Sputtering of Al surface with very-low-energy ions

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Surface Science 287/288 (1993) 21-24 North-Holland

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Sputtering of Al surface with very-low-energy Michio Okada

and Yoshitada

ions

Murata

The Institute for Solid State Physics, The University of Tokyo, 7-22-1, Roppongi, Minato-ku, Tokyo 106, Japan

Received 1 September 1992; accepted for publication 26 November 1992

Sputtering of AIf for N:, N+, and Ne+ incident ions was observed at the threshold energy of _ 20 eV. The polar-angle distribution of the sputtered Al + ions gives almost the same lobe position with different angles of incidence. In atomic-ion incidence at incident energies above 100 eV, a sharp structure was observed in the angular distribution of the sputtered AIf ions. The mechanisms of ion sputtering induced by very-low-energy ions (I 200 eV) are discussed.

1. Introduction

Sputtering is one of the most important dynamic processes occurring in fundamental studies of surface science and has been used and discussed in various applied fields, i.e. surface analysis, ion-beam processing, plasma deposition, plasma-wall interaction in nuclear fusion, cosmic technology, and so on. Nevertheless, among the fundamental studies of the sputtering phenomenon definitive experiments and theories are still lacking. Some theoretical approaches have been tried to elucidate the mechanism of secondary ion formation for an incident ion beam. The broken bond model by Yu [l] is successfully applied to sputtering of oxide materials. Another model is the tunneling model extended by Sroubek [2]. Other models are reviewed by Williams [3]. Recently, Diebold and Varga studied the influence of the primary-ion charge state on the secondary-ion production in the CO/Ni (111) system with very-low-energy Ne+, Ne2+, Kr+, and Kr2+ ions [4,5]. They proposed that the secondary ion is formed via ionization by the primary ion in a binary collision. Several direct knock-off processes in sputtering were proposed by Winters and Sigmund [6] and have been extended by Yamamura et al. [7]. 0039-6028/93/$06.00

We are much concerned with sputtering phenomena with very-low-energy reactive ions, because sputtering is one of the fundamental processes in ion-surface interactions and very-lowenergy reactive ions can gently and selectively modify the surface [8,9]. Very-low-energy ion beams below 100 eV have scarcely been applied to the study of sputtering phenomena due to the difficulty in sputtered-particle detection. The sputtered ions are easily detected with high sensitivity, but very-low-energy ions are strongly neutralized at the surface. Fortunately, we found sputtered ions for an Al target. The incident-energy region around the sputtering threshold is of great importance in order to study the beamsurface interaction, since only a few collision contribute to the sputtering in this energy region.

2. Experiment The experiments were performed with almost the same apparatus described previously [8,10]. The incident ion current was 0.1-60 nA even at very low energies (Ei I 100 eV>. The scattered ions were detected with a quadrupole mass filter. The trajectories of the ions after passing through the mass filter were bent by 90” with a deflecting plate mounted in front of an electron multiplier.

0 1993 - Elsevier Science Publishers B.V. All rights reserved

M. Okada, Y. Murata / Sputtering of Al surface with very-low-energy ions

22

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A voltage to the deflecting plate was applied to obtain the maximum intensity of Al+. The Al (111) surface was mechanically polished and was cleaned by a repetition of Ar+-ion bombardment and annealing at 470-670 K. Low-energy electron diffraction showed a sharp 1 X 1 pattern and Auger electron spectra showed no detectable contamination. The surface cleanliness was also verified by secondary-ion emission measurements.

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300 200 INCIDENT ENERGY (eV) 100

of incident Fig. 1. Sputtered Al+ ion yields as a function energy with Nz (circles), N+ (triangles), and Ne+ (squares) incidence. The incidence angle measured from the surface normal was 45” for closed marks and 60” for open marks. Specularly scattered ions along the [ll?!] azimuth were detected. The yield is given in arbitrary units.

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3. Results and discussion

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Fig. 1 shows the sputtered Al+ ion yield in specular direction along the [ll?!] azimuth as a function of the incident energy for N+, N:, and Ne+ ion incidence. The sputtered Al+ ion yield steeply increases with increasing impact energy. The threshold energy for the Al+ sputtering from the Al (111) surface is - 20 eV for both Nef and Nf incidence and is independent of the inci-

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Fig. 2. Angular distributions of sputtered AI+ ions in N+ incidence with incident energy Ei = 60 and 100 eV in the upper panel. Angular distributions of sputtered Al+ ions in Nef incidence with incident energy Ei = 50 and 100 eV in the lower panel. The incidence angle measured from the surface normal is 60” for the left of the figure and 45” for the right. The scale is arbitrary in each diagram.

dence angle. These values are well in agreement with calculated values [ll]. Figs. 2 and 3 show the polar-angle distributions of the sputtered Al+ ions for bombardment by N’ and Ne+, and by N:, respectively, at slightly higher than threshold energies. The scattering plane is selected as the plane including both the surface normal and the [llz] crystal azimuth. The symmetry center of the lobe for the whole of the incident ion species is located at the same position for both incidence angles 60” and 45”. Another prominent feature is that the lobe for atomic-ion incidence consists of two components at incident energies higher than 100 eV. One is a broad component, and the other is a sharp component which is located at a smaller angle from the surface normal to the broad component. On the other hand, in the case of N:

M. Okada, Y Murata / Sputtering of Al surface with very-low-energy ions

incidence, no sharp component was observed even at higher incident energies WlO and 200 eV). The secondary Al atom formation due to sputtering can be discussed based on a theoretical study by Yamamura et al. [12]. In their simulation for 50 eV Ar+ incidence on a Cu surface, the angular distribution of the sputtered neutral Cu atoms is under-cosine, and the exit-preferential angle corresponding to the minimum-energy-loss trajectory is independent of the incidence angle. Their simulation results agree well with the present results in the case of 60 eV N+ and 50 eV Nef incidence on an Al(111) surface. The agreement is considered to be caused by the fact that the mass ratio of projectile to target atom in their simulation is nearly the same as in the present experiment. The observed total sputtered-ion yield for N+ incidence is larger than that for Ne+ incidence, which can also be seen in the sputtered-ion yield in the specular direction shown in fig. 1. On the other hand, the calculated total sputtering yield

SO” INCIDENCE

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MECHANISM

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Fig. 4. Schematic illustration of sputtering processes via mechanisms I and II. In mechanism I, the primary recoil atom leading to the emission process is produced at the first collision of the projectile with a surface atom, while in mechanism II, the primary recoil atom leading to sputtering is created after a few collisions.

