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Laser-induced forward transfer of aluminium
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Applied Surface North-Holland
Science 52 (1991) 303-309
Laser-induced
applied surface science
forward transfer of aluminium
V. Schultze Physikalisch-Technisches
Institut, Helmholtzweg
4, D(O)-6900
Jena, Germany
and M. Wagner Institut fiir Festkiirperphysik, Received
Friedrich-Schiller-Uniuersitiit
31 May 1991; accepted
for publication
Jena, Max- Wien-Platz I, D(O)-6900
17 August
Jena, Germany
1991
The laser-induced forward transfer (LIFT) of aluminium is experimentally investigated for aluminium target thicknesses up to some pm. The aluminium is transferred to glazed ceramics and to silicon substrates with and without oxidized surface, respectively. Two types of laser beam sources are used: a Nd:YAG laser with a Gaussian beam profile and a Nd:glass laser system providing a homogenized flat top profile. The resulting deposition is determined by the removal mode of the Al from the target and by the interaction of the molten Al with the substrate. In a first removal mode (low laser intensities and/or thin targets) where melting through of the target precedes the onset of vaporization at the target front side the material can easily be transferred to the substrate. In a second mode (high laser intensities and/or thick targets) the blow-off process is characterized by higher vapor pressures which leads to disturbed deposits. The interaction of impacting material with the substrate causes the formation of droplets on oxidized surfaces due to the high surface tension of liquid metals.
1. Introduction
In the laser-induced forward transfer (LIFT) technique a thin film which is on the back side of an optically transparent support is irradiated through this support with a single laser pulse. So the film material is removed and transferred to a substrate which is on the opposite end of the support (fig. 1).
transparent support metal
target
Fig. 1. Principle picture of the laser-induced forward (LIFT) of a metal film onto a substrate. 0169-4332/91/$03.50
0 1991 - Elsevier
Science
film
transfer
Publishers
The whole process of material Kernoval from the support, the travel between support and substrate, and the landing on the substrate influences the final appearance of the deposit. The removal process was first described by Adrian and co-workers [l-3]. Corresponding to their model the LIFT process consists of the following sequence of events: (1) the laser pulse heats the front surface of the film until it melts; (2) the melt front propagates through the film until it reaches the back surface; (3) at about this time the front surface is superheated or close to the boiling point; and (4) at or close to meltthrough the metal vapor pressure at the front surface propels the molten metal film to the substrate. The theoretical descriptions made by Adrian and co-workers additionally showed that the time required for the melt front to reach the back side of the metal film - in the following called “target”
B.V. All rights reserved
304
V. Schultze, M. Wagner / Laser-induced forward transfer of aluminium
- increases with target thickness. Moreover, for higher laser intensities the frontside of thicker targets heats up above the boiling temperature before the target is molten through. Experimentally, in these papers the different removal modes of the target material are shown by their effect on .the appearance of the deposits on the substrate. The blow-off process itself was studied in more detail for Au and Cr targets by Baseman and Andreshak [4] and for Al targets by Schultze and Wagner [5]. Finally the removal process will affect the print patterns on the substrate. Qualitatively this was shown by Adrian and co-workers for the LIFT of Cu [l-3] and by Fogarassy et al. for YBaCuO and BiSrCaCuO [61. Starting from our quantitative assignment of the different removal modes to definite laser intensities for different target thicknesses [.5] the present paper describes the dependence of the appearance of deposited areas on the removal process for Al. In addition, the effect of the interaction with different substrates is shown. All experiments were performed at a laser wavelength of A = 1.06 pm using a Q-switched Nd:YAG laser with a Gaussian beam profile and a special Nd:glass laser system providing a homogenized flat top beam profile.
