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Cu interface studied by RBS, NRS, SEM techniques
Nuclear
Instruments
and Methods
in Physics
Research B45 (1990) 651-657
651
worth-Holland
ION BOMBARDMENT EFFECT ON A Nb/Cu BY BBS, NRS, SEM TECHNIQUES
INTERFACE
STUDIED
M.
EL BOUANANI, A. CHEVARIER, N. CHEVARIER, E. GERLIC, N. MONCOFFRE and M. STERN
Institut de Physique NuclPaire de Lyon, IN2P3_CNRS/UniversitP 69622 Villeurban~e Cedex, France
Claude Bernard 43, Bd du ii Nouembre 1928,
J.C. MAGNE and B. AUNE D$artement
de Physique %ucEaire, Centre d’Etudes NuclPaires de Saclay, BP
no.
2, 9119I Gif-stir-Yvette, France
Niobium is used to make cavities of high-energy particle accelerators because of its superconducting properties. The quality of radiofrequency superconducting cavities is closely related to the purity of the material surface. Tests are done on a single cell cavity made of a few pm thick niobium layer sputtered onto a copper cavity. The goal is to obtain adequate structure in order to get high acceleration fields as obtained in pure niobium cells. The cavity thus obtained should improve the thermal stability at high field. With this aim, carbon and oxygen contamination at the surface and at the interface of the Nb/Cu device are measured using RBS and NRS techniques. In order to simulate the energy deposition occurring during particle acceleration, Nb/Cu samples have been The element distribution evolution is then followed by RBS. irradiated with a 600 keV argon beam from 1Ol6 to 3 X lOI ions/cm’. The surface topography is evaluated using SEM.
2. ~~ete~zation
1. In~~uetion Radiofrequency superconducting cavity technology is an attractive solution for particle accelerator projects. Very pro~sing results have been obtained with bulk niobium cavities, but this material suffers from its relatively poor thermal conductivity [l]. Since the radiofrequency intensity lies in the first 100 nm depth, the deposition of thin niobium films on copper cavities has been developed 121. The homogeneity of the niobium coating film and the contamination at the niobium surface and at the Nb/Cu interface are important parameters to be checked. In the first part of this work we present the characterization of the niobium film using RBS and elastic scattering of high-energy o-particles. Such analyses allow measurements of the niobium film thickness, niobium and copper concentration profiles, as well as carbon, oxygen surface and interface contamination. In the second part we study the ion bombardment effects on the Nb/Cu interface. Nb/Cu samples are exposed to 600 keV Ar* ion bombardment in order to simulate the effect of deposit energy. Surface topography evaluation before and after irradiation is performed using the SEM technique. The ion beam analysis presented in the first part is used to profile the coating film elements. Special attention is paid to migration of copper atoms from the Nb/Cu interface to the niobium film surface. 0168-583X/~/$03.50 (North-Holland)
Q Elsevier Science Publishers
B.V.
of the niobium fitm
1 pm niobium films have been deposited at CERN on copper samples using the magnetron diode discharge sputtering technique [3] performed on rf cavities. These 10 X 30 mm’ samples are cut from strips placed in a test cavity. 2.1. R&Y profifing RBS of os-particles is used to measure the niobium film thickness uniformity, its stoichiometry and the interdiffusion of niobium and copper at the interface. Because of the 1 ,um coating, 5.7 MeV cl-particle energy is needed to separate the niobium component from the copper one. One example of a RBS spectrum is given in fig. 1. A theoretical simulation using the RBS yield is used to reproduce the experimental spectrum (fig. la). One can then deduce the niobium film thickness. Its absolute value depends on the stopping power for which the error may reach 10% but differences of thickness inside the same samples can be measured with an error of a few percent. From the simulation of RBS spectra one can also deduce the concentration profiles of niobium and copper (fig. lb). The observed interdiffusion depth between niobium and copper is about 200 pg/cm’. The computer-generate spectrum (fig. la) fits the experimental spectrum assuming the concentration profile given in fig. Ib. The calculated spectrum is X. COMPLEME~ARY
TECHNIQUE
652
M. Et Bouanani et al. / Ion bombardment effect on Nb/Cu interface
E,
L 5.7
MeV
8 =i72O
4 ENERGY
(MeV)
b
DEPTH
(M~cr~g.
