LOW ENERGY ION ASSISTED VAPOR DEPOSITION

X. W. ZHOU AND H. N. G. WADLEYDepartment of Materials Science and Engineering, University of Virginia,Charlottesville, VA 22903

ABSTRACT

The performance of multilayered thin film materials often depends sensitively upon the(physical) roughness and degree of (chemical) mixing at interfaces. Irradiation of a growthsurface with an assisting ion beam is often used to modify surface roughness. Moleculardynamics has been used to explore the use of low energy (less than 20 eV) Xe+ and Ar+ assisteddeposition of model Ni/Cu/Ni multilayers to control both physical roughness and chemicalmixing. The study indicated that under normal ion incidence condition, ion energies as low as 3eV could effectively flatten the relatively weakly bonded copper surface (and therefore the nickelon copper interface). Higher ion energies (at least 10 eV) were required to flatten the morestrongly bonded nickel surface. Chemical intermixing by an exchange mechanism between asurface atom and an underlying atom in an already deposited layer depended upon the bindingenergy of the already deposited layer. As a result, significant chemical mixing occurred as 9 eV(and above) ions impacted with nickel atoms on an already deposited copper surface. At a givenion incident energy, (the heavier) Xe÷ ions resulted in less roughness but more mixing. Amodulated ion assistance strategy in which no assisting ion beam was used while depositing thefirst few monolayers of each new metal layer was found to successfully reduce both interfacialroughness and interlayer mixing.

INTRODUCTION

Multilayer structures often possess properties not possessed by either of their constituents.For instance, multilayers containing ferromagnetic thin films (e.g., permalloy or cobalt) separatedby a thin conductor spacer (such as copper) can exhibit giant magnetoresistance (GMR) [ 1,2]. Theread heads of all computer disk drives manufactured today now use these GMR materials toincrease storage capacity [3). GMR materials are also being explored for the development of anew class of low cost, nonvolatile magnetic random access memories [3,4]. To achieve a highmagnetoresistance with small magnetic fields, the interfaces between adjacent layers must be bothflat and exhibit minimal intermixing of the layers. Various physical vapor deposition (PVD) pro-cesses have used in attempts to accomplish this. These studies indicate that GMR multilayersneed to be deposited at low temperatures to minimize the thermally activated interlayer diffusion[5]. However, low temperature deposition promotes rough surfaces and interfaces. Sputtering pro-cesses can be used to reduce this roughness presumably because of the energetic impact of theadatoms or working gas ions with the surface. Magnetron sputtering experiments have indicatedthat best GMR properties occur for intermediate energy deposition conditions [6]. A recentmolecular dynamics approach has investigated the effects of the incident metal atom energy uponthe interfacial roughness and intermixing of model Ni/Cu/Ni multilayers [7,8]. The results indi-cated that high incident energies successfully reduced the surface (and subsequent interface)roughness, but promoted intermixing by an exchange mechanism. The simulations led to the rec-ognition that a modulated metal atom energy deposition strategy, in which the first half of a newlayer was deposited with a thermalized flux and the remainder with a hyperthermal flux, could

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Mat. Res. Soc. Symp. Proc. Vol. 585 © 2000 Materials Research Society

significantly reduce both interfacial roughness and intermixing. These results also indicated thatbecause adatoms are thermalized in molecular beam epitaxy (MBE), sputtering or ion beam depo-sition are preferred processes for synthesizing high performance GMR multilayers [9].

An ion beam deposition (IBD) pro-cess is shown in Figure 1. A high energy(0.1-2 keV) inert ion beam from the pri- Substrate Ie O.

mary ion beam gun causes the sputtering lo' .assist \ V.of the metal target. The average energy of basinthe sputtered atoms increases with inci-dent ion energy. Because ion beam depo-sition avoids the use of a plasma, it can be Sputtered metaloperated at very low pressure. This mini- target atomsmizes the energy loss of the sputtered on beam Kr or Xe

atoms by background gas collisions dur-ing their transportation to the substrate.Hence, the average energy of the deposi- .. . Backgroundtion atoms can be directly controlled by pressure: Pethe ion acceleration voltage of the pri- \ ...3mary ion beam gun. The IBD method hasresulted in high quality GMR films [10]. Figure 1. Ion beam assisted ion beam deposition.The IBD approach also allows a second-ary assisting ion beam to be directed at the subtrate. In this ion assisted ion beam (IBAD)approach, the impact of the hyperthermal assisting ions with a growth surface promotes surfaceatom diffusion and significantly affect surface roughness. Its effects upon interfacial mixing dur-ing multilayer deposition are much less clear. Similar energetic ion impacts with growth surfacecan also occur during rf diode and magnetron sputter deposition. Here molecular dynamics simu-lations are used to investigate the effects of these assisting ions. The growth of a model Ni/Cu/Nimultilayer in the [I l l] direction has been modeled.

