Ion-induced radiation-enhanced diffusion of silver in nickel

Materials Science and Engineering, A l l 5 (1989) 223-227

223

Ion-induced Radiation-enhanced Diffusion of Silver in Nickel* D. MARTONt, J. FINE and G. P. CHAMBERS

Surface Science Division, National Institute of Standards and Technology, Gaithersburg, MD 20899 (U.S.A.) (Received September 16, 1988)

Abstract

Radiation-enhanced diffusion (RED) was observed during the Auger electron spectroscopy sputter depth profiling of multilayered Ag/Ni thin films. Broadening of the thin (4 nm) silver layers occurred during sputter profiling and resulted in silver Auger intensity profiles for each of these five layers which were asymmetric, exhibiting a steep leading edge followed by a more slowly decreasing tail. These findings can be interpreted in terms of interface broadening due to two main factors: (1) surface roughening--symmetric broadening," (2) RED of silver--asymmetric broadening. With this model it has been possible to separate these factors" and to determine the rate of RED. 1. Introduction

In a number of binary metallic systems, ion bombardment has been shown to cause radiation-enhanced diffusion (RED)[1-6]. Such radiation-enhanced material transport is also seen to occur at sputtered Ni-Ag interfaces and is a major factor in the modified depth-composition distributions obtained in sputter profiling of such interfaces [1, 2]. In order to determine the specific contribution of room temperature RED on sputter depth profile interface broadening, RED measurements have been carried out by steady state Auger sputter depth profiling of Ag/Ni multilayered thin film structures. The interface broadening observed in this system is determined by a number of factors, the main two being roughening and RED. The contributions of these two factors *Paper presented at the Sixth International Conference on Surface Modification of Metals by Ion Beams, Riva del Garda, Italy, September 12-16, 1988. tOn leave from the Technical University, Budapest, Hungary. 0921-5093/89/$3.5(1

can be described by a simple model based on the assumption that roughness leads to a symmetric profile shape, while an asymmetric shape is caused by RED. The results of such an analysis allow us to separate the contributions due to each of these two effects; this separation can be confirmed by an independent measurement of in situ surface roughness measurements using light scattering techniques [7]. This approach has the advantages that (1) the contributions to interface broadening obtained in this study will have direct bearing on profiles obtained in Auger electron spectroscopy ( A E S ) ( a n d secondary ion mass spectrometry) measurements, (2) it is applicable for both miscible and immiscible systems (such as Ag-Ni) and (3) the temperature dependence can also be studied [8]. Both RED and surface roughening that result from the ion bombardment of an Ag/Ni system were analysed using multilayered Ag/Ni thin film structures. These structures consist of five thin (3-4 nm) silver layers alternately deposited between six thick (50 nm) nickel layers. Such samples were produced by sputter deposition using polished silicon substrates that had been coated, in situ, with amorphous silicon prior to the deposition of the metal films; they are similar to those previously described [1 ]. Stylus measurements indicated that the r.m.s, surface roughness of the samples prior to sputtering was less than 1 nm. Sputter depth profiling of these Ag/Ni samples was carried out using Ar + ions of both 1 and 4 keV energy. A differentially pumped ion gun produced a fine focused beam rastered over about 2 mm x 2 mm, at total beam currents in the 50-500 nA range. Auger electron spectra excited by a primary electron beam of 2500 eV energy (50-200 nA) were recorded in an EN(E) mode; digital background subtraction techniques were employed to obtain high precision depth profiles. © Elsevier Sequoia/Printed in "I'l~eNetherlands

224

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60

90

120

Time (min)

Fig. 1. Auger depth profile of a Ni/Ag multilayer (Ep = 4 keV) (the left-hand ordinate is the relative N(E) intensity for silver; the right-hand ordinate is for nickel; +, measured silver data; - - - , nickel data are connected with straight lines; -- + -- a fit corresponding to eqn. (4) for the silver data.

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Time (min)

Fig. 2. Measurement of the defect range zo for Ep = 1 keV. The sputtering was stopped at time q; there was no sputtering until t2, when sputtering was resumed. It should be noted that the silver segregation time is minutes and that the amount segregated is only about 6% of a monolayer.

The elements detected were nickel (LMM fine, 844 eV), silver (MNN fine, 349 eV) and nitrogen (KIJ, line, 375 eV). A typical AES depth profile is shown in Fig. 1.

