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LaBx thin films prepared by magnetron sputtering
Applied Surface Science 70/71 (1993) 742-745 North-Holland
applied surface s c i e n c e
LaB x thin films prepared by magnetron sputtering A. K i n b a r a a, T. N a k a n o a, A. K o b a y a s h i a, S. B a b a a a n d T. K a j i w a r a a Department of Applied Physics, The University of Tokyo, Tokyo 113, Japan b Consumer Products Developments Laboratory, Mitsubishi Electric Corp., Kamakura 247, Japan Received 10 August 1992; accepted for publication 20 November 1992
L a B , (x = 0-6) thin films were prepared by magnetron sputtering using a LaB 6 target and Ar discharge gas. The composition of the films was investigated by the ICP method. It has been found that the composition strongly depended on the Ar discharge gas pressure. The film composition was mostly stoichiometric (x = 6) at low Ar pressures while the value of x decreased with increasing pressure. This nonstoichiometry is interpreted in terms of collision scattering of sputtered particles from the target by Monte Carlo simulation. T h e Ar pressure change induces the change of the crystal orientation of the films and this change is related with the work function of the film. T h e most appropriate Ar pressure for the preparation of the films used as electrodes is discussed.
1. Introduction LaB 6 thin film is a candidate for electrodes of plasma display devices because of its high secondary electron emission coefficient [1]. However, the physical properties of the LaB 6 film are strongly affected by the preparation condition of the films. In the previous paper, we have reported that large internal compressive stress generated in the sputter deposited LaB 6 thin films and their crystal structure investigated by X-ray diffraction showed remarkable dependence on Ar discharge gas pressure [2,3]. In the present paper, we p r e p a r e d films by magnetron sputtering using a LaB 6 target and Ar discharge gas. The dependence of film composition and of physical properties on the Ar discharge gas pressure were investigated. Deviation of the composition of the film from stoichiometry was interpreted in terms of a collision scattering process of the sputtered particles with As atoms in the discharge space. The most appropriate As gas pressure is discussed from the view point of
the stoichiometry and the secondary electron emission from the film.
2. Experimental results A conventional D C magnetron sputtering apparatus was used for the preparation of the films. The films were formed on glass substrates and their thickness ranged between 0.2 and 0.3/zm. A LaB 6 plate of 99.5% purity was mounted on an electrode as a target. The composition of LaB x thin films prepared in this experiment was investigated by the ICP (inductively coupled plasma discharge) method. The results on the atom number ratio of Boron and La, B / L a , are shown in fig. 1 as a function of Ar discharge gas pressure during the sputtering process. As is shown here, the depencence of the composition on the Ar pressure is remarkable. When the pressure is around 2.5 Pa or less, the values of B / L a are close to 6 but it decreased with pressure increase and when the pressure is
0169-4332/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
743
A. Kinbara et al. / LaB x thin films prepared by magnetron sputtering "S
20 • B (180eV) [] La (645eV)
rr
O O (510eV)
E
.E
Z
E o <.%
•
C (240eV)
"l1¢ t~ (D I
4
i
i
i i iiiii
1
i
10 Ar Pressure (Pa)
O
•
•
•
[] o
[] ©
[]
13._
O
,<
Fig. 1. Dependence of atom number ratio B/La on the Ar discharge gas pressure.
0
i 0
•1 20
t•
t 40
• t
i• 60
sputtering time (rain)
25 Pa, the ratio becomes about 4. The deficiency of B causes necessarily the change of the structure of the film. According to the equilibrium phase diagram of LaB x, the state of the film below 280°C is shown by, 0.0
a-La+LaB4,
4.8
LaB 4 + L a B 6 ,
5.3 < x ,
LaB 6 + (B).
Hence, it is considered that the state of the film is a mixture of LAB6, LaB4, La and B. However, as far as the films were investigated by X-ray diffraction, the traces of LaB4, B and La could not be detected. It is known that La and B atoms are easy to be oxidized if they are included in the atomic state. B atoms also generates carbides by the existance of carbon atoms. Auger electron spectroscopy (AES) observation showed that the ratio of the number of oxygen atoms to La in the film was approximately 0.15 and the value was almost independent of the Ar pressure during the sputtering. The depth profile of La, B, O and C atoms of a film observed by AES is shown in fig. 2. They seem to distribute uniformly in the film except for oxygen atoms at the surface. The oxygen and carbon atoms in the films combine with the La and B atoms to form an amorphous state or small crystallites. This may be a reason that we could not observe the traces of the other crystallites except for LaB 6. In order to study the state of the particles arriving at the substrate, we placed a quadrupole mass spectrometer at the place of the substrate
Fig. 2. AES depth profile of a sputtered LaB6 thin film. instead of the substrate for measuring the kinetic energies of ionized particles. An orifice was installed in front of the mass spectrometer. A mesh type electrode for applying a retardation potential was mounted close to the hole of the orifice. The average energies of the ions arriving at the substrate during the sputtering are shown in fig. 3 as a function of the Ar pressure. La ÷, B + and
40
30
c W
Ar + Ar 2+
•
La +
A
B+
O
20
O • A
A
2A
10
0
• (3
A
I
I
~
I
I Illll
I
10
I
Pa
Fig. 3. Average kinetic energy of species in Ar discharge gas during LaB6 sputtering.
