Mass Quadrupole Spectrometry for Infrared Laser-Generated Plasmas Detection

A nanosecond pulsed Nd:YAG laser, operating at the fundamental wavelength of 1064 nm and at an intensity of about 10 W/cm, was employed to irrad iate the Cu0.4W0.6 metallic alloy and the relative pure components (Cu and W) in vacuum to investigate about the ablation. Produced plasmas were characterized in terms of thermal and Coulomb interactions evaluating the equivalent temperatures and accelerat ion voltages developed in the non-equilibrium p lasma core. The particles emission, produced along the normal to the target surface, was investigated by measuring, with the special electrostatic mass quadrupole spectrometer Hiden EQP 300, neutral and ion energy distributions and fitting experimental data with the “Coulomb-Boltzmann-shifted” function. Results indicated that the metal alloy stoichiometry, in a first approximation, is well transported to the ion and neutral stoichiometry of the plas ma.


Introduction
Intense pulsed laser beams focused on a solid material produce ablation and formation of hot non-equilibriu m plasmas, which have been used for several applicat ions in these last years, such as deposition of thin films, generation of ions at high energy and charge state, treatment of surfaces, ion imp lantation, medical applications, etc. [1,2].
Energetic plas ma ions permit to induce ion imp lantation effects in the exposed surfaces, improving the film-substrate adhesion, to induce activation and growth of nanostructures, such as carbon nanotubes and fullerenes [3]. Energetic neutral emission determines the fractional ion izat ion of the plasma and play an important role in the plas ma properties (temperature, density, ion neutralization, charge exchange, ionization processes, etc.).
Processes developed inside the laser-generated plasma are very complex and depend on many different parameters, such as the laser intensity, target composition, irradiation conditions, etc. Thus, for their description, many effects, such as optical absorption, heat conduction, phase transitions, flu id dynamics, inverse Bremmstrahlung and self-focusing, were introduced [4].
Although these plasmas are in non-equilib riu m conditions and not really thermalized, the concept of ''equivalent temperature", referred to ions, electrons and neutrals in near local thermal equilibriu m (NLTE) conditions, is employed in order to characterize the plas ma properties, as reported in literature [5]. In this work a 3 ns Nd:YA G laser operating in the infrared region with relat ively high pulse energy is employed to irradiate copper, tungsten and the alloy of 40% Cu and 60% W, in vacuum. Obtained plas mas are investigated in terms of neutral and ion emission. The aim is to investigate about the ablation, in order to know if the alloy stoichiometry is maintained in the plasma ion and neutral co mponents, in the case of two metals having very different physical and chemical p roperties.

Materials and Methods
The employed laser is a Q-switched Nd:Yag pulsed laser, operating at 1064 n m wavelength, 3 ns pulse duration, 1-180 mJ pulse energy, in single pulse or repetition rate (1-10 Hz) mode. The laser beam is focused through a convergent lens on the solid target placed inside a vacuu m chamber at 10 -6 mbar. The optimu m focalizat ion distance (50 cm) was determined min imizing the spot dimension observed on the target. The spot diameter is about 1 mm and the incidence angle is 45°. This angle can be changed by the operator moving a vacuum feedthrough at which the target holder is fixed.
The employed targets consist of pure Cu and W sheets and an alloy of 40% Cu and 60% W, with a polished 1 cm 2 surface and 1 mm thickness. They can be moved vertically with the vacuum feedthrough, so that each laser shot can hit a fresh flat surface.
