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Macroscopic phenomena caused by high dose MeV energy He, Ne and Ar implantation
Nuclear Instruments North-Holland
and Methods
in Physics Research
Nuclear lnstnments 4% Methods in Physics Research SectIon B
B62 (1992) 377-383
Macroscopic phenomena caused by high dose MeV energy He, Ne and Ar implantation * F. Phszti Central Research Institute for Physics, P.O. Box 49, H-1525 Budapest, Hungary
High dose implantation of reaction product MeV energy (Y particles into the construction elements of future controlled thermonuclear reactors might lead to serious problems. Among others the material suffers from swelling, large mechanical stresses develop in it and the bubbles into which the He gas accumulates weaken the material. When the interbubble material breaks, a layer will be separated and the specimen will show macroscopic surface deformations. An alternative process is when the implanted layer becomes mechanically unstable under the built up compressive mechanical stress and becomes rippled without separating from the bulk. Radiation damage, creep and sputtering enhanced by other particles coming from the active zone lead to further complications. Extensive research was devoted in our laboratory to explore and understand the above mentioned processes. 4He+ ions from a 5 MeV Van de Graaff accelerator were applied to simulate a particles. Simultaneous radiation damage, creep and sputtering was simulated by heavier noble gas ions. A special technique was developed to perform quasi-simultaneous multiple energy implantation. A short insight into the achieved results will be given.
1. Introduction If one wants fusion to take place at the necessary rate in a thermonuclear reactor, the temperature in the active zone has to be very high, corresponding to several tens of keV, where the deuterium and tritium components are in plasma state [ll. The energetic (Y particles and neutrons from the D + T + He (3.52 MeV) + n (14.1 MeV) fusion reaction will penetrate the plasma losing a considerable part of their energy (especially the alphas) and hit the construction elements of the device. The caused damage and the accumulating He gas in the material may lead to serious problems. One of these is the macroscopic surface deformation by the following mechanism. If the implanted He gas is unable to escape, it will coalesce and precipitate to form gas filled bubbles in the host material. These bubbles may grow by capturing additional gas atoms and vacancies or by punching out interstitial dislocation loops. When they become large enough a crack parallel to the sample surface will be formed [2,3]. The layer separated from the substrate by this crack may contain a huge compressive lateral stress. This stress and/or the pressure of the gas accumulated in the cavity formed by the crack lifts covering layer up, deforms it plastically and may even tear it off from the bulk [4-61. If this process takes place on well defined, * Invited. 0168-583X/92/$05.00
0 1992 - Elsevier
Science
Publishers
usually circular spots, it is called blistering, otherwise exfoliation. If the detached layer falls off, the process is referred to as flaking [7]. These processes are very dangerous because the affected layer is soon removed from the surface and reaches the active zone of the reactor. The heavy impurities cool down the plasma and the economical self-sustaining nuclear fusion fails. The cold He gas that will be reemitted during the surface deformations, may have similar effects. The material loss in itself may also be a problem. Other phenomena, such as sputtering of plasma ions will also have their effect on the first wall and plasma purity, so it may become necessary to replace regularly the most affected wall components. It is evident that the interaction of the fusioning plasma and the reactor wall is a rather comlex problem, especially when synergism between the different effects and processes is also taken into account. If one wants to design a fusion reactor, the data about the different processes and their interactions are crucial. This is why wide scale model experiments are carried out in different laboratories all over the world. As reviewed by others (e.g. refs. [Sll]), many studies have been made over the last 15 years on the formation and evolution of surface deformations as a result of helium implants in the l-200 keV ion energy range. Our “plasma-wall interaction” group in CRIP, Budapest has been formed and joined this research in 1978. Our work is mainly based on a 5 MeV Van de
B.V. All rights
reserved
III. DAMAGE
STUDIES
378
F. P&zti / Macroscopic phenomena caused by implantation
Graaff accelerator and this focuses our investigations on the effects of MeV energy He ions. The aim of the present paper is to review our most important results.
2. Experimental Our accelerator provides a beam of l-2 keV energy spread and l-10 uA current. The beam is defocussed for implantations to at least 2 cm2 area, and only the central 4-9 mm2 part of it is cut out by a movable four sector slit and hits the target. In this way we can assure a reasonable homogeneity (about lo-20%). Electrostatic beam sweeping could considerably improve this value. The necessary device is under construction. Macroscopic surface deformations appear in an implanted gas concentration range of lo-60 at.% (see below). To reach this level in the case of monoenergetic He+ ions of a few MeV energy, a fluence of 10” ions/cm2 is necessary. In real vacuum a thin oil film forms on the surfaces. At the beam spot the ionisation cracks the oil molecules and the formed reactive fragments join into a nonvolatile hydrocarbon layer. In contrast to sputtering, macroscopic surface deformations are not too sensitive to surface contaminations, but during the long time that is needed to reach the necessary fluence, the hydrocarbon layer may grow to be thick enough to considerably distort the implanted depth profile and knockon implanted carbon may change the mechanical properties of the material. To avoid these problems the vacuum is kept in the lo-’ Pa range, and the oil is trapped by liquid nitrogen traps. In this way, we can keep the beam spot hydrocarbon free even during implantations of several days. The implanted fluence is determined from the beam area and the integrated charge carried by the implanted ions. The former is set by the four sector slit and checked by letting the beam on a paper for a short time, while the second is determined by a transmission Faraday cup [12]. If it is necessary to determine the lateral and depth distributions of the implanted gas atoms, in situ RBS analysis is also applied. For this purpose a silicon surface detector is mounted at 165 o scattering angle in the target chamber. To avoid its radiation damage, the detector is blinded by a movable shutter during implantation. To carry out the measurement, the shutter is removed and the beam spot size is reduced by closing the four sector slit. By proper slit positioning the whole beam spot can be scanned with a lateral resolution of 0.2 mm. For He profiling, proton RBS around 3 MeV is applied (e.g. ref. [13]). Here the cross section is about 300 times higher than the Rutherford value [14] and the background spectrum of the host material is normally smooth in the energy region where the He peak appears. The detection limit is about 0.1
at.%. For heavier noble gas implantations He RBS is applied [ 151. To determine the critical fluence where macroscopic surface deformations appear, the implanted spot is regularly observed by a telescope of 10 x magnification. Its resolution is about 50 urn, which is sufficient to identify only larger formations such as exfoliations. Smaller sized deformations like blistering can be detected by noticing enhanced light scattering at near reflecting angles. In some cases the sample is implanted at higher temperatures or it is annealed in situ after implantation [15,16]. For this purpose we built a special sample holder that can be heated up to 700 o C by a builtin 100 W quartz lamp. The sample holder is also movable by 50 mm in both X and Y directions using computer controlled stepping motors. To simulate more realistically the wide energy distribution of the (Y particles coming from a fusion reactor, the “quasi-simultaneous multiple energy implantation” method was developed [17]. Here the ions lose energy when penetrating an absorber foil mounted in front of the sample. The foil is periodically tilted by a simple mechanism to change its effective thickness, and as a consequence the energy of the particles bombarding the specimen is also time dependent. After reaching the desired dose, the samples are transferred into a scanning electron microscope for further investigations. To investigate their interiors, the exfoliations are opened with a sharp wolfram pin.
