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Sputtering of titanium and zirconium by fusion plasma impurity ions
Journal
of Nuclear
Materials
133&134
(1985)
301
301-304
SPUTTERING OF TITANIUM AND ZIRCONIUM BY FUSION PLASMA IMPURITY IONS E. HECHTL Phwik
- Deparmlenr,
Technische
~n~~ers~~~f ~~~nchen, D - 8046
Garchin~/~~nchen.
Federuri Republic
of German_>
J. BOHDANSKY Max - Plunck . Instiiur
fiir Plasmaphysik,
EURA TOM
Associarion,
D -8046 Gurching/Miinchen,
Federal
Republic
of Germon)
Sputtering yields of the getter materials titanium and zirconium for incident ion energies in the range of 100 eV to 20 keV are reported. In addition to ions of the respective target materials, Of and Ne i- ions were also used as projectiles. Measurements were made at room temperature and in the case of oxygen also at a target temperature of 550°C to investigate chemical effects. Experimental data are determined by the integral mass change method. Experimental results are compared with those calculated from an analytical relation based on a modified collision cascade theory.
1. Introduction Sputtering is the main impurity release mechanism in fusion devices. Therefore it is important to establish a broad data base on the sputtering behavior of relevant materials. The getter materials titanium and zirconium (and their compounds such as TIC) are possible coatings for the first wall of fusion machines. For these metals sputtering yields with light ions have been reported earlier [l]. In the present paper the sputtering effect of plasma impurity ions is studied. Data are reported on sputtering by oxygen and on selfsputtering of titanium and zirconium. Since sputtering with oxygen is expected to be temperature dependent, yields were measured at two different target temperatures, room temperature and 550°C (the possible upper limit for the temperature of the first wall in fusion reactors). At the temperature of 550°C the diffusion of oxygen into the target materials investigated here is still low, therefore the incident oxygen remains in the surface and an oxide layer is formed. Under these conditions the sputtering yield is influenced by oxide formation [2], and the sputtering behavior is different from purely physical sputtering. For comparison sputtering yields with Ne+ ions were also measured. In this latter case only physical sputtering occurs and the yields approximate the amount of physical sputtering with oxygen (because neon and oxygen have similar masses).
2. Experimental The ion bombardments were performed in a differentially pumped chamber which is linked to a Harwell-type isotope separator. The beam retardation system and the irradiation setup is described elsewhere i3.41. The temperature of the targets was measured with an infrared thermometer [S] calibrated with a micropyrometer [6]. The accuracy of the infrared thermometer is 0022-3115/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
5°C. The calibration was done at 700°C because this is the lowest temperature measurable with the micropyrometer. The base pressure in the sputtering chamber is 1 x 10e6 Pa. During operation this pressure rises to a few times lo-’ Pa. The sample size was 10 X 20 mm’, and the current density on the samples was about 0.1 mA/cm2. The samples were polished to mirror finish. The irradiation time was chosen to produce a mass loss of approximately 100 pg in each sample. Masses were determined with a Mettler ME 22 microbalance with an accuracy better than 1 pg. The sputtering yields are calculated from the mass change according to either of formulas (1) and (2). Y = - N,Am/M,N, Y-
-N,Am/M,N+
(1) 1.
