The effect of the angle of incidence on secondary ion yields of oxygen-bombarded solids

Nuclear Instruments and Methods in Physics Research 218 (1983) 307-311 North-Holland, Amsterdam

THE EFFECT OF THE ANGLE OF INCIDENCE OXYGEN-BOMBARDED SOLIDS

ON SECONDARY

307

ION YIELDS

OF

K. W I T T M A A C K Gesellschaft fi~r Strahlen- und Umweltforschung mbH, Physikalisch-Technische A bteilung, D- 8042 Neuherberg, Fed. Rep. Germar~v

Secondary ion emission from Si, Ge and GaAs bombarded with 3-12 keV (mostly 10 keV) 02 ~ ions has been investigated as a function of the angle of beam incidence, 0 ° ~<0 ~<60 °. The (peak) intensities of positive and negative secondary ions decrease with increasing 0. In contrast, the sputtering yields increase with increasing 0. In either case the effect is largest for Si and smallest for Ge. Upon changing 0 from 0 ° (normal beam incidence) to 60 ° the fractional ion yields of Si + and Ge + decrease by a factor of 2 x 103 and 30, respectively. Almost the same yield reduction is found with negative ions. The O + and O intensities exhibit roughly the same O-dependence as the fractional ion yields of the target atoms. For 0 < 45 ° the absolute intensities of O + and O - ions are largest for a Si target and smallest for a Ge target. The results suggest that the O + and O intensities provide semi-quantitative information about the stationary oxygen surface concentration which, for small 0, is high in Si but low in Ge.

1. Introduction

2. Experimental

In recent analyses of literature data we have shown that the m e c h a n i s m of secondary ion yield e n h a n c e m e n t due to sample b o m b a r d m e n t with oxygen ions is not yet well understood [1,2]. This is partly due to the fact that, contrary to previous assumptions [3], the stationary c o n c e n t r a t i o n of oxygen, established at the surface of the i o n - b o m b a r d e d sample, c a n n o t be calculated from simple models of ion retention in the presence of sputtering [4-6]. One of the essential shortcomings of these models is that the distribution of the depth of origin of sputtered particles [7] is not taken into account. It can be shown, however, that it is impossible to calculate the steady-state surface concentration of implanted oxygen atoms from simple balance equations (Ni~ = N ~ t ) unless the escape functions of probe atoms a n d target atoms are k n o w n [2]. Previous work concerning secondary ion yields of o x y g e n - b o m b a r d e d solids has been carried out at a fixed angle of b o m b a r d m e n t [1-3,8-10]. Only in a few cases the effect of the angle of incidence was considered [2,9,10], but the respective data were o b t a i n e d using different instruments. Analysis of intensity a n d sputtering yield ratios [2] revealed a systematic angular dependence. The purpose of this study was to employ one instrum e n t for measuring secondary ion yields as a function of the angle of incidence of O~- primary ions. Matrix effects were investigated by use of different target materials. In order to determine fractional ion yields, i.e. the m e a n n u m b e r of secondary ions detected per sputtered atom, the sputtering yields of the different samples were also measured.

The measurements were performed using the D I D A ion microprobe described elsewhere [11]. The samples were m o u n t e d on wedge-shaped supports fixed to the rotatable target manipulator. The tilt axis of the b o m b a r d e d samples was almost normal to the direction of b e a m propagation ( 0 x = 2 °) and parallel to the plane of secondary ion deflection. This orientation was assumed to minimize geometrical effects associated with the fixed position of the ion gun and the secondary ion mass spectrometer. The experiment were carried out at a base pressure of 1 x 10 -7 Pa or less. The samples were b o m b a r d e d with velocity-filtered 02 ~ ions focused to a spot < 30 /zm. The scan width of the b e a m was - 200 ffm × 200 f f m / c o s ~ , where 0 ~-Oy is the angle of incidence with respect to the surface normal. The b e a m current was 20 n A or less (measured with a F a r a d a y cage). Polished Si, Ge a n d G a A s samples were used as targets. The Si sample was uniformly doped with B ( - 0.04at.%). The impurity content of the G e a n d G a A s samples was u n k n o w n (but low). Prior to the measurem e n t s the samples were sputter cleaned and saturated with oxygen. For each ion species the pass energy of the secondary ion energy filter ( b a n d width - 2 eV) was optimized to transmit ions with the most p r o b a b l e energy ( " p e a k intensities"). S t a n d a r d electronic gating techniques were employed to reject ions originating from the rim of the sputtered craters. For measurements of the sputtering yield, the target current was increased to 200 hA. Using a scan width of 270/~m a n d sputtering times of 30 rain, the resulting crater depths ranged from 0.3 /~m to 2.8 ffm, sufficiently deep for an accurate

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K. Wittrnaack / The effect of the angle of incidence on secondary ion yields

d e t e r m i n a t i o n of the crater volume by means of a surface profilometer (Talysurf 10).

