Neutral and negative ion cluster emission during sputtering

Nuclear Instruments and Methods in Physics Research B62(1991) 43-46 North-Holland

telex Instruments & Methods inphysicsResearch Sechon B

Neutral and negative ion cluster emission during sputtering M.K . Abdullaeva, B .G. Atabaev and R. Dzabbarganov

Arifov Institute of Electronics, Uzbek Akademy of Sciences, Tashkent, 700143, USSR

Received 29 April 1991 There is a lack of a generally accepted model for polyatomic cluster formation under ion bombardment . A series of correlated investigations of cluster formation during graphite, silicon, and aluminum sputtering by alkali metal if" as is carried out to obtain new information concerning cluster formation mechanisms. The relative distribution of polyatomic negati,,e and neutral clusters was investigated and the cluster yield determined as a function of the primary ion energy . The ionization coefficients ß for sputtered carbon, silicon, and aluminum clusters were determined through post ionization of the neutral components . Theeffect of work function changes on the negative ion cluster yields was also studied. The results show that clusters mainly leave the sputtered surface as such . 1. Introduction It has been well established that sputtering occurs not only in the form of atoms (as was thought for a long time), but that the flux of sputtered particles contains a significant component of molecules and multiatomic clusters . Most often two models are used to explain the molecular particle emission during sputtering, namely the model of direct emission [2] and the recombination model of cluster formation [3]. The model of direct emission proposes that during ion bombardment the typical cluster configurations are formed on the surface and leave the surface as such. According to the recombination model, sputtering occurs only in the form of monoatomic particles and that molecules are formed as a result of recombination of individual atoms sputtered independently in the same collision cascade. There are experimental data supporting both models . Moreover, it is shown [4] that both mechanisms can exist simultaneously on the same surface and that mechanisms can be different for various charge states of sputtered molecular particles. Keeping these key ideas in mind, within the last few years in order to investigate the basis of the recombination model we have carried out the following four interrelated studies of sputtering of graphite, silicon and aluminum under alkaline metal ion bombardment: 1) The mass-spectra of negative-ion and neutral com ponents of graphite, silicon and aluminum sputtering have been studied. * First presented at 7th Ins. Conf. on Ion Beam Modification of Materials, Knoxville, TN, USA, September 9-14, 1990 .

2) The dependencies of Si and AI  cluster ion yields from silicon and aluminum targets on the energy and mass of the primary ion have been investigated . 3) Ionization coefficient ßn values have been determined for graphite, silicon and aluminum sputtering in the form of negative cluster ions by the method [1 ]. 4) The influence of the presence of an alkaline metal (potassium) film on the sputtered surface on cluster ionyields and the value of the coefficient of ionization /3  have been studied. 2. Experimental technique and results 2. I. Cluster sputtering of graphite, silicon and aluminum

Methods used for the investigation of solid negative-ion sputtering allows one to analyze the fragment composition of the particles formed during cathode sputtering both in the charged and neutral state. Mass-spectrometer analysis of negative-ion sputtering products (SIMS) gives information on the cluster ion relative content according to the number of atoms in them . Data on the relative content of clusters sputtered in the neutral state can be obtained using our neutral particle detector [1], based on the possibility of sputtered neutral component detection by the postionization of these species into negative ions in the process of their scattering from an auxiliary surface with an extremely small work function (SNMS) . The analysis by SIMS and SNMS methods of the products of graphite, silicon and aluminum sputtering

0168-583X/91/$03.50 0 1991 - Elsevier Science Publishers B.V. All rights reserved