2. Experimental
conditions
The Al targets were deposited on 1.5 mm thick glass supports by conventional high rate magnetron sputtering with a thickness of d = 0.8, 2.7 and 5.7 pm, respectively. Prior to the Al deposition in the same vacuum cycle an about 40 nm thick oxygen-containing NiCr layer was deposited as an adhesion and antireflexion layer. The laser irradiation was carried out with two different systems. For the investigation with the Gaussian laser intensity profile a Q-switched Nd: YAG laser with a pulse length of T = 120 ns (FWHM) was used in TEM,, mode. The laser beam was focused with a long distance objective or a single lens with 15.6 or 62 mm focal length, respectively. By additional defocusing the radius
of the Gaussian laser beam was varied in the range of rg = 14-300 pm. In a second series of experiments the homogenized beam of a Nd:glass laser system (A = 1.06 pm, pulse width r =:40 ns) was used. The laser system provided a flat top laser beam of rectangular cross section by applying a high-performance beam shaping and homogenizing optical system [7]. The residual intensity fluctuations across the irradiated target are less than &2%-3% on a macroscopic as well as on the microscopic scale. Therefore, the melt fronts produced by the laser irradiation are extremely smooth [S]. Target and substrate were mounted together on a step-motor-driven x-y table. All experiments were performed in air. Target and substrate were joined to each other by vacuum sucking. So target and substrate had a maximum distance of some micrometers. The substrates we normally used were 630 pm thick Al,O, ceramic slices with a glaze of about 75 pm thickness. The glaze consists of different oxides with SiO, as the main component and Al,O,, BaO, PbO and CaO as the most important aggregates. In some complementary experiments we used other substrates, namely bare 4 inch Si wafers and wafers with a thermally grown SiO, surface of about 1 pm thickness.
3. Results and discussion The irradiation of absorbing solid targets through a transparent support with the objective of a laser-induced forward transfer (LIFT) of the target at first leads to phase changes in the target. The thermal parameters of the target material determine the phase front dynamics in the target. The interaction of the phase changes, namely the movement of the melt front through the target and the onset of vaporization at the target/support interface are the principal factors determining the removal modes of the target material. For a given material the phase front dynamics can be influenced by the target thickness and the characteristics of the laser irradiation. Fig. 2 shows the characteristic times for the removal process, calculated with the heat balance
I/ Schultze, M. Wagner / Laser-induced forward transfer of aluminium
Fig. 2. Dependence of the burn-through time tgT and the moment tV of beginning vaporization at the front side of Al targets with thickness d on the incident laser intensity I. Higher front surface temperatures Tv and corresponding higher vapor pressures pa (compared to the normal pressure pa of 1 atm) are included.
integral method for a smooth laser beam profile [5]. The time tar is needed for the burn-through of the target (e.g. at this time after the beginning of the laser pulse the melt front reaches the back
305
surface of the target). At the time tv vaporization starts at the front side of the target. Both times are calculated for different target thicknesses. Fig. 2 shows that for a thin target with a thickness of only 0.8 pm and a moderate laser intensity Z the time t,, is the smallest characteristic time. So at first the melt front reaches the back side of the target. Later, namely at the time t,, vaporization starts at the target/support interface. Our calculations showed [5] that in this first removal mode with t,, < t, a vapor pressure of about 1 atm suffices to propel the liquid target material away from the support. The threshold intensity for the removal in this mode is that intensity which is high enough to achieve vaporization at the target/ support interface within the laser pulse, i.e. when the vaporization time t, equals the laser pulse length r. Gaussian laser intensity profile means decreasing intensity away from the beam center. Because the heat diffusion length in Al is small (only some micrometers for laser pulse lengths T of about 120 ns) zones with dimensions of some micrometers act independently of each other in the LIFT process. Thus, the removal essentially reproduces the spatial shape of the laser beam. Therefore, in the normal LIFT mode (i.e. tar < tv) vaporization starts within an area being smaller than the molten range (cf. the dependence of t,, and tv
Fig. 3. SEM pictures of an AI target with a thickness of d = 0.8 Km (left-hand side) and a glazed ceramic substrate side) after LIFT with a Gaussian laser beam profile and a laser intensity of I = 2.1 x 10’ W/cm*.
