Appl Phys A DOI 10.1007/s00339-009-5077-6

Measurement of femtosecond -induced damage and ablation thresholds in dielectrics

N. Sanner · O. Utéza · B. Bussiere · G. Coustillier · A. Leray · T. Itina · M. Sentis

Received: 2 September 2008 / Accepted: 15 December 2008 © Springer-Verlag 2009

Abstract The paper is focused on the importance of ac- peculiarity of using femtosecond pulses for processing ma- curate determination of surface damage/ablation thresh- terials is the possibility to separate in time the energy depo- old of a dielectric material irradiated with femtosecond sition (heating of electrons during the laser pulse) and the laser pulses. We show that different damage characteriza- damage appearance (energy relaxation occurring after the tion techniques and data treatment procedures from a single pulse). The energy is deposited in the material by nonlin- experiment provide complementary physical results char- ear absorption of photons via multiphoton or tunneling ef- acterizing laser–matter interaction. We thus compare and fects, followed by an avalanche mechanism leading to strong discuss two regression techniques, well adapted to the ionization. This free-electron initially enhances light measurement of threshold, and a statistical absorption, until a critical density is reached leading to a approach giving the laser damage threshold and further in- metal-like behavior of the dielectric. Then electron transport formation concerning the deterministic character of fem- and different energy relaxation channels eventually lead- tosecond damage. These two measurements are crucial for ing to damage and ablation (plasma expansion and/or mat- laser micromachining processes and high peak-power laser ter vaporization) are likely to occur in a quite complex technology in general. combination. The precise knowledge of these processes is crucial for the comprehensive understanding of the exper- · · PACS 42.70.CE 61.80.Ba 42.62.Eh imental observations and for predicting the damage and/or ablation thresholds of a material in the frame of micro- machining process development and high peak-power laser 1 Introduction technology in general. Part of the answer could be provided by parametric studies of laser-induced damage (and/or ab- Femtosecond are unique tools for micro- (nano-) lation) thresholds (LIDT/LIAT) with various experimental machining materials like transparent dielectrics, providing conditions (wavelength, pulse duration, polarization, num- benefits in terms of minimal invasiveness (reduced “Heat- Affected Zone”) and precision over longer laser pulses. The ber of pulses, material bandgap). However, even for similar experiments from different authors [1–6] reporting surface LIDT/LIAT measurements for fused silica samples irradi- N. Sanner () · O. Utéza · B. Bussiere · G. Coustillier · ated with single shot, ∼100 fs, 800 nm pulse, a large scat- A. Leray · T. Itina · M. Sentis tering of results, from 2 to 12 J/cm2, exists. Unfortunately, Laboratoire LP3, UMR 6182 CNRS—Université the dispersion of these measurements (F ≈ 10 J/cm2) is de la Méditerranée, Campus de Luminy, case 917, 13288 Marseille cedex 9, France largely superior to the absolute surface LIDT/LIAT value, e-mail: [email protected] thus preventing the precise determination of accurate data. Fax: +33-4-91829289 This issue is even more critical in the context of femtosecond laser-dielectric interaction as it is supposed to be extremely B. Bussiere Amplitude Technologies, 2 rue du Bois Chaland, CE2926, deterministic owing to its highly nonlinear nature [7]. In ad- 91029 Evry cedex, France dition, this particular feature is one of the main reasons why N. Sanner et al. ultrashort material processing is now acknowledged as a rel- evant technology for applications requiring a high level of accuracy. In particular, the spatial extent of the processed zone can be limited to the laser spot area for which the local fluence exceeds the material threshold, enabling to reach [8] or even beat [7] the diffraction-limited beam surface when the LIDT is surpassed only in the central region of the fo- cused distribution. This sharp ‘threshold’ ef- fect, arising from the highly nonlinear character of absorp- tion, is of prime interest for emerging nanomorphing appli- Fig. 1 Experimental setup. M: mirrors; Pol.: reflective Brewster po- cations, and is only obtained for pulse energies very close larizer; BD: beam dump; L:lensf = 100 mm to the material ablation threshold. The precise and reliable determination of material thresholds is then a crucial issue 2 Experimental setup for both fundamental and applicative breakthroughs. The problem of laser damage and ablation measurement The experimental setup is presented in Fig. 1.Thelaser in femtosecond regime has already been addressed in the source is a commercial S-pulse system from Amplitude Sys- literature. There are ex situ investigations of the diameter, tèmes, delivering 450 fs (controlled by a second-order auto- depth and morphology of damages by AFM [9], SEM [3], correlator), 1 kHz, 200 µJ pulses at 1025 nm. The intensity optical miscroscopy [10] or profilometry [11]. On the other distribution is Gaussian with a M2 factor equal to 1.3. The 2 hand, a multitude of in situ procedures are applied like time- M value was determined by studying the beam propaga- = of-flight [12], light scattering [13], time-resolved plasma tion with a long focal lens (f 300 mm) and a CCD Spiri- formation [14], time-resolved interference [15, 16], plasma con beam analyzer. The beam is linearly polarized, allow- radiation [17, 18], or transient reflectivity [9, 14]. Never- ing simple energy adjustment by means of a half-wave plate combined with a reflective polarizer used at the Brewster theless, there is no general agreement on the definition of angle. The incident beam on the sample is then s-polarized, thresholds (melting, damage, and ablation), and on the meth- whatever the tuning of energy. For small values of energy, ods of measurements (with their different detection limits). calibrated neutral density filters are added, in order to benefit Moreover, one observes a large variety of experimental se- from small and precise energy increment which is required tups, laser beam and material parameters (chemical material for an accurate LIDT determination. The beam is expanded composition, surface state, etc.). As a consequence, it ap- with