The abscissa is the sputtering time, which can be readily transformed to a depth scale using known layer thicknesses. The layered Ag/Ni system has been shown to exhibit surface segregation of silver when a silver layer buried between two nickel layers is sputter profiled at room temperature [1]. While that study was directed to the analysis of radiationinduced segregation (RIS), it showed also the role of RED in the transport of silver atoms to the surface. Similar segregation experiments have been carried out in order to obtain information about the distance between the sputtered surface and the underlaying silver layer at which segregation can initially be detected. We define z0 as the range of defects generated by the primary ion impact. A typical measurement from which z0 can be determined is shown in Fig. 2. When a depth profiling measurement is interrupted at some depth before the silver layer is reached and the silver Auger intensity is continuously monitored in time, it is possible to observe two, dramatically different outcomes: (1) silver either segregates to the surface (as in Fig. 2) or (2) it does not segregate. In the first case, the ion-induced defects were able to reach the silver layer; in the second case, however, the silver layer is still beyond the range of the defects. From a series of such measurements the range of the defects was determined with an accuracy of about 1 nm; we conclude that z0 = 15.3 nm for 1 keV ion bombardment and z0 = 18.4 nm for 4 keV ion bombardment. The depth profile evaluation procedure for each nickel and silver layer, shown in Fig. 1, is based on a series of assumptions. Each depth profile of a given individual silver layer is analysed independently and resembles a gaussian in shape but is asymmetric, having a steeper slope at earlier sputtering times. We propose that each of these silver profiles is the result of the superposition of a gaussian (symmetric) and a RED (asymmetric) profile. The width AZR of the gaussian is determined mainly by surface roughness, although a small contribution from other factors (the thickness of the silver layer, the escape depth le of Auger electrons, the interface broadening Azc due to cascade mixing and thermal diffusion) should also be taken into account: c = AzR

exp{-- zo/21 A-ZR

J

(1)

225

where c o depends on the silver layer thickness and z is the depth coordinate directed perpendicular to the surface, le = 0.8 nm (silver at 375 eV [9]) has been used for all measurements. Values of AZc for various sputtering ions and energies are available from calculations based on Monte Carlo methods [10]; the values used here are 2 nm for Ep = 1 keV and 4 nm for Ep = 4 keV bombardment. No thermal interdiffusion in the Ag/Ni system was observed even at elevated temperatures [8] and was therefore neglected in the present analysis. Furthermore, we assume that RED starts when the sputtered surface is at a distance of z0 from the silver layer and that it continues at a constant rate after that. This will lead, according to standard diffusion theory for a planar source of negligible thickness at z0, to a profile described by the equation [11]

c=c"(4Dt)-l/2 exp{ -(-z-z'')214Dt]

(2)

where D is the diffusion coefficient and t is the diffusion time. The depth z is related to time t through the sputtering rate S:

z =St

(3)

It should be noted that the time t for each silver layer is measured separately. Combining eqns. (1)-(3), we obtain an equation which describes the measured depth profiles:

c=co

AZR /_I/

4Dt + ~ - ]

xexp{_(St_zo)2(4Dt AZR2'-1}

(41

Depth profiles, containing c vs. t information, are used in a computer fitting program to calculate co, D and AZR. This equation can be used to obtain very precise fits to the asymmetric silver layer sputter depth profiles. The model described allows us to calculate interface broadening values for each sputtered silver layer. In this procedure, we first calculate the maximal silver concentration. The sputtering rate S is then evaluated on the assumption that the sputtering yield of the Ag-Ni mix is the weighted average of the elemental sputtering yields of silver and nickel. The variation in S within the silver layer was neglected owing to the

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Fig. 3. RED rates (obtained from AES depth profiling measurements) vs. sputtering rates: *, 1 keV incident ion energy; o, 4 keV incident ion energy. Each point shown corresponds to one complete profile through several interfaces. The error bars correspond to the standard deviation of the RED rates obtained in each such profile.

limited range in silver concentration observed in the silver depth profiles. The full interface width obtained in our measurements consists of a part due to RED. This part can be characterized by the quantity 8

AZD = - -

0

(5)

which plays an analogous role to AZR in eqn. (4). The analysis of the data clearly shows that AzD is independent of the depth, and so we conclude that the depth dependence of the interface width is due to the roughening of the surface. This is as expected since there is no reason why the RED process should be affected by the depth, whereas the roughening of the surface is expected to increase as the square root of the depth [12]. Diffusion coefficients obtained from experimental depth profiles of silver, using eqn. (5) to fit the data, are plotted in Fig. 3 as a function of sputtering rates SN~.Results obtained with both 1 and 4 keV sputtering energies Ep are described by the linear equations D=l.22x10-17+1.75x10 15SNi (r=0.9710; n = 4 3 ; E p = 1 keV)

(6a)

and D=l.35x10-17+9.05x10 15Sr~i (r =0.9764; n = 29; Ep = 4 keV)

(6b)

where D is in square centimetres per second and SNi is in nanometres per second; r is the regres-

226

sion coefficient and n is the number of data points taken into consideration. The diffusion rate of grain boundary diffusion for silver in nickel, extrapolated from high temperature data [13], is about O b = 3.3 x 10 -19 c m 2 s-~ at 300 K, a value considerably smaller than obtained here for the R E D rate. The present authors are not aware of any R E D rate measurements carried out on the Ag/Ni system, except for the RIS measurement cited above [1]. The diffusion rate was not evaluated in that study but can be estimated on the basis of the segregation rate constants which were obtained. This estimate yields D 1 = 4.7 × 10 -17 c m 2 S - 1 and D 4 = 1.9 x 10 -~6 cm 2 s-1 for 1 and 4 keV measurements respectively (the sputtering rates were $1 = 7 × 1 0 - 3 n m s - I a n d $4 = 3 . 1 × 1 0 - 2 n m s - l ) .

