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Expansion and backscattering of laser produced Fe plasma plume
Spectrochimica Acta Part B 68 (2012) 34–39
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part B journal homepage: www.elsevier.com/locate/sab
Expansion and backscattering of laser produced Fe plasma plume M. Bišćan ⁎, S. Milošević Institute of Physics, Bijenička 46, HR-10000 Zagreb, Croatia
a r t i c l e
i n f o
Article history: Received 22 July 2011 Accepted 16 January 2012 Available online 24 January 2012 Keywords: Backscattering Laser produced plasma Cavity ringdown spectroscopy Iron
a b s t r a c t Forward and backward moving atoms within the laser produced plasma plume were studied by means of a cavity ringdown spectroscopy. The plume was produced using a nanosecond Nd-YAG laser pulse illuminating stainless steel target in a vacuum or helium background gas. Measurements were done at pressures ranging from 10− 5 to 1 mbar. Atomic absorption line shapes of iron around 388 nm were measured above and below the target at different times after the ablation initiation. Changes in absorption line shapes were used to estimate kinetic parameters of the plasma plume. The observations were interpreted through modeling which takes into account the angular and velocity distributions of atoms in the expanding plume. The amount of backward scattered atoms was about 10% of the total number of particles. © 2012 Elsevier B.V. All rights reserved.
1. Introduction When an intense, focused laser pulse hits a solid or liquid target, material is evaporated and dense ionized hot plasma is produced. This ablation stage is then followed by rapid expansion into the vacuum or background gas environment. Because of the high plasma pressure just above the target surface, most of the ablated particles move out perpendicularly away from it. If the laser pulse is still present at this stage, it will interact with the expanding plume and further energize its particles. According to Harilal et al. [1] after the laser pulse is stopped, particles are no longer ablated from the surface. But, in order to explain two components of the plume, Mahmood et al. [2] introduced the possibility that second vaporization takes place using the energy from primary plasma. In both cases, developed plume then expands freely in a vacuum or low pressure environment. If the pressure is high enough, plume expansion is governed by its interaction with the background gas [1]. Also, due to the collisions within the plume itself or with the background gas certain percentage of atoms is back-reflected and moves back towards the target surface. This backward scattering of the ablated species is usually experimentally observed indirectly through the deposits on the target or using fast imaging techniques. Although its origin is inside the plume itself, Gonzalo et al. [3] found that backscattering is more prominent in the presence of an obstacle. The Monte Carlo simulations, [4–6] have shown that up to 20% of atoms could be back reflected depending on the background pressure. One of the features of the laser-produced plasmas (LPP) is the so-called plume splitting which describes the effect when forward moving plume splits into two or even three components with different velocities [1,2,7,8], the effect which could be important for pulsed laser
⁎ Corresponding author. Tel.: + 385 14698851. E-mail address: [email protected] (M. Bišćan). 0584-8547/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2012.01.010
deposition (PLD). As noticed by Wood [8] and Nevolin [9], particles from fast component can have considerable effect or even damage the growing film on the substrate. Avoiding these particles is therefore important for thin film deposition. One of the possibilities is to use the backscattered particles. Both the forward moving and backscattering atoms depend on many laser ablation (LA) parameters, such as the laser fluence, beam profile, focusing conditions, target size and shape, background gas, etc. Optimization of back flux on any of such parameters could be important in PLD [10,11]. In the present work we employed cavity ringdown spectroscopy (CRDS) technique [12,13] to measure kinetics of the Fe ground state atoms through measurements of spectral line shapes in the regions above and below the target. To demonstrate the method properly, care was taken to keep the laser ablation parameters fixed and the only variable parameter was pressure of the background gas. The observed line shapes were then interpreted through modeling which takes into account the angular distribution of atoms and their velocity distributions as described in [14,15]. The CRDS technique applied to atomic and molecular sources with preferential moving direction such as atomic beams (jets) or LPP plumes adds Doppler features to the measured absorption line shapes. As photons within the cavity move back and forth, atoms traversing the cavity axis with certain parallel velocity component experience both blue and red Doppler shifted absorptions. This may result in substantial line shape splitting [15,16]. If the experimental resolution is sufficient, this splitting provides information about the angular and velocity distribution of atoms. Freely expanding laser produced plasma plumes are characterized by angular emission distributions which can be described using Gauss-functions [17]. To some approximation these are cones pointing out from the target surface where the cone steepness changes with atomic mass and the laser spot size. This mainly describes atoms moving in forward direction out of the target surface. The distributions of back scattered particles, which are due to the
M. Bišćan, S. Milošević / Spectrochimica Acta Part B 68 (2012) 34–39
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collisions within the plume itself or with the background surrounding gas, are quite different as depicted from Monte Carlo simulations (see Fig. 7 in Ref. [4]).