3. Summary of results with MeV energy He implantation We will review the results of our MeV energy implantations in the light of model investigations via ion implantation at mainly l-200 keV ion energies at different laboratories. These latter will be referred to in a general way, rather than with individual references. The mentioned results and theoretical considerations are mainly reviewed in refs. [8-111. 3.1. Appearance At these relatively low energies, it was shown that the mentioned processes can take place in all solid materials including metals, semiconductors, ceramics, glasses, metallic glasses, sintered powders and carbonic materials of different crystal structure, etc. We have also investigated materials of different crystal structure (e.g. polycrystalline Au [18,19], Al [13,16,20,21], Incone1 and stainless steel [22], single crystalline Si [23,24] and Ni [25] and amorphous metallic glasses [26-291). It was found that instead of blistering, which is frequently observed in the low energy range, exfoliation and flak-
F. P&A
/ Macroscopic phenomena caused by implantation
ing takes place. We supposed, that there is a “transition energy” between the two processes and it is related to the width of “suppressed zones” surrounding the blisters [19]. From these, enough He migrates into the blisters to prevent the appearance of a new blister. If the radius of the blisters is small the blisters will never coalescence. Increasing the implantation energy the blister radius increases [4], so by overcoming the thickness of the suppressed zone, blister coalescence is then possible. At even higher energies one cannot observe separated blisters [19] but instead the exfoliation will expand in jumps when new blisters appear and join it [18,25]. It was also shown that the transition energy sharply increases with implantation temperature [16]. This fact is in accordance wih the speculation that the width of the suppressed zone sharply increases together with the diffusion coeficient. Hoping to get information about the formation mechanism of exfoliations,,we opened them. The traces of the primary blisters and of the expansion of the exfoliation in sudden jumps were clearly seen as rough contour lines on both the inner surface of the exfoliated layer and the surface left behind on the bulk [18]. This observation confirms our speculation on the mechanism laying behind the transition energy. The mentioned surfaces frequently contained small secondary blisters or suffered secondary flaking [18,19,22]. It was suggested, that this was due to the high He concentration in the vicinity of these surfaces. To make more direct experiments, the “quasi-multiple energy implantation” technique was developed and applied. It was confirmed that surface deformations may take place even when the implanted concentration is high at the sample surface [21,24]. This is in contradiction with the observations at smaller energies, where a channel network formed in the He containing layer. This network was open to the surface and the implanted gas escaped continuously through it. At high energies, however, this mechanism may sometimes fail. To get information on the appearance of the channel network, a special experiment was performed [13]. Here, a wide gas depth profile was implanted into Al by multiple energy implantation, then sharp monoenergetic peaks were added to it. The He distribution was regularly checked using in situ proton RBS. No gas redistribution was observed before the onset of exfohation. Also, the gas escaped through the holes in the exfoliation lid only from the layers of highest gas concentration. From this experiment one can draw the conclusion that the channel network appears more or less simultaneously with the surface deformation. 3.2. Critical dose In the work at lower energies when the implanted dose reaches
it was shown that, a critical @c value,
379
macroscopic surface deformations appear suddenly. @Jo has special interest because of its relation to the lifetime of the reactor wall. It soon became obvious that @c strongly depends on experimental parameters such as the target material, its lattice and macroscopic structure, temperature and surface roughness, as well as the implantation energy or energy distribution and the angle of incidence. It was generally supposed, that the ion energy, the angle of incidence and their distributions have their effect on Qc only via the critical peak concentration n,: Qc will be reached if the He concentration at the maximum of its depth distribution reaches n,. While this approximation can fail at low implantation energies, in our case its validity was clearly proven in experiments with widened implantation profiles [13,21,24,29] and at different implantation energies [15,16,22]. The effect of target material on n, is moderate. We found II, to be 22, 24, 31, 33, 38 and 45 at.% for Au [18], Al [13,16,20,21], Ni [2.5], stainless steel [22], metallic glass [26] and Si [23,24] respectively. The effect of alloying was demonstrated on an AlMgSi sample (n, = 32 at.% instead of 24 at.% for pure Al) [16]. In the case of pure and alloyed Al we found that when increasing the temperature, n, decreases in accordance to the yield strength or tensile strength of the material [15,16]. This change can be dramatic. For example n, decreases from 24 at.% at room temperature to 3 at.% at 530°C for pure Al. The experiments also showed that in this case, a post implantation annealing at the same temperature is equivalent to an elevated temperature during implantation. The same experiment [16] clearly demonstrated that blistering can occur even at 90% of the melting temperature T, of the material. This is in marked contrast to the rule of thumb found for lower implant energies saying that surface deformations do not occur above OST,. The reason is obvious: if the implanted profile is shallow, the cannels that may form at high temperatures by interbubble coalescence intercept the surface and as a result the gas escapes without any macroscopic surface deformation. If the implantation energy is high enough, the channels cannot reach the surface, so surface deformations are inevitable. 3.3. The effect of lateral stress As it was pointed out in the introduction, the effect of the huge compressive lateral stress generated by the extra material and damage introduced during implantation is crucial in these processes. Some of our experiments clearly demonstrated that this stress can also cause new types of phenomena that were unknown from experiments using lower implantation energies. If our sample was homogeneous (i.e. single crystalline Si or amorphous metallic glass), we observed a rather III. DAMAGE STUDIES
In ear& work with helium implants in the Iower energy range, there was considerable discussion on the relation between this thickness and the position of the He peak E% ~t~o~~gh some discussion ensued art the rage of Xaterai stress, d~s~Ia~ement damage and other factors, it was ev~~t~aI~y r~~o~~~~e~ that whiIe bubble swe~Iing [331 could ~robabi~ account for earlier dis~re~ancies~ some aspects were not totally resolved. Xn our ex~er~meuts the thickness of the detached layer always coincided with the depth of the He cuneentration peak within ex~er~meuta1 error. This was vafid even at irn~~~n~tinns of wide energy distribution [13,21,24,291. Moreover, if the depth distribution had two additional well separated, equivalent peaks, the surface layer detached a~t~r~at~vely at both of them f132. This operation excfudes the validity of specuiatians based on macroscopic effects.