(2)
N, is Avogadro’s number, ML the target atomic mass in g/mol (M, being the ion mass), Am the measured mass increase of the target, and N the primary ion dose. Eq. (1) allows to calculate the sputtering yield for sputtering with gaseous projectiles, i.e. when no buildup of the projectiles occurs. This formula does not take into account the mass of the implanted ions which is justified in large dose measurements, when the mass loss by sputtering is much higher than any mass gain due to the implantation of bombarding ions in the projected range. Eq. (2) yields selfsputtering values if reflected ions are also counted as sputtered target atoms. Titanium and zirconium getter gases when heated. A control experiment without impinging ion beam was made to determine the mass gain of the samples during heating. The samples were heated for an equal time interval and kept in the same vacuum as during ion bombardment. The mass of the gettered gases was then found by weighing the samples and was added to the mass loss of the sputtered targets. Selfsputtering below a certain energy results in a buildup of material (Y < I). Therefore oxygen molecules from the rest gas bomdarding the sputtering samples
302
E. Hechrl, J. Bohdansky
/ Sputrermg
(and sticking to the surface) will be buried by impinging ions of the target material. The partial pressure of oxygen in the sputtering chamber during bombardment is about 1 x 10m6 Pa. This oxygen pressure together with the ion current density used yields a concentration of oyxgen of no more than 1 at% in the layer built-up. The error in mass change due to this oxygen buildup is about 1% for titanium and about 0.3% for zirconium. The uncertainty in the yield data without error bars is estimated to be 5% except for the selfsputtering data below 1 keV and the yield data for 55O’C target temperature. In the two latter cases the uncertainty is estimated to be 10%. In the case of selfsputtering the uncertainty is larger because the yield is caiculated from the sum of two terms, viz buildup and sputtering. The higher uncertainty of the data at 550°C is caused by the mass gain due to gettering. Again, the measured mass change is a superposition, here between the sputtering loss and the gettering gain.
of tltumum
cl
and zircomum
-
TlTdNlUrj
!
10-Z L-L 50
1IO2
’
’ !I’j’/
1 i”
IO3
ION ENERGY Fig. 1. Selfsputtering
‘I”IO1
yields of titanium
and room temperature
/
as a function
at normal
using eq. (3)
given in table 2.
given by
3. Results and discussion
Y(E,,.O)=Q[3.441,/~
In(E,,/E,,+2.718)]
x f 1 + 6.355JW
The results are summarized in table 1 and plotted in figs. 1 to 4. In these figures the data are compared to an analytical relation presented by bold solid lines. A detailed discussion of this analytical relation has been published recently [7,8]. The analytical expression is
+ E()/E,&.882JE,/E;,
- 1.708)]
x [ 1 - t E,,/E,)2’3]
(1 - &h/E,,
)’
Table 1 Sputtering
yield data for titanium
and zirconium
bombarded
with ions of the respective
target materials,
0’
and Ne’
Zr
Target
Ti
Ion
Tif
0*
RT
RT
energy
550°C
Ne+
Zr+
0+
RT
RT
RT
Ne’
550°C
RT
(eV) 90 120 150 200 250 300 400 500 600 700 1000 1500 3000 5500 10000 20 000
0.011 0.090 0.12 0.24
0.148
0.34
0.24 0.036
0.039
0.34
0.079 0.025 0.066 0.119 0.22
0.48 0.62 1 .oo 1.79 2.18
mcidcnce
of ion energy. The solid
line represents the result of model calculations with the parameters
I”5x101
leV1
0.185 0.039 0.31 0.056
0.067 0.103 0.185 0.30 0.37 0.30 0.20
0.59
0.61
0.19 0.82 0.39 0.39 0.32
1.11 0.72
1 so 2.46 3.17
0.095 0.138 0.201 0.27 0.204 0.130
0.55 0.14 0.74 0.28 0.31 0.22
0.91 0.60
I (3)
E. Hechtl, J. Bohdansky / Sputtering of titanium and zirconrum Table 2 The constants
Q, Err
and Eu, for different
Ti+ --* Ti
O++Ti
Ne+ --) Ti
Zr+ -+ Zr
O-+ --f Zr
Net
3.7 118000 40
0.8 24800 200
2.6 33970 55
9.6 476000 155
0.8 45 800 150
2.7 60690 75
of the analytical
Q (atom/ion) %=(eV) E,,(eV)
,&2i lo2
I
/
,/,,
relation
/
1
L1l
IO3 ION ENERGY
Fig. and line with
303
IO&
I
I
1
i
5x10c
[eV1
2. Selfsputtering yields of zirconium at normal incidence room temperature as a function of ion energy. The solid represents the result of model calculations using eq. (3) the parameters given in table 2.
