Quantitative Auger analysis of silicides

Vacuum/volume 36/numbers 7-9/pages 433 to 435/1 986 Printed in Great Britain

0042-207X/8653.00 + .00 Pergamon Journals Ltd

Quantitative Auger analysis of silicides T Wirth,

M Procop

a n d H L a n g e , Central Institute of Electron Physics, Academy of Sciences of the GDR,

Berlin, GDR

Quantitative AES analysis is carried out on Cr-SL V-Si, and M o - S i systems. The treatment is based on the introduction of matrix and sputter correction factors. The matrix correction factors are calculated along the lines outlined in the literature. Experimental data obtained for mechanically cleaned surfaces of Cr 3SL V3SL V5Si3, VSi2, and MoSi 2 agree well with nominal composition ratios. The sputter correction factors were found to be strongly dependent on composition for all three systems. Comparison with theory for preferential sputtering as the dominant mechanism yields agreement with the experiment only for samples with low Si content. The larger sputter correction required with increasing Si content will be discussed in terms of recoil location and radiationinduced segregation.

1. Introduction Auger electron spectroscopy (AES) is an effective method for chemical analysis. However, problems arise in quantitative determination of components from AES depth profiling of composite materials. These are due to matrix effects on the standard elemental sensitivity factors and component redistributions due to sputter effects (preferential sputtering, knock-on, radiation-induced segregation). Different correction procedures have been proposed in the literature and successfully applied to metal alloys 1'2. We have tried to separate matrix and sputter corrections in AES analysis of depth profiling experiments on metal-nonmetal compounds. 2. Experimental The investigations were carried out on Cr3Si, V3Si, VsSi3, VSi2, and MoSi 2 bulk samples and on MoSi, amorphous layers of composition between x =0.8 and 2.0 as well as on polycrystalline CrSi 2 and CrSi layers on silicon substrates. The layer thickness was in all cases nearly 100 nm. The AES experiments were performed with a scanning Auger microprobe (PHI 590). The Auger electron spectra were recorded with a primary electron energy of 5 keV and a modulation voltage of 4 V. The angle of incidence between the electron beam and the surface normal was 60 °. Ion sputtering for depth profiling was performed with Ar ÷ ions of 1 keV energy and about 45 IrA cm- 2 current density at an oblique angle of incidence of 34.1 ° to the surface normal. The surface of the bulk samples was cleaned by scratching under uhv conditions. This excludes any sputter-induced changes of the surface layer and enables an experimental analysis of the matrix correction by determining the metal/silicon AES intensity ratio. 3. Results and discussion

denum silicides completely. In the case of Cr3Si some traces of carbon and oxygen are still detectable. As an example in Figure 1 the Auger spectrum of the MoSi 2 sample is shown. Calculations of the matrix correction to the elemental sensitivity factors were carried out taking into account changes in the atomic density, electron escape depth and backscattering. The matrix correction factor KM is defined by the equation

SAXA

IA,AB

],,AB -- KM ~

.

(I)

IA.AB and IB.AB are the Auger electron intensities of selected transitions in the atoms A and B, respectively, in the binary system AB. The ratio of the elemental sensitivity factors S A / S B is equal to the intensity ratio of the same transitions in the pure components J/

)J

MoSi2

/ ==

l

JQ

I

MoLMM

(x 2.5)

I ~ MNN

Si KLL (x 2.5) jSi

LVV

t 3OO

5(3O

21OO

Auger etectron energy(eV)

3.1. Matrix correction. The mechanical cleaning of the sample surface removes the impurities from the vanadium and molyb-

Figure 1. Auger electron spectra of MoSi2after cleaning the sample surface by scratching. 433

T Wirth et al:

