cm.In order to compare the obtained results, the applied laser fluence for each laser system is chosen to be about 6 times higher than the corresponding for given pulse duration ablation threshold.The latter is estimated by a dependence of the ablation depth versus laser fluence.In this way the laser fluences used in the experiments are: 20 J/cm 2 for ns experiments; 2 J/cm 2 for ps experiments, and 0.7 J/cm 2 for fs experiments.Note that at higher fluences at nanosecond laser processing, a decrease of the ablation rate is observed [13], due to interaction of the laser radiation with the ablated material.The ablation depth of the ablated area is estimated by pre-focusing of the samples surface and the ablation spot bottom, using optical microscope (OPTICA B-150).The surface morphology, and composition are analysed by SEM (Hitachi TM 4000), XPS (AXIS Supra electron spectrometer) and Raman spectrometer (DeltaNu, 785 nm), respectively.

Results and discussion
In order to quantify the ablation efficiency for the different laser pulse duration regimes used, ablation rate with meaning of ablation depth per pulse is defined.Figure 1 represents the dependence of this parameter on the applied laser pulse number for AlN ceramic.In all cases of pulse durations, after application of small number of laser pulses, the ablation depth per pulse is maximal, as with the increase of the pulse number it decreases.In the presented range, the dependence can be well approximated by a linear fit in logarithmic shale.There is a clear dependence of a decrease of the ablation rate with the decrease of the laser pulse duration.It can be seen at points of 100 and 1000 pulses where values for the three pulse durations are given.Single pulses with nanosecond duration results in ablation depth of few micrometer per pulse.In the case of femtosecond regime, the ablation depth in the presented range varies from few tens to few nanometers, which demonstrates the ability of femtosecond laser for precise material removal.

Figure 1.
Ablation depth per pulse for AlN as a function of the applied laser pulse number, for the three pulse duration regimes.Laser fluences are: 20 J/cm 2 for ns, 2 J/cm 2 for ps, and 0.7 J/cm 2 for fs regime.
The dependence of the ablation depth per pulse as a function of the applied laser pulse number for Si3N4 is given in figure 2. In this case, a clear decrease of the ablation rate with the pulse number increase is observed only for the case of ps regime of ablation.The application of nanosecond pulses, here also results in the highest ablation depth per pulse, as the value is about 1 µm and it is nearly constant in the studied pulse number interval.In the case of processing by femtosecond laser pulses, the values are lowest, of about ten of nanometers, as they also do not depend strongly on the change of the pulse number.

Figure 2.
Ablation depth per pulse for Si3N4 as a function of the applied laser pulse number, for the three pulse duration regimes.Laser fluences are: 20 J/cm 2 for ns, 2 J/cm 2 for ps, and 0.7 J/cm 2 for fs regime.
The ablation process strongly influences the morphology of the processed area.Figure 3 demonstrates SEM images of the ablation zones in AlN ceramic, for the three pulse duration regimes, and for application of 1000 pulses in all cases.An image of the native ceramic surface is also presented.The well-defined crystallites that form the native ceramic surface morphology are not present after processing.In the case of ablation with ns pulses, a porous surface, characterized by micrometer holes is formed.The smooth edges of the formed surface structures suggests of formation of melt.The ablation with the shorter laser pulses, results in formation of dome-like structures.In the areas between them, the ceramic surface is decorated by periodic structure, ripples.In the case of application of picosecond pulses, structures with different period are formed -a finer one with period of about 200 nm and second one with period of 1.2 µm.The femtosecond regime of ablation is characterized by a formation of ripple structure with period of about 200 nm at the presented conditions.The surface of the domes formed when ps and fs laser pulses are applied, is rough as substructure with characteristic dimensions in the range of tens of hundreds of nanometers can be seen.
The surface morphologies induced by the laser pulses with the different duration in Si3N4 ceramic are presented in figure 4. It shows SEM images for areas ablated by 1000 pulses, at the same fluences as mentioned in figure 3.An image, presenting the native ceramic surface, is also shown.It indicates that the surface morphology is defined by ceramic crystallites with a rod-like shape.The ablation with the laser pulses with the presented characteristics, results in a significant modification of the surface morphology.The crystallites seen in the surface of the native material are not present and a rough surface is formed.In the case of ns ablation, smoother edges of the surface structures are observed that also can be attributed to formation of melt.The structure formed at fs ablation is finest and the edges of the characteristic structures are sharpest.In all cases of pulse durations, formation of ripple structure can be seen in some areas.The applied laser fluences are 20 J/cm 2 for ns, 2 J/cm 2 for ps, and 0.7 J/cm 2 for fs regime.An image of the native surface morphology is also presented.