ion target combinations
Y( E,, 0) is the total sputtering yield at normal ion incidence. The ion energy is E,. (2, ETF, and E,, are three parameters depending on the ion target combination. Q is a fitting parameter, E,, is the Thomas Fermi energy, and E,, is the threshold energy for sputtering. In table 2 these parameters are given for the ion target combinations discussed in this work. The results for selfsputtering of Ti and Zr are in good agreement with the analytical relation (figs. 1 and 2). The sputtering process can be explained by the cascade theory [9] if corrections for the low ion energy regime are made [7]. Eq. (3) is convenient for the use in computer programs for plasma simulation. For sputtering by oxygen this model calculation is not expected to be relevant because the formation of surface oxides leads to changes of the surface binding energy and to depletion of the metal atoms in the surface layer, which is not included in the cascade theory [9]. Nevertheless, data of oxygen sputtering are in reasonable agreement with eq. (3) for ion energies between 500 eV and 5 keV. At lower and higher energies the agreement is only moderate. Therefore in the case of oxygen, eq. (3) can
I//,/j
,,,,;
IO3 ION
ENERGY
[eVl
Fig. 3. Sputtering yields of titanium versus ion energy. The projectiles are O+ and Ne+. The bold lines are drawn according to the analytical relation eq. (3) with the parameters given in table 2. In the case of 0’ bombardment, thin lines for the best fit are drawn also.
-+ Zr
ION ENERGY
10L
, /j 5x10L
1eV1
Fig. 4. Sputtering yields of zirconium versus ion energy. The projectiles are O+ and Ne+. The bold lines are drawn according to the analytical relation eq. (3) with the parameters given in table 2. In the case of 0.’ bombardment, thin lines for the best fit are drawn also.
be considered as an empirical fit in the energy range between 500 eV and 5 keV. The sputtering yield for 0’ predicted by the cascade theory is similar to that predicted for Net. The yields for both projectiles. O+ and Net. have been measured and the results are given in figs. 3 and 4. The drastic reduction of the yield values for oxygen sputtering of metals observed earlier [4]. is also seen here. At ion energies above 5 keV a temperature dependence of the sputtering yield seems to exist. Such an effect has been observed by different authors and may be attributed to sublimation [lo] or a spike effect [I 11.
4. Summary Sputtering yield measurements of titanium and zirconium are reported. Besides selfsputtering. O+ and Ne + ions have been used as projectiles. Sputtering yields for 0’ bombardment above 5 keV are higher at 550°C target temperature than those at room temperature. Comparison of the experimental data with an analytical expresslon showjs good agreement in the cases of selfsputtering. but only limited agreement for 0’ bomdardment.
Acknowledgements
The authors W. Ottenberger
are very indebted to R. Obermaier for valuable technical assistance.
and
References and W. Ottenberger. IPP-Report 111 J. Roth, J. Bohdansky Y/26 (1979). PI E. Hechtl. J. Bohdansky and J. Roth, J. Nucl. Mater. 103/104 (1981) 333. [31 E. Hechtl. Nucl. Instrum. Methods 186 (1981) 453. J. Nucl. Mater. 122/123 141 E. Hechtl and J. Bohdansky, (1984) 1431. by EL Technology, Carpinteria, [51 Infrared Thermometer California, USA. by Pyro-Werke GmbH Hannover, Heb161 Micro Pqromrter helstr. 5. Fed. Rep. of Germany. Nucl. Instrum. Methods Phys. Res. 82 171 J. Bohdansky. (1984) 587 R.A. Langly, J. Bohdansky. W. Eckstein, P. Mloduszewskl. J. Roth. E. Taglauer. E.W. Thomas, H. Verbeek and K.L. Wilson. Nucl. Fusion. Special Issue 1984. Data Compendium for Plasma-Surface interactions (IAEA, Vwnna 19X4). P. Sigmund. in: Sputtering by Particle Bombardment I. Ed. R. Behrisch (Springer-VerIag. Berlin 1981). M. Saidoh. Workshop on Synergistic Effects. Nagoya. Japan 1984. to be published in Radiation Effects. R. Kelly and N.Q. Lam, Radiation Effects 19 (1973) 39.
of Nuclear
Materials
133&134
(1985)
301
301-304
SPUTTERING OF TITANIUM AND ZIRCONIUM BY FUSION PLASMA IMPURITY IONS E. HECHTL Phwik
- Deparmlenr,
Technische
~n~~ers~~~f ~~~nchen, D - 8046
Garchin~/~~nchen.
