Ion beam mixing and phase formation in NiAi multilayers studied by RBS and PAC

SURFACE AND INTERFACE ANALYSIS, VOL. 17, 33&334 (1991)

Ion Beam Mixing and Phase Formation in Ni-A1 Multilayers Studied by RBS and PAC?

Thomas Weber, Klaus Peter Lieb and Michael Uhrmacher I1 Physikalisches Institut and Sonderforschungsbereich 345, Universitat Gottingen, D-3400 Gottingen, Germany

NcA~ bi- and multilayers were irradiated with 3M)-900 keV Xe ions and analysed by Rutherford backscattering spectrometry (RBS) and the perturbed gamma-ray angular correlation (PAC) method. As PAC measures the interaction of radioactive probe nuclei ("'In) with hyperfine fields, it is sensitive to changes in the nearest- neighbour surrounding induced by a phase transformation. The formation of the crystalline NiAl phase during irradiation was indeed detected by PAC; it was found to start at ion fluences below that required for complete mixing of the constituents. Similar results were obtained after 600 keV Ar irradiations. The interface mixing rate in NCAl bilayers due to 300-600 keV Xe ions was measured with RBS to be k = 4 3 3 ) x 10' di' at room temperature.

INTRODUCTION

Ion beam mixing (IBM) has developed into a powerful method with which to increase thin film adhesion and to form metastable alloy films with either crystalline or amorphous structures. When irradiated with noble gas ions, samples consisting of alternating layers of two metals exhibit several effects: surface atoms are sput- tered, the layers are mixed and new phases may form.'-6 In the case of light-ion irradiation, ballistic mixing is expected to be the predominant process and the mass transport through the interface is regarded as the consequence of collision cascades : the mixing rate should scale linearly with the energy F , deposited at the interface.' For heavy-ion irradiation, strong experimental evidence was found for IBM to be dominated by the collective motion of low-energy (c 1 ev) recoils. At such energies, chemical effects play an important role. In this so-called thermal spike region, mixing is expected to scale with F,2.6 Several empirical rules have been for- mulated concerning the formation of new phases by IBM. Liu and co-workers7 have pointed out that an amorphous binary alloy will be formed when the two constituent metals have different crystalline structures. Hung et ul.' have suggested that the structural and chemical complexity of the alloy phases play an important role and that complex phases will turn amorphous under irradiation. Although these rules are very helpful, exceptions seem to exist to all of them.

Rutherford backscattering spectrometry (RBS)9 has proved to be well suited for the investigation of ion irradiation effects in multilayered structures. This method is non-destructive and provides quantitative information on the depth distributions of the constituent materials. An ideal complement to RBS is an analysing method that reveals the phases present in the samples, i.e. methods such as x-ray diffraction or electron micros- copy. In the present paper we report, for the first time, on experiments in which RBS and the perturbed angular correlation (PAC) method were combined.

t Paper presented at QSA-6, London, November, 1990.

0142-242 119 1/0603305 $05.00 0 1991 by John Wiley & Sons, Ltd.

Over the last few years we have successfully applied PAC to measure, for example, magnetic phase tran- sitions," defect structures in metals'O~'' and structural or REDOX phase transformations in metals and metal oxide^.'^*'^ In a PAC experiment, the hyperfine interaction at the site of the probe nucleus is deduced from the angular correlation of its yy cascade, often the 171-245 keV cascade in '"Cd populated in the electron capture-decay of radioactive "'In. The intermediate state in '"Cd is influenced during its 84 ns half-life by the electric field gradient(s) produced by the neighbour- ing field generating charges. Roughly speaking, the perturbation of the gamma correlation pictures changes in the hyperfine field of the probe microsurrounding intro- duced by IBM and thus should provide the possibility of studying the early stages of the phase formation process.

In this paper we report on mixing experiments with Ni-A1 bi- and multilayers. The Ni-A1 phase diagram shows the presence of four phases:' NiAl is a highly ordered B2 alloy with a wide composition range from cNi = 45-60 at.%. It is stable during room-temperature Xe irradiation, as observed by Hung et a!.,* while NiAl,, Ni2Al, and Ni,Al become amorphous. After room-temperature Xe mixing of Ni-A1 multiplayers, the formation of crystalline NiAl has been observed by Jaouen et Here, we will focus on the dependence of the irradiation results on the ion species and fluence and try to correlate the results with the mixing rates obtained via RBS from bilayers.

