54 Nuclear Instruments and Methods in Physics Research B44 (1989) 54-60

North-Holland

HIGH FLUENCE XENON IRRADIATION OF ALUMINIUM: SPUTTERING AND SATURATION IMPLANTATION *

Thomas WEBER and Klaus-Peter LIEB

II. Physikalisches Institut, Universitiit Gijttingen, D-3400 Gb’ttingen, FRG

Received 17 April 1989 and in revised form 16 June 1989

Xenon ion irradiation of polycrystalline aluminium was studied at energies of 80, 250, 450 and 700 keV and at fluences between

10” and 3x10r7/cm2. The Xe depth distributions were measured by 900 keV a-particle Rutherford backscattering and the

sputtering and saturation yields were deduced. Additional optical micrograph and scanning electron microscopy measurements were

performed. At 80 keV the implantation is governed by surface sputtering. At the higher ion energies, saturation is mainly caused by

bubble and blister formation, leading to oversaturation effects and final gas release. Saturation also depends on a relaxation

mechanism in which the Xe content in the near surface region is depleted. Thermal blister formation and gas release were observed to

occur above 260 o C annealing temperature. Comparison with our previous measurements of 250 and 450 keV Kr+ ion implantation

in Al gives a consistent description of the heavy noble gas collection mechanism in this matrix.

1. Introduction

Ion beam implantation in metals is a versatile tech- nology for forming metastable alloys and compounds. It offers the potential to modify the properties of materi- als selectively. Precise knowledge of the basic effects connected with the irradiation process in the different fluence regions is very important. In recent years the implantation of noble gases in metals has been found to induce phenomena such as formation of inclusions con- taining solid gas at extremely high pressure [l-4]. At high fluences, saturation effects and gas release occur. Work in this field has mainly been done on He in metals [5-71, stimulated by first wall phenomena in future nuclear fusion devices. The many mechanisms influencing high-dose implantation profiles have been reviewed e.g. by Andersen [8].

The work reported here forms part of an extended study of basic effects of heavy ion implantation and ion beam mixing in metals and compounds [9,10] by means of ion beam depth profiling techniques like Rutherford backscattering (RBS), resonant nuclear reaction analysis (RNRA) and proton induced X-ray emission (PIXE). Measurements were carried out on model systems in order to explore the basic mechanisms, and on systems of technological interest (for instance TIN coatings). In a previous paper [9] we have shown the influence of sputtering, blistering, gas precipitation and gas release in Al after 250 keV and 450 keV Kr+ implantation. It

* Supported partially by Deutsches Bundesministerium fur Forschung und Technologie under contract 13N5453/7.

0168-583X/89/$03.50 0 Elsevier Science Publishers B.V.

(North-Holland)

was found that at low energies and low Kr fluences sputtering is the dominant process. At higher energies other mechanisms such as gas precipitation and blister- ing occur. These results suggested that similar effects may be generally observable in heavy noble gas irradia- tions of metals.

As an extension of our previous work we have now performed Xe+ and Xe*+ implantation experiments in Al at 80-700 keV beam energy and at fluences ranging from 1015 to 3 x 10” Xe ions/cm2. This system was chosen as it allows high resolution depth profiling by RBS, due to the large mass and charge ratio between the nuclei of the bulk material and the implanted species. The implantation process can thus be followed over a large energy range. The present study is also of method- ological interest since it demonstrates the application of a novel technique to measure sputtering yields of heavy implants.

2. Experimental procedure and data analysis

The implantations and analyses were carried out by means of the Gottingen ion implanter IONAS [ll]. Singly and doubly charged ‘32Xe ions were implanted into polycrystalline Al foils of 5N purity and of 8 X 10 mm2 size. The thickness of the foils was 2 mm (at 450 and 750 keV) and 50 ym (at 80 and 250 keV) respec- tively. During implantation the typical current density was 20 PA/cm2 for the Xe+ ions extracted from an SO55 duoplasmatron ion source, and 2 PA/cm2 for the Xe2+ ions produced by a Penning type ion source. The

T. Weber, K.-P. Lieb / Sputtering and saturation implantation of Xe in AI 55

samples were clamped onto a water cooled metal back- ing which kept them close to room temperature during the implantation. Homogeneous impl~tation over the 6 x 6 mm2 beam area was achieved with the help of an electrostatic x-y-sweep system.