A special electrostatic mass quadrupole spectrometer, the so-called Electrostatic Quadrupole Plasma (EQP Hiden 300), was emp loyed to monitor particles ejected fro m the plas ma along the target normal surface, with a mass range within 1 and 300 amu. The instrument detects neutrals and charged particles depending on the filament state (with 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 by an electrostatic ion deflection. It measures the particle energy with an accuracy of 1 eV and plots the energy distribution of neutral and charged species in the energy range 1 eV-1 keV. The spectra acquisition occurs operating at 10 Hz laser repetition rate, and it is triggered with the laser shot. EQP uses a 45° electrostatic deflection, in order to measure the ion energy, and a d ifferential pu mping at 10 -7 mbar p ressure. The instrument consists of four main sections: the ionisation source; the electrostatic energy filter; the mass filter; the secondary electron mult iplier (SEM ) detector. More details on the EQP instrument are reported in the literature[6].  Fig. 1 (a) shows a plot of the experimental setup and Fig. 1 (b) reports a scheme of the EQP detector.
EQP spectra were analysed in order to separate the neutral component fro m the ionic one and to plot particle energy distributions for various charge states.
The fits of experimental energy distributions were performed with the "Coulo mb-Bo ltzmann-shifted" function [7]: where A represents the normalization constant, m the atomic mass, E the total energy along the normal to the target surface, E K the adiabatic expansion energy in vacuum and E C the Coulo mb energy. The ablation yield was measured off-line by calculating the number of removed atoms per laser pulse from the laser-generated crater on the target. The crater profiles were investigated with the surface profiler (Tencor-P10), which depth and lateral resolution were about 10 n m and 1 µm, respectively.
A CCD camera was placed at 90° with respect to the target normal direction. It was triggered with the laser pulse in order to observe the visible light emitted fro m the plu me, integrating with a variable exposition time. Figure 2 shows a co mparison of crater profiles obtained irradiating with 50 laser shots, 1 Hz, three employed targets (Cu, W and Cu 0.4 W 0.6 ) in the same experimental conditions, at 0° incidence angle and 180 mJ pulse energy.
Spectra report the yield signal versus particle energy for neutral and ionized atoms. The fit of experimental data (points) indicates that particles have typical Bolt zmann distributions, peaked at different energies depending on their charge state. In particular, Cu neutrals are peaked at energies of about 35 eV and 46.5 eV for Cu and Cu 0.4 W 0.6 targets, respectively, demonstrating that Cu part icles energies increase in presence of W, due to the higher electron density of the produced plasma. Neutrals energy distributions contain direct informat ion on the plasma temperature. Along the normal to the target irradiation th is energy is due to t wo different contributions: the first is due to the thermal energy, E T = 3kT/ 2 and the second to the adiabatic expansion energy, E K = γkT/ 2, where k is the Bo ltzmann constant, T is the temperature, and γ is the adiabatic coefficient (1.67 for monoatomic species). The fit of experimental spectra indicates that the temperature for Cu and Cu 0.4 W 0.6 plasmas is 15 eV and 20 eV, respectively, demonstrating that it increases in presence of W due to the increase of the plasma electron density. Ion energy distributions contain informat ion not only about the temperature but also about Coulo mb interactions. In this case, in fact, a third contribution is involved in the total ion energy; this is the Coulo mb energy, E C = ze V 0 , where ze is the ion charge and V 0 is the equivalent acceleration voltage developed inside the non-equilib riu m plasma. First ionized Cu ato ms are peaked at about 112 eV and 136 eV for Cu and Cu 0.4 W 0.6 plasmas. Cu ions with higher charge states have a regular energy shift of about 75 eV and 90 eV for Cu and Cu 0.4 W 0.6 targets, respectively, for a total number of 4 and 8 charge states, respectively. All experimental data are reported in Table 1.    Figure 4 shows typical EQP spectra obtained ablating W (a) and Cu 0.4 W 0.6 (b) targets and detecting tungsten at the same experimental conditions as before. One of these properties is represented by the density of electrons of conduction in metals, wh ich are weakly bounded to the elemental ato ms. In general, for elements characterized by an high electron density value, there will be a higher amount of electrons in the dense vapor, immed iately after the laser ablat ion. These electrons absorb the laser photons energy through the inverse bremsstrahlung process reaching energies suitable to induce ionization processes.
