- Email: [email protected]
Angular distribution and expansion of laser ablation plumes measured by fast intensified charge coupled device photographs
.__ __ B
CQa
Nuclear Instruments and Methodsin Physics ResearchB 116 ( 1996)257-26 1
NIOMB
Beam Interactions with Materials 8 Atoms
Angular distribution and expansion of laser ablation plumes measl lred by fast intensified charge coupled device photographs A. Mele &*, A. Giardini Guidoni a, R. Kelly b, A. Miotello b, S. Orlando ‘, R. Teghil d, C. Fhrnini a a Dipartimento di Chimica, Vniversit& di Roma “La Sapienza”, Piauale A. More, 5-00185 Rome, Italy b Dipartimento di Fisica, Vniversith di Trento, Povo (TN), Italy ’ Istituto Materiali Speciali, CNR, Tito Scala (PZ), Italy ’ Dipartimento di Chimica, Vniversit& &Na Basilicata, Potenza. Italy
Abstract
Laser ablation plumes produced from various targets including metals, semiconductors, and superconductors have been photographed in real time with a gated, intensified charge coupled device (CCD) camera system. The expansion rates of the three-dimensional images have been measured in a wide range of delay times after the laser pulse by varying the beam parameters such as spot size, spot shape, and fluence. The analysis of the observed plumes leads to the angular distribution of the ablated material which is strongly related to the process of film growth by pulsed laser deposition.
1. Introduction
Pulsed laser deposition (PLD) has been increasingly applied to the preparation of thin films and coatings of a large number of materials. Laser ablation of a solid produces a plume (often but not always a plasma plume) which travels from the target to the substrate expanding along three perpendicular axes. In situ characterization of the plume as it moves and of the angular spread is very important in order to control thin film properties such as uniformity, morphology, and composition [l]. During PLD a background gas such as 0, or N, may be added to supplement a loss or to aid incorporation in the growing film. It is known, both from gas dynamics [2] and experiment [3], that plume expansion has a different trend in vacuum than in the presence of background gas. Moreover, angular profiles of film thickness and composition (i.e. stoichiometry) may be altered if a multicomponent target is being ablated [4]. The overall process has been interpreted in terms of Knudsen layer formation very close to the target surface, followed by an unsteady adiabatic expansion [5,6]. Several experimental methods have been used to measure the plume angular distribution. These are classified as film-based or probe-based [I]. The probe-methods analyze
* Corresponding author. Fax +39 6 490324, Tel. -I-39 6 49913307.
the high resolution optical emission or absorption of the laser plume. Ion probes have also been utilized to probe the expanding plume [7]. The film-based methods depend on determining ex situ the thickness profile of the deposited thin film. The present work deals with a direct method of probing the expanding plume by a three-dimensional fast photographic technique. The method is useful for studying plume dynamics during in situ pulsed laser deposition of a wide variety of materials: metals, semiconductors, and high T, superconductors. The effect of the laser parameters (spot size, spot shape, and fluence) has been investigated by observing temporally and spatially the expanding plume.
2. Experimental
Laser ablation was carried out in a high vacuum chamber equipped with several view ports [3]. Vacuum operating conditions were better than 1O-3 Pa. A rotating target (8 rpm) was irradiated at normal incidence either by an excimer laser (A = 248 nm, r= 18 ns, fluence 3-12 J/cm’, rectangular spot size 0.5-2.4 mm by 1.0 mm) or by a NQYAG laser (A = 532 nm, r = 6 ns, fluence 3-6 J/cm2, circular spot size 0.7 mm’). The larger dimension of the rectangular spot could be reduced by inserting a slit between the laser and the target. Independently of this, the fluence could be varied by tilting a coated fused silica plate through which the beam passed. A gated intensified
0168-583X/%/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved PII SO168-583X(96)00056-0
258
A. Mele et al./Nucl.
Instr. and Meth. in Phys. Res. B II6 (1996) 257-261
charge coupled device (CCD) camera (EG & G Model 1530-PUV+ LCI-I, 512X 512 pixels, 18 bits dynamic range) with spatial and temporal resolution of 1.50 pm and 5 ns was used to acquire the plume images for a single shot. Orthogonal lateral views of the plume expansion were made by recording the overall visible emission (400800 nm). A 50 mm Nikkor lens could observe the plume expansion over a 6 cm region starting at the target. The plasma emission intensity of the intensified CCD photographs was plotted in terms of 10% step contours. The targets were Cu disks, Sn disks, as well as Pbe,,Bi,,,Sr,Ca,Cu,O. (PbBSCCO) and SnSe pellets which were made from pressed powders.
