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Energy distribution of particles ejected by laser-generated aluminium plasma
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 252 (2006) 183–189 www.elsevier.com/locate/nimb
Energy distribution of particles ejected by laser-generated aluminium plasma L. Torrisi a
a,b,*
, F. Caridi a, A. Picciotto a, A. Borrielli
a
Dipartimento di Fisica, Universita` di Messina, Ctr. Papardo 31, S. Agata, 98166 Messina, Italy b INFN-Laboratori Nazionali del Sud, Via S. Sofia 44, Catania, Italy Received 15 June 2006 Available online 6 October 2006
Abstract A study of laser-generated plasma by visible laser ablation of aluminium, in high vacuum (3 · 107 mbar), by using 3 ns Nd:YAG laser radiation, is reported. Nanosecond pulsed ablation, at intensity of the order of 1010 W/cm2, gives an emission of neutral and charged particles. A special mass quadrupole spectrometer, operating in the 1–300 amu mass range, was used to analyse the energetic atomic emission produced by the laser ablation. Neutrals show typical Boltzmann distributions, directly correlated to the plasma temperature, while ions show Coulomb–Boltzmann-shifted distributions depending on their charge state. Surface profiles of the craters and microscopy investigations permitted to study the ablation threshold and the ablation yields. The multi-component structure of the plasma plume emission is investigated in terms of charge state, neutrals and ions temperature. A special regard is given to the study of the ion acceleration process occurring inside the plasma due to a high electrical field generated in the non-equilibrium plasma. The angular distributions of the neutral and ion particles are also presented and discussed. 2006 Published by Elsevier B.V. PACS: 41.75.Jv; 52.38.Kd Keywords: Laser ablation; Aluminium plasma; Plasma temperature; Electric field in laser-generated plasma
1. Introduction A pulsed laser beam can be focused on a solid material to cause ablation and plasma production. The plasma properties, such as the composition, temperature, density, fractional ionization and angular distribution, are investigated in order to understand the base mechanisms of plasma formation and development in vacuum. Multiple ionizations, excitations and de-excitations take place in the plasma as interactions between particles and between radiations and particles. Plasma contains energetic neutral atoms and molecules, ions, electrons, clusters and generate * Corresponding author. Address: Dipartimento di Fisica, Universita` di Messina, Ctr. Papardo 31, S. Agata, 98166 Messina, Italy. Tel.: +39 90 6765052; fax: +39 90 395004. E-mail address: [email protected] (L. Torrisi).
0168-583X/$ - see front matter 2006 Published by Elsevier B.V. doi:10.1016/j.nimb.2006.07.004
high photons emission. An useful tool to analyze the plasma particles ejected by the pulsed laser ablation species is the mass quadrupole spectrometer; it can provide particular information on the atomic and molecular composition of the plasma plume [1]. The pulsed laser ablation (PLA) technique is employed for several purposes: deposition of thin films, ion generation; X-ray emission; ion implantation; etc. A special interest of PLA concerns the high temperature plasma production at which ions at high charge state are involved. The extraction of the ions from the plasma makes it possible to inject them into special ion sources of ion accelerators. This process permits to increase the ionization efficiency and the ion extraction current of traditional ion sources, as recently developed at INFN-LNS of Catania permitting to inject ions into an electron cyclotron resonance (ECR) system [2].
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L. Torrisi et al. / Nucl. Instr. and Meth. in Phys. Res. B 252 (2006) 183–189
The deposition of thin films has been applied to a very wide variety of materials, ranging from simple metals to complex oxide compounds. In order to achieve a usefully high deposition rate, the laser intensity is chosen to heat the solid to somewhat above the normal boiling point. The technique has several advantages over conventional sputtering methods since stoichiometric transfer of the target material to the substrate is near perfect over small areas for most materials [3], and it can be useful in many scientific fields, such as microelectronics, chemistry, biomedicine and metallurgy. Another important application of PLA technique is the use of energetic ions to induce ion implantation effects in the plasma-exposed substrates, improving the film–substrate adhesion, inducing activation and growth of nanostructures [4] and modifying the physical and chemical properties of the implanted surfaces [5]. This work presents a study of laser ablation of aluminium, in vacuum, by using nanosecond pulses with 532 nm wavelength, and a study on the plume characteristics by measuring the neutral and ion emission through electrostatic filters of a special mass quadrupole spectrometer. The measurements are discussed within the framework of recent mechanistic theories of material ablation with ns pulsed laser. 2. Materials and methods A high vacuum chamber equipped with several side arms, each of which is sealed by a glass window to permit the optical observation of the target, is used for the laser– matter interaction. The chamber vacuum has a base pressure of 3 · 107 mbar. The incident laser beam is directed through one of the side arms, at 45 to the target surface normal, and it is brought to a focus at the target using a biconvex lens of 70 cm focal length. The target used in this work was a 2 cm · 4 cm surface and 1 mm thickness pure aluminium sheet supplied by Goodfellow. Gaussian laser beam profile, in single pulse or in 10 Hz repetition rate, with a maximum pulse energy of 170 mJ and a maximum intensity of 2.4 · 1010 W/cm2 was used. The incidence angle of the laser beam is /inc = 45 and the spot size at this angle was 0.24 mm2. The laser pulse energy was changed from 1.7 mJ up to 170 mJ, which corresponds to a fluence ranging from 0.69 J/cm2 to 69 J/cm2. A special mass quadrupole spectrometer (EQP Hiden 300) was employed to monitor the particles ejected from the plasma with a mass ranging between 1 and 300 amu [6]. The instrument detects neutrals and charged particles depending on the filament state. With the input filament ‘‘on’’ neutrals and ions are both detected, while with filament ‘‘off’’ only ions are detected. The charge state is monitored through the mass-to-charge ratio measurement performed with an electrostatic quadrupolar filter. The particle energy is measured by means of a 45 ion deflector. The instrument plots the energy distribution of neutral and charged species in 1 eV to 1 keV energy range.