regular wave pattern of ~rotrwsi~~s with tr~a~~~lar~y shaped cross section on the crack made surfaces after Baking, both on the remaining bulk and the detached Iayer (see fig, 1) f23,2&%-29~. The ~benome~o~ was explained by a stress mode& assuming that the lateral stress in the zone around the projected range of the ~rn~~ngi~g ions (R,) cam reach a crificaf Emit (ap~r~~atel~ a tenth of Young’s modulus1. At this stress value, the Iayer will be subjected to rne~han~~~l instability and become rippled with a wavelength of A = 4.3h,where h is the thickness of the impianted profile (or the projected range strangling in FWHM sense) [~3,3~~31], When fhc crack is pro~~~at~n~ in a layer of per~odi~~I]y ~han~~n~ stress caused by the rambling process, it should be disturbed, resultmg in wavelike structures. The rippbng was also directly observed on the sample surface if the irn~I~ted profile was close to the surface @4,32), It is worth noting, that this new type of surface ~~fo~at~on develop without the appearance of any crack in the material. The homo~eue~t~ of the sample is a necessary but not sufficient ~oadjt~on for the appearance of the wavehke patterns. If other types of surface deformation
The main .goal of these experiments was to ~~rn~are the effect of He and other noble gases in high dose MeV energy ~m~~antatia~s. We hoped that im~Iantation of heavier noble gas ions can simutate the fusion reactor environment in a more complex way: the effect of the. lattice damage of fast neutrons, the sputtering caused by the D-T fuei and other impurities, and that the bubble formation and surface defo~~~ions caused by high energy Me fusion products can be mad&d s~rn~lt~neons~~ f&321. ~~f~er~nt types of sarn~~~s were used in the experiments: anlor~bous meetailie glass, Single crystal Si and po~yc~stalline Al. The main conditions and results of ihcse experiments are presented in table 1, Fur ~m~ariso~, some experi~lents with He ions are also included in the table. The critical concentrations are lower for Ne and Ar ions than for He. This fact is partly connected to the different atomic volumes. The higher 1eveIs of radiation damage (see 5 in table I) can also ptay some rofe by ~nhan&in~ gas atom and vacancy ~~i~~tion, in metallic glasses, the amorpb~us state of the material makes a further increase in n, [%f, The relatively low values for Ne and Ar ions in this case can also be connected to re~~tai~i~tion under ion graduation @4-3@* In case of He ~~~~ant~t~~~,the projected range was Iarge and we unly observed flaking or exfoliation. For Ne and Ar, because of shaIIower ~rnp~ant~t~on depths, bfisters became more preferred. Sometimes~ however, the “‘blister” ~ensi~ was so high that it was mare
F. Pkti
I
ffl
I
I
/ Macroscopic phenomena caused by implantation
I
i+++
41
I
++++
I
I
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precise to speak about ripples on the surface instead of distinct blisters (fig. 2). At doses only slightly higher than the critical one the ripples were ordered into a wave pattern in accordance to our stress theory. Their sizes are also included in table 1. The mean wavelengths of these patterns were in good accordance with the stress model (see in the table), so it was reasonable to suppose that this phenomenon was also caused by the same stress driven mechanism. There remain, however, two important questions to answer: how can the amplitude of these ripples become large enough to be observed and why does not the stress relax before reaching the critical value. The possible explanations are as follows. Diffusion of host atoms, vancancies and implanted noble gas atoms as well as orientation and gliding of interstitial dislocation loops are also influenced by stress gradients. This leads, however, not only to stress release, but it can decrease the effective Young coefficient for long term processes (creep). Thus ripples can form far below the critical stress calculated from unimplanted conditions. The same mechanism can lead to the enhanced amplitude of ripples via transporting material to places where tensile stress arises under the layer moving away from the bulk. Gas migration from bubbles of smaller size onto larger ones (Ostwald ripening) can also play an important role. It is worth noting that there is no crack involved in this mechanism. Cylindrical or mushroom shaped blisters were also observed on Si samples. They may also be connected to the processes mentioned above. Here the bottom of the blister walls is the place where tensile stress arose, and continuous material transport built up the observed cylindrical walls. Similar formations were observed on the inner surface of a 252Cf neutron source capsule [37]. A detailed explanation of the phenomenon is also presented there. 5. Relevance for fusion reactors One can predict that surface deformations caused by thermalised He ions will not appear in fusion reactors, because: the erosion rate from other processes (i.e. sputtering) will be too high, and the critical He concentration will never be reached; the maximum gas concentration will be reached near to the surface first, and gas will escape through the channels that develop under such condition; the wall temperature will be above half of the melting point of the material, and gas will escape similarly to the previous case. Our results show that the last two points alone cannot guarantee against surface deformations by MeV energy He ions in a fusion reactor. If the competitive processes are somehow suppressed, and they do not III. DAMAGE STUDIES
382
F. P&zti / Macroscopic phenomena caused by implantation
Fig. 2. Rippling on a 0.7 MeV Ar implanted amorphous Metglass-2826A sample.
erode the surface with a rate that is high enough, then surface deformations will cause serious problems. The effect of radiation damage and the elevated wall temperature may decrease the critical gas concentration by a factor of 10, as compared to the values from classical experiments at room temperature and low ion energies.
