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The pulsed laser ablation deposition technique: a new deposition configuration for the synthesis of uniform films
Surface and Coatings Technology 180 – 181 (2004) 603–606
The pulsed laser ablation deposition technique: a new deposition configuration for the synthesis of uniform films D. Guidoa, L. Cultreraa, A. Perroneb,* b
a Physics Department, University of Lecce, via Arnesano, 73100 Lecce, Italy Physics Department and National Nanotechnology Laboratory of Istituto Nazionale di Fisica della Materia (INFM), University of Lecce, 73100 Lecce, Italy
Abstract The conventional configuration (substrate parallel and frontal to the target surface) of the pulsed laser ablation deposition technique has been modified for improving the film uniformity with acceptable deposition rates, leading to the ‘dynamic deposition configuration’. The novelty of this arrangement is the ability to place by computer-control the substrate synchronously in front of the plume axis compensating the plume deflection effect. The new geometrical configuration was used to deposit films of Ti, Al and Cu. All the deposited films exhibited both high thickness uniformity and remarkable deposition rates. Initial results of the experiments are explained and compared with the results obtained in the conventional configuration, in otherwise similar experimental conditions. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Pulsed laser ablation deposition; Plume deflection effect; Deposition rate; Thickness profile
1. Introduction Pulsed laser ablation deposition (PLAD) represents an efficient and low cost method for the growth of thin films of any material. In spite of its several advantages compared to alternative and competitive thin films deposition techniques it still presents some intrinsic and severe limitations. In addition to the drawback related to droplets on the film surfaces w1–3x, the low deposition rate and the inhomogeneities in the film thickness w4– 8x are the main restrictions, which prevent PLAD technique from emerging as technology for the deposition of coatings of commercial interest. The latter shortcomings arise from the surface roughening of the target, which provokes the plume deflection effect w4x. Many solutions have been suggested to reduce the roughening of the target surface. Among them, the rotation and translation of the target w9x and the laser scanning over a large-diameter rotating target w10–12x have been the most used. Nevertheless, the surface alteration is an intrinsic shortcoming and it cannot be eliminated. As an *Corresponding author. Tel.: q39-0832-297501; fax: q39-0832297505. E-mail address: [email protected] (A. Perrone).
alternative to these approaches, other geometrical configurations for depositing uniform films, called off-axis configurations, have also been reported w13–15x. The off-axis arrangements seem to be a good solution for obtaining films with predictable and reproducible growth processes and film uniformity. Nevertheless, the new configurations suffer from very low deposition rates. In this complex scenario of the PLAD technique we report a highly flexible automated deposition system making the PLAD technique suitable for obtaining films with remarkable uniform thickness profile and quite high deposition rates. In the new configuration the substrate motion, controlled by computer, continuously follows the symmetry axis of the plasma plume, compensating the plume deflection effect. The deposition arrangement was used to deposit films of Ti, Al and Cu, which exhibited high deposition rate and acceptable thickness profile. However, many other tests must be performed with the new arrangement. First of all, the applicability of this dynamic configuration to other materials must be still tested. Moreover, the obtained results indicate that the symmetry between the optical and material distribution, so far assumed, could not be always verified.
