- Email: [email protected]
Laser spectroscopic studies of pulsed-laser deposition process for high-Tc thin films
MATERIALS SCIENCE & ENGINEERING
B
Materials Science and Engineering B47 (1997) 64-69
Laser spectroscopic studies of pulsed-laser deposition process for high-T, thin films] Tatsuo Okada *, Mitsuo Depwtment
0s Electrical
Engineerin
g, Kyz~sh
University,
Maeda Hakoznlci,
Fulu~oica
812, Japan
Abstract In order to investigate the particle-behavior in the pulsed-laser deposition (PLD) process for high-temperature superconducting (high-T3 thin film fabrication, laser-spectroscopic techniques have been appIied. Laser-induced fluorescence spectroscopy (LIF) was used to obtain the detailed information on the behavior of non-radiative atomic and molecular species in their ground states. Mie scattering was also successfully used to monitor the production of the particulate in the ablated plume. Visualization of the propagating plume is also presented. 0 1997 Published by Elsevier Science S.A. Pulsed-laser deposition; Laser-induced fluorescence spectroscopy; Mie scattering; Rayleigh scattering; Fast imaging; Superconducting thin film; YBa,Cu,O, _ ,; Shock wave; Rotational temperature; Condensation
Keywords:
1. Introduction Pulsed-laser deposition (PLD) has been widely used to deposit a variety of thin films. In the case of high-T,
film deposition, bulk high-TT, superconducting materials (targets) are laser-ablated in an ambient oxygen gas (or other oxidizing gas) at a pressure around several tens of Pascal and thin films are deposited on a heated substrate placed at several tens of millimeters from the target. It is recognized that this simple deposition schemecan produce high-quality films with good reproducibility. Although advantages in PLD process have accelerated device-oriented studies of high-T, films, the details of the PLD process has not been fully understood yet. A variety of techniques has been applied to investigate the PLD process. Among them laser-induced fluorescence (LIF) is the most powerful method because of its high spatiotemporal resolution with high species selectivity and with capabilities of non-destructive and in-situ measurements. Using LIF, detailed behavior of speciesin their ground states, which are thought to be the major constituent in the plume, can be investigated. * Corresponding author. Fax: + 81 92 6312790; e-mail: [email protected] ’ Proceedings of the Engineering Foundation Conference on Materials processing and Advanced Appkzdtions of Lasers, Palm Coast, Fiorida, 1-6 May 1994.
The same optical system as used in LIF can be used to detect the production of particulate in the plume by Mie scattering. The capabilities of LIF and Mie scattering can be fully exploited by introducing a gated imageintensifier followed by a CCD camera. In this paper, the transport of molecular species through the ambient oxygen gas and the interaction of ablated specieswith a substrate are discussedbased on the results of LIF studies and on the visualization of propagating plume. The results of Rayleigh and Mie scattering experiments are also presented.
2. Experimental The experimental apparatus for LIF and Mie scattering consists of a vacuum chamber with a rotatable target-holder and with a heated substrateholder, an ArF or KrF excimer laser for laser ablation, a probe laser and a light-detection system. The experimental arrangement for imaging system is shown in Fig. 1. A sintered YBa,Cu,O, _ ,s(YBCO) target on the rotating holder was ablated by the KrF or ArF excimer laser. Ablation was performed in a vacuum or under an ambient oxygen gas. In the LIF experiments, a XeCl laser-pumped dye laser was used as the probe laser. For Mie scattering experiments, a XeCl laser was used as the probe laser. Temporal developments of the plume
0921-5107/97/$17.00 0 1997 Published by Elsevier Science S.A. All rights reserved. PUSO921-5107(96)01881-8
T. Oicada,
KrF
nr ArF
M.
Maeda
/MateCds
Science
nrd
Engineering
B47
(1997)
65
64-69
3. LIT studies
laser
/Heated
3.1. Tmnsport of ablated particles
substrate
Fig. 1. Experimental arrangement.
were measured by changing the delay time between the ablation laser and the probe laser. In the following, three types of the experiments were performed with different light detection systems. In the fist system, a photo multiplier was used as a detector, whereby the rotational temperature of ablated molecules, the TOF distributions of atomic and molecular spices, and the TOF distributions of particulate were measured [l-3]. In the second, a gated image-intensifier followed with a linear photo-diode array was used to measure one-dimensional (1D) density distribution of YO molecules, whereby detailed information of the spatiotemporal development of density distribution of YO molecules were investigated [3-51. This lD-LIF system has been upgraded into a 2D system by introducing a CCD camera and a data acquisition system, by which visualization of the PLD process became possible.