References [l] T.J. Dolan, Fusion Research (Pergamon, New York, 1982). [2] H. Trinkaus, Radiat. Eff. 78 (1983) 189. [3] J.H. Evans, J. Nucl. Mater. 68 (1977) 129. [4] M. Risch, J. Roth and B.M.U. Scherzer, Proc. Int. Symp. on Plasma Wall Interaction (Pergamon, Oxford, 1977) p. 391. [5] G.K. Erents and G.M. McCracken, Radiat. Eff. 18 (1973) 245. [6] 0. Auciello, in: Ion Bombardment Modification of Sur-
faces, eds. 0. Auciello and R. Kelly (Elsevier, Amsterdam, 1984) p. 1. Gy. [71 G. Mezey, F. Paszti, M. Fried, A. Manuaba, Vizkelethy, Cs. Hajdu and E. Kotai, Twenty Years of Plasma Physics, ed. B. McNamara (World Scientific, Philadelphia, 1984) p. 95. Bl S.K. Das and M. Kaminsky, Radiation Effects on Solid Surfaces, Advances in Chemistry Series 158, ed. M. Kaminsky (American Chemical Society, Washington, 1976) p. 112. 191 B.M.U. Scherzer, Sputtering by Particle Bombardment, Topics in Appl. Phys. 52, ed. R. Behrisch (Springer, Berlin, 1983) p. 271. of Mater. Sci. and Eng., ed. [lOI J.H. Evans, Enclyclopedia M.B. Bever (Pergamon, Oxford, 1983) p. 2123. [ll] MI. Guseva and Yu.V. Martynenko, Physics of Radiation Effects in Crystals, eds. R.A. Johnson and A.N. Orlov (Elsevier, Amsterdam, 1986) p. 621. [12] F. Paszti, A. Manuaba, C. Hajdu, A.A. Melo and M.F. Da Silva, Nucl. Instr. and Meth. B47 (1990) 187. [13] A. Manuaba, F. Paszti and E. Kotai, J. Nucl. Mater. 175 (1990) 158.
F. Pdszti / Macroscopic phenomena caused by implantation [14] L.C. Feldman and S. Picraux, Ion Beam Handbook for Materials Analysis, eds. J.W. Mayer and E. Rimini (Academic, New York, 1977) p. 109. [1.5] N.T. My, F. Paszti, A. Manuaba, G. Mezey and E. Kotai, J. Nucl. Mater 168 (1989) 76. [16] N.T. My, F. Paszti, G. Mezey, A. Manuaba, E. Kotai and J. Gyulai, J. Nucl. Mater. 165 (1989) 222. 1171 F. Paszti, M. Fried, A. Manuaba, G. Mezey, E. Kotai and T. Lohner, J. Nucl. Mater. 114 (1983) 330. [18] F. Paszti, L. Pogany, G. Mezey, E. Kbtai, A. Manuaba, L. P&s, J. Gyulai and T. Lohner, J. Nucl. Mater. 98 (1981) 11. [I91 G. Mezey, F. Paszti, L. Pogany, A. Manuaba, M. Fried, E. Kotai, T. Lohner, L. P&s and J. Gyulai, Ion Implantation Into Metals, ed. V. Ashworth (Pergamon, Oxford, 19821 p. 293. [20] N.T. My, G. Mezey, A. Manuaba, F. Paszti, L. Pogany, E. Kotai, M. Fried and L. P&s, Proc. 12th Europ. Conf. on Controlled Fusion and Plasma Physics, eds. L. P&s and A. Montvai (EPS, Budapest, 1985) p. 639. [21] N.T. My, A. Manuaba, G. Mezey, F. Paszti, Kotai, L. P&s, E. Klopfer, P. Kostka and M. Fried, Proc. 12th Europ. Conf. on Controlled Fusion and Plasma Physics, eds. L. P&s and A. Montvai (EPS, Budapest, 1985) p. 642. [22] F. Paszti, G. Mezey, L. Poglny, M. Fried, A. Manuaba, E. Kotai, T. Lohner and L. P&s, Nucl. Instr. and Meth. 209/210 (1983) 1001. [23] F. Paszti, Cs. Hajdu, A. Manuaba, N.T. My, E. Kotai, L. Pogany, G. Mezey, M. Fried, Gy. Vizkelethy and J. Gyulai, Nucl. Instr. and Meth. B7/8 (1985) 371. [24] F. Paszti, A. Manuaba, L. Pogany, Gy. Vizkelethy, M. Fried, E. Kotai, H.V. Suu, T. Lohner, L. P&s and G. Mezey, J. Nucl. Mater. 119 (1983) 26.
383
[25] F. Paszti and A. Manuaba, Proc. Int. Conf. on Physics of Irradiation Effects in Metals, 1991, Siofok, Hungary. [26] A. Manuaba, F. Paszti, L. Pogany, M. Fried, E. Kotai, G. Mezey, T. Lohner, I. Lovas, L. P&s and J. Gyulai, Nucl. Instr. and Meth. 199 (1982) 409. [27] F. Pbszti, M. Fried, L. PogLny, A. Manuaba, G. Mezey, E. Kotai, I. Lovas, T. Lohner and L. P&s, Nucl. Instr. and Meth. 209/210 (1983) 273. [28] F. Paszti, M. Fried, L. Pogany, A. Manuaba, G. Mezey, E, K6tai, I. Lovas, T. Lohner and L. P&s, Phys. Rev. B28 (1983) 5688. [29] M. Fried, L. Pogany, A. Manuaba, F. Paszti and C. Hajdu, Phys. Rev. B41 (1990) 3923. [30] C. Hajdu, F. Paszti, M. Fried and I. Lovas, Nucl. Instr. and Meth. B19/20 (1987) 607. [31] C. Hajdu, F. Paszti, I. Lovas and M. Fried, Phys. Rev. B41 (1990) 3920. [32] F. Paszti, Mater. Sci. Eng. A115 (1989) 57. [33] B. Emmoth, Radiat. Eff. 78 (1983) 365. [34] R.V. Nandekar and A.K. Tyagi, Radiat. Eff. Lett. 58(3) (1981) 91. [35] A.K. Tyagi, R.V. Nandekar and K. Krishan, J. Nucl. Mater. 114 (1983) 181. [36] N. Nayashi and I. Sakamoto, Phys. Lett. A88 (1982) 299. [37] W.R. McDonell, J. Nucl. Mater. 85/86 (1979) 1117. [38] J.P. Biersack and L.G. Haggmark, Nucl. Instr. and Meth. 174 (1980) 257. [39] N. Matsunami, Y. Yamamura, Y. Itikawa, N. Itoh, Y. Kazumata, S. Miyagawa, K. Morita and R. Shimizu, Institute of Plasma Physics (Nagoya University, Japan) reports, IPPJ-AM-14 (1980).
