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Diagnostics of laser ablated plasma plumes
Thin Solid Films 453 – 454 (2004) 562–572
Diagnostics of laser ablated plasma plumes S. Amorusoa,b,*, B. Toftmannc, J. Schouc, R. Velottaa,b, X. Wanga a
Coherentia INFM and Istituto Nazionale per la Fisica della Materia, Complesso Universitario di Monte S. Angelo, Via Cintia, I-80126 Napoli, Italy b Dipartimento di Scienze Fisiche, Universita` degli Studi di Napoli Federico II, Complesso Universitario di Monte S. Angelo, Via Cintia, I-80126 Napoli, Italy c Optics and Fluid Dynamics Department, Risø National Laboratory, DK-4000 Roskilde, Denmark
Abstract The effect of an ambient gas on the expansion dynamics of laser ablated plasmas has been studied for two systems by exploiting different diagnostic techniques. First, the dynamics of a MgB2 laser produced plasma plume in an Ar atmosphere has been investigated by space-and time-resolved optical emission spectroscopy. Second, deposition rate and fast ion probe measurements have been used to study the plume propagation dynamics during laser ablation of a silver target, over a large range of Ar background gas pressures (from high vacuum to f100 Pa). A comparative analysis of the experimental results allows us to identify different regimes of the plume expansion, going from a free plume at low pressure, through collisional and shockwave like hydrodynamic regimes at intermediate pressure, finally reaching a confined plume with subsequent thermalization of the plume particles at the largest pressure of the background gas. The experimental findings also show that a combination of complementary techniques, like optical emission spectroscopy, close to the target, and fast ion probe and deposition rate measurements at larger distances, can lead to a more detailed understanding of the laser ablated plasma plume propagation in a background gas. 䊚 2003 Elsevier B.V. All rights reserved. PACS: 34.50 Ion-molecule collisions; 52.50.J Laser produced plasma; 52.75.R Plasma application-film deposition; 79.20.D Laser ablation Keywords: Laser ablation; Spectroscopy; Electrical probe; Plume dynamics
1. Introduction During the past decade laser ablation has been utilized for a number of applications, emerging as a versatile technique for pulsed laser deposition (PLD), nanoparticles and clusters production, material sampling, etc. w1– 3x. Further progress in these applications requires a deeper understanding of the mechanisms of plasma plume formation and evolution. Laser ablation plasma is transient in nature, quickly evolving in space and time and depends strongly on processing parameters like laser fluence and pulse duration, ambient gas pressure and composition, etc. w1,2x. In spite of the extensive literature on the subject, the different mechanisms involved during a laser ablation process are rather complex and the expansion dynamics of the plasma *Corresponding author. Fax: q39-081-676376. E-mail address: [email protected] (S. Amoruso).
plume, in vacuum or in a background gas, are not still fully understood. In a number of papers we reported on the complex dynamics of multicomponent materials, namely YNi2B2C borocarbide targets, in high vacuum and on the effect on thin film deposition w4–6x. The main outcome of these investigations was the dependence of the spatial distribution of the emitted plume ions with the heavier species in its center and the lighter ones at its edges and the identification of target-to-substrate distance as a critical parameter for the optimal conditions for deposition of this material by PLD. Here we present an overview of our more recent experiments on plume dynamics in a background gas. In particular, we report results for two different systems by exploiting different investigation techniques. The first experiment, which was carried out at the plume diagnostic setup of the Coherentia-INFM center w7x, deals with the analysis of the expansion dynamics of the new
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.11.137
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MgB2 superconductor in Ar ambient gas by applying optical emission spectroscopy. In the second experiment performed at the existing setup in Risø National Laboratory w8,9x, combined diagnostic measurements of ion time-of-flight signals and deposition rate have been employed to investigate the propagation of a laser ablation plume from an elemental target, silver, into a background gas. In both cases, the expansion behavior in the background gas was studied at different pressures, showing interesting effects like plume-splitting and sharpening, excitationyionization revival, shock waves and plume thermalization. This paper is organized in five sections. In Sections 2 and 3 the experimental procedure and findings of the two experiments are discussed, respectively, while Section 4 contains a description of the plasma plume propagation into a background gas and a comparative interpretation of the experimental results. Finally, the summary and conclusions are given in Section 5. 2. MgB2 plasma dynamics investigated by optical emission spectroscopy The plasma dynamics of the plume formed during excimer laser (XeF, 351 nm, 20 ns) irradiation of a stoichiometric MgB2 target at a laser fluence of 3 J cmy2 has been investigated by space- and time-resolved optical emission spectroscopy. The target was mounted on a rotating holder and placed in a vacuum chamber with a residual pressure of f10y5 Pa. During the experiment, the chamber was filled with Ar gas and the pressure was varied in the range 10y2 –50 Pa. An optical system was used to image the plume onto the entrance slit of a monochromatoryspectrograph system (Jobin Yvon HR250), to have a 3:1 correspondence between the sampled area of the plume and the image, with a spatial resolution of f300 mm wsee Fig. 1ax. The monochromator was equipped with a turret of two interchangeable gratings (100 and 1200 grovesymm). One of the exit ports of the spectrograph was coupled to an ICCD camera (Andor Technology) operated in the vertical binning mode of the array to obtain spectral emission intensity vs. wavelength. In order to record the temporal profile of a selected emission line, the other exit port was coupled to a photomultiplier tube (PMT), whose output signal was registered by a 500 MHz digital oscilloscope (Tektronix TDS5054) triggered by a fast photodiode collecting the light scattered by the laser beam focusing lens. The collected light was guided to the selected output by means of a diverter mirror (M). This setup provides time- and space-resolved analysis of the emission from the constituent species within the plume, in particular in normal direction. In order to acquire the plume emission in the largest wavelength range the spectra were acquired with the 100 grovesymm grating, and an UV subtractive filter
Fig. 1. (a) Schematic of the experimental setup. M is a diverter mirror which switches between the intensified charge coupled device (ICCD) camera and the photomultiplier tube (PMT). (b) Typical spectrum of the MgB2 laser ablated plasma plume acquired at an Ar pressure of 7 Pa and at a distance ds2 mm from the target surface. An UV absorbing filter was used to avoid multiple orders from the emission lines from BI and MgI at wavelengths shorter than 300 nm.
was used to discern the emission lines from high order peaks of B and Mg lines with wavelengths shorter than 300 nm. Moreover, the 1200 grovesymm grating was used to obtain a more reliable and fine identification of the most significant lines. In Fig. 1b, a characteristic spectrum of the MgB2 plume in the spectral range 300– 800 nm is shown. The observed emission lines were identified according to standard data w10x and correspond to Mg and B neutrals and ions. Fig. 2 shows the emission intensity temporal profiles of the MgI 383 nm line observed at two different distances from the target surface w(a) ds3 mm, (b) ds 7 mmx, for three different Ar background gas pressures. These profiles represent a convolution of the signal from three very close MgI spectral emission lines w382.935, 383.229 and 383.23 nmx involving transitions between 3s3d 3D and 3s3p 3P0 levels, which cannot be resolved by our experimental setup. All the transitions are characterized by a lifetime of f10 ns. To facilitate the
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Fig. 2. Emission intensity temporal profiles of the MgI atoms at 383 nm at two distances from the target surface, (a) ds3 mm and (b) ds7 mm, and for three different pressures P.
