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Transport of ablated material through a water vapor atmosphere in pulsed laser deposition of hydroxylapatite
Applied Surface Science 186 (2002) 448±452
Transport of ablated material through a water vapor atmosphere in pulsed laser deposition of hydroxylapatite J.L. Arias*, M.B. Mayor, J. Pou, B. LeoÂn, M. PeÂrez-Amor Departamento de FõÂsica Aplicada, Universidade de Vigo, Lagoas-Marcosende 9, E-36200 Vigo, Spain
Abstract Hydroxylapatite (Ca10(PO4)6(OH)2) is a calcium phosphate used as a coating for dental and orthopaedical implants, because its composition and structure is similar to the mineral part of bone. As an alternative to the traditional plasma spray coating technique, pulsed laser deposition (PLD) has been applied. A hydroxylapatite target was ablated with an ArF laser in a water vapor pressure of 45 Pa to investigate the transport of the ablated material to the substrate. The substrate was placed at different distances from the target, inside and outside the plume. The distribution of coating thickness was measured by pro®lometry. The Ca/P ratio of the coatings was measured by EDAX, whereas their OH and CO3 2 content was evaluated by FT-IR spectroscopy. Inside the plume the thickness distributions correspond to an adiabatic expansion, while outside there is a diffusion of the species through the water vapor atmosphere to the substrate. The composition of the coatings also con®rms this behavior. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Hydroxylapatite; Pulsed laser deposition; Transport
1. Introduction Calcium phosphates, in particular hydroxylapatite (HA), are used as coatings to improve the biocompatibility of dental and orthopaedical metal implants [1], because their structure and composition is similar to the mineral part of bone. The commercial coating technique, plasma spraying, allows high deposition rates, but the coating to substrate adhesion and the control of stoichiometry and composition homogeneity are very poor [1]. As an alternative, pulsed laser deposition (PLD) has been applied. Until now, almost all the work has been focused in the production and characterization of the structure, composition and mechanical properties of these calcium phosphate *
Corresponding author. Tel.: 34-986-812216; fax: 34-986-812201. E-mail address: [email protected] (J.L. Arias).
coatings [2±5]. Moreover, the study of the transport of the ablated material has been limited to the analysis of the velocities of the fronts of the plume at different pressures and of some species by fast ICCD imaging and optical emission spectroscopy [6,7]. Herein, we investigate the relationship between the transport of the ablated material through a water vapor atmosphere and the deposition of the coatings. 2. Experimental The experiments were performed under the deposition conditions for which crystalline carbonated HA coatings are produced [4]. A sintered HA target (95% theoretical density) was ablated in a water vapor pressure of 45 Pa with a laser energy density of 1.6 J cm 2. This energy density was obtained by focusing the beam of a Lambda Physik LPX200 ArF excimer
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 7 3 4 - 6
J.L. Arias et al. / Applied Surface Science 186 (2002) 448±452
laser (wavelength l 193 nm, 20 ns pulse length, 20 Hz repetition rate) to a spot of 3:3 1:4 mm2 . The energy losses caused by the optical elements were determined before and after this set of experiments and were related to the point where the energy was measured during experiments. The energy absorption through the vacuum chamber by the water atmosphere was determine to be less than 0.03%. The c-Si (1 1 1) substrates were placed at different distances from the target (9±48 mm), inside and outside the visible plume. During deposition, the substrate temperature was maintained at 490 8C for all distances. The thickness distributions of the coatings were measured along both the horizontal (x) and vertical (y) axis (related to the long and short axis of the laser spot, respectively) with a Veeco Dektak3 pro®lometer. The Ca/P ratio of the coatings was determined by an energy dispersive X-ray analysis (EDAX), attached to a Philips XL30 scanning electron microscope (SEM). The measurements were calibrated with three standards, so that their estimated error was of 5%. The wt.% CO3 2 in the coatings was calculated from their infrared spectra [8], obtained with a BOMEM MB-100 Fourier transform infrared (FT-IR) spectrometer in transmission. XRD analysis of the coatings was carried out with a Siemens D-5000 diffractometer in y±2y con®guration, using Cu Ka radiation at 40 kV and 30 mA. The spectra were taken with a rotating sample and a 2y step size of 0.028 with a dwell time of 15 s.