Another important physical parameter is the ablation threshold, calculated through the approach given by Torrisi et al. [8]. For copper, that has a lower ablation threshold (1.04 J/cm 2 ) and a low boiling point, only a small fraction of the laser pulse energy is used for the evaporation. As a consequence, above the threshold the ablation yield is generally h igh, because of the remaining h igh laser pulse energy transferred to the plasma. On the contrary, for tungsten, with a higher value o f the ablation threshold (1.3 J/cm 2 ) and a high boiling point, a higher amount of the laser pulse energy is necessary to produce its evaporation in vacuum. The ab lation yield is lower as a consequence of the remain ing low pulse energy transferred to the plasma. Moreover, we expect that the mean energy transferred by the laser pulse to the atoms of the vapor is low when the target has a low evaporation point and high ab lation yield, as Cu for example, because many atoms co mpose a dense vapor [9].
On the contrary, the mean energy is high when the target has a high evaporation point and low ablation yield, as W for example, because the vapor is less dense. This is in good agreement with our experimental results detailed in Table 1.
Results obtained with EQP indicate that the maximu m charge state for the ablated targets obtainable at 180 mJ pulse energy is 4+ and 8+ for copper in Cu and Cu 0.4 W 0.6 plasmas, respectively, and 7+ and 9+ for tungsten in W and Cu 0.4 W 0.6 plasmas, respectively. This result is in good agreement with the ionizat ion potential values of the elements [10].
In fact, assuming for examp le an average plasma temperature of about 15 eV for copper in Cu plas ma, the mean thermal electron energy is about 23 eV.
Because the energy distribution has a tail up to about three times than the mean energy, this means that electrons with energies of about 76 eV are present in the plasma. At this energy level the expected ionizat ion is 4+, as reported in Figure 5. Likewise the expected ionizat ion is 7+ for tungsten in W plasma, 8+ for copper and 9+ for tungsten in Cu 0.4 W 0.6 plasma, respectively.
EQP data were analyzed as energy d istributions through fits that have been performed taking in consideration that the used ion charge state distributions should be in agreement with the trend of the electron ionizat ion cross section.  To this reason comparisons between normalized ion charge state distributions and ionization cross-sections based on Lotz theory [11] have been performed.
The cross-sections were calculated using the mean electron energy, E e , given by the best temperature fit, as reported in Fig. 5. Fig. 6 shows the comparison for EQP ion data elaboration for Cu (a) and W (b) targets irradiation, respectively.
The experimental d istributions of the ion yields for the various charge states (squares) and the calculated electron ionization cross section (Lotz theory) show a good agreement. Fig. 7 shows a co mparison between the normalized neutral and ion yields for copper (a) in Cu and Cu 0.4 W 0.6 plasmas, respectively, and for tungsten (b) in W and Cu 0.4 W 0.6 plasmas, respectively.
Experimental results follow the stoichiometry of emp loyed samples.
Thus results indicate that the metal alloy stoichiomet ry, in a first appro ximation, is well transported to the ion and neutral stoichiometry of the plasma.

Conclusions
The laser ablation of Cu, W and Cu 0.4 W 0.6 targets at 1064 nm was investigated by using an electrostatic deflector mass quadrupole spectrometer. At a laser intensity of about 10 10 W/cm 2 energy spectra show "Coulo mb-Bo ltzmann-shifted" distributions for various ion charge states. The total energy has three components: thermal, adiabatic and Coulo mb.
The neutral monoatomic species characterize the mean plasma temperature, which is about 20 eV at the maximu m fluence.
The energy shift of ion energy distributions gives a measure of the equivalent acceleration potential generated inside the plasma during the laser pulse irradiat ion. The corresponding electrical field, for d istances comparable with the Debye length (about 100 n m) and for an electron density of the order of 19x10 16 /cm 3 can be calculated as E = V 0 /λ D [13]. A maximu m value of about 11.5 M V/cm can be given as a first appro ximation.
The comparison of our results with literature data is good, confirming the valid ity of our experimental approaches. The original alloy stoichiometry of 40% Cu and 60% W is in first approximation found also in the plasma neutrals and ions.