COMPUTATIONAL METHODS

An embedded atom method (EAM) potential for binary Cu-Ni alloys was used to calculatethe interactions between Cu-Cu atoms, Ni-Ni atoms, and Cu-Ni atoms [11]. The EAM capturesthe local environment dependence of the potential, and hence realistically describes the energeticsnear defective crystal regions such as the growth surface and interfaces. A pairwise two-body uni-versal potential [12] was employed to define the interactions between the inert ions and the sur-face metal atoms. The universal potential is a suitable potential for this work because it was wellfitted to a vast amount of experimental data for low energy ion impacts with metal surfaces. Com-putational crystals were created by assigning the positions of atoms based on the lattice sites. Peri-odic boundary conditions were applied in the x- and z- directions (e.g., Figure 2). Ion assistanceand surface growth were simulated by injecting inert ions and metal atoms from random locationstowards the top (y) surface. Normal incident angle (0 = 00) was used for both assisting ions anddeposition atoms. The incident energy, Ei, and incident angle, 0, were introduced by assigning anappropriate velocity vector to each particle. A thermostat algorithm [13] was used to ensure thatall simulations were conducted at a fixed substrate temperature T = 300 K. The evolution of atom-

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istic structures were determined by solving for the trajectories of both lattice atoms and vaporphase particles using Newton's equations of motion.

RESULTS

Ni/Cu/Ni multilayer deposition was simulated for various assisting ion energies using a lowmetal atom deposition energy of 0.1 eV and an ion / metal flux ratio of 2. The resultant atomicconfigurations are shown as a function of ion energy in Figure 2. Here, dark and light spheres rep-resent nickel and copper atoms respectively. High interfacial roughness can be seen when no ionassistance was used, Figure 2(a). The roughness of the copper on nickel interface was higher thanthat of nickel on copper interface. The mixing of copper in the nickel layer near the nickel on cop-per interface was also much more significant than the mixing of nickel in the copper layer near thecopper on nickel interface. The interfacial roughness and the intermixing were both significantlyreduced at an ion energy of 0.5 eV, Figure 2(b). As the ion energy was increased from 0.5 to 3 eV,the roughness of the copper on nickel interface continued to decrease, but more intermixing wasobserved at the nickel on copper interface, Figure 2(c).

Ertil= 0 ,1eIon / metal = 2T 300K

(a) No ion bombardment (b) Exo = 0,5eV (c) Ex, = 3,0eVSy[•1111

'Cu on Ni. ....... Interfolace

20A NaNi on Cu

20A Cu interface

M'4 substtale /ZIIIOX (112)

Figure 2. Multilayer structure as a function of assisting Xe÷ ion energy.

The effects shown in Figure 2 are the result of ion interactions with a continuously evolvingcomplex surface morphology. To explore detailed mechanisms, ion impacts with a prescribed(model) rough surface was investigated without synchronized deposition. The initial surface con-figuration is shown in Figure 3(a). The effects of Xe÷ ion impacts are summarized in Figures 3(b)- 3(e). It can seen from Figure 3(b) that even a Xe÷ ion energy as low as 3 eV could fully flattenthe copper islands on a nickel crystal. However, the same 3 eV Xe÷ ions could not flatten thenickel island on a copper surface, Figure 3(c). Figure 3(d) indicates that increasing the Xe÷ energyto 12 eV did flatten the nickel on copper surface, but this caused extensive Ni-Cu mixing. Whennickel islands were formed on a more strongly bonded nickel (as opposed to copper) crystal sur-face, then 12 eV Xe÷ impacts could flatten the nickel islands without causing the exchangebetween the island nickel and the underlying nickel, Figure 3(e). Clearly, the nickel on copper sur-face is the only surface that cannot be satisfactorily synthesized with ion assistance because theion energy necessary for flattening the nickel islands are sufficiently high to cause nickel mixingwith the underlying copper.

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T = 300K (a,F - 0.5 ionsiA2 CLEx,&= variabley [ Cu (or t

S~ x [112]-elz [1 101

Before Xe+ impacts (b) After 3 eV Xe+ impacts(or Ni) Cu

JI IF) _"M _ t

(C) After 3 eV Xe+ impacts (d) After 12 eV Xe+ impacts (e) After 12 eV Xe+ impactsNi Ni Ni

Figure 3. Xe+ ion impact effects on different surfaces.