0257-8972/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2003.10.102
604
D. Guido et al. / Surface and Coatings Technology 180 – 181 (2004) 603–606
Table 1 Experimental parameters used for the deposition of Ti, Cu and Al films Target materials Substrate material Target–substrate distance Laser wavelength Laser pulse duration Laser spot size Laser fluence Total number of laser pulses Base pressure
Ti, Al, Cu Si (1 0 0) 7.5 cm 308 nm 30 ns 1.1 mm2 5 Jycm2 15 000 10y4 Pa
2. Experimental details The experiments were carried out in a high vacuum system with a base pressure of 10y4 Pa. The XeCl excimer laser (ls308 nm, ts30 ns) was operated at a repetition rate of 10 Hz. The laser beam was focused onto the targets at an incidence angle of approximately 458. In order to avoid fast drilling, the targets were rotated during the laser irradiation with a frequency of 3 Hz. All other experimental parameters used for the deposition of the present films are given in Table 1. In the new arrangement a substrate positioning-system meeting the following requirements was necessary: i. The dimension of the substrate positioning mechanism (mainly its horizontal dimensions) must be kept as small as possible to avoid interferences with the laser beam. ii. Positioning should be done by remote control. iii. Positioning should be done by on-line computer. A computer was used for controlling the correct substrate position and a digital camera (Panasonic NVDS28EG) recorded both the plume deflection and the substrate motion. Fig. 1 shows the experimental apparatus used in our experiments. A scanning electron microscope (SEM, Philips XL 20) was used to study the morphology of the target and film surface, and a
Fig. 1. Scheme of the apparatus for PLAD experiments. T, target; S, substrate; L, lens; PC, personal computer; MS, mass spectrometer. The dashed lines show the conventional configuration, while the arrow shows the substrate motion.
Fig. 2. Picture illustrating where the thickness of the films was measured. The laser beam was coming from the C point side. The radius of the films was approximately 5 mm.
profilometer (Alphastep) was used to deduce the thickness profile of the films in three points along a horizontal line as shown in Fig. 2. 3. Results The plume created during the laser ablation of the rotating targets (Ti, Al and Cu) was clearly visible to the naked eye. It was studied from the top flange of the PLAD system by a digital camera. As the number of laser pulsesysite increased, the plume deviation became more and more evident in the direction of the incoming laser beam. As an example, the plume images recorded by a digital camera during the laser irradiation of Al target are shown in Fig. 3. Each top-view image is the result of a single laser shot. The plasma plume exhibits a characteristic elliptical shape and expands in the forward direction. The maximum deflection angle is reached only when the laser ablation process is stabilized. The plume deflection angle as a function of laser pulsesysite was deduced for all the targets w4x and used to program the substrate motion. In this way we were able to place the substrate continuously in front of the plume axis compensating the plume deflection angle changes. In Fig. 4 the SEM micrographs of Ti, Al and Cu target surface after 15 000 laser pulses are shown. The topography of the target surface is quite different, in spite of this the plume deflection effect is always
Fig. 3. Aluminum plume images recorded after: (a) 2 pulsesysite; (b) 100 pulsesysite; (c) 300 pulsesysite; (d) 600 pulsesysite. Arrows indicate the direction of the laser beam.
D. Guido et al. / Surface and Coatings Technology 180 – 181 (2004) 603–606
605
In Table 2 the average thickness values of the films deposited in both the configurations and in similar experimental conditions are listed. The thickness value was measured with an Alphastep profilometer in three aligned points (Fig. 2). In those points a small droplet of silver paste was put in order to create a measurable step between the substrate surface and the deposited film. Each value given in table corresponds to the average of four measurements taken around the circularshape of the silver paste droplets we used. The mean deposition rate in the new arrangement was 0.38, 0.34 ˚ and 0.25 Aypulse for Ti, Al and Cu, respectively. It is evident that the uniformity of the films obtained with the new dynamic configuration is not only more satisfactory but also the deposition rate is higher. All the films deposited with the dynamic configuration show a high uniformity without any lowering of the deposition rate. On the contrary, in the case of Ti films, the deposition rate is even increased by a factor of approximately 2 when compared to the one obtained with the conventional configuration. The different increments of the values of the deposition rate, obtained with the new configuration for Ti, Al and Cu films, can be attributed to different mass distribution width of the ablated material in the plume. In particular, the high directionality of the plume observed during the laser ablation of Ti, produced by the specific oriented surfaces created on the irradiated target, may be the reason for the strong increase of the deposition rate. Closer examinations showed that the average thickness value of the films deposited with the conventional configuration increased towards the laser beam direction, just along the direction of the plume deflection. Thus, the uniformity is still rather acceptable, because during long laser irradiation experiments the conventional configuration becomes in every respect an off-axis configuration due to the plume deflection effect. Yet, it is worth noting that in the case of Ti film deposition an unexplainable trend of the average thickness value has been observed. Indeed, the maximum thickness is slightly shifted towards the right, corresponding to the side of arrival of the laser beam on the target, in spite of the fact that the substrate was perfectly in axis with the luminous plume, as recorded by the digital camera. These results demonstrate that the Fig. 4. SEM images of the morphology of the tracks of: (a) Ti; (b) Al and (c) Cu targets. Arrows indicate the direction of the laser beam.
