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R.f. sputtered tantalum films deposited in an oxygen doped atmosphere
Thin Solid Films - Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
R57
Short Communication R.F. sputtered tantalum •ms deposited in an oxygen doped atmosphere*
P.N. BAKER
Central Research Laboratory, Edwards High Vacuum International, Manor Royal, Crawley, Sussex (Gt. Britain)
(Received November 18, 1970)
In their work on tantalum thin Films, Sosniak e t aL 1 and Nakamura e t al. 2 show that differences in the deposition parameters lead to the two different phases body-centred cubic tantalum or fl-tantalum. Work in this laboratory with a radio frequency sputtering method shows that the two phases may be produced in the same system under nominally the same conditions. It is generally believed 1,2,3 that fl-tantalum is formed only in a clean vacuum system. Westwood and Livermore4 however, believe that fl-tantalum is an impurity phase precipitated to accomodate oxygen when the solubility limit in the b.c.c, phase is exceeded. They indicate that Gerstenberg and Calbick s may not have observed the fl-phase because the electrical and structural information was obtained from two different sets of films. This paper describes the electrical and structural properties of tantalum films obtained by r.f. sputtering in an argon atmosphere doped with various concentrations of oxygen. Structural information was obtained, in the same way as reported by Westwood and Livermore, by means of an X-ray diffractometer. The pumping station used was an Edwards 12 inch coating unit, type E12E, with stainless-steel diffusion pump, baffle valve and base plate. A liquid-nitrogen trap immediatel~ above the diffusion pump and an activated-alumina trap above the rotary pump reduced water vapour in the chamber and backstreaming of pump fluids. An ultimate pressure of 4 x 10 -6 torr was obtained. The chamber was an open-ended glass cylinder, 30 cm in diameter and 36 cm in length, sealed by Viton L shaped gaskets at each end. The top of the chamber was formed by a circular aluminium plate which supported the copper water-cooled substrate table. A gas mixing cylinder was connected both by a quarter inch Speedivalve and a needle valve to the vacuum chamber. The gas pressure for sputtering was controlled with the needle valve and by adjustment of the baffle valve conductance. A non-grounded sputtering system was used,/.e., the discharge was struck between two electrodes both insulated from earth. The self-excited oscillator operated at 13.6 MHz. ~From work conducted at the Central Research Laboratory, Edwards High Vacuum International, for the M.Sc. degree at Brighton College of Technology (now Brighton Polytechnic). Thin Solid Films, 6 (1970) R57-R60
R58
SHORT COMMUNICATIONS
The electrode heads were water cooled and the targets resting on top were 4.6 cm diam. discs cut from high-purity tantalum sheet. The substrate-to-electrode distance was 4 cm. A coil was positioned around the chamber so that a magnetic field of approximately 25 gauss was produced parallel to the substrate-electrode direction. This increased the deposition rate by increasing the gas ionisation. A shutter consisting of several leaves of stainless steel supported on shaft passing through the baseplate was positioned so that the substrates were protected during a presputtering period. After presputtering the shutter was removed from the deposition area by rotation and withdrawal of the supporting shaft. Film depositions were made onto 5 cm x 5 cm x 1 mm soda glass cover slides which had been cleaned with teepol and degreased with iso-propanol. Immediately after cleaning, a slide was securely clamped to the substrate table, using two small stainless-steel corner clips, directly above one of the electrodes. The chamber and mixing cylinder were then evacuated to an ultimate pressure of 4 x 10-6 torr after which the mixing cylinder was sealed off and certain amounts of argon and oxygen to it. This gas mixture was leaked into the sputtering chamber until the pressure rose to 4 x 10 -3 torr. A presputtering period of 20 min onto the shutter was initiated to clean the tantalum targets and was followed by another pump down to remove desorbed gases due to heating of the electrode shields. The discharge was struck again and after a short period the shutter was removed to allow deposition for 8 min at a pressure of 4 x 10 -3 torr. The discharge voltage was approximately 2 kV and the current 20 mA, which lead to deposition rates of 350 A/min. Film thickness was measured by weighing slides before and after deposition and was checked in a number of cases using multiple beam interferometr'. Measurement of film resistance at 20°C and - 196°C gave values for resistivity and te perature coefficient of resistance. The crystal structure of a number of films was found without removing them from their substrates by use of the diffractometer. B.c.c. films were characterised by diffraction peaks at d = 2.35 A and 1.19 A , corresponding to the (110) and (220) reflections. ~ films were characterised by peaks at d = 2.66 A and 1.34 A, correspondinl
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300 2'00 I00
g
0 O'OI
•1
PERCENTAGE
OFOXYGEN
!