For the atomic configurations shown in Figure 3, the surface roughness can be quantified bythe fraction of atoms remaining above the first island layer, and the degree of mixing can be mea-sured by the mixing probability. The surface roughness and the mixing probability after 1500(Xe+ and Ar+) ion impacts are plotted in Figures 4(a) and 4(b) as a function of ion energy for thenickel on copper surface. Figure 4 indicates that flattening and mixing were less for the lighterAr+ ion impacts, but the general trend was similar.

1.0- 04 0.5 I

Surface roughness ' MixingCL 0.8 "- Irio macs • 0.4-//

0.*Ar ion impacts .- *Ar ion impacts-Xe ion impacts 0 Xe ion impacts

)-0. -0.3E00

Lto4 -0.4 0 0.2

0.00

0.0 o _o ___0.0_........_-____-__.... - ' "

0 3 6 9 12 15 0 3 6 9 12 15Ion energy Ei., (eV) Ion energy E,,, (eV)

Figure 4. Roughness and mixing of nickel on copper surface as a function of ion energy.

MECHANISMS OF ION IMPACT EFFECTS

To understand the mechanism of these assisting ion effects, individual ion impacts weresimulated. Figures 5(a) and 5(b) show three ion impacts with a copper and a nickel surface respec-tively. On the right of the copper island shown in Figure 5(a), a 4 eV Xe÷ bombarded a cluster of

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three copper atoms near the edge of the island. Because the binding energy of copper atoms islow, this ion impact caused one copper atom to jump to a lower surface. The ion impact thereforepromoted step flow growth and the flattening of the copper surface (or the nickel on copper inter-face). At the center of the copper island, a 3 eV Xe÷ impacted a single nickel atom. At the front ofthe island, a 0.1 eV Xe÷ ion collided with a single nickel atom attached to the edge of the island.During both impacts, the weakly bonded underlying copper lattice was penetrated, resulting in theexchange of the nickel atom with a copper atom.

(a) Xe+ impact on copper (b) Xe+ impact on nickel

(1) t = 0.0 ps (1) t = 0.0 ps

(0 1 eV)(3 eV) (3 eV)(12 eV)(8 eV)

Nao1o CuIatom 41 [11o 1

Figure 5. Mechanisms of iimpact effects.

Figure 5(b) shows that even a 8 eV Xea ion could not cause the flattening of the three nickel

atom cluster. Because nickel has a higher cohesive energy than copper, ion impact induced atomicjumps required significantly higher ion energies, and the nickel surface (or the copper on nickelinterface) tended to remain rougher than the copper surface. Figure 5(b) also shows that atomicexchange between a copper atom and an underlying nickel atom did not occur during a 12 eV ionimpact at the center of the nickel island or a 3 eV ion impact at the edge of the island. This isbecause the strongly bonded nickel lattice could not be penetrated at low ion energies. As a result,the nickel on copper interface is more chemically intermixed than the copper on nickel interface.

Mixing was promoted at low ion energy near ledge sites. Reducing the ledge density duringgrowth may therefore also reduce mixing.

DISCUSSION

Low energy ion assistance cannot reduce the nickel on copper surface roughness withoutcausing mixing. An alternative approach is to use a modulated ion assistance in which the firstfew monolayers of a new material are deposited without ion assistance and the remainder of thatmaterial is deposited with ion assistance. To explore this idea, modulated ion assisted depostionwas simulated as a function of ion energy for a low (metal) deposition energy of 0.1 eV and an ionmetal flux ratio of 3. Figure 6 shows that the multilayer interfaces can be flattened without induc-ing mixing by modulated ion assistance scheme.

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I03 = 0' ~Xe / metal = 3T = 300K

(a) Ex.= 0.5eV

20A Ni

'20A Cu

(b) Ex, = 5.OeV

Figure 6. Effects of ion modulation.

CONCLUSIONS

- Molecular dynamics simulations of ion assisteddeposition of multilayers indicated that:

Yl'111 1. 3 eV ion impacts can flatten a Cu surface.I Higher ion energies (> 10 eV) are required to

V-Xe on flatten a nickel surface.2. Copper mixing in the nickel layer near the

nickel on copper interface is much more signifi-cant than nickel mixing in the copper layer nearthe copper on nickel interface. Unlike other sur-faces, ion assistance cannot fully flatten thenickel on copper surface without causing mix-ing.

3. A modulated ion assistance can result in a flatnickel on copper surface without inducing mix-ing.

ACKNOWLEDGEMENTS

We are grateful to the Defence Advanced ResearchProjects Agency (A. Tsao and S. Wolf, ProgramManagers) and the National Aeronautics and SpaceAdministration for support of this work throughNASA grants NAGW 1692 and NAG- I-1964.

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