Investigation of Metal Diffusion at Dielectric Surface by Ellipsometry

183

INVESTIGATION OF METAL DIFFUSION AT DIELECTRIC SURFACE BY ELLIPSOMETRY L.Ghita and C.Ghita Central Institute of Physics - I.F.T .A.R. Bucharest - Magurele, P.D.Box MG-7, Romania

The possibility to measure, with a high degree of precision, small modifications of the polarization state of the reflected light created the premise to apply the ellipsometry to a great variety of surface phenomena /1/. The present paper takes into consideration the study of the thermal diffusion of some atomic species at the surface of a bulk material by this method. This case, little explored by ellipsometry, presents a great practical interest in different fields such as electronic industry, glass industry, etc. The ellipsometric investigation of this kind of processes is justified by the precision of the method, its rapidity, relative low cost per installation and the possibility to adapt it to on-line measurements. We have performed the study of the thermal diffusion process of 5n in float glass samples by the determination, with a standard ellipsometer /2, 3/, of the ellipsometric parameters (6.,

'1.\"'),

characterizing the polarization state of the reflected light on the

surface of a bulk material. We present and discuss the results obtained for four samples (51 - 54) of plane glass, cut from plates of different origins (France, Soviet Union, Belgium, West Germany). We have measured

D. ,,,¥, at the incidence angle of 70° and :A. = 546 nm on both

faces of the samples. Table I present the values -V-FF and "\fCF' corresponding to the free face and respectively contact face of the glass with the melted metal in the drawing bath. The nFF and nCF are the corresponding refractive indices calculated by means of

6. ,

'\f

determined on both faces, from the fundamental ellipsometric relation for

dielectric bulk material. The variation of

A from the free to the contact face and even from a sample to

another is smaIl at the given incidence angle, thus its contribution to the calculation of n manifests at the most 4-th decimal. The experimental errors in D., '\r are:!: 0,02° with our instrument, which affects n with + 1.10-3•

184

Table I Sample

'\1' FF

'\fCF

01\1"

nFF

nCF

&'n.102

51

20,241°

19,867°

0,375°

1,517

1,533

1,6

52

20,358°

19,954 0

0,400°

1,512

1,529

1,7

53

20,504°

19,917°

0,583°

1,506

1,531

2,5

54

20,104°

19,566°

0,538°

1,523

1,546

2,3

'-V

The decreasing of CF and increasing of nCF, visible in Tab.l for all samples, indicate a raising of density at the surface in the contact region, explained by the diffusion of the metal (Sn) from the bath. The difference in the values ~1\r and

dn for the samples 5It

52 and 53, 54 can be explained both by the variations in composition of the glass (see nFF) and in the technological parameters (bath temperature, sample thickness, drawing speed, supplementary treatments, etc.). Measurements of the ellipticity /4/ of the reflected light on both faces, at the Brewster angle of the bulk material, have evidenced, for all the mentioned samples, noticeable differences between the free and contact faces. Attempts to correlate the data obtained by these two types of experiments are in our view. An ellipsometric programme destined to evaluate the height of the diffusion layer, considering different presumable morphological models and technological parameters of the diffusion process, is in progress. REFERENCES 1.

Azzam R.M.A. and Bashara N.M., Ellipsometry and Polarized Light, 2nd print.,

2.

Baltog I., Bi:idanau I., Constantinescu M., GhitEi C., Ghita L. and Vlahovici N.,

North-Holland Publ.Comp., Amsterdam, 1979. 5tud.Cerc.Fiz., 26(1974), 569. 3.

Ghita C., Ghita L., Baltog I., Contantinescu M. and Muscalu G.L., European Optical Con f., Rydzyna-Poland, May 1983, 56.

4.

Bennett J.M. and King R.J., J.Appl.Optics, 9(1970), 236.