Fig. 1 shows the experimental set-up photo (a) and the scheme of the EQP instrument (b) employed for the mass analysis. EQP is placed at 45 with respect to the incidence laser beam, along the normal to the target surface hdet = 0. It is triggered with the laser shot and it measures the plasma conditions for different exposition times. Spectra are collected operating at 10 Hz repetition rate and sampling the plasma with 3 ms dwell time. The Electrostatic Quadrupole Plasma (EQP) Analyser is a high-transmission 45 sector field ion energy analyser and quadrupole mass spectrometer. Mass spectra and energy spectra may be acquired, allowing detailed analysis of positive ions and neutrals. EQP signal trends in intensity may be plotted versus mass and versus time. EQP system analyses the energy and the mass-to-charge state ratio distributions of ions, neutrals or radicals generated in the plasma source. EQP is differentially pumped with respect to the laser–matter interaction chamber and it works at a vacuum of 1 · 107 mbar. The instrument is made up by four main sections: the ionization source, the electrostatic energy filter, the mass filter and the detector. The neutrals of the plasma enter into the mass quadrupole spectrometer through the sampling orifice (diameter = 100 lm) mounted on the probe front end, and they diffuse into the ion source. Here they are subject to bombardment by electrons generated from a filament and they are ionized at a charge state 1+. The ion beam obtained in
Fig. 1. Experimental set-up (a) and scheme of the EQP detector (b).
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this way is focused afterwards by a lens system, which does not change the beam energy, and it arrives at the 45 sector field of the ion energy analyser. Here, by setting rightly the voltage between the electrodes, only ions with a fixed energy go through this filter and arrives to the quadrupolar mass filter. This one is used to select only ions with a fixed mass-to-charge state ratio, which cross it with a sinusoidal path and arrives at the detector, a secondary electron multiplier (SEM) with a sensitivity of 1 part per million (ppm). EQP spectra were analysed in order to separate the neutral component from the ionic component. The fits of the experimental energy distributions were performed with Boltzmann functions through the software program ‘‘Peakfit’’ [7]. A surface profiler (Tencor P-10) was employed in order to measure the crater depth and shapes and to calculate the mass ablated versus the laser fluence. The lateral surface profiler sensitivity is 10 nm and the depth sensitivity is 1 nm. 3. Results The ablation yield of aluminium was calculated through the depth profile of the ablated craters. Fig. 2(a) shows a typical profile of the Al crater produced by 50 shots laser ablation at 100 mJ. The depth profile is performed along the crater diameter and it has approximately a rotational invariance. The inset of the figure shows an optical microscope image of the crater. The depth profile permits to calculate the ablation yield, given in terms of amount of removed mass (or atoms) per laser pulse, versus the laser fluence, as reported in Fig. 2(b). This result shows the existence of an ablation threshold, below which no ablation occurs. Above the threshold the ablation yield increases linearly with the laser fluence. The experimental ablation threshold is 2.04 mJ, corresponding to a fluence of 0.85 J/ cm2, in good agreement with the classic ablative model proposed by Torrisi et al. [8]. According to this model the fluence threshold, Fth, is due to the minimum amount of energy density needed to evaporate, in vacuum, the irradiated aluminium material, as calculated by the following expression: pffiffiffiffiffiffiffiffiffi F th ffi q ðd þ D sÞ ðcs DT þ kf þ ke Þ=ð1 RÞ; ð1Þ where q is the aluminium density (2.7 g/cm3), d is the radiation depth penetration (3.5 nm), D is the thermal diffusivity (97.5 · 106 m2/s), s is the laser pulse duration (3 ns), cs is the specific heat (0.9 J/g K), DT is the temperature rise (640.57 K), kf (10.79 kJ/mol) and ke (293.4 kJ/mol) are the latent heat of fusion and evaporation, respectively, and R (0.72) is the surface reflectivity at the evaporation temperature. Eq. (1) gives a threshold value of 2.21 J/ cm2. However, by assuming negligible the reflectivity, due to the not flat surface during the melting-sublimation phase and to the high energy absorbed by the vapour during its interaction with the laser shot, this approach gives a theo-
Fig. 2. Crater depth profile and optical microscope image of the crater obtained with 50 laser shots at 100 mJ laser energy (a) and ablation yield versus laser pulse energy and laser fluence (b).
