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Particles interaction with obstacles in a pulsed laser deposition system
Applied Surface Science 248 (2005) 466–469 www.elsevier.com/locate/apsusc
Particles interaction with obstacles in a pulsed laser deposition system A. Marcu a,*, C. Grigoriu a, K. Yatsui b a
Laser Department, National Institute for Laser, Plasma and Radiation Physics, Atomistilor 409, MG-BOX 36, Bucharest-Magurele, Romania b Extreme Energy Density Research Institute, Nagaoka University of Technology, Nagaoka 940-2188, Niigata, Japan Available online 12 April 2005
Abstract A new and simplified three-dimensional laser ablation plume model is proposed for simulating particles-obstacles interaction for non-standard pulsed laser deposition (PLD) systems such as the ‘‘shadow mask’’, also known as the‘‘eclipse method’’, and plasma reflection (PLD/PR). The model is based on the direct Monte-Carlo method and a 3D finite-elements mesh. During deposition, for a significant fraction of the ablated particles, we observed that trajectories are affected by the obstacle even if no direct interaction takes place between the particle and the obstacle. A comparison between experimental and simulation results for thin film deposition by PLD/PR technique is presented. The observed effects regarding ablation plume particles interaction with obstacle and macro-particles are described. # 2005 Elsevier B.V. All rights reserved. PACS: 81.15; 52.65 Keywords: PLD/PR; Surface quality; Plume behavior; Droplets
1. Introduction Pulsed laser ablation became a useful technique for many applications in the past few decades [1,2]. The purpose of the ablation is not always to remove material from a surface, but in many cases to collect or deposit materials on other surfaces, such as in the pulsed laser deposition (PLD) process, or the production of nanoparticles and nanoclusters. How* Corresponding author. Fax: +40 21 4574467. E-mail address: [email protected] (A. Marcu).
ever, besides the advantages of being a clean and good technique for almost every material, PLD also has some drawbacks. Some of them dealt with in this paper are related to the difficulties of selecting the size of particles to obtain a good surface quality in PLD, or for nanoparticles production. Many techniques have been studied to avoid the ‘droplets’ formation on the surface of thin films deposited by PLD [3–16]. Some are based on the possibility to select the particles during their propagation from the target to the substrate. Thus, various geometries are used for deposition, e.g. axis-
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.03.073
A. Marcu et al. / Applied Surface Science 248 (2005) 466–469
467
off deposition [4], plasma reflection [11], ‘eclipse’ deposition [12–15]. These techniques have been developed mainly experimentally. Some theoretical models for ablated particles (plume) propagation have been developed, beginning with the plume luminous front propagation and its related shock-wave, in an ambient gas [17]. The particle evolution inside the plume during propagation has been approached later by analytical and numerical models. The direct Monte-Carlo technique seems to be the most suitable approach for simulations [18]. Fig. 1. Schematics of the experimental setup.
2. Modeling In this paper we present a simplified 3D model for studying the plume propagation in various deposition systems while interacting with the ambient gas and various obstacles. We consider all particles having a similar spherical shape. The particles mass and diameter are the average mass (57 a.u.) and average ˚ ) of the ablated target particles. atomic diameter (8 A At the emission time the ablated particles speed vector and spatial position are considered to have a Gaussian distributions with an average value of 40 km/s, perpendicular to the target surface. The model is based on the direct Monte-Carlo simulation and a 3D rectangular finite-elements mesh over the investigated area. Mechanical interactions are considered, using the ‘hard sphere model’ for interactions between particles or with obstacles. The Newton’s equations of impulse (1) and kinetic energy (2) rule the particle collisions and a linear movement is assumed between consecutive collisions. p1 ! þ p2 ! ¼ p01 ! þ p02 ! (1) Ec1 þ Ec2 ¼ Ec01 þ Ec02
(2)
While interacting with an obstacle, some of the particles are reflected by classical mechanics rules considering a perfect elastic collision and some simply stick on the surface at the interaction point, thus being deposited. We have defined a deposition probability (25%), estimated by comparing the quantities of ablated material, deposited material and lost material, in the experimental processes. The finite-elements mesh is aimed only to provide us local information about average interaction con-
ditions (speed, free path, etc.). It consists of rectangular cells over the investigated area, each having a pre-defined constant number of particles. Gravity and other forces are neglected here.