an afocal system providing a beam radius w = 4mm pears unavoidable to observe a large scattering of threshold at 1/e2, and is focused onto the surface of the sample with a values, which makes the comparisons between experiments standard plano-convex BK7 lens of 100 mm focal length. very delicate. The target consists of the most widely studied transpar- In this paper, we present different techniques for sur- ent dielectric material, i.e. fused silica (Heraeus HOQ310, face LIDT/LIAT measurements used in the experiments thickness 2 mm and diameter 25 mm). To compare the sur- with femtosecond laser pulses. The proposed techniques are face LIDT/LIAT results deduced from the three methods based on a single experimental setup but different post- presented below, and because these thresholds may depend mortem analysis and data treatment. Assumptions respon- on surface imperfections (scratches, cracks, grooves, etc.), sible for systematic errors are considered. We show that in- roughness, exact chemical composition or contamination, all formation on laser ablation and/or laser damage threshold experiments are performed on the same SiO2 sample pol- ished with standard optical quality. The sample is mounted of a material are preferentially obtained, depending on the on a three-axis computer-controlled translation stage and its applied technique and procedure of treatment of the experi- position is carefully adjusted by combined energy-scan and mental data. z-scan procedures, allowing to precisely locate the surface The paper is organized as follows. Section 2 describes at the waist position with an accuracy much better than the the setup configuration including precise description of the Rayleigh range, ensuring the accuracy of the measurements. laser source, the diagnostics and the experimental protocol. As an example, for the 100 mm focal lens used in this ex- The appropriate definitions of material damage and abla- periment, the half Rayleigh range is ∼350 µm and the final tion thresholds are discussed in the same section. Then we z-scan step amounts to 10 µm. A far-field imaging system is present in Sect. 3 the measurement and data exploitation also implemented. This system consists of the focusing op- techniques for surface LIDT/LIAT determination, which are tics itself combined with a CCD camera and its objective, compared and discussed in Sect. 4. providing real-time visualization of the target surface with Measurement of femtosecond laser-induced damage and ablation thresholds in dielectrics a high magnification. The target surface is illuminated with tor is measured and subtracted from the recovered single- incoherent grazing red light (Stocker Yale Specbright LED shot amplifier energy value. Note the pulse energy ratio, −9 spotlight centered at 630 nm), so that the imaging system E1shot,oscillator/Esingle-shot,amplifier ∼ 10 , is largely inferior is the equivalent of an in situ dark-field microscope, which to any material modification threshold, even under multi- collects the reflected and scattered light coming from per- shot irradiation [13, 19]. The pre- and post-pulses are mini- manent damages induced by the laser pulses. This setup en- mized by a fine tuning of the extraction delay of the regener- ables to reveal damages smaller than the laser waist at the ative amplifier (contrast ratio ∼250). The single-shot regime focus. All experiments are performed under ambient air in a is obtained by triggering of the internal Pockels cell, leading single-shot regime, and the sample is systematically moved to shot-to-shot fluctuations ≤20%. These fluctuations are at- to a fresh zone after each shot, even if no damage is detected. tributed to the non-optimal management of the instant of ex- Indeed, multi-shot experiments imply incubation effects due traction of the pulse from the regenerative amplification loop to accumulation of pulses on the same site, which consider- when operating the laser in that single-pulse regime. This ably lower the threshold [3] and lead to results more delicate could be optimized by implementing an additional Pockels to interpret. Our damage diagnostic is based on ex situ op- cell outside the laser, gating a single pulse from the nominal tical and/or atomic force (AFM) microscopy analysis. Note kilohertz pulse train. Nevertheless, during the experiments, that the laser-induced modifications of the material are com- the amplitude of shot-to-shot fluctuations can be controlled patible with the accuracy and resolution of these characteri- by a photodiode. zation tools. In order to calculate the fluence, the parameter “surface” We define damage as a permanent irreversible modifi- has to be evaluated. In the frame of LIDT/LIAT measure- cation of the morphology of the material surface, which ments, the surface can be defined in two ways. On the one can be detected by an AFM or an optical microscope with hand, one can consider the real beam size in the plane of the adapted magnification and illumination. Transient reversible target as it can be characterized by a beam analyzer with a phase changes or permanent structural modifications (lead- CCD camera or a knife-edge scanning system. On the other ing to different material properties like a refractive index hand, one can extrapolate the beam size from laser-induced rise) without any change in surface topology are, thus, not transformation measurements of the surface of a material as considered as damage. When the surface modification is ac- first suggested by Liu for 20-ps long Gaussian pulses [20]. companied by material removal, we speak about ablation. Dealing with femtosecond laser pulses, one should con- The ablation of the material is easily put in evidence by the sider the transport of the laser beam onto the target and, AFM, enabling to measure the ablated volume, and quali- in particular, the impact of the laser focusing system on tatively by the optical microscope by varying the focus to the beam characteristics (waist and pulse duration) at the detect the formation of a crater. focal plane, where the sample is studied. In all the exper- In short-pulse regime (nanosecond to femtosecond) and iments described hereafter, the beam power P is below 2 for a given laser source (pulse duration and wavelength), the critical power (P = 3.77λ ) for self-focusing in air cr 8πn0n2 the usual parameter to express the surface LIDT/LIAT of (P ∼ 2–20 MW