The propagation of ablated particles was visualized by a fast-gated imaging of the emissiveparticles and two-dimensional LIF (2D-LIF) with a sheet excitation beam. In 2D-LIF, the second harmonic of a Q-switched Nd3 + :YAG laser was used as the probe laser. It was found, as described in the following section, that when the second harmonics of Nd:YAG laser and a XeCl excimer laser were used as an excitation source, intense fluorescence was produced due to the accidental coincidence between the excitation wavelengths and the band structures of ablated molecules. Examples of the images at an ambient oxygen gas pressure of 67 Pa are shown in Fig. 2, where the gate width of the exposure was 50 ns. Images of emissive particles at different times after the ablation are shown in Fig. 2(a) and images of emissive particles with LIF images are shown in Fig. 2(b). The sensitivities for different delay times are not the same and not corrected in Fig. 2, but the sensitivities for Fig. 2(a) and (b) were the same for the same delay time. Main difference between the images of the emissive particles and LIF images is in their distributions. In the case of the emissive particles’ images, a luminous front is propagating and is the most intense near the leading front. It is thought that the luminous front corresponds to the shock front driven by the ablated particles. In the case of 2D-LIF image, on the other hand, nonemissive molecular species spread behind the luminous front. The outer edge of the LIF image coincided with the luminous front of emissive species. LIF signals were
(a>
Target
I
(b)
3 8
Target
10
20
30
Time after ablation (us) Fig. 2. Images of plume at different delay times after ablation. (a) Images of emissive species and (b) images of emissive species plus LIF.
66
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hf. Maedu
/Maferials
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and Engineering
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Pa ps- 2 over the pressure range from 40 to 240 Pa and at an ablation fluence of 0.9 J cmF2. 3.2. htemal energy distributions
Time
after
ablittion
(ps)
Fig. 3. Flight distance of YO molecules as a function of time after ablation.
higher at the outlining-edge rather than at the center of the plume, indicating that non-emissive molecular species were formed in the luminous front by the chemical reaction of ablated atoms and ions with oxygen gas. In understanding the results of the emission spectroscopy, it is important to keep in mind the presence of nonemissive particles as shown here. At present, species selected 2D-LIF measurements are in progress using a dye laser as a probe laser. The transport of molecular species has been measured precisely by one-dimensional LIF. YO molecules were excited via the Q1 + RQZl (O-O) band of the A2111,, 2-X2x transition near 613 nm with a sheet dye laser beam (about 0.5 mm in width and 40 mm in height) propagating along the pellet surface. The resultant fluorescence light was detected by a linear photo-diode array (Hamamatsu C2326) with a gated image-intensifier (Hamamatsu C2349). Measurements were performed in-situ during the deposition at an ablation fluence of 0.9 J crnmz. The positions of the leading edgesof the propagating front are plotted as a function of time after ablation (time of flight) in Fig. 3 for different ambient gas pressures. It is clearly seen that the propagation is decelerated during the propagation and stopped at a i?nite distance which becomes shorter as the gas pressure increased. The characteristics of the propagation was analyzed based on the point source blast wave model. According to the model, the position of the expansion front R from the target surface is given as a function of the time t after ablation as follows; R = (k/~~)~.*(t)~.~
The gas temperature (rotational temperature) was measured using the excitation spectra of the A21’11,2X2C (0,O) band of YO molecules. Simulated spectra at various rotational temperatures are shown in Fig. 4. The present band system is not sensitive for the rotational temperature of more than 2000 K. Typical examples of the excitation spectra are shown in Fig. 5 along with fitted theoretical curves. Measurements were performed at 30 mm from the target surface without the substrate and an ablation fluence was 0.9 J cmm2. At an oxygen pressure of 6.7 Pa and at 14 ps after ablation, shown in Fig. 5(a), the pressure is too low to drive the blast wave. In this casethe rotational temperature was about 1250 K and was comparable with that in vacuum. In the expansion phase according to the blast wave, the increase of the rotational temperature at the leading edge of the expanding plume was clearly observed, as shown in Fig. 5(b). This strongly supports the initiation of the blast wave in PLD process. The fitted temperature of 2000 K is the lowest value because the spectra is not sensitive to the temperature of more than 2000 K as mentioned above. In the diffusion phase, the cooling of the rotational temperature is observed, but the temperature was still kept around 1000 K as shown in Fig. 5(c). These results shows that the position of the substrate relative to the expansion distance affects the temperature of incoming plume onto the substrate. 3.3. Plume-substrate interaction Images of emissive particles and non-emissive molecules near a substrate are shown in Fig. 6. The
(1)
where p stands for the static pressure of the ambient, assuming a spherical expansion from the ablated pointsource. The R-t plot for the present experimental data at a pressure of 67 Pa is plotted in Fig. 3. The fitted k value was 7.2 x 10 cm5 Pa psm2. It is found that the shock model described the propagation characteristics well. The results are consistent with the fact that the leading edge of LIF images overlapped with the luminous shock front as shown in Fig. 2. Similar fittings were performed for other data in Fig. 3 and the fitted k values were in the range from 7.2 x 10 to 9.3 x 10 cm,
613.0
613.5 Wavelength
(nm)
614.0
Fig. 4. Normalized simulated spectra of YO Q, + QR,,,(O,O) band of the A’II,,z-X2C transition. The molecular constants used gave a discrepancy of about 0.2 nm for the band head position. The positions shown by arrows were used as reference points in fitting with the experimental spectra [3].