3. Results and discussion The expansion of the plasma plumes obtained by irradiating with the rectangular beam of the excimer laser was measured from the intensified CCD photographs. The plumes were analyzed in three dimensions by lateral views in both the xz and yz planes. z is the axis normal ,to the target, whereas x is normally the larger and y is normally the smaller dimension of the laser spot (Fig. 1). The plumes were found to have an ellipsoidal shape with three distinct axes. As in previous.work [8] the lateral expansions (x and y) were found to be “rotated” by 90“ with respect to the laser spot (Fig. 1). We hasten to add, however, that the “rotation” was not a formal rotation in which the plume passed continuously from one orientation to the other but rather a situation in which (referring to Fig. 1) the transport of particles was more effective in the y than in the x direction. This has been confirmed by numerical solution of the gas-dynamic equations in two dimensions [8,9] where we find an unrotated situation at very short times but a well defined symmetry change at longer times. Tentatively, the effect can be explained in terms of the directionality of the flow such that transport normal to a side is favored over transport from region near a comer because it is less divergent. The effect has been confirmed for square, rectangular, and triangular laser spots [9,10]. The materials examined, namely Cu, Sn, SnSe and PbBSCCO, have all shown very similar behaviour. A typical example is shown in Fig. 2 for plumes generated from Cu and PbBSCCO targets at 600 ns delay time viewed from two lateral axes at 90” with respect to the laser spot as in Fig. 1. It can be seen that both plumes have an ellipsoidal shape, that for Cu having dimensions x = 5 mm, y = 12 mm, Z= 12 mm and that for PbBSCCO having dimensions x = 12 mm, y = 17 mm, z = 21 mm. A correlation between the laser spot shape and the lateral spread of the luminous material is indicated. According to the gas-dynamics calculations [S,9] the role of the spot shape in affecting the shape of the plume begins in the period immediately after particle release when the lateral
expansion is relatively slight. This was also confirmed experimentally from photographs at very short delay times when the plume was close to the target (50-60 ns). The shape changes would, by necessity, cease when the particles no longer interacted according to gas-dynamics but went into free flight. We have assumed that the light emitting particles in the plume are representative of all materials ejected, i.e. that the emitting and non-emitting states have similar velocities. A detailed analysis of the effect of spot size on the plume generated from Cu is shown in Fig. 3. A set of three density contours, obtained by varying one dimension of the rectangular spot and maintaining the other fixed, always at the same fluence (12 J/cm2), is shown. The contours of these frames depict the change of the plume viewed in the xz. plane, i.e. the plane corresponding to the reduced dimension of the spot (Fig. 1). The plume shape viewed in the yz plane, i.e. the plane corresponding to the unreduced dimension y = 1 mm, remained the same (12 + 1 mm). From this sequence it may be observed that by reducing the larger dimension from 2.0 to 0.5 mm the lateral spread increases. These data confirm previous findings that film thickness profiles become less forward peaked as the spot dimension diminishes [4]. Results for Cu of the type seen in Fig. 3 are plotted in Fig. 4a in terms of the parameter p of the angular distribution fl f3) a cos pl.?,which is a reasonable approximation to describe the plume shape in three-dimensional space [l]. In this case 0 is measured on the 10% intensity contour. The plot shows the trend of p when the large (x) dimension of the rectangular laser beam is diminished from 2.4 to 0.5 at the same fluence of 12 J/cm’. The data fit a linear plot with the tendency to a value of 0 5 p I 1 for small spot sizes. It should be noted that for different time delays from the laser pulse and thus for various distances from the target, the plumes have similar values of p, i.e. the shape
/-\ I/
EXPANDING PLUME \
Fig. 1. Solid: sketch showing the lateral (xy) axes of a rectangular 1ase.r spot appropriate to a laser pulse directed at normal incidence towards a target. Dashed: approximate xy symmetry of the expanding plume, showing that the larger side of the spot becomes the smaller side of the plume.
