Effect of Plasma Shielding on Laser Ablation Rate of Pure Metals at Reduced Pressure

Transcription

1 SURFACE AND INTERFACE ANALYSIS Surf. Interface Anal. 27, (1999) Effect of Plasma Shielding on Laser Ablation Rate of Pure Metals at Reduced Pressure J. M. Vadillo, J. M. Fernández Romero, C. Rodríguez and J. J. Laserna* Department of Analytical Chemistry, Faculty of Sciences, University of Málaga, Málaga, Spain The ablation rate expressed as the amount of removed material per laser shot was calculated for pure metal samples under different experimental conditions: laser fluence ( J cm 2 ), buffer gas (air, He and Ar) and gas pressure ( mbar). Fluence values covered the range between the plasma threshold (~1 2 J cm 2 for most elements) and 16.7 J cm 2. The 581 nm output of an excimer-pumped dye laser was used. Results pointed out a strong dependence of ablation rate on experimental parameters. At high fluence, the ablated material efficiently attenuates the incoming laser radiation (plasma shielding) and reduces the ablation rate. The extent of this shielding effect depend also on the experimental variables (buffer gas, pressure) and sample nature. These studies are useful to determine the amount of ablated material as a function of experimental parameters, to understand the extension of the shielding process and to establish the conditions under which it may be avoided. Copyright 1999 John Wiley & Sons, Ltd. KEYWORDS: laser-induced plasmas; plasma shielding effect; laser ablation; LIBS INTRODUCTION Laser ablation has found a wide range of applications, including thin-film fabrication, 1,2 microfabrication, 3,4 and medical applications, 5 where the knowledge of amount of mass removed as a function of the experimental variables (including laser energy, atmospheric conditions, laser wavelength and sample type) is critical to control and model the process. Furthermore, laser ablation has found a growing interest in chemical analysis, particularly as a way to introduce solid samples into inductively coupled plasma (ICP) systems 6,7 or by laserinduced breakdown spectrometry (LIBS). In LIBS, at a certain energy (characteristic for each matrix) exceeding the material s breakdown threshold, the sample becomes dissociated, atomized and partially ionized to form a high-temperature and high-electron-density plasma. This microplasma may be analysed by optical emission spectrometry. Spectrally and temporally resolved detection of the specific atomic emission will reveal analytical information about the elemental composition of the sample. 8 Laser induced breakdown spectrometry is chiefly performed in air at atmospheric pressure, 9 because working in a vacuum hinders one of the main LIBS advantages: easy sample-handling. However, different experiments have been done with several buffer gases or vacuum conditions * Correspondence to: J. J. Laserna, Department of Analytical Chemistry, Faculty of Sciences, University of Málaga, Málaga, Spain. laserna@uma.es Contract/grant sponsor: Spanish Dirección General de Investigación, Ciencia y Tecnología, Ministerio de Educación y Cultura, Madrid; Contract/grant number: PB Contract/grant sponsor: Spanish Secretaría de Estado de Universidades, Investigación y Desarrollo. Contract/grant sponsor: University of Córdoba and Junta de Andalucía; Contract/grant number: DGICYT PB On leave from Department of Analytical Chemistry, Faculty of Sciences, University of Córdoba, Spain. in an attempt to decrease the high background emission typically associated with LIB spectra or to tailor the excitation conditions. 10 Furthermore, optical monitoring of laser plasmas at moderate to high vacuum levels and many different inert and reactive atmospheres is required in specific instances (laser-ablation deposition, emission spectroscopy in the low-uv region, etc). Although the vacuum conditions introduce a degree of complexity in the experimental set-up, the non-invasive characteristics of LIBS (because only optical access to the sample is required) makes the technique ideal for optical monitoring of high-vacuum processes, including advanced laser processing of materials. 11 Depending on the application, different fluence conditions are used. The high-fluence regime is preferred for processes looking for more stable excitation plasma conditions (i.e. LIBS or laser ablation coupled to ICP); however, in thin-film production, low fluences are chosen to make laser vaporization the more significant process. Unfortunately, the physical processes involved in laser beam interaction with metallic targets, dynamic expansion of the plume and many non-lineal phenomena taking part in the process are very matrix- and fluence-dependent and remain, to date, not well-explained. 12 For instance, laser radiation absorption in the plasma plume, specially at high fluences, plays a substantial role in the evaporation process and in the spectral emission in laser microanalysis. The influence of the ablated material leaving the surface during the laser pulse has to be taken into account 13,14 because it attenuates the incoming laser radiation by absorption or scattering, affecting optical monitoring due to the non-linear behaviour of the emission line intensities at moderate fluences. 15,16 The advantages of tailoring the excitation conditions in LIBS have been described in a previous work 10 where depth-resolved analysis of coated materials under air and Ar at 1000 and 1 mbar was performed. The detrimental effect of redeposition of previously ablated material on depth resolution was proved, as well as the advantages CCC /99/ $17.50 Received 3 May 1999 Copyright 1999 John Wiley & Sons, Ltd. Revised 17 July 1999; Accepted 17 July 1999