2. Experiment and method The experimental details are similar as described earlier [18] with main changes regarding the shape and position of the target. To probe the forward moving and back scattered atoms we set experiment as illustrated in Fig. 1. Probing the plume directly above the target is sensitive to forward moving atoms, while probing directly below the target detects only the backscattered atoms. Here we used a NdYAG laser (1064 nm) of 5 ns pulse duration at a repetition rate of 5 Hz to ablate a stainless steel rod, with diameter of 6 mm, placed inside the vacuum chamber (with diameter of 40 cm). The target was rotated in order to avoid effects of drilling and heating. The chamber was evacuated by means of a diffusion pumps down to a 10 − 5 mbar. Higher pressure was achieved using a flow of helium gas. The laser beam was focused using a 33 cm lens to a spot size of 0.25 × 102 cm2 yielding a fluence of 26 J/cm2. For the absorption measurements, excimer pumped dye laser (pulse length 15 ns, Δν= 0.17 cm− 1) operating with BiBuQ (LC 3860) laser dye was used. Ringdown signal was detected using a photomultiplier tube, digitized and averaged using an 8-bit digital oscilloscope and then sent to a computer. Fig. 2 shows the typical cavity ringdown curves in a semi-logarithmic scale for cases when excimer pumped dye laser is tuned to the absorption line center (black dots) and out of the resonance (gray dots, the empty cavity case). When the dye laser is non-resonant, CRDS curve is a simple exponential decay, while for the resonant absorption, CRDS curve shows non-exponential decay due to density time evolution of absorbing atoms. The radius of curvature of cavity mirrors is 50 cm and their maximum reflectivity is around 388 nm where absorption from the iron ground state occurs (Fe I transition a 5D3 → z5D30, see Fig. 3 thick arrow). The dye laser intensity used for the CRDS was kept low enough to avoid saturation effects [19] which would cause signal decrease in the line center. The line shapes were evaluated at different time delays tk within time windows Δt which define temporal resolution as described in Ref. [13]. Spatial resolution is determined by the laser beam waist at a cavity center and was measured to be about 1 mm. The distance d of the cavity axis to the target surface was changed by means of a stepping motor in synchronization with the position of lens used to focus the laser beam to the target surface. Each measured line shape is an average of 24 measurements; eight measurements were averaged at oscilloscope to give one ringdown curve and the dye laser was scanned over given wavelength interval three times.
Here, τ and τ0 are the lifetimes of the probing laser pulse inside the cavity with and without absorbing medium, respectively. 1/(c ⋅ τ) and 1/(c ⋅ τ0) are the corresponding losses. L is the total length of a cavity and labs is the length of absorbing medium. In order to find absorption coefficient kλ one has to know the width labs of the plasma plume (absorbing medium) at a given distance d. According to [20], integral of absorption coefficient is a function of a density of absorbing atoms:
Fig. 1. Experimental setup for observation (a) above and (b) below the target.
Fig. 3. The excitation scheme to probe Fe population.
Fig. 2. A typical ringdown curves and temporal resolution scheme.
In the CRDS measurements, measured spectrum represents the wavelength dependence of absorption loss inside the cavity. The absorption coefficient kλ can be expressed as: kλ ¼
1 1 h −1 i − cm labs ⋅c τ τ0 L
∫kν dν ¼
·
λ20 g 2 N h −1 −1 i cm s · 8πg1 τ
ð1Þ
ð2Þ
Here, ν is the frequency of light, g1 and g2 are statistical weights of lower and upper level, N is the density of atoms in lower level, λ0 is
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M. Bišćan, S. Milošević / Spectrochimica Acta Part B 68 (2012) 34–39
the central wavelength of absorption line and τ is the mean life time of upper level (not to be confused with τ in Eq. (1)). If effective absorption lengths labs are known, the densities of particles can be calculated, as will be described below. To interpret the experimental results with theory we used forward convolution modeling and calculated absorption line shapes using Gaussian function for line profile [14]. This simple model takes into account all relevant experimental parameters regarding specific CRDS observation (temporal and spatial resolution) and assumes the angular and velocity distribution of atoms within the plume. For the angular distribution we used Gaussian distribution with σ as a fitting parameter. According to Sristava et al. [17], Gaussian distribution is the most suited for describing angular shape of a freely expanding laser produced plumes. For the velocity distribution we used shifted Maxwell–Boltzmann function with two parameters, mean velocity and temperature. Temperature gives an estimate of overall spread of velocity. In order to describe the expansion of backscattering particles, angular distributions were modified to include the effect of shielding by the target. Around a central axis these distributions have a hole which extends to a cutoff angle given in Table 1. Inside this hole the distribution is zero and outside it follows a Gaussian shape. In simulations, the backscattering of particles is similar to the forward moving, the differences are angular distributions and the position and orientation of the source. Forward moving particles emerge from a point source at the surface of the target and move upward while backscattering particles emerge from a point source at some effective height h above the target and move downward. As it has been shown in Refs. [14,15], CRDS absorption line shape profiles are quite sensitive to angular distribution of atoms, since atoms moving with velocity component parallel to the cavity axis experience both red and blue Doppler shift. Doppler broadening or splitting can then be used as a measure of parallel velocity component. The larger the splitting, the larger the parallel velocity component. On the other hand, time delay tk which gives maximum intensity can be used to estimate perpendicular velocity component. Combining these two, one can get the mean velocity of the expanding plume. This velocity is then used in theoretical model to calculate line shapes. This simulation gives relative intensities as all calculated line shapes were scaled to the one with maximum intensity. 3. Results and discussion 3.1. Line shapes measurements above the target Fig. 4 shows the line shapes measured and calculated at a given distance d above the target versus time delay tk after the ablation initiation. The y-scale corresponds to labs ⋅ kν/L (loss [cm − 1]). The time delay tk is indicated in each row and Δt was 0.75 μs. Measured line shapes were obtained after averaging three subsequent dye laser scans. As the plume expands, at early time delays tk, line shapes are mainly influenced by the atoms moving perpendicularly away from the target surface because these atoms are the first to enter the probing axis. Therefore, the line shape is dominantly a single Gauss profile. At later time delays, most of the forward moving atoms have passed through the probing axis and the influence of atoms with larger parallel velocity component rises. Accordingly, at tk = 1750 μs and d = 13 mm
Table 1 Parameters of angular and velocity distributions used to obtain the best fit to the experimental line shapes.