744
A. Kinbara et al. / LaB x thin films prepared by magnetron sputtering
Ar 2+ ions have similar energies. Ar + ions have comparatively large energies in the region of small Ar gas pressure.
O.1Pa B z
z 2~
3. Consideration Special notice should be paid in considering the LaB 6 sputtering because the difference of atomic weight between La (138.9) and B (10.8) is extremely large. This means that the scattering probability of those atoms in the discharge space by Ar atoms is very different from each other. The scattering of La atoms sputtered from the target by the Ar atoms is small and thereby, La atoms are comparatively easy to arrive at the substrate. On the other hand, B atoms are apt to be scattered by Ar. Some of them arrive at the substrate but some are deposited at the chamber wall or evacuated by the vacuum pump. Behavior of the La and B atoms after they are emitted from the target surface is simulated by the Monte Carlo method. We assume that the angle distribution of atoms sputtered from the target is expressed in terms of cosine law and the atoms collide with Ar atoms moving at the uniform velocity corresponding to the room temperature. Ar, La and B atoms are considered to be rigid spheres, diameters of which are estimated by their cross section data. The collision among those spheres are assumed to be completely elastic. Traces of a La atom and a B atom emitted from a point on the target are investigated until they reached the substrate or the target. We investigated 1000 atoms of La and B atoms in total. We counted all the atoms arrived at the substrate and returned to the target. The results of the simulation are shown in figs. 4 and 5. In fig. 5, in the space between the target and the substrate, the probability of thermalization of those atoms is also shown but the details will be discussed elsewhere. In these figures, thick arrows indicate the starting point of the sputtered atoms. The z-axis is taken normal to the target and the substrate surface. As shown in fig. 4, when the Ar pressure is 0.1 Pa or less, the scattering of La and B by Ar
Fig. 4. Simulated results by the Monte Carlo method on frequency of arriving La and B atoms on the target and the substrate at Ar pressure of 0.1 Pa. atoms is small and most of those atoms arrive at the substrate. The distribution of the sputtering direction is likely to be conserved at the substrate. On the other hand, when the Ar pressure is 3 Pa, the effect of the scattering by Ar atoms becomes large particularly for B atoms as shown in fig. 5. La atoms are not so affected by the scattering, but most of the B atoms return again to the target. Returned B atoms distribute on the target. This is equivalent to the enhancement of B atom motion parallel to the target surface. The nonstoichiometry of the films prepared at higher Ar pressure is interpreted qualitatively by this scattering effect. The returned B atoms to the substrate can again be sputtered and some of them can arrive at the substrate. On the other hand, the resputtering of the film induced by comparatively high
3Pa
Fig. 5. Simulated results by the Monte Carlo method on frequency of arriving La and B atoms on the target and the substrate at Ar pressure of 3 Pa.