(right-hand
306
Fig. 4. SEM pictures
V. Schultze, M. Wagner / Laser-induced forward transfer of aluminium
of an Al target with a thickness of d = 5.7 pm (left-hand side) and a glazed ceramic substrate after LIFT with a Gaussian laser beam profile and a laser intensity of I = 7.8 x lo7 W/cm2.
on the laser intensity in fig. 2). Consequently, the target material will completely be removed within a circle having a radius determined by the threshold intensity for vaporization. The vapor pressure needed for the removal is always on the order of 1 atm. Fig. 3 shows that this “soft” removal is also reflected by the pattern of the material deposited on the substrate. The material is transferred from the target to the glazed ceramic substrate without severe modifications. The removed area in the target and the size of the deposit on the substrate exhibit equal dimensions. Higher laser intensities or thicker targets involve the case where the target has not been completely molten before the boiling point at normal pressure at the front side of the target is reached (fig. 2). Our experimental investigations concerning the removal process [5] showed that in this mode the target temperature will further increase until burn-through occurs. During this time the front surface of the target becomes increasingly superheated. Thus, higher vapor pressures arise. This effect is indicated in fig. 2 by the corresponding tv curves. For a Gaussian laser beam this behavior has the consequence that only within the radius where burn-through is reached molten material can be pushed away, but the propelling vapor pressure is
(right-hand
side)
very high. Because this pressure also acts in radial direction and is highest in the spot center (corresponding to the laser intensity profile) the removed material is pushed to the edges of the deposited range on the substrate (fig. 4). During this outward-directed motion some portion of the melt resolidifies. In this second mode of the LIFT process the increase of vapor pressure with laser intensity (cf.
Fig. 5. SEM picture of a glazed ceramic substrate after LIFT of an Al target with a thickness of d = 2.7 pm with a flat top laser beam profile and a laser intensity of I = 1.5 x lo8 W/cm’.
V. Schultze, M. Wagner / Laser-induced forward transfer of aluminium
fig. 2) also causes increasing dimensions of the patterns on the substrate. For example, for the removal in a target of 5.7 ,um thickness the ratio of the diameter of the deposit on the substrate to that of the removed area in the target varies from N 1 for the threshold intensity for the removal of the Al (i.e. the laser intensity when the burnthrough time t,, equals the laser pulse length 7) to 2.5 for ten times this threshold value. In conclusion, the interaction of the position of the melt front and the vaporizing zone in the target - not only in vertical but also lateral direction - causes different distributions of the de-
307
posit on the substrate. Especially in the second LIFT mode the material is preferentially located at the edges of the forward-transferred range. The SEM pictures of the substrate show that (additionally to the distribution of material which is caused by the removal process) the deposited material tends to form droplets on the glazed ceramic substrates. This process was investigated separately by using a homogenized flat top laser beam profile. In this case no lateral action of the vapor pressure will be superimposed to the normal component. Fig. 5 illustrates the result of such a LIFT
Fig. 6. SEM pictures of a glazed ceramic substrate after LIFT of a lattice-like structured Al target with a thickness of d = 5.7 pm. The LIFT was performed with a flat top laser beam profile and a laser intensity of I = 7.4 X 10’ W/cm2 (top) and I = 9.1 X 10’ W/cm2 (bottom).