,
,.
I 0
IO
I00
IN T H E
SPUTTERING
ATMOSPHERE
Fig.1. Variation of the resistivity of r.f. sputtered tantalum f'tlms with the percentage of oxygen in the argon. Thin Solid Films, 6 (1970) R57-R60
SHORT COMMUNICATIONS
3
1500
b.cx.
R59
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500
~
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o
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b,¢~,
UJ
O,Ot
O1.1 ......
!
I0
I
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i _
I00
PERCENTAGE OF OXYGEN IN THE SPU'I:TERING ATMOSPHERE
Fig.2. Variation of the temperature coefficient of resistance of r.f. sputtered tantalum f'rims with the percentage of oxygen in the argon.
to the (200) and (400) reflections. These four were the major diffraction peaks observed by Westwood and Livermore. For films deposited in pure argon the resistivities were found to range from 100 to 400 ta~2 cm and t.c.r.'s from + 1300 to - 1 0 0 p.p.m./°C. The film with the lowest resistivity exhibited only the b.c.c, diffraction peaks whilst the film with the highest resistivity showed only the/3 peaks. A Film with intermediate resistivity had peaks from both b.c.c, and ~-tantalum. A series of films was then reactivity sputtered with oxygen. In Fig.1 the resistivities of the films are plotted against the percentage of oxygen in the sputtering atmosphere. Fig.2 shows the t.c.r.'s for the same films. Below an oxygen concentration of 4% the films fall into two groups (1) with resistivities of between 300 and 400 ta[2 cm and near zero t.c.r.'s and (2) with lower resistivities and much larger positive t.c.r.'s. The diffractometer traces showed that #-tantalum predominated in the first group and b.c.c, tantalum in the second. Above an oxygen concentration of 4% the resistivity rises and the t,c.r, falls sharply until apparently constant values are reached. Diffraction peaks from b.c.c, tantalum only were recorded in this high oxygen concentration region. Thus summarising, f'tlms with the electrical and structural characteristics of b.c.c. tantalum were observed over the whole range of oxygen concentrations in the sputtering atmosphere whereas/~-tantalum was observed only when the oxygen concentration was less than 4%. This indicates that there is not a particular oxygen concentration above which ~-tantalum is deposited to accomodate extra gas. Another factor, such as the presence or absence of water vapour, as suggested by Nakamura et at, probably decides which phase is formed. Many thanks are due to the members of the Central Research Laboratory, and Brighton College of Technology for their helpful advice, especially Dr. L. Holland, Director of Research and Development, Edwards High Vacuum International, and for his permission to publish this work. Thin Solid Films, 6 (1970) R57-R60
R60
SHORT COMMUNICATIONS
REFERENCES 1 2 3 4 5
J. Sosniak, W.J. Polito and G.A. Rozgong, J. AppL Phys., 38 (1967) 3041. M. Nakamura, M. Fujimori and Y. Nishimura, Japan. J. Appt Phy~, 9 (1970) 557. R.B. Marcus and S. Quigley, Thin Solid Films, 2 (1968) 467. W.D. Westwood and F.C. Livermore, Thin Solid Films, 5 (1970) 407. D. Gerstenberg and C.J. Calbick, J. AppL Phys., 35 (1964) 402.