Temperature (K) Velocity (105 cm/s) Width (°) Cut off (°)
Above
Below
15000 8.7 25
6000 3.3 35 40
Fig. 4. Measured and calculated line shapes at distance d = 13 mm above the target for different time delays tk (ΔT = 0.75 μs) with fluence of 26 J/cm2 and pressure of 2 × 10− 5 mbar.
the line shape shows double Gauss profile, due to Doppler splitting. Appearance of splitting is delayed in time for larger distances d. The effective absorption length labs can be obtained by modeling or visual inspection, latter being unreliable as the absorbing medium could be non-emitting. As the plasma plume is modeled with the Gaussian angular distribution, its labs at some distance d was calculated as width corresponding to the parameter σ. Using the Eq. (2), the lifetime of the upper level τ of 81.1 ns, found in [21], and labs of 12 mm maximum density of Fe I atoms in the state a 5D3 moving in forward direction is measured to be around 2.2 × 1011 cm− 3 at a given fluence of 26 J/cm 2. This is the density measured at time tk = 1 μs and distance d = 13 mm. Because angular distributions are Gaussian functions, calculated density is the mean density of atoms, in above-mentioned state, in region through which the probing beam passes. Comparing experimental and simulated line shapes we see that Doppler splitting can be described solely by free expansion of the plasma plume. Influence of backscattering atoms can at these distances d be neglected, as will be shown later. Above considerations hold for pressure of around 10− 5 mbar. The best fit parameters are given in Table 1. 3.2. Line shapes measurements below the target Below the target only contribution from the backscattered atoms is measured. Fig. 5 shows the time sequence of line shapes measured and calculated at a given distance d. One can see that absorption losses are order of magnitude smaller compared to the case above the target, as shown in Fig. 4, a sign that only a fraction of atoms is backscattered. Those atoms that have sufficiently large perpendicular velocity component are blocked by the target itself and this results in missing intensity around the line center, the line shape splitting due to the Doppler effect is more pronounced. The amount of splitting gives in this case a parallel velocity component to be around
M. Bišćan, S. Milošević / Spectrochimica Acta Part B 68 (2012) 34–39
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Fig. 6. Integrated loss at different distances d below the target at 2 × 10− 5 mbar.
Fig. 5. Measured and calculated line shapes at distance d = − 7 mm below the target and for different time delays tk (ΔT = 0.75 μs) with fluence of 26 J/cm2 and pressure of 2 × 10− 5 mbar.
3 × 10 5 cm/s. Assuming that total velocity is not larger than velocity of forward moving atoms and taking into account geometrical factors (size and shape of the target and overall dimensions of the chamber), boundary values for perpendicular component can be estimated to 1.5–8.5 × 10 5 cm/s. These values then give the upper (3.9 mm) and lower (0.3 mm) heights h above the target from where most of the atoms are scattered. Uncertainty in the origin position consequently gives uncertainty in perpendicular velocity component and angular shielding by the target. Nevertheless, if we make a simulation at those above-mentioned extreme distances, we get two different results. For minimum height of 0.3 mm and shielding of 65°, Doppler splitting is correct but integrated intensity is not. On the other hand, result is opposite for maximum height of 3.9 mm and shielding of 4°. So the best fit is in between. Fig. 6 shows how integrated loss changes with distance |d| increasing. We see that at each |d| integrated loss is different and reaches maximum at a different time. Better estimation of velocities and heights h can be obtained if we plot the time position of maximum integrated loss tmax versus distance d below the target. In this manner, almost linear dependence is obtained, as shown in Fig. 7. As the backscattered particles occur just above the target surface, this linear dependence suggests that after that they move at a constant velocity away from their origin. Backscattered cloud then follows free expansion. By fitting this data to a function t(d): dþh t ðdÞ ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 9 v 1− ð3þh Þ2
backscattering contribution which may arise from collisions with chamber walls is expected in time periods larger than 20 μs and can be neglected in our time window. The same fit gives us also the effective height h above the target at which the backscattering occurs (h = 1 ± 2 mm). In Fig. 8 measured and calculated integrated losses above (a) and below (b) the target are shown versus time. It can be seen that particles do not travel as a continuous stream but are bunched together and their density reaches maximum at some time. Above the target time distribution is narrow while below the target it is much broader and with maximum delayed in this case by around 5 μs. These distributions correspond to a group of fast atoms with approximately the same velocity, and a group of slower atoms, respectively. Comparison with the simulation shows that backscattered atoms below the target can also be described by a freely expanding plume. Best fit parameters are given in Table 1. Note that the intensity scale is an order of magnitude smaller for the backscattered atoms and proportional to overall number of absorbing particles inside the probing beam. We estimated that about 10% of atoms are back scattered. This ratio is obtained by dividing time and
ð3Þ
we get the mean velocity vm = 2.7 ± 0.6 × 10 5 cm/s to be compared with the velocity of atoms moving in forward direction which was around 1 × 10 6 cm/s. Height h and distance d in Eq. (3) are expressed in mm. Since the closest chamber surface was 10 cm above the target,
Fig. 7. Time position of maximum integrated loss tmax versus distance d below the target at 2 × 10− 5 mbar.