A. Mele et al./Nucl.
Instr. and Meth. in Phys. Res. B 116 (1996) 257-261
is conserved. The effect of laser fluence on the angular distribution in the xz plane of the plume generated from Cu is reported in Fig. 4b. A marked decrease of p from 9 to 5 as a result of varying the fluence from 12 to 6 J/cm2 is found. Similar behaviour is observed also for a SnSe target. This interesting result indicates that the plume has a tendency to become symmetrical along the three axes x, y, z when the fluence of the laser beam is decreased (p = 1). At the same time the lateral spread increases. A broadened distribution is typical at low fluence also for other systems [ 1,7]. Experiments with a circular beam from a Nd-YAG laser (532 nm) show that, at a laser fluence of 12 J/cm’, the plume expansion is ellipsoidal and symmetrical in the xz and yz planes with the axes z > x = y. This could be expected from the geometry of the spot. The same evolution of plume shape found for a rectangular spot as a
259
function of fluence is observed. Thus, the plume tends to become symmetrical along the three axes x, y, z when the fluence of the laser beam is decreased. The nearly spherical shape of the plume with both a rectangular and a spherical spot suggests an effusive regime corresponding to a removal of material by normal vaporization (as distinct from explosive boiling [11,12]). This suggests that within distances where the substrate is usually located, that is about 20-30 mm, the angular distribution will be independent of distance. The analysis of the plume evolution of all materials studied, namely Cu, Sn, SnSe, and PbBSCCO, has confirmed this result. It is apparent from the contours of the rectangular spot of Fig. 3 and from the circular spot (to be published) that the spot size and shape of the laser beam have a marked effect on the plume symmetry. This effect is very general for all materials and persists at long distances from the
Fig. 2. Overall emission from Cu ((a) and (b)) and PbBSCCO(cc) and (d>) targets laser-irradiated at 248 mn. The light intensity is shown in terms of two-dimensional 10% density contours (smoothened). The maxima are indicated in (b) to (d) as a solid circle. Details: delay time 600 ns, exposure time 100 ns, fluence 12 J/cm*, rectangular laser spot (2.4 X 1 mm*). The views, as indicated in each case, are either in the xz or the yz plane of the spot. Grid step: 2 mm.
A. Mele et al./Nucl.
260
Instr. and Meth. in Phys. Res. B 116 (1996) 257-261
target. A closer examination of the results of the present investigation shows an additional effect which is due to the energy distribution within the laser spot. It is known from the profiles of the energy maps, either of an excimer laser with a rectangular beam or of a Nd-YAG laser with a circular beam, that the energy for both is higher in the middle than in the periphery of the beam. This has the effect of a non-uniform removal of material from the target. This view is supported from a comparison of the plume contours obtained by a square spot from an excimer laser beam by reducing the larger lateral dimension down to that of the smaller one. A homogeneous distribution of the energy on both sides would provide an identical plume symmetry as observed for a circular spot [13]. However, this is not the case for the excimer laser because the unreduced small dimension side has kept the energy distri-
TARGET
1 (4
600
TARGET
ns
I(b)]
101
I
x DIMENSION 10
I
OF LASER
I
1
SPOT
(mm)
,
L
hv(248nm)--Cu
I
1
TARGET
1
Fig. 3. Set of three 10% density contours (smoothened) from photographs of a Cu disk laser-irradiated at 248 nm. Details: delay time 600 ns, exposure time 100 ns, fluence 12 J/cm’. The rectangular laser spot dimensions are 2.0X 1.0 mm* (a), 1.0X 1.0 mm* (b), 0.5 X 1.0 mm* (cl. The views are in the w plane of the spot. Grid step: 2 mm.
6
8 ENERGY
L
I
12
10 FLUENCE
J
(J/cm*)
Fig. 4. Angular distribution in terms of cos@ from data for a laser-irradiated Cu disk as in Fig. 3. (a) Plot of p versus a varying laser spot dimension (x). the other dimension remaining fixed at 1.0 mm. (b) Plot of p versus fluence. In the case of(b) the spot dimensions are 2.4X 1.0 mm*. The views are in the xz plane of the spot for both (a) and (b).