This technique is based on the measurement of the damage 3.2 Volume-regression technique area of the material, here with an optical microscope. It was first proposed for indirectly recovering the intensity distri- Another regression technique can also be implemented by bution of a 20-picosecond laser [20] based on the fact that using measurements of the ablated volume. Fig. 3 shows the the damage size depends on the pulse fluence. For the con- volume of ablated material (averaged over 6 points) mea- sidered range of pulse duration, this method provides the sured by an atomic force microscopy system (AFM XE-100 LIDT of the fused silica target by means of damage diame- from Park Systems, Inc.) versus the laser pulse fluence. The ter measurements. The radial fluence distribution at the fo- linear shape of this curve might help for identification of = cal point of the Gaussian beam is given by [26]: F(r) predominant absorption channels. − 2 2 − 2 2 2Emeas 2r /w0 = 2r /w0 2 e Fmease ,Emeas being the measured It is now well established that photo-ionization and πw0 laser energy, r the radial coordinate and w0 the beam waist electron–electron impact ionization are responsible for the radius measured at 1/e2. The main assumption states that, if optical breakdown in dielectrics [27–29], but their relative the material is not damaged at the distance r from the center importance strongly depends on laser parameters (mainly of the beam, the corresponding value of fluence F(r)equals intensity and pulse duration, as pointed out in reference Measurement of femtosecond laser-induced damage and ablation thresholds in dielectrics channel in our experimental laser conditions is most prob- ably related to avalanche ionization, which is predicted to scale linearly with the laser intensity according to standard kinetic models [27]. This result further confirms previous findings [31], though the latter were obtained in a multi- shot regime. Finally, with this volume-regression technique, the threshold fluence (resp. energy) is then measured to be 2 Fth = 4.25 J/cm (resp. Eth = 7.65 µJ), in agreement with the first regression method relying on diameter measure- ments.