3. Simulation results and discussions For investigating the ablated particles interactions with an obstacle we use here the experimental data obtained for the case of a pulsed laser deposition with plasma reflection (PLD/PR) [11,19]. The experimental setup consists of a quasi-conventional PLD system, except for a plain obstacle ’reflector’ between the target and substrate. A simplified scheme of the system is presented in Fig. 1 and the main system parameters in Table 1. The comparison of the theoretical and experimental results for the deposited particle is given in Fig. 2 for the substrate surface. The deposited thin film (a) has a strong thickness variation with a maximum value of about 0.16 mm and a 50% decrease towards the edges of the 2 cm wide substrate. The simulation gives deposition rate spatial variation (c) for a single pulse. Table 1 Typical experimental system parameters Laser
Target Ambient gas
Wavelength Pulse energy Pulse duration Frequency Fluency Composition Composition Pressure
1064 nm 200 mJ 7 ns 10 Hz 2.5 J/cm2 YBa2Cu3O7x O2 25 Pa
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A. Marcu et al. / Applied Surface Science 248 (2005) 466–469
Extrapolating this deposition rate curve for N = 18,000 pulses as in the experiment, we obtain a theoretical film thickness variation curve (b). The theoretical curve has a maximum value of 80 nm, representing only half of the experimental curve maximum value and a slightly smaller thickness decrease towards the edges. These differences occur due to neglecting the electrical interaction between the ablated particles. Thus, in the simulation, the ablated particles spread faster than in the experiment. Making the same comparison at earlier times, for example at the reflector surface, one can observe considerable smaller differences (see Fig. 3). Thus, the experimental thickness variation of Fig. 3a has a similar decreasing tendency as the deposition rate (c).
By extrapolating the deposition rate curve for N = 18,000 pulses, we obtain a theoretical thickness curve, with maximum value of about 8.5 mm as in the experiment and a very similar curve shape. This means that, for earlier times, the results are in very good agreement with the experiment. By counting the total number of particles deposited on the reflector surface in our simulation and considering the deposition probability coefficient chosen, we observed the following effects. Even if most particles’ trajectories are strongly affected by the obstacle, for our experimental conditions of 25 Pa Oxygen, at least 12% of the particles do not have a direct interaction with the obstacle. The percentage increases significantly by decreasing the pressure, reaching about 40% in vacuum. Since not all the particles which interact with the obstacle are deposited and part of them, getting in the vicinity of the obstacle might interact more than once, we expect the actual non-interacting particles percent to be even higher, i.e. 12–40%. Considering a similar behavior for big particles or ‘droplets’ during the deposition processes, the interaction with other particles in the plume and with the ambient gas particles might drive them in the substrate area, allowing for an explanation of the presence of ‘droplets’ on substrates, for PLD/PR or ‘eclipse’ deposition techniques. The simulated behavior of droplets in a PLD/PR system with a starting position at the center of the target and with initial speed of 20 km/s, half of that of the plume fine particles, is given in Fig. 4. Particles are ejected from the target all with the same parameters, except of mass. It was observed that
Fig. 3. Thickness and deposition rate on the reflector surface in PLD/PR (a) experimental thickness, (b) theoretical thickness and (c) deposition rate.
Fig. 4. Droplets representative trajectories in PLD/PR with mass of: (a) 27, (b) 50, (c) 75, (d) 100 and (e) 150 times the mass of the fine particles.
Fig. 2. Thickness and deposition rate on the substrate in PLD/PR (a) experimental thickness, (b) theoretical thickness and (c) deposition rate.
A. Marcu et al. / Applied Surface Science 248 (2005) 466–469
by increasing mass, particles tend to have smoother trajectories and become harder to deflect from their trajectories. Thus, a stronger deviation angle may be possible for lighter particles, driven into the deposition area by the plume fine particles, explaining the lower quality thin films experimentally obtained in these areas.
4. Conclusions We have proposed a new and simplified model for plasma expansion based on DMC and a finiteelements mesh giving us information about particles interaction with obstacles in PLD systems. We noticed that a significant fraction (up to 12 40%) of the emitted particles in a PLD system does not have a direct interaction with the obstacles, thus rather being driven by the interaction with other particles by a ‘fluid like’ behavior. This phenomenon may occur even for the droplets behavior, giving a possible explanation for the droplets presence in techniques such as PLD/PR or the ‘eclipse’. When choosing a droplet reduction technique for depositing thin films, the obstacle size and position with respect to the plume fine particles should thus be also carefully analyzed.
Acknowledgment Part of this simulation work has been run on National Center of Information Technology – ‘‘Colaborator’’ under Professor V. Cristea’s coordination.
