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Sputtering and angular distribution of steel 316L irradiated by He ions
941
Journal of Nuclear Materials 162-164 (1989) 941-944 North-Holland, Amsterdam
SPUTTERING AND ANGULAR DISTRIBUTION OF STEEL 316L IRRADIATED BY He IONS B. EMMOTH
I, H. BERGSAKERI
and C.H. WU 2
’ Research I~titute of Physics, ~ss~~at~on Euraiom - NFR, S-104 05 St~khol~
Sweden 2 The NET-Team, IPP, Max-Planck-Institute of Physics, D-8046 Garching bei Miinchen, Fed. Rep. Germany.
Key words: helium, steel 316L, sputtering, angular distribution, topography Helium ions bombarded a polished surface of steel 316L, in order to investigate sputtering, angular ~s~butions and ion-induced surface topography. For perpendicular sputtering a wide, or under-cosine, distribution of sputtered particles was found. The sputtering yield, measured by the collector method, was in agreement with other methods, 0.16 atoms per ion for 4 keV He ions. Sputtering at glancing incidence, resulted in a peaked distribution about 45O away from the beam in the incidence plane. Apparantly the angular distribution is not very sensitive to the angle of incidence for ion impact in the range 45 to 60’ from the normal. The original flat surface changed its topography due to different sputtering yields of differently oriented grains, with a rougher surface as the result.
1.
Introduction
In all present medium and large size tokamaks metals exist as impurities. The main reason is that metallic surfaces are exposed to plasma somewhere, although effort are made to shield by graphite tiles as much as possible. Even for fully carbonized machines metal impurities are present [l]. A film of carbon is likely to be damaged and deteriorate with time and the possibility will be open for metal release. Additional carbonization may temporarily bury impurities. Alpha particles bombarding a surface cause considerable damage on metals by sputtering and blistering. The distribution of alpha particles escaping from the plasma is not well described, they may be thermal&d with the possibility of localized enhanced loads due to ripple-induced losses 121. A candidate as structure material for the first wall in NET is steel 316L. If the concept total cover by thick graphite can not be realized helium will hit the steel surface resulting in release of high-Z metal impurities. The angular distribution of sputtered particles is important for the optimization of the wall construction. For a polyc~stalline material like 316L, the result of erosion of nearby grains is interesting since this can cause a much rougher surface than the original one already after low fluences. In this work, a low energy beam of helium ions was used to irradiate steel samples of 316L, with the aim to investigate low energy
ion-surface interaction using a collector method [3,4], while in a subsequent study high energy helium ions and combined ion beam impact are investigated. In principle, the technique used here was to coltect sputtered particles on the surface of a semi-circular collector holder with centre at the position of the sample. Due to the expected symmetry of sputtered particle flux, the collector was mounted in the plane of the target normal and the direction of the helium beam, the incidence plane. After irradiation the target was retracted and the collector could be turned to face a high energy beam, without venting the chamber between irradiation and analysis, for a Rutherford backscattering investigation using a particle detector and corresponding electronics.
2. Experimental The sputtering low energy beam current was measured by a Faraday cup (FC), before the target was moved into position for sputtering within the surrounding collector. During sputte~ng the current was momtored on the target without electron suppression and after the end of sputtering the beam current was again checked by the FC. The total fluence could be integrated from the target current corrected by a factor given by the FC measurements. After sputtering the target was retracted and the collector rotated in order to
0022-3115/89/$03.50 @ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
942
B. Emmoth et al. / Sputtenng
face the high energy beam, 2.0 MeV, used for backscattering analysis of the collector surface. The collector surface was covered by an Al foil, with high purity > 99.999%. Sticking of heavier sputtered atoms to a surface consisting of a lighter mass material should be high [5]. All internal movements in the experimental chamber were done in situ without breaking the vacuum conditions. The basic pressure throughout the experiments was in the range of lo-’ Torr. The collector surface was analysed by Rutherford backscattering spectrometry, and the different components of steel 316L could be resolved and compared to the bulk composition (at%); Cr 16-18, Ni 10-14, MO 2-3, Fe balance. Cr is obviously possible to separate in a straightforward manner by a computer fit [6], while it is needed to input more information to fit the two components Fe and Ni. MO is well separated in the spectrum. In the fitting procedure of MO the isotopic composition is used in the program. For the total sputtering yield the sum of all components of 316L is used, but the statistical accuracy was sufficient for a study of preferential sputtering effects and differential distibutions of sputtered species, by comparing results from spectra and the known bulk composition.