comparison, the emission profile for Ps0.5 Pa has been multiplied by a factor 0.4 at ds3 mm and 0.05 at ds 7 mm, respectively. At a distance ds3 mm wsee Fig. 2ax, we observe a very intense emission at a pressure of 0.5 Pa, with a peak maximum at a time tMf150 ns and a tail extending to not more than f1 ms. By increasing the pressure to Ps7 Pa, the peak signal intensity reduces by factor f3, while tM is still f150 ns. Then, at Ps 23 Pa the appearance of a long tail, which extends up to f 5 ms, is observed. At ds7 mm, Fig. 2b, the peak signal intensity reduces by factor f20 by increasing the pressure from Ps0.5 to 7 Pa, while the time at which the peak maximum is observed, tMf300 ns, remains almost unchanged. Then, at Ps23 Pa, a second delayed component appears in the emission profile with a maximum at f4 ms and a very long tail, which extends up to f70 ms. Moreover, a fast peak characterized by a signal level and timing quite similar to the one observed at lower pressure is still present. Similar features have been observed for the other spectral lines investigated. The analysis of the temporal emission profiles reveals that this peak splitting effect is observed only above a distance of 3 mm at a pressure of 23 Pa. This shows that plume splitting appeared during the later stage of the plasma expansion as a consequence of plumebackground gas interaction. The plume splitting effect has been observed for plume ions expanding into an ambient gas, and has been thoroughly discussed in the literature w11–14x. More recently, by studying the expansion of an Al plume into a background air w15x, Hariral et al. have reported this effect also for neutrals, in agreement with our present
observation. Nevertheless, it remains just one stage of the complex plume dynamics in an ambient atmosphere. In Fig. 3a the integrated yield of excited species emission as a function of the distance d from the target is reported. The integrated yield has been obtained by integrating the whole area under the emission profile, and gives an indication of the amount of plume excited species reaching a given distance d. The data show that going from 0.5 to 7 Pa, the emission intensity decreases at almost all distances, whereas at Ps23 Pa it grows up again, becoming even larger than that observed at Ps0.5 Pa, for distances d larger 3 mm. Similar results have been observed for Mg ions w7,16x. Coincidentally, at an Ar pressure of 7 Pa, the mean free path l of an Mg atom in Ar becomes of the order of f3 mm, as estimated by using the equations given by Westwood w17x. This means that the plume species start to experience collisions with the background gas atoms in this pressure range on a distance of few millimeters. In this respect, it is worth observing that a Mg atom in a collision with an Ar atom, changes, on average, its direction by f 668 and loses f60% of its kinetic energy w17x. Thus, the strong reduction of plume luminescence can be ascribed to the escape of excited plume species from the luminescence region. To support this interpretation, in Fig. 3b we show the ratio between the integrated emission yield at a pressure of 7 and 0.5 Pa, rw7 Pax. The ratio between 23 and 0.5 Pa, rw23 Pax is also reported for comparison. We observe that at 7 Pa, rw7 Pax is fairly well described by an exponential decrease, rw7 Pax A exp(ydy l) with ls (3.7"0.5) mm, while a completely different behavior
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is observed for rw23 Pax. This suggests that at Ps23 Pa a different plume dynamics occurs, which influences the excitationyionization kinetics of the expanding plume. To elucidate the underlying mechanisms leading to the delayed component observed at Ps23 Pa, we have shown the dependence of the time tM at which the maximum in the emission intensity profile is observed as a function of the distance d from the target. We have made d–tM plots for the three different pressures as shown in Fig. 4. At 23 Pa only the d–tM plot for the delayed component is shown, while for the fast peak almost the same behavior as at lower pressure was observed. We note that the plume expansion in the early stage (-0.5 ms) is linear irrespective of the background pressure, then at longer distances and times a different behavior is observed at the larger pressures investigated. In particular, at 0.5 Pa a linear dependence is observed over the whole range of distances for which emission could be detected, resembling a behavior similar to a free-plume expansion in high vacuum. The velocity estimated from the d–tM plots at this pressure results of (2.4"0.1)=104 mys for the Mg neutral atoms and (2.9"0.3)=104 mys for the Mg ions. At larger pressures, the dependence is no longer linear at distances d larger than 7 mm at Ps7 Pa and 3 mm at Ps23 Pa, respectively. There is a clear distance-related pressure braking effect on the emitting species. Thus, we observe, both for ions and neutrals, a fast component which moves with almost the free expansion velocity, and a delayed component which appears at larger pressures, above a specific distance, and whose dynamics is strongly influenced by the pressure of the background gas. Zel’dovich and Raizer have shown that the expansion of a spherical blast wave is nearly self-similar with a
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Fig. 4. Distance d from the target surface as a function of the time tM at which the maximum in the emission intensity profile is registered for the 383 nm MgI line at different Ar background pressures. At 23 Pa only the d–tM data for the delayed component is shown, while for the fast peak almost the same behavior as at lower pressures is observed. The solid line and dashed lines are fits to dAtM and dAtaM with as0.3, respectively. The inset shows an enlarged view of the first microsecond.
velocity gradient such that the velocity of the front R as a function of the time t can be described by RAt a with as0.4 w18x. In the present experiment, we are not following specifically the temporal evolution of the plume front R, but rather the dependence of the time tM at which the maximum in the intensity emission profile occurs at the distance d. Nevertheless, we have shown that such a parameter is normally representative
Fig. 3. (a) Integrated emission intensity as a function of the distance d from the target surface for the MgI 383 nm line. The lines are a guide to the eye. (b) Ratio between the integrated emission yield at a pressure of 7 and 0.5 Pa, rw7 Pax, and 23 and 0.5 Pa, rw23 Pax, respectively. The solid line represent a fit to an exponential decrease behavior rw7xA exp(ydyl), with ls(3.7"0.5) mm. The dashed line is a guide to the eye.
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Fig. 5. Axial distribution of Mg excited atoms obtained by the emission intensity of the MgI 383 nm line for different time delays tD after the laser pulse and at three different Ar background gas pressure: (a)–(d) Ps0.5 Pa; (b)–(e) Ps7 Pa; (c)–(f) Ps23 Pa. The lines are a guide to the eye.
of the plume front dynamics w7x. In fact, the temporal evolution of the distance d as a function of the maximum emission time tM for the delayed component at 23 Pa follows fairly well the dependence dAtaM, with an expansion coefficient af0.3, thus showing a shockwave-like propagation of the plume at this pressure. The background gas pressure also influences the spatial properties of the expanding plume. To investigate this effect we have analyzed the spatial distribution of the emission intensity along the normal to the target surface as a function of the time delay tD with respect to the laser pulse. Such plots give a snapshot of the axial profiles, and they are shown in Fig. 5a–f for the three different pressures investigated and at different
values of tD. The experimental data points have been obtained by integrating over a 100 ns interval centered at tD. The intensity distributions show that the background gas largely affect the plume spatial evolution, leading to two major effects: (i) by increasing the pressure the plume emission becomes more confined; (ii) a strong increase in the emission is observed at Ps 23 Pa compared with Ps7 Pa. In particular, at low pressure wPs0.5 Pa, see Fig. 5a– dx, the plume emission intensity drops down rapidly in the first microsecond, while the position of the intensity maximum dM moves away. Then, at later times wtG1 ms, see Fig. 5dx, the plume emission extends up to more than 10 mm from the target showing two spatial com-
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ponents. The fast one, corresponding to the trailing edge of the highly forward-directed expanding plume, moves away from the target while its emission drops down rapidly. On the contrary, the second one shows an almost stationary behavior as a function of the distance d, while its luminescence decreases on a timescale of few microseconds. The latter one can be ascribed to very slow plume atoms, which are thermally emitted in the late stage of target evaporation, as already observed during laser ablation of different materials by ICCD photography w19x and time-of-flight mass spectrometry w20x. By increasing the pressure to Ps7 Pa wsee Fig. 5bx, we observe a decrease in the overall emission, and the luminescent plume appears more confined, while the fast component observed at the lower pressure and later times is no longer present wsee Fig. 5ex, as a consequence of collisions between plume species and background gas atoms. Finally, when the pressure is increased to Ps23 Pa wsee Fig. 5c–fx, a much more intense plume background interaction occurs, leading to a stronger confinement at early times (up to f1 ms) accompanied by a much larger emission from the plume core, with respect to the lower pressures. Moreover, a different temporal dynamics is observed with the maximum of emission increasing, going from 0.1 to 0.5 ms, and then slowly decreasing as a function of time. Then, at longer times wsee Fig. 5fx the tail of the spatial distribution extends to larger distances and shows an increase of the number of excited species at the plume front with respect to Ps 7 Pa due to the interaction dynamics with the background gas. This is in good agreement with the observation of a shock wave-like dynamics in the d–tM plot at this pressure. In this regime, the expanding plume compresses the surrounding gas like a piston due to a large number of collisions between the ablated and background gas atoms. The adjoint mass of the ambient gas at the plume edge breaks the plume expansion resulting in a large deviation from the free expansion when the mass of the gas surrounding the leading edge of the plume becomes comparable with the plume mass w18x. The generation of a shock wave produces a redistribution of kinetic and thermal energies between the plume and the ambient gas, leading to a transfer of part of the particle flux velocity into plume thermal energy, then resulting in plume heating w21x. This process mainly affects the plume front w21x and can lead to the observed increase of excited species in the plume tail, at larger distances and longer times wsee Fig. 4fx, as well as to the different behavior of the integrated emission yield wsee Fig. 3x, at Ps23 Pa with respect to Ps7 Pa. To show more clearly this effect we report in Fig. 6 the variation of the relative emission intensity as a function of the distance d from the target for different time delays tD at Ps23 Pa. The relative intensity distributions have been obtained by normalizing the
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Fig. 6. Relative emission intensity spatial profiles for different time delays tD after the laser pulse at Ps23 Pa.