469
References [1] D.B. Chrisey, G.K. Hubler, ‘Pulsed Laser Deposition of Thin Films’, New York, 1994. [2] E. Fogarassy, S. Lazare, Laser Ablation of Electronic Materials Basic Mechanisms and Applications, Elevier, 1992. [3] E.V. Pchen, A.V. Valashikin, S.I. Kronosvobodtsev, B. Brunner, K.F. Renk, Appl. Phys. Lett. 63 (1995) 2292. [4] B. Holzapfel, B. Roas, L. Shultz, P. Bauer, G. SaemannIschenko, Appl. Phys. Lett. 61 (1992) 3178. [5] G. Koren, A. Gupta, R.J. Baseman, M.I. Lutwyche, R.B. Laibowitz, Appl. Phys. Lett. 55 (1989) 2450. [6] F. Tcheliebou, S. Baik, J. Appl. Phys. 80 (1996) 7046. [7] R.E. Leuchymer, J.S. Horwitz, D.B. Chrisey, J. Appl. Phys. 83 (1998) 5477. [8] H.L. Spindler, R.M. Gilgenbach, J.S. Lash, Appl. Phys. Lett. 68 (1996) 4345. [9] S.G. Lee, D.S. Hwang, Y.K. Park, J.C. Lash, Appl. Phys. Lett. 83 (1998) 5477. [10] G. Koren, R.J. Baseman, A. Gupta, M.I. Lutwyche, R.B. Laibowitz, Appl. Phys. Lett. 56 (1990) 2144. [11] A. Marcu, C. Grigoriu, W. Jiang, K. Yatsui, SPIE Proc. 4762 (2001) 210. [12] K. Kinobuchi, K. Kinoshita, H. Ishibashi, T. Kobayashi, Jpn. J. Appl. Phys. 33 (1994) 113. [13] K. Kinoshita, H. Ishibashi, T. Kobayashi, Jpn. J. Apply. Phys. 33 (1994) L417. [14] M. Tachiki, T. Kobayashi, Jpn. J. Apply. Phys. 38 (1999) 3642. [15] D.W. Kim, S.M. Oh, S.H. Lee, T.W. Noh, J. Appl. Phys. 85 (1999) 2016. [16] E.G. Gamaly, A.V. Rode, B. Luther-Davies, J. Appl. Phys. 85 (1999) 4213. [17] P.E. Dyer, A. Issa, P.H. Key, Appl. Phys. Lett. 57 (1990) 186. [18] T.E. Itina, W. Marine, M. Autric, J. Appl. Phys. 92 (7) (1997) 3536. [19] A. Marcu, C. Grigoriu, W. Jiang, K. Yatsui, SPIE Proc. 4430 (2000) 241.
Particles interaction with obstacles in a pulsed laser deposition system A. Marcu a,*, C. Grigoriu a, K. Yatsui b a
Laser Department, National Institute for Laser, Plasma and Radiation Physics, Atomistilor 409, MG-BOX 36, Bucharest-Magurele, Romania b Extreme Energy Density Research Institute, Nagaoka University of Technology, Nagaoka 940-2188, Niigata, Japan Available online 12 April 2005
Abstract A new and simplified three-dimensional laser ablation plume model is proposed for simulating particles-obstacles interaction for non-standard pulsed laser deposition (PLD) systems such as the ‘‘shadow mask’’, also known as the‘‘eclipse method’’, and plasma reflection (PLD/PR). The model is based on the direct Monte-Carlo method and a 3D finite-elements mesh. During deposition, for a significant fraction of the ablated particles, we observed that trajectories are affected by the obstacle even if no direct interaction takes place between the particle and the obstacle. A comparison between experimental and simulation results for thin film deposition by PLD/PR technique is presented. The observed effects regarding ablation plume particles interaction with obstacle and macro-particles are described. # 2005 Elsevier B.V. All rights reserved. PACS: 81.15; 52.65 Keywords: PLD/PR; Surface quality; Plume behavior; Droplets
1. Introduction Pulsed laser ablation became a useful technique for many applications in the past few decades [1,2]. The purpose of the ablation is not always to remove material from a surface, but in many cases to collect or deposit materials on other surfaces, such as in the pulsed laser deposition (PLD) process, or the production of nanoparticles and nanoclusters. How* Corresponding author. Fax: +40 21 4574467. E-mail address: [email protected] (A. Marcu).
ever, besides the advantages of being a clean and good technique for almost every material, PLD also has some drawbacks. Some of them dealt with in this paper are related to the difficulties of selecting the size of particles to obtain a good surface quality in PLD, or for nanoparticles production. Many techniques have been studied to avoid the ‘droplets’ formation on the surface of thin films deposited by PLD [3–16]. Some are based on the possibility to select the particles during their propagation from the target to the substrate. Thus, various geometries are used for deposition, e.g. axis-
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.03.073
A. Marcu et al. / Applied Surface Science 248 (2005) 466–469
467
off deposition [4], plasma reflection [11], ‘eclipse’ deposition [12–15]. These techniques have been developed mainly experimentally. Some theoretical models for ablated particles (plume) propagation have been developed, beginning with the plume luminous front propagation and its related shock-wave, in an ambient gas [17]. The particle evolution inside the plume during propagation has been approached later by analytical and numerical models. The direct Monte-Carlo technique seems to be the most suitable approach for simulations [18]. Fig. 1. Schematics of the experimental setup.