T. Olcada,
613
wavelength
M.
Maedn
/Materials
Science
and Engineering
B47 (1997)
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614
Fig. 5. Measured excitation spectra of YO A-X
band. Solid lines are theoretical curves at temperature given in each figure.
substrate was a Si wafer and was placed at about 30 mm from the target surface. An ambient oxygen gas pressure was 33 Pa. Images of the emissive particles at different delay times are shown in Fig. 6(a), and LIF images excited by the second harmonics of the Nd:YAG laser plus those of emissive particles are shown in Fig. 6(b). The sensitivities for different delay times are not the same and not corrected in Fig. 6, but the sensitivities for Fig. 6(a) and (b) were the same for the same delay time. At 12 ~.lsafter the ablation, the leading edge of the plume just touched on the substrate. The shape of the leading edge turned to be flattened by being compressed by the rear part of the plume and the density near the substrate increased rapidly, as can be seen in the LIF image at 14 us. The increase of the density suggeststhat the sticking coefficient of the particles is lessthan unity at this stage. The high density region near the substrate were reflected back towards the target. After 30 us, the LIF images became hint gradually in keeping the similar distribution as that at 30 us. After this stage, molecular speciesare transported by the diffusion. Similar behavior has been reported in the imaging of emissive particles by Geoheagan [6]. The distributions of YO molecules near the substrate were measured in situ by lD-LID during the deposition with a heated MgO substrate which was placed at 35 mm from the target surface. The spatial distributions of YO along the substrate normal at different delay times after ablation are-shown in Fig. 7(a)-(d) for different oxygen gas pressures of 13, 33, 53 and 100 Pa. At a delay time of 8 us in Fig. 7(a) and (b), the background emission from the ejected plume could not be neglected, so the distributions at 8 ms were obtained by subtracted the background emission obtained without dye laser excitation from the signal obtained with excitation. But due to the shot-to-shot fluctuation of the signal, it is not sure if the dips around the center was real distribution or not. The rapid decrease of the signals in Fig. 8(a) just in front of the substrate was partly due to the sensitivity of the detection system, and the small humps at the substrate were due to the dye laser light scattered by the substrate edge. At a lower pressure of 13 Pa shown in Fig. 7(a), the particle-plume collided with the substrate during the
expansion. In this case, the same behavior as that observed in Fig. 6 can be observed. It can be seen that YO molecules did not stick directly on the substrate, rather reflected or stagnated in front of the substrate. At a higher oxygen pressure of 100 Pa, propagation of YO molecules were almost stopped just in front of the substrate. Therefore, the propagation was not influenced by the presence of the substrate. As a result, little peaking was observed. Afterwards, the leading edge gradually flowed onto the substrate with time and the peak position rather moved to the pellet, suggesting that YO molecules were diffusing at this stage.
4. Rayleigh and Mie scattering experiments LIF studies showed that the initiation and propagation of the blast wave play an important role in PLD processesin an ambient gas. When the ideal blast wave is driven, the ablated particles push the ambient oxygen gas just as a piston. As a result, compressed layer and depleted region are formed in front of the ablated plume and behind it, respectively. It is also shown that the shock propagation stopped at a finite distance from the target surface. At this diffusion phase, the cooling of the plume was observed and this may lead to the condensation of the ablated particles into clusters or particulate. To investigate these phenomena accompanying the PLD process, Rayleigh and Mie scattering experiments were conducted. Because the Rayleigh scattering cross section is proportional to the 4th power of the probe frequency, a XeCl excimer laser used for the dye laser excitation was used as a probe laser. A detection limit was accessedby filling the vacuum chamber with an oxygen gas and a detection limit of 13 Pa was achieved for the oxygen gas at room temperature, after suppressing the stray light. Fig. 8 shows the intensities of the scattered light at an oxygen gas the target surface as the function of the pressure of 27 Pa at 30 mm from delay time between the KrF laser for ablation and the probe XeCl laser. In this case, the temporal change from 10 ms to 0.9 s was recorded. There observed two components; one was peaked at around 20 ms after the ablation and the other was peaked at around 100 ms. To identify the
T. Okndu,
hf. Muedu
/ Marei-ids
Science
md Engineering
B47 (I 997) 64-69
(a>
0
3
(b)
-Target
- Substrate 40 mm
40
11
Fig,
6. Images
of plumes
12
14 Time after ablation
interacting
with Si substrate.