A. Mele et al./Nucl.
600
.z
1 TARGET
Instr. and Meth. in Phys. Res. B 116 (1996) 257-261
ns
(
I600
TARGET
261
ns 1
i(b)I
Fig. 5. Set of two-dimensional 10% density contours (smoothened) from photographs of a Cu disk laser-irradiated at 248 nm. The maxima are indicated as a solid circle. Details: delay time 600 ns, exposure time 100 ns, fluence 12 J/cm*, square laser spot (0.8 X 0.8 mm2). The views, as indicated in each case, are either in the xz or the yz plane of the spot. Grid step: 2 mm.
bution whereas
the other side acquires
a more homoge-
neous distribution when the original dimension is reduced. The latter distribution produces the plume of Fig. 5a in the ZJ plane, the unchanged side that of Fig. 5b in the zy plane. The angular distribution of the expanding plume is a direct measurement of the particles velocities in a three-dimensional space. From the results of Figs. 2 and 3 it appears that the highest velocity is normal to the target and that the lowest velocities are for lateral expansion. Various factors may lead to such behaviour. For example, it may be the result of complex phenomena occurring by the interaction of the laser beam with a solid surface, such that a number of layers of the material are ejected and a pressure profile develops at the interface which characterizes the flow of material. This would be especially true at higher fluences when explosive rather than normal vaporization can be expected [11,12]. The expansion n&ma1 and parallel to the target evidently acquires different pressure gradients [ 13,141. The highest gradient is expected to be along the target normal (z axis) and the lowest gradients along the x and y axes. The gradients themselves are in turn effected by the size and shape of the laser beam. Finally we note that conditions of normal vaporization lead to low particle densities and the distributions are characterized by low powers of cos 6. The flow of the particles under conditions of explosive boiling is characterized by high number densities. The flow conditions are then characterized by a Knudsen layer, where the distribution is discontinuous [ 11,121. This boundary layer is followed fist by an unsteady adiabatic expansion and finally by free flight. Depending on the persistance of adiabatic
expansion phase, the angular distributions show strong forward peaking of the type cos% with p >> 1.
References [I] K.L. Saenger, in Pulsed Laser Deposition of Thin Films (than. 71, eds. D.B. Chrisev and G.K. Hubler (Wiley, New York, 1994) p. 199. 01 R. Kelly, A. Miotello, B. Braren, A. Gupta and K. Casey, Nucl. Instr. and Meth B 65 (1992) 187. [31 T. Di Palma, S. Orlando, A. Giardini Guidoni, A.J. Paul, J.W. Hastie and A. Mele, Appl. Surf. Sci. 86 (1995) 68. [41 R,E. Muenchausen, K.M. Hubbard, S. Foltyn, R.G. Esteer, N.S. Nogar and C. Jenkins, Appl. Phys. Lett. 56 (1990) 578. 151 R. Kelly and R.W. Dreyfus, Surf. Sci. 198 (1988) 263. t61 R. Kelly and A. Miotello, Appl. Phys. B 57 (1993) 145. [iI D.B. Geohegan, in: Pulsed Laser Deposition of Thin Films (chap. 51, eds. D.B. Chrisey and G.K. Hubler (Wiley, New York, 1994) p. 115. k4 A. Mele, A. Giardmi, R. Kelly, A. Miotello, S. Orlando and R. Teghil, Appl. Surf. Sci. 96-98 (1996) 102. 191 A. Miotello and R. Kelly, Appl. Phys. Lett. 61 (1992) 2784. 1101 R. Kelly and A. Miotello, Nucl. Jnstr. and Meth. B 91 (1994) 682. illI R. Kelly and A. Miotello, Appl. Surf. Sci. 96-98 (1996) 205. [I21 R. Kelly, A. Miotello, A. Mele, A. Giardini Guidoni, J.W. Hastie, P.K. Schenck and H. Okabe, Surf. Sci. (submitted). [I31 R.W. Ohse, J.F. Bebelot, A. Frezzotti, K.A. Long and J. Magill, High Temp. High Press. 2 (1980) 537. [I41 N.F. Ramsey, Molecular Beams (Clarendon Press, Oxford, 1956).