Quantitative Auger analysis of silicides

A and B, recorded under identical experimental conditions as IA,ABand Ia,AB- ZA/ZB denotes the concentration ratio of the two components, K n can be calculated using tabulated atomic densities, and formulae given for the escape depth and the backscattering factor by Seah and Dench 3 and Shimizu4, respectively. A more detailed description of our matrix correction procedure has been published 5. The calculated correction factor K u proved to be weakly dependent on the layer composition. From the metal/silicon intensity ratio we get by means of the sensitivity and matrix correction factors the chemical composition XSi/.¥rnetal to within 10% for MoSi2, the V-silicides, and for Cr3Si in case of Si KLL transition. The result for Cr3Si using the Si LVV transition will be influenced by the traces of impurities still on the surface. The results are presented in Table 1. It should be added, that for good agreement between calculated and nominal compositions the high modulation voltage is essential 5. 3.2. Sputter correction. In order to separate sputter and matrix corrections for the alloy compositions studied, the steady-state intensity of the silicon and the metal signals obtained after 1 keV sputter cleaning has been measured as a function of composition. The dependence was decomposed into contributions due to matrix correction and sputter-induced alterations. It is found that for the Si signal both contributions nearly compensate, whereas for the metal the sputter influence dominates the matrix correction. This is in agreement with the changing of the Si and metal signals of the scratched surfaces with subsequent sputtering. After sputtering the surface concentration Xs and Xs are related to the bulk concentrations by the sputter correction factor K s , XA X s - Ks X~"

X AS

(2)

The surface concentrations were determined from measured intensities and matrix-corrected sensitivity factors according to equation (1). With the known bulk concentrations equation (2) gives K s , shown in Figure 2 as solid lines. It is seen from this figure that K s for the Cr-Si and V-Si systems has a steeper composition

1.0

~ 08

-'-'-x...... 7

~.wCr_Si

Mo-Si

. . . .

~ 06 8

N,

~ 04 T

I

O2

I

04

I

06

If

I

02

I

I

04

0.6

~f

I 02

I 04

I 06

08

Silicon concentrotion Xsi

Figure 2. Sputter correction factor K s determined according to equation (2) for low-energy(filledcircles)and high-energy(open circles)Auger electron transitions (see Table 1) for the systems(a) ~Si, (b) C~Si and (c) Mo-Si as a function of nominal sample composition. Dashed lines represent K s given by the theory of preferential sputtering. dependence than for the M ~ S i system. Moreover, K s for the high-energy transitions decreases less strongly with increasing Si content than for the low-energy transitions in all three systems. The K s values are valid for the conditions of ion beam sputtering given in Section 2. Altered ion beam parameters have a small influence on K s only. Increasing the ion energy up to 5 keV results in a weakly increased K s for all investigated silicides 6. For the Mo-Si system the influence of ion current density and ion mass was investigated too. Increasing the current density to 100 #A cm-2 at 1 keV enlarges K s by 5%. Using Xe ions we have found Ks(Ar) = (1.03 + 0.01)Ks(Xe ). For comparison we have calculated K s from the sputter yield ratio using Sigmund's formula for preferential sputtering 7 with Kelly's expression for the surface binding energy ratio 8 and values for the surface binding energy given by Anderson9. The results are given in Figure 2 as dashed lines. As seen in the figure, the composition dependence of the theoretical K s values, although being rather weak, is stronger for V Si and Cr Si systems than for the Mo Si system which is in

Table 1. Intensity ratios of Auger electron transitions, ratios of elemental Auger sensitivity factors, corrections to elemental sensitivity factors and composition ratios for scratched MoSi2, Cr3Si, VSi2, VsSi3, and V3Si surfaces Sample MoSi2