intensity profiles of Fig. 5 to the integral of the emission yield. We note that the fraction of luminescence from the plume front gradually increases at larger time delays tD. This shows that an even larger number of excited species is present at the plume front during the propagation of the plume species into the background gas as a consequence of plume heating. The results analyzed in this section clearly show that the interaction with the background gas, above certain distance and pressures, strongly affect the temporal and spatial dynamics of the plume, but also influences the plume excitationyionization kinetics. 3. Silver plasma dynamics investigated by ion probe and quartz crystal techniques The effect of the pressure of the background gas on the dynamics of a laser ablation plasma plume produced by a simple one component target, silver, in Ar background gas in a large pressure interval (from f10y4 to f102 Pa) has been investigated by using combined measurements of ion signals and deposition rates. The experiments were carried out at the existing setup at Risø National Laboratory w8,9x. A frequency-tripled Nd:YAG laser beam (ls355 nm, pulse width f6 ns FWHM, laser fluence f2.5 J cmy2) was used in the present study to ablate a silver target in a vacuum chamber with a residual pressure of 10y5 Pa. The laser beam was focused to a circular beam spot on the target at normal incidence. The ion flux was measured by using a planar Langmuir probe, oriented to face the target spot, located at a distance of 75 mm from the target and at an angle of f208 with respect to the normal to the target surface. The probe collecting area was a 2=2 mm2 square
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Fig. 7. Time-of-flight profiles of the silver ions as a function of the Ar background gas pressure.
copper plate insulated on the rear side, similarly to those in Ref. w9x. During the ion collection, the probe was biased at y10 V. The collected ion current was measured by acquiring the voltage signal developed across a load resistor by a 500 MHz digital oscilloscope. The amplitude of the probe signals on a new target spot was found to reach a constant regime after approximately 50 laser shots. All the measurements were carried out after such a target conditioning with the ion flux signals averaged over five consecutive shots. The deposition rate was measured by a quartz crystal microbalance (QCM), with a 6-mm diameter active area, located at a distance of 80 mm from the target and at an angle of f238 with respect to the normal to the target surface. Each run was taken on a fresh target spot, typically with 200–1000 pulses with a repetition frequency of 0.1 Hz and with a subsequent relaxation time for the crystals of 4–6 h. Fig. 7 shows the influence of the Ar background on the ion time-of-flight (TOF) signals for different values of the ambient gas pressure, P. Each signal has been normalized to its own maximum value to facilitate the comparison. The fast narrow peak at almost zero time is due to photoelectrons from the metal of the probe tip induced by scattered laser photons and UV photons from the plume. The additional peak with a short arrival time observed at larger pressures is presumably due to background gas atoms ionized in the immediate vicinity of the collector by UV light or fast electrons from the plasma w22,23x and collected on the biased probe. Then, the peak with a maximum at f5 ms and extending up to f20 ms, at the lower pressure, is due to the collection of plasma plume ions reaching the probe. This single
peak structure was observed from high vacuum up to a pressure of f4 Pa, where the signal temporal profile is mainly characterized by the appearance of a tail which extends to longer times as the pressure increases. By further increasing the pressure, the long tail transforms, at first, into a shoulder, and then, at Pf5 Pa, into a second delayed peak of slow ions. This means that ion plume splitting at such an Ar background pressure occurs at distances of several centimeters from the target surface w11,12,14x. At even higher gas pressure the arrival time of the delayed ion peak increases, while the number of fast ions (first peak) becomes progressively smaller than that in the second delayed peak. We also analyzed the collected ion yield Yi and the deposition rate as a function of pressure P. The data are reported in Fig. 8a,b. We clearly observe that the number of ions reaching the Langmuir probe is essentially constant and equal to the vacuum yield up to a pressure of f0.5 Pa wsee Fig. 8a, insetx. This indicates that the slight broadening of the signal is due to fast ions experiencing elastic collisions with the background gas atoms with do not significantly deflect them out of the collecting solid angle in this pressure interval. In this respect, it has to be noticed that in a collision between Ag and Ar atoms the Ar atoms are efficiently pushed away, because of the large difference between the masses of projectile and target species. In particular, the direction of the Ag atoms changes, on average, only of f178 while the loss of kinetic energy is f40% w17x. A progressive decrease of Yi is observed for pressures larger than 1 Pa such that the collected ion yield is reduced at one-half of the value observed in high vacuum for an Ar pressure Pf5 Pa, while the long tail
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pressure, the peak merges into the fast background ionization peak. The delayed ion component shows a clear shock-wave-like dependence, tAP 0.5 w18x, in the pressure range f 5–10 Pa and is then somewhat delayed at higher pressure. This indicates a further decrease of the kinetic energy towards a complete plume thermalization with background gas atoms. 4. Physical description of laser produced plasma plume expansion into an ambient gas
Fig. 8. (a) Collected ion yield and (b) atoms deposition rate as a function of background gas pressure. In the insets the data are shown on a linear-logarithmic scales plot to show the behavior at very low pressures. The lines are guide to the eye.
in the ion signal becomes more and more significant wsee Fig. 7x. At larger pressure, plume splitting occurs in the ion signal and the delayed component start to become more and more significant wsee Fig. 7x, whereas the collected yield continues to slow down. Finally, at the largest pressure (PG20 Pa) a quasi-stationary regime is reached, in which the collected ion yield slowly decreases as a function of the pressure. A similar behavior is observed for the deposited atoms on the QCM shown in Fig. 8b. Eventually from the observed values in high vacuum an ionization degree of the order of f30% can be estimated at the detection distance. To further elaborate on the effect of ambient gas on the plasma dynamics, in Fig. 9 we report the time of arrival (obtained by considering the time at which the maximum in the ion TOF-signals is observed) of the fast and slow peak of the collected ion flux as a function of pressure. The scattering of the arrival time of the delayed peak at high pressure is due to signal noise which does not allow a very precise identification of the peak maximum. It is interesting to note that the fast ions propagate through the background gas almost without deceleration up to a pressure of f10 Pa. At higher
In nanosecond laser ablation, target evaporation begins just after the impact of the leading edge of the laser pulse on the sample surface. The interaction of the following part of the laser beam with the vapor in the vicinity of the target surface leads to strong heating and ionization of the vapor and to plasma formation. Initially, the ablated atoms and ions undergo collisions in a high density region near the sample surface, leading to a highly directional expansion along the normal to the target surface. In high vacuum condition, and for background gas pressures so low that the mean free path of the ablated particles is larger than the characteristic length of the experiment, an adiabatic expansion occurs with the plasma plume rapidly reaching an inertial stage. In this stage almost all the initial thermal energy deposited in the laser produced plasma has been converted into directed plume kinetic energy (free-plume). A physical description of plume expansion in this regime has been modeled by using self-similar profiles of the plume thermodynamic variables based on gas-dynamics and energy conservation equations w24x. In an ambient gas, the expansion of the laser produced plasma plume can be quite complex and strongly dependent on a number of properties like atomic mass of the plume and background atoms, initial plume energy and density, etc. w7,21,25,29x. Obviously, if the initial mass and energy of the laser produced plasma is not high enough, the plume can be so faint that no hydrodynamic effects can take place and ablated particles just dilute into the ambient gas. However, when an energetic plasma is formed, the plume expansion into the background gas can pass through different stages, which can involve plume confinement, splitting and snowplowing effects, plume-gas mixing, shock wave formation, etc. The expansion of a laser produced plasma plume into an ambient gas has been treated by different authors addressing different aspects of the process. For example, the evolution of spherical laser plume has been analytically modeled in terms of internal and external shock wave formation to describe the different stages of plume expansion by Arnold et al. w21x. Gas-dynamics equations with collisional plume-gas interactions have been considered by Wood et al. to explain plume splitting in Ref. w26x. In addition, other numerical models based on the
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Fig. 9. Peak arrival time vs. pressure for the two observed ion peaks. The solid line corresponds to a shock-wave-like behavior, tAP 0.5, while the dashed line is a guide to the eye.
