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Internal stress in IBS films
Materials Science and Engineering, 98 (1988) 519 522
519
Internal Stress in IBS Films* H. K O N D O and T. M I Z O G U C H I
Faculty ol" Science, Gakushuin University, l 5 I Mejiro, Toshimaku, Tokyo 171 (Japan)
Abstract The internal stress in films prepared by ion beam sputtering (IBS) was investigated as a function of the acceleration voltage V~ of the sputtering argon ions and thefilm thickness. IBS films of nickel and zirconium have strong compressive stresses of the order of -lO~°dvn cm 2 at V~ = l.l kV, which decrease with decreasing acceleration voltage. The atomic peening effect of incident atoms with high kinetic energy is considered to make a dominant contribution to the internal stress,
X-ray diffraction for nickel films shows that the lattice spacing perpendicular to the film plane increases because of a strong in-plane compressive stress. The effecth;e value El(1 + q), where E is Young's modulus and q is the Poisson ratio, of the films is only 22% of the bulk value. The internal stress is reduced to the order of - 107 to - 108 dyn cm 2 when the atomic arrangement becomes amorphous, i.e. in compositionally modulated Ni-Zr flms.
can penetrate into the films or knock atoms on the surface into the film. This is called the atomic peening effect and is characteristic of sputter deposition. This peening effect is expected to cause compressive stress in the film, and many sputter-deposited films have compressive stresses. There have been many reports about internal stress in sputtered films prepared by diode sputtering or magnetron sputtering [5-8]. In this work, films were prepared by ion beam sputtering (IBS), in which argon ions were generated and accelerated in an ion gun, and sputter and deposition took place in a higher vacuum ( 10 3 Pa) than that used for diode sputtering. Internal stress in IBS films was studied as a function of the acceleration voltage of argon ions, and the thickness of the films. Besides the several elemental films (silicon, iron, cobalt, nickel and zirconium), compositionally modulated amorphous Ni-Zr films were also prepared and their internal stress investigated.
2. Experimental procedure 1. Introduction The internal stress in thin films deposited on substrates, in general, depends on the preparation conditions. Many studies on the internal stress in vapour-deposited films have been made and its origins have been discussed. Typical features of metal films prepared by thermal vapour deposition under vacuum are that it is tensile, its magnitude is of the order of (107-108) dyn cm 2, and it becomes independent of thickness above a certain film thickness (300-1000 A). The origin of tensile stress is considered to lie in the condensation force between atoms in adjoining islands or columns which sometimes appear during the growth of films [ 1-4]. In contrast to thermal vapour deposition in which the kinetic energy of incident depositing atoms is of the order of 0.1 eV, sputter deposition provides many more energetic incident atoms, whose kinetic energy is of the order of 10 eV. The energetic incident atoms *Paper presented at the Sixth International Conference on Rapidly Quenched Metals, Montreal, August 3-7. 1987. 0025-5416/88/$3.50
Thin films were prepared by ion beam sputtering (Fig. I). After the chamber was evacuated to a pressure of 1 x 10 4 p a by a c r y o p u m p , argon gas was introduced to a Kufman-type sputter ion gun for plasma generation. The deposition was performed at a pressure of 2.6 x 10 _3 Pa, which is lower by a factor of 10 2 10--3 than the pressure for diode sputtering or magnetron sputtering. An elemental target was sputtered by argon ion ejected from the ion gun, with a typical acceleration voltage of 1.1 kV, and an ion beam current of 12 mA. The sputtered atoms whose kinetic energy is supposed to be about 10 eV were deposited on the substrate. The target holder has two elemental targets on each side, as shown in Fig. 1. By rotating it at a certain time interval, compositionally modulated films were also prepared by sputtering two elemental targets alternately. Pyrex glass disc substrates 0.5 mm thick and 26 mm in diameter were used. Film thickness was estimated from the mass and density of bulk material. By IBS deposition, the flat disc substrate was bent to a convex disc to become a spherical mirror with its ;.~ Elsevier Sequoia/Printed in The Netherlands
520
0" (x101° dynlcm
SUBSTRATE
2)
[]
RTURE 1 0
Accelerating voltage(kV) 0.5 1.0 1.5 i i i
$
PUMP q HAt
Fig. 1. Schematic diagram of ion beam sputtering system.