III. DAMAGE
STUDIES
and Methods
in Physics Research
Nuclear lnstnments 4% Methods in Physics Research SectIon B
B62 (1992) 377-383
Macroscopic phenomena caused by high dose MeV energy He, Ne and Ar implantation * F. Phszti Central Research Institute for Physics, P.O. Box 49, H-1525 Budapest, Hungary
High dose implantation of reaction product MeV energy (Y particles into the construction elements of future controlled thermonuclear reactors might lead to serious problems. Among others the material suffers from swelling, large mechanical stresses develop in it and the bubbles into which the He gas accumulates weaken the material. When the interbubble material breaks, a layer will be separated and the specimen will show macroscopic surface deformations. An alternative process is when the implanted layer becomes mechanically unstable under the built up compressive mechanical stress and becomes rippled without separating from the bulk. Radiation damage, creep and sputtering enhanced by other particles coming from the active zone lead to further complications. Extensive research was devoted in our laboratory to explore and understand the above mentioned processes. 4He+ ions from a 5 MeV Van de Graaff accelerator were applied to simulate a particles. Simultaneous radiation damage, creep and sputtering was simulated by heavier noble gas ions. A special technique was developed to perform quasi-simultaneous multiple energy implantation. A short insight into the achieved results will be given.
1. Introduction If one wants fusion to take place at the necessary rate in a thermonuclear reactor, the temperature in the active zone has to be very high, corresponding to several tens of keV, where the deuterium and tritium components are in plasma state [ll. The energetic (Y particles and neutrons from the D + T + He (3.52 MeV) + n (14.1 MeV) fusion reaction will penetrate the plasma losing a considerable part of their energy (especially the alphas) and hit the construction elements of the device. The caused damage and the accumulating He gas in the material may lead to serious problems. One of these is the macroscopic surface deformation by the following mechanism. If the implanted He gas is unable to escape, it will coalesce and precipitate to form gas filled bubbles in the host material. These bubbles may grow by capturing additional gas atoms and vacancies or by punching out interstitial dislocation loops. When they become large enough a crack parallel to the sample surface will be formed [2,3]. The layer separated from the substrate by this crack may contain a huge compressive lateral stress. This stress and/or the pressure of the gas accumulated in the cavity formed by the crack lifts covering layer up, deforms it plastically and may even tear it off from the bulk [4-61. If this process takes place on well defined, * Invited. 0168-583X/92/$05.00
0 1992 - Elsevier
Science
Publishers
usually circular spots, it is called blistering, otherwise exfoliation. If the detached layer falls off, the process is referred to as flaking [7]. These processes are very dangerous because the affected layer is soon removed from the surface and reaches the active zone of the reactor. The heavy impurities cool down the plasma and the economical self-sustaining nuclear fusion fails. The cold He gas that will be reemitted during the surface deformations, may have similar effects. The material loss in itself may also be a problem. Other phenomena, such as sputtering of plasma ions will also have their effect on the first wall and plasma purity, so it may become necessary to replace regularly the most affected wall components. It is evident that the interaction of the fusioning plasma and the reactor wall is a rather comlex problem, especially when synergism between the different effects and processes is also taken into account. If one wants to design a fusion reactor, the data about the different processes and their interactions are crucial. This is why wide scale model experiments are carried out in different laboratories all over the world. As reviewed by others (e.g. refs. [Sll]), many studies have been made over the last 15 years on the formation and evolution of surface deformations as a result of helium implants in the l-200 keV ion energy range. Our “plasma-wall interaction” group in CRIP, Budapest has been formed and joined this research in 1978. Our work is mainly based on a 5 MeV Van de
B.V. All rights
reserved
III. DAMAGE
STUDIES
378
F. P&zti / Macroscopic phenomena caused by implantation
Graaff accelerator and this focuses our investigations on the effects of MeV energy He ions. The aim of the present paper is to review our most important results.