449
Fig. 1. Growth rate in the center of the deposit vs. target-substrate distance L. The dotted line indicates the estimated plume length Lp 22 mm.
3. Results and discussion The growth rate in the center of the deposit strongly decreases for target-substrate distances, L, higher than 9 mm (Fig. 1), while the normalized thickness distributions (Fig. 2), obtained by dividing by the thickness in the center (Table 1), are more uniform. They can be ®tted to a sum of two power cosine functions (Table 1, dashed lines in Fig. 2), showing that the weight of the more forward-peaked component is
Fig. 2. Normalized thickness distributions along: (a) horizontal (x) and (b) vertical (y) axis of the coatings obtained at L 9 mm ($), 18 mm (D), 28 mm (*), 37 mm (&), and 48 mm (^). Fits to A cos3q y B cos3r y in dashed lines and to Eq. (1) in solid lines.
450
J.L. Arias et al. / Applied Surface Science 186 (2002) 448±452
Table 1 Fits of the normalized thickness distributions to A cos3q y B cos3r y, along with the absolute thickness in the center of the coatings, and the calculated plume length L (mm)
9 18 28 37 48
A cos3q y B cos3r y Ax
qx
Bx
rx
Ay
qy
By
ry
0.7 0.65 0.56 0.32 0.10
21 23 22 90 130
0.3 0.35 0.44 0.64 0.90
6.9 4.5 4.8 6.0 7.0
0.80 0.39 0.19 0 0
11.9 31 80 0 0
0.2 0.61 0.81 1.00 1.00
1.0 1.0 1.0 2.9 1.7
lower when L is larger. The variation in the values of the exponents does not indicate the evolution of two distinct components, but that there are two terms of a cosine power development of an analytical function [9±11]. The high values of the exponents indicate that the ablated material expansion is more like an adiabatic expansion than a normal evaporation [9]. Therefore, a valid analytical function could be the thickness distributions obtained for a three-dimensional adiabatic expansion of a non-ionized plume in vacuum [11]: D0 D x; y; L ; (1) 2 2 2 x kx =L y2 ky2 =L2 13=2 where D0 is the thickness in the center, kx Vz =Vx , ky Vz =Vy , Vz the longitudinal expansion velocity of the plume and Vx, Vy the transversal expansion velocities. This solution is only valid as an approximation for the expansion in a water vapor atmosphere if the chamber pressure p0 is low [10] (p0 45 Pa < pp E=Vi 2:9 107 Pa; E total pulse energy; Vi volume of initial vapor calculated from the plume velocities [6] and the spot dimensions) and the laser energy density is low enough to avoid atom ionization (<2 J cm 2 in ultraviolet nanosecond laser ablation [12]). However, the ®t of the normalized thickness distributions to the function given by Eq. (1) only gives a result as good as with the cosine power terms for L 9 mm (solid line in Fig. 2). Therefore, that solution is not completely valid. Nevertheless, it is known that in presence of a gas the adiabatic expansion stops, forming a stationary plume, and the material continues expanding by diffusion through the atmosphere [10,13]. The length of this stationary plume, Lp, can be estimated to be 22 mm (Table 1)
Lp (mm)
Ê) Thickness in the center (A
23 22 21 23 22
155400 23500 16200 18100 12700
by the following expression [10,13]: 1=3g E ; Lp A g 11=3g Vi g 1=3g p0
(2)