(a) Images
of emissive
origin of these signals, we have examined the spectra and the temporal wave forms of the scattered lights. As a result, we have concluded that the faster component was the fluorescence generated from molecules which were excited by a XeCl laser light due to an accidental coincidence of the laser wavelength and the molecular band. Actually the TOF distribution was comparable with that of YO molecules. This strong fluorescence lights conquered a weak Rayleigh scattered light, which was expected to be generated from the oxygen layer compressed by the ablation driven blast wave. On the other hand, the slower component delayed very much compared with the TOF distributions observed for atoms and molecules in the ablated plume. We have concluded that this component is due to the scattering (Mie or Rayleigh) from particluates or clusters. The TOF distributions of Mie scattered signals at various ambient pressures are shown in Fig. 9 for a flight distance of 30 mm. The increases of signals on the
20 (ps)
mm
30
species and (b) images
of emissive
species
plus LIF.
left-hand sides of each curves were due to the fluorescence lights. In vacuum, Mie scattered signals peaked at around 500 us, giving a TOF velocity of 60 m s-l. This velocity is comparable with that of the particulate generated from the target directly, which has been reported by Geoheagan based on the imaging of the particulates [7]. It is believed that the origin of the Mie scattered signal in vacuum are the particulate generated from the target. On the other hand, Mie scattered signals at a higher ambient gas are delayed by more than 100 ms after ablation. The intensities of signals are dramatically increased with the gas pressure. For example, the signal intensity increased by two orders of magnitude by increasing an oxygen pressure from 27 to 106 Pa. We believed that the production of particulates due to condensation is responsible for the scattering signals. We are planning to perform the polarization dependent Mie scattering experiment to evaluate the diameter of particulate. Then information on the origin will be obtained.
[bl Oxygen pressure
Distance
from target
(mm)
Fig. 7. Distributions of YO molecules different delay times after ablation.
Distance
near heated
from target (mm)
MgO
substrate
O.bOl
at Fig. 8. Temporal
0.1 Time after
change
10 ablntion
of intensities
1000 (ms)
of scattered
signal.
T. Olcacla, M. Oxygen -0
Mae&
/Materials
Science
and Engineering
B47 (1997)
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69
image acquisition system, Professor H. Tsuji for developing the simulation code for the excitation spectrum of YO, Professors M. Takeo, K. Enpuku and Dr. T. Kisu for the evaluation of the deposited films. This work was supported in part by the Grant-in-Aid for Scientific Research on Priority Area from the Ministry of Education, Science and Culture of Japan.
pressure (Pa) 67
References Time after ablation Fig. 9. Temporal pressures.
changes
of scattered
intensities
(ms) for different
oxygen
5. Summary
We have shown the usefulness of the laser spectroscopic diagnostics of PLD. Especially, imaging LIF provides direct information on the dynamic behavior of the majority constituent in the PLD atmosphere of high-T, film deposition. This technique can be easily extended to monitor the particulate by Mie scattering. Such measurements are now in progress. Acknowledgements
The author would like to thank Y. Nakata and Y. Sasaki and W.K.A. Kumuduni for their assistance in the experiments, Professors K. Muraoka and K. Uchino for lending us the KrF excimer laser and the
[I] T. Okada, N. Shibamaru, Y. Nakayama and M. Maeda, Inlzestigation of behavior of particles generated from laser-ablated using laser-induced fluorescence, Appl. YBa,Cu,O, -x target Phys. Lett., 60 (1992) 941. [2] T. Okada, N. Shibamaru, Y. Nakayama and M. Maeda, Time-offlight measurement of particles with laser-induced fluorescence in the plume produced by laser ablation of YBazCu307-, target, Jpn. J. Appl. Phys., 31 (1992) L367. [3] W.K.A. Kumuduni, Y. Nakata, T. Okada and M. Maeda, Transport of YO molecules produced by ArF laser ablation of YBa,Cu,O,-, in ambient oxygen gas, J. Appl. Phys., 74 (1993) 7510. [4] W.K.A. Kumuduni, Y. Nakata, T. Okada and M. Maeda, Spatial distribution of YO molecules ejected from Iaser-ablated YBazCuj07-, Appl. Phys., B58 (1994) 289. [5] Y. Nakata, W.K.A. Kumuduni, T. Okada and M. Maeda, Plume-substrate interaction in pulsed-laser deposition of hightemperature superconducting thin films, Appl. Phys. Lett.. 64 (1994) 2599. [6] D.B. Geohegan, Fast diagnostics of laser ablation used for pulsed laser deposition, Tech. Digest ConJ Laser and Electra-Optics, Baltimore, Muy 1993. [7] D.B. Geohegan, Imaging and blackbody emission spectra of particulates generated in the KrF-laser ablation of BN and YBa,Cu,O,-,, Appl. Phys. Len., 62 (1993) 1463.