3.3 Statistical approach

This method is usually used for laser damage studies with long pulses (typically nanosecond). In this regime, laser Fig. 3 Ablated volume vs. laser fluence. The data of ablated volume damage exhibits a probabilistic behavior, contrary to the ex- are averaged over 6 points; the vertical error bars shown in the figure pected deterministic one for ultra-short pulses. In fact, in the correspond to the amplitude of the standard deviation. The threshold is determined by the fluence value attained when the linear fit goes down nanosecond regime, damage occurrence is more dependent to V = 0 on impurity concentration than on intrinsic material proper- ties, therefore leading to a non-deterministic behavior [37] and to damage dimensions which are not directly related to [30]). Concerning photo-ionization mechanisms, the inten- the laser beam spot size. The technique used for the LIDT sities used here (I<1013 W/cm2) involve for the Keldysh  determination relies, thus, on statistical experiments. For parameter (defined by γ = ω/e m cnε E /I ,see[30]) e 0 g constant laser fluence below the intrinsic material threshold, a value ∼3, suggesting that multiphoton absorption pre- damage will occur if the laser spot hits a defect while no vails upon tunneling ionization for the generation of very damage will be observed if no defect stands in the laser fo- first free-electrons. In our case a minimum of eight pho- cus area. As a result, the answer is binary, that is to say we tons of 1.21 eV energy is required to transfer an elec- only consider the presence or absence of any damage after tron from the valence to the conduction band (fused silica the laser shot [37]. bandgap E = 8.9 eV). This population of seed electrons g Here, we use this technique for precisely determining undergoes a further rise by impact ionization, usually de- the surface LIDT of the sample irradiated with femtosec- scribed by avalanche ionization models for dielectrics [27, ond pulses. We measure the probability to produce damage 29]. However, the respective contributions and probable in- for a set of different fixed laser energies. For each energy terplay of the two processes are still under discussion [29] case, 50 laser shots are applied on 50 different sites (single- due to contradictory existing experimental results [27, 31, shot regime) of the fused silica sample which is examined 32]. This is a fundamental question because the ratio be- afterwards under optical microscopy to count the number tween photo-excited and impact-ionized electrons enables of damaged sites. Only the occurrence of the damage is to estimate the temporal evolution of the electron density reported in that approach without any need of quantitative in the material and hence the transient absorption of the measurement of a physical data like the diameter of a dam- laser energy during the pulse (in other words, the dynam- aged or ablated zone. This procedure automatically mini- ics of the laser energy deposition in the material) and the mizes the errors related to the precision and sensibility of dynamics of the plasma formation. The absorption itself is the diagnostic system and of the operator as well. This last indeed a quite complex phenomenon as the free-electron point can be particularly important when dealing with (sub-) plasma generated by the initial part of the pulse can either micrometer laser damage, close to the limit of resolution and absorb the later part more efficiently or act as a plasma mir- detection of a classical optical microscope. ror and reflect most of the incident energy [33, 34]. In a The results are plotted on Fig. 4. Two LIDT values can 2 complementary experiment [35], we measured a linear evo- be extracted: Fth,low = 2.2J/cm (resp. Eth,low = 3.9µJ) 2 lution of the material absorption with respect to laser flu- and Fth,high = 3.5J/cm (resp. Eth,high = 6.2µJ).Thelow ence increase (for the range Fth ≤ F ≤ 3Fth), in agreement threshold value is the highest fluence for which the damage with other published results [5, 36]. A linear rise of the ab- probability equals zero. Surface damage begins to appear lated volume with the pulse fluence was thus expected close (but not systematically) if the pulse fluence is just superior. to threshold, and we therefore infer the damage threshold The high threshold is the lowest fluence for which damage by extrapolating the linear regression to zero. This exper- is systematically produced for each of the 50 trials. In Ta- imental observation suggests that the dominant absorption ble 1, high- and low-thresholds are reported, and also what N. Sanner et al. Table 1 Energy and fluence thresholds measured with the three independent techniques (see the text for details). Fluence thresholds are calculated using the experimentally beam waist, w0 = 10.7 µm. When possible, the calculated beam waist inferred from the treatment of the experimental data is given for comparison