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M. Bišćan, S. Milošević / Spectrochimica Acta Part B 68 (2012) 34–39
a
b
Fig. 8. Time dependence of integrated line intensities at (a) distances d = 13 mm above and (b) d = − 12 mm below the target surface with fluence of 26 J/cm2 and pressure of 2 × 10− 5 mbar.
wavelength integrated line intensities from below and above the target and taking into account shielding by the target. 3.3. Pressure effects As the integrals in Fig. 6 are proportional to the number of absorbing particles, their sum is, at zero approximation, expected to be inversely proportional to the distance d above the target, as shown in Fig. 9. Fitting the data at 5 × 10 − 5 mbar to a function a/d b yields the value of parameter b to be − 1.0 ± 0.2. Similar values are obtained for measurements on higher pressures. From these results it can be concluded that at these background pressures backscattering atoms emerge from a small space just above the target surface. Fig. 10 shows how absorption line shapes change with an increase of background helium pressure for measurements above (left) and below (right) the target at time tmax. It can be seen that raising the pressure to 4 × 10 − 3 mbar has little influence indicating that this pressure is not high enough to alter the angular and velocity distribution significantly. Rising the pressure even further to 2 × 10 − 1 mbar, the amount of absorbing particles decreases and line changes its shape. Above the target it appears as though a line splitting is present. Around 10 − 1 mbar the backscattered absorption signal slightly decreases and Fe atomic line is not anymore split indicating a change in angular distribution of backscattered atoms. This coincides with formation of a clear spherical light ball above the target. Approaching 1 mbar pressure signal decrease is accompanied by a higher background noise and saturation effects due to high density of atoms. In Fig. 11 time dependence of integrated loss is plotted for different background pressures at distances d = 13 mm (a) and d = 18 mm (b) above and d =
Fig. 10. Line shapes at distances d = 13 mm above (left column) and d = − 7 mm below (right column) the target surface at different He pressures.
− 7 mm (c) and d = − 12 mm (d) below the target surface. It can be seen that raising the pressure does not change the concentration above the target appreciably, although by visual inspection plasma plume is smaller. Increasing the pressure causes the overall number of particles at d = − 7 mm to also increase. This is attributed to containment of backscattered Fe atoms near the target. At distance of
a
b
c
d
Fig. 9. Sum of integrals from Fig. 8 versus distance d below the target surface at different He pressures.
Fig. 11. Pressure dependence of integrated loss at distances d=18 mm (a) and d=13 mm (b) above and d=−7 mm (c) and d=−12 mm (d) below the target surface evaluated at tmax.
M. Bišćan, S. Milošević / Spectrochimica Acta Part B 68 (2012) 34–39
d = − 12 mm below the target, this effect remains but is much less pronounced. 4. Conclusions In conclusions, we have shown that line shapes and their time dependence, measured by the CRDS, can be simulated using a simple model which takes into account only a few parameters, most important of which are the angular and velocity distributions. Comparing measured and simulated line shapes, the effective absorption length was deduced and the number density of absorbing atoms in one atomic state above the target calculated. From the time positions of maxima of integrated line shapes, the mean velocity and effective origin position of backscattered atoms were obtained. We found that, at low pressures, significant backscattering occurs only at early times after the ablation and just above the target surface. Backscattered atoms also have a smaller velocity than those forward moving ones. By comparing integrated line shapes above and below the target, one can get the ratio of forward and backward moving particles, in this case ≈10%. The CRDS measurements can therefore be used as a means of monitoring the backscattered signal and its optimization during deposition from multicomponent targets or in other applications of LPP. Acknowledgments The results shown are part of investigation performed within the project no. 035-0352851-2856 supported by the Ministry of Science, Education and Sports of the Republic of Croatia. References [1] S.S. Harilal, C.V. Bindhu, M.S. Tillack, F. Najmabadi, A.C. Gaeris, Internal structure and expansion dynamics of laser ablation plumes into ambient gases, J. Appl. Phys. 93 (2003) 2380–2388. [2] S. Mahmood, R.S. Rawat, M.S.B. Darby, M. Zakaullah, S.V. Springham, T.L. Tan, P. Lee, On the plume splitting of pulsed laser ablated Fe and Al plasmas, Phys. Plasmas 17 (2010) 103105.
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[3] J. Gonzalo, J. Siegel, A. Perea, D. Puerto, V. Resta, M. Galvan-Sosa, C.N. Afonso, Imaging self-sputtering and backscattering from the substrate during pulsed laser deposition of gold, Phys. Rev. B 76 (2007) 035435. [4] T. Itina, W. Marine, M. Autric, Monte Carlo simulation of pulsed laser ablation from two-component target into diluted ambient gas, J. Appl. Phys. 82 (1997) 3536–3542. [5] A.A. Morozov, Thermal model of pulsed laser ablation: back flux contribution, Appl. Phys. A 79 (2004) 997–999. [6] A. Marcu, C. Grigoriu, K. Yatsui, Particles interaction with obstacles in a pulsed laser deposition system, Appl. Surf. Sci. 248 (2005) 466–469. [7] S.S. Harilal, B. O'Shay, M.S. Tillack, Spectroscopic characterization of laser-induced plasma, J. Appl. Phys. 98 (2005) 013306. [8] R.F. Wood, K.R. Chen, J.N. Leboeuf, A.A. Puretzky, D.B. Geohegan, Dynamics of plume propagation and splitting during pulsed-laser ablation, Phys. Rev. Lett. 79 (1997) 1571–1574. [9] V.N. Nevolin, V.Y. Fominskii, A.G. Gnedovets, R.I. Romanov, Pulsed laser deposition of thin-film coatings using an antidroplet shield, Tech. Phys. 54 (2009) 1681–1688. [10] L. Egerhazi, Z. Geretovszky, T. Csako, T. Szorenyi, Atomic force microscopic characterization of films grown by inverse pulsed laser deposition, Appl. Surf. Sci. 252 (2006) 4661–4666. [11] J.J. Lin, L.S. Loh, P. Lee, T.L. Tan, S.V. Springham, R.S. Rawat, Effects of targetsubstrate geometry and ambient gas pressure on FePt nanoparticles synthesized by pulsed laser deposition, Appl. Surf. Sci. 255 (2009) 4372–4377. [12] I.B. Gornushkin, U. Panne, Radiative models of laser-induced plasma and pumpprobe diagnostics relevant to laser-induced breakdown spectroscopy, Spectrochim. Acta Part B 65 (2010) 345–359. [13] I. Labazan, S. Milošević, Laser vaporized Li2, Na2, K2, and LiNa molecules observed by cavity ring-down spectroscopy, Phys. Rev. A 68 (2003) 032901. [14] N. Krstulović, N. Čutić, S. Milošević, Spatial and temporal probing of a laser-induced plasma plume by cavity ringdown spectroscopy, Spectrochim. Acta