3. Results Fig. 1 shows the result of perpendicular sputtering by 4 keV helium irradiation of steel 316L. All black dots correspond to measured values shown in a polar diagram. The arrow pointing to one of the dots demonstrates the relative amount of sputtered particles in the particular direction as well as the statistical error, which is about 10%. The circle shows what would be expected for a pure cosine distribution. The measured distribution shows a broadening to the sides, as compared to a pure cosine this one is under-cosine. By fitting a ~0~~0, function, where 0, is the emission angle with respect to the normal, the total yield can be determined by integration. The best fit was obtained by a = 0.7, and the sputtering yield for the sum of different steel components was found to be 0.16, which is very similar to earlier reported results [7-91. The collector method gives here a result very close to the weight-loss method, or within expected errors in all earlier determinations with other methods. For further studies of the angular distribution, sputtering was performed at two other angles of incidence, 45 and 60 O. In case of 45 O, a peaked distribution was found in the specular direction, not shown here. Fig. 2 shows the result of sputtering 60” from the
and angular distribution
of steel 316L
_l
Ion Beam
Target 316 L
Fig. 1. Angular differential sputtering yields of steel 316L for 4 keV He bombardment at normal incidence (arbitrary units). normal. The distribution is still peaked 45” from the normal and in opposite direction, similar to sputtering at 45O incidence angle, although the total yield has been increased as expected [lo]. At these energies the shape of the angular distribution is relatively unsensitive for different beam impact angles at least in the range studied here. From the fitting procedure it is found that the amount of Cr relative the other steel components in all ejection angles is about 23%, which is more than is expected from the bulk concentration. Fig. 3 shows two photographs, fig. 3a corresponds to the original surface before implantation and after polishing. The small black points are defects and structure damage. Fig. 3b shows the surface after implantation to the typical irradiation dose 0.1 C, and for the same magnification x280. The grain structure is now very pronounced due to the different sputtering yields for differently oriented grains. At a larger magnification, not shown here, there appears to be rows with bubbles at an early state of blistering. Further investigations of this matter is in progress. 4. Discussion and conclusions We have found that the sputtering yield determined for normal incidence sputtering of steel 316L by 4 keV
943 Max. at
l .
.
Tnrget
316i. Fig. 2. The angular distribution of steel material sputtered by a helium ion beam 60° from the normal. The distribution is peaked 45 * from the normal in the incidence plane.
Fig. 3. (a) The or&,inaI polished steel surface before implantation at the magnification X 280. (b) The same surface after 0.1 C helium ioa impact. The grain structure is now more pronounced due to different sputtering yiefds of differently oriented grains, at the magnification X 280.
944
B. Emmoth et al. / Sputtering
He ions, Y = 0.16, is in agreement with other methods. The angular distribution is found to be “wider” than a cosinus (cf. fig. l), which is typical for sputtering of a metal with low surface binding energy flO]. The distribution can best be described by a function propotional to COSV, where cy= 0.7, and the amount of sputtered material is depending on the ejection angle and the total irradiation dose. For different angle of incidence, and the same energy, the sputtering yield increased as expected, and ejected material was peaked 45’ away from the beam in the incidence plane, apparently independently of the angle of incidence in the range 45 to 60 O. This information should be useful for the first wall construction. The different components of steel 316L were resolved and studied independently. One observation was that Cr on the collector surface corresponded to 23 at%, instead of the bulk composition 14-18 at%. Our interpretation is that since Cr is enriched at the surface of steel, either the erosion due to sputtering of the sample surface was not sufficient to reach bulk ~n~entration, or Cr diffused to the surface during irradiation. For the different ejection angles the same relative concentration was found in all directions indicating that the distribution of Cr and Fe + Ni were the same or similar within the statistical error. In case of MO, statistics was poor, however, a wide or more pronounced under-cosine ~st~bution seemed to be the case, but this still has to be investigated in more detail. In an experiment where a binary compound was studied [ll] it was also found that the angle for maximum differential sputtering was larger for the heavier component, which seem to be similar to our result. Surface segregation may also be a contributing effect [12]. In conclusion, results from elementary and two-component targets are essentially also valid for technical steels. The irradiated surface was investigated by mi-
and angular disiribution
ofsteel316.L
croscopy before and after helium implantation. The surface topography was clearly changed due to different sputtering of different grains. This is expected to result in an increasing surface roughness with increasing ion dose, also for low energy impact of helium, as was the case in this work. The low solubility of helium in metals is probably the cause of what appears to be bubble arrays within the grains after sputtering.