EXPERIMENTAL PROCEDURE

Thin-film layered samples were prepared by sequential electron-gun evaporation of Ni and Al. During evaporation, the vacuum in the chamber increased from a base pressure of 5 x mbar to - 5 x lo-' mbar. Two different sample configurations were prepared : bilayers for studying mixing effects and multilayers for phase formation experiments. The bilayers consisted of 30 or 40 nm of Ni on top of 150 nm of Al, deposited onto A1 substrates. In this way, A1 oxidation at the NiAl

Received 13 November 1990 Accepted I February 1990

Ni-AI MULTILAYERS 331

interface was greatly reduced. The second set of samples consisted of Ni-A1 multilayers on A1 or A120, substrates, with individual layer thicknesses ranging from 20 to 30 nm and adjusted to obtain equiatomic film composition after mixing. The total thickness was typi- cally 200 nm. In order to prevent a reaction between substrate and layer stack, sapphire substrates were chosen for these samples, which were annealed after- wards.

One batch of bilayers was irradiated with 300-600 keV Xe ions at fluences of 2,4 and 6 x 1015 ions cm-2. All implantation fluxes were kept below 1.5 pA cmP2. During implantation, the targets were mounted on a water-cooled copper backing in order to keep them at room temperature. In the phase formation experiments, the multilayers were implanted with -10" '"In ions at 400 keV. The calculated projected range of such ions is N 100 nm. In the course of the PAC experiments, the samples were gradually mixed by 900 keV Xe2 + or 600 keV Ar' ions up to doses of 12 x 1015 Xe cn-' and 24 x 10'' Ar Before and after each mixing step, PAC spectra were taken with a set-up of four NaI detectors arranged in a 90" geometry. A detailed description of the PAC apparatus and the method of data reduction is given in Ref. 12. Backscattering mea- surement using 900 keV a-particles were performed before the first and after each irradiation step. All irradiations and RBS analyses were carried out at the Got- tingen ion implanter ION AS.'^

RESULTS AND DISCUSSION

Mixing and sputtering of Al-Ni bilayers by xenon

The RBS spectra displayed in Fig. 1 show the broaden- ing of the Ni-A1 bilayer interface during the 450 keV Xe irradiation. In order to determine the amount of mixing and surface sputtering, the spectra were fitted by use of the computer code RUMP.16 The interfaces were

described by simulation of an error-function-like Ni concentration profile

c(x) = c0/2[1 - erqx - 4/J2021 Here, o2 denotes the interface variance and d the thickness of the Ni layer. For all energies, the changes of o2 and d were found to depend linearly on the fluence @, as illustrated in Fig. 2. Table 1 summarizes the measured Ni sputtering yields YNi = Ad/A@ and Ni-A1 mixing rates k = Aa2/A@, which were obtained from least- square fits to the data. The sputtering yields YNi = 7-10 are in good agreement with theoretical calculations, according to the well-known Sigmund model," which predicts YNi to decrease slightly to about Y = 10.9 at 300 keV and to Y = 9.9 at 600 keV. Since all previous experimental work on this system has been performed in the low-energy region below 3 keV," only the results of Almen and Bruce can be compared to our data. These authors obtained a value of YNi = 7.6 for 45 keV Xe irradiation."