2. Experimental Our accelerator provides a beam of l-2 keV energy spread and l-10 uA current. The beam is defocussed for implantations to at least 2 cm2 area, and only the central 4-9 mm2 part of it is cut out by a movable four sector slit and hits the target. In this way we can assure a reasonable homogeneity (about lo-20%). Electrostatic beam sweeping could considerably improve this value. The necessary device is under construction. Macroscopic surface deformations appear in an implanted gas concentration range of lo-60 at.% (see below). To reach this level in the case of monoenergetic He+ ions of a few MeV energy, a fluence of 10” ions/cm2 is necessary. In real vacuum a thin oil film forms on the surfaces. At the beam spot the ionisation cracks the oil molecules and the formed reactive fragments join into a nonvolatile hydrocarbon layer. In contrast to sputtering, macroscopic surface deformations are not too sensitive to surface contaminations, but during the long time that is needed to reach the necessary fluence, the hydrocarbon layer may grow to be thick enough to considerably distort the implanted depth profile and knockon implanted carbon may change the mechanical properties of the material. To avoid these problems the vacuum is kept in the lo-’ Pa range, and the oil is trapped by liquid nitrogen traps. In this way, we can keep the beam spot hydrocarbon free even during implantations of several days. The implanted fluence is determined from the beam area and the integrated charge carried by the implanted ions. The former is set by the four sector slit and checked by letting the beam on a paper for a short time, while the second is determined by a transmission Faraday cup [12]. If it is necessary to determine the lateral and depth distributions of the implanted gas atoms, in situ RBS analysis is also applied. For this purpose a silicon surface detector is mounted at 165 o scattering angle in the target chamber. To avoid its radiation damage, the detector is blinded by a movable shutter during implantation. To carry out the measurement, the shutter is removed and the beam spot size is reduced by closing the four sector slit. By proper slit positioning the whole beam spot can be scanned with a lateral resolution of 0.2 mm. For He profiling, proton RBS around 3 MeV is applied (e.g. ref. [13]). Here the cross section is about 300 times higher than the Rutherford value [14] and the background spectrum of the host material is normally smooth in the energy region where the He peak appears. The detection limit is about 0.1
at.%. For heavier noble gas implantations He RBS is applied [ 151. To determine the critical fluence where macroscopic surface deformations appear, the implanted spot is regularly observed by a telescope of 10 x magnification. Its resolution is about 50 urn, which is sufficient to identify only larger formations such as exfoliations. Smaller sized deformations like blistering can be detected by noticing enhanced light scattering at near reflecting angles. In some cases the sample is implanted at higher temperatures or it is annealed in situ after implantation [15,16]. For this purpose we built a special sample holder that can be heated up to 700 o C by a builtin 100 W quartz lamp. The sample holder is also movable by 50 mm in both X and Y directions using computer controlled stepping motors. To simulate more realistically the wide energy distribution of the (Y particles coming from a fusion reactor, the “quasi-simultaneous multiple energy implantation” method was developed [17]. Here the ions lose energy when penetrating an absorber foil mounted in front of the sample. The foil is periodically tilted by a simple mechanism to change its effective thickness, and as a consequence the energy of the particles bombarding the specimen is also time dependent. After reaching the desired dose, the samples are transferred into a scanning electron microscope for further investigations. To investigate their interiors, the exfoliations are opened with a sharp wolfram pin.
3. Summary of results with MeV energy He implantation We will review the results of our MeV energy implantations in the light of model investigations via ion implantation at mainly l-200 keV ion energies at different laboratories. These latter will be referred to in a general way, rather than with individual references. The mentioned results and theoretical considerations are mainly reviewed in refs. [8-111. 3.1. Appearance At these relatively low energies, it was shown that the mentioned processes can take place in all solid materials including metals, semiconductors, ceramics, glasses, metallic glasses, sintered powders and carbonic materials of different crystal structure, etc. We have also investigated materials of different crystal structure (e.g. polycrystalline Au [18,19], Al [13,16,20,21], Incone1 and stainless steel [22], single crystalline Si [23,24] and Ni [25] and amorphous metallic glasses [26-291). It was found that instead of blistering, which is frequently observed in the low energy range, exfoliation and flak-
F. P&A
/ Macroscopic phenomena caused by implantation
ing takes place. We supposed, that there is a “transition energy” between the two processes and it is related to the width of “suppressed zones” surrounding the blisters [19]. From these, enough He migrates into the blisters to prevent the appearance of a new blister. If the radius of the blisters is small the blisters will never coalescence. Increasing the implantation energy the blister radius increases [4], so by overcoming the thickness of the suppressed zone, blister coalescence is then possible. At even higher energies one cannot observe separated blisters [19] but instead the exfoliation will expand in jumps when new blisters appear and join it [18,25]. It was also shown that the transition energy sharply increases with implantation temperature [16]. This fact is in accordance wih the speculation that the width of the suppressed zone sharply increases together with the diffusion coeficient. Hoping to get information about the formation mechanism of exfoliations,,we opened them. The traces of the primary blisters and of the expansion of the exfoliation in sudden jumps were clearly seen as rough contour lines on both the inner surface of the exfoliated layer and the surface left behind on the bulk [18]. This observation confirms our speculation on the mechanism laying behind the transition energy. The mentioned surfaces frequently contained small secondary blisters or suffered secondary flaking [18,19,22]. It was suggested, that this was due to the high He concentration in the vicinity of these surfaces. To make more direct experiments, the “quasi-multiple energy implantation” technique was developed and applied. It was confirmed that surface deformations may take place even when the implanted concentration is high at the sample surface [21,24]. This is in contradiction with the observations at smaller energies, where a channel network formed in the He containing layer. This network was open to the surface and the implanted gas escaped continuously through it. At high energies, however, this mechanism may sometimes fail. To get information on the appearance of the channel network, a special experiment was performed [13]. Here, a wide gas depth profile was implanted into Al by multiple energy implantation, then sharp monoenergetic peaks were added to it. The He distribution was regularly checked using in situ proton RBS. No gas redistribution was observed before the onset of exfohation. Also, the gas escaped through the holes in the exfoliation lid only from the layers of highest gas concentration. From this experiment one can draw the conclusion that the channel network appears more or less simultaneously with the surface deformation. 3.2. Critical dose In the work at lower energies when the implanted dose reaches
it was shown that, a critical @c value,
379
