Rutherford Back Scattering and Auger Electron Spectroscopy Analysis of the Oxygen Distribution in the AI-Ag System Evaporated at

Torr

Alicia Oliver and Federico Garcia Santibafiez Instituto de Fisica, Universidad Nacional Aut6noma de MCxico, Apdo. Postal 20-364, MCxico 20, D. F.

An analysis of the AI-Ag system on thin films carried out through Rutherford back scattering and Auger electron spectroscopy combined with ion sputtering is reported. The films were prepared by evaporation on vitreous carbon under vacuum pressures of 1 X Torr. The results show different distributions of oxygen depending upon the order of A1 evaporation.

INTRODUCTION

The interdiffusion of the system Ag-A1 in thin films at temperatures below 200 "C has important consequences for optical applications. The interdiffusion of this system as a function of heat treatment has been studied through Rutherford back scattering (RBS) by Westmoreland' and through optical reflectivity by Weaver.2 In both works the films were prepared by evaporation at press- ures of 2-5 x lo-' Torr, with typical thickness of 2000 and 3000 A. However, in the reported results obtained by RBS, the phase Ag2AI which is reported as the predominant phase responsible for the reflectivity changes of the system in the optical reflectivity studies does not appear. W h e other hand, in the work perfor- med by RBS it is shown that if the aluminium film is evaporated first onto the substrate, an intermediate film is not needed to form a diffusion barrier at 135 "C. They suggest that, given the vacuum conditions under which the films were prepared, the possible formation of an aluminium oxide layer represents a barrier to diffusion.

It is kndwn that for a vacuum below 1 x lo-' Torr, commonly used on industrial applications, the aluminium films trap a great amount of oxygen during their formation. It has been observed that impurities in the interface can affect the formation of a particular intermediate phase, form barriers and subsequently modify the evolution of the ~ y s t e m . ~

This work reports studies to obtain the concentration profile of the Al-Ag system, analysed by RBS and Auger electron spectroscopy (AES), for evaporated films at working pressures of 1 x Torr.

EXPERIMENTAL

The sample substrates were made of polished vitreous carbon. Metal films wer'e deposited sequentially under a vacuum of 1 x Torr. The prepared sample con- sisted of three films in the sequence Al/Ag/Al/C. In no case was the vacuum broken between sequential metal evaporation. The approximate thicknesses of the

films were of 2000 8, for silver, and 400 8, and respectively for aluminium.

700 8,

The sample profile was first obtained by Rutherford back scattering of protons. Proton beams with energies of 700, 600 and 400 keV were obtained from a Van de Graaff accelerator with a scattering angle 0 of 165 O ,

and incidence angles a of 45 O and 90 O . The experi- mental geometry is shown in Fig. 1. The calibration of energy was carried out by means of a gold film deposited on Si02.

The concentration profiles were also obtained with AES combined with ion sputtering by means of a PHI model 590 ESCA-AUGER system. The electron beam energy was 3 keV and the argon ion beam energy was 2 keV.

DATA TREATMENT

The RBS data were three point smoothed and the back- ground subtracted when required. The energy loss in the silver film was determined using the known criteria4 which take into account the detector resolution and the energy straggling. The energy losses in other elements were determined with the aid of the curve given by Rickards' that deconvolutes the detector resolution. Stopping power values given in published tables6 were used, and the stopping powers for compounds were obtained by use of Bragg's rule.

DETECTOR 1 1 .ip BEAM

e i TARGET e i &TARGET

Figure 1. Notation for the proton scattering experiment; a is the incidence angle and @ the scattering angle.

CCC-Ol42-2421/81/0003-0226 $01 S O 226 SURFACE AND INTERFACE ANALYSIS, VOL. 3, NO. 5, 1981 @ Heyden & Son Ltd, 1981

RUTHERFORD BACK SCATTERING AND AUGER ELECTRON SPECTROSCOPY ANALYSIS

Ln

z 3

t- n n

L

a k m a z

n

w

a:

- J

t-

AI/Ag/AI /C

E.700 KeV

e= 1650

a.90" f

E ( K e V )

Figure 2. Energy spectrum of scattered protons with 700 keV initial energy, a = 90' and 0 = 165", from thin films AI/Ag/AI/C, showing the energy position of thin films.

RESULTS

RBS experiment

Figure 2 shows the spectrum of the back-scattered pro- tons of 700 keV obtained at an incidence angle a of 90 O. From the kinematic factors and energy losses, it was possible to determine the presence of three films in the sequence in which they were deposited (Al(i), Ag, Al(& as well as the oxygen associated with the aluminium film at the surface O(,), and to the aluminium between the substrate and silver O(+ The detail of .the spectrum of the contribution from the carbon deposited by the proton beam on the sample surface is shown in Fig. 3.