Federuri Republic
of German_>
J. BOHDANSKY Max - Plunck . Instiiur
fiir Plasmaphysik,
EURA TOM
Associarion,
D -8046 Gurching/Miinchen,
Federal
Republic
of Germon)
Sputtering yields of the getter materials titanium and zirconium for incident ion energies in the range of 100 eV to 20 keV are reported. In addition to ions of the respective target materials, Of and Ne i- ions were also used as projectiles. Measurements were made at room temperature and in the case of oxygen also at a target temperature of 550°C to investigate chemical effects. Experimental data are determined by the integral mass change method. Experimental results are compared with those calculated from an analytical relation based on a modified collision cascade theory.
1. Introduction Sputtering is the main impurity release mechanism in fusion devices. Therefore it is important to establish a broad data base on the sputtering behavior of relevant materials. The getter materials titanium and zirconium (and their compounds such as TIC) are possible coatings for the first wall of fusion machines. For these metals sputtering yields with light ions have been reported earlier [l]. In the present paper the sputtering effect of plasma impurity ions is studied. Data are reported on sputtering by oxygen and on selfsputtering of titanium and zirconium. Since sputtering with oxygen is expected to be temperature dependent, yields were measured at two different target temperatures, room temperature and 550°C (the possible upper limit for the temperature of the first wall in fusion reactors). At the temperature of 550°C the diffusion of oxygen into the target materials investigated here is still low, therefore the incident oxygen remains in the surface and an oxide layer is formed. Under these conditions the sputtering yield is influenced by oxide formation [2], and the sputtering behavior is different from purely physical sputtering. For comparison sputtering yields with Ne+ ions were also measured. In this latter case only physical sputtering occurs and the yields approximate the amount of physical sputtering with oxygen (because neon and oxygen have similar masses).
2. Experimental The ion bombardments were performed in a differentially pumped chamber which is linked to a Harwell-type isotope separator. The beam retardation system and the irradiation setup is described elsewhere i3.41. The temperature of the targets was measured with an infrared thermometer [S] calibrated with a micropyrometer [6]. The accuracy of the infrared thermometer is 0022-3115/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
5°C. The calibration was done at 700°C because this is the lowest temperature measurable with the micropyrometer. The base pressure in the sputtering chamber is 1 x 10e6 Pa. During operation this pressure rises to a few times lo-’ Pa. The sample size was 10 X 20 mm’, and the current density on the samples was about 0.1 mA/cm2. The samples were polished to mirror finish. The irradiation time was chosen to produce a mass loss of approximately 100 pg in each sample. Masses were determined with a Mettler ME 22 microbalance with an accuracy better than 1 pg. The sputtering yields are calculated from the mass change according to either of formulas (1) and (2). Y = - N,Am/M,N, Y-
-N,Am/M,N+
(1) 1.