dynamics of shock wave formation and propagation has been also proposed by Bulgakov and Bulgakova w27x. More recently, a hybrid approach based on the combination of continuous gas-dynamical modeling and direct Monte Carlo simulation has been used to describe plume expansion of Al neutral ground-state atoms in oxygen environment by Itina et al. w28x. The experimental results discussed in the previous sections allow us to give a physical description of the laser produced plume expansion into an ambient atmosphere. In particular, the combination of complementary techniques, like optical emission spectroscopy, fast ion probe and deposition rate measurements, enables us to investigate the effects of the background gas both at short and long distances from the target surface following the behavior of neutrals, ions and excited species. From the experimental findings reported in the previous sections, we can identify three different regimes of the plume expansion as a function of the background gas pressure: (a) free-plume, where the plume propagation is nearly vacuum-like; (b) collisional regime, where more and more plume particles from the plume front start to collide with background gas atoms. Above a certain pressure a strong plume-background interaction occurs leading to shock wave generation and plume splitting; (c) plume thermalization, where the plume becomes strongly confined and, above a certain distance, the ablated species tend to diffuse out the plume core into the ambient gas. In the following, the basic features of the different regimes will be analyzed and discussed. (a) Free-plume: At low pressures, the plume moves away from the target with a constant average velocity,
while adiabatically expanding, resembling the plume expansion in high vacuum conditions. During expansion, plume density and temperature gradually drop down, leading to the evolution of the MgB2 plume emission at a pressure of 0.5 Pa observed in Fig. 5a. However, the arrival time of the first ion peak wsee Fig. 9x as well as the number of collected silver atoms and ions wsee Fig. 8x at a distance of several cm from the target is almost independent of the pressure up to Pf1 Pa. (b) Collisional regime: At intermediate pressures, when the mean free path of the ablated species becomes comparable with the observation distances, collisions with the background gas atoms start to play a role. Initially, the collisions mainly involve ablated species at the plume edge, and influence the overall plume propagation dynamics only slightly. This effect can be observed in the evolution of the temporal maximum of plume emission, tM, at Ps7 Pa wsee Fig. 4x, where we observe a small delay only above a distance df7 cm from the target surface. The effect on plume emission can be clearly observed by comparing the axial distribution of the plume emission at 0.5 and 7 Pa. We clearly observe that while the emission close to the target is almost comparable at the two pressures, the luminescence at 7 Pa is progressively reduced at larger distances from the target as a result of collisions suffered by emitting species. The reduction of the emission signal can be ascribed to scattering processes as confirmed by the dependence of the integrated emission yield on the distance d shown in Fig. 3. A behavior in agreement with the analysis reported above can also be draw out from the dependence of the Langmuir probe and QCM data observed in Figs. 7 and 8. In fact, by increasing
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the pressure of the background gas, we observe no significant variation in the arrival time of the Ag ions peak, up to a pressure of f4 Pa, while the collected yield is gradually attenuated. When the ambient gas pressure is further increased, a stronger plume-background gas interaction regime occurs w7,21,25,27–29,29x. Through an increasing number of collisions the ablated materials begins to effectively push away the ambient gas atoms. As a result, both the plume and the adjacent background gas start to compress. Then, the transition to a hydrodynamic regime of the plume propagation takes place, with the plume acting as a piston on the surrounding atmosphere, whose counteraction breaks the ablated atoms expansion. This leads to plume confinement, which also influences the plume core. This is shown by the appearance of a delayed component in the temporal profiles of the emission intensity at Ps23 Pa wsee Fig. 2x and of the ions time-of-flight distribution above a pressure of f4 Pa wsee Fig. 7x. The delayed component becomes more and more important as the ambient gas pressure increases, as clearly observed in Fig. 7. Nevertheless, a small number of fast species is yet observable in both cases and even at particularly high pressure. This indicates that a small fraction of the energetic species present at the leading edge of the plume still penetrate into the ambient gas and reaches the detection region with negligible delay. The analysis of the delayed peak temporal dynamics shows that the plume-gas hydrodynamics coupling results in the generation of a shock-wave-like motion, as observed by the distance–time plot at 23 Pa, in Fig. 4, and by the arrival time vs. pressure, in the interval 5–10 Pa, in Fig. 9. The formation of a shock-wave-like expansion leads to plume confinement and plume heating effect compared to free-plume, and an increase of the overall plume emission is observed at Ps23 Pa as shown in Fig. 5c–e. In particular, plume confinement results in a larger amount of emitted photons from the inner part of the plume due to the formation of a high density peak in this area w28,29x. It also tends to increase excitationyionization at the plume front as a result of plume heating w7,21x, due to transfer of plume stream velocity into thermal energy, as observed in Figs. 3 and 6. However, we observe that the amount of atoms and ions reaching a collector located at several centimeters from the target surface decreases with the ambient gas pressure. During the hydrodynamic regime, the plume is braked mainly along the normal to the target surface, because the larger the expansion velocity, the larger the adjoint mass due to background gas. This leads to a significant modification of the plume angular distribution in this regime w29x, which results in the reduction of the number of atoms reaching the QCM and the Langmuir probe, as observed in Fig. 8. Moreover, the
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decrease of the collected ions yield observed in Fig. 8a can be partly ascribed to an enhancement of three-body recombination of the delayed ions as a consequence of plume confinement. (c) Thermalization: At still higher pressures, namely above 10 Pa for silver in Ar ambient gas, a thermalization stage is gradually approached. This is noticeably seen in Figs. 8 and 9. Fig. 8 shows a rapid decrease of the slope of the collected yield as a function of the pressure above f10 Pa. This is well correlated to the rise of the delayed peak arrival time observed at the same pressures in Fig. 9, indicating the transition from a shock-wave-like propagation of the plume to a diffusion-like motion of the plume ions into the background gas followed by a thermalization w7,25,29x. At this background gas pressures also, the remaining highenergy ions starts to break, finally confusing with the background ionization peak, in Fig. 7. 5. Conclusions In summary, two experiments exploiting different diagnostic techniques have been carried out in order to investigate laser produced plasma plume dynamics in a background gas. In the first one, reported in Section 2, space- and time-resolved optical emission spectroscopy has been used to investigate the effect on an Ar atmosphere on the dynamics of an MgB2 plasma plume. In the second experiment, described in Section 3, fast ion probe and deposition rate measurements have been used to study the propagation dynamics of ablated ions and atoms from a silver target in Ar background gas, over a large range of pressures. From a comparative analysis of the experimental findings we have demonstrated the evolution of the plume expansion through different regimes as a function of the ambient gas pressure. In particular, by increasing the pressure, the plume propagation dynamics goes from a vacuum-like free-plume expansion to a diffusion-like regime, where the ablated species become gradually thermalized with background gas atoms, passing through collisional and shock-wave-like hydrodynamic regimes at intermediate pressures. Additionally, the experimental data also show that the combination of complementary techniques, like optical emission spectroscopy, close to the target, and fast ion probe and deposition rate measurement at larger distances leads to a more detailed understanding of the laser ablated plasma dynamics. However, due to the complexity of the physical processes leading to the different features observed in the present work, we believe that further detailed experimental and theoretical studies will help to completely clarify the propagation dynamics of a laser ablation plasma plume in gas environment.
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References w1x B.B. Chrisey, G.K. Hubler (Eds.), Pulsed Laser Deposition of Thin Films, John Wiley and Sons, New York, 1994. w2x J.C. Miller, R.F. Haglund (Eds.), Laser Desorption and Ablation—Experimental Methods in the Physical Sciences, 30, Academic Press, 1998. w3x A.A. Puretzky, D.B. Geohegan, X. Fan, S.J. Pennycook, Appl. Phys. Lett. 76 (2000) 182. w4x X. Wang, S. Amoruso, R. Bruzzese, N. Spinelli, A. Tortora, R. Velotta, C. Ferdeghini, G. Grassano, W. Ramadan, Chem. Phys. Lett. 353 (2002) 1. w5x V. Ferrando, S. Amoruso, E. Bellingeri, R. Bruzzese, P. ` R. Velotta, X. Wang, C. Ferdeghini, Manfrinetti, D. Marre, Supercond. Sci. Technol. 16 (2003) 241. w6x X. Wang, S. Amoruso, M. Armenante, A. Boselli, R. Bruzzese, N. Spinelli, R. Velotta, Opt. Laser Eng. 39 (2003) 179. w7x S. Amoruso, R. Bruzzese, N. Spinelli, R. Velotta, M. Vitiello, X. Wang, Phys. Rev. B 67 (2003) 224 503. w8x B. Toftmann, J. Schou, J.G. Lunney, Phys. Rev. B 67 (2003) 104 101. w9x B. Toftmann, J. Schou, T.N. Hansen, J.G. Lunney, Phys. Rev. Lett. 84 (2000) 3998. w10x W.C. Martin, J. Sugar, A. Musgrove, G.R. Dalton, W.L. Wiese, J.R. Fuhr, D.E. Kelleher, NIST Database for Atomic Spectroscopy, NIST, Gaithersburg-MD, 1995. w11x R.F. Wood, K.R. Chen, J.N. Leboeuf, A.A. Puretzky, D.B. Geohegan, Phys. Rev. Lett. 79 (1997) 1571. w12x R.F. Wood, J.N. Leboeuf, D.B. Geohegan, A.A. Puretzky, K.R. Chen, Phys. Rev. B 58 (1998) 1533. w13x A. Miotello, R. Kelly, Appl. Surf. Sci. 138–139 (1999) 44.