B
Materials Science and Engineering B47 (1997) 64-69
Laser spectroscopic studies of pulsed-laser deposition process for high-T, thin films] Tatsuo Okada *, Mitsuo Depwtment
0s Electrical
Engineerin
g, Kyz~sh
University,
Maeda Hakoznlci,
Fulu~oica
812, Japan
Abstract In order to investigate the particle-behavior in the pulsed-laser deposition (PLD) process for high-temperature superconducting (high-T3 thin film fabrication, laser-spectroscopic techniques have been appIied. Laser-induced fluorescence spectroscopy (LIF) was used to obtain the detailed information on the behavior of non-radiative atomic and molecular species in their ground states. Mie scattering was also successfully used to monitor the production of the particulate in the ablated plume. Visualization of the propagating plume is also presented. 0 1997 Published by Elsevier Science S.A. Pulsed-laser deposition; Laser-induced fluorescence spectroscopy; Mie scattering; Rayleigh scattering; Fast imaging; Superconducting thin film; YBa,Cu,O, _ ,; Shock wave; Rotational temperature; Condensation
Keywords:
1. Introduction Pulsed-laser deposition (PLD) has been widely used to deposit a variety of thin films. In the case of high-T,
film deposition, bulk high-TT, superconducting materials (targets) are laser-ablated in an ambient oxygen gas (or other oxidizing gas) at a pressure around several tens of Pascal and thin films are deposited on a heated substrate placed at several tens of millimeters from the target. It is recognized that this simple deposition schemecan produce high-quality films with good reproducibility. Although advantages in PLD process have accelerated device-oriented studies of high-T, films, the details of the PLD process has not been fully understood yet. A variety of techniques has been applied to investigate the PLD process. Among them laser-induced fluorescence (LIF) is the most powerful method because of its high spatiotemporal resolution with high species selectivity and with capabilities of non-destructive and in-situ measurements. Using LIF, detailed behavior of speciesin their ground states, which are thought to be the major constituent in the plume, can be investigated. * Corresponding author. Fax: + 81 92 6312790; e-mail: [email protected] ’ Proceedings of the Engineering Foundation Conference on Materials processing and Advanced Appkzdtions of Lasers, Palm Coast, Fiorida, 1-6 May 1994.
The same optical system as used in LIF can be used to detect the production of particulate in the plume by Mie scattering. The capabilities of LIF and Mie scattering can be fully exploited by introducing a gated imageintensifier followed by a CCD camera. In this paper, the transport of molecular species through the ambient oxygen gas and the interaction of ablated specieswith a substrate are discussedbased on the results of LIF studies and on the visualization of propagating plume. The results of Rayleigh and Mie scattering experiments are also presented.
2. Experimental The experimental apparatus for LIF and Mie scattering consists of a vacuum chamber with a rotatable target-holder and with a heated substrateholder, an ArF or KrF excimer laser for laser ablation, a probe laser and a light-detection system. The experimental arrangement for imaging system is shown in Fig. 1. A sintered YBa,Cu,O, _ ,s(YBCO) target on the rotating holder was ablated by the KrF or ArF excimer laser. Ablation was performed in a vacuum or under an ambient oxygen gas. In the LIF experiments, a XeCl laser-pumped dye laser was used as the probe laser. For Mie scattering experiments, a XeCl laser was used as the probe laser. Temporal developments of the plume
0921-5107/97/$17.00 0 1997 Published by Elsevier Science S.A. All rights reserved. PUSO921-5107(96)01881-8
T. Oicada,
KrF
nr ArF
M.
Maeda
/MateCds
Science
nrd
Engineering
B47
(1997)
65
64-69
3. LIT studies
laser
/Heated
3.1. Tmnsport of ablated particles
substrate
Fig. 1. Experimental arrangement.
were measured by changing the delay time between the ablation laser and the probe laser. In the following, three types of the experiments were performed with different light detection systems. In the fist system, a photo multiplier was used as a detector, whereby the rotational temperature of ablated molecules, the TOF distributions of atomic and molecular spices, and the TOF distributions of particulate were measured [l-3]. In the second, a gated image-intensifier followed with a linear photo-diode array was used to measure one-dimensional (1D) density distribution of YO molecules, whereby detailed information of the spatiotemporal development of density distribution of YO molecules were investigated [3-51. This lD-LIF system has been upgraded into a 2D system by introducing a CCD camera and a data acquisition system, by which visualization of the PLD process became possible.