Cm-21
Fig. 1. (a) Experimental and calculated (full line) RBS spectra on a Nb/Cu sample. Incident beam is perpendicularto the target. (b) Mass composition of niobium and copper vs depth. Full line: niobium; dashed line: copper. significantly changed (outside the error bars) on top of the Nb signal if one assumes at least 1% Cu weight content in the niobium films. Therefore the Rl3S technique allows us to suspect that there is no copper diffusion up to the surface niobium films within this sensitivity knit.
2.2. Alpha-elastic scattering analysis The scattering cross sections of high-energy o-particks on light elements such as carbon and oxygen are not only governed by Ru~erford scattering but have a nuclear component which can be important. The ad-
M. El Bouanani et al. / Ion bombardment effect on Nb/Cu
ENERGY
interface
(MeVl
OXYGENINTERFACE 4
f . . .. *,‘.’ *.‘- . ; .
_.
2
3
653
.
:I * . . I f ; \/I
4 ENERGY
J
5
6
’
7
(MeVl
Fig. 2. Energy spectra obtained at 7.5 and 7.7 MeV incident energies. X. COMPLEMENTARY
TECHNIQUES
654
M. El Bouanani et al. / Ion bombardment effect on Nb/Cu
vantage of such a feature in the profiling of oxygen and carbon contamination has been emphasized [4]. 2.2.1. Oxygen contamination measurements Concerning oxygen profiling the trick is to take advantage of a cross section plateau of 750 mb/sr in the 7.3-1.5 MeV energy range. The surface oxygen analysis in the Nb/Cu device is then performed with 7.5 MeV o-particles. The sensitivity limit is lOi atoms/cm2 and the surface resolution is 30 pg/cm2. Because of the 1 pm niobium coating it is necessary to raise the incident energy up to 7.7 MeV in order to profile interface oxygen contamination. Due to the straggling effect the interface depth resolution increases up to 50 pg/cm2. Typical experimental spectra are given in fig. 2. The oxygen contamination at the niobium surface or at the Nb/Cu interface is given by the ratio between the oxygen area peak A, and one channel content HN,, corresponding to the scattered e-particles at the niobium surface. For this single channel one can associate an analysed thickness Ax and consequently a number of niobium atoms per cm2, NNb. The oxygen content No [atoms/cm21 is then: N
_
o
A~ Nie(d~/dQ),b HN,
’
(da/dQ)o
If the oxygen distribution is much larger than the depth resolution it is possible to make a simulation of the whole experimental spectrum. In such a calculation the RBS yield on niobium and copper as well as the oxygen cY-scattering component are added. In the Nb/Cu samples the surface oxygen contamination was
2
interface
included in the surface depth resolution but the interface oxygen distribution was broad. Results are given in fig. 3. The interface oxygen contamination is spread over the whole niobium-copper interdiffusion zone. But it is important to notice that oxygen is shifted inside the niobium film. Such a result can be explained by the well known niobium-oxygen affinity. Moreover, we observe that such interface oxygen contamination is not constant but increases in some points where blisters are produced in the niobium film. For all the niobium samples the surface oxygen contamination is equal to 2 X 1016 atoms/cm2 on less than 40 pg/cm2 at the niobium film surface. At the Nb/Cu interface the oxygen content is 1017 atoms/cm2 extended over 300 pg/cm2.
2.2.2. Carbon contamination measurements In this analysis we take advantage of a cross section plateau of 420 mb/sr in the 5.3-5.7 MeV energy range. Such a measurement is performed simultaneously with the RBS profiling mentioned above, but now we have to analyse the low-energy part of the spectra. The carbon sensitivity is 5 x lOi atoms/cm2 and the surface depth resolution is 25 pg/cm2. In the Nb/Cu device the carbon contamination is much less than the oxygen contamination. Therefore it is not possible to get an interface carbon profiling but only a mean value (- 2 X 1016 atoms/cm2) corresponding to the involved depth (fig. 3). The surface carbon contamination is less than 8 x 1015 atoms/cm2, standing on the first 25 pg/cm2 of the niobium film.
/:::‘: x l/50
I
+-
\
+
Interface
/
!