Part B 63 (2008) 1233–1239. [15] N. Krstulović, N. Čutić, S. Milošević, Cavity ringdown spectroscopy of collinear dual-pulse laser plasmas in vacuum, Spectrochim. Acta Part B 64 (2009) 271–277. [16] A. Yalin, V. Surla, Velocity measurements by cavity ringdown spectroscopy, Opt. Lett. 30 (2005) 3219–3221. [17] S. Srivastava, K. Rohr, Scaling law of angular emission distributions of laser ablated particle pulses from monoatomic and compound targets, Nucl. Instrum. Methods Phys. Res. Sect. B 237 (2005) 497–506. [18] I. Labazan, S. Milošević, Observation of lithium dimers in laser produced plume by cavity ring-down spectroscopy, Chem. Phys. Lett. 352 (2002) 226–233. [19] I. Labazan, S. Rudić, S. Milošević, Nonlinear effects in cavity ringdown spectroscopy of lithium vapour, Chem. Phys. Lett. 320 (2000) 613–622. [20] A.C.G. Mitchell, M.W. Zemansky, Resonance Radiation and Excited Atoms, The Sysdios of the Cambridge University Press, Cambridge, 2nd ed., 1961. [21] T.R. O'Brian, M.E. Wickliffe, J.E. Lawler, W. Whaling, J.W. Brault, Lifetimes, transition probabilities, and level energies in Fe I, J. Opt. Soc. Am. B 8 (1991) 1185–1201.
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part B journal homepage: www.elsevier.com/locate/sab
Expansion and backscattering of laser produced Fe plasma plume M. Bišćan ⁎, S. Milošević Institute of Physics, Bijenička 46, HR-10000 Zagreb, Croatia
a r t i c l e
i n f o
Article history: Received 22 July 2011 Accepted 16 January 2012 Available online 24 January 2012 Keywords: Backscattering Laser produced plasma Cavity ringdown spectroscopy Iron
a b s t r a c t Forward and backward moving atoms within the laser produced plasma plume were studied by means of a cavity ringdown spectroscopy. The plume was produced using a nanosecond Nd-YAG laser pulse illuminating stainless steel target in a vacuum or helium background gas. Measurements were done at pressures ranging from 10− 5 to 1 mbar. Atomic absorption line shapes of iron around 388 nm were measured above and below the target at different times after the ablation initiation. Changes in absorption line shapes were used to estimate kinetic parameters of the plasma plume. The observations were interpreted through modeling which takes into account the angular and velocity distributions of atoms in the expanding plume. The amount of backward scattered atoms was about 10% of the total number of particles. © 2012 Elsevier B.V. All rights reserved.
1. Introduction When an intense, focused laser pulse hits a solid or liquid target, material is evaporated and dense ionized hot plasma is produced. This ablation stage is then followed by rapid expansion into the vacuum or background gas environment. Because of the high plasma pressure just above the target surface, most of the ablated particles move out perpendicularly away from it. If the laser pulse is still present at this stage, it will interact with the expanding plume and further energize its particles. According to Harilal et al. [1] after the laser pulse is stopped, particles are no longer ablated from the surface. But, in order to explain two components of the plume, Mahmood et al. [2] introduced the possibility that second vaporization takes place using the energy from primary plasma. In both cases, developed plume then expands freely in a vacuum or low pressure environment. If the pressure is high enough, plume expansion is governed by its interaction with the background gas [1]. Also, due to the collisions within the plume itself or with the background gas certain percentage of atoms is back-reflected and moves back towards the target surface. This backward scattering of the ablated species is usually experimentally observed indirectly through the deposits on the target or using fast imaging techniques. Although its origin is inside the plume itself, Gonzalo et al. [3] found that backscattering is more prominent in the presence of an obstacle. The Monte Carlo simulations, [4–6] have shown that up to 20% of atoms could be back reflected depending on the background pressure. One of the features of the laser-produced plasmas (LPP) is the so-called plume splitting which describes the effect when forward moving plume splits into two or even three components with different velocities [1,2,7,8], the effect which could be important for pulsed laser
⁎ Corresponding author. Tel.: + 385 14698851. E-mail address: [email protected] (M. Bišćan). 0584-8547/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2012.01.010
deposition (PLD). As noticed by Wood [8] and Nevolin [9], particles from fast component can have considerable effect or even damage the growing film on the substrate. Avoiding these particles is therefore important for thin film deposition. One of the possibilities is to use the backscattered particles. Both the forward moving and backscattering atoms depend on many laser ablation (LA) parameters, such as the laser fluence, beam profile, focusing conditions, target size and shape, background gas, etc. Optimization of back flux on any of such parameters could be important in PLD [10,11]. In the present work we employed cavity ringdown spectroscopy (CRDS) technique [12,13] to measure kinetics of the Fe ground state atoms through measurements of spectral line shapes in the regions above and below the target. To demonstrate the method properly, care was taken to keep the laser ablation parameters fixed and the only variable parameter was pressure of the background gas. The observed line shapes were then interpreted through modeling which takes into account the angular distribution of atoms and their velocity distributions as described in [14,15]. The CRDS technique applied to atomic and molecular sources with preferential moving direction such as atomic beams (jets) or LPP plumes adds Doppler features to the measured absorption line shapes. As photons within the cavity move back and forth, atoms traversing the cavity axis with certain parallel velocity component experience both blue and red Doppler shifted absorptions. This may result in substantial line shape splitting [15,16]. If the experimental resolution is sufficient, this splitting provides information about the angular and velocity distribution of atoms. Freely expanding laser produced plasma plumes are characterized by angular emission distributions which can be described using Gauss-functions [17]. To some approximation these are cones pointing out from the target surface where the cone steepness changes with atomic mass and the laser spot size. This mainly describes atoms moving in forward direction out of the target surface. The distributions of back scattered particles, which are due to the
M. Bišćan, S. Milošević / Spectrochimica Acta Part B 68 (2012) 34–39
35
collisions within the plume itself or with the background surrounding gas, are quite different as depicted from Monte Carlo simulations (see Fig. 7 in Ref. [4]).