745
A. Kinbara et aL / LaB x thin films prepared by magnetron sputtering
energy Ar + ions shown in fig. 3 may change the composition. Consequently, the results shown in fig. 5 do not quantitatively connect with the composition ratio shown in fig. 1 but this figure suggests that the loss of the B atoms is large during the sputtering. In the previous p a p e r [2,3], we have pointed out that the internal stress in the films increases with the decrease of the Ar pressure and in the range 0.1-0.01 Pa, sometimes spontaneous peeling of the films from the substrates occurs although stoichiometric films are obtained. Hence, it is preferable to prepare films in the pressure range of 1 Pa or in the higher pressure range to obtain small stress films. We have also pointed out that the internal stress or strain energy affects the orientation of crystal surfaces of LaB 6. The strain energy densities stored in the crystallites is related to the crystal orientation. We calculated the crystallite energy densities as a function of the internal strain parallel to a crystal plane along with the calculation by Vook and Witt [4] and the effect of the energy on the orientation was considered according to the idea similar to that by H u a n g ct al. [5] where elastic constants of LaB 6 calculated by T a n a k a et al. [6] were used. We took into account the surface free energy of the crystal in the calculation. Surface free energies of LaB 6 have been evaluated from the dangling bond density of boron atoms at each crystal surface [2]. The surface free energies of (100), (110) and (111) surfaces were estimated to be 4.7, 3.7 and 3.0 J / m 2 respectively. Thus we could estimate the dependence of the energy density on the internal strain. The calculated results are shown in fig. 6. Because the internal stress is usually parallel to the film surface, the thick curve in the figure indicates the most probable orientation on the substrate. As the work function of (100) plane is the smallest, stress free state is the most preferable from the view point of electron emission. In order to obtain a stress free film, Ar gas pressure should be larger than 1 Pa or more. In summary, the low pressure sputtering is preferable to obtain the stoichiometric LaB 6 thin films but it induces the high internal strain and
Strain Energy U
~
Uloo=
•
0.47£z+ a 'X,,,
Ullo = 0 . 3 7 ~ + b
0.31 e2+ c
0
Strain Fig. 6. Energy density of crystallite of various orientation as functions of strain.
leads to the generation of the crystal surface (111) of high work function. The moderate discharge gas pressure was found to be in the range of 1 Pa.
Acknowledgements This work is partly supported by the Grant-inAid for the Scientific Research from the Ministry of Education, Culture and Art. We express our sincere thanks to the Advanced Energy Corp. for providing us an electric power supply for the sputtering.
References [1] Y. Okamoto, T. Aida and S. Shinada, Jpn. J. Appl. Phys. 26 (1987) 1722. [2] T. Nakano, S. Baba, A. Kobayashi, A. Kinbara, T. Kajiwara and K. Watanabe, J. Vac. Sci. Technol. A 9 (1991) 547. [3] T. Kajiwara, T. Urakabe, K. Sano, K. Fukuyama, S. Baba, T. Nakano and A. Kinbara, Vacuum 41 (1990) 1224. [4] W.R. Vook and F. Witt, J. Appl. Phys. 36 (1965) 2169. [5] T.C. Huang, G. Lim, F. Parmigiani and E. Kay, J. Vac. Sci. Techol. A 3 (1985) 2161. [6] T. Tanaka, J. Yoshimoto, M. Ishii, E. Bannai and S. Kawai, Solid State Commun. 22 (1977) 203.
applied surface s c i e n c e
LaB x thin films prepared by magnetron sputtering A. K i n b a r a a, T. N a k a n o a, A. K o b a y a s h i a, S. B a b a a a n d T. K a j i w a r a a Department of Applied Physics, The University of Tokyo, Tokyo 113, Japan b Consumer Products Developments Laboratory, Mitsubishi Electric Corp., Kamakura 247, Japan Received 10 August 1992; accepted for publication 20 November 1992
L a B , (x = 0-6) thin films were prepared by magnetron sputtering using a LaB 6 target and Ar discharge gas. The composition of the films was investigated by the ICP method. It has been found that the composition strongly depended on the Ar discharge gas pressure. The film composition was mostly stoichiometric (x = 6) at low Ar pressures while the value of x decreased with increasing pressure. This nonstoichiometry is interpreted in terms of collision scattering of sputtered particles from the target by Monte Carlo simulation. T h e Ar pressure change induces the change of the crystal orientation of the films and this change is related with the work function of the film. T h e most appropriate Ar pressure for the preparation of the films used as electrodes is discussed.
1. Introduction LaB 6 thin film is a candidate for electrodes of plasma display devices because of its high secondary electron emission coefficient [1]. However, the physical properties of the LaB 6 film are strongly affected by the preparation condition of the films. In the previous paper, we have reported that large internal compressive stress generated in the sputter deposited LaB 6 thin films and their crystal structure investigated by X-ray diffraction showed remarkable dependence on Ar discharge gas pressure [2,3]. In the present paper, we p r e p a r e d films by magnetron sputtering using a LaB 6 target and Ar discharge gas. The dependence of film composition and of physical properties on the Ar discharge gas pressure were investigated. Deviation of the composition of the film from stoichiometry was interpreted in terms of a collision scattering process of the sputtered particles with As atoms in the discharge space. The most appropriate As gas pressure is discussed from the view point of
the stoichiometry and the secondary electron emission from the film.