308
V. Schultze, M. Wagner / Laser-induced forward transfer of aluminium
process. The figure clearly reveals the droplet formation on the glazed ceramic substrate. This effect might be a serious problem for every kind of LIFT of metals because of the high surface tension of all liquid metals [9,103. Two ways were tried to overcome the problem of droplet formation of Al deposited with the LIFT method. At first we used other substrate materials because - corresponding to Young’s equation - the free surface energy of the substrate also influences the behavior of a liquid material on the corresponding substrate. In fact, Al can be transferred to bare Si substrates without any remarkable effect due to the interaction between Al and Si, whereas oxidized Si substrates again lead to a droplet formation of the Al. However, when the Al shall be used for any electrical purpose, e.g. for a connection on a chip, a dielectric substrate surface is necessary for electrical isolation. Here a solution could be found by an operation at a laser intensity which only weakly exceeds the threshold value for the removal in the target. This can be seen in fig. 6. The figure shows the result of a LIFT of an Al target which was previously lattice-like structured in a photolithographic cycle with crossing stripes of 22 pm width and a lattice constant of 130 pm. This lattice with a thickness of 5.7 pm was irradiated with the flat top intensity profile. For a laser intensity of I = 6.8 x 10’ W/cm2 no removal could be observed. For I = 9.1 x 10’ W/cm* the target structure is already obscured on the glazed ceramic substrate by the droplet formation. For the LIFT just at the threshold intensity Z = 7.4 X 10’ W/cm* the material obviously will possess little excess energy above the melting point. Thus, it freezes in the form it hits the substrate. So there is no time to reach the hydrodynamic equilibrium and to form droplets. So in a narrow range of laser intensity a target structure can be transferred without droplet formation to a substrate with an oxide on its surface. However, it should be underlined that already the target has to be patterned in the finally desired layout of the deposit and that the LIFT must be performed with an even laser profile. Only then laterally different vapor pressures at the target/
support interface, which would lead to a change of the patterning during the removal, can be avoided. Furthermore, the laser intensity has exactly to be tuned to the removal threshold.
4. Summary The appearance of deposits produced by the LIFT technique depends on the removal process of the material in the target as well as on the interaction of the incoming molten material with the substrate. The removal process manifests itself in the distribution of the material on the substrate. In a first removal mode (low laser intensities and/or thin targets) where melting through of the target precedes the onset of vaporization the target material is transferred to the substrate without significant modifications caused by the removal process. In the second mode (high laser intensities and/or thick targets) where the temperature at the target/support interface exceeds the vaporization temperature at normal pressure before the target is molten through the deposition results become worse. The higher vapor pressure at the target/ support interface pushes an increasing amount of material to the edges of the deposited area at the expense of its inner range. The interaction of the incoming material with a substrate with oxidic surface leads to droplet formation because of the high surface tension of liquid metals. This can be suppressed when the removal proceeds just at or slightly above the threshold intensity for a removal. In this case the material possesses only little energy above the melting point and it freezes before the hydrodynamic equilibrium is reached, i.e. before the melt is able to form droplets. Prerequisite to such a LIFT process is a very exactly tuned laser intensity and a smooth intensity profile.
Acknowledgements
The authors whish to thank Dr. P. Dittrich and Mr. H. Ziehl for the manufacturing of the SEM pictures.
K Schultze, M. Wagner / Laser-induced forward transfer of aluminium
This work was supported by the Keramische Werke Hermsdorf GmbH, Tridelta AG.
References [l] J. Bohandy, B.F. Kim and F.J. Adrian, J. Appl. Phys. 60 (1986) 1538. [2] F.J. Adrian, J. Bohandy, B.F. Kim, A.N. Jette and R. Thompson, J. Vat. Sci. Technol. B 5 (1987) 1490. [3] J. Bohandy, B.F. Kim, F.J. Adrian and A.N. Jette, J. Appl. Phys. 63 (1988) 1158.