(2)
N, is Avogadro’s number, ML the target atomic mass in g/mol (M, being the ion mass), Am the measured mass increase of the target, and N the primary ion dose. Eq. (1) allows to calculate the sputtering yield for sputtering with gaseous projectiles, i.e. when no buildup of the projectiles occurs. This formula does not take into account the mass of the implanted ions which is justified in large dose measurements, when the mass loss by sputtering is much higher than any mass gain due to the implantation of bombarding ions in the projected range. Eq. (2) yields selfsputtering values if reflected ions are also counted as sputtered target atoms. Titanium and zirconium getter gases when heated. A control experiment without impinging ion beam was made to determine the mass gain of the samples during heating. The samples were heated for an equal time interval and kept in the same vacuum as during ion bombardment. The mass of the gettered gases was then found by weighing the samples and was added to the mass loss of the sputtered targets. Selfsputtering below a certain energy results in a buildup of material (Y < I). Therefore oxygen molecules from the rest gas bomdarding the sputtering samples
302
E. Hechrl, J. Bohdansky
/ Sputrermg
(and sticking to the surface) will be buried by impinging ions of the target material. The partial pressure of oxygen in the sputtering chamber during bombardment is about 1 x 10m6 Pa. This oxygen pressure together with the ion current density used yields a concentration of oyxgen of no more than 1 at% in the layer built-up. The error in mass change due to this oxygen buildup is about 1% for titanium and about 0.3% for zirconium. The uncertainty in the yield data without error bars is estimated to be 5% except for the selfsputtering data below 1 keV and the yield data for 55O’C target temperature. In the two latter cases the uncertainty is estimated to be 10%. In the case of selfsputtering the uncertainty is larger because the yield is caiculated from the sum of two terms, viz buildup and sputtering. The higher uncertainty of the data at 550°C is caused by the mass gain due to gettering. Again, the measured mass change is a superposition, here between the sputtering loss and the gettering gain.
of tltumum
cl
and zircomum
-
TlTdNlUrj
!
10-Z L-L 50
1IO2
’
’ !I’j’/
1 i”
IO3
ION ENERGY Fig. 1. Selfsputtering
‘I”IO1
yields of titanium
and room temperature
/
as a function
at normal
using eq. (3)
given in table 2.
given by
3. Results and discussion
Y(E,,.O)=Q[3.441,/~
In(E,,/E,,+2.718)]
x f 1 + 6.355JW
The results are summarized in table 1 and plotted in figs. 1 to 4. In these figures the data are compared to an analytical relation presented by bold solid lines. A detailed discussion of this analytical relation has been published recently [7,8]. The analytical expression is
+ E()/E,&.882JE,/E;,
- 1.708)]
x [ 1 - t E,,/E,)2’3]
(1 - &h/E,,
)’
Table 1 Sputtering
yield data for titanium
and zirconium
bombarded
with ions of the respective
target materials,
0’
and Ne’
Zr
Target
Ti
Ion
Tif
0*
RT
RT
energy
550°C
Ne+
Zr+
0+
RT
RT
RT
Ne’
550°C
RT
(eV) 90 120 150 200 250 300 400 500 600 700 1000 1500 3000 5500 10000 20 000
0.011 0.090 0.12 0.24
0.148
0.34
0.24 0.036
0.039
0.34
0.079 0.025 0.066 0.119 0.22
0.48 0.62 1 .oo 1.79 2.18
mcidcnce
of ion energy. The solid
line represents the result of model calculations with the parameters
I”5x101
leV1
0.185 0.039 0.31 0.056
0.067 0.103 0.185 0.30 0.37 0.30 0.20
0.59
0.61
0.19 0.82 0.39 0.39 0.32
1.11 0.72
1 so 2.46 3.17
0.095 0.138 0.201 0.27 0.204 0.130
0.55 0.14 0.74 0.28 0.31 0.22
0.91 0.60
I (3)
E. Hechtl, J. Bohdansky / Sputtering of titanium and zirconrum Table 2 The constants
Q, Err
and Eu, for different
Ti+ --* Ti
O++Ti
Ne+ --) Ti
Zr+ -+ Zr
O-+ --f Zr
Net
3.7 118000 40
0.8 24800 200
2.6 33970 55
9.6 476000 155
0.8 45 800 150
2.7 60690 75
of the analytical
Q (atom/ion) %=(eV) E,,(eV)
,&2i lo2
I
/
,/,,
relation
/
1
L1l
IO3 ION ENERGY
Fig. and line with
303
IO&
I
I
1
i
5x10c
[eV1
2. Selfsputtering yields of zirconium at normal incidence room temperature as a function of ion energy. The solid represents the result of model calculations using eq. (3) the parameters given in table 2.