where A is a geometrical factor of the plume calculated from the thickness distributions and g the adiabatic coef®cient. An average value of g 1:5 was chosen to take into account the species detected by Serra and Morenza [7] in the ablation of HA. Therefore, two of the substrates were selected to be inside the plume (L 9 and 18 mm) and the other three outside (L 28, 37, and 48 mm). The calculated area under the curve (which gives an idea of the total material deposited) is halved by increasing the substrate distance from 9 to 18 mm (Table 2), while it increases from 18 to 48 mm. This fact denotes that inside the plume there is a loss of material by interaction between the ablated material and the substrate (plume re¯ection [14], sputtering by the incident particles [15] or desorption from the surface due to an excessive kinetic energy [16]). The surface of the coatings obtained inside shows a great damage compared to those obtained outside (Fig. 3), probably
Table 2 Area under the curves calculated by integrating the A cos3q y B cos3r y (Table 1) and multiplying by the thickness in the center L (mm) 9 18 28 37 48
Area under the curve (10
8
m2)
x-Axis
y-Axis
3.4 1.2 1.5 2.5 2.8
5.7 2.5 3.5 4.8 4.7
J.L. Arias et al. / Applied Surface Science 186 (2002) 448±452
451
Fig. 3. SEM micrographs of the surface of the coatings obtained: (a) inside L 9 mm and (b) outside L 48 mm the plume.
caused by this interaction. In spite of the loss of material from 9 to 18 mm, the strong diminishing of the growth rate in the center of the coating (Fig. 1) can be only explained by the dispersion of the ablated material by collisions with water molecules during the adiabatic expansion. The Ca/P ratio of the coatings obtained inside the plume is higher than outside (Fig. 4), which is more similar to that of the target (1.70). The effect of the adiabatic expansion is an enrichment of the center of the coating in the heavier species, which follow a narrower distribution than the lighter ones [9]. Therefore, there is an enrichment of calcium (Ca) and calcium oxide (CaxOy) compared to phosphorous (P) (species detected in the plume by Serra et al. [6]). Afterwards, during the diffusion stage, the narrow distributions behave more like a point source, while the wide ones act like a plane wave [17]. Therefore, the Ca and CaxOy distributions expand more than that of P and, thus, the Ca/P ratio is lower outside the plume. Regarding the CO3 2 concentration, it is lower inside the plume. It is probably desorbed as CO2 from the coating, due to the heating produced by the impingement of high velocity particles.
Fig. 4. Ca/P ratio (*) and wt.% CO3 2 (^) vs. target-substrate distance L. The dotted line indicates the estimated plume length Lp 22 mm.
4. Conclusions The transport of material through a water vapor atmosphere in PLD of HA takes place in two stages: an adiabatic expansion till the formation of a stationary plume, and a subsequent diffusion to the substrate.
452
J.L. Arias et al. / Applied Surface Science 186 (2002) 448±452
Both stages in¯uence on the coating structure and composition. For dental and orthopaedical applications of calcium phosphate coatings produced by PLD, the more suitable target-substrate distances are those greater than the stationary plume length. The coatings are more homogeneous in thickness, while at shorter distances the coatings also present undesired phases and surface damage. Acknowledgements SEM, EDAX, XRD and pro®lometry were performed at the CACTI (University of Vigo). Authors wish to thank Prof. J. Arends for providing some of the standards used for EDAX calibration. This work was partially supported by CICYT (MAT93-0271), UE CRAFT (BRE2.CT94.1533), MINER (PATI 665/ 95), Xunta de Galicia (INFRA93, INFRA94-58) and Universidade de Vigo. References [1] M.J. Filiaggi, N.A. Combs, R.M. Pilliar, J. Biomed. Mater. Res. 25 (1991) 1211.