309
and J.C. Andreshak, Mater. Res. Sot. [41 R.J. Baseman Symp. Proc. 100 (1988) 627. Dl V. Schultze and M. Wagner, Appl. Phys. A 53 (1991) 241. [61E. Fogarassy, C. Fuchs, F. Kerheve, G. Hauchecorn and J. Perriere, J. Mater. Res. 4 (1989) 1082. [71 M. Wagner, H.D. Geiler and D. Wolff, Meas. Sci. Technol. 1 (1990) 1193. KYM. Wagner, A. Witzmann and H.D. Geiler, Appl. Surf. Sci. 43 (1989) 260. Dl R.N. Lyon, Ed., Liquid Metals Handbook (US Govt. Printing Off., Washington, DC, 1952) p. 41. DO1C.J. Smithells, Metals Reference Book, Vol. II, 2nd ed. (Butterworths, London, 1955) p. 639.
Applied Surface North-Holland
Science 52 (1991) 303-309
Laser-induced
applied surface science
forward transfer of aluminium
V. Schultze Physikalisch-Technisches
Institut, Helmholtzweg
4, D(O)-6900
Jena, Germany
and M. Wagner Institut fiir Festkiirperphysik, Received
Friedrich-Schiller-Uniuersitiit
31 May 1991; accepted
for publication
Jena, Max- Wien-Platz I, D(O)-6900
17 August
Jena, Germany
1991
The laser-induced forward transfer (LIFT) of aluminium is experimentally investigated for aluminium target thicknesses up to some pm. The aluminium is transferred to glazed ceramics and to silicon substrates with and without oxidized surface, respectively. Two types of laser beam sources are used: a Nd:YAG laser with a Gaussian beam profile and a Nd:glass laser system providing a homogenized flat top profile. The resulting deposition is determined by the removal mode of the Al from the target and by the interaction of the molten Al with the substrate. In a first removal mode (low laser intensities and/or thin targets) where melting through of the target precedes the onset of vaporization at the target front side the material can easily be transferred to the substrate. In a second mode (high laser intensities and/or thick targets) the blow-off process is characterized by higher vapor pressures which leads to disturbed deposits. The interaction of impacting material with the substrate causes the formation of droplets on oxidized surfaces due to the high surface tension of liquid metals.
1. Introduction
In the laser-induced forward transfer (LIFT) technique a thin film which is on the back side of an optically transparent support is irradiated through this support with a single laser pulse. So the film material is removed and transferred to a substrate which is on the opposite end of the support (fig. 1).
transparent support metal
target
Fig. 1. Principle picture of the laser-induced forward (LIFT) of a metal film onto a substrate. 0169-4332/91/$03.50
0 1991 - Elsevier
Science
film
transfer
Publishers
The whole process of material Kernoval from the support, the travel between support and substrate, and the landing on the substrate influences the final appearance of the deposit. The removal process was first described by Adrian and co-workers [l-3]. Corresponding to their model the LIFT process consists of the following sequence of events: (1) the laser pulse heats the front surface of the film until it melts; (2) the melt front propagates through the film until it reaches the back surface; (3) at about this time the front surface is superheated or close to the boiling point; and (4) at or close to meltthrough the metal vapor pressure at the front surface propels the molten metal film to the substrate. The theoretical descriptions made by Adrian and co-workers additionally showed that the time required for the melt front to reach the back side of the metal film - in the following called “target”
B.V. All rights reserved
304
V. Schultze, M. Wagner / Laser-induced forward transfer of aluminium
- increases with target thickness. Moreover, for higher laser intensities the frontside of thicker targets heats up above the boiling temperature before the target is molten through. Experimentally, in these papers the different removal modes of the target material are shown by their effect on .the appearance of the deposits on the substrate. The blow-off process itself was studied in more detail for Au and Cr targets by Baseman and Andreshak [4] and for Al targets by Schultze and Wagner [5]. Finally the removal process will affect the print patterns on the substrate. Qualitatively this was shown by Adrian and co-workers for the LIFT of Cu [l-3] and by Fogarassy et al. for YBaCuO and BiSrCaCuO [61. Starting from our quantitative assignment of the different removal modes to definite laser intensities for different target thicknesses [.5] the present paper describes the dependence of the appearance of deposited areas on the removal process for Al. In addition, the effect of the interaction with different substrates is shown. All experiments were performed at a laser wavelength of A = 1.06 pm using a Q-switched Nd:YAG laser with a Gaussian beam profile and a special Nd:glass laser system providing a homogenized flat top beam profile.