-0.5
deposited film on the outer side. The radius of curvature of the disc with IBS films is twice the focal distance of the spherical mirror which was measured by a simple optical system. The stress is calculated from [3, 91
-1.0
Ni film
Eb 2
a
6(1 - q)rd
|
(1)
where r is the radius of curvature of the disc, d is the thickness of the films, E = 6 . 2 x 10 ~l d y n c m 2 is Young's modulus, q = 0.24 is the Poisson ratio and b = 0.5 mm is the thickness of the substrate. The structure and strain of the crystalline lattice in the films were measured by X-ray diffraction. 3. Results and discussion
The internal stress in IBS nickel films is shown in Fig. 2 as a function of acceleration voltage of argon ions. These films were prepared keeping the ion beam current constant at 12 mA: they are about 2000 A, thick. Figure 2 indicates that the internal stress, which is compressive, increases linearly with increasing acceleration voltage up to 1.1 kV, and seems to saturate at about - 1.1 x 10 ~° dyn cm -2. The deposition rate changed with the acceleration voltage of argon ion for a constant total ion beam Current, as shown in Fig. 3. This is supposed to be because of the change of beam intensity profile, since only the central part of the wide ion beam is effective in sputtering the target. To examine the effects of the deposition rate on the internal stress, films were prepared with different deposition rates by changing the total ion beam current, but keeping the acceleration voltage constant (at 0.8 kV). In this case, the internal stresses in the films were observed to be almost equal. Therefore, the internal stress in films prepared by IBS depends on the acceleration voltage, i.e. the kinetic energy of sputtering argon ions, which affects the kinetic energy of sputtered atoms from the target and also of reflected argon ions or neutral argon
-1.5 Fig. 2. Internal stress v s . acceleration voltage. The sign of compressive stress is assigned as negative.
atoms. These experimental results suggest the importance of the atomic peening effect for the compressive stress in IBS films. The internal stress in nickel and zirconium films is shown in Fig. 4 as a function of film thickness. In the region of the film thickness above 500/~, the internal
t.i
E <
30
v
o O k.
,-- 20
°Io
u)
O Q. O
-o 10
at
vO
0.5 1.0 1.5 acceleration voltage (kV)
Fig. 3. The relationship between acceleration voltage and deposition rate.
521
C)"(x 101°dynlcm 2)
2O (deg) 69.0
d(A) 2000 3000
1000
0
4000
tx
68.5
-0.5
[]
68.C
[]
-1.0
D
• [] []
o
[] D
•
%u •
[]
•
67"50
0.1 sin2¢
-17.
0.2
Fig. 4. The scattering angle 20 of (111) reflection of nickel in the films for different scattering vectors; q5 is the angle between the scattering vector and the normal to the film plane. stress is constant and its magnitude is of the order of 101° dyn cm 2, which exceeds the yield stress of the bulk metals. The internal stresses of other elemental films (silicon, iron and cobalt) are shown in Table 1. They have also compressive stresses of the order of 10 ~° dyn cm 2 The results of X-ray diffraction for nickel films indicates that the lattice spacing for (111) (200) and (220) planes perpendicular to the film plane increases compared with those in bulk nickel. The increases of the lattice spacing normal to the film plane are caused by the compressive stress in the film plane as expected by -
el.,,i~
__
2qa E
(2)
To get reliable qualitative value of the strain parameter, the diffraction measurements were performed TABLE 1
Zr Ni Co Si Fe
Internal stress of some
elemental
films
Film thickness
Deposition rate
Internal stress
(A)
(/~s
( x 10mdyncm -2)
1855 2406 2861 2640 3461
0.309 0.501 0.53 0.734 0.48
I)
1.05 1.14 1.04 0.639 1.63
Ni film Zr film Ni-Zr film
Fig. 5. Internal stress in films of various thickness: L', ~, &, indicate nickel, zirconium and Ni Zr amorphous modulated films respectively.