CQa
Nuclear Instruments and Methodsin Physics ResearchB 116 ( 1996)257-26 1
NIOMB
Beam Interactions with Materials 8 Atoms
Angular distribution and expansion of laser ablation plumes measl lred by fast intensified charge coupled device photographs A. Mele &*, A. Giardini Guidoni a, R. Kelly b, A. Miotello b, S. Orlando ‘, R. Teghil d, C. Fhrnini a a Dipartimento di Chimica, Vniversit& di Roma “La Sapienza”, Piauale A. More, 5-00185 Rome, Italy b Dipartimento di Fisica, Vniversith di Trento, Povo (TN), Italy ’ Istituto Materiali Speciali, CNR, Tito Scala (PZ), Italy ’ Dipartimento di Chimica, Vniversit& &Na Basilicata, Potenza. Italy
Abstract
Laser ablation plumes produced from various targets including metals, semiconductors, and superconductors have been photographed in real time with a gated, intensified charge coupled device (CCD) camera system. The expansion rates of the three-dimensional images have been measured in a wide range of delay times after the laser pulse by varying the beam parameters such as spot size, spot shape, and fluence. The analysis of the observed plumes leads to the angular distribution of the ablated material which is strongly related to the process of film growth by pulsed laser deposition.
1. Introduction
Pulsed laser deposition (PLD) has been increasingly applied to the preparation of thin films and coatings of a large number of materials. Laser ablation of a solid produces a plume (often but not always a plasma plume) which travels from the target to the substrate expanding along three perpendicular axes. In situ characterization of the plume as it moves and of the angular spread is very important in order to control thin film properties such as uniformity, morphology, and composition [l]. During PLD a background gas such as 0, or N, may be added to supplement a loss or to aid incorporation in the growing film. It is known, both from gas dynamics [2] and experiment [3], that plume expansion has a different trend in vacuum than in the presence of background gas. Moreover, angular profiles of film thickness and composition (i.e. stoichiometry) may be altered if a multicomponent target is being ablated [4]. The overall process has been interpreted in terms of Knudsen layer formation very close to the target surface, followed by an unsteady adiabatic expansion [5,6]. Several experimental methods have been used to measure the plume angular distribution. These are classified as film-based or probe-based [I]. The probe-methods analyze
* Corresponding author. Fax +39 6 490324, Tel. -I-39 6 49913307.
the high resolution optical emission or absorption of the laser plume. Ion probes have also been utilized to probe the expanding plume [7]. The film-based methods depend on determining ex situ the thickness profile of the deposited thin film. The present work deals with a direct method of probing the expanding plume by a three-dimensional fast photographic technique. The method is useful for studying plume dynamics during in situ pulsed laser deposition of a wide variety of materials: metals, semiconductors, and high T, superconductors. The effect of the laser parameters (spot size, spot shape, and fluence) has been investigated by observing temporally and spatially the expanding plume.
2. Experimental
Laser ablation was carried out in a high vacuum chamber equipped with several view ports [3]. Vacuum operating conditions were better than 1O-3 Pa. A rotating target (8 rpm) was irradiated at normal incidence either by an excimer laser (A = 248 nm, r= 18 ns, fluence 3-12 J/cm’, rectangular spot size 0.5-2.4 mm by 1.0 mm) or by a NQYAG laser (A = 532 nm, r = 6 ns, fluence 3-6 J/cm2, circular spot size 0.7 mm’). The larger dimension of the rectangular spot could be reduced by inserting a slit between the laser and the target. Independently of this, the fluence could be varied by tilting a coated fused silica plate through which the beam passed. A gated intensified
0168-583X/%/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved PII SO168-583X(96)00056-0
258
A. Mele et al./Nucl.
Instr. and Meth. in Phys. Res. B II6 (1996) 257-261
charge coupled device (CCD) camera (EG & G Model 1530-PUV+ LCI-I, 512X 512 pixels, 18 bits dynamic range) with spatial and temporal resolution of 1.50 pm and 5 ns was used to acquire the plume images for a single shot. Orthogonal lateral views of the plume expansion were made by recording the overall visible emission (400800 nm). A 50 mm Nikkor lens could observe the plume expansion over a 6 cm region starting at the target. The plasma emission intensity of the intensified CCD photographs was plotted in terms of 10% step contours. The targets were Cu disks, Sn disks, as well as Pbe,,Bi,,,Sr,Ca,Cu,O. (PbBSCCO) and SnSe pellets which were made from pressed powders.