After implantation, RBS depth profiling was per- formed with 900 keV e-particles. Two silicon surface barrier detectors of 13 keV energy resolution (FWHM) were mounted at +- 165” with respect to the o-beam which was focussed to 1.5 mm diameter. The RBS spectra were converted into depth distributions by applying standard formulae used in backscattering anal- ysis [12]. The metric depth scale was calculated using the density of pure Al. For lower Xe concentrations this is a good appro~mation to the unknown density in the Xe doped region.

The temperature dependence of the implantation profiles was studied with samples which had been doped with 450 keV Xe+ ions up to a peak concentration of 10 at.%. Each sample was then heated individually for 30 min at lop6 mbar at temperatures between 230 and 440°C. The temperature stability of +5“ C was controlled by a Pt-PtRh-thermocouple which was in contact with the target.

The total amount of Xe retained in the samples after high fluence implantation, i.e. the saturation yield S in units of at./cm’, is simply obtained by integrating the profile over the full depth. The evaluation of the sputtering yield Y required a more intricate method of analysis. In order to obtain Y, the profiles were compared with calculated ones in the low-dose range far below saturation, where no implanted material was lost by outgassing or blistering. In the calculation the im- plantation process was divided into a sufficiently high number of single implantation steps (typically 100). In each step uniform sputtering was considered so that the new surface moves into the former inner part of the sample. In addition, range shortening caused by the changing target composition due to the increasing Xe content was taken into account. An experimental depth profile taken at very low fluence (typically 2-3 X

10’5/cm2) was chosen as the undisturbed reference distribution against which the changes in profile after the low fluence implantations could be measured.

As to the interpretation of the sputtering yields obtained in this manner, the influence of already im- planted Xe on the sputtering has to be discussed. According to Sigmund’s sputtering theory [13] the mass dependent factor a(M2/M1) will be increased due to the increasing average target mass M2 during implanta- tion. This causes a higher sputtering yield than predic- ted for a pure Al sample. The order of magnitude of this effect can be estimated: according to ref. [13], a(Mz/M,) for the system Xe/Xe - which corresponds to a pure Xe-target - is by 50% higher than that for Al/Xc. The yield data, however, were obtained in the

fluence region, where the average Xe concentration is less than 3 at.%. Therefore the sputtering yield can be estimated to be increased by 1.5% at most. In addition, sputtering occurs in the top surface layers where the Xe concentration is even lower. We conclude, that this mechanism can be neglected within the accuracy of the analysing method.

3. Results

3.1. Implantation at 80 keV

The Xe profiles after 80 keV implantation at several fluences are illustrated in fig. 1. With increasing dose + the depth of the maximum Xe concentration is moving towards the surface. At a fluence of & = 2 X 10’6/cm2, saturation is reached and the profile does not change any more when further increasing +. The maximum concentration is found at the surface and decreases at larger depths. The collected amount of Xe ions first rises linearly with @I and then reaches a saturation value of S = (1.85 i 0.05) 10’“/cm2, as shown in fig. 2. This is in qualitative agreement with an implantation process dominated by sputtering from the sample surface and by range shortening, due to the influence of previously implanted Xe atoms. Indeed, a calculation described above nicely parametrizes the data if a sputtering yield of Y = 6 _i 2 is assumed.

On the average, saturation is reached as soon as for every collected Xe ion one Xe gas atom leaves the target. Although the calculations describe the shape of the experimental profiles at low and medium fluences quite well, they fail in the case of the saturation profile at depths smaller than 50 nm. This can be seen by

i- 80 ke'i XC?--AI

F luence 0 033 x 10'%m'

b 078 I 10'6/cm2 C 10 x iO'6/Cm* d 3L x 10'6/cm2

e colculoted saturation profIle

Depth Inml

Fig. 1. Xe depth profiles measured after 80 keV implantation

into Al. The saturation profile is observed at fluences higher

than 2X 10’6/cmz. The theoretical saturation profile was calculated taking sputtering and range shortening into consid-

eration.

56 T. Weber, K.-P. Lieb / Sputtering and saturation implantation of Xe in Al

- 80 keV Xe*-Al "E 2-

02 __i__~__- --f

-D c

s /

; I-

/ : P

P /x c

0 Y /x

is / OX 0 1 2 3 L 5

Xe’ Fluence @ 11d6/cm21

Fig. 2. Measured Xe collection curve at 80 keV implantation energy.

comparing the curve in fig. l(d) with the calculated saturation profile l(e). An additional mechanism must be operative which reduces the storage of injected ions.