These D values obtained from a completely different measurement are in good agreement with those reported here. Several other researchers have reported measurements of ion-induced RED in various two-component systems; their observed diffusion rates are of the same order of magnitude as those reported here [3-6]. In Fig. 3, diffusion rate data obtained during the same sputter profile (at constant sputtering rate) from a number of silver layers at different depths are shown as a single point, with an error bar corresponding to the combined scatter of the sputtering rate and diffusion rate in the various layers. The diffusion rates are plotted as a function of the sputtering rates rather than of the current densities, as the latter were not easily measurable. Since the sputtering rate is proportional to the current density for ions of a given energy, eqns. (6) describe the linear dependence of the measured diffusion rates on the ion beam current density. It is quite clear that the R E D rates are essentially proportional to the sputtering rates; this result is quite significant since it is basic to understanding the role of defect annihilation in R E D mechanisms. Diffusion processes in R E D take place by means of defects that have been generated as a result of energetic collisional events within a solid; unlike thermal diffusion, these defects are produced in a temperature-independent manner. We propose the following mechanism for ioninduced RED. In solids, collisional processes result in the generation of point defects within the volume of the collision cascade itself. The lifetime of these defects is not long enough to allow them to diffuse much beyond the collision cas-

cade; so they, consequently, cannot give rise to the extensive diffusion observed in RED. The short-rived point defects may also combine to form complex defects which may have rather long lifetimes and may themselves diffuse. Atom migration is made possible by encounters with these long-lived complex defects which can extend far below the sputtered surface. Further evidence that R E D processes involve defect complexes has been obtained from measurements of the temperature dependence of the diffusion rate [8]. Defects may be trapped at sinks, may dissociate or may recombine with other defects; only the latter process can be expected to depend on the energy and/or current density of the primary ions. Since the R E D rate does depend linearly on the sputtering rate, the recombination process may be excluded. The fact that diffusion takes place on a time scale of minutes (both in experiments described here and in ref. 1), allows us to conclude that defect complexes, rather than point defects, are responsible for the diffusion observed. In summary, R E D measurements have been made by Auger sputter depth profiling Ag/Ni multilayered thin films. The interface broadening observed is determined mainly by surface roughness and RED; these two factors can be separated to show that the roughness contribution is depth dependent, while the R E D component is not. Effective diffusion ranges which extend far beyond the ion range were measured and diffusion rates were determined that are proportional to the rate of sputtering. It is suggested that complex defects are involved in the diffusion process. This study also suggests that RED may contribute substantially to interface broadening in some multicomponent systems which have relatively low activation energies of diffusion.

Acknowledgment The authors would like to thank B. Navin~ek (J. Stefan Institute, Ljubljana, Yugoslavia) who fabricated the high quality thin film structures.

References 1 J. Fine, T. D. Andreadis and F. Davarya, Nucl. Instrum. Methods, 209 (1983) 521. 2 D. Marton, J. Fine and G. P. Chambers, in M. Grunze, H. J. Kreuzer and J. J. Weimer (eds.), Diffusion at Interfaces:

227 Microscopic Concepts, Springer, Berlin, 1988, p. 111. 3 D.G. Swartzfager, S. B. Ziemeeki and M. J. Kelley, J. Vac. Sci. Technol., 19 (1981) 185. 4 J. E. Hobbs and A. D. Marwick, Nucl. Instrum. Methods B, 9(1985) 169. 5 R.S. Li and T. Koshikawa, Surf. Sci., 151 (1985) 459. 6 N.Q. Lain and H. A. Hoff, Surf. Sci., 193 (1988) 353. 7 D. Marton and J. Fine, submitted to Thin Solid Films. 8 D. Marton, J. Fine and G. P. Chambers, Phys. Rev. Lett., 61 (1988)2697.

9 J. C. Ashley and C. J. Tung, Surf. Interface Anal,, 4 (1982) 52. 10 E Davarya, M. L. Roush, J. Fine, T. D. Andreadis and O. E Goktepe, J. Vac. Sci. Technol. A, 1 (1983) 467. 11 P. G. Shewmon, Diffusion in Solids, McGraw-Hill, New York, 1963. 12 D. Marton and J. Fine, Thin Solid Films, 151 (1987) 433. 13 A. B. Vladimirov, V. N. Kaygorodov, S. M. Klotsman and I. Sh. Trakhtenberg, Phys. Met. Metalloved., 45 (1979) 100.