[2] P. Baeri, L. Torrisi, N. Marino, G. Foti, Appl. Surf. Sci. 54 (1992) 210. [3] C.M. Cotell, D.B. Chrisey, K.S. Grabowski, J.A. Sprague, J. Appl. Biomater. 3 (1992) 87. [4] B. Mayor, J. Arias, S. Chiussi, F. GarcõÂa, J. Pou, B. LeoÂn, M. PeÂrez-Amor, Thin Solid Films 317 (1998) 363. [5] J.M. FernaÂndez-Pradas, L. CleÁries, G. Sardin, J.L. Morenza, J. Mater. Res. 14 (1999) 4715. [6] P. Serra, J. Palau, M. Varela, J. Esteve, J.L. Morenza, J. Mater. Res. 10 (1995) 473. [7] P. Serra, J.L. Morenza, J. Mater. Res. 13 (1998) 1132. [8] J. Arends, C.L. Davidson, Calcif. Tiss. Res. 18 (1975) 65± 79. [9] K.L. Saenger, J. Appl. Phys. 70 (1991) 5629. [10] S.I. Anisimov, B.S. Luk'yanchuk, A. Luches, Appl. Surf. Sci. 96±98 (1996) 24. [11] F. Antoni, C. Fuchs, E. Fogarassy, Appl. Surf. Sci. 96±98 (1996) 50. [12] R.E. Russo, X. Mao, in: J.C. Miller, R.F. Haglund (Eds.), Laser Ablation and Desorption, Academic Press, San Diego, CA, 1998, pp. 375±405. [13] P.E. Dyer, A. Issa, P.H. Key, Appl. Surf. Sci. 46 (1990) 89. [14] D.B. Geohegan, in: D.B. Chrisey, G.K. Hubler (Eds.), Pulsed Laser Deposition of Thin Films, Wiley, New York, 1994, pp. 55±88. [15] S. Ma, S.K. Hau, K.H. Wong, P.W. Chan, C.L. Choy, Appl. Phys. Lett. 69 (1996) 2030. [16] M. Tyunina, J. Levoska, S. LeppaÈvuori, J. Appl. Phys. 83 (1998) 5489. [17] J. Gonzalo, C.N. Afonso, J.M. Ballesteros, A. Grosman, C. Ortega, J. Appl. Phys. 82 (1997) 3129.
Transport of ablated material through a water vapor atmosphere in pulsed laser deposition of hydroxylapatite J.L. Arias*, M.B. Mayor, J. Pou, B. LeoÂn, M. PeÂrez-Amor Departamento de FõÂsica Aplicada, Universidade de Vigo, Lagoas-Marcosende 9, E-36200 Vigo, Spain
Abstract Hydroxylapatite (Ca10(PO4)6(OH)2) is a calcium phosphate used as a coating for dental and orthopaedical implants, because its composition and structure is similar to the mineral part of bone. As an alternative to the traditional plasma spray coating technique, pulsed laser deposition (PLD) has been applied. A hydroxylapatite target was ablated with an ArF laser in a water vapor pressure of 45 Pa to investigate the transport of the ablated material to the substrate. The substrate was placed at different distances from the target, inside and outside the plume. The distribution of coating thickness was measured by pro®lometry. The Ca/P ratio of the coatings was measured by EDAX, whereas their OH and CO3 2 content was evaluated by FT-IR spectroscopy. Inside the plume the thickness distributions correspond to an adiabatic expansion, while outside there is a diffusion of the species through the water vapor atmosphere to the substrate. The composition of the coatings also con®rms this behavior. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Hydroxylapatite; Pulsed laser deposition; Transport
1. Introduction Calcium phosphates, in particular hydroxylapatite (HA), are used as coatings to improve the biocompatibility of dental and orthopaedical metal implants [1], because their structure and composition is similar to the mineral part of bone. The commercial coating technique, plasma spraying, allows high deposition rates, but the coating to substrate adhesion and the control of stoichiometry and composition homogeneity are very poor [1]. As an alternative, pulsed laser deposition (PLD) has been applied. Until now, almost all the work has been focused in the production and characterization of the structure, composition and mechanical properties of these calcium phosphate *
Corresponding author. Tel.: 34-986-812216; fax: 34-986-812201. E-mail address: [email protected] (J.L. Arias).
