SURFACE A N D INTERFACE ANALYSIS, VOL. 10, 409-415 (1987)

Investigation of the Sampling Volume in Secondlary Ion Microanalysis 111. Peculiarities of Multi-element Metal Specimens Under Oxygen Bombardment

H. Mai Zentralinstitut fiir Festkorperphysik and Werkstofforschung der Akademie der Wissenschaften der DDR, Dresden-8027, Postfach, GDR

The modification caused by oxygen primary ion bombardment in homogeneous multi-element metal specimens has been studied. Irivestigations of the appropriate sample region of Fe40Ni40P,4B6 and Fe81Si,.5C2B,3.5 metallic glasses by transmission electron microscopy, electron diffraction and scanning Auger microanalysis revealed an inhomogeneous steady-state sampling volume. Chemical decomposition of the initial matrix, precipitation of oxide components anti formation of crystalline phases have been proven.

INTRODUCTION

Some problems of secondary ion mass spectrometry (SIMS) investigations of pure element matrices have already been explained in preceding papers.'.' The pro- cesses involved in primary ion (PI)/solid interaction become much more complicated in the case where two or more matrix elements have to be considered. Thus their elucidation is less advanced than in the pure ele- ment case. Since the commercial availability of micro- homogeneous multi. element standards is rather

appropriate model specimens of a satisfactory single-phase constitution in extended sample regions can be obtained only by particular preparation. Besides appropriate thin film alloys5 various compounds of metallic glasses have been successfully applied in SIMS.6-'" The followiing explanations will refer to gen- eral considerations of PI/solid modification' and point out some peculiarities observed by oxygen bombard- ment of typical quaternary metallic glasses.

The processes of oxidation and formation of finely dispersed metal/oxide phase microstructures as con- sequences of oxygen implantation in pure elements have already been considered.2 Additional effects of PI/solid interaction in the case of multi-element specimens have to be expected from the more complex processes of atomic redistribution within the total sampling volume V, of SIMS. Preferential sputtering, recoil demixing and at least a temporary decomposition of the initial matrix could be accompanied by:

( i ) a biased migration of highly mobile matrix atoms towards the surface of V, or other dominating sinks of diffusion,

(ii) partial oxidation of the V, material (i.e. the pre- ferred oxidation of particular matrix elements in analogy to internal oxidation processes), and

(iii) formation of more-complex new phases and new total microstructures of V, during solid state reac- tions and recrystallization.

Whereas for the behaviour of minor components in 'pure' elements a comprehensive review has recently

0142-242 1 / 87: 080409-07 $05.00 @ 1987 by John Wiley & Sons Ltd

been published," the information about oxygen- implantation-modified binary alloys is still rather

and for SIMS of actual multi-element matrices the characterization of V, has not been described pre- viously (except in Refs. 7, 14 and 15).

Thus the effects to be expected from the present experiments will be considered at least partially as a qualitative analogy to

(i) results observed for Ar+ bombardment (where radiation enhanced diffusion, surface segrega-

. tion of one of the matrix elements, and structural

(ii) results observed for 0' bombardment of pure elements and binary alloys (where surface segre- gation of the implant has been ~tated'~,'~,' '),

(iii) results observed for 0' bombardment of pure elements and binary alloys (where formation of new phases within the near-surface region has

(iv) results observed for Ni+ bombardment of Ni crystals containing Si and Mn in solution (<0.2 at O/o ,22).

modification have been r e p ~ r t e d ~ * ' ~ , ' ~ - ' ~ ),t

1, and been fOUnd2.12.13,19,21

EXPERIMENTAL

The application of metallic glasses as model specimens for SIMS experiments with multi-element matrices needs some clarification concerning their particular properties. Metallic glasses generally have to be considered as ther- modynamically metastable substances involving an atomic arrangement that shows no long-range order (-amorphous). They consist usually of metallic com- ponents and a certain amount of glass-forming elements (e.g. B, C , Si, P). The change of their solid structure is observed under elevated specimen temperature ( TPr - 600-800 K), and no influence has been reported so far

t The terminology used in Ref. 16 could be misleading since segrega- tion should merely influence the amount of preferential sputtering. The effect itself is caused by the type of interaction between PI and solid.