2. Experimental
conditions
The Al targets were deposited on 1.5 mm thick glass supports by conventional high rate magnetron sputtering with a thickness of d = 0.8, 2.7 and 5.7 pm, respectively. Prior to the Al deposition in the same vacuum cycle an about 40 nm thick oxygen-containing NiCr layer was deposited as an adhesion and antireflexion layer. The laser irradiation was carried out with two different systems. For the investigation with the Gaussian laser intensity profile a Q-switched Nd: YAG laser with a pulse length of T = 120 ns (FWHM) was used in TEM,, mode. The laser beam was focused with a long distance objective or a single lens with 15.6 or 62 mm focal length, respectively. By additional defocusing the radius
of the Gaussian laser beam was varied in the range of rg = 14-300 pm. In a second series of experiments the homogenized beam of a Nd:glass laser system (A = 1.06 pm, pulse width r =:40 ns) was used. The laser system provided a flat top laser beam of rectangular cross section by applying a high-performance beam shaping and homogenizing optical system [7]. The residual intensity fluctuations across the irradiated target are less than &2%-3% on a macroscopic as well as on the microscopic scale. Therefore, the melt fronts produced by the laser irradiation are extremely smooth [S]. Target and substrate were mounted together on a step-motor-driven x-y table. All experiments were performed in air. Target and substrate were joined to each other by vacuum sucking. So target and substrate had a maximum distance of some micrometers. The substrates we normally used were 630 pm thick Al,O, ceramic slices with a glaze of about 75 pm thickness. The glaze consists of different oxides with SiO, as the main component and Al,O,, BaO, PbO and CaO as the most important aggregates. In some complementary experiments we used other substrates, namely bare 4 inch Si wafers and wafers with a thermally grown SiO, surface of about 1 pm thickness.
3. Results and discussion The irradiation of absorbing solid targets through a transparent support with the objective of a laser-induced forward transfer (LIFT) of the target at first leads to phase changes in the target. The thermal parameters of the target material determine the phase front dynamics in the target. The interaction of the phase changes, namely the movement of the melt front through the target and the onset of vaporization at the target/support interface are the principal factors determining the removal modes of the target material. For a given material the phase front dynamics can be influenced by the target thickness and the characteristics of the laser irradiation. Fig. 2 shows the characteristic times for the removal process, calculated with the heat balance
I/ Schultze, M. Wagner / Laser-induced forward transfer of aluminium
Fig. 2. Dependence of the burn-through time tgT and the moment tV of beginning vaporization at the front side of Al targets with thickness d on the incident laser intensity I. Higher front surface temperatures Tv and corresponding higher vapor pressures pa (compared to the normal pressure pa of 1 atm) are included.
integral method for a smooth laser beam profile [5]. The time tar is needed for the burn-through of the target (e.g. at this time after the beginning of the laser pulse the melt front reaches the back
305
surface of the target). At the time tv vaporization starts at the front side of the target. Both times are calculated for different target thicknesses. Fig. 2 shows that for a thin target with a thickness of only 0.8 pm and a moderate laser intensity Z the time t,, is the smallest characteristic time. So at first the melt front reaches the back side of the target. Later, namely at the time t,, vaporization starts at the target/support interface. Our calculations showed [5] that in this first removal mode with t,, < t, a vapor pressure of about 1 atm suffices to propel the liquid target material away from the support. The threshold intensity for the removal in this mode is that intensity which is high enough to achieve vaporization at the target/ support interface within the laser pulse, i.e. when the vaporization time t, equals the laser pulse length r. Gaussian laser intensity profile means decreasing intensity away from the beam center. Because the heat diffusion length in Al is small (only some micrometers for laser pulse lengths T of about 120 ns) zones with dimensions of some micrometers act independently of each other in the LIFT process. Thus, the removal essentially reproduces the spatial shape of the laser beam. Therefore, in the normal LIFT mode (i.e. tar < tv) vaporization starts within an area being smaller than the molten range (cf. the dependence of t,, and tv
Fig. 3. SEM pictures of an AI target with a thickness of d = 0.8 Km (left-hand side) and a glazed ceramic substrate side) after LIFT with a Gaussian laser beam profile and a laser intensity of I = 2.1 x 10’ W/cm*.