References [l] B. Emmoth, M. Rubel, H. Bergstier, P. Wienhold and J. Winter, in: these Proe. (PSI-S), J. NucI. Mater. 162-164 (1989) 409. f2] F. Engelmamx, M. ChazaIon, M.F.A. Harrison, ES. Hotston, F. Moons and G. Vieder, J. Nucl. Mater. 145-147 (1987) 154. [3] S. Nagata, H. Bergs%ker, B. Emmoth and L. Ilyinsky, Nucl. Instr. and Meth. B18 (1987) 515. f4] H. Bergstier, S. Nagata and 3. Emmoth, .I. Nuci. Mater. 145-147 (1987) 364. [5] B. Emmoth and H. Bergs%ker, Nucl. Instr. and Meth., in press. [6] H. Bergsaker, Thesis, KTH, Stockholm 1987, ISBN 917170-915-O. [7] J. Bohdansky, H.L. Bay and J. Roth, in: Proc. 7th Int. Vacuum Congress and 3rd Int. Conf. on Solid Surfaces, Eds. R. Dobrozemsky, F. Ridenauer, F.P. Viebock and A. Breth (private publication, Wien, 1977) p. 1509. [8] H. van Seefeld, H. Scbmidl, R. Behrisch and B.M. Schemer, J. Nucl. Mater. 63. (1976) 215. [9] E. Hintz, D. Rusbtildt, B. Schweer, J. Bohdansky, J. Roth and P.A. Martinelli, J. Nucl. Mater. 93 & 94 (1980) 656. [lo] J. Bohdansky, Nuclear Fusion, Speeiai Issue (1984) 61. [ll] J. Roth, J. Bohdansky and W. Eckstein, Nucl. Ins&. and Meth. 218 (1983) 751. [12] H.H. Andersen et al., Nucl. Instr. and Meth. 209-210 (1983) 487.
Journal of Nuclear Materials 162-164 (1989) 941-944 North-Holland, Amsterdam
SPUTTERING AND ANGULAR DISTRIBUTION OF STEEL 316L IRRADIATED BY He IONS B. EMMOTH
I, H. BERGSAKERI
and C.H. WU 2
’ Research I~titute of Physics, ~ss~~at~on Euraiom - NFR, S-104 05 St~khol~
Sweden 2 The NET-Team, IPP, Max-Planck-Institute of Physics, D-8046 Garching bei Miinchen, Fed. Rep. Germany.
Key words: helium, steel 316L, sputtering, angular distribution, topography Helium ions bombarded a polished surface of steel 316L, in order to investigate sputtering, angular ~s~butions and ion-induced surface topography. For perpendicular sputtering a wide, or under-cosine, distribution of sputtered particles was found. The sputtering yield, measured by the collector method, was in agreement with other methods, 0.16 atoms per ion for 4 keV He ions. Sputtering at glancing incidence, resulted in a peaked distribution about 45O away from the beam in the incidence plane. Apparantly the angular distribution is not very sensitive to the angle of incidence for ion impact in the range 45 to 60’ from the normal. The original flat surface changed its topography due to different sputtering yields of differently oriented grains, with a rougher surface as the result.
1.
Introduction
In all present medium and large size tokamaks metals exist as impurities. The main reason is that metallic surfaces are exposed to plasma somewhere, although effort are made to shield by graphite tiles as much as possible. Even for fully carbonized machines metal impurities are present [l]. A film of carbon is likely to be damaged and deteriorate with time and the possibility will be open for metal release. Additional carbonization may temporarily bury impurities. Alpha particles bombarding a surface cause considerable damage on metals by sputtering and blistering. The distribution of alpha particles escaping from the plasma is not well described, they may be thermal&d with the possibility of localized enhanced loads due to ripple-induced losses 121. A candidate as structure material for the first wall in NET is steel 316L. If the concept total cover by thick graphite can not be realized helium will hit the steel surface resulting in release of high-Z metal impurities. The angular distribution of sputtered particles is important for the optimization of the wall construction. For a polyc~stalline material like 316L, the result of erosion of nearby grains is interesting since this can cause a much rougher surface than the original one already after low fluences. In this work, a low energy beam of helium ions was used to irradiate steel samples of 316L, with the aim to investigate low energy
ion-surface interaction using a collector method [3,4], while in a subsequent study high energy helium ions and combined ion beam impact are investigated. In principle, the technique used here was to coltect sputtered particles on the surface of a semi-circular collector holder with centre at the position of the sample. Due to the expected symmetry of sputtered particle flux, the collector was mounted in the plane of the target normal and the direction of the helium beam, the incidence plane. After irradiation the target was retracted and the collector could be turned to face a high energy beam, without venting the chamber between irradiation and analysis, for a Rutherford backscattering investigation using a particle detector and corresponding electronics.