Thin Solid Films, 6 (1970) R57-R60
R57
Short Communication R.F. sputtered tantalum •ms deposited in an oxygen doped atmosphere*
P.N. BAKER
Central Research Laboratory, Edwards High Vacuum International, Manor Royal, Crawley, Sussex (Gt. Britain)
(Received November 18, 1970)
In their work on tantalum thin Films, Sosniak e t aL 1 and Nakamura e t al. 2 show that differences in the deposition parameters lead to the two different phases body-centred cubic tantalum or fl-tantalum. Work in this laboratory with a radio frequency sputtering method shows that the two phases may be produced in the same system under nominally the same conditions. It is generally believed 1,2,3 that fl-tantalum is formed only in a clean vacuum system. Westwood and Livermore4 however, believe that fl-tantalum is an impurity phase precipitated to accomodate oxygen when the solubility limit in the b.c.c, phase is exceeded. They indicate that Gerstenberg and Calbick s may not have observed the fl-phase because the electrical and structural information was obtained from two different sets of films. This paper describes the electrical and structural properties of tantalum films obtained by r.f. sputtering in an argon atmosphere doped with various concentrations of oxygen. Structural information was obtained, in the same way as reported by Westwood and Livermore, by means of an X-ray diffractometer. The pumping station used was an Edwards 12 inch coating unit, type E12E, with stainless-steel diffusion pump, baffle valve and base plate. A liquid-nitrogen trap immediatel~ above the diffusion pump and an activated-alumina trap above the rotary pump reduced water vapour in the chamber and backstreaming of pump fluids. An ultimate pressure of 4 x 10 -6 torr was obtained. The chamber was an open-ended glass cylinder, 30 cm in diameter and 36 cm in length, sealed by Viton L shaped gaskets at each end. The top of the chamber was formed by a circular aluminium plate which supported the copper water-cooled substrate table. A gas mixing cylinder was connected both by a quarter inch Speedivalve and a needle valve to the vacuum chamber. The gas pressure for sputtering was controlled with the needle valve and by adjustment of the baffle valve conductance. A non-grounded sputtering system was used,/.e., the discharge was struck between two electrodes both insulated from earth. The self-excited oscillator operated at 13.6 MHz. ~From work conducted at the Central Research Laboratory, Edwards High Vacuum International, for the M.Sc. degree at Brighton College of Technology (now Brighton Polytechnic). Thin Solid Films, 6 (1970) R57-R60
R58
SHORT COMMUNICATIONS
The electrode heads were water cooled and the targets resting on top were 4.6 cm diam. discs cut from high-purity tantalum sheet. The substrate-to-electrode distance was 4 cm. A coil was positioned around the chamber so that a magnetic field of approximately 25 gauss was produced parallel to the substrate-electrode direction. This increased the deposition rate by increasing the gas ionisation. A shutter consisting of several leaves of stainless steel supported on shaft passing through the baseplate was positioned so that the substrates were protected during a presputtering period. After presputtering the shutter was removed from the deposition area by rotation and withdrawal of the supporting shaft. Film depositions were made onto 5 cm x 5 cm x 1 mm soda glass cover slides which had been cleaned with teepol and degreased with iso-propanol. Immediately after cleaning, a slide was securely clamped to the substrate table, using two small stainless-steel corner clips, directly above one of the electrodes. The chamber and mixing cylinder were then evacuated to an ultimate pressure of 4 x 10-6 torr after which the mixing cylinder was sealed off and certain amounts of argon and oxygen to it. This gas mixture was leaked into the sputtering chamber until the pressure rose to 4 x 10 -3 torr. A presputtering period of 20 min onto the shutter was initiated to clean the tantalum targets and was followed by another pump down to remove desorbed gases due to heating of the electrode shields. The discharge was struck again and after a short period the shutter was removed to allow deposition for 8 min at a pressure of 4 x 10 -3 torr. The discharge voltage was approximately 2 kV and the current 20 mA, which lead to deposition rates of 350 A/min. Film thickness was measured by weighing slides before and after deposition and was checked in a number of cases using multiple beam interferometr'. Measurement of film resistance at 20°C and - 196°C gave values for resistivity and te perature coefficient of resistance. The crystal structure of a number of films was found without removing them from their substrates by use of the diffractometer. B.c.c. films were characterised by diffraction peaks at d = 2.35 A and 1.19 A , corresponding to the (110) and (220) reflections. ~ films were characterised by peaks at d = 2.66 A and 1.34 A, correspondinl
OJtlO7
r-~-?,.*,-
I,IO 7
o
G >" I.-
SOO 4O0
I-u3 W
300 2'00 I00
g
0 O'OI
•1
PERCENTAGE
OFOXYGEN
!