3. Results and discussion The expansion of the plasma plumes obtained by irradiating with the rectangular beam of the excimer laser was measured from the intensified CCD photographs. The plumes were analyzed in three dimensions by lateral views in both the xz and yz planes. z is the axis normal ,to the target, whereas x is normally the larger and y is normally the smaller dimension of the laser spot (Fig. 1). The plumes were found to have an ellipsoidal shape with three distinct axes. As in previous.work [8] the lateral expansions (x and y) were found to be “rotated” by 90“ with respect to the laser spot (Fig. 1). We hasten to add, however, that the “rotation” was not a formal rotation in which the plume passed continuously from one orientation to the other but rather a situation in which (referring to Fig. 1) the transport of particles was more effective in the y than in the x direction. This has been confirmed by numerical solution of the gas-dynamic equations in two dimensions [8,9] where we find an unrotated situation at very short times but a well defined symmetry change at longer times. Tentatively, the effect can be explained in terms of the directionality of the flow such that transport normal to a side is favored over transport from region near a comer because it is less divergent. The effect has been confirmed for square, rectangular, and triangular laser spots [9,10]. The materials examined, namely Cu, Sn, SnSe and PbBSCCO, have all shown very similar behaviour. A typical example is shown in Fig. 2 for plumes generated from Cu and PbBSCCO targets at 600 ns delay time viewed from two lateral axes at 90” with respect to the laser spot as in Fig. 1. It can be seen that both plumes have an ellipsoidal shape, that for Cu having dimensions x = 5 mm, y = 12 mm, Z= 12 mm and that for PbBSCCO having dimensions x = 12 mm, y = 17 mm, z = 21 mm. A correlation between the laser spot shape and the lateral spread of the luminous material is indicated. According to the gas-dynamics calculations [S,9] the role of the spot shape in affecting the shape of the plume begins in the period immediately after particle release when the lateral
expansion is relatively slight. This was also confirmed experimentally from photographs at very short delay times when the plume was close to the target (50-60 ns). The shape changes would, by necessity, cease when the particles no longer interacted according to gas-dynamics but went into free flight. We have assumed that the light emitting particles in the plume are representative of all materials ejected, i.e. that the emitting and non-emitting states have similar velocities. A detailed analysis of the effect of spot size on the plume generated from Cu is shown in Fig. 3. A set of three density contours, obtained by varying one dimension of the rectangular spot and maintaining the other fixed, always at the same fluence (12 J/cm2), is shown. The contours of these frames depict the change of the plume viewed in the xz. plane, i.e. the plane corresponding to the reduced dimension of the spot (Fig. 1). The plume shape viewed in the yz plane, i.e. the plane corresponding to the unreduced dimension y = 1 mm, remained the same (12 + 1 mm). From this sequence it may be observed that by reducing the larger dimension from 2.0 to 0.5 mm the lateral spread increases. These data confirm previous findings that film thickness profiles become less forward peaked as the spot dimension diminishes [4]. Results for Cu of the type seen in Fig. 3 are plotted in Fig. 4a in terms of the parameter p of the angular distribution fl f3) a cos pl.?,which is a reasonable approximation to describe the plume shape in three-dimensional space [l]. In this case 0 is measured on the 10% intensity contour. The plot shows the trend of p when the large (x) dimension of the rectangular laser beam is diminished from 2.4 to 0.5 at the same fluence of 12 J/cm’. The data fit a linear plot with the tendency to a value of 0 5 p I 1 for small spot sizes. It should be noted that for different time delays from the laser pulse and thus for various distances from the target, the plumes have similar values of p, i.e. the shape
/-\ I/
EXPANDING PLUME \
Fig. 1. Solid: sketch showing the lateral (xy) axes of a rectangular 1ase.r spot appropriate to a laser pulse directed at normal incidence towards a target. Dashed: approximate xy symmetry of the expanding plume, showing that the larger side of the spot becomes the smaller side of the plume.