2. Experimental The sputtering low energy beam current was measured by a Faraday cup (FC), before the target was moved into position for sputtering within the surrounding collector. During sputte~ng the current was momtored on the target without electron suppression and after the end of sputtering the beam current was again checked by the FC. The total fluence could be integrated from the target current corrected by a factor given by the FC measurements. After sputtering the target was retracted and the collector rotated in order to
0022-3115/89/$03.50 @ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
942
B. Emmoth et al. / Sputtenng
face the high energy beam, 2.0 MeV, used for backscattering analysis of the collector surface. The collector surface was covered by an Al foil, with high purity > 99.999%. Sticking of heavier sputtered atoms to a surface consisting of a lighter mass material should be high [5]. All internal movements in the experimental chamber were done in situ without breaking the vacuum conditions. The basic pressure throughout the experiments was in the range of lo-’ Torr. The collector surface was analysed by Rutherford backscattering spectrometry, and the different components of steel 316L could be resolved and compared to the bulk composition (at%); Cr 16-18, Ni 10-14, MO 2-3, Fe balance. Cr is obviously possible to separate in a straightforward manner by a computer fit [6], while it is needed to input more information to fit the two components Fe and Ni. MO is well separated in the spectrum. In the fitting procedure of MO the isotopic composition is used in the program. For the total sputtering yield the sum of all components of 316L is used, but the statistical accuracy was sufficient for a study of preferential sputtering effects and differential distibutions of sputtered species, by comparing results from spectra and the known bulk composition.
3. Results Fig. 1 shows the result of perpendicular sputtering by 4 keV helium irradiation of steel 316L. All black dots correspond to measured values shown in a polar diagram. The arrow pointing to one of the dots demonstrates the relative amount of sputtered particles in the particular direction as well as the statistical error, which is about 10%. The circle shows what would be expected for a pure cosine distribution. The measured distribution shows a broadening to the sides, as compared to a pure cosine this one is under-cosine. By fitting a ~0~~0, function, where 0, is the emission angle with respect to the normal, the total yield can be determined by integration. The best fit was obtained by a = 0.7, and the sputtering yield for the sum of different steel components was found to be 0.16, which is very similar to earlier reported results [7-91. The collector method gives here a result very close to the weight-loss method, or within expected errors in all earlier determinations with other methods. For further studies of the angular distribution, sputtering was performed at two other angles of incidence, 45 and 60 O. In case of 45 O, a peaked distribution was found in the specular direction, not shown here. Fig. 2 shows the result of sputtering 60” from the
and angular distribution
of steel 316L
_l
Ion Beam
Target 316 L
Fig. 1. Angular differential sputtering yields of steel 316L for 4 keV He bombardment at normal incidence (arbitrary units). normal. The distribution is still peaked 45” from the normal and in opposite direction, similar to sputtering at 45O incidence angle, although the total yield has been increased as expected [lo]. At these energies the shape of the angular distribution is relatively unsensitive for different beam impact angles at least in the range studied here. From the fitting procedure it is found that the amount of Cr relative the other steel components in all ejection angles is about 23%, which is more than is expected from the bulk concentration. Fig. 3 shows two photographs, fig. 3a corresponds to the original surface before implantation and after polishing. The small black points are defects and structure damage. Fig. 3b shows the surface after implantation to the typical irradiation dose 0.1 C, and for the same magnification x280. The grain structure is now very pronounced due to the different sputtering yields for differently oriented grains. At a larger magnification, not shown here, there appears to be rows with bubbles at an early state of blistering. Further investigations of this matter is in progress. 4. Discussion and conclusions We have found that the sputtering yield determined for normal incidence sputtering of steel 316L by 4 keV
943 Max. at
l .