The propagation of ablated particles was visualized by a fast-gated imaging of the emissiveparticles and two-dimensional LIF (2D-LIF) with a sheet excitation beam. In 2D-LIF, the second harmonic of a Q-switched Nd3 + :YAG laser was used as the probe laser. It was found, as described in the following section, that when the second harmonics of Nd:YAG laser and a XeCl excimer laser were used as an excitation source, intense fluorescence was produced due to the accidental coincidence between the excitation wavelengths and the band structures of ablated molecules. Examples of the images at an ambient oxygen gas pressure of 67 Pa are shown in Fig. 2, where the gate width of the exposure was 50 ns. Images of emissive particles at different times after the ablation are shown in Fig. 2(a) and images of emissive particles with LIF images are shown in Fig. 2(b). The sensitivities for different delay times are not the same and not corrected in Fig. 2, but the sensitivities for Fig. 2(a) and (b) were the same for the same delay time. Main difference between the images of the emissive particles and LIF images is in their distributions. In the case of the emissive particles’ images, a luminous front is propagating and is the most intense near the leading front. It is thought that the luminous front corresponds to the shock front driven by the ablated particles. In the case of 2D-LIF image, on the other hand, nonemissive molecular species spread behind the luminous front. The outer edge of the LIF image coincided with the luminous front of emissive species. LIF signals were
(a>
Target
I
(b)
3 8
Target
10
20
30
Time after ablation (us) Fig. 2. Images of plume at different delay times after ablation. (a) Images of emissive species and (b) images of emissive species plus LIF.
66
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hf. Maedu
/Maferials
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Pa ps- 2 over the pressure range from 40 to 240 Pa and at an ablation fluence of 0.9 J cmF2. 3.2. htemal energy distributions
Time
after
ablittion
(ps)
Fig. 3. Flight distance of YO molecules as a function of time after ablation.
higher at the outlining-edge rather than at the center of the plume, indicating that non-emissive molecular species were formed in the luminous front by the chemical reaction of ablated atoms and ions with oxygen gas. In understanding the results of the emission spectroscopy, it is important to keep in mind the presence of nonemissive particles as shown here. At present, species selected 2D-LIF measurements are in progress using a dye laser as a probe laser. The transport of molecular species has been measured precisely by one-dimensional LIF. YO molecules were excited via the Q1 + RQZl (O-O) band of the A2111,, 2-X2x transition near 613 nm with a sheet dye laser beam (about 0.5 mm in width and 40 mm in height) propagating along the pellet surface. The resultant fluorescence light was detected by a linear photo-diode array (Hamamatsu C2326) with a gated image-intensifier (Hamamatsu C2349). Measurements were performed in-situ during the deposition at an ablation fluence of 0.9 J crnmz. The positions of the leading edgesof the propagating front are plotted as a function of time after ablation (time of flight) in Fig. 3 for different ambient gas pressures. It is clearly seen that the propagation is decelerated during the propagation and stopped at a i?nite distance which becomes shorter as the gas pressure increased. The characteristics of the propagation was analyzed based on the point source blast wave model. According to the model, the position of the expansion front R from the target surface is given as a function of the time t after ablation as follows; R = (k/~~)~.*(t)~.~
The gas temperature (rotational temperature) was measured using the excitation spectra of the A21’11,2X2C (0,O) band of YO molecules. Simulated spectra at various rotational temperatures are shown in Fig. 4. The present band system is not sensitive for the rotational temperature of more than 2000 K. Typical examples of the excitation spectra are shown in Fig. 5 along with fitted theoretical curves. Measurements were performed at 30 mm from the target surface without the substrate and an ablation fluence was 0.9 J cmm2. At an oxygen pressure of 6.7 Pa and at 14 ps after ablation, shown in Fig. 5(a), the pressure is too low to drive the blast wave. In this casethe rotational temperature was about 1250 K and was comparable with that in vacuum. In the expansion phase according to the blast wave, the increase of the rotational temperature at the leading edge of the expanding plume was clearly observed, as shown in Fig. 5(b). This strongly supports the initiation of the blast wave in PLD process. The fitted temperature of 2000 K is the lowest value because the spectra is not sensitive to the temperature of more than 2000 K as mentioned above. In the diffusion phase, the cooling of the rotational temperature is observed, but the temperature was still kept around 1000 K as shown in Fig. 5(c). These results shows that the position of the substrate relative to the expansion distance affects the temperature of incoming plume onto the substrate. 3.3. Plume-substrate interaction Images of emissive particles and non-emissive molecules near a substrate are shown in Fig. 6. The
(1)
where p stands for the static pressure of the ambient, assuming a spherical expansion from the ablated pointsource. The R-t plot for the present experimental data at a pressure of 67 Pa is plotted in Fig. 3. The fitted k value was 7.2 x 10 cm5 Pa psm2. It is found that the shock model described the propagation characteristics well. The results are consistent with the fact that the leading edge of LIF images overlapped with the luminous shock front as shown in Fig. 2. Similar fittings were performed for other data in Fig. 3 and the fitted k values were in the range from 7.2 x 10 to 9.3 x 10 cm,
613.0
613.5 Wavelength
(nm)
614.0
Fig. 4. Normalized simulated spectra of YO Q, + QR,,,(O,O) band of the A’II,,z-X2C transition. The molecular constants used gave a discrepancy of about 0.2 nm for the band head position. The positions shown by arrows were used as reference points in fitting with the experimental spectra [3].