retical fluence threshold of 0.64 J/cm2, in good agreement with the experimental datum. At the maximum used energy of 170 mJ, i.e. at a fluence of 69 J/cm2, the ablation yield, Y, corresponds to about 6.3 · 1015 Al atoms/pulse and, by supposing negligible the reflectivity, to a chemical yield of 0.59 Al atoms/ 100 eV photons. Fig. 3 reports the integral neutral yield (counts/s) (a) and the integral Al1+ yield (b) versus the laser energy. Two typical EQP neutral and single ionized spectra are reported in the insets of Fig. 3(a) and (b), respectively. From Fig. 3(a), it is possible to observe a fast neutral yield (peak integral, c/s) increase just above the ablation threshold, up to about 25 mJ pulse energy, a maintenance of a maximum yield up to about 40 mJ and a slowly yield decreasing with the pulse energy at higher laser energy. Near the threshold the neutral yield increases fast with the laser energy due to initial high vaporization rate, while at high energy the laser radiation, interacting with the vapour, produces ionization effects, increasing at high energy the fractional ionization of the plasma and decreasing the
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Fig. 4. Spectra comparison of the neutral and ion energy distributions for Al0, Al1+, Al2+ and Al3+ at low laser energy (25 mJ) (a) and at high laser energy (170 mJ) (b). Fig. 3. Integral neutrals yield versus laser pulse energy (a) and Al1+ integral yield versus laser pulse energy (b).
neutral yield. As a first approximation, the neutral yield, Y(Al0), decreases linearly with the laser pulse energy, EL, given in mJ. Fig. 3(b) shows an integral Al1+ yield increases with the laser pulse energy, indicating that the amount of ions produced just above the ablation threshold increases with the laser pulse energy, as result of the high ionising processes induced by the laser irradiation and reaches a saturation value at about 100 mJ. Here the experimental threshold is about 3 mJ, in agreement with the theoretical prediction obtainable increasing the neutral threshold with the energy amount due to the first Al ionization potential (5.98 eV) of all removed atoms per pulse. Fig. 4 shows two spectra comparison of the ion energy distributions obtained ablating the Al target at low pulse energy, with 25 mJ (a) and at high pulse energy, with 170 mJ (b). Ion spectra were obtained detecting with filament ‘‘off’’, in order to detect only ions without neutrals,
and by fixing the mass-to-charge ratios at 27 amu, 13.5 amu and 9 amu, corresponding to the detection of Al1+, Al2+ and Al3+, respectively. For comparison, spectra shows also the corresponding neutral energy distributions, obtained with filament ‘‘on’’ and by subtracting the ions contribution. It is possible to observe that by increasing the charge-state the energetic distribution is shifted towards higher energy, according to the Coulomb–Boltzmann-shifted model proposed by Torrisi et al. [9], and that the ion yields decreases exponentially with the charge-state, as expected by Shirkov–Lotz theory of the ionization crosssections [10]. Particularly, a mean value of 70 eV/chargestate shift and 100 eV/charge-state shift were measured at 25 mJ and 170 mJ laser pulse energy, respectively. This result indicates that inside the non-equilibrium plasma is generated an equivalent voltage of 70 V and 100 V in the two cases above mentioned, respectively, which produces ion acceleration along the normal to the target surface. Moreover, from the neutral detection at the maximum laser fluence, it is possible to calculate directly a maximum equivalent plasma temperature kT ¼ 2E=3 ¼ 47 eV, corresponding to 5.5 · 105 K.
L. Torrisi et al. / Nucl. Instr. and Meth. in Phys. Res. B 252 (2006) 183–189
Fig. 5(a) shows the comparison between the neutral yields and the ionic yields versus laser energy. At the ablation threshold (2.5 mJ) only neutrals are emitted. The single ionization occurs just above the ablation threshold, at about 3 mJ pulse energy; the double ionization at 3.3 mJ and the third ionization at about 3.32 mJ. Generally the ion yield increases with the laser pulse energy, except for the neutral species, due to the increment of the ionization processes. At 170 mJ laser energy the amount of Al1+ is about two times higher with respect to the neutrals one. This does not mean that the neutrals amount is lower than single ionized species, because the angular distribution measurements demonstrated that the neutral emission is larger with respect to the ion emission, as will be presented in the following. The same figure reports that the Al2+ integral yield is about one order magnitude lower with respect to Al1+ one and that Al3+ is about 3 order of magnitude lower with respect to Al1+ integral yield. Evaluating the relative amount of the integral ion and neutral yields and the angular distributions of neutrals
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and ion species, it was possible to calculate the fractional ionization of the plasma. It increases from about zero, at the ablation threshold, up to about 50%, at a pulse energy of 170 mJ. Fig. 5(b) reports the mean energy of the neutral and ion energy distributions as a function of the laser pulse energy and of the ion charge-state. It is possible to observe that, at 100 mJ laser energy, the neutral mean energy is 60 eV, while this energy assumes the values of 80 eV, 160 eV, 240 eV for the charge states Al1+, Al2+ and Al3+, respectively. Results indicate a regular energy shift with the charge state, which can be valued of 80 eV/charge state at 100 mJ energy. In the range of the investigated laser energy the mean energy of the ion distributions and the energy shifts of the distributions do not change significantly with the laser pulse energy. Ions are characterised by energies higher with respect to the neutrals, due not only to the plasma temperature but also to the Coulomb interactions at which the charges are submitted inside the plasma. The mean thermal velocity of monatomic neutral species is rffiffiffiffiffiffiffiffi 3kT vT ¼ ; ð2Þ m where k is the Boltzmann constant, T is the temperature and m is the Al mass. At the maximum used laser energy the plasma temperature is 47 eV, thus is vT = 2.2 · 104 m/s. The velocity of the plasma expansion in vacuum depends on the temperature by the adiabatic expansion model rffiffiffiffiffiffiffiffi ckT vk ¼ ; ð3Þ m where c = 1.67 is the adiabatic coefficient for monatomic species. At the maximum laser energy is vk = 1.7 · 104 m/s. The velocity of the ions, due only to the Coulomb acceleration, depends on the equivalent voltage, V0, according to the classical model rffiffiffiffiffiffiffiffiffiffiffiffi 2ezV 0 vC ¼ ; ð4Þ m
Fig. 5. Integral yield for neutral and ions versus laser energy (a) and mean energy of the Boltzmann distributions versus laser energy (b).