(right-hand
306
Fig. 4. SEM pictures
V. Schultze, M. Wagner / Laser-induced forward transfer of aluminium
of an Al target with a thickness of d = 5.7 pm (left-hand side) and a glazed ceramic substrate after LIFT with a Gaussian laser beam profile and a laser intensity of I = 7.8 x lo7 W/cm2.
on the laser intensity in fig. 2). Consequently, the target material will completely be removed within a circle having a radius determined by the threshold intensity for vaporization. The vapor pressure needed for the removal is always on the order of 1 atm. Fig. 3 shows that this “soft” removal is also reflected by the pattern of the material deposited on the substrate. The material is transferred from the target to the glazed ceramic substrate without severe modifications. The removed area in the target and the size of the deposit on the substrate exhibit equal dimensions. Higher laser intensities or thicker targets involve the case where the target has not been completely molten before the boiling point at normal pressure at the front side of the target is reached (fig. 2). Our experimental investigations concerning the removal process [5] showed that in this mode the target temperature will further increase until burn-through occurs. During this time the front surface of the target becomes increasingly superheated. Thus, higher vapor pressures arise. This effect is indicated in fig. 2 by the corresponding tv curves. For a Gaussian laser beam this behavior has the consequence that only within the radius where burn-through is reached molten material can be pushed away, but the propelling vapor pressure is
(right-hand
side)
very high. Because this pressure also acts in radial direction and is highest in the spot center (corresponding to the laser intensity profile) the removed material is pushed to the edges of the deposited range on the substrate (fig. 4). During this outward-directed motion some portion of the melt resolidifies. In this second mode of the LIFT process the increase of vapor pressure with laser intensity (cf.
Fig. 5. SEM picture of a glazed ceramic substrate after LIFT of an Al target with a thickness of d = 2.7 pm with a flat top laser beam profile and a laser intensity of I = 1.5 x lo8 W/cm’.
V. Schultze, M. Wagner / Laser-induced forward transfer of aluminium
fig. 2) also causes increasing dimensions of the patterns on the substrate. For example, for the removal in a target of 5.7 ,um thickness the ratio of the diameter of the deposit on the substrate to that of the removed area in the target varies from N 1 for the threshold intensity for the removal of the Al (i.e. the laser intensity when the burnthrough time t,, equals the laser pulse length 7) to 2.5 for ten times this threshold value. In conclusion, the interaction of the position of the melt front and the vaporizing zone in the target - not only in vertical but also lateral direction - causes different distributions of the de-
307
posit on the substrate. Especially in the second LIFT mode the material is preferentially located at the edges of the forward-transferred range. The SEM pictures of the substrate show that (additionally to the distribution of material which is caused by the removal process) the deposited material tends to form droplets on the glazed ceramic substrates. This process was investigated separately by using a homogenized flat top laser beam profile. In this case no lateral action of the vapor pressure will be superimposed to the normal component. Fig. 5 illustrates the result of such a LIFT
Fig. 6. SEM pictures of a glazed ceramic substrate after LIFT of a lattice-like structured Al target with a thickness of d = 5.7 pm. The LIFT was performed with a flat top laser beam profile and a laser intensity of I = 7.4 X 10’ W/cm2 (top) and I = 9.1 X 10’ W/cm2 (bottom).