T. Olcada,
613
wavelength
M.
Maedn
/Materials
Science
and Engineering
B47 (1997)
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614
Fig. 5. Measured excitation spectra of YO A-X
band. Solid lines are theoretical curves at temperature given in each figure.
substrate was a Si wafer and was placed at about 30 mm from the target surface. An ambient oxygen gas pressure was 33 Pa. Images of the emissive particles at different delay times are shown in Fig. 6(a), and LIF images excited by the second harmonics of the Nd:YAG laser plus those of emissive particles are shown in Fig. 6(b). The sensitivities for different delay times are not the same and not corrected in Fig. 6, but the sensitivities for Fig. 6(a) and (b) were the same for the same delay time. At 12 ~.lsafter the ablation, the leading edge of the plume just touched on the substrate. The shape of the leading edge turned to be flattened by being compressed by the rear part of the plume and the density near the substrate increased rapidly, as can be seen in the LIF image at 14 us. The increase of the density suggeststhat the sticking coefficient of the particles is lessthan unity at this stage. The high density region near the substrate were reflected back towards the target. After 30 us, the LIF images became hint gradually in keeping the similar distribution as that at 30 us. After this stage, molecular speciesare transported by the diffusion. Similar behavior has been reported in the imaging of emissive particles by Geoheagan [6]. The distributions of YO molecules near the substrate were measured in situ by lD-LID during the deposition with a heated MgO substrate which was placed at 35 mm from the target surface. The spatial distributions of YO along the substrate normal at different delay times after ablation are-shown in Fig. 7(a)-(d) for different oxygen gas pressures of 13, 33, 53 and 100 Pa. At a delay time of 8 us in Fig. 7(a) and (b), the background emission from the ejected plume could not be neglected, so the distributions at 8 ms were obtained by subtracted the background emission obtained without dye laser excitation from the signal obtained with excitation. But due to the shot-to-shot fluctuation of the signal, it is not sure if the dips around the center was real distribution or not. The rapid decrease of the signals in Fig. 8(a) just in front of the substrate was partly due to the sensitivity of the detection system, and the small humps at the substrate were due to the dye laser light scattered by the substrate edge. At a lower pressure of 13 Pa shown in Fig. 7(a), the particle-plume collided with the substrate during the
expansion. In this case, the same behavior as that observed in Fig. 6 can be observed. It can be seen that YO molecules did not stick directly on the substrate, rather reflected or stagnated in front of the substrate. At a higher oxygen pressure of 100 Pa, propagation of YO molecules were almost stopped just in front of the substrate. Therefore, the propagation was not influenced by the presence of the substrate. As a result, little peaking was observed. Afterwards, the leading edge gradually flowed onto the substrate with time and the peak position rather moved to the pellet, suggesting that YO molecules were diffusing at this stage.
4. Rayleigh and Mie scattering experiments LIF studies showed that the initiation and propagation of the blast wave play an important role in PLD processesin an ambient gas. When the ideal blast wave is driven, the ablated particles push the ambient oxygen gas just as a piston. As a result, compressed layer and depleted region are formed in front of the ablated plume and behind it, respectively. It is also shown that the shock propagation stopped at a finite distance from the target surface. At this diffusion phase, the cooling of the plume was observed and this may lead to the condensation of the ablated particles into clusters or particulate. To investigate these phenomena accompanying the PLD process, Rayleigh and Mie scattering experiments were conducted. Because the Rayleigh scattering cross section is proportional to the 4th power of the probe frequency, a XeCl excimer laser used for the dye laser excitation was used as a probe laser. A detection limit was accessedby filling the vacuum chamber with an oxygen gas and a detection limit of 13 Pa was achieved for the oxygen gas at room temperature, after suppressing the stray light. Fig. 8 shows the intensities of the scattered light at an oxygen gas the target surface as the function of the pressure of 27 Pa at 30 mm from delay time between the KrF laser for ablation and the probe XeCl laser. In this case, the temporal change from 10 ms to 0.9 s was recorded. There observed two components; one was peaked at around 20 ms after the ablation and the other was peaked at around 100 ms. To identify the
T. Okndu,
hf. Muedu
/ Marei-ids
Science
md Engineering
B47 (I 997) 64-69
(a>
0
3
(b)
-Target
- Substrate 40 mm
40
11
Fig,
6. Images
of plumes
12
14 Time after ablation
interacting
with Si substrate.