macroscopic surface deformations appear suddenly. @Jo has special interest because of its relation to the lifetime of the reactor wall. It soon became obvious that @c strongly depends on experimental parameters such as the target material, its lattice and macroscopic structure, temperature and surface roughness, as well as the implantation energy or energy distribution and the angle of incidence. It was generally supposed, that the ion energy, the angle of incidence and their distributions have their effect on Qc only via the critical peak concentration n,: Qc will be reached if the He concentration at the maximum of its depth distribution reaches n,. While this approximation can fail at low implantation energies, in our case its validity was clearly proven in experiments with widened implantation profiles [13,21,24,29] and at different implantation energies [15,16,22]. The effect of target material on n, is moderate. We found II, to be 22, 24, 31, 33, 38 and 45 at.% for Au [18], Al [13,16,20,21], Ni [2.5], stainless steel [22], metallic glass [26] and Si [23,24] respectively. The effect of alloying was demonstrated on an AlMgSi sample (n, = 32 at.% instead of 24 at.% for pure Al) [16]. In the case of pure and alloyed Al we found that when increasing the temperature, n, decreases in accordance to the yield strength or tensile strength of the material [15,16]. This change can be dramatic. For example n, decreases from 24 at.% at room temperature to 3 at.% at 530°C for pure Al. The experiments also showed that in this case, a post implantation annealing at the same temperature is equivalent to an elevated temperature during implantation. The same experiment [16] clearly demonstrated that blistering can occur even at 90% of the melting temperature T, of the material. This is in marked contrast to the rule of thumb found for lower implant energies saying that surface deformations do not occur above OST,. The reason is obvious: if the implanted profile is shallow, the cannels that may form at high temperatures by interbubble coalescence intercept the surface and as a result the gas escapes without any macroscopic surface deformation. If the implantation energy is high enough, the channels cannot reach the surface, so surface deformations are inevitable. 3.3. The effect of lateral stress As it was pointed out in the introduction, the effect of the huge compressive lateral stress generated by the extra material and damage introduced during implantation is crucial in these processes. Some of our experiments clearly demonstrated that this stress can also cause new types of phenomena that were unknown from experiments using lower implantation energies. If our sample was homogeneous (i.e. single crystalline Si or amorphous metallic glass), we observed a rather III. DAMAGE STUDIES
In ear& work with helium implants in the Iower energy range, there was considerable discussion on the relation between this thickness and the position of the He peak E% ~t~o~~gh some discussion ensued art the rage of Xaterai stress, d~s~Ia~ement damage and other factors, it was ev~~t~aI~y r~~o~~~~e~ that whiIe bubble swe~Iing [331 could ~robabi~ account for earlier dis~re~ancies~ some aspects were not totally resolved. Xn our ex~er~meuts the thickness of the detached layer always coincided with the depth of the He cuneentration peak within ex~er~meuta1 error. This was vafid even at irn~~~n~tinns of wide energy distribution [13,21,24,291. Moreover, if the depth distribution had two additional well separated, equivalent peaks, the surface layer detached a~t~r~at~vely at both of them f132. This operation excfudes the validity of specuiatians based on macroscopic effects.
regular wave pattern of ~rotrwsi~~s with tr~a~~~lar~y shaped cross section on the crack made surfaces after Baking, both on the remaining bulk and the detached Iayer (see fig, 1) f23,2&%-29~. The ~benome~o~ was explained by a stress mode& assuming that the lateral stress in the zone around the projected range of the ~rn~~ngi~g ions (R,) cam reach a crificaf Emit (ap~r~~atel~ a tenth of Young’s modulus1. At this stress value, the Iayer will be subjected to rne~han~~~l instability and become rippled with a wavelength of A = 4.3h,where h is the thickness of the impianted profile (or the projected range strangling in FWHM sense) [~3,3~~31], When fhc crack is pro~~~at~n~ in a layer of per~odi~~I]y ~han~~n~ stress caused by the rambling process, it should be disturbed, resultmg in wavelike structures. The rippbng was also directly observed on the sample surface if the irn~I~ted profile was close to the surface @4,32), It is worth noting, that this new type of surface ~~fo~at~on develop without the appearance of any crack in the material. The homo~eue~t~ of the sample is a necessary but not sufficient ~oadjt~on for the appearance of the wavehke patterns. If other types of surface deformation
The main .goal of these experiments was to ~~rn~are the effect of He and other noble gases in high dose MeV energy ~m~~antatia~s. We hoped that im~Iantation of heavier noble gas ions can simutate the fusion reactor environment in a more complex way: the effect of the. lattice damage of fast neutrons, the sputtering caused by the D-T fuei and other impurities, and that the bubble formation and surface defo~~~ions caused by high energy Me fusion products can be mad&d s~rn~lt~neons~~ f&321. ~~f~er~nt types of sarn~~~s were used in the experiments: anlor~bous meetailie glass, Single crystal Si and po~yc~stalline Al. The main conditions and results of ihcse experiments are presented in table 1, Fur ~m~ariso~, some experi~lents with He ions are also included in the table. The critical concentrations are lower for Ne and Ar ions than for He. This fact is partly connected to the different atomic volumes. The higher 1eveIs of radiation damage (see 5 in table I) can also ptay some rofe by ~nhan&in~ gas atom and vacancy ~~i~~tion, in metallic glasses, the amorpb~us state of the material makes a further increase in n, [%f, The relatively low values for Ne and Ar ions in this case can also be connected to re~~tai~i~tion under ion graduation @4-3@* In case of He ~~~~ant~t~~~,the projected range was Iarge and we unly observed flaking or exfoliation. For Ne and Ar, because of shaIIower ~rnp~ant~t~on depths, bfisters became more preferred. Sometimes~ however, the “‘blister” ~ensi~ was so high that it was mare
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/ Macroscopic phenomena caused by implantation
I
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precise to speak about ripples on the surface instead of distinct blisters (fig. 2). At doses only slightly higher than the critical one the ripples were ordered into a wave pattern in accordance to our stress theory. Their sizes are also included in table 1. The mean wavelengths of these patterns were in good accordance with the stress model (see in the table), so it was reasonable to suppose that this phenomenon was also caused by the same stress driven mechanism. There remain, however, two important questions to answer: how can the amplitude of these ripples become large enough to be observed and why does not the stress relax before reaching the critical value. The possible explanations are as follows. Diffusion of host atoms, vancancies and implanted noble gas atoms as well as orientation and gliding of interstitial dislocation loops are also influenced by stress gradients. This leads, however, not only to stress release, but it can decrease the effective Young coefficient for long term processes (creep). Thus ripples can form far below the critical stress calculated from unimplanted conditions. The same mechanism can lead to the enhanced amplitude of ripples via transporting material to places where tensile stress arises under the layer moving away from the bulk. Gas migration from bubbles of smaller size onto larger ones (Ostwald ripening) can also play an important role. It is worth noting that there is no crack involved in this mechanism. Cylindrical or mushroom shaped blisters were also observed on Si samples. They may also be connected to the processes mentioned above. Here the bottom of the blister walls is the place where tensile stress arose, and continuous material transport built up the observed cylindrical walls. Similar formations were observed on the inner surface of a 252Cf neutron source capsule [37]. A detailed explanation of the phenomenon is also presented there. 5. Relevance for fusion reactors One can predict that surface deformations caused by thermalised He ions will not appear in fusion reactors, because: the erosion rate from other processes (i.e. sputtering) will be too high, and the critical He concentration will never be reached; the maximum gas concentration will be reached near to the surface first, and gas will escape through the channels that develop under such condition; the wall temperature will be above half of the melting point of the material, and gas will escape similarly to the previous case. Our results show that the last two points alone cannot guarantee against surface deformations by MeV energy He ions in a fusion reactor. If the competitive processes are somehow suppressed, and they do not III. DAMAGE STUDIES
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F. P&zti / Macroscopic phenomena caused by implantation
Fig. 2. Rippling on a 0.7 MeV Ar implanted amorphous Metglass-2826A sample.