308
V. Schultze, M. Wagner / Laser-induced forward transfer of aluminium
process. The figure clearly reveals the droplet formation on the glazed ceramic substrate. This effect might be a serious problem for every kind of LIFT of metals because of the high surface tension of all liquid metals [9,103. Two ways were tried to overcome the problem of droplet formation of Al deposited with the LIFT method. At first we used other substrate materials because - corresponding to Young’s equation - the free surface energy of the substrate also influences the behavior of a liquid material on the corresponding substrate. In fact, Al can be transferred to bare Si substrates without any remarkable effect due to the interaction between Al and Si, whereas oxidized Si substrates again lead to a droplet formation of the Al. However, when the Al shall be used for any electrical purpose, e.g. for a connection on a chip, a dielectric substrate surface is necessary for electrical isolation. Here a solution could be found by an operation at a laser intensity which only weakly exceeds the threshold value for the removal in the target. This can be seen in fig. 6. The figure shows the result of a LIFT of an Al target which was previously lattice-like structured in a photolithographic cycle with crossing stripes of 22 pm width and a lattice constant of 130 pm. This lattice with a thickness of 5.7 pm was irradiated with the flat top intensity profile. For a laser intensity of I = 6.8 x 10’ W/cm2 no removal could be observed. For I = 9.1 x 10’ W/cm* the target structure is already obscured on the glazed ceramic substrate by the droplet formation. For the LIFT just at the threshold intensity Z = 7.4 X 10’ W/cm* the material obviously will possess little excess energy above the melting point. Thus, it freezes in the form it hits the substrate. So there is no time to reach the hydrodynamic equilibrium and to form droplets. So in a narrow range of laser intensity a target structure can be transferred without droplet formation to a substrate with an oxide on its surface. However, it should be underlined that already the target has to be patterned in the finally desired layout of the deposit and that the LIFT must be performed with an even laser profile. Only then laterally different vapor pressures at the target/
support interface, which would lead to a change of the patterning during the removal, can be avoided. Furthermore, the laser intensity has exactly to be tuned to the removal threshold.
4. Summary The appearance of deposits produced by the LIFT technique depends on the removal process of the material in the target as well as on the interaction of the incoming molten material with the substrate. The removal process manifests itself in the distribution of the material on the substrate. In a first removal mode (low laser intensities and/or thin targets) where melting through of the target precedes the onset of vaporization the target material is transferred to the substrate without significant modifications caused by the removal process. In the second mode (high laser intensities and/or thick targets) where the temperature at the target/support interface exceeds the vaporization temperature at normal pressure before the target is molten through the deposition results become worse. The higher vapor pressure at the target/ support interface pushes an increasing amount of material to the edges of the deposited area at the expense of its inner range. The interaction of the incoming material with a substrate with oxidic surface leads to droplet formation because of the high surface tension of liquid metals. This can be suppressed when the removal proceeds just at or slightly above the threshold intensity for a removal. In this case the material possesses only little energy above the melting point and it freezes before the hydrodynamic equilibrium is reached, i.e. before the melt is able to form droplets. Prerequisite to such a LIFT process is a very exactly tuned laser intensity and a smooth intensity profile.
Acknowledgements
The authors whish to thank Dr. P. Dittrich and Mr. H. Ziehl for the manufacturing of the SEM pictures.
K Schultze, M. Wagner / Laser-induced forward transfer of aluminium
This work was supported by the Keramische Werke Hermsdorf GmbH, Tridelta AG.
References [l] J. Bohandy, B.F. Kim and F.J. Adrian, J. Appl. Phys. 60 (1986) 1538. [2] F.J. Adrian, J. Bohandy, B.F. Kim, A.N. Jette and R. Thompson, J. Vat. Sci. Technol. B 5 (1987) 1490. [3] J. Bohandy, B.F. Kim, F.J. Adrian and A.N. Jette, J. Appl. Phys. 63 (1988) 1158.
309
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