where e is the electron charge and z the charge-state. At the maximum laser energy the distribution shift is 100 eV thus the equivalent acceleration voltage is V0 = 100 V. At this value the corresponding Coulomb velocity assumes the value of 2.6 · 104 m/s for the Al1+ ions and of 4.6 · 104 m/s for the Al3+ ions. The equivalent acceleration voltage, V0, developed inside the non-equilibrium plasma, depends on the laser energy and accelerates ions proportionally to their charge state. It is directed along the normal to the target surface, as demonstrated by the measurements of angular distribution of the ion velocities. The measurements of angular distribution of ejected particles from laser-generated plasma assume a great importance in the understanding of the mechanisms of
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ion production. The measurements demonstrated that the maximum particle velocity is directed along the normal to the target surface. Neutrals show a larger angular distribution while ions have a distribution more and more narrow increasing their charge state, a result in good agreement with the literature [9]. Fig. 6(a) shows the comparison of the angular distributions for neutrals and ions at their mean energy produced by 50 mJ laser pulse. The given angular distributions are compared at their mean energy, by considering for Al0 particles the angular distribution of the neutrals at 56.7 eV, for Al1+ ions the distribution at 78 eV, for Al2+ ions the distribution at 154 eV and for Al3+ ions the distribution at 230 eV. The plot indicates that the velocity is directed mainly along the normal to the target surface and that it decreases strongly within ± 30 for the charge states ranging between 3+ and 2+ while it decreases slowly at 20% of the maximum value for the charge state 1+ at ±30. For neutrals the strong decreasing occurs at angles higher than 70. Fig. 6(b) reports the comparison of the angular distributions by polar projection for the Al yields of neutrals and ions produced by 50 mJ laser pulse. The yield angular distributions demonstrate that the higher charge states show a narrow angular distribution more and more peaked around
the target-normal direction increasing the charge state, in agreement with the literature [9]. 4. Discussion and conclusions Due to the high difficulty to measure the energy of neutral species, the literature is poor of data concerning the neutral energy distributions of particles ejected from laser-generated plasma. Obtained results confirm two theoretical expectations concerning that the neutrals energy increases with the laser fluence, due to the plasma temperature increase, and that the neutrals yield decreases with the laser fluence, due to the ionization processes increase. The energy distributions follow a Boltzmann function, which width depends directly on the plasma temperature. Ions have a kinetic energy higher with respect to neutrals, due to the Coulomb accelerations in the direction of the normal to the target surface. Along the target-normal direction the ion velocity depends on the sum vT + vk + vC, while orthogonally to this direction the ion velocity is zero and only the thermal velocity vT occurs. Thus, the particle velocity is not isotropic, according to the literature [9]. This result indicates that ions are subjected to a Coulomb acceleration due to a high electrical field generated inside the non-equilibrium plasma, which value can be calculated assuming it to be developed on a distance comparable with the Debye length. Preliminary evaluations of this electric field, presented in a previous work [11], indicated that its value is high and of the order of some MeV/cm. The measurements are based on the evaluation of the temperature by the Boltzmann distribution obtained by EQP and by optical spectroscopy of ionized species, which give respectively a temperature of 47 eV and 15 eV. The plasma density measurements were evaluated by the optical spectroscopy with the Boltzmann method giving a value of the order of 1016 cm3 [12,13]. Acknowledgements Authors thank the INFN-5th National Committee, which supported this work (the PLATONE experiment). References
Fig. 6. Angular distributions for neutrals and ions of the mean Boltzmann distributions (a) and of the ablation yields (b).
[1] M.C. Kelly, G.G. Gomlak, V.G. Panayotov, C. Cresson, J. Rodney, B.D. Koplitz, Appl. Surf. Sci. 127–129 (1998) 988. [2] S. Gammino, L. Torrisi, G. Ciavola, et al., J. Appl. Phys. 96 (5) (2004) 2961. [3] G.C. Tyrrell, T. York, N. Cherief, D. Givord, J. Lawler, J.G. Lunney, M. Buckley, I.W. Boyd, Microelectron. Eng. 25 (1994) 247. [4] L. Torrisi, S. Gammino, A.M. Mezzasalma, et al., Appl. Surf. Sci. 217 (2003) 319. [5] L. Torrisi, A.M. Visco, A. Valenza, Rad. Eff. Def. Solids 158 (2003) 621. [6] Hiden Catalogue. Available from:, 2005. [7] http://cranessoftware.com/products/systat/2006. [8] L. Torrisi, G. Ciavola, S. Gammino, L. Ando`, A. Barna`, L. Laska, J. Krasa, Rev. Sci. Instr. 71 (11) (2000) 4330. [9] L. Torrisi, L. Ando`, S. Gammino, L. Laska, J. Appl. Phys. 91 (5) (2002) 4685.