2 1010 J. M. VADILLO ET AL. of working in a vacuum. However, there was a lack of sensitivity due to the decrease of the emission line intensities. In the present work, the effect of laser fluence, buffer gas (air, Ar and He) and pressure ( mbar) on the averaged ablation rate (AAR) for different pure metals will be studied. The results will serve to understand the extension of the shielding process and to establish the experimental conditions under which it may be avoided. EXPERIMENTAL Instrumental A detailed description of the same experimental set-up may be found in a previous work, 10 so no redundant information will be provided here. Briefly, the system consists of an excimer-pumped dye laser operating at 581 nm. The laser beam (in the fluence range Jcm 2 ) was focussed directly onto the samples using a quartz planoconvex lens. The fluence used was high enough to ablate the samples and to generate a microplasma that was characterized by its optical emission spectrum. The samples were placed within a vacuum chamber, where the effect of different surrounding conditions (gas type and vacuum level) was easily demonstrated. Materials The metals Zn, Al, Cu, Ni, Fe, Mo, W and Ti (250 µm nominal thickness, 99.9% purity) were purchased from Sigma (St Louis, MI). Prior to analysis, the samples were cleaned with methanol and rinsed with deionized water. These elements were chosen because they differed largely in their physical properties (summarized in Table 1). Measurement of ablation rates In order to calculate the ablation rate for the materials used, different foils were placed on a laboratoryconstructed polymethylmethacrylate (PMMA) holder and the laser was fired repeatedly at the same position, eroding the sample continuously. Care was taken to keep the same geometrical configuration between different experiments, particularly the lens-to-sample distance and the laser energy. After a number of laser shots the sample was drilled and the incident laser was observed clearly through the transparent PMMA holder, as evidence of the whole depletion of the foil. Because the ablation rate is not constant along the sample thickness, the AAR (in µm per shot) is the calculated value for the number of laser shots required to drill the foil (250 µm). This method has been described previously to determine the ablated mass per pulse in 100 µm thick stainless-steel foils. 17 The reproducibility in calculation of the AAR values, expressed as the relative standard deviation (RSD, %), was better than 5% for five measurements in the same day. This result was observed regardless of the sample, whereas the day-to-day values differed notably as a consequence of the variations in the output energy of the pumping laser. Bearing in mind these two facts, the AAR values were calculated in a single-run basis on the same day for comparative purposes. RESULTS AND DISCUSSION Effect of fluence on ablation rate The effect of laser fluence on AAR was checked using the experimental procedure described above for six metals differing largely in their physical properties (Al, Cu, Mo, Ti, W and Zn). The results, summarized in Table 2, cover the fluence interval from 1.3 Jcm 2 (close to the ablation threshold of the cited elements) to 16.7 Jcm 2. A diaphragm placed after the amplification cuvette of the dye laser kept the beam diameter constant. In order to observe the trend regardless of the absolute AAR value for each element, data were normalized to the low fluence AAR value within each set. The representation is Table 2. Calculated averaged ablation rate (AAR) values (in µm per pulse) for different pure metals as a function of laser fluence in air at atmospheric pressure Fluence (J cm 2 ) Al Cu Mo Ti W Zn Table 1. Physical constants for different elements a Tb Tf Lv Lf Cp K (K) (K) (g cm 3 ) (J g 1 ) (J g 1 ) (J K 1 kg 1 ) (W mk 1 ) H R Zn Al Cu Ni Fe Mo W Ti a T b, boiling point; T f, melting point;, density;l v, latent heat of evaporation; L f, latent heat of fusion; C p,specific heat; K, thermal conductivity; H, hardness; R, reflectivity (at 532 nm). Surf. Interface Anal. 27, (1999) Copyright 1999 John Wiley & Sons, Ltd.