With regard to the mixing effect, it was noticed that the largest mixing rate occurs when the ion range R , exceeds the layer thickness by -30%. Usually, the mixing rates are discussed in terms of the deposited energy F, . This quantity (listed in Table 1) was calculated by means of the Monte Carlo program TRIM,20 taking into account the recoil cascades. For the case of Xe- and Ar-irradiated TiN films on AlMg, substrates, the importance of the recoil cascades for atomic transport has recently been demonstrated by Miiller et aL21 These authors have shown that mixing occurs even if

Table 1. Sputtering yield YN,, mixing rate k aod deposited energy F,, for ACNi bdayers irradiated with Xe ions

E Initial k klF, (keV) thickness y,, (A ~ 1 0 ' ) ( e v k ' ) (A5ev-q)

300 30 7.5 (10) 480 (40) 420 114 (11) 450 30 8.5 (15) 455 (40) 380 120 (11) 450 40 8.5 (15) 450 (30) 395 114 (8) 600 40 10.0 (15) 405 (30) 332 122 (9)

450 keV Xe - > N iAB i laye r

400 500 600 700

Energy (keV) Figure 1. Rutherford backscattering spectra of a 40 nm Ni-AI bilayer taken before and after 450 keV Xe irradiations with the doses given.

332 T. WEBER, K. P. LIEB AND M. UHRMACHER

42

40

38 E

u 36 \

34

32

450 keV Xe

- > 40 nm N i N

I I I I

0 2 4 6 8

@ / 1015/cm2

40000 1 1

1 450 keV Xe /I - > 40 nm N i U

30000

<U

Q \ 20000 ru

loooot k=455(30)-10* A4 1

0 / 1015/cm2

Figure 2. Dependence of the Ni film thickness d (a) and NiAl interface variance uz (b) on the Xe ion fluence @. The slopes are the sputtering yield YNi = M/A@ and mixing rate k = Au2/A@.

the primary ions do not penetrate the interface. Thus, not only is the nuclear stopping of the primary ions responsible for the mixing effect, but also the damage created over the whole lifetime of the damage cascade. We arrive at a mixing rate of k = 4.5(3) x lo4 A" and a mixing efficiency of k/F, = 108(5) A eV-'. Results of Ma et al." on the mixing of Ni-A1 bilayers near 77 K give a smaller mixing efficiency than the room- temperature data presented here. This difference could point to a non-negligible contribution of radiation- enhanced diffusion (RED). On the other hand, experiments of Mantl and co-workersZ3 on the mixing of Ni marker layers in A1 matrices did not reveal any differ- ences in mixing rates in the temperature range 18-300 K. Of course, the marker geometry might not give directly comparable results.

Phase formation

The development of the Ni concentration profiles measured via RBS after successive 900 keV Xe irradiations of Ni-A1 multilayers is displayed in Fig. 3. The layers are mixed quite uniformly over the whole multilayer stack until complete mixing with an equiatomic NiAl concentration is reached for fluences @ > 10 x lo" C I - I - ~ . As can be seen in Fig. 3, at the bottom of the layer stack intermixing with the A1 substrate occurs.

Energy (keW

Figure 3. Rutherford backscattering spectra from Xe-irradiated Ni-AI multilayers at different fluences. Only the Ni part of the spectra is presented.

This does not influence the phase formation experiments, since the number of "'In probe atoms at these large depths is negligible.

The development of the PAC spectra, i.e. the perturbation functions R(t) and their Fourier transforms, is shown in Fig. 4. They clearly reveal the AI-Ni to AlNi phase transformation : the ' "In-doped as-deposited samples show the well-known 93 MHz Larmor frequency of a '"Cd probe nucleus on a substitutional (ferromagnetic) Ni lattice site.24 The cubic non- magnetic surrounding of probe atoms in the A1 sub- layers produces no electric field gradient and thus leads to a quadrupole frequency distribution around zero. After irradiation with 4 x 1015 Xe cm-', the fraction with the Ni Larmor frequency has disappeared and the formation of a crystallline NiAl microsurrounding is proved by a change in the frequency pattern that is typical for '"Cd impurities in NiA1.2s Up to a fluence of 12 x lo" cm-', the frequency distribution does not change any more with dose. X-ray diffraction measurements corroborate the observed phase formation.

Ni-AI MULTILAYERS

0 06-

333

Ni-Al-Multilayer .

as deposited '

0.0 as deposited

0 03- 0 C6

a o 2 x 10"Xe/cmz

12x1015~r /cm2 .

0 0 '1

Figure 4. Perturbed angular correlation perturbation functions R(t) mixed with 900 keV Xe at different fluences.

