Determination of the Grain Boundary Diffusion Coefficients in Thin Film Au-Ag Couples by Auger Electron Spectroscopy

170

oaTERMINATION OF THE GRAIN BOUNDARY OIFFUSION COEFFICIENTS IN THIN FILM Au-Ag COUPLES BY AUGER ELECTRON SPECTROSCOPY Antoni 6ukaluk Institute of Mathematics and PhySics, Academy of Technology and Agriculture, ul. S. Kaliskiego 7, 85-790 Bydgoszcz, Poland. Thin film polycrystalline Au-Ag couples were prepared by evaporating of silver film and subsequently depositing gold layer onto the Ag film. For depth profile analysis Ni substrates were used, on which Ag films of thicknesses 800 nm and Au films of thicknesses 100 nm were deposited. For the analysis based on the growth of Ag layer on Au surface a structure of 410 nm Ag + + 118 nm Au has been evaporated on the glass substrate. For measuring grain boundary diffusivity from the profila analysis the whipple /1/ solution was used. To extract the grain boundary diffusion coefficient O'it is convenient to plot the composition profile as a function of the distance from the interface y to the power 6/5 on a semi log scale. In such a plot of InC vs. y6/5, the part due to the grain boundary diffusion should be a linear function. The grain boundary diffusion coefficient can be determined from the relationship /2/ OIS

'Cln~

-0.661· ( 'a 6j S y

) -5/3.

(4 ) tO

1/2

( 1 )

where 8' is the effective grain bounda ry width, 'a InC/ '0 y6/5 is the slope of the grain boundary part of the profile, 0 and O· are the lattice and grain boundary diffusion coefficients,respectively, and t is the time of aging. Assuming after OeBonte et al, that SO - 0.5 nm /3/, activation energies of 1.2 eV and 1.3 eV and Arrhenius equations: o • - 0.76 • exp / -1.2 eV/kT / CID2 S -1 9 2 -1 o - 5.83· exp / -1.3 eV/kT / CII S , P have been obtained by the use of 1nterdiffusion coefficients at the interface /Og/ and inside the Au film /0'/ /4/, in the temp perature range of 523 ~ - 648 K.

171

Diffusion of Ag through the Au film induces growth of a silver layep on the gold surface. The amount of Ag at the surface is proportional to the total surface cpverage 8. This coverage may be obtained from the relationships /5.6/ : hS

..

h~

.. (1-

k

1-(1-

where:h~, tively, 8

s)t

i=l

8

i

' 6

i - 1,

a)1'1=1 -:. 8 {i3i - (1t

( 2 )

a)e

9j"a i - ( j +1>} .

( 3 )

JD1

h:

are the normalized Au and Ag layer signals, respecis the coverage of th~ i-th monolayer, a and s are i the attenuation factors of Auger lines for the Ag layer and the Au substrate, respectively, and 9i 1s the relative backscattering factor of the i-th monolayer. The linear extrapolation of the results of e vs. time dependence ought to intersect the time axis at the point t i DL 2/ 60 ' / 7/ , where L is the Au-layer thickness. Using this relation the 0' value at 383 K has been found to be 7.7.10- 14 cm 2s- 1 • The value of O'calculated in such a way is larger than the value of 3.0-10- 16 cm2s- 1 obtained from extrapolation of the results determined ftom depth profile analysis. The reason of this discrepancy may be discontinuity of the growing Ag layer, which can cause an overestimation of the amount of silver at the surface and, as a consequence, overestimation of D' value. REFERENCES.

11/ Whipple R.T.P.: Philos. Meg. /2/ Le Claire A.O.: Sr.

/3/ /4/ /5/ /6/

45 (1954) 1225.

a,

Appl. Phys. 14 (1963) 351. DeSonte W.~ •• Poete ~.M•• Melliar-Smith C.M. and Levesque R.: ~. Appl. Phys. 46 (1975) 4284. Bukaluk A.: S~rf. Interface Anel. 5 (1983) 20. Siuda R.: Surf. Sci. 140 (1984) 472. Bukaluk A••Rozwedowski ft •• Siude R.: Appl. Phys. A 34 (1984) 193.

/7/ Crank ~.: The Mathematics of Diffusion (Oxford Univer8ity Press, London. 1956 ) • p. 47.