(a) Images
of emissive
origin of these signals, we have examined the spectra and the temporal wave forms of the scattered lights. As a result, we have concluded that the faster component was the fluorescence generated from molecules which were excited by a XeCl laser light due to an accidental coincidence of the laser wavelength and the molecular band. Actually the TOF distribution was comparable with that of YO molecules. This strong fluorescence lights conquered a weak Rayleigh scattered light, which was expected to be generated from the oxygen layer compressed by the ablation driven blast wave. On the other hand, the slower component delayed very much compared with the TOF distributions observed for atoms and molecules in the ablated plume. We have concluded that this component is due to the scattering (Mie or Rayleigh) from particluates or clusters. The TOF distributions of Mie scattered signals at various ambient pressures are shown in Fig. 9 for a flight distance of 30 mm. The increases of signals on the
20 (ps)
mm
30
species and (b) images
of emissive
species
plus LIF.
left-hand sides of each curves were due to the fluorescence lights. In vacuum, Mie scattered signals peaked at around 500 us, giving a TOF velocity of 60 m s-l. This velocity is comparable with that of the particulate generated from the target directly, which has been reported by Geoheagan based on the imaging of the particulates [7]. It is believed that the origin of the Mie scattered signal in vacuum are the particulate generated from the target. On the other hand, Mie scattered signals at a higher ambient gas are delayed by more than 100 ms after ablation. The intensities of signals are dramatically increased with the gas pressure. For example, the signal intensity increased by two orders of magnitude by increasing an oxygen pressure from 27 to 106 Pa. We believed that the production of particulates due to condensation is responsible for the scattering signals. We are planning to perform the polarization dependent Mie scattering experiment to evaluate the diameter of particulate. Then information on the origin will be obtained.
[bl Oxygen pressure
Distance
from target
(mm)
Fig. 7. Distributions of YO molecules different delay times after ablation.
Distance
near heated
from target (mm)
MgO
substrate
O.bOl
at Fig. 8. Temporal
0.1 Time after
change
10 ablntion
of intensities
1000 (ms)
of scattered
signal.
T. Olcacla, M. Oxygen -0
Mae&
/Materials
Science
and Engineering
B47 (1997)
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image acquisition system, Professor H. Tsuji for developing the simulation code for the excitation spectrum of YO, Professors M. Takeo, K. Enpuku and Dr. T. Kisu for the evaluation of the deposited films. This work was supported in part by the Grant-in-Aid for Scientific Research on Priority Area from the Ministry of Education, Science and Culture of Japan.
pressure (Pa) 67
References Time after ablation Fig. 9. Temporal pressures.
changes
of scattered
intensities
(ms) for different
oxygen
5. Summary
We have shown the usefulness of the laser spectroscopic diagnostics of PLD. Especially, imaging LIF provides direct information on the dynamic behavior of the majority constituent in the PLD atmosphere of high-T, film deposition. This technique can be easily extended to monitor the particulate by Mie scattering. Such measurements are now in progress. Acknowledgements
The author would like to thank Y. Nakata and Y. Sasaki and W.K.A. Kumuduni for their assistance in the experiments, Professors K. Muraoka and K. Uchino for lending us the KrF excimer laser and the
[I] T. Okada, N. Shibamaru, Y. Nakayama and M. Maeda, Inlzestigation of behavior of particles generated from laser-ablated using laser-induced fluorescence, Appl. YBa,Cu,O, -x target Phys. Lett., 60 (1992) 941. [2] T. Okada, N. Shibamaru, Y. Nakayama and M. Maeda, Time-offlight measurement of particles with laser-induced fluorescence in the plume produced by laser ablation of YBazCu307-, target, Jpn. J. Appl. Phys., 31 (1992) L367. [3] W.K.A. Kumuduni, Y. Nakata, T. Okada and M. Maeda, Transport of YO molecules produced by ArF laser ablation of YBa,Cu,O,-, in ambient oxygen gas, J. Appl. Phys., 74 (1993) 7510. [4] W.K.A. Kumuduni, Y. Nakata, T. Okada and M. Maeda, Spatial distribution of YO molecules ejected from Iaser-ablated YBazCuj07-, Appl. Phys., B58 (1994) 289. [5] Y. Nakata, W.K.A. Kumuduni, T. Okada and M. Maeda, Plume-substrate interaction in pulsed-laser deposition of hightemperature superconducting thin films, Appl. Phys. Lett.. 64 (1994) 2599. [6] D.B. Geohegan, Fast diagnostics of laser ablation used for pulsed laser deposition, Tech. Digest ConJ Laser and Electra-Optics, Baltimore, Muy 1993. [7] D.B. Geohegan, Imaging and blackbody emission spectra of particulates generated in the KrF-laser ablation of BN and YBa,Cu,O,-,, Appl. Phys. Len., 62 (1993) 1463.