In the spectrum obtained at an incidence angle a of 45", shown in Figs 4 and 5 , the contributions due to oxygen O(s) and aluminium Al(s) on the surface are not visible, due to the shift and broadening at low energies

Al/Ag/AI/C

E= 700 KeV

a=90°

e= I 650

O[,)i :"

500 5 50 600

E(KeV)

Figure 3. Detail of energy spectrum presented in Fig. 2, showing the energy positions of thin films.

Al /Ag /AI/C

E= 700 KeV

e.1650

a= 45"

t

0 100 200 300 400 500 600 700

E(KeVI

Figure 4. Energy spectrum of scattered protons with 700 keV initial energy, a =45" and 0 = 165", from thin films AI/Ag/AI/C, showing the energy positions of thin films.

of the silver and interior aluminium peaks. However, it is clearly seen that the carbon C(s) on the Ale) surface and the oxygen O(i) related to the interior aluminium are separated. Experiments at energies of 600 and 400 keV at incidence angles a of 45 O and 90 O were per- formed to check this. From the form of the aluminium and oxygen peaks it is possible to say that the film of aluminium plus oxygen is not homogen~us .~

AES experiment

The Auger spectrum obtained from the surface of the sample is shown in Fig.6. The combination of AES with argon ion sputtering of the sample enabled oxygen to be detected through the films. Figure 7 gives a plot of atomic concentration for the detected elements as a function of sputter time. It can be appreciated that the oxygen distribution in the surface aluminium is different from that in the second aluminium film. In the surface

I

? z 3

> a: n a

t m

a

z

a w

n

- _I

>

AI/Ag/AI/C

E = 700 KeV

8 = 165"

a = 45'

OIII I

450 50q 550

E (KeV)

Figure 5. Detail of energy spectrum presented in Fig. 4, showing the energy positions of thin films.

@ Heyden & Son Ltd, 1981 SURFACE AND INTERFACE ANALYSIS, VOL. 3, NO. 5, 1981 227

A. OLIVER AND F. G. SANTIBAGEZ

I

400 800 1200 1600 2000

ELECTRON ENERGY (eV)

Figure 6. Auger spectrum from surface of sample AI/Ag/AI/C.

film, oxygen concentrations are seen at the vacuum-A1 interface as well as at the AI-Ag interface. The oxygen in the interior aluminium film, follows the profile concen- tration of the aluminium along its thickness. As the films were evaporated sequentially without breaking the vacuum, it is possible that the first aluminium film could trap more oxygen, due to the presence of oxygen in the aluminium itself before evaporation, and therefore the second aluminium film is less contaminated.

CONCLUSIONS

The spectra obtained by AES corroborate the informa- tion from the analysis by RBS. The results suggest that for a vacuum of 1 x lop4 Torr, the oxygen impurity from the filament and from the aluminium itself, before evaporation, contribute substantially to the oxygen dis- tribution through the thickness of the film.

n Al/Ag/Al/C

0 3 6 9 12 \5 18 21 24

SPUTTER TIME (MIN)

Figure 7. Atomic concentration versus sputter time for sample AI/Ag/AI/C obtained by combination of AES and argon ion sputtering.

Because Westmoreland and Weisenberger utilized SiOz as the substrate in their analysis, they could not see the oxygen in their aluminium films, and hence it is possible that the difference in the interdiffusion for aluminium films evaporated both before and after the silver, are due to a distribution difference of oxygen in the film than to an oxide on the surface, since the evaporation of the samples was carried out at pressures higher than 1 x lop5 Torr. These differences in the oxy- gen distribution probably also influence the formation of intermediate phases during interdiff usion by means of the temperature. Efforts to understand this behaviour are in progress.

Acknowledgements

The authors are indebted to L. Cota for his help in the AES analysis, and to R. Carmona for his help in the RBS analysis.

REFERENCES

1. J. E. Westmoreland and W. H. Weisenberger, Thin Solid Films

2. C. Weaver and L. C. Brown, Pbilos. Mag. 17, 881 (1968). 3. C. Canali, C. Catellani, M. Prudenziati, W. H. Waldin and C. A.

Evans, Appl. Phys. Lett. 31,43 (1977). 4. W. Chu, J. W. Mayer and M. A. Nicolet, Backscattering Spec-

troscopy, Chapters 2,3,4. Academic Press, New York (1978). 5. J. Rickards, Nucl. Instrum. Methods 152,585 (1978).

6. L. C. Northcliffe and R. F. Schilling, Nuclear Data Tables, A7,

7. A. Hiraki, E. Lugjjo and J. W. Mayer, J. Appl. Pbys. 43, 3643

Received 20 April 1981; accepted 22 June 1981

@ Heyden & Son Ltd, 1981

19, 349 (1973). 233 (1970).

(1972).

228 SURFACE AND INTERFACE ANALYSIS, VOL. 3, NO. 5, 1981 @ Heyden & Son Ltd, 1981