1 45” INCIDENCE

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Fig. 3. Angular distributions of sputtered Al+ ions in N$ incidence with incident energy E, = 60, 100, and 200 eV. See fig. 2.

of neutral atoms for the 50 eV N+ incidence is nearly the same as that for the 50 eV Ne+ incidence in the normal-incidence geometry [ 111. In the present experiment, only sputtered Al+ ions were detected and detection of the neutral species was not carried out. Since the mass ratio of the projectile to the target atom is nearly the same in both cases, the sputtering yield of the neutral atoms for the N+ incidence is considered to be nearly equal to that for the Ne+ incidence. So, the above-mentioned experimental result shows that the ionization probability of Al for N+ incidence is larger than that for Ne+ incidence. Hence, ionization via formation of a quasi-molecule between the incident ion and a target atom contributes to the Al+ ion formation. The sputtering process near the threshold region can be discussed based on a-few-collisions model represented by mechanisms I and II, shown in fig. 4 [6,12]. In mechanism II, ionization results from the velocity-dependent interaction of an electronic level of a “sputtered atom” with the

24

M. Okada, Y. Murata / Sputtering of AI surface with very-low-energy ions

surface electronic structure of the metal. In mechanism I, in addition to the above-mentioned ionization process, ionization occurs via the formation of a quasi-molecular state between an incident “ion” and a target atom. So, Al+ ions are considered to be more easily formed in the case of Nf ions because of the many holes of the N+ ion. Therefore, the present experimental results show that Al+ sputtering via mechanism 1 is more dominant than mechanism II. Moreover, mechanism I-A is considered to be most dominant, since the angular distribution of the sputtered Al+ ions is independent of the angle of incidence. A sharp structure of the angular distribution of the sputtered Al’ ions was observed in the atomic-ion incidence as seen in fig. 2. The sharp structure is proposed to be due to the sputtering mechanism proposed by Lehmann and Sigmund [13], which leads to Wehner spots [14]. In this mechanism, the sputtering of an atom is initiated by a collision between it and one of its close neighbors beneath the surface. So, the spot center should be located at 8 = 35” from the surface normal, while experimentally, the sharp structure was observed at 9 = 45”-50”. We detected Al+ ions with energies below a few tens of eV in the present experiment. So, the sharp structure is shifted away from the surface normal by a refraction-like interaction due to the surface barrier and the image potential. On the other hand, in the N: ion incidence, the sharp structure was not observed. The result is caused by the fact that the NC ions cannot easily penetrate, leading to a small perturbation to the second layer in the molecular ion incidence which originates in the size of the projectile. Another possibility of the origin of the sharp structure is the rainbow-like structure. Two possible origins of the rainbow structure can be considered: one is due to rainbow scattering taking place along the exit trajectory, and the other is

caused by rainbow scattering of primary particles. Both processes are unlikely to be the cause of the sharp structure, as will be described elsewhere n51.

Acknowledgments

We wish to thank Prof. A.W. Kleyn for fruitful discussions. This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, and Culture. M.O. acknowledges JSPS Fellowships for Japanese Junior Scientists.

References [l] [2] [3] [4] [5]

[6] [7] [8] [9] [IO] [ll]

[12]

1131 [14] [15]

M.L. Yu, Nucl. Instr. Meth. B 18 (1973) 542. Z. Sroubek, Spectrochim. Acta 44B (1989) 317. P. Williams, Surf. Sci. 90 (1979) 588. U. Diebold and P. Varga, Surf. Sci. 241 (1991) Lh. U. Diebold and P. Varga, in: Desorption Induced by Electronic Transitions-DIET IV (Springer, Berlin, 19901 p. 193. H.F. Winters and P. Sigmund, J. Appl. Phys. 45 (1974) 4760. Y. Yamamura, Y. ltikawa and N. Itoh, IPPJ-AM-26, Institute of Plasma Physics, Nagoya University (1983). H. Akazawa and Y. Murata, J. Chem. Phys. 88 (1988) 3317. S.R. Kasi, H. Kang, C.S. Sass and J.W. Rabalais, Surf. Sci. Rep. 10 (1989) 1. H. Akazawa and Y. Murata, J. Chem. Phys. 92 (1990) 5551. N. Matsunami, Y. Yamamura, Y. Itikawa, N. Itoh, Y. Kazumata, S. Miyagawa, K. Morita, R. Shimizu and H. Tawara, At. Data Nucl. Data Tables 31 (1984) 1. Y. Yamamura, Y. Mizuno and H. Kimura, Nucl. Instrum. Methods B 13 (1986) 393; Y. Yamamura and Y. Mizuno, IPPJ-AM-40, Institute of Plasma Physics, Nagoya University (19831. C. Lehmann and P. Sigmund, Phys. Status Solidi 16 (1966) 507. G.K. Wehner, J. Appl. Phys. 26 (19551 1056. M. Okada and Y. Murata, unpublished.