L. Torrisi et al. / Nucl. Instr. and Meth. in Phys. Res. B 252 (2006) 183–189 [10] G.D. Shirkov, G. Zschornack, in: G.D. Shirkov, G. Zschornack (Eds.), Electron Impact Ion Sources for Charged Heavy Ions, F. Vieweg & Sohn, Publ., Gottingen, 1996. [11] L. Torrisi, S. Gammino, L. Laska, J. Krasa, K. Rohlena, J. Wolowski, in: Proc. SPPT 2006, Prague, 26 June 2006, p. 580.
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[12] L. Torrisi, L. Ryc´, D. Margarone, A. Borrielli, in: Proc. SPPT 2006, Prague, 26 June 2006, p. 571. [13] A. Picciotto, L. Torrisi, A.M. Mezzasalma, in: Proc. PPLA 2003, Messina and Catania, Italy, 18–19 September 2003, p. 191.
Nuclear Instruments and Methods in Physics Research B 252 (2006) 183–189 www.elsevier.com/locate/nimb
Energy distribution of particles ejected by laser-generated aluminium plasma L. Torrisi a
a,b,*
, F. Caridi a, A. Picciotto a, A. Borrielli
a
Dipartimento di Fisica, Universita` di Messina, Ctr. Papardo 31, S. Agata, 98166 Messina, Italy b INFN-Laboratori Nazionali del Sud, Via S. Sofia 44, Catania, Italy Received 15 June 2006 Available online 6 October 2006
Abstract A study of laser-generated plasma by visible laser ablation of aluminium, in high vacuum (3 · 107 mbar), by using 3 ns Nd:YAG laser radiation, is reported. Nanosecond pulsed ablation, at intensity of the order of 1010 W/cm2, gives an emission of neutral and charged particles. A special mass quadrupole spectrometer, operating in the 1–300 amu mass range, was used to analyse the energetic atomic emission produced by the laser ablation. Neutrals show typical Boltzmann distributions, directly correlated to the plasma temperature, while ions show Coulomb–Boltzmann-shifted distributions depending on their charge state. Surface profiles of the craters and microscopy investigations permitted to study the ablation threshold and the ablation yields. The multi-component structure of the plasma plume emission is investigated in terms of charge state, neutrals and ions temperature. A special regard is given to the study of the ion acceleration process occurring inside the plasma due to a high electrical field generated in the non-equilibrium plasma. The angular distributions of the neutral and ion particles are also presented and discussed. 2006 Published by Elsevier B.V. PACS: 41.75.Jv; 52.38.Kd Keywords: Laser ablation; Aluminium plasma; Plasma temperature; Electric field in laser-generated plasma
1. Introduction A pulsed laser beam can be focused on a solid material to cause ablation and plasma production. The plasma properties, such as the composition, temperature, density, fractional ionization and angular distribution, are investigated in order to understand the base mechanisms of plasma formation and development in vacuum. Multiple ionizations, excitations and de-excitations take place in the plasma as interactions between particles and between radiations and particles. Plasma contains energetic neutral atoms and molecules, ions, electrons, clusters and generate * Corresponding author. Address: Dipartimento di Fisica, Universita` di Messina, Ctr. Papardo 31, S. Agata, 98166 Messina, Italy. Tel.: +39 90 6765052; fax: +39 90 395004. E-mail address: [email protected] (L. Torrisi).
0168-583X/$ - see front matter 2006 Published by Elsevier B.V. doi:10.1016/j.nimb.2006.07.004
high photons emission. An useful tool to analyze the plasma particles ejected by the pulsed laser ablation species is the mass quadrupole spectrometer; it can provide particular information on the atomic and molecular composition of the plasma plume [1]. The pulsed laser ablation (PLA) technique is employed for several purposes: deposition of thin films, ion generation; X-ray emission; ion implantation; etc. A special interest of PLA concerns the high temperature plasma production at which ions at high charge state are involved. The extraction of the ions from the plasma makes it possible to inject them into special ion sources of ion accelerators. This process permits to increase the ionization efficiency and the ion extraction current of traditional ion sources, as recently developed at INFN-LNS of Catania permitting to inject ions into an electron cyclotron resonance (ECR) system [2].