1
‘f
I
7
T--
I.I I
0
0
i
l
f
100
.zoa
300
aa
500
6aa
ma
‘1 i
\
+
, I I
wa
930
1 lo
DEPTH Wcrog .&i-Z)
Fig. 3. Surface and interface oxygen and carbon distributions (full line: oxygen, dashed line: carbon). In order to guide eye, the niobium distribution is displayed( x l/50).
M. El Bouanani et al. / Ion bombardment effect on Nb/Cu
3. Effect of Ar + ion irradiation on the Nb/Cu
sample
Superconductivity cavities, when they act in the normal accelerating conditions, are certainly exposed to irradiation effects. In order to study these radiation effects the Nb/Cu device is exposed to Ar+ bombardment whereby the deposit energy and the created defects can be studied. 3.1. Experimental
procedure
In this study, the thickness of the coating niobium film has been set to 1200 A (about 10 times smaller than the real thickness of niobium fihn of the copper cavity). This choice is imposed by the Ar+ irradiation requirement. It also allows us to better study the Nb/Cu interface because of less straggling in the RBS measurements. Concerning the Ar+ ion energy choice, two conditions have to be met: the Ar+ ions must lose energy prior to crossing the Nb/Cu interface in order to create a maximum defect at the interface. Using the TRIM program [5] we get Ar+ ion and vacancy distributions in this Nb/Cu bilayer for different argon incident energies. Such theoretical predictions lead to a required irradiation energy of 600 keV. The deposition technique developed at CERN has been used to make the Nb/Cu device. The irradiations have been per-
interface
655
formed at room temperature in a 2 x lo-’ mbar vacuum. The doses vary from 1016 to 3 X 101’ ions/cm’. During irradiation the sample temperature is controlled at approximately 298 K. Since all our analyses are based on RBS and NRS techniques, scanning electron microscopy (SEM) was necessary to monitor the niobium surface topography, especially after ion beam irradiation. We point out that no important change in the niobium surface topography after Ar’ ion bombardment is observed. Therefore a reliable 01 RBS analysis can be performed. RBS and c1 scattering analysis have been performed before and after sample irradiation in order to get niobium, copper and oxygen profiles. 3.2. Evolution
of niobium,
copper,
oxygen
concentration
profiles RF3S is used to characterize the Nb/Cu interface modification. An o-particle incident energy of 2.5 MeV has been used. A superposition of experimental spectra before and after Arc ion beam irradiation (with a dose equal to 2 X 10” atoms/cm*) is given in fig. 4. The spectrum corresponding to the irradiated sample is artificially shifted in abscissa in order to take into account niobium sputtering during irradiation. Such a shift allows us to get a clear comparison at the Nb/Cu
Ecxr 2_5 MeV 8 =
J
17Z0
I
J ’
.
.
(100 CHANNEL
NIOBIUM
.
*
1000
NUMBER
Fig. 4. Superposition of RBS spectra before (full line) and after (dotted line) Ar ion beam irradiation. The dotted line spectrum is shifted to better reveal the interface change. X. COMPLEMENTARY TECHNIQUES
656
M. Et Bowtnani et al. / Ion bo~bard~enf effect on Nb/Cu inferface
I . interface
a
f
300
OEPTH(Microg .Cm-21
I f interface tL
200
250
0
DEPTH omicron .Cm-21 Fig. 5. (a} Mass composition of niobium, copper and oxygen (X 10) before irradiation. (b) Mass composition of niobium, copper and oxygen (X 10) after irradiation. Full line: niobium; dashed line: copper; dotted line: oxygen.
interface. A RBS computer [6] simulation program is used to reproduce the two experiments RBS spectra and to describe the concentration profile evolutions of niobium and copper before and after irradiation. Concerning interface oxygen contamination, the oxygen profile is obtained from 7.5 MeV (Y scattering analysis. The niobium, copper and oxygen distributions versus depth are plotted in figs. 5a and 5b.