2. Experimental results A conventional D C magnetron sputtering apparatus was used for the preparation of the films. The films were formed on glass substrates and their thickness ranged between 0.2 and 0.3/zm. A LaB 6 plate of 99.5% purity was mounted on an electrode as a target. The composition of LaB x thin films prepared in this experiment was investigated by the ICP (inductively coupled plasma discharge) method. The results on the atom number ratio of Boron and La, B / L a , are shown in fig. 1 as a function of Ar discharge gas pressure during the sputtering process. As is shown here, the depencence of the composition on the Ar pressure is remarkable. When the pressure is around 2.5 Pa or less, the values of B / L a are close to 6 but it decreased with pressure increase and when the pressure is
0169-4332/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
743
A. Kinbara et al. / LaB x thin films prepared by magnetron sputtering "S
20 • B (180eV) [] La (645eV)
rr
O O (510eV)
E
.E
Z
E o <.%
•
C (240eV)
"l1¢ t~ (D I
4
i
i
i i iiiii
1
i
10 Ar Pressure (Pa)
O
•
•
•
[] o
[] ©
[]
13._
O
,<
Fig. 1. Dependence of atom number ratio B/La on the Ar discharge gas pressure.
0
i 0
•1 20
t•
t 40
• t
i• 60
sputtering time (rain)
25 Pa, the ratio becomes about 4. The deficiency of B causes necessarily the change of the structure of the film. According to the equilibrium phase diagram of LaB x, the state of the film below 280°C is shown by, 0.0
a-La+LaB4,
4.8
LaB 4 + L a B 6 ,
5.3 < x ,
LaB 6 + (B).
Hence, it is considered that the state of the film is a mixture of LAB6, LaB4, La and B. However, as far as the films were investigated by X-ray diffraction, the traces of LaB4, B and La could not be detected. It is known that La and B atoms are easy to be oxidized if they are included in the atomic state. B atoms also generates carbides by the existance of carbon atoms. Auger electron spectroscopy (AES) observation showed that the ratio of the number of oxygen atoms to La in the film was approximately 0.15 and the value was almost independent of the Ar pressure during the sputtering. The depth profile of La, B, O and C atoms of a film observed by AES is shown in fig. 2. They seem to distribute uniformly in the film except for oxygen atoms at the surface. The oxygen and carbon atoms in the films combine with the La and B atoms to form an amorphous state or small crystallites. This may be a reason that we could not observe the traces of the other crystallites except for LaB 6. In order to study the state of the particles arriving at the substrate, we placed a quadrupole mass spectrometer at the place of the substrate
Fig. 2. AES depth profile of a sputtered LaB6 thin film. instead of the substrate for measuring the kinetic energies of ionized particles. An orifice was installed in front of the mass spectrometer. A mesh type electrode for applying a retardation potential was mounted close to the hole of the orifice. The average energies of the ions arriving at the substrate during the sputtering are shown in fig. 3 as a function of the Ar pressure. La ÷, B + and
40
30
c W
Ar + Ar 2+
•
La +
A
B+
O
20
O • A
A
2A
10
0
• (3
A
I
I
~
I
I Illll
I
10
I
Pa
Fig. 3. Average kinetic energy of species in Ar discharge gas during LaB6 sputtering.
744
A. Kinbara et al. / LaB x thin films prepared by magnetron sputtering
Ar 2+ ions have similar energies. Ar + ions have comparatively large energies in the region of small Ar gas pressure.
O.1Pa B z
z 2~
3. Consideration Special notice should be paid in considering the LaB 6 sputtering because the difference of atomic weight between La (138.9) and B (10.8) is extremely large. This means that the scattering probability of those atoms in the discharge space by Ar atoms is very different from each other. The scattering of La atoms sputtered from the target by the Ar atoms is small and thereby, La atoms are comparatively easy to arrive at the substrate. On the other hand, B atoms are apt to be scattered by Ar. Some of them arrive at the substrate but some are deposited at the chamber wall or evacuated by the vacuum pump. Behavior of the La and B atoms after they are emitted from the target surface is simulated by the Monte Carlo method. We assume that the angle distribution of atoms sputtered from the target is expressed in terms of cosine law and the atoms collide with Ar atoms moving at the uniform velocity corresponding to the room temperature. Ar, La and B atoms are considered to be rigid spheres, diameters of which are estimated by their cross section data. The collision among those spheres are assumed to be completely elastic. Traces of a La atom and a B atom emitted from a point on the target are investigated until they reached the substrate or the target. We investigated 1000 atoms of La and B atoms in total. We counted all the atoms arrived at the substrate and returned to the target. The results of the simulation are shown in figs. 4 and 5. In fig. 5, in the space between the target and the substrate, the probability of thermalization of those atoms is also shown but the details will be discussed elsewhere. In these figures, thick arrows indicate the starting point of the sputtered atoms. The z-axis is taken normal to the target and the substrate surface. As shown in fig. 4, when the Ar pressure is 0.1 Pa or less, the scattering of La and B by Ar
Fig. 4. Simulated results by the Monte Carlo method on frequency of arriving La and B atoms on the target and the substrate at Ar pressure of 0.1 Pa. atoms is small and most of those atoms arrive at the substrate. The distribution of the sputtering direction is likely to be conserved at the substrate. On the other hand, when the Ar pressure is 3 Pa, the effect of the scattering by Ar atoms becomes large particularly for B atoms as shown in fig. 5. La atoms are not so affected by the scattering, but most of the B atoms return again to the target. Returned B atoms distribute on the target. This is equivalent to the enhancement of B atom motion parallel to the target surface. The nonstoichiometry of the films prepared at higher Ar pressure is interpreted qualitatively by this scattering effect. The returned B atoms to the substrate can again be sputtered and some of them can arrive at the substrate. On the other hand, the resputtering of the film induced by comparatively high
3Pa
Fig. 5. Simulated results by the Monte Carlo method on frequency of arriving La and B atoms on the target and the substrate at Ar pressure of 3 Pa.