w14x T.N. Hansen, J. Schou, J.G. Lunney, Appl. Surf. Sci. 138–139 (1999) 184. w15x S.S. Hariral, C.V. Bindhu, M.S. Tillack, F. Najmabadi, A.C. Gaeris, J. Phys. D. Appl. Phys. 35 (2002) 2935. w16x S. Amoruso, R. Bruzzese, N. Spinelli, R. Velotta, X. Wang, C. Ferdeghini, Appl. Phys. Lett. 80 (2003) 4315. w17x W.D. Westwood, J. Vac. Sci. Technol. 15 (1978) 1. w18x Ya.B. Zel’dovich, Yu.P. Raizer, Physics of Shock Waves and High-Temperature Hydrodynamic Phenomena, Academic Press, New York, 1966. w19x D.B. Geohegan, Appl. Phys. Lett. 60 (1992) 2732. w20x X. Wang, S. Amoruso, M. Armenante, R. Bruzzese, N. Spinelli, R. Velotta, Appl. Surf. Sci. 168 (2000) 100. w21x N. Arnold, J. Gruber, J. Heitz, Appl. Phys. A: Mater. Sci. Process. 69 (1999) S87. w22x S. Amoruso, M. Armenante, R. Bruzzese, N. Spinelli, R. Velotta, X. Wang, Appl. Phys. Lett. 75 (1999) 7. w23x R.C. Issac, G.K. Varier, P. Gopinath, S.S. Harilal, V.P.N. Nampoori, C.P.G. Vallabhan, Appl. Phys. A: Mater. Sci. Process. 67 (1998) 557. w24x S.I. Anisimov, D. Bauerle, ¨ B.S. Luk’yanchuk, Phys. Rev. B 48 (1993) 12 076. w25x B. Toftmann, S. Amoruso, J. Schou, J.G Lunney, to be published. w26x R.F. Wood, R.K. Chen, J.N. Leboeuf, A.A. Puretzky, D.B. Geohegan, Phys. Rev. Lett. 79 (1997) 1571. w27x A.V. Bulgakov, N.M. Bulgakova, J. Phys. D 31 (1998) 693. w28x T.E. Itina, J. Hermann, P. Delaporte, M. Sentis, Phys. Rev. E 66 (2002) 066406. w29x S. Amoruso, B. Toftmann, J. Schou, to be published.
Diagnostics of laser ablated plasma plumes S. Amorusoa,b,*, B. Toftmannc, J. Schouc, R. Velottaa,b, X. Wanga a
Coherentia INFM and Istituto Nazionale per la Fisica della Materia, Complesso Universitario di Monte S. Angelo, Via Cintia, I-80126 Napoli, Italy b Dipartimento di Scienze Fisiche, Universita` degli Studi di Napoli Federico II, Complesso Universitario di Monte S. Angelo, Via Cintia, I-80126 Napoli, Italy c Optics and Fluid Dynamics Department, Risø National Laboratory, DK-4000 Roskilde, Denmark
Abstract The effect of an ambient gas on the expansion dynamics of laser ablated plasmas has been studied for two systems by exploiting different diagnostic techniques. First, the dynamics of a MgB2 laser produced plasma plume in an Ar atmosphere has been investigated by space-and time-resolved optical emission spectroscopy. Second, deposition rate and fast ion probe measurements have been used to study the plume propagation dynamics during laser ablation of a silver target, over a large range of Ar background gas pressures (from high vacuum to f100 Pa). A comparative analysis of the experimental results allows us to identify different regimes of the plume expansion, going from a free plume at low pressure, through collisional and shockwave like hydrodynamic regimes at intermediate pressure, finally reaching a confined plume with subsequent thermalization of the plume particles at the largest pressure of the background gas. The experimental findings also show that a combination of complementary techniques, like optical emission spectroscopy, close to the target, and fast ion probe and deposition rate measurements at larger distances, can lead to a more detailed understanding of the laser ablated plasma plume propagation in a background gas. 䊚 2003 Elsevier B.V. All rights reserved. PACS: 34.50 Ion-molecule collisions; 52.50.J Laser produced plasma; 52.75.R Plasma application-film deposition; 79.20.D Laser ablation Keywords: Laser ablation; Spectroscopy; Electrical probe; Plume dynamics
1. Introduction During the past decade laser ablation has been utilized for a number of applications, emerging as a versatile technique for pulsed laser deposition (PLD), nanoparticles and clusters production, material sampling, etc. w1– 3x. Further progress in these applications requires a deeper understanding of the mechanisms of plasma plume formation and evolution. Laser ablation plasma is transient in nature, quickly evolving in space and time and depends strongly on processing parameters like laser fluence and pulse duration, ambient gas pressure and composition, etc. w1,2x. In spite of the extensive literature on the subject, the different mechanisms involved during a laser ablation process are rather complex and the expansion dynamics of the plasma *Corresponding author. Fax: q39-081-676376. E-mail address: [email protected] (S. Amoruso).
plume, in vacuum or in a background gas, are not still fully understood. In a number of papers we reported on the complex dynamics of multicomponent materials, namely YNi2B2C borocarbide targets, in high vacuum and on the effect on thin film deposition w4–6x. The main outcome of these investigations was the dependence of the spatial distribution of the emitted plume ions with the heavier species in its center and the lighter ones at its edges and the identification of target-to-substrate distance as a critical parameter for the optimal conditions for deposition of this material by PLD. Here we present an overview of our more recent experiments on plume dynamics in a background gas. In particular, we report results for two different systems by exploiting different investigation techniques. The first experiment, which was carried out at the plume diagnostic setup of the Coherentia-INFM center w7x, deals with the analysis of the expansion dynamics of the new
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.11.137
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MgB2 superconductor in Ar ambient gas by applying optical emission spectroscopy. In the second experiment performed at the existing setup in Risø National Laboratory w8,9x, combined diagnostic measurements of ion time-of-flight signals and deposition rate have been employed to investigate the propagation of a laser ablation plume from an elemental target, silver, into a background gas. In both cases, the expansion behavior in the background gas was studied at different pressures, showing interesting effects like plume-splitting and sharpening, excitationyionization revival, shock waves and plume thermalization. This paper is organized in five sections. In Sections 2 and 3 the experimental procedure and findings of the two experiments are discussed, respectively, while Section 4 contains a description of the plasma plume propagation into a background gas and a comparative interpretation of the experimental results. Finally, the summary and conclusions are given in Section 5. 2. MgB2 plasma dynamics investigated by optical emission spectroscopy The plasma dynamics of the plume formed during excimer laser (XeF, 351 nm, 20 ns) irradiation of a stoichiometric MgB2 target at a laser fluence of 3 J cmy2 has been investigated by space- and time-resolved optical emission spectroscopy. The target was mounted on a rotating holder and placed in a vacuum chamber with a residual pressure of f10y5 Pa. During the experiment, the chamber was filled with Ar gas and the pressure was varied in the range 10y2 –50 Pa. An optical system was used to image the plume onto the entrance slit of a monochromatoryspectrograph system (Jobin Yvon HR250), to have a 3:1 correspondence between the sampled area of the plume and the image, with a spatial resolution of f300 mm wsee Fig. 1ax. The monochromator was equipped with a turret of two interchangeable gratings (100 and 1200 grovesymm). One of the exit ports of the spectrograph was coupled to an ICCD camera (Andor Technology) operated in the vertical binning mode of the array to obtain spectral emission intensity vs. wavelength. In order to record the temporal profile of a selected emission line, the other exit port was coupled to a photomultiplier tube (PMT), whose output signal was registered by a 500 MHz digital oscilloscope (Tektronix TDS5054) triggered by a fast photodiode collecting the light scattered by the laser beam focusing lens. The collected light was guided to the selected output by means of a diverter mirror (M). This setup provides time- and space-resolved analysis of the emission from the constituent species within the plume, in particular in normal direction. In order to acquire the plume emission in the largest wavelength range the spectra were acquired with the 100 grovesymm grating, and an UV subtractive filter
Fig. 1. (a) Schematic of the experimental setup. M is a diverter mirror which switches between the intensified charge coupled device (ICCD) camera and the photomultiplier tube (PMT). (b) Typical spectrum of the MgB2 laser ablated plasma plume acquired at an Ar pressure of 7 Pa and at a distance ds2 mm from the target surface. An UV absorbing filter was used to avoid multiple orders from the emission lines from BI and MgI at wavelengths shorter than 300 nm.