.
Tnrget
316i. Fig. 2. The angular distribution of steel material sputtered by a helium ion beam 60° from the normal. The distribution is peaked 45 * from the normal in the incidence plane.
Fig. 3. (a) The or&,inaI polished steel surface before implantation at the magnification X 280. (b) The same surface after 0.1 C helium ioa impact. The grain structure is now more pronounced due to different sputtering yiefds of differently oriented grains, at the magnification X 280.
944
B. Emmoth et al. / Sputtering
He ions, Y = 0.16, is in agreement with other methods. The angular distribution is found to be “wider” than a cosinus (cf. fig. l), which is typical for sputtering of a metal with low surface binding energy flO]. The distribution can best be described by a function propotional to COSV, where cy= 0.7, and the amount of sputtered material is depending on the ejection angle and the total irradiation dose. For different angle of incidence, and the same energy, the sputtering yield increased as expected, and ejected material was peaked 45’ away from the beam in the incidence plane, apparently independently of the angle of incidence in the range 45 to 60 O. This information should be useful for the first wall construction. The different components of steel 316L were resolved and studied independently. One observation was that Cr on the collector surface corresponded to 23 at%, instead of the bulk composition 14-18 at%. Our interpretation is that since Cr is enriched at the surface of steel, either the erosion due to sputtering of the sample surface was not sufficient to reach bulk ~n~entration, or Cr diffused to the surface during irradiation. For the different ejection angles the same relative concentration was found in all directions indicating that the distribution of Cr and Fe + Ni were the same or similar within the statistical error. In case of MO, statistics was poor, however, a wide or more pronounced under-cosine ~st~bution seemed to be the case, but this still has to be investigated in more detail. In an experiment where a binary compound was studied [ll] it was also found that the angle for maximum differential sputtering was larger for the heavier component, which seem to be similar to our result. Surface segregation may also be a contributing effect [12]. In conclusion, results from elementary and two-component targets are essentially also valid for technical steels. The irradiated surface was investigated by mi-
and angular disiribution
ofsteel316.L
croscopy before and after helium implantation. The surface topography was clearly changed due to different sputtering of different grains. This is expected to result in an increasing surface roughness with increasing ion dose, also for low energy impact of helium, as was the case in this work. The low solubility of helium in metals is probably the cause of what appears to be bubble arrays within the grains after sputtering.
References [l] B. Emmoth, M. Rubel, H. Bergstier, P. Wienhold and J. Winter, in: these Proe. (PSI-S), J. NucI. Mater. 162-164 (1989) 409. f2] F. Engelmamx, M. ChazaIon, M.F.A. Harrison, ES. Hotston, F. Moons and G. Vieder, J. Nucl. Mater. 145-147 (1987) 154. [3] S. Nagata, H. Bergs%ker, B. Emmoth and L. Ilyinsky, Nucl. Instr. and Meth. B18 (1987) 515. f4] H. Bergstier, S. Nagata and 3. Emmoth, .I. Nuci. Mater. 145-147 (1987) 364. [5] B. Emmoth and H. Bergs%ker, Nucl. Instr. and Meth., in press. [6] H. Bergsaker, Thesis, KTH, Stockholm 1987, ISBN 917170-915-O. [7] J. Bohdansky, H.L. Bay and J. Roth, in: Proc. 7th Int. Vacuum Congress and 3rd Int. Conf. on Solid Surfaces, Eds. R. Dobrozemsky, F. Ridenauer, F.P. Viebock and A. Breth (private publication, Wien, 1977) p. 1509. [8] H. van Seefeld, H. Scbmidl, R. Behrisch and B.M. Schemer, J. Nucl. Mater. 63. (1976) 215. [9] E. Hintz, D. Rusbtildt, B. Schweer, J. Bohdansky, J. Roth and P.A. Martinelli, J. Nucl. Mater. 93 & 94 (1980) 656. [lo] J. Bohdansky, Nuclear Fusion, Speeiai Issue (1984) 61. [ll] J. Roth, J. Bohdansky and W. Eckstein, Nucl. Ins&. and Meth. 218 (1983) 751. [12] H.H. Andersen et al., Nucl. Instr. and Meth. 209-210 (1983) 487.