A. Mele et al./Nucl.
Instr. and Meth. in Phys. Res. B 116 (1996) 257-261
is conserved. The effect of laser fluence on the angular distribution in the xz plane of the plume generated from Cu is reported in Fig. 4b. A marked decrease of p from 9 to 5 as a result of varying the fluence from 12 to 6 J/cm2 is found. Similar behaviour is observed also for a SnSe target. This interesting result indicates that the plume has a tendency to become symmetrical along the three axes x, y, z when the fluence of the laser beam is decreased (p = 1). At the same time the lateral spread increases. A broadened distribution is typical at low fluence also for other systems [ 1,7]. Experiments with a circular beam from a Nd-YAG laser (532 nm) show that, at a laser fluence of 12 J/cm’, the plume expansion is ellipsoidal and symmetrical in the xz and yz planes with the axes z > x = y. This could be expected from the geometry of the spot. The same evolution of plume shape found for a rectangular spot as a
259
function of fluence is observed. Thus, the plume tends to become symmetrical along the three axes x, y, z when the fluence of the laser beam is decreased. The nearly spherical shape of the plume with both a rectangular and a spherical spot suggests an effusive regime corresponding to a removal of material by normal vaporization (as distinct from explosive boiling [11,12]). This suggests that within distances where the substrate is usually located, that is about 20-30 mm, the angular distribution will be independent of distance. The analysis of the plume evolution of all materials studied, namely Cu, Sn, SnSe, and PbBSCCO, has confirmed this result. It is apparent from the contours of the rectangular spot of Fig. 3 and from the circular spot (to be published) that the spot size and shape of the laser beam have a marked effect on the plume symmetry. This effect is very general for all materials and persists at long distances from the
Fig. 2. Overall emission from Cu ((a) and (b)) and PbBSCCO(cc) and (d>) targets laser-irradiated at 248 mn. The light intensity is shown in terms of two-dimensional 10% density contours (smoothened). The maxima are indicated in (b) to (d) as a solid circle. Details: delay time 600 ns, exposure time 100 ns, fluence 12 J/cm*, rectangular laser spot (2.4 X 1 mm*). The views, as indicated in each case, are either in the xz or the yz plane of the spot. Grid step: 2 mm.
A. Mele et al./Nucl.
260
Instr. and Meth. in Phys. Res. B 116 (1996) 257-261
target. A closer examination of the results of the present investigation shows an additional effect which is due to the energy distribution within the laser spot. It is known from the profiles of the energy maps, either of an excimer laser with a rectangular beam or of a Nd-YAG laser with a circular beam, that the energy for both is higher in the middle than in the periphery of the beam. This has the effect of a non-uniform removal of material from the target. This view is supported from a comparison of the plume contours obtained by a square spot from an excimer laser beam by reducing the larger lateral dimension down to that of the smaller one. A homogeneous distribution of the energy on both sides would provide an identical plume symmetry as observed for a circular spot [13]. However, this is not the case for the excimer laser because the unreduced small dimension side has kept the energy distri-
TARGET
1 (4
600
TARGET
ns
I(b)]
101
I
x DIMENSION 10
I
OF LASER
I
1
SPOT
(mm)
,
L
hv(248nm)--Cu
I
1
TARGET
1
Fig. 3. Set of three 10% density contours (smoothened) from photographs of a Cu disk laser-irradiated at 248 nm. Details: delay time 600 ns, exposure time 100 ns, fluence 12 J/cm’. The rectangular laser spot dimensions are 2.0X 1.0 mm* (a), 1.0X 1.0 mm* (b), 0.5 X 1.0 mm* (cl. The views are in the w plane of the spot. Grid step: 2 mm.
6
8 ENERGY
L
I
12
10 FLUENCE
J
(J/cm*)
Fig. 4. Angular distribution in terms of cos@ from data for a laser-irradiated Cu disk as in Fig. 3. (a) Plot of p versus a varying laser spot dimension (x). the other dimension remaining fixed at 1.0 mm. (b) Plot of p versus fluence. In the case of(b) the spot dimensions are 2.4X 1.0 mm*. The views are in the xz plane of the spot for both (a) and (b).