3 PLASMA SHIELDING EFFECT ON LASER ABLATION OF METALS 1011 Figure 1. Variation of AAR versus fluence showing the non-linear behaviour. The data (normalized to the lowest fluence) were obtained in air at atmospheric pressure. shown in Fig. 1. The AAR shows a non-lineal increase with fluence, reaching a saturation level at ¾5.3 Jcm 2 ¾0.2GWcm 2. For Al and Zn this value was slightly higher and the plateau was observed at ¾8 J cm 2 ¾0.3GWcm 2. In order to check the dependence of this cut-off value with the vacuum level, the same experiment was performed but at a lower pressure of 10 5 mbar. The same trend was observed in the curves although, as expected, larger AAR values were obtained due to free expansion of the plasma. This non-linear dependence of material removal with fluence is in good agreement with that obtained previously by monitoring the intensity in the generated plasma. 18 The explanation for this fact is the shielding of the exposed surface by the plasma formed above it. The laser-induced surface plasma becomes optically dense at a high power density, and the later part of the laser pulse energy will interact with the plasma, to be absorbed or reflected. This effect is wavelength dependent because the plasma resonant frequency, p, is described by the equation 11 p D 4n e e 2 /m e 1/2 D 8.9 ð 10 3 n e 1/2 where n e is the electron number density cm 3 in the plasma, e is the electron charge and m e is the electron mass. In order for substantial absorption to occur in the plasma, 1 / p ¾ 1, where 1 is the laser frequency; otherwise, the laser radiation is reflected. Because none of the variables except for electron density are altered in the equation, the change in the slope observed in Fig. 1 for the different materials and experimental conditions seems to suggest a change in the electron density of the plasma. However, no experimental approaches have been devised to determine this point. Ablation rate at reduced pressure in air Although LIBS is preferably performed in air at atmospheric pressure, in some instances (thin-film deposition, micropatterning) it is interesting to understand the effect of fluence on AAR and emission intensity at different vacuum levels. The studies were developed in the range mbar for different elements. The results are plotted for Al, Fe and Ti in Fig. 2. As shown, the AAR shows an increase with decreasing pressure from 1000 to Copyright 1999 John Wiley & Sons, Ltd. Surf. Interface Anal. 27, (1999)

4 1012 J. M. VADILLO ET AL. ¾250 mbar, regardless of the fluence and sample type. These results are in agreement with similar experiments where the ablation deposition rate was measured using a quartz microbalance. 19 This effect is explained on the basis of plasma expansion at low pressure because the ablation plume can expand freely. Another interesting conclusion may be extracted from Fig. 2. As shown, the AAR reachs a plateau at ¾200 mbar for every metal, regardless of the fluence. This indicates that the amount of material removal remains constant at a given back pressure, and it is high enough to allow complete masking of the incident beam. Thus, the useful energy reaching the sample would be quite similar, as well as the AAR values. This is proved for different elements: an increase in fluence of ¾300% produces a change in the AAR of <25% in each instance. The emission intensity of the Ti line at nm was also checked and the results are plotted in Fig. 3 for different fluences between 1.3 and 13.2 J cm 2. As shown, an increased vacuum produces a decrease in the net intensity. This fact has been explained previously 20,21 as an effect of shadowing due to the larger amount of sample vaporized at low pressure than at atmospheric pressure. Furthermore, plasma confinement should be taken into account. At atmospheric pressure, the hot plasma formed results not only in strong analytical emission but also in an intense background continuum. In a low pressure atmosphere, the rapid expansion of plasma causes a weakening of the signal intensity due to the rapid escape of emissive species from the observation region (the so-called optical time-of-flight). Effect of buffer gas on ablation rate The efficiency of laser ablation will depend obviously on the composition and pressure of the surrounding atmosphere. Although working under different conditions than air at atmospheric pressure introduces some complexity in a LIBS set-up and hinders one of its advantages, the understanding of the AAR under other atmospheric conditions may be relevant. For this purpose, the AAR values were obtained in Ar and He atmospheres at 1000 mbar as well as in air at 1 mbar. For a better comprehension of the variation, the data were normalized to the AAR in air at 1000 mbar. The results are shown in Fig. 4 for Ti, W, Fe, Cu and Mo at different fluences, namely 5.3, 7.7 and Figure 2. Variation of AAR with pressure, fluence and sample type. Surf. Interface Anal. 27, (1999) Figure 3. Effect of pressure and fluence on the emission line intensity. Data wereobtained from 1000to 0.1mbar, monitoring the Ti(I) line at nm. Copyright 1999 John Wiley & Sons, Ltd.