coatings [2±5]. Moreover, the study of the transport of the ablated material has been limited to the analysis of the velocities of the fronts of the plume at different pressures and of some species by fast ICCD imaging and optical emission spectroscopy [6,7]. Herein, we investigate the relationship between the transport of the ablated material through a water vapor atmosphere and the deposition of the coatings. 2. Experimental The experiments were performed under the deposition conditions for which crystalline carbonated HA coatings are produced [4]. A sintered HA target (95% theoretical density) was ablated in a water vapor pressure of 45 Pa with a laser energy density of 1.6 J cm 2. This energy density was obtained by focusing the beam of a Lambda Physik LPX200 ArF excimer
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 7 3 4 - 6
J.L. Arias et al. / Applied Surface Science 186 (2002) 448±452
laser (wavelength l 193 nm, 20 ns pulse length, 20 Hz repetition rate) to a spot of 3:3 1:4 mm2 . The energy losses caused by the optical elements were determined before and after this set of experiments and were related to the point where the energy was measured during experiments. The energy absorption through the vacuum chamber by the water atmosphere was determine to be less than 0.03%. The c-Si (1 1 1) substrates were placed at different distances from the target (9±48 mm), inside and outside the visible plume. During deposition, the substrate temperature was maintained at 490 8C for all distances. The thickness distributions of the coatings were measured along both the horizontal (x) and vertical (y) axis (related to the long and short axis of the laser spot, respectively) with a Veeco Dektak3 pro®lometer. The Ca/P ratio of the coatings was determined by an energy dispersive X-ray analysis (EDAX), attached to a Philips XL30 scanning electron microscope (SEM). The measurements were calibrated with three standards, so that their estimated error was of 5%. The wt.% CO3 2 in the coatings was calculated from their infrared spectra [8], obtained with a BOMEM MB-100 Fourier transform infrared (FT-IR) spectrometer in transmission. XRD analysis of the coatings was carried out with a Siemens D-5000 diffractometer in y±2y con®guration, using Cu Ka radiation at 40 kV and 30 mA. The spectra were taken with a rotating sample and a 2y step size of 0.028 with a dwell time of 15 s.
449
Fig. 1. Growth rate in the center of the deposit vs. target-substrate distance L. The dotted line indicates the estimated plume length Lp 22 mm.
3. Results and discussion The growth rate in the center of the deposit strongly decreases for target-substrate distances, L, higher than 9 mm (Fig. 1), while the normalized thickness distributions (Fig. 2), obtained by dividing by the thickness in the center (Table 1), are more uniform. They can be ®tted to a sum of two power cosine functions (Table 1, dashed lines in Fig. 2), showing that the weight of the more forward-peaked component is
Fig. 2. Normalized thickness distributions along: (a) horizontal (x) and (b) vertical (y) axis of the coatings obtained at L 9 mm ($), 18 mm (D), 28 mm (*), 37 mm (&), and 48 mm (^). Fits to A cos3q y B cos3r y in dashed lines and to Eq. (1) in solid lines.