Technique (used diagnostic) Regression diameter (optical microscope) Regression volume (AFM) Statistical (optical microscope) Low High Mean

Fluence threshold (J/cm2) 3.7 4.25 2.2 3.5 2.8 Energy threshold (µJ) 6.7 7.65 3.9 6.2 5.0 w0 (retrieved) (µm) 8.7 – – – –

old measurements are worth being underlined. The first two techniques (diameter- and volume-regression) integrate by definition the response of the material to the laser excitation through the quantitative measurement of changes induced on a physical parameter of the sample (diameter of the af- fected zone or of the drilled crater on the surface target, ab- lated volume, etc.). The diameter-regression technique re- lies on the assumption that the measured diameter of the damage is equal to the diameter of the Gaussian beam for a fixed fluence level. In other words, this technique assumes that the material damaging is deterministic. The method is, therefore, not adapted to threshold determination with laser pulses longer than a few tens of picoseconds, for which the Fig. 4 Damage probability vs. laser fluence. The high- and damaged region can extend to a significantly larger area than low-fluence thresholds are defined respectively by damage probabili- the irradiated zone, mostly because of thermal effects [38]. ties equal to 1 and 0 On the contrary, this technique is especially relevant for fem- tosecond laser pulses, which are known to drastically reduce can be called the mean-threshold, corresponding to the en- surrounding unwanted effects on the material target (e.g. ergy needed to obtain a 50% damage probability. thermal or mechanical damage [39, 40]) for pulse energies close to the threshold. This is due both to a better localiza- tion of energy absorption provided by its nonlinear nature, 4 Discussion and also to the relatively low dependence on the presence of defects for which absorption cross sections are smaller [41], The experimental results are summarized in Table 1.Eval- thus theoretically favoring intrinsic phenomenon. Now, the uating fluence thresholds is not totally straightforward due linearity of the fit in logarithmic scale (see Fig. 2) can be to the Gaussian intensity distribution of the focal spot. In- discussed when incident laser energies are very close to the deed, the material response is sensitive to the local peak threshold level. The corresponding points are not reported fluence (that is why a smooth intensity distribution with- on the graph because optical microscopy does not offer suf- out any hot spots is of prime importance) which is differ- ficient spatial resolution to carry out accurate measurements ent from the average value, integrated over the whole cross- (see for instance Fig. 5a). On the opposite, when the inci- sectional area measured by a power-meter placed in front of dent energy is high, the damages can exhibit a very irregular the beam. It is the reason why, in the previous section and spatial shape resulting from evident mechanical cracking or in Table 1, the fluence threshold was calculated by taking spallation (see for instance Fig. 5c). These points are ob- into account the surface correction considering the equiva- viously not considered because the diameter measurements = 2 lent “top-hat” laser beam surface (S πw0/2), a uniform are not representative. Actually, Fig. 5b shows typical dam- cylindrical beam with the same total energy and peak flu- ages, the diameter of which can be non-ambiguously mea- ence [26]. The laser fluence threshold Fth was then simply sured. As a result, the diameter-regression technique often = 2 expressed by: Fth 2Eth/πw0. This expression yields the does not consider a set of data fully representative of the threshold twice larger than that usually given in the litera- Gaussian beam (in other words, the feet and the peak of ture. the Gaussian beam can roughly be ignored in that analy- At this point, important remarks concerning the relia- sis), thus potentially inducing error in the derivation of the bility and the significance of the different material thresh- energy threshold and beam waist from the data. Measurement of femtosecond laser-induced damage and ablation thresholds in dielectrics quantitative data which have to be unambiguously defined and further measured. As precise measurement of these quantitative data can be challenging at fluences very close to the LIDT threshold, they are systematically ignored, fi- nally not leading to the determination of the damage thresh- old but of the laser ablation threshold (LIAT) of the ma- terial. Now, it should be emphasized that although arising from totally independent techniques (different equipment for damage measurements and different data treatment), the