,
,.
I 0
IO
I00
IN T H E
SPUTTERING
ATMOSPHERE
Fig.1. Variation of the resistivity of r.f. sputtered tantalum f'tlms with the percentage of oxygen in the argon. Thin Solid Films, 6 (1970) R57-R60
SHORT COMMUNICATIONS
3
1500
b.cx.
R59
+
°E'~" i000 h~ L) 7"
500
~
o
o
-500
kU r~
U/ U-
¢c
°t t)
I I I II
-I xlO s
+ 4-
~" _ 5 x l O s
b,¢~,
UJ
O,Ot
O1.1 ......
!
I0
I
IO
i _
I00
PERCENTAGE OF OXYGEN IN THE SPU'I:TERING ATMOSPHERE
Fig.2. Variation of the temperature coefficient of resistance of r.f. sputtered tantalum f'rims with the percentage of oxygen in the argon.
to the (200) and (400) reflections. These four were the major diffraction peaks observed by Westwood and Livermore. For films deposited in pure argon the resistivities were found to range from 100 to 400 ta~2 cm and t.c.r.'s from + 1300 to - 1 0 0 p.p.m./°C. The film with the lowest resistivity exhibited only the b.c.c, diffraction peaks whilst the film with the highest resistivity showed only the/3 peaks. A Film with intermediate resistivity had peaks from both b.c.c, and ~-tantalum. A series of films was then reactivity sputtered with oxygen. In Fig.1 the resistivities of the films are plotted against the percentage of oxygen in the sputtering atmosphere. Fig.2 shows the t.c.r.'s for the same films. Below an oxygen concentration of 4% the films fall into two groups (1) with resistivities of between 300 and 400 ta[2 cm and near zero t.c.r.'s and (2) with lower resistivities and much larger positive t.c.r.'s. The diffractometer traces showed that #-tantalum predominated in the first group and b.c.c, tantalum in the second. Above an oxygen concentration of 4% the resistivity rises and the t,c.r, falls sharply until apparently constant values are reached. Diffraction peaks from b.c.c, tantalum only were recorded in this high oxygen concentration region. Thus summarising, f'tlms with the electrical and structural characteristics of b.c.c. tantalum were observed over the whole range of oxygen concentrations in the sputtering atmosphere whereas/~-tantalum was observed only when the oxygen concentration was less than 4%. This indicates that there is not a particular oxygen concentration above which ~-tantalum is deposited to accomodate extra gas. Another factor, such as the presence or absence of water vapour, as suggested by Nakamura et at, probably decides which phase is formed. Many thanks are due to the members of the Central Research Laboratory, and Brighton College of Technology for their helpful advice, especially Dr. L. Holland, Director of Research and Development, Edwards High Vacuum International, and for his permission to publish this work. Thin Solid Films, 6 (1970) R57-R60
R60
SHORT COMMUNICATIONS
REFERENCES 1 2 3 4 5
J. Sosniak, W.J. Polito and G.A. Rozgong, J. AppL Phys., 38 (1967) 3041. M. Nakamura, M. Fujimori and Y. Nishimura, Japan. J. Appt Phy~, 9 (1970) 557. R.B. Marcus and S. Quigley, Thin Solid Films, 2 (1968) 467. W.D. Westwood and F.C. Livermore, Thin Solid Films, 5 (1970) 407. D. Gerstenberg and C.J. Calbick, J. AppL Phys., 35 (1964) 402.
Thin Solid Films, 6 (1970) R57-R60