After irradiation the depth of the ~ter~ffusion zone is increased by a factor 2 but no measurable change in the oxygen distribution is observed. We compare our data with results previously obtained in an ion beam mixing study on the Nb/Cu bilayer system [7]. In this work the mixing is characterized by the quantity Dt/@F,, where Dt is the square root of the diffusion length, @ the ion dose and
M. El Bouanani et al. / Ion bombardment
is the amount of damage energy deposited per angstrom normal to the interface. It can be deduced from TRIM calculations. Such a procedure enables the mixing to be compared on the basis of damage energy. The comparison shows that mixing between copper and niobium in our complex interface (including carbon and oxygen contamination) is around three times larger than the one measured [7] on a clean Nb/Cu interface. The interface oxide does not act as a barrier as was previously observed [S] in the case of the Cu/Al interface. F,,
4. Conclusions With RBS and high-energy e-scattering analysis it is possible to measure the niobium, copper, carbon and oxygen distributions in the coating film and at the Nb/Cu interface. The weight content sensitivity is 1% for copper and niobium profiling, 0.5% for carbon and 0.1% for oxygen. Changes at the Nb/Cu interface after irradiation of the sample are measured. A large increase in the interdiffusion zone is observed; however, the copper atoms do not reach the niobium film surface.
effect on Nb/Cu
interface
657
References [ll Progress Report of the Nuclear Physics Department, Centre d’Etudes Nucltaires de Saclay, Note CEA N-2550. [2] C. Benvenuti, D. Bloess, E. Chiaveri, N. Hilleret, M. Minestrini and W. Weingarten Proc. 3rd Workshop on Rf Superconductivity, ed. K.W. Shepard, Argonne National Laboratory (1987). [3] C. Benvenutti, N. Circelli and M. Hauer, Appl. Phys. Lett. 45 (1984) 583. [4] A. Chevarier, N. Chevarier, P. Deydier, H. Jaffrezic, N. Moncoffre, M. Stern and J. Tousset, J. Trace Microprobe Techn. 6 (1988) 1. [5] J.P. Biersack and L.G. Haggmark, Nucl. Instr. and Meth. 174 (1980) 257. [6] A. Chevarier and H. Jaffrezic, Progress Report, LYCEN 8932. [7] R.S. Averback, D. Peak and L.J. Thompson, Appl. Phys. A-39 (1986) 59. [8] F. Besenbacher, J. Bottinger, S.K. Nielsen and H.J. Whitlow, Appl. Phys. A-29 (1985) 141.
X. COMPLEMENTARY
TECHNIQUES
Instruments
and Methods
in Physics
Research B45 (1990) 651-657
651
worth-Holland
ION BOMBARDMENT EFFECT ON A Nb/Cu BY BBS, NRS, SEM TECHNIQUES
INTERFACE
STUDIED
M.
EL BOUANANI, A. CHEVARIER, N. CHEVARIER, E. GERLIC, N. MONCOFFRE and M. STERN
Institut de Physique NuclPaire de Lyon, IN2P3_CNRS/UniversitP 69622 Villeurban~e Cedex, France
Claude Bernard 43, Bd du ii Nouembre 1928,
J.C. MAGNE and B. AUNE D$artement
de Physique %ucEaire, Centre d’Etudes NuclPaires de Saclay, BP
no.
2, 9119I Gif-stir-Yvette, France
Niobium is used to make cavities of high-energy particle accelerators because of its superconducting properties. The quality of radiofrequency superconducting cavities is closely related to the purity of the material surface. Tests are done on a single cell cavity made of a few pm thick niobium layer sputtered onto a copper cavity. The goal is to obtain adequate structure in order to get high acceleration fields as obtained in pure niobium cells. The cavity thus obtained should improve the thermal stability at high field. With this aim, carbon and oxygen contamination at the surface and at the interface of the Nb/Cu device are measured using RBS and NRS techniques. In order to simulate the energy deposition occurring during particle acceleration, Nb/Cu samples have been The element distribution evolution is then followed by RBS. irradiated with a 600 keV argon beam from 1Ol6 to 3 X lOI ions/cm’. The surface topography is evaluated using SEM.