with various scattering vectors inclined by q5 = 0 - 3 0 from the normal of the film plane. The in-plane stress can be estimated from the dependence of the lattice spacing d (or scattering angle 20 of Bragg reflection) on q5 as follows [10] O" - -
E do - d± ( I + q) sin24~ d±
E cot 0 20± - 204, 2( 1 + q) sin2qb
(3)
The results of the diffraction experiments are shown in Fig. 4, from which we obtain the slope 2(0± - 04,)/sin2q~. The left-hand side of eqn. (3) is estimated from eqn. ( I ) as a = 1.19 × 10")dyn cm 2 for the film prepared with 1.1 kV. Combining these two experimental results for stress and strain, we obtain E l+q
-
3.44 x 1011
which is only 22% of the bulk value E
21.4 × 101l
1+ q
I + 0.336
- 1.60 x 1012 dyn cm 2
Under the extraordinarily large compressive stress which exceeds the yield stress for bulk materials, there must be a complex stress distribution for crystallite in the film. Each crystalline grain in the film seems to experience a much larger microscopic stress than average, which causes a larger change of the lattice spacing than expected.
522 Compositionally modulated amorphous films of nickel and zirconium with modulation wavelength of about 30 .A were also prepared. The internal stress of the films is much reduced, to the order of - 107 to - l08 dyn cm -2, as shown in Fig. 5, and is less than the internal stresses of elemental crystalline nickel or zirconium films by a factor of 10 -2. X-ray diffraction showed no crystalline Bragg peak but a broad halo, which indicates that the atomic arrangement in these films is amorphous. When the sputtered atoms with high kinetic energy from elemental nickel and zirconium targets deposit alternatively and successively, the atomic interdiffusion and alloying takes place during deposition, and the amorphous phase may be stabilized. Since there is no long-range atomic ordering in the amorphous phase, atoms can easily adjust the local atomic arrangement to reduce the macroscopic internal stress, as in the viscous medium. In the crystalline phase, however, if the grain size of crystallites is larger than the range of atomic peening, it becomes difficult to release the internal stress because of the rigidity from long-range atomic ordering.
4. Conclusions Films prepared by ion beam sputtering have strong compressive internal stresses of the order of
- 101° dyn cm -2. Under the extraordinary large compressive stress, the effective value of E/( 1 + q) of nickel films turns out to be only 22% of the bulk value. The internal stress in amorphous films was reduced to the order of - 107 to - 108 dyn cm -2.
Acknowledgment The authors are grateful to Prof. H. Sekizawa for valuable discussion.
References 1 K. Kinosita and H. Kondo, J. Phys. Soc. Jpn., 15(1960), 1339. K. Kinosita, K. Maki, K. Nakamizo and K. Takeuchi, Jpn. J. Appl. Phys., 6(1967) 42. 2 H. S, Story and R. W. Hoffman. Proc. Phys. Soc. B, 70 (1957) 950. 3 J. D. Finegan and R. W. Hoffman, J. Appl. Phys., 30 (1950) 597. 4 P. Chaudhari, J. Vac. Sci. Technol., 9(1971) 520. 5 C. T. Wu, Thin Solid Films, 64(1979) 103. 6 J. A. Thornton J. Tabock and D. W. Hoffman, Thin Solid Films, 64(1979) 111. 7 D. W. Hoffman and J. A. Thornton, Thin Solid Films, 40 (1977) 355; Thin Solid Films, 45(1977) 387. 8 J. A. Thornton and D. W. Hoffman, J. Vac. Sci. Technol., 14(1977) 164. 9 L. I. Maissel and R. Glang (eds.), Handbook of Thin Film Technology, McGraw-Hill, New York. 10 B. D. Cullity, Elements of X-Ray Diffraction, 2nd edn., Addison-Wesley, New York.