Received 18 August 1986 Accepted 6 January 1987

410 H. MA1

Figure 1. TEM micrographs and ED patterns of typical metallic glass specimens: (a), (b). (c)-initial state, and (d), (e), (f)-oxygen implanted Fe4,,Ni,P,,B,.

as a result of the incidence of electrons in transition electron microscopy (TEM) studies. This can be explained by the various transient energy densities avail- able in the different interaction processes. The reaction amorphous + crystalline demands activation energies of, typically, E, - 140-680 J ~ m - ~ . Whereas the order of magnitude of the mean specific energy loss under elec- tron bombardment, A E, , merely approaches levels of - J ~ m - ~ ; thermal experiments and ion implanta- tion can easily supply the energy densities mentioned above.6 Thus, under SIMS conditions, structural changes are quite probable and effects occurring in this connec- tion have to be carefully considered before generalizing them to crystalline matrices too. In contrast, no pro- nounced influence on specimen state is expected while investigating the model samples by selected electron techniques such as TEM and scanning Auger micro- analysis (SAM).

Sample preparation

The average composition of the Fe40Ni40P14B6 and Fe81Si3.5C2B13.5 metallic glasses has been determined by various techniques6 and the amorphous state and ele- mental composition of particular specimens have been verified by electron diffraction (ED) and electron probe microanalysis (EPMA) prior to oxygen bombardment (see Fig. l(c)).

The specimens had been prepared before for TEM investigations (TEMscan, JEOL, Tokyo, Japan) by elec- trolytical thinning (acetic acid/perchloric acid - 10 : 1). After oxygen ion bombardment ( E ( PI) - 20 keV, l60:, N(P1) - 5 x 10” ions cm-2, ion microprobe mass analy- ser (IMMA), ARL, Sunland, California) TEM mappings and ED patterns have been taken for V, characterization (cf. Fig. 1 in Ref. 2).

For SAM investigations a series of graded exposures has been applied within an area of -1 mm’. Unfortu-

nately the PI density had to be changed from i(P1)- 5 x A cm-’ to cover an exposure range N(P1) - 5 x l O I 4 to 1.4 x 1019 ions cm-2. Thus the kinetic conditions of the two high exposure runs might consider- ably differ from those of lower exposure. The analytical parameters applied in TEM and SAM, except SAM depth profiling, (PI density i(Ar+) - 0.04 mA cm-’, sput- ter rate us - 4 nm min-I are comparable to those for Fe in Ref. 2.

to 2 x

TEM and ED results

A detailed explanation of these experiments has already been The short summary of the results obtained from Fe40Ni40P14B6 and Fe8,Si3.5CzB 13.5 will merely con- sider a more advanced interpretation of the data. Figure 1 presents typical TEM mappings of the initial specimen state and of the state after implantation for Fe40Ni40P14B6. Two different phases of the microstruc- ture can easily be distinguished within the implanted specimen. Since fine crystallites of phase I ( d , - 100 A) are covered by a thin film of phase I1 (df- 20 A) the presence of a cell-structure could be supposed (Fig. 1 (e), for definition see Ref. 23). However, a preferred disper- sion structure may be deduced from Fe81Si3.5czB13.5 (Fig. 2(a)).

The appropriate ED patterns are given in Fig. l(c) and (f) for Fe40Ni40P14B6 in initial and implanted state respectively. The marked polycrystalline pattern (Fig. l ( f ) ) is superimposed by the diffuse pattern of the unaffected ‘substrate’ beneath V,. It is clearly distin- guished from those obtained for Fe and Fe81Si3.5C2B13.5 which show unambiguously a spinel structure (Fig. 2(b)), right side). Thus a prevailing contribution of a component different from Fe304, y- Fe203 or NiFe,04 is to be expected. From d spacing comparisons (Table 1) most of the reflexes observed can be ascribed to the

INVESTIGATION OF SAMPLING VOLUME IN SIMA. 111 41 1

Table 1. d spacings in Fe&li,P,,B6 after O2 implantation

Fe,,NI,P,,B,(Ox 1 range of d spacings'

dMl A

2.41-2.38

2.11-2.08

1.77-1.75

1.49-1.47

1.2fS1.25

1.22-1.20

1.04-1.02

0.964 90

Data of selected equilibrium compositionsb Fe'3 NiO y-Fe,O, Fe,O, NiFe,O,

- 2.413 2.408 2.422 2.408 (111) (222) (222) (222)

-- 2.090 2.085 2.098 2.085 (002) (004) (004) (004) - - - 1.778

- 1.478 1.474 1.483 1.474

1.2!53 1.260 1.257 1.265 1.257 (22.2) (113) (226) (226) (226)

(233)

(022) (044) (044) (044

-- 1.207 - 1.211 1.230 (222) (444) (136)

(004) (008) (008) - 1.045 1.043 - 1.043

-- 0.935 0.932 0.938 0.932 (024) (048) (048) (048)

a Errors of R measurement aiid reproducibility of camera length involved.