2. ~~ete~zation
1. In~~uetion Radiofrequency superconducting cavity technology is an attractive solution for particle accelerator projects. Very pro~sing results have been obtained with bulk niobium cavities, but this material suffers from its relatively poor thermal conductivity [l]. Since the radiofrequency intensity lies in the first 100 nm depth, the deposition of thin niobium films on copper cavities has been developed 121. The homogeneity of the niobium coating film and the contamination at the niobium surface and at the Nb/Cu interface are important parameters to be checked. In the first part of this work we present the characterization of the niobium film using RBS and elastic scattering of high-energy o-particles. Such analyses allow measurements of the niobium film thickness, niobium and copper concentration profiles, as well as carbon, oxygen surface and interface contamination. In the second part we study the ion bombardment effects on the Nb/Cu interface. Nb/Cu samples are exposed to 600 keV Ar* ion bombardment in order to simulate the effect of deposit energy. Surface topography evaluation before and after irradiation is performed using the SEM technique. The ion beam analysis presented in the first part is used to profile the coating film elements. Special attention is paid to migration of copper atoms from the Nb/Cu interface to the niobium film surface. 0168-583X/~/$03.50 (North-Holland)
Q Elsevier Science Publishers
B.V.
of the niobium fitm
1 pm niobium films have been deposited at CERN on copper samples using the magnetron diode discharge sputtering technique [3] performed on rf cavities. These 10 X 30 mm’ samples are cut from strips placed in a test cavity. 2.1. R&Y profifing RBS of os-particles is used to measure the niobium film thickness uniformity, its stoichiometry and the interdiffusion of niobium and copper at the interface. Because of the 1 ,um coating, 5.7 MeV cl-particle energy is needed to separate the niobium component from the copper one. One example of a RBS spectrum is given in fig. 1. A theoretical simulation using the RBS yield is used to reproduce the experimental spectrum (fig. la). One can then deduce the niobium film thickness. Its absolute value depends on the stopping power for which the error may reach 10% but differences of thickness inside the same samples can be measured with an error of a few percent. From the simulation of RBS spectra one can also deduce the concentration profiles of niobium and copper (fig. lb). The observed interdiffusion depth between niobium and copper is about 200 pg/cm’. The computer-generate spectrum (fig. la) fits the experimental spectrum assuming the concentration profile given in fig. Ib. The calculated spectrum is X. COMPLEME~ARY
TECHNIQUE
652
M. Et Bouanani et al. / Ion bombardment effect on Nb/Cu interface
E,
L 5.7
MeV
8 =i72O
4 ENERGY
(MeV)
b
DEPTH
(M~cr~g.
Cm-21
Fig. 1. (a) Experimental and calculated (full line) RBS spectra on a Nb/Cu sample. Incident beam is perpendicularto the target. (b) Mass composition of niobium and copper vs depth. Full line: niobium; dashed line: copper. significantly changed (outside the error bars) on top of the Nb signal if one assumes at least 1% Cu weight content in the niobium films. Therefore the Rl3S technique allows us to suspect that there is no copper diffusion up to the surface niobium films within this sensitivity knit.
2.2. Alpha-elastic scattering analysis The scattering cross sections of high-energy o-particks on light elements such as carbon and oxygen are not only governed by Ru~erford scattering but have a nuclear component which can be important. The ad-
M. El Bouanani et al. / Ion bombardment effect on Nb/Cu
ENERGY
interface
(MeVl
OXYGENINTERFACE 4
f . . .. *,‘.’ *.‘- . ; .
_.
2
3
653
.
:I * . . I f ; \/I
4 ENERGY
J
5
6
’
7
(MeVl
Fig. 2. Energy spectra obtained at 7.5 and 7.7 MeV incident energies. X. COMPLEMENTARY
TECHNIQUES
654
M. El Bouanani et al. / Ion bombardment effect on Nb/Cu
vantage of such a feature in the profiling of oxygen and carbon contamination has been emphasized [4]. 2.2.1. Oxygen contamination measurements Concerning oxygen profiling the trick is to take advantage of a cross section plateau of 750 mb/sr in the 7.3-1.5 MeV energy range. The surface oxygen analysis in the Nb/Cu device is then performed with 7.5 MeV o-particles. The sensitivity limit is lOi atoms/cm2 and the surface resolution is 30 pg/cm2. Because of the 1 pm niobium coating it is necessary to raise the incident energy up to 7.7 MeV in order to profile interface oxygen contamination. Due to the straggling effect the interface depth resolution increases up to 50 pg/cm2. Typical experimental spectra are given in fig. 2. The oxygen contamination at the niobium surface or at the Nb/Cu interface is given by the ratio between the oxygen area peak A, and one channel content HN,, corresponding to the scattered e-particles at the niobium surface. For this single channel one can associate an analysed thickness Ax and consequently a number of niobium atoms per cm2, NNb. The oxygen content No [atoms/cm21 is then: N
_
o
A~ Nie(d~/dQ),b HN,
’
(da/dQ)o
If the oxygen distribution is much larger than the depth resolution it is possible to make a simulation of the whole experimental spectrum. In such a calculation the RBS yield on niobium and copper as well as the oxygen cY-scattering component are added. In the Nb/Cu samples the surface oxygen contamination was
2
interface
included in the surface depth resolution but the interface oxygen distribution was broad. Results are given in fig. 3. The interface oxygen contamination is spread over the whole niobium-copper interdiffusion zone. But it is important to notice that oxygen is shifted inside the niobium film. Such a result can be explained by the well known niobium-oxygen affinity. Moreover, we observe that such interface oxygen contamination is not constant but increases in some points where blisters are produced in the niobium film. For all the niobium samples the surface oxygen contamination is equal to 2 X 1016 atoms/cm2 on less than 40 pg/cm2 at the niobium film surface. At the Nb/Cu interface the oxygen content is 1017 atoms/cm2 extended over 300 pg/cm2.