519
Internal Stress in IBS Films* H. K O N D O and T. M I Z O G U C H I
Faculty ol" Science, Gakushuin University, l 5 I Mejiro, Toshimaku, Tokyo 171 (Japan)
Abstract The internal stress in films prepared by ion beam sputtering (IBS) was investigated as a function of the acceleration voltage V~ of the sputtering argon ions and thefilm thickness. IBS films of nickel and zirconium have strong compressive stresses of the order of -lO~°dvn cm 2 at V~ = l.l kV, which decrease with decreasing acceleration voltage. The atomic peening effect of incident atoms with high kinetic energy is considered to make a dominant contribution to the internal stress,
X-ray diffraction for nickel films shows that the lattice spacing perpendicular to the film plane increases because of a strong in-plane compressive stress. The effecth;e value El(1 + q), where E is Young's modulus and q is the Poisson ratio, of the films is only 22% of the bulk value. The internal stress is reduced to the order of - 107 to - 108 dyn cm 2 when the atomic arrangement becomes amorphous, i.e. in compositionally modulated Ni-Zr flms.
can penetrate into the films or knock atoms on the surface into the film. This is called the atomic peening effect and is characteristic of sputter deposition. This peening effect is expected to cause compressive stress in the film, and many sputter-deposited films have compressive stresses. There have been many reports about internal stress in sputtered films prepared by diode sputtering or magnetron sputtering [5-8]. In this work, films were prepared by ion beam sputtering (IBS), in which argon ions were generated and accelerated in an ion gun, and sputter and deposition took place in a higher vacuum ( 10 3 Pa) than that used for diode sputtering. Internal stress in IBS films was studied as a function of the acceleration voltage of argon ions, and the thickness of the films. Besides the several elemental films (silicon, iron, cobalt, nickel and zirconium), compositionally modulated amorphous Ni-Zr films were also prepared and their internal stress investigated.
2. Experimental procedure 1. Introduction The internal stress in thin films deposited on substrates, in general, depends on the preparation conditions. Many studies on the internal stress in vapour-deposited films have been made and its origins have been discussed. Typical features of metal films prepared by thermal vapour deposition under vacuum are that it is tensile, its magnitude is of the order of (107-108) dyn cm 2, and it becomes independent of thickness above a certain film thickness (300-1000 A). The origin of tensile stress is considered to lie in the condensation force between atoms in adjoining islands or columns which sometimes appear during the growth of films [ 1-4]. In contrast to thermal vapour deposition in which the kinetic energy of incident depositing atoms is of the order of 0.1 eV, sputter deposition provides many more energetic incident atoms, whose kinetic energy is of the order of 10 eV. The energetic incident atoms *Paper presented at the Sixth International Conference on Rapidly Quenched Metals, Montreal, August 3-7. 1987. 0025-5416/88/$3.50
Thin films were prepared by ion beam sputtering (Fig. I). After the chamber was evacuated to a pressure of 1 x 10 4 p a by a c r y o p u m p , argon gas was introduced to a Kufman-type sputter ion gun for plasma generation. The deposition was performed at a pressure of 2.6 x 10 _3 Pa, which is lower by a factor of 10 2 10--3 than the pressure for diode sputtering or magnetron sputtering. An elemental target was sputtered by argon ion ejected from the ion gun, with a typical acceleration voltage of 1.1 kV, and an ion beam current of 12 mA. The sputtered atoms whose kinetic energy is supposed to be about 10 eV were deposited on the substrate. The target holder has two elemental targets on each side, as shown in Fig. 1. By rotating it at a certain time interval, compositionally modulated films were also prepared by sputtering two elemental targets alternately. Pyrex glass disc substrates 0.5 mm thick and 26 mm in diameter were used. Film thickness was estimated from the mass and density of bulk material. By IBS deposition, the flat disc substrate was bent to a convex disc to become a spherical mirror with its ;.~ Elsevier Sequoia/Printed in The Netherlands
520
0" (x101° dynlcm
SUBSTRATE
2)
[]
RTURE 1 0
Accelerating voltage(kV) 0.5 1.0 1.5 i i i
$
PUMP q HAt
Fig. 1. Schematic diagram of ion beam sputtering system.