ion target combinations
Y( E,, 0) is the total sputtering yield at normal ion incidence. The ion energy is E,. (2, ETF, and E,, are three parameters depending on the ion target combination. Q is a fitting parameter, E,, is the Thomas Fermi energy, and E,, is the threshold energy for sputtering. In table 2 these parameters are given for the ion target combinations discussed in this work. The results for selfsputtering of Ti and Zr are in good agreement with the analytical relation (figs. 1 and 2). The sputtering process can be explained by the cascade theory [9] if corrections for the low ion energy regime are made [7]. Eq. (3) is convenient for the use in computer programs for plasma simulation. For sputtering by oxygen this model calculation is not expected to be relevant because the formation of surface oxides leads to changes of the surface binding energy and to depletion of the metal atoms in the surface layer, which is not included in the cascade theory [9]. Nevertheless, data of oxygen sputtering are in reasonable agreement with eq. (3) for ion energies between 500 eV and 5 keV. At lower and higher energies the agreement is only moderate. Therefore in the case of oxygen, eq. (3) can
I//,/j
,,,,;
IO3 ION
ENERGY
[eVl
Fig. 3. Sputtering yields of titanium versus ion energy. The projectiles are O+ and Ne+. The bold lines are drawn according to the analytical relation eq. (3) with the parameters given in table 2. In the case of 0’ bombardment, thin lines for the best fit are drawn also.
-+ Zr
ION ENERGY
10L
, /j 5x10L
1eV1
Fig. 4. Sputtering yields of zirconium versus ion energy. The projectiles are O+ and Ne+. The bold lines are drawn according to the analytical relation eq. (3) with the parameters given in table 2. In the case of 0.’ bombardment, thin lines for the best fit are drawn also.
be considered as an empirical fit in the energy range between 500 eV and 5 keV. The sputtering yield for 0’ predicted by the cascade theory is similar to that predicted for Net. The yields for both projectiles. O+ and Net. have been measured and the results are given in figs. 3 and 4. The drastic reduction of the yield values for oxygen sputtering of metals observed earlier [4]. is also seen here. At ion energies above 5 keV a temperature dependence of the sputtering yield seems to exist. Such an effect has been observed by different authors and may be attributed to sublimation [lo] or a spike effect [I 11.
4. Summary Sputtering yield measurements of titanium and zirconium are reported. Besides selfsputtering. O+ and Ne + ions have been used as projectiles. Sputtering yields for 0’ bombardment above 5 keV are higher at 550°C target temperature than those at room temperature. Comparison of the experimental data with an analytical expresslon showjs good agreement in the cases of selfsputtering. but only limited agreement for 0’ bomdardment.
Acknowledgements
The authors W. Ottenberger
are very indebted to R. Obermaier for valuable technical assistance.
and
References and W. Ottenberger. IPP-Report 111 J. Roth, J. Bohdansky Y/26 (1979). PI E. Hechtl. J. Bohdansky and J. Roth, J. Nucl. Mater. 103/104 (1981) 333. [31 E. Hechtl. Nucl. Instrum. Methods 186 (1981) 453. J. Nucl. Mater. 122/123 141 E. Hechtl and J. Bohdansky, (1984) 1431. by EL Technology, Carpinteria, [51 Infrared Thermometer California, USA. by Pyro-Werke GmbH Hannover, Heb161 Micro Pqromrter helstr. 5. Fed. Rep. of Germany. Nucl. Instrum. Methods Phys. Res. 82 171 J. Bohdansky. (1984) 587 R.A. Langly, J. Bohdansky. W. Eckstein, P. Mloduszewskl. J. Roth. E. Taglauer. E.W. Thomas, H. Verbeek and K.L. Wilson. Nucl. Fusion. Special Issue 1984. Data Compendium for Plasma-Surface interactions (IAEA, Vwnna 19X4). P. Sigmund. in: Sputtering by Particle Bombardment I. Ed. R. Behrisch (Springer-VerIag. Berlin 1981). M. Saidoh. Workshop on Synergistic Effects. Nagoya. Japan 1984. to be published in Radiation Effects. R. Kelly and N.Q. Lam, Radiation Effects 19 (1973) 39.