44

MX Abdullaem et al. / Ion cluster emission

of these target surfaces under bombardment by cesium ions with an energy E =2.5-3.0 keV shows that multiatomic clusters, both in the form of negatively charged and neutral charge state clusters (fig. 1) are present in the mass spectra. The relative distribution of negative-ion clusters correlates with their electron affinity [5] . A specific feature of sputtered multiatomic cluster interaction with an auxiliary surface under scattering is that the conversion into negative ions reduces in efficiency for multiatomic clusters with growth in the number of atoms n in the cluster. Therefore though neutral multiatomic clusters form a more substantial part of the sputtered flux, the mass spectra of the neutral clusters are less abundant in comparison with the mass spectra of negative cluster ions, and there is asharperdecrease in the neutral cluster emission with growth in the number of atoms n in the cluster in comparison with cluster ion emission [6]. 2.2. The dependence of cluster ion yield on energy and

mass of the primary ions

To investigate the influence of the mean sputtering coefficient on cluster formation we have studied the

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Fig. 2. The yield of cluster ions Sin- and Aln- normalized relative to the yield of two-atom cluster ions Si . and ÀJZ versus primary ion energy. O, a, ® are the data obtained during the bombardment of silicon target by cesium ; rubidium and sodium ions respectively; o are the data reported by VVittmaack [111 ; e, n, O is the yield of AIR under cesium ion bombardment . dependence of Si  and A]  cluster ion yield from silicon and aluminum targets on the energy (0 .5-3.0 keV) and mass of alkaline metals primary ions [7,8). The resulting experimental data (fig. 2) show that the Si n and Al  cluster ion normalized yields are independent of the energy and mass of the primaryions. These results contradict the conclusions of cluster formation by the recombination model [3,71 and support the model of direct emission [8). 2.3. Ionization coefficient determination for graphite, silicon and aluminum sputtering

NunrLer

of akm it elast`era

i n

Fig. 1. Thedistribution of the relative number of negative-ion and neutral clusters versus the number of atoms in the clusters:O-Aln- ;ti-Ale; m -Cn- ; 9 -C~O; O-Sin- ; 0 -Si% .

Experimental determination of the ionization coefficient, ßn, which is known to be one of the most important quantitative parameters characterizing the efficiency of negative ion formation at a solid surface by ion beam bombardment, is still a difficult problem. Such measurements depend on the fact that to obtain reproducible values of ß; the ionic and neutral components of the sputtering should be determined in one experiment on the same target. The most difficult problem in such experiments is the measurement of the number of sputtered neutral atoms. Ionization coefficient values for graphite . silicon and aluminum

M.IL Abdullaem et al. / Ion

45

cluster emission

Table 1 The values of ß;; the ionization coefficient, for graphite, silicon and aluminum sputtering in the form of negative ions measured by the method of ref. [1] Ionization Ele~.tron Negative Conditions on the coefficie ,~t affinity energy ion surface EA [eVl 0- 1%1 1.2629±0.003 C; 11 .0 The best surface 3.54 ±0.05 CZ 33 .0 activation 4.0 1.8 ; 2.5 ; 1.3 by potassium C3 10.0 3.5 C,4 No potassium on surface

The best surface activation by potassium

Potassium thick film on cold surface Thebest surface activation by potassium

si si -

8.0

Sit 0-

28 .0±2.5 36.0±2.5 20.0±2 .5

0-

37 .5±2.5

Al -

sputtering in the form of negative ions (Cn , Sin, Aln) were measured by the method given in reference [1], which was proposed for the determination of ionization coefficients during monoatomic sputtering and which is based on determine the neutral component of the sputtering by post ionization into negative ions in the process of scattering from an auxiliary surface with an extremely small work function . We have determined /3 values corresponding to various conditions existing on a sputtered surface when potassium is continuously coated in the process of its cooling after high temperature heating [5]. The results of the Pn measurements (see table 1) show that (a) method [1] allows one to determine the ionization coefficients of the particles sputtered in the form of multiatomic clusters, (b) there is a strong dependence of ionization coefficient (ß ) values on the sputtered surface work function and (c) there is a correlation between PR and EA, the electron affinity energy, in eV. 3. Discussion 3.1. The influence of the presence of a potassium film on the sputtered surface on cluster ion yield

The determination of the correlation between the secondary negative ion yield and the change in the work function of the sputtered surface is a fundamental result of this investigation of the character and