-0.5
deposited film on the outer side. The radius of curvature of the disc with IBS films is twice the focal distance of the spherical mirror which was measured by a simple optical system. The stress is calculated from [3, 91
-1.0
Ni film
Eb 2
a
6(1 - q)rd
|
(1)
where r is the radius of curvature of the disc, d is the thickness of the films, E = 6 . 2 x 10 ~l d y n c m 2 is Young's modulus, q = 0.24 is the Poisson ratio and b = 0.5 mm is the thickness of the substrate. The structure and strain of the crystalline lattice in the films were measured by X-ray diffraction. 3. Results and discussion
The internal stress in IBS nickel films is shown in Fig. 2 as a function of acceleration voltage of argon ions. These films were prepared keeping the ion beam current constant at 12 mA: they are about 2000 A, thick. Figure 2 indicates that the internal stress, which is compressive, increases linearly with increasing acceleration voltage up to 1.1 kV, and seems to saturate at about - 1.1 x 10 ~° dyn cm -2. The deposition rate changed with the acceleration voltage of argon ion for a constant total ion beam Current, as shown in Fig. 3. This is supposed to be because of the change of beam intensity profile, since only the central part of the wide ion beam is effective in sputtering the target. To examine the effects of the deposition rate on the internal stress, films were prepared with different deposition rates by changing the total ion beam current, but keeping the acceleration voltage constant (at 0.8 kV). In this case, the internal stresses in the films were observed to be almost equal. Therefore, the internal stress in films prepared by IBS depends on the acceleration voltage, i.e. the kinetic energy of sputtering argon ions, which affects the kinetic energy of sputtered atoms from the target and also of reflected argon ions or neutral argon
-1.5 Fig. 2. Internal stress v s . acceleration voltage. The sign of compressive stress is assigned as negative.
atoms. These experimental results suggest the importance of the atomic peening effect for the compressive stress in IBS films. The internal stress in nickel and zirconium films is shown in Fig. 4 as a function of film thickness. In the region of the film thickness above 500/~, the internal
t.i
E <
30
v
o O k.
,-- 20
°Io
u)
O Q. O
-o 10
at
vO
0.5 1.0 1.5 acceleration voltage (kV)
Fig. 3. The relationship between acceleration voltage and deposition rate.
521
C)"(x 101°dynlcm 2)
2O (deg) 69.0
d(A) 2000 3000
1000
0
4000
tx
68.5
-0.5
[]
68.C
[]
-1.0
D
• [] []
o
[] D
•
%u •
[]
•
67"50
0.1 sin2¢
-17.
0.2
Fig. 4. The scattering angle 20 of (111) reflection of nickel in the films for different scattering vectors; q5 is the angle between the scattering vector and the normal to the film plane. stress is constant and its magnitude is of the order of 101° dyn cm 2, which exceeds the yield stress of the bulk metals. The internal stresses of other elemental films (silicon, iron and cobalt) are shown in Table 1. They have also compressive stresses of the order of 10 ~° dyn cm 2 The results of X-ray diffraction for nickel films indicates that the lattice spacing for (111) (200) and (220) planes perpendicular to the film plane increases compared with those in bulk nickel. The increases of the lattice spacing normal to the film plane are caused by the compressive stress in the film plane as expected by -
el.,,i~
__
2qa E
(2)
To get reliable qualitative value of the strain parameter, the diffraction measurements were performed TABLE 1
Zr Ni Co Si Fe
Internal stress of some
elemental
films
Film thickness
Deposition rate
Internal stress
(A)
(/~s
( x 10mdyncm -2)
1855 2406 2861 2640 3461
0.309 0.501 0.53 0.734 0.48
I)
1.05 1.14 1.04 0.639 1.63
Ni film Zr film Ni-Zr film
Fig. 5. Internal stress in films of various thickness: L', ~, &, indicate nickel, zirconium and Ni Zr amorphous modulated films respectively.