2. Experiment and method The experimental details are similar as described earlier [18] with main changes regarding the shape and position of the target. To probe the forward moving and back scattered atoms we set experiment as illustrated in Fig. 1. Probing the plume directly above the target is sensitive to forward moving atoms, while probing directly below the target detects only the backscattered atoms. Here we used a NdYAG laser (1064 nm) of 5 ns pulse duration at a repetition rate of 5 Hz to ablate a stainless steel rod, with diameter of 6 mm, placed inside the vacuum chamber (with diameter of 40 cm). The target was rotated in order to avoid effects of drilling and heating. The chamber was evacuated by means of a diffusion pumps down to a 10 − 5 mbar. Higher pressure was achieved using a flow of helium gas. The laser beam was focused using a 33 cm lens to a spot size of 0.25 × 102 cm2 yielding a fluence of 26 J/cm2. For the absorption measurements, excimer pumped dye laser (pulse length 15 ns, Δν= 0.17 cm− 1) operating with BiBuQ (LC 3860) laser dye was used. Ringdown signal was detected using a photomultiplier tube, digitized and averaged using an 8-bit digital oscilloscope and then sent to a computer. Fig. 2 shows the typical cavity ringdown curves in a semi-logarithmic scale for cases when excimer pumped dye laser is tuned to the absorption line center (black dots) and out of the resonance (gray dots, the empty cavity case). When the dye laser is non-resonant, CRDS curve is a simple exponential decay, while for the resonant absorption, CRDS curve shows non-exponential decay due to density time evolution of absorbing atoms. The radius of curvature of cavity mirrors is 50 cm and their maximum reflectivity is around 388 nm where absorption from the iron ground state occurs (Fe I transition a 5D3 → z5D30, see Fig. 3 thick arrow). The dye laser intensity used for the CRDS was kept low enough to avoid saturation effects [19] which would cause signal decrease in the line center. The line shapes were evaluated at different time delays tk within time windows Δt which define temporal resolution as described in Ref. [13]. Spatial resolution is determined by the laser beam waist at a cavity center and was measured to be about 1 mm. The distance d of the cavity axis to the target surface was changed by means of a stepping motor in synchronization with the position of lens used to focus the laser beam to the target surface. Each measured line shape is an average of 24 measurements; eight measurements were averaged at oscilloscope to give one ringdown curve and the dye laser was scanned over given wavelength interval three times.
Here, τ and τ0 are the lifetimes of the probing laser pulse inside the cavity with and without absorbing medium, respectively. 1/(c ⋅ τ) and 1/(c ⋅ τ0) are the corresponding losses. L is the total length of a cavity and labs is the length of absorbing medium. In order to find absorption coefficient kλ one has to know the width labs of the plasma plume (absorbing medium) at a given distance d. According to [20], integral of absorption coefficient is a function of a density of absorbing atoms:
Fig. 1. Experimental setup for observation (a) above and (b) below the target.
Fig. 3. The excitation scheme to probe Fe population.
Fig. 2. A typical ringdown curves and temporal resolution scheme.