erode the surface with a rate that is high enough, then surface deformations will cause serious problems. The effect of radiation damage and the elevated wall temperature may decrease the critical gas concentration by a factor of 10, as compared to the values from classical experiments at room temperature and low ion energies.
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F. Pdszti / Macroscopic phenomena caused by implantation [14] L.C. Feldman and S. Picraux, Ion Beam Handbook for Materials Analysis, eds. J.W. Mayer and E. Rimini (Academic, New York, 1977) p. 109. [1.5] N.T. My, F. Paszti, A. Manuaba, G. Mezey and E. Kotai, J. Nucl. Mater 168 (1989) 76. [16] N.T. My, F. Paszti, G. Mezey, A. Manuaba, E. Kotai and J. Gyulai, J. Nucl. Mater. 165 (1989) 222. 1171 F. Paszti, M. Fried, A. Manuaba, G. Mezey, E. Kotai and T. Lohner, J. Nucl. Mater. 114 (1983) 330. [18] F. Paszti, L. Pogany, G. Mezey, E. Kbtai, A. Manuaba, L. P&s, J. Gyulai and T. Lohner, J. Nucl. Mater. 98 (1981) 11. [I91 G. Mezey, F. Paszti, L. Pogany, A. Manuaba, M. Fried, E. Kotai, T. Lohner, L. P&s and J. Gyulai, Ion Implantation Into Metals, ed. V. Ashworth (Pergamon, Oxford, 19821 p. 293. [20] N.T. My, G. Mezey, A. Manuaba, F. Paszti, L. Pogany, E. Kotai, M. Fried and L. P&s, Proc. 12th Europ. Conf. on Controlled Fusion and Plasma Physics, eds. L. P&s and A. Montvai (EPS, Budapest, 1985) p. 639. [21] N.T. My, A. Manuaba, G. Mezey, F. Paszti, Kotai, L. P&s, E. Klopfer, P. Kostka and M. Fried, Proc. 12th Europ. Conf. on Controlled Fusion and Plasma Physics, eds. L. P&s and A. Montvai (EPS, Budapest, 1985) p. 642. [22] F. Paszti, G. Mezey, L. Poglny, M. Fried, A. Manuaba, E. Kotai, T. Lohner and L. P&s, Nucl. Instr. and Meth. 209/210 (1983) 1001. [23] F. Paszti, Cs. Hajdu, A. Manuaba, N.T. My, E. Kotai, L. Pogany, G. Mezey, M. Fried, Gy. Vizkelethy and J. Gyulai, Nucl. Instr. and Meth. B7/8 (1985) 371. [24] F. Paszti, A. Manuaba, L. Pogany, Gy. Vizkelethy, M. Fried, E. Kotai, H.V. Suu, T. Lohner, L. P&s and G. Mezey, J. Nucl. Mater. 119 (1983) 26.
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[25] F. Paszti and A. Manuaba, Proc. Int. Conf. on Physics of Irradiation Effects in Metals, 1991, Siofok, Hungary. [26] A. Manuaba, F. Paszti, L. Pogany, M. Fried, E. Kotai, G. Mezey, T. Lohner, I. Lovas, L. P&s and J. Gyulai, Nucl. Instr. and Meth. 199 (1982) 409. [27] F. Pbszti, M. Fried, L. PogLny, A. Manuaba, G. Mezey, E. Kotai, I. Lovas, T. Lohner and L. P&s, Nucl. Instr. and Meth. 209/210 (1983) 273. [28] F. Paszti, M. Fried, L. Pogany, A. Manuaba, G. Mezey, E, K6tai, I. Lovas, T. Lohner and L. P&s, Phys. Rev. B28 (1983) 5688. [29] M. Fried, L. Pogany, A. Manuaba, F. Paszti and C. Hajdu, Phys. Rev. B41 (1990) 3923. [30] C. Hajdu, F. Paszti, M. Fried and I. Lovas, Nucl. Instr. and Meth. B19/20 (1987) 607. [31] C. Hajdu, F. Paszti, I. Lovas and M. Fried, Phys. Rev. B41 (1990) 3920. [32] F. Paszti, Mater. Sci. Eng. A115 (1989) 57. [33] B. Emmoth, Radiat. Eff. 78 (1983) 365. [34] R.V. Nandekar and A.K. Tyagi, Radiat. Eff. Lett. 58(3) (1981) 91. [35] A.K. Tyagi, R.V. Nandekar and K. Krishan, J. Nucl. Mater. 114 (1983) 181. [36] N. Nayashi and I. Sakamoto, Phys. Lett. A88 (1982) 299. [37] W.R. McDonell, J. Nucl. Mater. 85/86 (1979) 1117. [38] J.P. Biersack and L.G. Haggmark, Nucl. Instr. and Meth. 174 (1980) 257. [39] N. Matsunami, Y. Yamamura, Y. Itikawa, N. Itoh, Y. Kazumata, S. Miyagawa, K. Morita and R. Shimizu, Institute of Plasma Physics (Nagoya University, Japan) reports, IPPJ-AM-14 (1980).
III. DAMAGE
STUDIES