2.2.2. Carbon contamination measurements In this analysis we take advantage of a cross section plateau of 420 mb/sr in the 5.3-5.7 MeV energy range. Such a measurement is performed simultaneously with the RBS profiling mentioned above, but now we have to analyse the low-energy part of the spectra. The carbon sensitivity is 5 x lOi atoms/cm2 and the surface depth resolution is 25 pg/cm2. In the Nb/Cu device the carbon contamination is much less than the oxygen contamination. Therefore it is not possible to get an interface carbon profiling but only a mean value (- 2 X 1016 atoms/cm2) corresponding to the involved depth (fig. 3). The surface carbon contamination is less than 8 x 1015 atoms/cm2, standing on the first 25 pg/cm2 of the niobium film.
/:::‘: x l/50
I
+-
\
+
Interface
/
!
1
‘f
I
7
T--
I.I I
0
0
i
l
f
100
.zoa
300
aa
500
6aa
ma
‘1 i
\
+
, I I
wa
930
1 lo
DEPTH Wcrog .&i-Z)
Fig. 3. Surface and interface oxygen and carbon distributions (full line: oxygen, dashed line: carbon). In order to guide eye, the niobium distribution is displayed( x l/50).
M. El Bouanani et al. / Ion bombardment effect on Nb/Cu
3. Effect of Ar + ion irradiation on the Nb/Cu
sample
Superconductivity cavities, when they act in the normal accelerating conditions, are certainly exposed to irradiation effects. In order to study these radiation effects the Nb/Cu device is exposed to Ar+ bombardment whereby the deposit energy and the created defects can be studied. 3.1. Experimental
procedure
In this study, the thickness of the coating niobium film has been set to 1200 A (about 10 times smaller than the real thickness of niobium fihn of the copper cavity). This choice is imposed by the Ar+ irradiation requirement. It also allows us to better study the Nb/Cu interface because of less straggling in the RBS measurements. Concerning the Ar+ ion energy choice, two conditions have to be met: the Ar+ ions must lose energy prior to crossing the Nb/Cu interface in order to create a maximum defect at the interface. Using the TRIM program [5] we get Ar+ ion and vacancy distributions in this Nb/Cu bilayer for different argon incident energies. Such theoretical predictions lead to a required irradiation energy of 600 keV. The deposition technique developed at CERN has been used to make the Nb/Cu device. The irradiations have been per-
interface
655
formed at room temperature in a 2 x lo-’ mbar vacuum. The doses vary from 1016 to 3 X 101’ ions/cm’. During irradiation the sample temperature is controlled at approximately 298 K. Since all our analyses are based on RBS and NRS techniques, scanning electron microscopy (SEM) was necessary to monitor the niobium surface topography, especially after ion beam irradiation. We point out that no important change in the niobium surface topography after Ar’ ion bombardment is observed. Therefore a reliable 01 RBS analysis can be performed. RBS and c1 scattering analysis have been performed before and after sample irradiation in order to get niobium, copper and oxygen profiles. 3.2. Evolution
of niobium,
copper,
oxygen
concentration
profiles RF3S is used to characterize the Nb/Cu interface modification. An o-particle incident energy of 2.5 MeV has been used. A superposition of experimental spectra before and after Arc ion beam irradiation (with a dose equal to 2 X 10” atoms/cm*) is given in fig. 4. The spectrum corresponding to the irradiated sample is artificially shifted in abscissa in order to take into account niobium sputtering during irradiation. Such a shift allows us to get a clear comparison at the Nb/Cu
Ecxr 2_5 MeV 8 =
J
17Z0
I
J ’
.