Cr3Si

VSi 2

VsSi 3

V3Si

434

Auger electron transition

Intensity ratio

Ssi/SmetaI

KM

Xs~/Xm¢t, 1

SiL23VV MoM4N22N45 SiKL2L2 MoLaMgsM4s SiL23VV CrL3M23M45 SiKL2L2 CrL3M23M45

3.32

1.27

1.33

1.96

5.8

2.5

1.27

1.8

0.27

0.66

1.40

0.29

0.10

0.21

1.39

0.34

1.52

0.57

1.30

2.05

0.49

0.18

1.29

2.09

0.51

0.57

1.29

0.69

0.15

0.18

1.26

0.64

0.24

0.57

1.30

0.32

0.08

0.18

1.26

0,36

SiL23VV

VL3M23M45 SiKLzL2 VL3M23M45 SiL23VV VL3Mz3M45 SiKL2L2 VL3M23M45 SiL23VV VL3M23M45 SiKL2L2 VLaM23M45

T Wirth et al: Quantitative Auger analysis of silicides

qualitative agreement with the experiment. Comparison with the experiment is only meaningful for the low-energy Auger transitions, as these carry information on composition changes in the 0.5-1 nm depth range where preferential sputtering should be the dominating mechanism of component redistribution. According to Figure 2 the experimental and theoretical values are in approximate agreement only for low Si content. The strong deviation from the sputter theory in the region of high Si content is due to a drastic change of the altered layer induced by sputtering. As an example, in Figure 3 the 1 keV Ar + depth profiles of the altered surface layer of VaSi and VSi 2 as they develop during 5 keV Ar + bombardment to steady state are shown. Whereas in the case of VaSi the Auger intensities vary only weakly, ion bombardment of VSi 2 creates an altered layer extending some nanometres below the surface with metal enrichment and Si depletion. Therefore the surface concentration ratio XSsi/)(so will be influenced by the altered layer profile. In general we have observed that composition changes are small for silicides with low Si content but considerably stronger for the Sirich silicides 1°. The value of K s depends on the bulk composition as well as on the escape depth of Auger electrons used for the analysis. This explains the smaller dependence of K s using the high energy Auger transitions in agreement with Figure 2. In a preceding paper 6 concentration profiles like Figure 3 were interpreted as being a result of preferential sputtering and recoil

V (a)

.$

si 20O

4O0

6

(b] Si

4O0

Sputter time Is)

Figure 3. AES depth profile of the altered surface layer of (a) VaSi and (b) VSi2 sample after sputtering with 5 keV Ar + ions.

4. Conclusions The analysis presented allows us to separate matrix and sputter effects and to determine the sputter correction as a function of composition. It shows that matrix correction procedures for AES intensity ratios may be successfully applied to some metal-nonmetal compounds. The sputter correction factor has to be determined by experiments. At present the theoretical models cannot provide this correction. Clearly further investigations are necessary to identify the dominating mechanism for the development of the altered layer profile.

References

I

200

location. This conclusion was drawn from the qualitative agreement between our altered layer profile and Monte Carlo simulations by Roush et all1 considering binary systems with equal surface binding energies but different masses of the two components. Recently the Monte Carlo simulations were repeated by M611er and Eckstein 12. They could not confirm the results of Roush et al and found depletion of the lighter component below the surface up to a depth about equal to the mean projected range of the incident ions. This depletion can explain the mass and composition dependence of K s but not profiles like in Figure 3 and their temporal development with increasing ion dose 6. Our results, especially the weak dependence of K s on the ion mass, current density, and beam energy, are in accordance with the effect of surface segregation too 13. In this model the depth profile of Figure 3 would be the result of preferential sputtering, radiation enhanced diffusion, and radiation induced segregation of Si 14.

1 p H Holloway, Surface Sci, 66, 479 (1977). 2 p M Hall and J M Morabito, Surface Sci, 83, 391 (1979). 3 M P Seah and W A Dench, Surface Interface Analysis, 1, 2 (1979). 4 R Shimizu, Japan J appl Phys, 22, 1631 (1983). 5 T Wirth, M Procop and H Lange, Surface Interface Analysis, g, 7 (1986). 6 T Wirth, V Atzrodt and H Lange, Phys Status Solidi (a), 82, 459 (1984). v p Sigmund, in Topics of Applied Physics. (Edited by R Behrisch), vol 47. Springer, Berlin (1981). 8 R Kelly, Surface Sci, 100, 85 (1980). 9 H H Anderson, Appl Phys, 18, 131 (1979). lo T Wirth and H Lange, to be published. 11 M L Roush, O F Goktepe, T D Andreadis and F Davaria, Nucl Instrum Meth, 194, 611 (1982). 12 W M611erand W Eckstein, Nucl Instrum Meth, B2, 814 (1984). 13 j Kirschner, Nucl lnstrurn Meth, B718, 742 (1985). 14 R Kelly, Surface Interface Analysis, 7, 1 (1985).

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