In sputtering of a multicomponent system, one has to deal with mass and bonding effects, which may lead to preferential sputtering [14]. In a collision cascade the lighter element of a two-component system is sputtered preferentially whenever bonding effects can be neglect- ed. In the present case Xe enrichment by a factor of two would be predicted [14]. On the other hand, because of differences in the surface binding energy, the sputtering of Xe which is the the high vapor pressure component, is enhanced and would produce an enrichment of Al. These considerations, however, affect only the few top nm at the surface which would not be resolved by the RBS method.

At saturation the near-surface region is heavily damaged by irradiation. The high-dose Xe implantation leads to considerable radiation damage and additional stress is induced by the Xe collected in the material.

Under such conditions, outdiffusion of Xe enhanced by stress has been observed in silicon [15]. This observation may also explain the reduced concentration in Al near

the surface.

3.2. Implantation at 450 and 700 keV

The Xe profiles measured after 450 keV implanta- tion up to I#I = 0.6 X 10’7/cm2 are shown in fig. 3. At these fluences all implanted ions are retained in the sample. The retained Xe amount increases linearly with + and the profiles shift towards the surface. A sputter- ing calculation taking range shortening into considera- tion gives a good description of the data if the sputter- ing yield is assumed to be Y = 3.5 + 1.5. For higher fluences, however, the results again deviate from the behaviour one would expect from pure sputtering. At C#I = 1.15 x 10i7/cm2 the depth profile narrows and a peak develops on top of the broad profile typical for the lower fluences (see fig. 4). This hints strongly at precipi- tation of Xe. When exceeding this fluence, the peak

! I

8- 650 keV Xe+-AI Fluence I cr?l

0 002 IlO’7 b 010 ,10’7 ,

c 038 x1017 d 058 110”

b

0 0

0 100 200 300 Depth inml

Fig. 3. 450 keV depth profiles at low fluences + 5 5.8 X 10’6/cm2.

concentration decreases from 16 at.% to 11 at.%. For fluences $J 2 2 x 10i7/cm2 the profile does not change any more and has reached its saturation shape which, however, is different from the one found at 80 keV. The maximum concentration now is localized at a depth of 70 nm and not at the surface.

The 450 keV collection curve illustrated in fig. 5 rises linearly with $I up to a dose of 1 X 10i7/cm2. Above a

Depth (nm)

Fig. 4. Xe depth profiles after 450 keV irradiation with fluences

higher than 1 x lO”/cm’.

T. Weber, K.-P. Lieb / Sputtering and saturaiion implantation of Xe in Al 51

/ xd-A’ x L50 keV

o 250 keV

Xe Fluence $110'7/~m21

Fig. 5. Xe collection curves at 250 and 450 keV in Al.

critical fluence the concentration decreases to a satura- tion value of S = (0.85 f 0.05) x lO”/cm*. The fluence region (1.0-1.8) lO”/cm can be considered as “over- saturated”. This is different from the collection model described above. SEM measurements revealed heavy blistering of the surface, beginning at 0 = 1.25 x lO”/cm*. For lower fluences closed blister caps were noticed. As an example fig. 6 shows the surface mor- phology of a sample irradiated with 1.8 X lO”/cm*. The blister diameter is of the order of the ion range (= 120 nm).

Compared to the 80 keV implantation, the ion range is higher and sputtering is less important. This means that much higher concentrations can be achieved before

700keV Xe"-AI

* * i

* t h

P * 0 2 *

x” 0' 0 1 2 3 L 5 6

Xe Fluence c$ 110'71cm21

Fig. 7. Collection curve of 700 keV Xe2+ in Al.

sputtering-induced loss of Xe starts and a super- saturated state far beyond solubility is generated. The Xe forms blisters, whenever the critical concentration of 14 at.% is exceeded. After gas release out of the heavily damaged surface region one expects a relatively smaller Xe concentration near the surface than in the case of the 80 keV implantation. The measured saturation pro- files show this behaviour: the concentration decreases to zero at the surface.