.
(100 CHANNEL
NIOBIUM
.
*
1000
NUMBER
Fig. 4. Superposition of RBS spectra before (full line) and after (dotted line) Ar ion beam irradiation. The dotted line spectrum is shifted to better reveal the interface change. X. COMPLEMENTARY TECHNIQUES
656
M. Et Bowtnani et al. / Ion bo~bard~enf effect on Nb/Cu inferface
I . interface
a
f
300
OEPTH(Microg .Cm-21
I f interface tL
200
250
0
DEPTH omicron .Cm-21 Fig. 5. (a} Mass composition of niobium, copper and oxygen (X 10) before irradiation. (b) Mass composition of niobium, copper and oxygen (X 10) after irradiation. Full line: niobium; dashed line: copper; dotted line: oxygen.
interface. A RBS computer [6] simulation program is used to reproduce the two experiments RBS spectra and to describe the concentration profile evolutions of niobium and copper before and after irradiation. Concerning interface oxygen contamination, the oxygen profile is obtained from 7.5 MeV (Y scattering analysis. The niobium, copper and oxygen distributions versus depth are plotted in figs. 5a and 5b.
After irradiation the depth of the ~ter~ffusion zone is increased by a factor 2 but no measurable change in the oxygen distribution is observed. We compare our data with results previously obtained in an ion beam mixing study on the Nb/Cu bilayer system [7]. In this work the mixing is characterized by the quantity Dt/@F,, where Dt is the square root of the diffusion length, @ the ion dose and
M. El Bouanani et al. / Ion bombardment
is the amount of damage energy deposited per angstrom normal to the interface. It can be deduced from TRIM calculations. Such a procedure enables the mixing to be compared on the basis of damage energy. The comparison shows that mixing between copper and niobium in our complex interface (including carbon and oxygen contamination) is around three times larger than the one measured [7] on a clean Nb/Cu interface. The interface oxide does not act as a barrier as was previously observed [S] in the case of the Cu/Al interface. F,,
4. Conclusions With RBS and high-energy e-scattering analysis it is possible to measure the niobium, copper, carbon and oxygen distributions in the coating film and at the Nb/Cu interface. The weight content sensitivity is 1% for copper and niobium profiling, 0.5% for carbon and 0.1% for oxygen. Changes at the Nb/Cu interface after irradiation of the sample are measured. A large increase in the interdiffusion zone is observed; however, the copper atoms do not reach the niobium film surface.
effect on Nb/Cu
interface
657
References [ll Progress Report of the Nuclear Physics Department, Centre d’Etudes Nucltaires de Saclay, Note CEA N-2550. [2] C. Benvenuti, D. Bloess, E. Chiaveri, N. Hilleret, M. Minestrini and W. Weingarten Proc. 3rd Workshop on Rf Superconductivity, ed. K.W. Shepard, Argonne National Laboratory (1987). [3] C. Benvenutti, N. Circelli and M. Hauer, Appl. Phys. Lett. 45 (1984) 583. [4] A. Chevarier, N. Chevarier, P. Deydier, H. Jaffrezic, N. Moncoffre, M. Stern and J. Tousset, J. Trace Microprobe Techn. 6 (1988) 1. [5] J.P. Biersack and L.G. Haggmark, Nucl. Instr. and Meth. 174 (1980) 257. [6] A. Chevarier and H. Jaffrezic, Progress Report, LYCEN 8932. [7] R.S. Averback, D. Peak and L.J. Thompson, Appl. Phys. A-39 (1986) 59. [8] F. Besenbacher, J. Bottinger, S.K. Nielsen and H.J. Whitlow, Appl. Phys. A-29 (1985) 141.
X. COMPLEMENTARY
TECHNIQUES