present; therefore, in order to compensate this effect, the substrate position is controlled by computer. The computer sends pulses to the stepping motor, which drives the substrate to stay continuously in front of the plume. The most important requirement for obtaining high-quality films is to achieve a precise control of the substrate motion.
Table 2 Average thickness values in nm measured in both the configurations Point A
Point B
Point C
Titanium
Conventional Dynamic
290 550
320 580
330 590
Aluminum
Conventional Dynamic
430 500
460 510
470 510
Cupper
Conventional Dynamic
260 380
290 380
320 370
606
D. Guido et al. / Surface and Coatings Technology 180 – 181 (2004) 603–606
assumed symmetry between optical and mass distribution could not be always true. 4. Conclusions A modified PLAD configuration was conceived for the deposition of thin films with remarkable uniform thickness profiles and acceptable deposition rates. The capabilities of the new deposition configuration (‘dynamic deposition configuration’) in the case of metallic targets have been shown. Using the dynamic configuration a significant increase in the uniformity of the films has been obtained. Extension of this new deposition configuration to other materials, such as semiconductors, dielectrics and oxides is expected. However, further investigations are still necessary for verifying the symmetry between the optical and ablated material distribution. Acknowledgments Dr M.L. Protopapa, who performed the thickness analysis, is gratefully acknowledged. One of the authors (A.P.) thanks the National Nanotechnology Laboratory for the financial support to this research activity.
References w1x I.N. Mihailescu, E. Gyorgy, V.S. Teodorescu, et al., J. Appl. Phys. 86 (1999) 7123. w2x T. Smausz, N. Kresz, B. Hopp, Appl. Surf. Sci. 177 (2000) 66. w3x J.D. Suh, G.Y. Sung, Physica C 235–240 (1994) 571. w4x A. Perrone, A. Zocco, L. Cultrera, D. Guido, A. Forleo, Appl. Surf. Sci. 197–198 (2002) 251. w5x T.P. O’Brien, J.F. Lawer, J.G. Lunney, W.J. Blau, Mater. Sci. Eng. B 13 (1992) 9. w6x Y. Hiroshima, T. Ishiguro, I. Urata, et al., J. Appl. Phys. 79 (1996) 3572. w7x R. Perez Casero, F. Kerherve’, J.P. Enar, J. Perrier, P. Regnier, Appl. Surf. Sci. 54 (1992) 147. w8x R. Tomov, V. Tsaneva, V. Tsanev, D. Ouzounov, J. Low Temp. Phys. 105 (1996) 1307. w9x N. Arnold, D. Bauerle, ¨ Appl. Phys. A 68 (1999) 363. w10x J.A. Greer, M.D. Tabat, J. Vac. Sci. Technol. A 13 (3) (1995) 1175. w11x I.J. Jeon, D. Kim, J.S. Song, J.H. Her, D.R. Lee, K.B. Lee, Appl. Phys. A 70 (2000) 235. w12x A. Jacquot, B. Lenoir, M.O. Boffoue, A. Dauscher, Appl. Phys. A 69 (1999) 195. w13x K.B. Erington, N.J. Ianno, Mater. Res. Soc. Proc. 191 (1990) 115. w14x R.J. Kennedy, Thin Solid Films 214 (1992) 223. w15x A. Perrone, L. Cultrera, A. Dima, et al., Jap. J. Appl. Phys. 42 (7A) (2000) 4181.