450
J.L. Arias et al. / Applied Surface Science 186 (2002) 448±452
Table 1 Fits of the normalized thickness distributions to A cos3q y B cos3r y, along with the absolute thickness in the center of the coatings, and the calculated plume length L (mm)
9 18 28 37 48
A cos3q y B cos3r y Ax
qx
Bx
rx
Ay
qy
By
ry
0.7 0.65 0.56 0.32 0.10
21 23 22 90 130
0.3 0.35 0.44 0.64 0.90
6.9 4.5 4.8 6.0 7.0
0.80 0.39 0.19 0 0
11.9 31 80 0 0
0.2 0.61 0.81 1.00 1.00
1.0 1.0 1.0 2.9 1.7
lower when L is larger. The variation in the values of the exponents does not indicate the evolution of two distinct components, but that there are two terms of a cosine power development of an analytical function [9±11]. The high values of the exponents indicate that the ablated material expansion is more like an adiabatic expansion than a normal evaporation [9]. Therefore, a valid analytical function could be the thickness distributions obtained for a three-dimensional adiabatic expansion of a non-ionized plume in vacuum [11]: D0 D x; y; L ; (1) 2 2 2 x kx =L y2 ky2 =L2 13=2 where D0 is the thickness in the center, kx Vz =Vx , ky Vz =Vy , Vz the longitudinal expansion velocity of the plume and Vx, Vy the transversal expansion velocities. This solution is only valid as an approximation for the expansion in a water vapor atmosphere if the chamber pressure p0 is low [10] (p0 45 Pa < pp E=Vi 2:9 107 Pa; E total pulse energy; Vi volume of initial vapor calculated from the plume velocities [6] and the spot dimensions) and the laser energy density is low enough to avoid atom ionization (<2 J cm 2 in ultraviolet nanosecond laser ablation [12]). However, the ®t of the normalized thickness distributions to the function given by Eq. (1) only gives a result as good as with the cosine power terms for L 9 mm (solid line in Fig. 2). Therefore, that solution is not completely valid. Nevertheless, it is known that in presence of a gas the adiabatic expansion stops, forming a stationary plume, and the material continues expanding by diffusion through the atmosphere [10,13]. The length of this stationary plume, Lp, can be estimated to be 22 mm (Table 1)
Lp (mm)
Ê) Thickness in the center (A
23 22 21 23 22
155400 23500 16200 18100 12700
by the following expression [10,13]: 1=3g E ; Lp A g 11=3g Vi g 1=3g p0
(2)
where A is a geometrical factor of the plume calculated from the thickness distributions and g the adiabatic coef®cient. An average value of g 1:5 was chosen to take into account the species detected by Serra and Morenza [7] in the ablation of HA. Therefore, two of the substrates were selected to be inside the plume (L 9 and 18 mm) and the other three outside (L 28, 37, and 48 mm). The calculated area under the curve (which gives an idea of the total material deposited) is halved by increasing the substrate distance from 9 to 18 mm (Table 2), while it increases from 18 to 48 mm. This fact denotes that inside the plume there is a loss of material by interaction between the ablated material and the substrate (plume re¯ection [14], sputtering by the incident particles [15] or desorption from the surface due to an excessive kinetic energy [16]). The surface of the coatings obtained inside shows a great damage compared to those obtained outside (Fig. 3), probably
Table 2 Area under the curves calculated by integrating the A cos3q y B cos3r y (Table 1) and multiplying by the thickness in the center L (mm) 9 18 28 37 48
Area under the curve (10
8
m2)
x-Axis
y-Axis
3.4 1.2 1.5 2.5 2.8
5.7 2.5 3.5 4.8 4.7
J.L. Arias et al. / Applied Surface Science 186 (2002) 448±452
451
Fig. 3. SEM micrographs of the surface of the coatings obtained: (a) inside L 9 mm and (b) outside L 48 mm the plume.
caused by this interaction. In spite of the loss of material from 9 to 18 mm, the strong diminishing of the growth rate in the center of the coating (Fig. 1) can be only explained by the dispersion of the ablated material by collisions with water molecules during the adiabatic expansion. The Ca/P ratio of the coatings obtained inside the plume is higher than outside (Fig. 4), which is more similar to that of the target (1.70). The effect of the adiabatic expansion is an enrichment of the center of the coating in the heavier species, which follow a narrower distribution than the lighter ones [9]. Therefore, there is an enrichment of calcium (Ca) and calcium oxide (CaxOy) compared to phosphorous (P) (species detected in the plume by Serra et al. [6]). Afterwards, during the diffusion stage, the narrow distributions behave more like a point source, while the wide ones act like a plane wave [17]. Therefore, the Ca and CaxOy distributions expand more than that of P and, thus, the Ca/P ratio is lower outside the plume. Regarding the CO3 2 concentration, it is lower inside the plume. It is probably desorbed as CO2 from the coating, due to the heating produced by the impingement of high velocity particles.