ASTM card index.

data of NiO, too. These expectations are also promoted by the absence of spinel major reflexes (i.e. (022), (111)).

SAM results

Since experimental limitations cause a marked surface contamination during specimen transport between IMMA, SAM and TEM, as well as inside TEM, the characterization of the uppermost atomic layers of V, is prevented. It can be easily shown by Fig. 3 (initial state,

( a 1

Figure 2. TEM micrograph and ED patterns from various crystalline phases observed after oxygen implantation. (a) TEM micrograph of Fe81Si,,C,B,,,, (b) ED patterns, left: Fe40Ni40P14B, (NiO structure), and, right: Fe,,Si, 5C2B135 (spinel structure).

similar results for all implanted areas) how in particular the low energy signals of FeM", NiM" and PLMM are suppressed by the carbon contamination (curve a ) which will disappear after a slight sputter cleaning (- 10 A, curve b ) . But this sputter influence removes the intrinsic

I (3

I

500 1000 Kinetic energy , eV

Figure 3. Elemental survey from non-implanted Fe,oNi,,P,,B, obtained by SAM: (a)-initial surface state, and (b)-after sputter removal of -10 A.

412 H. MA1

“0 1 2 Sputtertime lrnin

Figure 4. SAM depth profiles from non-implanted specimen Fe,Ni4,P14B6. N : bulk composition determined by EPMA: cB=4.2 at%, cp=16.4at%, cFe= 34.6 at% and cNi=40.4 at%. 0: SAM data for clean surface, before influence of preferential Ar+ sputtering dominates: cB=5.5 at%, cP=14.3 at%, cFe= 34.5 at% and cNi=41.3at%.

‘surface’ of V, too. Thus, from all depth profiles shown in the following, no information about A K , usually supposed to be the SI origin, can be obtained.

To give an estimate of methodical influence on SAM results (Ar+ sputtering, electron beam) the depth profiles of the initial sample state have been recorded (Fig. 4). The appropriate bulk composition found by EPMA ( Bi ) is measured by SAM (0) as soon as an oxygen surface contamination has been removed (= sputtered layer - 10 A). Continued sputtering results in a marked increase of Fe and B surface concentration, whereas the

Ni and P signals show a decrease suggesting a conversion of Fe/Ni ratio and change in B and P contents. Effects of a similar order of magnitude should be involved also in the data taken from implanted areas.

The depth profiles through a typical steady-state V, are shown in Fig. 5. Across the major region of the oxygen profile (up to sputtertime t,, - 6 min) a consider- able decomposition of the initial compound is observed. The steady-state composition is dominated by Ni in particular, where high oxygen concentrations are observed too. Whereas only a minor decrease of the Ni

Ni

0 10 2.0 4.0 8.0 Spuftertime/min

Figure 5. SAM depth profiles from oxygen implanted Fe40Ni40P14B6 ( N ( P l ) y 8 x 10’’ ions cm-‘, steady-state conditions). - 0 : surface concentration normalized without regard of C concentration.

INVESTIGATION OF SAMPLING VOLUME I N SIMA. I11 413

Ni

N (PI) /"o; ions ern-2

Figure 6. Temporal behaviour of the composition of a V, near-surface region during pre-sputtering interval (Fe,,Ni,P,,tl, specimen, SAM measurements from five IMMA craters having graded exposures).

content is observed with the marked oxygen slope the Fe profile is really anticorrelated from t,, - 3 min, until bulk is approached. Wilhin the 'upper half' of V, it has been depleted to 50% of the bulk level. But, the largest relative changes of atomic concentrations are found for the glass-formers B and P, respectively. They are heavily depleted within the uppermost lOOA of V,. There B concentration remains below the detection level of SAM and P is found to be ( 5 at%. To maintain these condi- tions in a steady-state an appropriate migration of these elements towards the surface is evident. The oxide state of Fe, Ni and P within this region is indicated by the peak shifts of the MNh and LMM lines. AE for Fe is found to be - 5 eV, whereas Ni gives -3-5 eV. The P signal shows the lines for elemental and oxidized phos- phorus as well. It is hard to say whether the elemental peak within V, is true or caused by oxide decomposition. For similar implantation conditions the virtual oxygen concentration czx has been calculated (x-axis normaliz- ation to co+ 0 at tSp- 8 min, see eq. (14) in Ref. 1). It shows a quite good agreement to that measured by SAM for lower levels, but, near V, surface a marked deviation has been found.