The Xe profiles obtained after 700 keV ‘32Xe2i implantation gave very similar results. From the low- fluence data up to $I = 0.8 x 10”/cm2, the sputtering yield Y = 2 + 1 was found. Due to the limited current of doubly-charged Xe ions from the Penning ion source (4

PA) and the very long implantation times required (up to 10 h), the high fluence region was scanned with less

Fig. 6. Blistering of the surface of an Al sample irradiated with 1.8 x 10” Xe+/cm2.

58 T. Weber, K.-P. Lieb / Sputtering and saturation implantation of Xe in Al

data points. Nevertheless, a saturation yield of S = 1.55 k 0.05 x 10i7/cmz was deduced at this energy. The collection curve is given in fig. 7. Visible blisters were found to occur at 9 2 2.0 X 10’7/cm2.

3.3. Implantation at 250 keV

At the intermediate implantation energy of 250 keV we wanted to verify whether high or low energy condi- tions prevail. Because of the expected lower ion range of 90 nm and a higher sputtering yield a critical concentra- tion for blistering might not be reached. However, the results of the 250 keV implantation series turned out to be in qualitative agreement with the high energy be- haviour: Up to a fluence of 0.7 X 10i7/cm2 the implan- tation is again sputtering-governed giving a sputtering yield of Y = 6.0 + 1.5. At 0.8 X 10’7/cm2 the profile narrows and, for higher fluences, the collected Xe con- tent decreases leading to a saturation value of S = 0.5 x lO”/cm*. The maximum Xe concentration occurs at a depth of 40 nm and the Xe profile decreases linearly to the surface. The collection curve is also shown in fig. 5.

4. Thermal blister formation

Xenon depth distributions obtained after 30 min annealing at temperature T = 230440°C are dis-

, I

L50keV'32XG-A

IO- T, = 275OC

T,=320°C

0 100 200 300

Depth lnml

Fig. 8. Xe depth profiles after different thermal treatments.

Tl°Cl

L50 LOO 350 300

1o-“- Matrix A, 0 xe x Kr

IL 1.5 16 1.7 1.8 lOOO/TlK-‘I

Fig. 9. Arrhenius plot of the effective diffusion constants D’

obtained from the amount of extracted Xe and Kr.

played in fig. 8. No change in the depth distribution of the implanted Xe was observed below 260 ’ C where Xe gets mobile. The profile narrows and starts to be shifted towards the surface. The maximum concentration is found at 115 nm. This is the depth of highest radiation damage, according to calculations with the computer code TRIM [16].

At higher temperatures gas release starts and the amount of Xe decreases steadily. The implantation pro- file changes to a double peak structure. The minimum now occurs at a depth where the maximum concentra- tion had been found just before loss of Xe began. Gas release is correlated with blistering of the surface, as micrograph photography revealed. At 260 o C blistering starts to be visible and at 290 o C the whole surface is blistered uniformly. The blister size of about 1 pm in diameter shows the thermal character of the blistering mechanism.

Despite the complicated nature of the gas reemission process, it has become customary to define an effective diffusion constant D’ of the gas release process. This constant can be estimated from the ratio M/M, of Xe extracted at times t and t = co, according to the expres- sion

M/M, = 1 - (S/T’) exp( -T2D’t/4L2);

L denotes the average implantation depth. From the data of fig. 8 D’ is estimated to be of the order of 5 x 10-13 to 10-1s cm2/s. As shown in fig. 9, D’

follows an Arrhenius type temperature dependence

D’(T) a exp( -AH’/kT)

from which we deduce an “effective migration enthalpy” AH’ = 0.6 eV.

5. Comparison with the results for Kr in Al and conclu- sions

In order to arrive at a consistent description of the collection process we now compare the present results

T. Weber, K.-P. Lieb / Sputtering and saturation implantation of Xe in AI 59

Table 1 Sputtering yield Y, saturation yield S and critical fluence $I~,,,~

after Xe and Kr implantations into Al

E Range Sputtering Saturation Critical (keV) (nm) yield yield fluence

S (cmm2) %,,,t (cm-*)

Xenon “‘Xe 80 45 6 (2) 1.85 (5)x10” -

250 90 6.0 (15) 0.53 (3)X10” 1.0 (1)X10”

450 165 3.5 (15) 0.85 (5)x 10” 1.2 (1)x 10”

700 260 2 (1) 1.55 (10)X10” 1.9 (1)X10”

Krypton X4Kr 250 150 2.5 (10) 1.10 (5)X10” 1.4 (1)X10”