The pulsed laser ablation deposition technique: a new deposition configuration for the synthesis of uniform films D. Guidoa, L. Cultreraa, A. Perroneb,* b
a Physics Department, University of Lecce, via Arnesano, 73100 Lecce, Italy Physics Department and National Nanotechnology Laboratory of Istituto Nazionale di Fisica della Materia (INFM), University of Lecce, 73100 Lecce, Italy
Abstract The conventional configuration (substrate parallel and frontal to the target surface) of the pulsed laser ablation deposition technique has been modified for improving the film uniformity with acceptable deposition rates, leading to the ‘dynamic deposition configuration’. The novelty of this arrangement is the ability to place by computer-control the substrate synchronously in front of the plume axis compensating the plume deflection effect. The new geometrical configuration was used to deposit films of Ti, Al and Cu. All the deposited films exhibited both high thickness uniformity and remarkable deposition rates. Initial results of the experiments are explained and compared with the results obtained in the conventional configuration, in otherwise similar experimental conditions. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Pulsed laser ablation deposition; Plume deflection effect; Deposition rate; Thickness profile
1. Introduction Pulsed laser ablation deposition (PLAD) represents an efficient and low cost method for the growth of thin films of any material. In spite of its several advantages compared to alternative and competitive thin films deposition techniques it still presents some intrinsic and severe limitations. In addition to the drawback related to droplets on the film surfaces w1–3x, the low deposition rate and the inhomogeneities in the film thickness w4– 8x are the main restrictions, which prevent PLAD technique from emerging as technology for the deposition of coatings of commercial interest. The latter shortcomings arise from the surface roughening of the target, which provokes the plume deflection effect w4x. Many solutions have been suggested to reduce the roughening of the target surface. Among them, the rotation and translation of the target w9x and the laser scanning over a large-diameter rotating target w10–12x have been the most used. Nevertheless, the surface alteration is an intrinsic shortcoming and it cannot be eliminated. As an *Corresponding author. Tel.: q39-0832-297501; fax: q39-0832297505. E-mail address: [email protected] (A. Perrone).
alternative to these approaches, other geometrical configurations for depositing uniform films, called off-axis configurations, have also been reported w13–15x. The off-axis arrangements seem to be a good solution for obtaining films with predictable and reproducible growth processes and film uniformity. Nevertheless, the new configurations suffer from very low deposition rates. In this complex scenario of the PLAD technique we report a highly flexible automated deposition system making the PLAD technique suitable for obtaining films with remarkable uniform thickness profile and quite high deposition rates. In the new configuration the substrate motion, controlled by computer, continuously follows the symmetry axis of the plasma plume, compensating the plume deflection effect. The deposition arrangement was used to deposit films of Ti, Al and Cu, which exhibited high deposition rate and acceptable thickness profile. However, many other tests must be performed with the new arrangement. First of all, the applicability of this dynamic configuration to other materials must be still tested. Moreover, the obtained results indicate that the symmetry between the optical and material distribution, so far assumed, could not be always verified.