In the CRDS measurements, measured spectrum represents the wavelength dependence of absorption loss inside the cavity. The absorption coefficient kλ can be expressed as: kλ ¼
1 1 h −1 i − cm labs ⋅c τ τ0 L
∫kν dν ¼
·
λ20 g 2 N h −1 −1 i cm s · 8πg1 τ
ð1Þ
ð2Þ
Here, ν is the frequency of light, g1 and g2 are statistical weights of lower and upper level, N is the density of atoms in lower level, λ0 is
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the central wavelength of absorption line and τ is the mean life time of upper level (not to be confused with τ in Eq. (1)). If effective absorption lengths labs are known, the densities of particles can be calculated, as will be described below. To interpret the experimental results with theory we used forward convolution modeling and calculated absorption line shapes using Gaussian function for line profile [14]. This simple model takes into account all relevant experimental parameters regarding specific CRDS observation (temporal and spatial resolution) and assumes the angular and velocity distribution of atoms within the plume. For the angular distribution we used Gaussian distribution with σ as a fitting parameter. According to Sristava et al. [17], Gaussian distribution is the most suited for describing angular shape of a freely expanding laser produced plumes. For the velocity distribution we used shifted Maxwell–Boltzmann function with two parameters, mean velocity and temperature. Temperature gives an estimate of overall spread of velocity. In order to describe the expansion of backscattering particles, angular distributions were modified to include the effect of shielding by the target. Around a central axis these distributions have a hole which extends to a cutoff angle given in Table 1. Inside this hole the distribution is zero and outside it follows a Gaussian shape. In simulations, the backscattering of particles is similar to the forward moving, the differences are angular distributions and the position and orientation of the source. Forward moving particles emerge from a point source at the surface of the target and move upward while backscattering particles emerge from a point source at some effective height h above the target and move downward. As it has been shown in Refs. [14,15], CRDS absorption line shape profiles are quite sensitive to angular distribution of atoms, since atoms moving with velocity component parallel to the cavity axis experience both red and blue Doppler shift. Doppler broadening or splitting can then be used as a measure of parallel velocity component. The larger the splitting, the larger the parallel velocity component. On the other hand, time delay tk which gives maximum intensity can be used to estimate perpendicular velocity component. Combining these two, one can get the mean velocity of the expanding plume. This velocity is then used in theoretical model to calculate line shapes. This simulation gives relative intensities as all calculated line shapes were scaled to the one with maximum intensity. 3. Results and discussion 3.1. Line shapes measurements above the target Fig. 4 shows the line shapes measured and calculated at a given distance d above the target versus time delay tk after the ablation initiation. The y-scale corresponds to labs ⋅ kν/L (loss [cm − 1]). The time delay tk is indicated in each row and Δt was 0.75 μs. Measured line shapes were obtained after averaging three subsequent dye laser scans. As the plume expands, at early time delays tk, line shapes are mainly influenced by the atoms moving perpendicularly away from the target surface because these atoms are the first to enter the probing axis. Therefore, the line shape is dominantly a single Gauss profile. At later time delays, most of the forward moving atoms have passed through the probing axis and the influence of atoms with larger parallel velocity component rises. Accordingly, at tk = 1750 μs and d = 13 mm
Table 1 Parameters of angular and velocity distributions used to obtain the best fit to the experimental line shapes.
Temperature (K) Velocity (105 cm/s) Width (°) Cut off (°)
Above
Below
15000 8.7 25
6000 3.3 35 40
Fig. 4. Measured and calculated line shapes at distance d = 13 mm above the target for different time delays tk (ΔT = 0.75 μs) with fluence of 26 J/cm2 and pressure of 2 × 10− 5 mbar.
the line shape shows double Gauss profile, due to Doppler splitting. Appearance of splitting is delayed in time for larger distances d. The effective absorption length labs can be obtained by modeling or visual inspection, latter being unreliable as the absorbing medium could be non-emitting. As the plasma plume is modeled with the Gaussian angular distribution, its labs at some distance d was calculated as width corresponding to the parameter σ. Using the Eq. (2), the lifetime of the upper level τ of 81.1 ns, found in [21], and labs of 12 mm maximum density of Fe I atoms in the state a 5D3 moving in forward direction is measured to be around 2.2 × 1011 cm− 3 at a given fluence of 26 J/cm 2. This is the density measured at time tk = 1 μs and distance d = 13 mm. Because angular distributions are Gaussian functions, calculated density is the mean density of atoms, in above-mentioned state, in region through which the probing beam passes. Comparing experimental and simulated line shapes we see that Doppler splitting can be described solely by free expansion of the plasma plume. Influence of backscattering atoms can at these distances d be neglected, as will be shown later. Above considerations hold for pressure of around 10− 5 mbar. The best fit parameters are given in Table 1. 3.2. Line shapes measurements below the target Below the target only contribution from the backscattered atoms is measured. Fig. 5 shows the time sequence of line shapes measured and calculated at a given distance d. One can see that absorption losses are order of magnitude smaller compared to the case above the target, as shown in Fig. 4, a sign that only a fraction of atoms is backscattered. Those atoms that have sufficiently large perpendicular velocity component are blocked by the target itself and this results in missing intensity around the line center, the line shape splitting due to the Doppler effect is more pronounced. The amount of splitting gives in this case a parallel velocity component to be around
M. Bišćan, S. Milošević / Spectrochimica Acta Part B 68 (2012) 34–39
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Fig. 6. Integrated loss at different distances d below the target at 2 × 10− 5 mbar.
Fig. 5. Measured and calculated line shapes at distance d = − 7 mm below the target and for different time delays tk (ΔT = 0.75 μs) with fluence of 26 J/cm2 and pressure of 2 × 10− 5 mbar.