µm ns µm fs µm ps µm Native AlN
In order to estimate the composition changes due to the ablation process, XPS analyses of the ablated zone are performed.Figure 5 (a) shows Al2p and (b) Si2p XPS spectra and their convolutions, for ablated areas in AlN and Si3N4, respectively, and for processing at the different pulse durations.The convolutions and identifications are obtained on the basis of apparatus integrated software (ESCApeTM of Kratos Analytical Ltd.).Data about the native ceramic surfaces are also presented in figure 5.The ablation is performed at the conditions given in figure 3. The surface of the native AlN consists of AlN, Al-O, and Al-OH compounds.The ablation process generally results to a change of the amount of the different phases and to formation of new.In the case of nanosecond ablation formation of metallic aluminium (Al-Al) is detected.This is confirmed by a finding that in these conditions the surface becomes electrically conductive.The spectrum of the material formed after fs ablation is dominated by Al-C phase.Still the origin of the carbon compound is not clear, but it can be related to a presence of some organic material on the ceramic surface that during ablation form Al-C phase.It is still present, due to the lower ablation depth for this regime.For the presented conditions, the application of ps pulses leads to efficient preservation of AlN in the ablated area.The native Si3N4 ceramic consists of Si3N4, Si-N-O (sinoite), and Si-O.For nanosecond and picosecond ablation the formed material is mainly oxide and smaller amount of Si-OH.As in the case of AlN, the ablation with femtosecond laser pulses leads to formation of dominant amount of a carbon containing ophase, Si-C.On the surface of the processed ceramic, Si3N4 is not detected at any regime.
The process of laser ablation of AlN and Si3N4 ceramics is realized by different mechanisms depending on the absorbed energy and the developed temperature [12].The applied laser fluences are 20 J/cm 2 for ns, 2 J/cm 2 for ps, and 0.7 J/cm 2 for fs regime.Data about the native ceramics are also presented.
The mechanism that requires the lowest energy, is decomposition of the ceramics, where nitrogen is ejected and liquid Al or Si remains on the surface.When the absorbed energy reaches a certain level, direct ejection of aluminium or silicon nitride can be realized.The change of the laser pulse duration influences the ratio between the rate of the energy input and velocity of the relaxation mechanisms into the material.When the surface of a material is heated, the propagation of the heat wave into the material can be characterized by a specific length lth = (kτp) 1/2 , which shows the spatial dimensions of the propagation of the heat during the laser pulse duration, τp.k is the thermal diffusivity of the material.With the decrease of the pulse duration, the heat is localized in the absorption volume during the laser pulse, which results in a strong overheating.Thus, for ps and especially for fs regime, the ablation of ceramics could be regarded as ejection of nitrides.If decomposition which leaves Al or Si is realized it is in a very limited volume [12].In this case, presence of the native ceramic material could be expected in the ablated volume, as it is observed for example, in the case of ablation of AlN with ps pulses (see figure 5).These effects are also responsible for the negligible formation of melt in the case of application ps and fs pulses, as mentioned above.Since the ablation process in this study is realized in air, the formation of aluminium and silicon oxides is expected.Considering the ablation rate, one can consider also the laser absorption mechanisms that can depend on the laser pulse duration.For application of ns pulses, linear absorption dominates as the absorption coefficient for AlN and Si3N4 is in order of 10 2 cm -1 [13,14].At this value the laser radiation propagates into the material at depth of tens of micrometers.When the incident energy overcomes the threshold, an abrupt ejection of significant amount of material can be realized.This may explain the highest ablation rate for ns ablation regime.For the short pulse regime, realization of nonlinear absorption can be expected, due to the high intensities reached.This reduces the radiation depth propagation and in addition to the localization of the heat in the near-surface zone, leads to ablation of thinner surface layer.

Conclusions
In this paper, main characteristics of the ablation process of AlN and Si3N4 ceramics induced by laser pulses with nanosecond, picosecond and femtosecond duration are presented.It is found that the ablation rate (ablation depth per pulse) decreases with the increase of the applied laser pulse number for AlN ceramic.For Si3N4 such effect is seen only for picosecond processing.The ablation rate expresses a weak dependence on the pulse number when application of nanosecond and femtosecond pulses are used.For both ceramics the ablation rate is highest at nanosecond regime and lowest for the femtosecond one.The ablation process, at all pulse durations, results in significant change of the surface morphology, as in most of the cases formation of ripple structures is observed.These changes are also accompanied by modification of the material's composition, as formation of oxides is usually realized.The femtosecond ablation results in efficient formation of carbon-containing compounds.The differences in the ablation characteristics for the different pulse duration regimes can be explained by the realization of different mechanisms of ceramic decomposition -direct ejection of the basic ceramic material at picosecond and femtosecond regime, and efficient ejection predominantly of nitrogen and formation of Al or Si layer on the surface of the ablation zone, for nanosecond ablation.The obtained results can be used for a precise ablation and surface structuring of nitride ceramics.

Figure 3 .
Figure 3. SEM images of areas in AlN ceramic processed by 1000 laser pulses with different pulse duration.The applied laser fluences are 20 J/cm 2 for ns, 2 J/cm 2 for ps, and 0.7 J/cm 2 for fs regime.An image of the native surface morphology is also presented.Inserts show magnified images of the surface.

Figure 4 .
Figure 4. SEM images of areas in Si3N4 ceramic processed by 1000 laser pulses with different pulse duration.The applied laser fluences are 20 J/cm 2 for ns, 2 J/cm 2 for ps, and 0.7 J/cm 2 for fs regime.An image of the native surface morphology is also presented.

Figure 5 .
Figure 5. Al2p and Si2p XPS spectra and their convolutions for ablated areas in AlN -(a) and Si3N4 -(b), fabricated at the different pulse durations.The applied laser fluences are 20 J/cm 2 for ns, 2 J/cm 2 for ps, and 0.7 J/cm 2 for fs regime.Data about the native ceramics are also presented.

Table 1 .
Laser systems used in the experiments on laser processing of nitride ceramics.