0257-8972/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2003.10.102
604
D. Guido et al. / Surface and Coatings Technology 180 – 181 (2004) 603–606
Table 1 Experimental parameters used for the deposition of Ti, Cu and Al films Target materials Substrate material Target–substrate distance Laser wavelength Laser pulse duration Laser spot size Laser fluence Total number of laser pulses Base pressure
Ti, Al, Cu Si (1 0 0) 7.5 cm 308 nm 30 ns 1.1 mm2 5 Jycm2 15 000 10y4 Pa
2. Experimental details The experiments were carried out in a high vacuum system with a base pressure of 10y4 Pa. The XeCl excimer laser (ls308 nm, ts30 ns) was operated at a repetition rate of 10 Hz. The laser beam was focused onto the targets at an incidence angle of approximately 458. In order to avoid fast drilling, the targets were rotated during the laser irradiation with a frequency of 3 Hz. All other experimental parameters used for the deposition of the present films are given in Table 1. In the new arrangement a substrate positioning-system meeting the following requirements was necessary: i. The dimension of the substrate positioning mechanism (mainly its horizontal dimensions) must be kept as small as possible to avoid interferences with the laser beam. ii. Positioning should be done by remote control. iii. Positioning should be done by on-line computer. A computer was used for controlling the correct substrate position and a digital camera (Panasonic NVDS28EG) recorded both the plume deflection and the substrate motion. Fig. 1 shows the experimental apparatus used in our experiments. A scanning electron microscope (SEM, Philips XL 20) was used to study the morphology of the target and film surface, and a
Fig. 1. Scheme of the apparatus for PLAD experiments. T, target; S, substrate; L, lens; PC, personal computer; MS, mass spectrometer. The dashed lines show the conventional configuration, while the arrow shows the substrate motion.
Fig. 2. Picture illustrating where the thickness of the films was measured. The laser beam was coming from the C point side. The radius of the films was approximately 5 mm.
profilometer (Alphastep) was used to deduce the thickness profile of the films in three points along a horizontal line as shown in Fig. 2. 3. Results The plume created during the laser ablation of the rotating targets (Ti, Al and Cu) was clearly visible to the naked eye. It was studied from the top flange of the PLAD system by a digital camera. As the number of laser pulsesysite increased, the plume deviation became more and more evident in the direction of the incoming laser beam. As an example, the plume images recorded by a digital camera during the laser irradiation of Al target are shown in Fig. 3. Each top-view image is the result of a single laser shot. The plasma plume exhibits a characteristic elliptical shape and expands in the forward direction. The maximum deflection angle is reached only when the laser ablation process is stabilized. The plume deflection angle as a function of laser pulsesysite was deduced for all the targets w4x and used to program the substrate motion. In this way we were able to place the substrate continuously in front of the plume axis compensating the plume deflection angle changes. In Fig. 4 the SEM micrographs of Ti, Al and Cu target surface after 15 000 laser pulses are shown. The topography of the target surface is quite different, in spite of this the plume deflection effect is always
Fig. 3. Aluminum plume images recorded after: (a) 2 pulsesysite; (b) 100 pulsesysite; (c) 300 pulsesysite; (d) 600 pulsesysite. Arrows indicate the direction of the laser beam.
D. Guido et al. / Surface and Coatings Technology 180 – 181 (2004) 603–606
605
In Table 2 the average thickness values of the films deposited in both the configurations and in similar experimental conditions are listed. The thickness value was measured with an Alphastep profilometer in three aligned points (Fig. 2). In those points a small droplet of silver paste was put in order to create a measurable step between the substrate surface and the deposited film. Each value given in table corresponds to the average of four measurements taken around the circularshape of the silver paste droplets we used. The mean deposition rate in the new arrangement was 0.38, 0.34 ˚ and 0.25 Aypulse for Ti, Al and Cu, respectively. It is evident that the uniformity of the films obtained with the new dynamic configuration is not only more satisfactory but also the deposition rate is higher. All the films deposited with the dynamic configuration show a high uniformity without any lowering of the deposition rate. On the contrary, in the case of Ti films, the deposition rate is even increased by a factor of approximately 2 when compared to the one obtained with the conventional configuration. The different increments of the values of the deposition rate, obtained with the new configuration for Ti, Al and Cu films, can be attributed to different mass distribution width of the ablated material in the plume. In particular, the high directionality of the plume observed during the laser ablation of Ti, produced by the specific oriented surfaces created on the irradiated target, may be the reason for the strong increase of the deposition rate. Closer examinations showed that the average thickness value of the films deposited with the conventional configuration increased towards the laser beam direction, just along the direction of the plume deflection. Thus, the uniformity is still rather acceptable, because during long laser irradiation experiments the conventional configuration becomes in every respect an off-axis configuration due to the plume deflection effect. Yet, it is worth noting that in the case of Ti film deposition an unexplainable trend of the average thickness value has been observed. Indeed, the maximum thickness is slightly shifted towards the right, corresponding to the side of arrival of the laser beam on the target, in spite of the fact that the substrate was perfectly in axis with the luminous plume, as recorded by the digital camera. These results demonstrate that the Fig. 4. SEM images of the morphology of the tracks of: (a) Ti; (b) Al and (c) Cu targets. Arrows indicate the direction of the laser beam.