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The deposition of thin films has been applied to a very wide variety of materials, ranging from simple metals to complex oxide compounds. In order to achieve a usefully high deposition rate, the laser intensity is chosen to heat the solid to somewhat above the normal boiling point. The technique has several advantages over conventional sputtering methods since stoichiometric transfer of the target material to the substrate is near perfect over small areas for most materials [3], and it can be useful in many scientific fields, such as microelectronics, chemistry, biomedicine and metallurgy. Another important application of PLA technique is the use of energetic ions to induce ion implantation effects in the plasma-exposed substrates, improving the film–substrate adhesion, inducing activation and growth of nanostructures [4] and modifying the physical and chemical properties of the implanted surfaces [5]. This work presents a study of laser ablation of aluminium, in vacuum, by using nanosecond pulses with 532 nm wavelength, and a study on the plume characteristics by measuring the neutral and ion emission through electrostatic filters of a special mass quadrupole spectrometer. The measurements are discussed within the framework of recent mechanistic theories of material ablation with ns pulsed laser. 2. Materials and methods A high vacuum chamber equipped with several side arms, each of which is sealed by a glass window to permit the optical observation of the target, is used for the laser– matter interaction. The chamber vacuum has a base pressure of 3 · 107 mbar. The incident laser beam is directed through one of the side arms, at 45 to the target surface normal, and it is brought to a focus at the target using a biconvex lens of 70 cm focal length. The target used in this work was a 2 cm · 4 cm surface and 1 mm thickness pure aluminium sheet supplied by Goodfellow. Gaussian laser beam profile, in single pulse or in 10 Hz repetition rate, with a maximum pulse energy of 170 mJ and a maximum intensity of 2.4 · 1010 W/cm2 was used. The incidence angle of the laser beam is /inc = 45 and the spot size at this angle was 0.24 mm2. The laser pulse energy was changed from 1.7 mJ up to 170 mJ, which corresponds to a fluence ranging from 0.69 J/cm2 to 69 J/cm2. A special mass quadrupole spectrometer (EQP Hiden 300) was employed to monitor the particles ejected from the plasma with a mass ranging between 1 and 300 amu [6]. The instrument detects neutrals and charged particles depending on the filament state. With the input filament ‘‘on’’ neutrals and ions are both detected, while with filament ‘‘off’’ only ions are detected. The charge state is monitored through the mass-to-charge ratio measurement performed with an electrostatic quadrupolar filter. The particle energy is measured by means of a 45 ion deflector. The instrument plots the energy distribution of neutral and charged species in 1 eV to 1 keV energy range.
Fig. 1 shows the experimental set-up photo (a) and the scheme of the EQP instrument (b) employed for the mass analysis. EQP is placed at 45 with respect to the incidence laser beam, along the normal to the target surface hdet = 0. It is triggered with the laser shot and it measures the plasma conditions for different exposition times. Spectra are collected operating at 10 Hz repetition rate and sampling the plasma with 3 ms dwell time. The Electrostatic Quadrupole Plasma (EQP) Analyser is a high-transmission 45 sector field ion energy analyser and quadrupole mass spectrometer. Mass spectra and energy spectra may be acquired, allowing detailed analysis of positive ions and neutrals. EQP signal trends in intensity may be plotted versus mass and versus time. EQP system analyses the energy and the mass-to-charge state ratio distributions of ions, neutrals or radicals generated in the plasma source. EQP is differentially pumped with respect to the laser–matter interaction chamber and it works at a vacuum of 1 · 107 mbar. The instrument is made up by four main sections: the ionization source, the electrostatic energy filter, the mass filter and the detector. The neutrals of the plasma enter into the mass quadrupole spectrometer through the sampling orifice (diameter = 100 lm) mounted on the probe front end, and they diffuse into the ion source. Here they are subject to bombardment by electrons generated from a filament and they are ionized at a charge state 1+. The ion beam obtained in
Fig. 1. Experimental set-up (a) and scheme of the EQP detector (b).
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this way is focused afterwards by a lens system, which does not change the beam energy, and it arrives at the 45 sector field of the ion energy analyser. Here, by setting rightly the voltage between the electrodes, only ions with a fixed energy go through this filter and arrives to the quadrupolar mass filter. This one is used to select only ions with a fixed mass-to-charge state ratio, which cross it with a sinusoidal path and arrives at the detector, a secondary electron multiplier (SEM) with a sensitivity of 1 part per million (ppm). EQP spectra were analysed in order to separate the neutral component from the ionic component. The fits of the experimental energy distributions were performed with Boltzmann functions through the software program ‘‘Peakfit’’ [7]. A surface profiler (Tencor P-10) was employed in order to measure the crater depth and shapes and to calculate the mass ablated versus the laser fluence. The lateral surface profiler sensitivity is 10 nm and the depth sensitivity is 1 nm. 3. Results The ablation yield of aluminium was calculated through the depth profile of the ablated craters. Fig. 2(a) shows a typical profile of the Al crater produced by 50 shots laser ablation at 100 mJ. The depth profile is performed along the crater diameter and it has approximately a rotational invariance. The inset of the figure shows an optical microscope image of the crater. The depth profile permits to calculate the ablation yield, given in terms of amount of removed mass (or atoms) per laser pulse, versus the laser fluence, as reported in Fig. 2(b). This result shows the existence of an ablation threshold, below which no ablation occurs. Above the threshold the ablation yield increases linearly with the laser fluence. The experimental ablation threshold is 2.04 mJ, corresponding to a fluence of 0.85 J/ cm2, in good agreement with the classic ablative model proposed by Torrisi et al. [8]. According to this model the fluence threshold, Fth, is due to the minimum amount of energy density needed to evaporate, in vacuum, the irradiated aluminium material, as calculated by the following expression: pffiffiffiffiffiffiffiffiffi F th ffi q ðd þ D sÞ ðcs DT þ kf þ ke Þ=ð1 RÞ; ð1Þ where q is the aluminium density (2.7 g/cm3), d is the radiation depth penetration (3.5 nm), D is the thermal diffusivity (97.5 · 106 m2/s), s is the laser pulse duration (3 ns), cs is the specific heat (0.9 J/g K), DT is the temperature rise (640.57 K), kf (10.79 kJ/mol) and ke (293.4 kJ/mol) are the latent heat of fusion and evaporation, respectively, and R (0.72) is the surface reflectivity at the evaporation temperature. Eq. (1) gives a threshold value of 2.21 J/ cm2. However, by assuming negligible the reflectivity, due to the not flat surface during the melting-sublimation phase and to the high energy absorbed by the vapour during its interaction with the laser shot, this approach gives a theo-
Fig. 2. Crater depth profile and optical microscope image of the crater obtained with 50 laser shots at 100 mJ laser energy (a) and ablation yield versus laser pulse energy and laser fluence (b).