was used to discern the emission lines from high order peaks of B and Mg lines with wavelengths shorter than 300 nm. Moreover, the 1200 grovesymm grating was used to obtain a more reliable and fine identification of the most significant lines. In Fig. 1b, a characteristic spectrum of the MgB2 plume in the spectral range 300– 800 nm is shown. The observed emission lines were identified according to standard data w10x and correspond to Mg and B neutrals and ions. Fig. 2 shows the emission intensity temporal profiles of the MgI 383 nm line observed at two different distances from the target surface w(a) ds3 mm, (b) ds 7 mmx, for three different Ar background gas pressures. These profiles represent a convolution of the signal from three very close MgI spectral emission lines w382.935, 383.229 and 383.23 nmx involving transitions between 3s3d 3D and 3s3p 3P0 levels, which cannot be resolved by our experimental setup. All the transitions are characterized by a lifetime of f10 ns. To facilitate the
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Fig. 2. Emission intensity temporal profiles of the MgI atoms at 383 nm at two distances from the target surface, (a) ds3 mm and (b) ds7 mm, and for three different pressures P.
comparison, the emission profile for Ps0.5 Pa has been multiplied by a factor 0.4 at ds3 mm and 0.05 at ds 7 mm, respectively. At a distance ds3 mm wsee Fig. 2ax, we observe a very intense emission at a pressure of 0.5 Pa, with a peak maximum at a time tMf150 ns and a tail extending to not more than f1 ms. By increasing the pressure to Ps7 Pa, the peak signal intensity reduces by factor f3, while tM is still f150 ns. Then, at Ps 23 Pa the appearance of a long tail, which extends up to f 5 ms, is observed. At ds7 mm, Fig. 2b, the peak signal intensity reduces by factor f20 by increasing the pressure from Ps0.5 to 7 Pa, while the time at which the peak maximum is observed, tMf300 ns, remains almost unchanged. Then, at Ps23 Pa, a second delayed component appears in the emission profile with a maximum at f4 ms and a very long tail, which extends up to f70 ms. Moreover, a fast peak characterized by a signal level and timing quite similar to the one observed at lower pressure is still present. Similar features have been observed for the other spectral lines investigated. The analysis of the temporal emission profiles reveals that this peak splitting effect is observed only above a distance of 3 mm at a pressure of 23 Pa. This shows that plume splitting appeared during the later stage of the plasma expansion as a consequence of plumebackground gas interaction. The plume splitting effect has been observed for plume ions expanding into an ambient gas, and has been thoroughly discussed in the literature w11–14x. More recently, by studying the expansion of an Al plume into a background air w15x, Hariral et al. have reported this effect also for neutrals, in agreement with our present
observation. Nevertheless, it remains just one stage of the complex plume dynamics in an ambient atmosphere. In Fig. 3a the integrated yield of excited species emission as a function of the distance d from the target is reported. The integrated yield has been obtained by integrating the whole area under the emission profile, and gives an indication of the amount of plume excited species reaching a given distance d. The data show that going from 0.5 to 7 Pa, the emission intensity decreases at almost all distances, whereas at Ps23 Pa it grows up again, becoming even larger than that observed at Ps0.5 Pa, for distances d larger 3 mm. Similar results have been observed for Mg ions w7,16x. Coincidentally, at an Ar pressure of 7 Pa, the mean free path l of an Mg atom in Ar becomes of the order of f3 mm, as estimated by using the equations given by Westwood w17x. This means that the plume species start to experience collisions with the background gas atoms in this pressure range on a distance of few millimeters. In this respect, it is worth observing that a Mg atom in a collision with an Ar atom, changes, on average, its direction by f 668 and loses f60% of its kinetic energy w17x. Thus, the strong reduction of plume luminescence can be ascribed to the escape of excited plume species from the luminescence region. To support this interpretation, in Fig. 3b we show the ratio between the integrated emission yield at a pressure of 7 and 0.5 Pa, rw7 Pax. The ratio between 23 and 0.5 Pa, rw23 Pax is also reported for comparison. We observe that at 7 Pa, rw7 Pax is fairly well described by an exponential decrease, rw7 Pax A exp(ydy l) with ls (3.7"0.5) mm, while a completely different behavior
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is observed for rw23 Pax. This suggests that at Ps23 Pa a different plume dynamics occurs, which influences the excitationyionization kinetics of the expanding plume. To elucidate the underlying mechanisms leading to the delayed component observed at Ps23 Pa, we have shown the dependence of the time tM at which the maximum in the emission intensity profile is observed as a function of the distance d from the target. We have made d–tM plots for the three different pressures as shown in Fig. 4. At 23 Pa only the d–tM plot for the delayed component is shown, while for the fast peak almost the same behavior as at lower pressure was observed. We note that the plume expansion in the early stage (-0.5 ms) is linear irrespective of the background pressure, then at longer distances and times a different behavior is observed at the larger pressures investigated. In particular, at 0.5 Pa a linear dependence is observed over the whole range of distances for which emission could be detected, resembling a behavior similar to a free-plume expansion in high vacuum. The velocity estimated from the d–tM plots at this pressure results of (2.4"0.1)=104 mys for the Mg neutral atoms and (2.9"0.3)=104 mys for the Mg ions. At larger pressures, the dependence is no longer linear at distances d larger than 7 mm at Ps7 Pa and 3 mm at Ps23 Pa, respectively. There is a clear distance-related pressure braking effect on the emitting species. Thus, we observe, both for ions and neutrals, a fast component which moves with almost the free expansion velocity, and a delayed component which appears at larger pressures, above a specific distance, and whose dynamics is strongly influenced by the pressure of the background gas. Zel’dovich and Raizer have shown that the expansion of a spherical blast wave is nearly self-similar with a
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Fig. 4. Distance d from the target surface as a function of the time tM at which the maximum in the emission intensity profile is registered for the 383 nm MgI line at different Ar background pressures. At 23 Pa only the d–tM data for the delayed component is shown, while for the fast peak almost the same behavior as at lower pressures is observed. The solid line and dashed lines are fits to dAtM and dAtaM with as0.3, respectively. The inset shows an enlarged view of the first microsecond.
velocity gradient such that the velocity of the front R as a function of the time t can be described by RAt a with as0.4 w18x. In the present experiment, we are not following specifically the temporal evolution of the plume front R, but rather the dependence of the time tM at which the maximum in the intensity emission profile occurs at the distance d. Nevertheless, we have shown that such a parameter is normally representative
Fig. 3. (a) Integrated emission intensity as a function of the distance d from the target surface for the MgI 383 nm line. The lines are a guide to the eye. (b) Ratio between the integrated emission yield at a pressure of 7 and 0.5 Pa, rw7 Pax, and 23 and 0.5 Pa, rw23 Pax, respectively. The solid line represent a fit to an exponential decrease behavior rw7xA exp(ydyl), with ls(3.7"0.5) mm. The dashed line is a guide to the eye.
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Fig. 5. Axial distribution of Mg excited atoms obtained by the emission intensity of the MgI 383 nm line for different time delays tD after the laser pulse and at three different Ar background gas pressure: (a)–(d) Ps0.5 Pa; (b)–(e) Ps7 Pa; (c)–(f) Ps23 Pa. The lines are a guide to the eye.