The atomic rearrangement and phase formation are initiated by the damage cascade created by the incom- ing ions. Compared to thermally activated phase tran- sitions observed by means of PAC,'? the perturbation functions R(t) are damped and rather broad peaks occur in the Fourier spectra, sometimes on top of even

-3 23u 3: :00 200

t(ns)

0 2 0 4 0 6

Frequency (GHz

and equivalent Fourier transforms of "In-doped Ni-AI multilayers

broader distributions. This indicates that irradiation defects, as well as an imperfect atomic rearrangement during the phase transformation, are still present. Fan and Collins25 attributed the two quadrupole interactions, o1 = 128 and 141 MHz, to defects in the AlNi lattice (trapped monovacancies). The striking simi-

Frequency (GHz)

Figure 5. Ni-AI to NiAl phase transformation induced by 600 keV ATZ+ irradiation as observed via the PAC method.

3 34 T. WEBER, K. P. LIEB A N D M. UHRMACHER

larities of the R(t) spectra in Figs 4 and 5, however, give reason to assume that further PAC measurements may eventually reveal more details on how the phase transformation develops, as was recently shown by Wegner et

During an isochromal annealing cycle of the ion- mixed samples, the frequency pattern stayed fairly con- stant up to 200"C, indicating that no further rearrangement of the lattice occurs. Above 300 "C, the sample turned amorphous, as indicated by the broad distribution centred around the NiAl frequencies.

NiAl phase formation was also observed for Ar irradiations at 600 keV (see Fig. 5). The results are comparable to those obtained with Xe ions, but the required Ar doses are higher by a factor of six. This turns out to be the difference in nuclear stopping power at these energies and may be an indication against a thermal spike mechanism being responsible for the phase formation. The Ar ions produce a less dense secondary (damage) cascade and thus are not expected to create a spike.

for the COO -+ Co304 transition.

CONCLUSION

In this paper we have demonstrated that the com- bination of RBS and PAC is well suited to gather information on the early stages of the ion mixing process in the system Ni-Al, after Xe and Ar irradiations. This system has been chosen because it combines a mass ratio of the two constituents, which is very favourable for RBS, and the known hyperfine interactions of the '"Cd probe nuclei in Ni, A1 and the unknown inter- metallic phase AlNi. Furthermore, the formation of AlNi via ion beam mixing had been verified pre- vi~usly. '~

Rutherford backscattering spectrometry on the Xe mixed bilayers ave a rather high mixing rate of k = 433) x lo4 f14 at room temperature, correspond-

ing to a mixing efficiency of k / F , = 108(5) A eV-'. The 30% difference with respect to the 77 K data by Ma et al.," which may indicate some small contribution from radiation-enhanced diffusion, needs further clarification and low-temperature mixing experiments with Ar, Kr and Xe ions are under way.

NiAl phase formation was found to take place after Xe and Ar ion bombardments of AI-Ni multilayers. It sets in at fluences below those required to induce complete mixing. Rutherford backscattering spectrometry and PAC reveal different facets of the mixing process. Rutherford backscattering spectrometry directly pictures the variation of the concentration profiles at the interface and thus provides information on the radiation-induced atomic transport process with a depth resolution of - 10 nm. Perturbed angular correlation is sensitive to the local distribution of hyperfine fields within a spatial range of several nanometres around the probe atoms. In the case of multilayers, RBS requires constituents of considerable contrast in atomic mass, i.e. 27: 58 in the present case. Perturbed angular correlation may therefore be more generally suited for application in systems with comparable mass numbers; PAC also easily differentiates between crystalline and amorphous phases and is sensitive to radiative point defects, either trapped at the probe atoms or at larger distances. However, one should note that the PAC spectra are considerably damped, as expected in the presence of radiation defects and/or local lattice distor- tions. An alternative explanation should consider that a mixed multilayer may contain a variety of similar microsurroundings, which cause a distribution of electric field gradients (EFGs). Further work is necessary for gaining a better understanding of the complex PAC spectra.

Acknowledgements

We are grateful to D . Purschke, Th. Wenzel, A. Bartos and D . Wiarda for their help during the experiments and the PAC analysis.

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Ion beam mixing and phase formation in NiAi multilayers studied by RBS and PAC

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