retical fluence threshold of 0.64 J/cm2, in good agreement with the experimental datum. At the maximum used energy of 170 mJ, i.e. at a fluence of 69 J/cm2, the ablation yield, Y, corresponds to about 6.3 · 1015 Al atoms/pulse and, by supposing negligible the reflectivity, to a chemical yield of 0.59 Al atoms/ 100 eV photons. Fig. 3 reports the integral neutral yield (counts/s) (a) and the integral Al1+ yield (b) versus the laser energy. Two typical EQP neutral and single ionized spectra are reported in the insets of Fig. 3(a) and (b), respectively. From Fig. 3(a), it is possible to observe a fast neutral yield (peak integral, c/s) increase just above the ablation threshold, up to about 25 mJ pulse energy, a maintenance of a maximum yield up to about 40 mJ and a slowly yield decreasing with the pulse energy at higher laser energy. Near the threshold the neutral yield increases fast with the laser energy due to initial high vaporization rate, while at high energy the laser radiation, interacting with the vapour, produces ionization effects, increasing at high energy the fractional ionization of the plasma and decreasing the
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Fig. 4. Spectra comparison of the neutral and ion energy distributions for Al0, Al1+, Al2+ and Al3+ at low laser energy (25 mJ) (a) and at high laser energy (170 mJ) (b). Fig. 3. Integral neutrals yield versus laser pulse energy (a) and Al1+ integral yield versus laser pulse energy (b).
neutral yield. As a first approximation, the neutral yield, Y(Al0), decreases linearly with the laser pulse energy, EL, given in mJ. Fig. 3(b) shows an integral Al1+ yield increases with the laser pulse energy, indicating that the amount of ions produced just above the ablation threshold increases with the laser pulse energy, as result of the high ionising processes induced by the laser irradiation and reaches a saturation value at about 100 mJ. Here the experimental threshold is about 3 mJ, in agreement with the theoretical prediction obtainable increasing the neutral threshold with the energy amount due to the first Al ionization potential (5.98 eV) of all removed atoms per pulse. Fig. 4 shows two spectra comparison of the ion energy distributions obtained ablating the Al target at low pulse energy, with 25 mJ (a) and at high pulse energy, with 170 mJ (b). Ion spectra were obtained detecting with filament ‘‘off’’, in order to detect only ions without neutrals,
and by fixing the mass-to-charge ratios at 27 amu, 13.5 amu and 9 amu, corresponding to the detection of Al1+, Al2+ and Al3+, respectively. For comparison, spectra shows also the corresponding neutral energy distributions, obtained with filament ‘‘on’’ and by subtracting the ions contribution. It is possible to observe that by increasing the charge-state the energetic distribution is shifted towards higher energy, according to the Coulomb–Boltzmann-shifted model proposed by Torrisi et al. [9], and that the ion yields decreases exponentially with the charge-state, as expected by Shirkov–Lotz theory of the ionization crosssections [10]. Particularly, a mean value of 70 eV/chargestate shift and 100 eV/charge-state shift were measured at 25 mJ and 170 mJ laser pulse energy, respectively. This result indicates that inside the non-equilibrium plasma is generated an equivalent voltage of 70 V and 100 V in the two cases above mentioned, respectively, which produces ion acceleration along the normal to the target surface. Moreover, from the neutral detection at the maximum laser fluence, it is possible to calculate directly a maximum equivalent plasma temperature kT ¼ 2E=3 ¼ 47 eV, corresponding to 5.5 · 105 K.