2. Modeling In this paper we present a simplified 3D model for studying the plume propagation in various deposition systems while interacting with the ambient gas and various obstacles. We consider all particles having a similar spherical shape. The particles mass and diameter are the average mass (57 a.u.) and average ˚ ) of the ablated target particles. atomic diameter (8 A At the emission time the ablated particles speed vector and spatial position are considered to have a Gaussian distributions with an average value of 40 km/s, perpendicular to the target surface. The model is based on the direct Monte-Carlo simulation and a 3D rectangular finite-elements mesh over the investigated area. Mechanical interactions are considered, using the ‘hard sphere model’ for interactions between particles or with obstacles. The Newton’s equations of impulse (1) and kinetic energy (2) rule the particle collisions and a linear movement is assumed between consecutive collisions. p1 ! þ p2 ! ¼ p01 ! þ p02 ! (1) Ec1 þ Ec2 ¼ Ec01 þ Ec02
(2)
While interacting with an obstacle, some of the particles are reflected by classical mechanics rules considering a perfect elastic collision and some simply stick on the surface at the interaction point, thus being deposited. We have defined a deposition probability (25%), estimated by comparing the quantities of ablated material, deposited material and lost material, in the experimental processes. The finite-elements mesh is aimed only to provide us local information about average interaction con-
ditions (speed, free path, etc.). It consists of rectangular cells over the investigated area, each having a pre-defined constant number of particles. Gravity and other forces are neglected here.
3. Simulation results and discussions For investigating the ablated particles interactions with an obstacle we use here the experimental data obtained for the case of a pulsed laser deposition with plasma reflection (PLD/PR) [11,19]. The experimental setup consists of a quasi-conventional PLD system, except for a plain obstacle ’reflector’ between the target and substrate. A simplified scheme of the system is presented in Fig. 1 and the main system parameters in Table 1. The comparison of the theoretical and experimental results for the deposited particle is given in Fig. 2 for the substrate surface. The deposited thin film (a) has a strong thickness variation with a maximum value of about 0.16 mm and a 50% decrease towards the edges of the 2 cm wide substrate. The simulation gives deposition rate spatial variation (c) for a single pulse. Table 1 Typical experimental system parameters Laser
Target Ambient gas
Wavelength Pulse energy Pulse duration Frequency Fluency Composition Composition Pressure
1064 nm 200 mJ 7 ns 10 Hz 2.5 J/cm2 YBa2Cu3O7x O2 25 Pa
468
A. Marcu et al. / Applied Surface Science 248 (2005) 466–469
Extrapolating this deposition rate curve for N = 18,000 pulses as in the experiment, we obtain a theoretical film thickness variation curve (b). The theoretical curve has a maximum value of 80 nm, representing only half of the experimental curve maximum value and a slightly smaller thickness decrease towards the edges. These differences occur due to neglecting the electrical interaction between the ablated particles. Thus, in the simulation, the ablated particles spread faster than in the experiment. Making the same comparison at earlier times, for example at the reflector surface, one can observe considerable smaller differences (see Fig. 3). Thus, the experimental thickness variation of Fig. 3a has a similar decreasing tendency as the deposition rate (c).
By extrapolating the deposition rate curve for N = 18,000 pulses, we obtain a theoretical thickness curve, with maximum value of about 8.5 mm as in the experiment and a very similar curve shape. This means that, for earlier times, the results are in very good agreement with the experiment. By counting the total number of particles deposited on the reflector surface in our simulation and considering the deposition probability coefficient chosen, we observed the following effects. Even if most particles’ trajectories are strongly affected by the obstacle, for our experimental conditions of 25 Pa Oxygen, at least 12% of the particles do not have a direct interaction with the obstacle. The percentage increases significantly by decreasing the pressure, reaching about 40% in vacuum. Since not all the particles which interact with the obstacle are deposited and part of them, getting in the vicinity of the obstacle might interact more than once, we expect the actual non-interacting particles percent to be even higher, i.e. 12–40%. Considering a similar behavior for big particles or ‘droplets’ during the deposition processes, the interaction with other particles in the plume and with the ambient gas particles might drive them in the substrate area, allowing for an explanation of the presence of ‘droplets’ on substrates, for PLD/PR or ‘eclipse’ deposition techniques. The simulated behavior of droplets in a PLD/PR system with a starting position at the center of the target and with initial speed of 20 km/s, half of that of the plume fine particles, is given in Fig. 4. Particles are ejected from the target all with the same parameters, except of mass. It was observed that
Fig. 3. Thickness and deposition rate on the reflector surface in PLD/PR (a) experimental thickness, (b) theoretical thickness and (c) deposition rate.