Fig. 5 Photographs of laser-induced damage in SiO2 at low (a), two LIAT thresholds obtained by the two regression tech- medium (b) and high (c) fluence. The analysis is made using the optical niques (diameter and ablated volume) are in good agreement microscope (magnification × 100) 2 2 (Fth,diameter = 3.7J/cm and Fth,ablated volume = 4.25 J/cm ). The third independent procedure is the statistical tech- The second regression technique using AFM volume nique. This technique does not need any physical assump- measurements is less questionable, even if the same diffi- tions (neither on the material answer nor the nature and/or culty of accurately measuring an ablated volume very close precision of the measurement of the physical data) since it is to the threshold still exists. Indeed, near the threshold, the based on the observation of the damage occurrence, which surface topography can be altered showing small pits and/or just depends on the sensibility of the used diagnostics. This pikes, melted zones, etc., but not leading to a measurement technique is therefore particularly well adapted to the mea- of an ablated volume. Now, in the considered fluence range surement of the damage threshold (rather than the ablation just above the threshold (see Fig. 3), the evolution of the threshold) of a material as it does not reside on any quanti- ablated volume with the pulse fluence follows a linear law tative measurement of a physical data. Such a measurement making it easy to extrapolate the fluence threshold from the then can provide different and complementary information with respect to the regression techniques which are mostly ablated volume data set. This determination of the fluence devoted to laser ablation threshold analysis of a material. threshold is done without any assumption concerning either Actually, the low threshold (F = 2.2J/cm2) has to be the focal spot energy distribution or the deterministic char- th,low related to the laser damage of the material (LIDT), which is acter of the material threshold. The AFM analysis can also significantly lower than the LIAT threshold obtained by the provide an independent estimation of the fluence threshold regression techniques (F = 3.7J/cm2). Interestingly, the and beam waist, by using the diameter-regression technique th high threshold (F = 3.5J/cm2) is measured to be close presented in Sect. 3.1 with AFM measured data, respectively th,high to the LIAT threshold. Note that the determination of this giving F = 3.3J/cm2 (E = 5.95 µJ) and w = 10.15 th th 0 high LIDT threshold is important for the technical develop- µm (in good agreement with the waist measured with the ment of material processing applications, because it corre- beam analyzer). The discrepancy in the inferred F , E th th sponds to the lowest fluence for which the modification of and w0 data illustrates the high sensitivity of the regres- the processed material is assured. Therefore, this operating sion techniques, related to the dispersion of the quantitative laser condition should be appropriate in terms of minimal data (crater diameter and ablated volume), the accuracy of invasiveness and optimal processing/machining quality. the numerical fit and the resolution of the diagnostic tools Furthermore, unlike the two previous regression-based (optical microscope and AFM). As a result, the regression techniques, the statistical study can easily include a large techniques are in general less accurate than the statistical number of shots, thus enabling to average both shot-to-shot approach for which the vertical error bar is considerably fluctuations of the laser energy and spatial heterogeneity of minimized when using a sufficient number of trials and an the SiO2 target. High accuracy and reliability are then ex- adapted diagnostic tool. Note also that deviations from the pected using this technique of LIDT characterization. An- theoretical waist value, which was experimentally confirmed other valuable interest is that this technique does not require by the beam analyzer, can be partly attributed to potential to measure the damage dimensions (which could be not triv- experimental errors concerning the positioning of the sur- ial to carry for example on biological tissues) and could face with respect to the beam focus location and diameter then been implemented as an in situ diagnostic, thus signif- measurements in a misfit or incomplete fluence range (with icantly increasing the treatment speed. Moreover, the statis- respect to the fluence threshold). tical method also provides additional information about the Finally, it is interesting to consider the physical meaning sharpness of the threshold, that is to say the deterministic of the information given by the two regression techniques. character of laser damaging. Note that the fluence difference 2 We believe that these regression methods determine the ab- between high and low LIDT values is FL→H = 1.3J/cm lation threshold (LIAT), rather than the damage threshold and seems to be rather high to describe what is usually be- (LIDT). This is largely due to the fact that they both utilize lieved to be a sharply deterministic phenomenon. Part of N. Sanner et al.

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