of the plume front dynamics w7x. In fact, the temporal evolution of the distance d as a function of the maximum emission time tM for the delayed component at 23 Pa follows fairly well the dependence dAtaM, with an expansion coefficient af0.3, thus showing a shockwave-like propagation of the plume at this pressure. The background gas pressure also influences the spatial properties of the expanding plume. To investigate this effect we have analyzed the spatial distribution of the emission intensity along the normal to the target surface as a function of the time delay tD with respect to the laser pulse. Such plots give a snapshot of the axial profiles, and they are shown in Fig. 5a–f for the three different pressures investigated and at different
values of tD. The experimental data points have been obtained by integrating over a 100 ns interval centered at tD. The intensity distributions show that the background gas largely affect the plume spatial evolution, leading to two major effects: (i) by increasing the pressure the plume emission becomes more confined; (ii) a strong increase in the emission is observed at Ps 23 Pa compared with Ps7 Pa. In particular, at low pressure wPs0.5 Pa, see Fig. 5a– dx, the plume emission intensity drops down rapidly in the first microsecond, while the position of the intensity maximum dM moves away. Then, at later times wtG1 ms, see Fig. 5dx, the plume emission extends up to more than 10 mm from the target showing two spatial com-
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ponents. The fast one, corresponding to the trailing edge of the highly forward-directed expanding plume, moves away from the target while its emission drops down rapidly. On the contrary, the second one shows an almost stationary behavior as a function of the distance d, while its luminescence decreases on a timescale of few microseconds. The latter one can be ascribed to very slow plume atoms, which are thermally emitted in the late stage of target evaporation, as already observed during laser ablation of different materials by ICCD photography w19x and time-of-flight mass spectrometry w20x. By increasing the pressure to Ps7 Pa wsee Fig. 5bx, we observe a decrease in the overall emission, and the luminescent plume appears more confined, while the fast component observed at the lower pressure and later times is no longer present wsee Fig. 5ex, as a consequence of collisions between plume species and background gas atoms. Finally, when the pressure is increased to Ps23 Pa wsee Fig. 5c–fx, a much more intense plume background interaction occurs, leading to a stronger confinement at early times (up to f1 ms) accompanied by a much larger emission from the plume core, with respect to the lower pressures. Moreover, a different temporal dynamics is observed with the maximum of emission increasing, going from 0.1 to 0.5 ms, and then slowly decreasing as a function of time. Then, at longer times wsee Fig. 5fx the tail of the spatial distribution extends to larger distances and shows an increase of the number of excited species at the plume front with respect to Ps 7 Pa due to the interaction dynamics with the background gas. This is in good agreement with the observation of a shock wave-like dynamics in the d–tM plot at this pressure. In this regime, the expanding plume compresses the surrounding gas like a piston due to a large number of collisions between the ablated and background gas atoms. The adjoint mass of the ambient gas at the plume edge breaks the plume expansion resulting in a large deviation from the free expansion when the mass of the gas surrounding the leading edge of the plume becomes comparable with the plume mass w18x. The generation of a shock wave produces a redistribution of kinetic and thermal energies between the plume and the ambient gas, leading to a transfer of part of the particle flux velocity into plume thermal energy, then resulting in plume heating w21x. This process mainly affects the plume front w21x and can lead to the observed increase of excited species in the plume tail, at larger distances and longer times wsee Fig. 4fx, as well as to the different behavior of the integrated emission yield wsee Fig. 3x, at Ps23 Pa with respect to Ps7 Pa. To show more clearly this effect we report in Fig. 6 the variation of the relative emission intensity as a function of the distance d from the target for different time delays tD at Ps23 Pa. The relative intensity distributions have been obtained by normalizing the
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Fig. 6. Relative emission intensity spatial profiles for different time delays tD after the laser pulse at Ps23 Pa.
intensity profiles of Fig. 5 to the integral of the emission yield. We note that the fraction of luminescence from the plume front gradually increases at larger time delays tD. This shows that an even larger number of excited species is present at the plume front during the propagation of the plume species into the background gas as a consequence of plume heating. The results analyzed in this section clearly show that the interaction with the background gas, above certain distance and pressures, strongly affect the temporal and spatial dynamics of the plume, but also influences the plume excitationyionization kinetics. 3. Silver plasma dynamics investigated by ion probe and quartz crystal techniques The effect of the pressure of the background gas on the dynamics of a laser ablation plasma plume produced by a simple one component target, silver, in Ar background gas in a large pressure interval (from f10y4 to f102 Pa) has been investigated by using combined measurements of ion signals and deposition rates. The experiments were carried out at the existing setup at Risø National Laboratory w8,9x. A frequency-tripled Nd:YAG laser beam (ls355 nm, pulse width f6 ns FWHM, laser fluence f2.5 J cmy2) was used in the present study to ablate a silver target in a vacuum chamber with a residual pressure of 10y5 Pa. The laser beam was focused to a circular beam spot on the target at normal incidence. The ion flux was measured by using a planar Langmuir probe, oriented to face the target spot, located at a distance of 75 mm from the target and at an angle of f208 with respect to the normal to the target surface. The probe collecting area was a 2=2 mm2 square
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Fig. 7. Time-of-flight profiles of the silver ions as a function of the Ar background gas pressure.
copper plate insulated on the rear side, similarly to those in Ref. w9x. During the ion collection, the probe was biased at y10 V. The collected ion current was measured by acquiring the voltage signal developed across a load resistor by a 500 MHz digital oscilloscope. The amplitude of the probe signals on a new target spot was found to reach a constant regime after approximately 50 laser shots. All the measurements were carried out after such a target conditioning with the ion flux signals averaged over five consecutive shots. The deposition rate was measured by a quartz crystal microbalance (QCM), with a 6-mm diameter active area, located at a distance of 80 mm from the target and at an angle of f238 with respect to the normal to the target surface. Each run was taken on a fresh target spot, typically with 200–1000 pulses with a repetition frequency of 0.1 Hz and with a subsequent relaxation time for the crystals of 4–6 h. Fig. 7 shows the influence of the Ar background on the ion time-of-flight (TOF) signals for different values of the ambient gas pressure, P. Each signal has been normalized to its own maximum value to facilitate the comparison. The fast narrow peak at almost zero time is due to photoelectrons from the metal of the probe tip induced by scattered laser photons and UV photons from the plume. The additional peak with a short arrival time observed at larger pressures is presumably due to background gas atoms ionized in the immediate vicinity of the collector by UV light or fast electrons from the plasma w22,23x and collected on the biased probe. Then, the peak with a maximum at f5 ms and extending up to f20 ms, at the lower pressure, is due to the collection of plasma plume ions reaching the probe. This single
peak structure was observed from high vacuum up to a pressure of f4 Pa, where the signal temporal profile is mainly characterized by the appearance of a tail which extends to longer times as the pressure increases. By further increasing the pressure, the long tail transforms, at first, into a shoulder, and then, at Pf5 Pa, into a second delayed peak of slow ions. This means that ion plume splitting at such an Ar background pressure occurs at distances of several centimeters from the target surface w11,12,14x. At even higher gas pressure the arrival time of the delayed ion peak increases, while the number of fast ions (first peak) becomes progressively smaller than that in the second delayed peak. We also analyzed the collected ion yield Yi and the deposition rate as a function of pressure P. The data are reported in Fig. 8a,b. We clearly observe that the number of ions reaching the Langmuir probe is essentially constant and equal to the vacuum yield up to a pressure of f0.5 Pa wsee Fig. 8a, insetx. This indicates that the slight broadening of the signal is due to fast ions experiencing elastic collisions with the background gas atoms with do not significantly deflect them out of the collecting solid angle in this pressure interval. In this respect, it has to be noticed that in a collision between Ag and Ar atoms the Ar atoms are efficiently pushed away, because of the large difference between the masses of projectile and target species. In particular, the direction of the Ag atoms changes, on average, only of f178 while the loss of kinetic energy is f40% w17x. A progressive decrease of Yi is observed for pressures larger than 1 Pa such that the collected ion yield is reduced at one-half of the value observed in high vacuum for an Ar pressure Pf5 Pa, while the long tail
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pressure, the peak merges into the fast background ionization peak. The delayed ion component shows a clear shock-wave-like dependence, tAP 0.5 w18x, in the pressure range f 5–10 Pa and is then somewhat delayed at higher pressure. This indicates a further decrease of the kinetic energy towards a complete plume thermalization with background gas atoms. 4. Physical description of laser produced plasma plume expansion into an ambient gas
Fig. 8. (a) Collected ion yield and (b) atoms deposition rate as a function of background gas pressure. In the insets the data are shown on a linear-logarithmic scales plot to show the behavior at very low pressures. The lines are guide to the eye.
in the ion signal becomes more and more significant wsee Fig. 7x. At larger pressure, plume splitting occurs in the ion signal and the delayed component start to become more and more significant wsee Fig. 7x, whereas the collected yield continues to slow down. Finally, at the largest pressure (PG20 Pa) a quasi-stationary regime is reached, in which the collected ion yield slowly decreases as a function of the pressure. A similar behavior is observed for the deposited atoms on the QCM shown in Fig. 8b. Eventually from the observed values in high vacuum an ionization degree of the order of f30% can be estimated at the detection distance. To further elaborate on the effect of ambient gas on the plasma dynamics, in Fig. 9 we report the time of arrival (obtained by considering the time at which the maximum in the ion TOF-signals is observed) of the fast and slow peak of the collected ion flux as a function of pressure. The scattering of the arrival time of the delayed peak at high pressure is due to signal noise which does not allow a very precise identification of the peak maximum. It is interesting to note that the fast ions propagate through the background gas almost without deceleration up to a pressure of f10 Pa. At higher
In nanosecond laser ablation, target evaporation begins just after the impact of the leading edge of the laser pulse on the sample surface. The interaction of the following part of the laser beam with the vapor in the vicinity of the target surface leads to strong heating and ionization of the vapor and to plasma formation. Initially, the ablated atoms and ions undergo collisions in a high density region near the sample surface, leading to a highly directional expansion along the normal to the target surface. In high vacuum condition, and for background gas pressures so low that the mean free path of the ablated particles is larger than the characteristic length of the experiment, an adiabatic expansion occurs with the plasma plume rapidly reaching an inertial stage. In this stage almost all the initial thermal energy deposited in the laser produced plasma has been converted into directed plume kinetic energy (free-plume). A physical description of plume expansion in this regime has been modeled by using self-similar profiles of the plume thermodynamic variables based on gas-dynamics and energy conservation equations w24x. In an ambient gas, the expansion of the laser produced plasma plume can be quite complex and strongly dependent on a number of properties like atomic mass of the plume and background atoms, initial plume energy and density, etc. w7,21,25,29x. Obviously, if the initial mass and energy of the laser produced plasma is not high enough, the plume can be so faint that no hydrodynamic effects can take place and ablated particles just dilute into the ambient gas. However, when an energetic plasma is formed, the plume expansion into the background gas can pass through different stages, which can involve plume confinement, splitting and snowplowing effects, plume-gas mixing, shock wave formation, etc. The expansion of a laser produced plasma plume into an ambient gas has been treated by different authors addressing different aspects of the process. For example, the evolution of spherical laser plume has been analytically modeled in terms of internal and external shock wave formation to describe the different stages of plume expansion by Arnold et al. w21x. Gas-dynamics equations with collisional plume-gas interactions have been considered by Wood et al. to explain plume splitting in Ref. w26x. In addition, other numerical models based on the
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Fig. 9. Peak arrival time vs. pressure for the two observed ion peaks. The solid line corresponds to a shock-wave-like behavior, tAP 0.5, while the dashed line is a guide to the eye.