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Fig. 5(a) shows the comparison between the neutral yields and the ionic yields versus laser energy. At the ablation threshold (2.5 mJ) only neutrals are emitted. The single ionization occurs just above the ablation threshold, at about 3 mJ pulse energy; the double ionization at 3.3 mJ and the third ionization at about 3.32 mJ. Generally the ion yield increases with the laser pulse energy, except for the neutral species, due to the increment of the ionization processes. At 170 mJ laser energy the amount of Al1+ is about two times higher with respect to the neutrals one. This does not mean that the neutrals amount is lower than single ionized species, because the angular distribution measurements demonstrated that the neutral emission is larger with respect to the ion emission, as will be presented in the following. The same figure reports that the Al2+ integral yield is about one order magnitude lower with respect to Al1+ one and that Al3+ is about 3 order of magnitude lower with respect to Al1+ integral yield. Evaluating the relative amount of the integral ion and neutral yields and the angular distributions of neutrals
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and ion species, it was possible to calculate the fractional ionization of the plasma. It increases from about zero, at the ablation threshold, up to about 50%, at a pulse energy of 170 mJ. Fig. 5(b) reports the mean energy of the neutral and ion energy distributions as a function of the laser pulse energy and of the ion charge-state. It is possible to observe that, at 100 mJ laser energy, the neutral mean energy is 60 eV, while this energy assumes the values of 80 eV, 160 eV, 240 eV for the charge states Al1+, Al2+ and Al3+, respectively. Results indicate a regular energy shift with the charge state, which can be valued of 80 eV/charge state at 100 mJ energy. In the range of the investigated laser energy the mean energy of the ion distributions and the energy shifts of the distributions do not change significantly with the laser pulse energy. Ions are characterised by energies higher with respect to the neutrals, due not only to the plasma temperature but also to the Coulomb interactions at which the charges are submitted inside the plasma. The mean thermal velocity of monatomic neutral species is rffiffiffiffiffiffiffiffi 3kT vT ¼ ; ð2Þ m where k is the Boltzmann constant, T is the temperature and m is the Al mass. At the maximum used laser energy the plasma temperature is 47 eV, thus is vT = 2.2 · 104 m/s. The velocity of the plasma expansion in vacuum depends on the temperature by the adiabatic expansion model rffiffiffiffiffiffiffiffi ckT vk ¼ ; ð3Þ m where c = 1.67 is the adiabatic coefficient for monatomic species. At the maximum laser energy is vk = 1.7 · 104 m/s. The velocity of the ions, due only to the Coulomb acceleration, depends on the equivalent voltage, V0, according to the classical model rffiffiffiffiffiffiffiffiffiffiffiffi 2ezV 0 vC ¼ ; ð4Þ m
Fig. 5. Integral yield for neutral and ions versus laser energy (a) and mean energy of the Boltzmann distributions versus laser energy (b).
where e is the electron charge and z the charge-state. At the maximum laser energy the distribution shift is 100 eV thus the equivalent acceleration voltage is V0 = 100 V. At this value the corresponding Coulomb velocity assumes the value of 2.6 · 104 m/s for the Al1+ ions and of 4.6 · 104 m/s for the Al3+ ions. The equivalent acceleration voltage, V0, developed inside the non-equilibrium plasma, depends on the laser energy and accelerates ions proportionally to their charge state. It is directed along the normal to the target surface, as demonstrated by the measurements of angular distribution of the ion velocities. The measurements of angular distribution of ejected particles from laser-generated plasma assume a great importance in the understanding of the mechanisms of
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ion production. The measurements demonstrated that the maximum particle velocity is directed along the normal to the target surface. Neutrals show a larger angular distribution while ions have a distribution more and more narrow increasing their charge state, a result in good agreement with the literature [9]. Fig. 6(a) shows the comparison of the angular distributions for neutrals and ions at their mean energy produced by 50 mJ laser pulse. The given angular distributions are compared at their mean energy, by considering for Al0 particles the angular distribution of the neutrals at 56.7 eV, for Al1+ ions the distribution at 78 eV, for Al2+ ions the distribution at 154 eV and for Al3+ ions the distribution at 230 eV. The plot indicates that the velocity is directed mainly along the normal to the target surface and that it decreases strongly within ± 30 for the charge states ranging between 3+ and 2+ while it decreases slowly at 20% of the maximum value for the charge state 1+ at ±30. For neutrals the strong decreasing occurs at angles higher than 70. Fig. 6(b) reports the comparison of the angular distributions by polar projection for the Al yields of neutrals and ions produced by 50 mJ laser pulse. The yield angular distributions demonstrate that the higher charge states show a narrow angular distribution more and more peaked around
the target-normal direction increasing the charge state, in agreement with the literature [9]. 4. Discussion and conclusions Due to the high difficulty to measure the energy of neutral species, the literature is poor of data concerning the neutral energy distributions of particles ejected from laser-generated plasma. Obtained results confirm two theoretical expectations concerning that the neutrals energy increases with the laser fluence, due to the plasma temperature increase, and that the neutrals yield decreases with the laser fluence, due to the ionization processes increase. The energy distributions follow a Boltzmann function, which width depends directly on the plasma temperature. Ions have a kinetic energy higher with respect to neutrals, due to the Coulomb accelerations in the direction of the normal to the target surface. Along the target-normal direction the ion velocity depends on the sum vT + vk + vC, while orthogonally to this direction the ion velocity is zero and only the thermal velocity vT occurs. Thus, the particle velocity is not isotropic, according to the literature [9]. This result indicates that ions are subjected to a Coulomb acceleration due to a high electrical field generated inside the non-equilibrium plasma, which value can be calculated assuming it to be developed on a distance comparable with the Debye length. Preliminary evaluations of this electric field, presented in a previous work [11], indicated that its value is high and of the order of some MeV/cm. The measurements are based on the evaluation of the temperature by the Boltzmann distribution obtained by EQP and by optical spectroscopy of ionized species, which give respectively a temperature of 47 eV and 15 eV. The plasma density measurements were evaluated by the optical spectroscopy with the Boltzmann method giving a value of the order of 1016 cm3 [12,13]. Acknowledgements Authors thank the INFN-5th National Committee, which supported this work (the PLATONE experiment). References
Fig. 6. Angular distributions for neutrals and ions of the mean Boltzmann distributions (a) and of the ablation yields (b).
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[12] L. Torrisi, L. Ryc´, D. Margarone, A. Borrielli, in: Proc. SPPT 2006, Prague, 26 June 2006, p. 571. [13] A. Picciotto, L. Torrisi, A.M. Mezzasalma, in: Proc. PPLA 2003, Messina and Catania, Italy, 18–19 September 2003, p. 191.