The behhviour of V, composition near its surface, for increasing N ( Pl), is reflected by Fig. 6 . The points ( x ) are obtained by SAM after f S P = 1 min sputter removal. Thus, an indication for the evolution of decomposition of the initial V, and the elemental transport towards its surface is obtained. At present we desist from con- clusions about the formation of definite compounds throughout the pre-sputtering stage due to pronounced concentration gradients within V, and the contribution of different matrix layers to the various Auger signals. The comparison of the measured oxygen concentration and c,*, (calculated) reflects two deviations. On the one hand a parallel shift is observed at the beginning (which might be explained by the different scales of c,,(SAM) and c&). On the other hand an excess of measured

oxygen around N ( PI) - 10'' ions cm-2, compared to measured steady-state level, is found, which is not to be expected from theory due to idealized assumptions that do not involve an atomic redistribution within V,.

Behaviour of SI signals during pre-sputter stage

In comparison with the simple structured Z(S1) charac- teristics obtained from pure elements2 the matrix lines from a compound sample show rather complex intensity variations (Fig. 7). After removal of surface contamina- tion (e.g. native oxide layer) each of the characteristics for P, Fe and Ni pass through a, more or less, pronoun- ced relative maximum for N ( PI) < 10l6 ions cm-2, whereas B already shows its abolute maximum in this region. From SAM data (Table 2 and Fig. 6 ) it can be concluded that Ni passes through its maximum 'surface' concentration there, whereas cFe and cp show a steady decrease from bulk concentration and cB is below the limit of detection. The implanted oxygen near V, surface is now found to be of the order of magnitude cox-- 10 at%. Whereas the relative maxima just explained are found at different N ( PI) levels, succeeding relative minima are found for all elements very close to N ( PI) - loL6 ionscm-2. From these data. in accordance with

Table 2. SAM determined composition of instantaneous near- surface regions during pre-sputter stage, concentrations in at%

(A) PI ( C ) (D)

0 12.5 37.5 42.5 37.5 B <1 <1 <1 < l P 8 3.5 -1 1-2 Fe 30 27.5 25.5 24 Ni 4 1 34 29 36

414 H. MA1

15

1G

5

Figure 7. Temporal behaviour of the atomic SI intensities, I(SI), from matrix elements during pre-sputtering interval (Fe,Ni4,P146, specimen, IMMA depth profiles, PI: "O;, E(PI)-20 keV, M(PI)- 1.7 x 1014 ions ctW2 s-I). c,,(SAM): measured oxygen concentration, c&: calculated virtual oxygen content, and (A) to (D): various time increments during pre-sputter stage, for appropriate near-surface compositions see Table 2.

continued implantation, the I( SI) increase and approach maximum levels again for different N ( PI). For B a relative maximum is obtained first. Then P and Fe follow already before the maximum concentration of the implanted species is measured. Finally, Ni passes its maximum value close to the maximum of measured cox and the minimum of measured cNi (see Table 2). At higher implantation doses the I(S1) values show an almost constant progress, while a steady-state cox - 37 at% is attained close to V, surface and the initial matrix elements have cB < 1 at%, cp - 2 at%, cFe - 25 ato% and cNi - 35 at%. For N(P1) > 1OI6 ions cm-' at least a partial correlation between 'surface' cox and I(S1) is found, but not between the atomic secondary ion signals and the matrix element concentrations. Since real segregation effects within the few uppermost layers of V, could not be examined in our experiments the predominant region for SI creation still remains outside of our consideration!

CONCLUSIONS

From an initially almost homogeneous multi-element specimen (metallic glass Fe40Ni40P,4B6) a non-

monotonous temporal behaviour of the SI currents is observed during the pre-sputtering interval. In addition to a steady increase of the implanted oxygen concentra- tion within the total V, the modification process of the actual V, surface layers, as well as of the remainder of V,, are supposed to be connected with this behaviour.

The investigations of phase modification inside V, by SAM, TEM and ED show, as consequences of the oxy- gen PI implantation:

(i) decomposition of the metallic glass compound (ii) precipitation of oxide phases, and

(iii) formation of dispersion and cell-like microstruc-

Thus, the oxygen implantation in SIMS creates a steady- state sampling volume that is inhomogeneous and differs in atomic order and composition widely from the initial specimen state.

tures.

Acknowledgements

The author wishes to thank Dr H.-D. Bauer (ZFW) and Dipl. Phys. W. KieBling (ZFTM) for their contribution in the experimental work (ED, SAM). He is indebted to Prof. Dr W. Pompe (corresp. member of the Acad. of Sci. GDR) for his critical and valuable comments throughout the preparation of the manuscript.

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