450 230 2 (1) 1.70 (7)X10” 2.2(1)X10”

with our previous RBS measurements after Kr implan- tation into Al (ref. [9]). Table 1 summarizes the sputter- ing yields Y, saturation yields S and the critical fluences

L,, measured at the different Xe and Kr ion energies. In fig. 10 the sputtering yields evaluated from the low- fluence data [17] tabulated by Andersen and Bay [18] and from the high-fluence data by Almen and Bruce [19] and the present work are compared with the theo- retical and/or semi-empirical predictions by Sigmund [12], Gries and Strydom [20] and Matsunami et al. [21]. While Sigmund’s approach systematically overestimates

10'

loo

0 I, 0’

10-l /’ - Sigmund

- --Gr!es.Strydom

c “/ - --Mols”“om, et LII

G , / , ,

’ lo'- Kr -Al z T;

IO' IO0 10' E IkeVl

1 I

lo2 IO'

Fig. 10. The evaluated sputtering yields taken from [9,17-191

and the present work and compared to calculations according

to Sigmund [13], Gries and Strydom [20] and Matsunami et al.

Pll.

I

2.0- x Kr-AI

o Xe-AI

1.5-

:I

,,' a

l.O- /'

/ $' '

0.5- ,,' 'P.'

// 1 I I

0 50 100 150 200 250 :

Ion Rangeinm)

IC IO

Fig. 11. Dependence of the saturation yield S on the ion range.

the experimental sputtering yields, a somewhat better agreement is obtained with the predictions of refs. [20] and [21].

As to the saturation collection of heavy noble gases in Al, our results at high energies are in qualitative agreement with the effects observed for He irradiation of metals [5-71. Blistertng which is not observed at 80 keV Xe implantation, evidently depends on the balance between sputtering yield and ion range. A sufficiently high gas concentration is achieved if the implanted region is not too quickly removed via sputtering. A comparison between the results for Xe and Kr again indicates, that the ion range is the dominant parameter. In both systems the collection curves for comparable implantation depths are very close to each other (250 keV Kr+/450 keV Xe+ and 450 keV Kr+/700 keV Xe2+) and the obtained values for &,,,, and S are in agreement. The saturation yield depends linearly on the ion range, as can be seen in fig. 11.

None the less. the results in the two systems are not in perfect agreement: the saturation yield achieved in the Kr irradiation is 24% higher. This might be explained in terms of radiation damage. The damage created by Xe is higher than the one in the case of Kr [16]. Therefore the conditions for precipitation and blistering are fulfilled at lower fluences and the critical fluence is also lower. At saturation the concentration gradient in the near-surface region is by a factor of two smaller for Xe than for Kr which again indicates heavier damaging of the host material and leads to a reduced saturation yield S.

Our previous results on the diffusion of implanted Kr in Al are also inserted into fig. 9. Thermal blistering starts at the same temperature as observed with Xe. Since Kr was implanted at 250 keV to a comparable depth (table 1) and up to the same peak concentration, this suggests that the same mechanisms are responsible in both cases. Noble gases in metals are essentially insoluble. Therefore they tend to precipitate and form bubbles. Kr and Xe bubbles in Al at concentrations

60 T Weber, K.-P. Lieb / Sputtering and saturation implantation of Xe in Al

comparable to the ones discussed here have recently been investigated by several groups [l-4]. Very detailed observations on the formation, melting and epitaxial regrowth of Kr inclusions in Al single crystals have been made [4,22,23]. Bubble formation was found to start at typical concentration of 2 at.%. The Xe and Kr bubbles have similar sizes with diameters of typically some nm.

Diffusion into the zone of highest radiation damage with subsequent blister formation is a bubble diffusion process. According to Kelly [24], the process depends on the bubble size and the host material. Since the bubble diameters are comparable, the similar behaviour of Kr and Xe implanted into the same depth is not surprising. The highest probability for coalescence of bubbles and blister formation is indeed at the depth of highest gas concentration. Therefore the process leads to loss of gas mainly out of this depth. The spurs towards the surface and into the material are less influenced. This explains the observed double-peak pro- files at higher temperatures.

The authors would like to thank D. Purschke for his expert running of IONAS, Dr. M. Uhrmacher for stimulating discussions and Dr. T. Heinrichs for assis- tance in taking the electron microscopy pictures.

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