745
A. Kinbara et aL / LaB x thin films prepared by magnetron sputtering
energy Ar + ions shown in fig. 3 may change the composition. Consequently, the results shown in fig. 5 do not quantitatively connect with the composition ratio shown in fig. 1 but this figure suggests that the loss of the B atoms is large during the sputtering. In the previous p a p e r [2,3], we have pointed out that the internal stress in the films increases with the decrease of the Ar pressure and in the range 0.1-0.01 Pa, sometimes spontaneous peeling of the films from the substrates occurs although stoichiometric films are obtained. Hence, it is preferable to prepare films in the pressure range of 1 Pa or in the higher pressure range to obtain small stress films. We have also pointed out that the internal stress or strain energy affects the orientation of crystal surfaces of LaB 6. The strain energy densities stored in the crystallites is related to the crystal orientation. We calculated the crystallite energy densities as a function of the internal strain parallel to a crystal plane along with the calculation by Vook and Witt [4] and the effect of the energy on the orientation was considered according to the idea similar to that by H u a n g ct al. [5] where elastic constants of LaB 6 calculated by T a n a k a et al. [6] were used. We took into account the surface free energy of the crystal in the calculation. Surface free energies of LaB 6 have been evaluated from the dangling bond density of boron atoms at each crystal surface [2]. The surface free energies of (100), (110) and (111) surfaces were estimated to be 4.7, 3.7 and 3.0 J / m 2 respectively. Thus we could estimate the dependence of the energy density on the internal strain. The calculated results are shown in fig. 6. Because the internal stress is usually parallel to the film surface, the thick curve in the figure indicates the most probable orientation on the substrate. As the work function of (100) plane is the smallest, stress free state is the most preferable from the view point of electron emission. In order to obtain a stress free film, Ar gas pressure should be larger than 1 Pa or more. In summary, the low pressure sputtering is preferable to obtain the stoichiometric LaB 6 thin films but it induces the high internal strain and
Strain Energy U
~
Uloo=
•
0.47£z+ a 'X,,,
Ullo = 0 . 3 7 ~ + b
0.31 e2+ c
0
Strain Fig. 6. Energy density of crystallite of various orientation as functions of strain.
leads to the generation of the crystal surface (111) of high work function. The moderate discharge gas pressure was found to be in the range of 1 Pa.
Acknowledgements This work is partly supported by the Grant-inAid for the Scientific Research from the Ministry of Education, Culture and Art. We express our sincere thanks to the Advanced Energy Corp. for providing us an electric power supply for the sputtering.
References [1] Y. Okamoto, T. Aida and S. Shinada, Jpn. J. Appl. Phys. 26 (1987) 1722. [2] T. Nakano, S. Baba, A. Kobayashi, A. Kinbara, T. Kajiwara and K. Watanabe, J. Vac. Sci. Technol. A 9 (1991) 547. [3] T. Kajiwara, T. Urakabe, K. Sano, K. Fukuyama, S. Baba, T. Nakano and A. Kinbara, Vacuum 41 (1990) 1224. [4] W.R. Vook and F. Witt, J. Appl. Phys. 36 (1965) 2169. [5] T.C. Huang, G. Lim, F. Parmigiani and E. Kay, J. Vac. Sci. Techol. A 3 (1985) 2161. [6] T. Tanaka, J. Yoshimoto, M. Ishii, E. Bannai and S. Kawai, Solid State Commun. 22 (1977) 203.