7.0

1 .385 ±0 .005 1.9 t0.3 1.465 ±0.005

0.4

regularities of negative ion secondary emission. An exponential dependence of the negative ion yield on the change in the work function 0O I=B exp(-AO/e)- type was shown in all our experiments. Strong correlation between ionization probability and AO suggests a non-local interaction and has motivated the development of an electron tunneling model for the ionization of the sputtered atoms. In the model of electron tunneling developed by Norskov and Lundqvist, especially for the case of secondary negative ion emission [9], the probability of negative ion formation for a large class of systems, to a first approximation, is described by the expression ON-L « C exp[-(0- EA)] /e,

where -P is the work function of the sputtered surface; EA is the energy of the sputtered atom's electron affinity; e is a parameterwhich depends on the normal component of ion velocity and character of its electron interaction with the surface. The values of ionization coefficient p; under sputtering in the form of monoatomic negative ions, determined both by methods of ref. [1] and by the approximation of ref. [10] are in good agreement with the Norskov-Lundgvist electron tunneling model (fig. 3). The study of the dynamics of the change in cluster ion C;, Si ;, A] ; yield versus the thickness and the

46

M.IC Abdullaeva et al. / Ion cluster emission

Fig. 3. The relationship between ionization coefficient ß, under solid target sputtering by positive cesium ions, and the value of (ßm,ß - EA), where Om, is the sputtered surface work function ait it's optimum coating with alkaline metal film. 0 - (1); o - [to]. rate of the deposition of a potassium film on the surface of sputtered targets showed that the increase in emission, when the submonolayer films of alkali metals are on the sputtered surface, characteristic of secondary emission of monoatomic ions, is not always true for secondary, ion emission of clusters and occurs only at so"ne optimal rate of potassium film deposition and density of the bombarding ion beam. It should be noted that the film of alkali metal (potassium) of optimal thickness greatly increases cluster ion yield for n>2 . The dependence of cluster ion Sin-, and Aln yield on the density of potassium film on sputtered surfaces of silicon and aluminum (i.e . on the change in the surface work function) are presented in fig . 4 and allows one to conclude that the relationship between negative-ion cluster yield and the change in surface work function AO is the exponential character of eq. (1). Also the relative change of these yields with reduction in the work function correlates with cluster electron affinity. However, there are some deviations from an exponential dependence when the work function is close to the minimum. On the basis of the above obtained data relating to a strict correlation between the negative ionic cluster yield I. and the change of sputtered surface work function 0O, we can assume that the surface ioniza-

Fig . 4. The yield of cluster ions Si- and Ah versus potassium film density on sputtered Si and AI surfaces. tion of clusters can be explained by the electron tunneling model as in the case of sputtered atom ionization . References [11 A.H. Ayukhanov and E. Turmashev, J . Techn. Phys. 47 (1977) 1234 . [21 A . Benninghoven, Surf. Sci. 35 (1973) 427; SIMS II, Springer Series in Chemical Physics, vol. 9 (Springer, New-York, 1979) p. 116 . [31 G.P . Konnen, A. Tip and A .E. de Vries, Radiat . Eff. 21 (1974) 269, Radiat . Eff. 26 (1975) 23. [41 M .L. Yu, Phys . Rev. B24 (1981) 1147. [51 A.H . Ayukhanov, M.K. Abdullaeva and E. Turmashev, J. Tech . Phys. 56 (1986) 2210. [61 H. Gnaser ,and W.O. Hofer, Appl . Phys. A48 (1989) 261. [71 W. Gerhard, J . Physik. B22 (1975) 31 . [8) K. Wittmaack, Phys . Lett . 69A (1979) 322 . [9) J.K. Norskov and B .I. Lundqvist, Phys. Rev. B19 (1979) 5661 . [101 M.K. Abdullaeva, A .H . Ayukhanov and U .B . Shamsijev, Radiat . Eff. 19 (1973) 225 . [111 K . Wittmaack, Surf. Sci. 126 (1983) 573.