Fig. 4. Ca/P ratio (*) and wt.% CO3 2 (^) vs. target-substrate distance L. The dotted line indicates the estimated plume length Lp 22 mm.
4. Conclusions The transport of material through a water vapor atmosphere in PLD of HA takes place in two stages: an adiabatic expansion till the formation of a stationary plume, and a subsequent diffusion to the substrate.
452
J.L. Arias et al. / Applied Surface Science 186 (2002) 448±452
Both stages in¯uence on the coating structure and composition. For dental and orthopaedical applications of calcium phosphate coatings produced by PLD, the more suitable target-substrate distances are those greater than the stationary plume length. The coatings are more homogeneous in thickness, while at shorter distances the coatings also present undesired phases and surface damage. Acknowledgements SEM, EDAX, XRD and pro®lometry were performed at the CACTI (University of Vigo). Authors wish to thank Prof. J. Arends for providing some of the standards used for EDAX calibration. This work was partially supported by CICYT (MAT93-0271), UE CRAFT (BRE2.CT94.1533), MINER (PATI 665/ 95), Xunta de Galicia (INFRA93, INFRA94-58) and Universidade de Vigo. References [1] M.J. Filiaggi, N.A. Combs, R.M. Pilliar, J. Biomed. Mater. Res. 25 (1991) 1211.
[2] P. Baeri, L. Torrisi, N. Marino, G. Foti, Appl. Surf. Sci. 54 (1992) 210. [3] C.M. Cotell, D.B. Chrisey, K.S. Grabowski, J.A. Sprague, J. Appl. Biomater. 3 (1992) 87. [4] B. Mayor, J. Arias, S. Chiussi, F. GarcõÂa, J. Pou, B. LeoÂn, M. PeÂrez-Amor, Thin Solid Films 317 (1998) 363. [5] J.M. FernaÂndez-Pradas, L. CleÁries, G. Sardin, J.L. Morenza, J. Mater. Res. 14 (1999) 4715. [6] P. Serra, J. Palau, M. Varela, J. Esteve, J.L. Morenza, J. Mater. Res. 10 (1995) 473. [7] P. Serra, J.L. Morenza, J. Mater. Res. 13 (1998) 1132. [8] J. Arends, C.L. Davidson, Calcif. Tiss. Res. 18 (1975) 65± 79. [9] K.L. Saenger, J. Appl. Phys. 70 (1991) 5629. [10] S.I. Anisimov, B.S. Luk'yanchuk, A. Luches, Appl. Surf. Sci. 96±98 (1996) 24. [11] F. Antoni, C. Fuchs, E. Fogarassy, Appl. Surf. Sci. 96±98 (1996) 50. [12] R.E. Russo, X. Mao, in: J.C. Miller, R.F. Haglund (Eds.), Laser Ablation and Desorption, Academic Press, San Diego, CA, 1998, pp. 375±405. [13] P.E. Dyer, A. Issa, P.H. Key, Appl. Surf. Sci. 46 (1990) 89. [14] D.B. Geohegan, in: D.B. Chrisey, G.K. Hubler (Eds.), Pulsed Laser Deposition of Thin Films, Wiley, New York, 1994, pp. 55±88. [15] S. Ma, S.K. Hau, K.H. Wong, P.W. Chan, C.L. Choy, Appl. Phys. Lett. 69 (1996) 2030. [16] M. Tyunina, J. Levoska, S. LeppaÈvuori, J. Appl. Phys. 83 (1998) 5489. [17] J. Gonzalo, C.N. Afonso, J.M. Ballesteros, A. Grosman, C. Ortega, J. Appl. Phys. 82 (1997) 3129.