A. Mele et al./Nucl.
600
.z
1 TARGET
Instr. and Meth. in Phys. Res. B 116 (1996) 257-261
ns
(
I600
TARGET
261
ns 1
i(b)I
Fig. 5. Set of two-dimensional 10% density contours (smoothened) from photographs of a Cu disk laser-irradiated at 248 nm. The maxima are indicated as a solid circle. Details: delay time 600 ns, exposure time 100 ns, fluence 12 J/cm*, square laser spot (0.8 X 0.8 mm2). The views, as indicated in each case, are either in the xz or the yz plane of the spot. Grid step: 2 mm.
bution whereas
the other side acquires
a more homoge-
neous distribution when the original dimension is reduced. The latter distribution produces the plume of Fig. 5a in the ZJ plane, the unchanged side that of Fig. 5b in the zy plane. The angular distribution of the expanding plume is a direct measurement of the particles velocities in a three-dimensional space. From the results of Figs. 2 and 3 it appears that the highest velocity is normal to the target and that the lowest velocities are for lateral expansion. Various factors may lead to such behaviour. For example, it may be the result of complex phenomena occurring by the interaction of the laser beam with a solid surface, such that a number of layers of the material are ejected and a pressure profile develops at the interface which characterizes the flow of material. This would be especially true at higher fluences when explosive rather than normal vaporization can be expected [11,12]. The expansion n&ma1 and parallel to the target evidently acquires different pressure gradients [ 13,141. The highest gradient is expected to be along the target normal (z axis) and the lowest gradients along the x and y axes. The gradients themselves are in turn effected by the size and shape of the laser beam. Finally we note that conditions of normal vaporization lead to low particle densities and the distributions are characterized by low powers of cos 6. The flow of the particles under conditions of explosive boiling is characterized by high number densities. The flow conditions are then characterized by a Knudsen layer, where the distribution is discontinuous [ 11,121. This boundary layer is followed fist by an unsteady adiabatic expansion and finally by free flight. Depending on the persistance of adiabatic
expansion phase, the angular distributions show strong forward peaking of the type cos% with p >> 1.
References [I] K.L. Saenger, in Pulsed Laser Deposition of Thin Films (than. 71, eds. D.B. Chrisev and G.K. Hubler (Wiley, New York, 1994) p. 199. 01 R. Kelly, A. Miotello, B. Braren, A. Gupta and K. Casey, Nucl. Instr. and Meth B 65 (1992) 187. [31 T. Di Palma, S. Orlando, A. Giardini Guidoni, A.J. Paul, J.W. Hastie and A. Mele, Appl. Surf. Sci. 86 (1995) 68. [41 R,E. Muenchausen, K.M. Hubbard, S. Foltyn, R.G. Esteer, N.S. Nogar and C. Jenkins, Appl. Phys. Lett. 56 (1990) 578. 151 R. Kelly and R.W. Dreyfus, Surf. Sci. 198 (1988) 263. t61 R. Kelly and A. Miotello, Appl. Phys. B 57 (1993) 145. [iI D.B. Geohegan, in: Pulsed Laser Deposition of Thin Films (chap. 51, eds. D.B. Chrisey and G.K. Hubler (Wiley, New York, 1994) p. 115. k4 A. Mele, A. Giardmi, R. Kelly, A. Miotello, S. Orlando and R. Teghil, Appl. Surf. Sci. 96-98 (1996) 102. 191 A. Miotello and R. Kelly, Appl. Phys. Lett. 61 (1992) 2784. 1101 R. Kelly and A. Miotello, Nucl. Jnstr. and Meth. B 91 (1994) 682. illI R. Kelly and A. Miotello, Appl. Surf. Sci. 96-98 (1996) 205. [I21 R. Kelly, A. Miotello, A. Mele, A. Giardini Guidoni, J.W. Hastie, P.K. Schenck and H. Okabe, Surf. Sci. (submitted). [I31 R.W. Ohse, J.F. Bebelot, A. Frezzotti, K.A. Long and J. Magill, High Temp. High Press. 2 (1980) 537. [I41 N.F. Ramsey, Molecular Beams (Clarendon Press, Oxford, 1956).