Fig. 4. Droplets representative trajectories in PLD/PR with mass of: (a) 27, (b) 50, (c) 75, (d) 100 and (e) 150 times the mass of the fine particles.
Fig. 2. Thickness and deposition rate on the substrate in PLD/PR (a) experimental thickness, (b) theoretical thickness and (c) deposition rate.
A. Marcu et al. / Applied Surface Science 248 (2005) 466–469
by increasing mass, particles tend to have smoother trajectories and become harder to deflect from their trajectories. Thus, a stronger deviation angle may be possible for lighter particles, driven into the deposition area by the plume fine particles, explaining the lower quality thin films experimentally obtained in these areas.
4. Conclusions We have proposed a new and simplified model for plasma expansion based on DMC and a finiteelements mesh giving us information about particles interaction with obstacles in PLD systems. We noticed that a significant fraction (up to 12 40%) of the emitted particles in a PLD system does not have a direct interaction with the obstacles, thus rather being driven by the interaction with other particles by a ‘fluid like’ behavior. This phenomenon may occur even for the droplets behavior, giving a possible explanation for the droplets presence in techniques such as PLD/PR or the ‘eclipse’. When choosing a droplet reduction technique for depositing thin films, the obstacle size and position with respect to the plume fine particles should thus be also carefully analyzed.
Acknowledgment Part of this simulation work has been run on National Center of Information Technology – ‘‘Colaborator’’ under Professor V. Cristea’s coordination.
469
References [1] D.B. Chrisey, G.K. Hubler, ‘Pulsed Laser Deposition of Thin Films’, New York, 1994. [2] E. Fogarassy, S. Lazare, Laser Ablation of Electronic Materials Basic Mechanisms and Applications, Elevier, 1992. [3] E.V. Pchen, A.V. Valashikin, S.I. Kronosvobodtsev, B. Brunner, K.F. Renk, Appl. Phys. Lett. 63 (1995) 2292. [4] B. Holzapfel, B. Roas, L. Shultz, P. Bauer, G. SaemannIschenko, Appl. Phys. Lett. 61 (1992) 3178. [5] G. Koren, A. Gupta, R.J. Baseman, M.I. Lutwyche, R.B. Laibowitz, Appl. Phys. Lett. 55 (1989) 2450. [6] F. Tcheliebou, S. Baik, J. Appl. Phys. 80 (1996) 7046. [7] R.E. Leuchymer, J.S. Horwitz, D.B. Chrisey, J. Appl. Phys. 83 (1998) 5477. [8] H.L. Spindler, R.M. Gilgenbach, J.S. Lash, Appl. Phys. Lett. 68 (1996) 4345. [9] S.G. Lee, D.S. Hwang, Y.K. Park, J.C. Lash, Appl. Phys. Lett. 83 (1998) 5477. [10] G. Koren, R.J. Baseman, A. Gupta, M.I. Lutwyche, R.B. Laibowitz, Appl. Phys. Lett. 56 (1990) 2144. [11] A. Marcu, C. Grigoriu, W. Jiang, K. Yatsui, SPIE Proc. 4762 (2001) 210. [12] K. Kinobuchi, K. Kinoshita, H. Ishibashi, T. Kobayashi, Jpn. J. Appl. Phys. 33 (1994) 113. [13] K. Kinoshita, H. Ishibashi, T. Kobayashi, Jpn. J. Apply. Phys. 33 (1994) L417. [14] M. Tachiki, T. Kobayashi, Jpn. J. Apply. Phys. 38 (1999) 3642. [15] D.W. Kim, S.M. Oh, S.H. Lee, T.W. Noh, J. Appl. Phys. 85 (1999) 2016. [16] E.G. Gamaly, A.V. Rode, B. Luther-Davies, J. Appl. Phys. 85 (1999) 4213. [17] P.E. Dyer, A. Issa, P.H. Key, Appl. Phys. Lett. 57 (1990) 186. [18] T.E. Itina, W. Marine, M. Autric, J. Appl. Phys. 92 (7) (1997) 3536. [19] A. Marcu, C. Grigoriu, W. Jiang, K. Yatsui, SPIE Proc. 4430 (2000) 241.