dynamics of shock wave formation and propagation has been also proposed by Bulgakov and Bulgakova w27x. More recently, a hybrid approach based on the combination of continuous gas-dynamical modeling and direct Monte Carlo simulation has been used to describe plume expansion of Al neutral ground-state atoms in oxygen environment by Itina et al. w28x. The experimental results discussed in the previous sections allow us to give a physical description of the laser produced plume expansion into an ambient atmosphere. In particular, the combination of complementary techniques, like optical emission spectroscopy, fast ion probe and deposition rate measurements, enables us to investigate the effects of the background gas both at short and long distances from the target surface following the behavior of neutrals, ions and excited species. From the experimental findings reported in the previous sections, we can identify three different regimes of the plume expansion as a function of the background gas pressure: (a) free-plume, where the plume propagation is nearly vacuum-like; (b) collisional regime, where more and more plume particles from the plume front start to collide with background gas atoms. Above a certain pressure a strong plume-background interaction occurs leading to shock wave generation and plume splitting; (c) plume thermalization, where the plume becomes strongly confined and, above a certain distance, the ablated species tend to diffuse out the plume core into the ambient gas. In the following, the basic features of the different regimes will be analyzed and discussed. (a) Free-plume: At low pressures, the plume moves away from the target with a constant average velocity,
while adiabatically expanding, resembling the plume expansion in high vacuum conditions. During expansion, plume density and temperature gradually drop down, leading to the evolution of the MgB2 plume emission at a pressure of 0.5 Pa observed in Fig. 5a. However, the arrival time of the first ion peak wsee Fig. 9x as well as the number of collected silver atoms and ions wsee Fig. 8x at a distance of several cm from the target is almost independent of the pressure up to Pf1 Pa. (b) Collisional regime: At intermediate pressures, when the mean free path of the ablated species becomes comparable with the observation distances, collisions with the background gas atoms start to play a role. Initially, the collisions mainly involve ablated species at the plume edge, and influence the overall plume propagation dynamics only slightly. This effect can be observed in the evolution of the temporal maximum of plume emission, tM, at Ps7 Pa wsee Fig. 4x, where we observe a small delay only above a distance df7 cm from the target surface. The effect on plume emission can be clearly observed by comparing the axial distribution of the plume emission at 0.5 and 7 Pa. We clearly observe that while the emission close to the target is almost comparable at the two pressures, the luminescence at 7 Pa is progressively reduced at larger distances from the target as a result of collisions suffered by emitting species. The reduction of the emission signal can be ascribed to scattering processes as confirmed by the dependence of the integrated emission yield on the distance d shown in Fig. 3. A behavior in agreement with the analysis reported above can also be draw out from the dependence of the Langmuir probe and QCM data observed in Figs. 7 and 8. In fact, by increasing
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the pressure of the background gas, we observe no significant variation in the arrival time of the Ag ions peak, up to a pressure of f4 Pa, while the collected yield is gradually attenuated. When the ambient gas pressure is further increased, a stronger plume-background gas interaction regime occurs w7,21,25,27–29,29x. Through an increasing number of collisions the ablated materials begins to effectively push away the ambient gas atoms. As a result, both the plume and the adjacent background gas start to compress. Then, the transition to a hydrodynamic regime of the plume propagation takes place, with the plume acting as a piston on the surrounding atmosphere, whose counteraction breaks the ablated atoms expansion. This leads to plume confinement, which also influences the plume core. This is shown by the appearance of a delayed component in the temporal profiles of the emission intensity at Ps23 Pa wsee Fig. 2x and of the ions time-of-flight distribution above a pressure of f4 Pa wsee Fig. 7x. The delayed component becomes more and more important as the ambient gas pressure increases, as clearly observed in Fig. 7. Nevertheless, a small number of fast species is yet observable in both cases and even at particularly high pressure. This indicates that a small fraction of the energetic species present at the leading edge of the plume still penetrate into the ambient gas and reaches the detection region with negligible delay. The analysis of the delayed peak temporal dynamics shows that the plume-gas hydrodynamics coupling results in the generation of a shock-wave-like motion, as observed by the distance–time plot at 23 Pa, in Fig. 4, and by the arrival time vs. pressure, in the interval 5–10 Pa, in Fig. 9. The formation of a shock-wave-like expansion leads to plume confinement and plume heating effect compared to free-plume, and an increase of the overall plume emission is observed at Ps23 Pa as shown in Fig. 5c–e. In particular, plume confinement results in a larger amount of emitted photons from the inner part of the plume due to the formation of a high density peak in this area w28,29x. It also tends to increase excitationyionization at the plume front as a result of plume heating w7,21x, due to transfer of plume stream velocity into thermal energy, as observed in Figs. 3 and 6. However, we observe that the amount of atoms and ions reaching a collector located at several centimeters from the target surface decreases with the ambient gas pressure. During the hydrodynamic regime, the plume is braked mainly along the normal to the target surface, because the larger the expansion velocity, the larger the adjoint mass due to background gas. This leads to a significant modification of the plume angular distribution in this regime w29x, which results in the reduction of the number of atoms reaching the QCM and the Langmuir probe, as observed in Fig. 8. Moreover, the
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decrease of the collected ions yield observed in Fig. 8a can be partly ascribed to an enhancement of three-body recombination of the delayed ions as a consequence of plume confinement. (c) Thermalization: At still higher pressures, namely above 10 Pa for silver in Ar ambient gas, a thermalization stage is gradually approached. This is noticeably seen in Figs. 8 and 9. Fig. 8 shows a rapid decrease of the slope of the collected yield as a function of the pressure above f10 Pa. This is well correlated to the rise of the delayed peak arrival time observed at the same pressures in Fig. 9, indicating the transition from a shock-wave-like propagation of the plume to a diffusion-like motion of the plume ions into the background gas followed by a thermalization w7,25,29x. At this background gas pressures also, the remaining highenergy ions starts to break, finally confusing with the background ionization peak, in Fig. 7. 5. Conclusions In summary, two experiments exploiting different diagnostic techniques have been carried out in order to investigate laser produced plasma plume dynamics in a background gas. In the first one, reported in Section 2, space- and time-resolved optical emission spectroscopy has been used to investigate the effect on an Ar atmosphere on the dynamics of an MgB2 plasma plume. In the second experiment, described in Section 3, fast ion probe and deposition rate measurements have been used to study the propagation dynamics of ablated ions and atoms from a silver target in Ar background gas, over a large range of pressures. From a comparative analysis of the experimental findings we have demonstrated the evolution of the plume expansion through different regimes as a function of the ambient gas pressure. In particular, by increasing the pressure, the plume propagation dynamics goes from a vacuum-like free-plume expansion to a diffusion-like regime, where the ablated species become gradually thermalized with background gas atoms, passing through collisional and shock-wave-like hydrodynamic regimes at intermediate pressures. Additionally, the experimental data also show that the combination of complementary techniques, like optical emission spectroscopy, close to the target, and fast ion probe and deposition rate measurement at larger distances leads to a more detailed understanding of the laser ablated plasma dynamics. However, due to the complexity of the physical processes leading to the different features observed in the present work, we believe that further detailed experimental and theoretical studies will help to completely clarify the propagation dynamics of a laser ablation plasma plume in gas environment.
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