3 × 10 5 cm/s. Assuming that total velocity is not larger than velocity of forward moving atoms and taking into account geometrical factors (size and shape of the target and overall dimensions of the chamber), boundary values for perpendicular component can be estimated to 1.5–8.5 × 10 5 cm/s. These values then give the upper (3.9 mm) and lower (0.3 mm) heights h above the target from where most of the atoms are scattered. Uncertainty in the origin position consequently gives uncertainty in perpendicular velocity component and angular shielding by the target. Nevertheless, if we make a simulation at those above-mentioned extreme distances, we get two different results. For minimum height of 0.3 mm and shielding of 65°, Doppler splitting is correct but integrated intensity is not. On the other hand, result is opposite for maximum height of 3.9 mm and shielding of 4°. So the best fit is in between. Fig. 6 shows how integrated loss changes with distance |d| increasing. We see that at each |d| integrated loss is different and reaches maximum at a different time. Better estimation of velocities and heights h can be obtained if we plot the time position of maximum integrated loss tmax versus distance d below the target. In this manner, almost linear dependence is obtained, as shown in Fig. 7. As the backscattered particles occur just above the target surface, this linear dependence suggests that after that they move at a constant velocity away from their origin. Backscattered cloud then follows free expansion. By fitting this data to a function t(d): dþh t ðdÞ ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 9 v 1− ð3þh Þ2
backscattering contribution which may arise from collisions with chamber walls is expected in time periods larger than 20 μs and can be neglected in our time window. The same fit gives us also the effective height h above the target at which the backscattering occurs (h = 1 ± 2 mm). In Fig. 8 measured and calculated integrated losses above (a) and below (b) the target are shown versus time. It can be seen that particles do not travel as a continuous stream but are bunched together and their density reaches maximum at some time. Above the target time distribution is narrow while below the target it is much broader and with maximum delayed in this case by around 5 μs. These distributions correspond to a group of fast atoms with approximately the same velocity, and a group of slower atoms, respectively. Comparison with the simulation shows that backscattered atoms below the target can also be described by a freely expanding plume. Best fit parameters are given in Table 1. Note that the intensity scale is an order of magnitude smaller for the backscattered atoms and proportional to overall number of absorbing particles inside the probing beam. We estimated that about 10% of atoms are back scattered. This ratio is obtained by dividing time and
ð3Þ
we get the mean velocity vm = 2.7 ± 0.6 × 10 5 cm/s to be compared with the velocity of atoms moving in forward direction which was around 1 × 10 6 cm/s. Height h and distance d in Eq. (3) are expressed in mm. Since the closest chamber surface was 10 cm above the target,
Fig. 7. Time position of maximum integrated loss tmax versus distance d below the target at 2 × 10− 5 mbar.
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M. Bišćan, S. Milošević / Spectrochimica Acta Part B 68 (2012) 34–39
a
b
Fig. 8. Time dependence of integrated line intensities at (a) distances d = 13 mm above and (b) d = − 12 mm below the target surface with fluence of 26 J/cm2 and pressure of 2 × 10− 5 mbar.
wavelength integrated line intensities from below and above the target and taking into account shielding by the target. 3.3. Pressure effects As the integrals in Fig. 6 are proportional to the number of absorbing particles, their sum is, at zero approximation, expected to be inversely proportional to the distance d above the target, as shown in Fig. 9. Fitting the data at 5 × 10 − 5 mbar to a function a/d b yields the value of parameter b to be − 1.0 ± 0.2. Similar values are obtained for measurements on higher pressures. From these results it can be concluded that at these background pressures backscattering atoms emerge from a small space just above the target surface. Fig. 10 shows how absorption line shapes change with an increase of background helium pressure for measurements above (left) and below (right) the target at time tmax. It can be seen that raising the pressure to 4 × 10 − 3 mbar has little influence indicating that this pressure is not high enough to alter the angular and velocity distribution significantly. Rising the pressure even further to 2 × 10 − 1 mbar, the amount of absorbing particles decreases and line changes its shape. Above the target it appears as though a line splitting is present. Around 10 − 1 mbar the backscattered absorption signal slightly decreases and Fe atomic line is not anymore split indicating a change in angular distribution of backscattered atoms. This coincides with formation of a clear spherical light ball above the target. Approaching 1 mbar pressure signal decrease is accompanied by a higher background noise and saturation effects due to high density of atoms. In Fig. 11 time dependence of integrated loss is plotted for different background pressures at distances d = 13 mm (a) and d = 18 mm (b) above and d =
Fig. 10. Line shapes at distances d = 13 mm above (left column) and d = − 7 mm below (right column) the target surface at different He pressures.
− 7 mm (c) and d = − 12 mm (d) below the target surface. It can be seen that raising the pressure does not change the concentration above the target appreciably, although by visual inspection plasma plume is smaller. Increasing the pressure causes the overall number of particles at d = − 7 mm to also increase. This is attributed to containment of backscattered Fe atoms near the target. At distance of
a
b
c
d
Fig. 9. Sum of integrals from Fig. 8 versus distance d below the target surface at different He pressures.
Fig. 11. Pressure dependence of integrated loss at distances d=18 mm (a) and d=13 mm (b) above and d=−7 mm (c) and d=−12 mm (d) below the target surface evaluated at tmax.
M. Bišćan, S. Milošević / Spectrochimica Acta Part B 68 (2012) 34–39
d = − 12 mm below the target, this effect remains but is much less pronounced. 4. Conclusions In conclusions, we have shown that line shapes and their time dependence, measured by the CRDS, can be simulated using a simple model which takes into account only a few parameters, most important of which are the angular and velocity distributions. Comparing measured and simulated line shapes, the effective absorption length was deduced and the number density of absorbing atoms in one atomic state above the target calculated. From the time positions of maxima of integrated line shapes, the mean velocity and effective origin position of backscattered atoms were obtained. We found that, at low pressures, significant backscattering occurs only at early times after the ablation and just above the target surface. Backscattered atoms also have a smaller velocity than those forward moving ones. By comparing integrated line shapes above and below the target, one can get the ratio of forward and backward moving particles, in this case ≈10%. The CRDS measurements can therefore be used as a means of monitoring the backscattered signal and its optimization during deposition from multicomponent targets or in other applications of LPP. Acknowledgments The results shown are part of investigation performed within the project no. 035-0352851-2856 supported by the Ministry of Science, Education and Sports of the Republic of Croatia. References [1] S.S. Harilal, C.V. Bindhu, M.S. Tillack, F. Najmabadi, A.C. Gaeris, Internal structure and expansion dynamics of laser ablation plumes into ambient gases, J. Appl. Phys. 93 (2003) 2380–2388. [2] S. Mahmood, R.S. Rawat, M.S.B. Darby, M. Zakaullah, S.V. Springham, T.L. Tan, P. Lee, On the plume splitting of pulsed laser ablated Fe and Al plasmas, Phys. Plasmas 17 (2010) 103105.
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