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3. Results and discussion

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Stationary intensities of positive and negative seco n d a r y ions, measured as a function of the angle of incidence, are presented in figs. 1 - 3 for Si, G e and G a A s targets, respectively, b o m b a r d e d with 10 keV O ] ions. The data are normalized to an integrated primary ion charge of 1 nC a n d an isotopic a b u n d a n c e of 100%. In order to get a fairly complete picture of the changes induced by a variation of the angle of incidence, the intensities of O + a n d O ions as well as of oxygen-carrying molecular ions ( M O ÷, M 2 0 +, M O 2 ) were measured in addition to the matrix ions M + and M - . ( F o r the sake of clarity figs. 1 - 3 depict only data for a limited n u m b e r of secondary ion species.) The results o b t a i n e d with the different target materials are seen to be qualitatively similar. At n e a r - n o r m a l incidence, 0 ~< 15 °, the intensities are almost constant. (The small n o n - n o r m a l c o m p o n e n t 0~ = 2 ° is neglected in this work). As # exceeds 20 ° to 30 ° the intensities decrease more or less rapidly with increasing 0. In this

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region the effects are quantitatively different for the different matrices. For example, the intensity of Si + decreases by more than two orders of magnitude upon increasing 0 from 15 ° to 60 ° (fig. 4). The same 0-variation reduces the intensity of G e +, G a + and As + be only one order of m a g n i t u d e (figs. 2 a n d 3). A similar difference is observed with negatively charged matrix ions

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M - . (The intensity of G a - is very low, - 2 c o u n t s / n C at normal beam incidence.) A number of experiments have been carried out in order to verify the absence of instrumental effects. A m o n g others the position of the sample surface was varied in the z-direction, i.e. the direction of beam propagation. The intensities observed after properly setting the strengths of the electric field of the secondary ion energy filter were found to be almost independent of z, for Az ~< 2 mm. It should also be noted that, as a consequence of the somewhat crude method of sample mounting, electric means for optimizing the secondary ion transport in the y-direction turned out to be indispensible (the y-direction is normal to the deflection plane of the secondary ion energy filter). In a separate series of experiments we have studied the effect of the angle of incidence on secondary ion emission from Si bombarded with inert gas ions [12]. Compared to the present results the v%dependence of the secondary ion intensities was found to be rather small. Under bombardment with 5 keV Ar + ions, for example, the intensity of Si + (Si-) ions increased (decreased) by a factor of 1.7 (2.8) as 0 was increased from 0 ° to 60 °. Accordingly, we attribute the pronounced intensity variations in figs. 1-3 mostly to an angular dependent variation of the degree of ionization, introduced by differences in the stationary oxygen content of

309

the samples. Recent measurements [13]-have in fact shown that, for 15 keV O~- bombardment of Si, the stationary surface concentration of oxygen, as seen by Auger electron spectroscopy, decreases markedly as # is raised from 0 ° to 45 °. At normal incidence, the nearsurface regions of Si are almost completely converted to SiO 2 [13]. Taking into account that the degree of ionization of Si + and Si- inreases with a high power of the mean oxygen concentration of the sample [14], the results of figs. 1-3 are thus easy to understand in semi-quantitative terms. In our previous work [14] we also noted a direct proportionality between the intensities of Si + and O emitted from oxygen-doped Si under Ar + bombardment. The results of figs. 1-3 suggest a similar correlation. However, the present study relates to O~bombardment. A direct comparison with the results of ref. 14 may not be advisable, therefore. Inspection of figs. 1-3 reveals the well-known feature [1,3] that the intensities of the matrix ions M + and M differ by orders of magnitude. This effect is mostly due to differences in the ionization potential and the electron affinity of the elements [1,3,15]. In contrast to the matrix ions, the O - probe ions, reemitted from Si, Ge and GaAs, exhibit fairly similar intensities, as illustrated in fig. 4. Equally important is the finding that the O / O + intensity ratios are rather insensitive to the choice of the target material. The process of oxygen reemission during oxygen bombardment is not well understood at present. It would appear that at least three processes have to be taken into account, (1) backscattering, (2) collisional sputtering and (3) thermal or beam-enhanced outdiffusion. O - and O + ions emitted with energies below several 100 eV can be assumed to be ejected mostly via collisional sputtering. The fraction of previously injected atoms actually available for sputter ejection, however, will depend upon the extent of reemission via backscattering and outdiffusion. The results of fig. 4 suggest that the latter two processes are matrix-specific, Si showing the largest and Ge the smallest capacity for near-surface storage of oxygen continuously supplied by ion implantation. As the angle of incidence exceeds - 4 5 ° the stationary surface concentration of oxygen becomes largely independent of the matrix. This is indicative of the dominance of surface depletion via a direct recoil mechanism. The increasing nuclear energy deposition near the surface may also play a role. Measurements like those presented in figs. 1-3 have been carried out at O~- energies between 3 and 12 keV. In agreement with other studies involving oxygen primary ions [2], the results were found to be almost independent of the probe energy. In order to get further insight into the effect of the angle of incidence on secondary ion yields we have measured the sputtering yields. The results are depicted