5 PLASMA SHIELDING EFFECT ON LASER ABLATION OF METALS 1013 Figure 4. Variation of AAR with buffer gas. The experiments were carried out in He and Ar at atmospheric pressure and air at 1 mbar. For a better comparison, the data were normalized with those obtained in air at 1000 mbar J cm 2. In general, the effect of He at 1000 mbar and air at 1 mbar is quite similar (not only in the trend, but also numerically), increasing the AAR slightly. This effect is more acute with increased fluences. With regard to Ar, the overall effect is a decrease of the AAR; this effect is independent of fluence. The result for Ar is not surprising because the absorption of laser radiation by the plasma is more significant in Ar than in He or in air; thus, more absorption of the laser energy reaching the sample necessarily implies that less sample is vaporized. 22 Comparison between theoretical and calculated ablation rates The theoretical ablation rate (TAR, in µm per pulse) values were estimated from the expression used to calculate the ablated mass, which involved the main physical and thermal parameters affecting laser ablation. 23 The ablated volume is identical because the foil is completely drilled, so the equation was conveniently converted into length units introducing the density of each element. The final expression is F 1 R TAR D C p T b T 0 C L v ] where F D fluence (in J cm 2 ), R D reflectivity at 581 nm; C p D specific heat (in J K 1 kg 1 ), T b D boiling point (in K), T 0 D room temperature (in K), L v D latent heat of evaporation (in J kg 1 ) and D density (in kg cm 3 ). The comparison between TAR and AAR at different fluences is presented in Fig. 5. As shown, at low fluences (close to the ablation threshold) the AAR Copyright 1999 John Wiley & Sons, Ltd. Surf. Interface Anal. 27, (1999)

6 1014 J. M. VADILLO ET AL. Figure 5. Comparison between AAR and TAR at different fluence values. The comparison was made in air at atmospheric pressure. is two to four times larger than TAR (with the only exception of W). A similar situation may be found in the literature when monitoring the emission intensity, 24 where the emission takes larger values at low fluence. This behaviour has been explained as an increased degree of elemental fractionation at the low fluence regime. Increased fluences produce an inverse behaviour: the TAR values becomes larger than AAR. This fact may be taken as an overestimation because the equation guesses a 100% coupling in the laser energy, without taking into account plasma shielding. At ¾5.3 J cm 2, both values are quite similar for almost every studied element. CONCLUSIONS Our measurements point out that there is a strong influence of the ablation rate with experimental parameters, especially laser fluence. From a given value, the ablated material efficiently attenuates the incoming laser radiation and reduces the ablation rate. The amount and effect of this shielding depend also on the experimental variables and the sample type (buffer gas, pressure). The mechanisms by which the ambient atmosphere and the pressure influence the generation processes of the plasma still remain unclear. Further studies are needed in order to understand how the difference in atmosphere will change the physical quantities, such as the amount of varporized sample and the excitation temperature, and how the sample characteristics will affect the plasma development. If bulk analysis is the objective and there is no limitation in terms of light collection, it seems clear that the common LIBS configuration (in air at atmospheric pressure) presents no limitations. However, when depth profiling is going to be carried out, it could be important to control the volume of the sample that has interacted with the incident laser. Although the first choice could be the use of different laser fluences to increase the ablation rate and thus increase the number of excited atoms, it has been demonstrated that increased fluences do not yield larger AAR values. However, the amount of ablated mass that is atomized and contributes to the emission lines, or is simply vaporized, has not yet been stated. Further studies must be directed towards this. The use of He as a buffer gas introduces advantages in some elements that increase their AAR in a similar manner to that in air at low pressure. In this sense, an He gas-jet directed to the sample surface would be more useful and less tedious than using a vacuum chamber. The lower AAR values were obtained with Ar, which seems to be the best alternative if laser ablation is to be used for depth-profiling analytical purposes. On this point, the use of low pressure is not a goal because the AAR is higher in all instances. However, the non-specific background at early times that is typically obtained in optical emission spectra may be diminished considerably and its use may be justified in some instances. Anyway, the effects of using moderate low pressures (in the range mbar) are similar to those obtained at much lower pressures (<10 1 mbar) and, again, the experimental setup can be greatly simplified. Acknowledgements This work was supported by the Spanish Dirección General de Investigación, Ciencia y Tecnología (Ministerio de Educación y Cultura, Madrid, Project PB ). J.M.V. thanks the Spanish Secretaría de Estado de Universidades, Investigación y Desarrollo for providing a fellowship. J.M.F.R. wants to acknowledge the University of Córdoba and Junta de Andalucía for supporting his stay (Project DGICyT PB ). REFERENCES 1. Inam A, Hedge MS, Wu XD, Venkatesan T, England P, Micelli PF, ChaseES, ChangCC, Tarascon JM, Wachtman JB. Appl. Phys. Lett. 1988; 53: 908. Surf. Interface Anal. 27, (1999) 2. Mao XL, Russo RE, Liu HBV, Ho JC. Appl. Phys. Lett. 1990; 57: Sugioka K, Toyoda K. Appl. Phys I. 1990; 29: Copyright 1999 John Wiley & Sons, Ltd.

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