with various scattering vectors inclined by q5 = 0 - 3 0 from the normal of the film plane. The in-plane stress can be estimated from the dependence of the lattice spacing d (or scattering angle 20 of Bragg reflection) on q5 as follows [10] O" - -
E do - d± ( I + q) sin24~ d±
E cot 0 20± - 204, 2( 1 + q) sin2qb
(3)
The results of the diffraction experiments are shown in Fig. 4, from which we obtain the slope 2(0± - 04,)/sin2q~. The left-hand side of eqn. (3) is estimated from eqn. ( I ) as a = 1.19 × 10")dyn cm 2 for the film prepared with 1.1 kV. Combining these two experimental results for stress and strain, we obtain E l+q
-
3.44 x 1011
which is only 22% of the bulk value E
21.4 × 101l
1+ q
I + 0.336
- 1.60 x 1012 dyn cm 2
Under the extraordinarily large compressive stress which exceeds the yield stress for bulk materials, there must be a complex stress distribution for crystallite in the film. Each crystalline grain in the film seems to experience a much larger microscopic stress than average, which causes a larger change of the lattice spacing than expected.
522 Compositionally modulated amorphous films of nickel and zirconium with modulation wavelength of about 30 .A were also prepared. The internal stress of the films is much reduced, to the order of - 107 to - l08 dyn cm -2, as shown in Fig. 5, and is less than the internal stresses of elemental crystalline nickel or zirconium films by a factor of 10 -2. X-ray diffraction showed no crystalline Bragg peak but a broad halo, which indicates that the atomic arrangement in these films is amorphous. When the sputtered atoms with high kinetic energy from elemental nickel and zirconium targets deposit alternatively and successively, the atomic interdiffusion and alloying takes place during deposition, and the amorphous phase may be stabilized. Since there is no long-range atomic ordering in the amorphous phase, atoms can easily adjust the local atomic arrangement to reduce the macroscopic internal stress, as in the viscous medium. In the crystalline phase, however, if the grain size of crystallites is larger than the range of atomic peening, it becomes difficult to release the internal stress because of the rigidity from long-range atomic ordering.
4. Conclusions Films prepared by ion beam sputtering have strong compressive internal stresses of the order of
- 101° dyn cm -2. Under the extraordinary large compressive stress, the effective value of E/( 1 + q) of nickel films turns out to be only 22% of the bulk value. The internal stress in amorphous films was reduced to the order of - 107 to - 108 dyn cm -2.
Acknowledgment The authors are grateful to Prof. H. Sekizawa for valuable discussion.
References 1 K. Kinosita and H. Kondo, J. Phys. Soc. Jpn., 15(1960), 1339. K. Kinosita, K. Maki, K. Nakamizo and K. Takeuchi, Jpn. J. Appl. Phys., 6(1967) 42. 2 H. S, Story and R. W. Hoffman. Proc. Phys. Soc. B, 70 (1957) 950. 3 J. D. Finegan and R. W. Hoffman, J. Appl. Phys., 30 (1950) 597. 4 P. Chaudhari, J. Vac. Sci. Technol., 9(1971) 520. 5 C. T. Wu, Thin Solid Films, 64(1979) 103. 6 J. A. Thornton J. Tabock and D. W. Hoffman, Thin Solid Films, 64(1979) 111. 7 D. W. Hoffman and J. A. Thornton, Thin Solid Films, 40 (1977) 355; Thin Solid Films, 45(1977) 387. 8 J. A. Thornton and D. W. Hoffman, J. Vac. Sci. Technol., 14(1977) 164. 9 L. I. Maissel and R. Glang (eds.), Handbook of Thin Film Technology, McGraw-Hill, New York. 10 B. D. Cullity, Elements of X-Ray Diffraction, 2nd edn., Addison-Wesley, New York.