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Fig. 7. The same as fig. 6 but for negative secondary ions. in fig. 5. Significant differences in the 0 - d e p e n d e n c e of the sputtering yields of Si, G e a n d G a A s are immediately evident. The results for Si are in good agreem e n t with data reported by M o r g a n et al. [10]. Moreover, the sputtering yield ratios are found to agree well with literature data compiled previously [2]. On the basis of the ideas outlined above, the results

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of fig. 5 may be interpreted as follows. At large angles of incidence at which the stationary oxygen surface c o n c e n t r a t i o n ('~ is low, the sputtering yield Y~ corresponds to the n u m b e r to be expected for b o m b a r d m e n t with (hypothetical) inert gas ions of the same mass as oxygen. As 0 is reduced, Y~ decreases more rapidly the higher c ~ becomes. In accordance with fig. 4 the effect is largest for Si, Y ~ ( 6 0 ° ) / Y ~ ( O °) = 11 + 1, and smallest for Ge, Y ~ ( 6 0 ° ) / Y ~ ( O °)= 2.8 + 0.2. For clean samples b o m b a r d e d with inert gas ions one would expect Y ~ ( 6 0 ° ) / Y ~ ( O ° ) = 1 / c o s ' O = 2 m where 1 ~< m ~< 2 [16]. The results of fig. 5 thus imply that c ~ ( G e ) was quite low even for 0 = 0 ° whereas in the case of Si(O < 30 °) a significant fraction of the collisionally sputtered material must have been oxygen atoms. G a A s constitutes an intermediate case. Knowing the sputtering yields we can derive fractional ion yields of the matrix elements. The results (cf. figs. 6 and 7) provide yet a n o t h e r example of the differences between Si and Ge. U p o n increasing 0 from 0 ° to 60 ° the fractional ion yield of Si + drops by a factor of 2 × 103 whereas the respective n u m b e r for G e + is 30. Note again that the negative ions exhibit almost the same O-dependence as the positive ions (cf. figs. 6 and 7).

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Fig. 6. Fractional ion yields of positive secondary ions emitted from oxygen-bombarded Si, Ge and GaAs as a function of the angle of beam incidence.

In this study we have d e m o n s t r a t e d that the effect of the angle of incidence on secondary ion yields of o x y g e n - b o m b a r d e d samples is strongly matrix-specific. The results can be interpreted in terms of differences in

K. Wittmaack / The effect of the angle of incidence on secondary ion yields the stationary surface concentration of oxygen which manifests itself in a corresponding yield of O - and O ÷ ions as well as in a reduction of the partial sputtering yield of the matrix. As far as the mechanism of secondary ion formation is concerned, the present work supports previous findings [14]. It would appear, however, that more detailed conclusions can only be derived by taking the angular dependent changes of the secondary ion energy distributions into account.

References [1] K. Wittmaack, J. Appl. Phys. 52 (1981) 527. [2] K. Wittmaack, Appl. Surf. Sci. 9 (1981) 315. [3] V.R. Deline, W. Katz, C.D. Evans, Jr. and P. Williams, Appl. Phys. Lett. 33 (1978) 832. [4] G. Carter, J.S. Colligon and J.H. Leck, Proc. Phys. Soc. 79 (1962) 299.

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[5] F. Schulz and K. Wittmaack, Rad. Eft. 29 (1976) 31. [6] N. Warmoltz, H.W. Werner and A.E. Morgan, Surf. Interf. Anal. 2 (1980) 46. [7] G. Falcone and P. Sigmund, Appl. Phys. 25 (1981) 307. [8] D.P. Leta and G.H. Morrison, Anal. Chem. 52 (1980) 277. [9] A.E. Morgan, H.A.M. de Grefte and H.J. Tolle, J. Vac. Sci. Technol. 18 (1981) 164. [10] A.E. Morgan, H.A.M. de Grefte, N. Warmoltz, H.W. Werner and H.J. Tolle, Appl. Surf. Sci. 7 (1981) 372. [11] K. Wittmaack, Vacuum 32 (1982) 65. [12] K. Wittmaack, presented at the 10th Int. Conf. on Atomic Collisions in Solids, to be published in Nucl. Instr. and Meth. [13] W. Reuter and K. Wittmaack, Appl. Surf. Sci. 5 (1980) 221. [14] K. Wittmaack, Surf. Sci. 118 (1981) 168. [15] P. Williams, W. Katz and C.A. Evans, Jr., Nucl. Instr. and Meth. 168 (1980) 373. [16] H.H. Andersen and H.L. Bay, Sputtering by ion bombardment I, ed. R. Behrisch (Springer, Berlin, 1981) p. 145.