present; therefore, in order to compensate this effect, the substrate position is controlled by computer. The computer sends pulses to the stepping motor, which drives the substrate to stay continuously in front of the plume. The most important requirement for obtaining high-quality films is to achieve a precise control of the substrate motion.
Table 2 Average thickness values in nm measured in both the configurations Point A
Point B
Point C
Titanium
Conventional Dynamic
290 550
320 580
330 590
Aluminum
Conventional Dynamic
430 500
460 510
470 510
Cupper
Conventional Dynamic
260 380
290 380
320 370
606
D. Guido et al. / Surface and Coatings Technology 180 – 181 (2004) 603–606
assumed symmetry between optical and mass distribution could not be always true. 4. Conclusions A modified PLAD configuration was conceived for the deposition of thin films with remarkable uniform thickness profiles and acceptable deposition rates. The capabilities of the new deposition configuration (‘dynamic deposition configuration’) in the case of metallic targets have been shown. Using the dynamic configuration a significant increase in the uniformity of the films has been obtained. Extension of this new deposition configuration to other materials, such as semiconductors, dielectrics and oxides is expected. However, further investigations are still necessary for verifying the symmetry between the optical and ablated material distribution. Acknowledgments Dr M.L. Protopapa, who performed the thickness analysis, is gratefully acknowledged. One of the authors (A.P.) thanks the National Nanotechnology Laboratory for the financial support to this research activity.
References w1x I.N. Mihailescu, E. Gyorgy, V.S. Teodorescu, et al., J. Appl. Phys. 86 (1999) 7123. w2x T. Smausz, N. Kresz, B. Hopp, Appl. Surf. Sci. 177 (2000) 66. w3x J.D. Suh, G.Y. Sung, Physica C 235–240 (1994) 571. w4x A. Perrone, A. Zocco, L. Cultrera, D. Guido, A. Forleo, Appl. Surf. Sci. 197–198 (2002) 251. w5x T.P. O’Brien, J.F. Lawer, J.G. Lunney, W.J. Blau, Mater. Sci. Eng. B 13 (1992) 9. w6x Y. Hiroshima, T. Ishiguro, I. Urata, et al., J. Appl. Phys. 79 (1996) 3572. w7x R. Perez Casero, F. Kerherve’, J.P. Enar, J. Perrier, P. Regnier, Appl. Surf. Sci. 54 (1992) 147. w8x R. Tomov, V. Tsaneva, V. Tsanev, D. Ouzounov, J. Low Temp. Phys. 105 (1996) 1307. w9x N. Arnold, D. Bauerle, ¨ Appl. Phys. A 68 (1999) 363. w10x J.A. Greer, M.D. Tabat, J. Vac. Sci. Technol. A 13 (3) (1995) 1175. w11x I.J. Jeon, D. Kim, J.S. Song, J.H. Her, D.R. Lee, K.B. Lee, Appl. Phys. A 70 (2000) 235. w12x A. Jacquot, B. Lenoir, M.O. Boffoue, A. Dauscher, Appl. Phys. A 69 (1999) 195. w13x K.B. Erington, N.J. Ianno, Mater. Res. Soc. Proc. 191 (1990) 115. w14x R.J. Kennedy, Thin Solid Films 214 (1992) 223. w15x A. Perrone, L. Cultrera, A. Dima, et al., Jap. J. Appl. Phys. 42 (7A) (2000) 4181.