THE S I L I C O N I Z t N G OF R E F R A C T O R Y M E T A L S

UNDER N O N E Q U I L I B R I U M C O N D I T I O N S

E. P. N e c h t p o r e n k o , V. M. K r i v o r u c h k o , a n d

Physicotechnical Institute, Academy of Sciences, UkrSSR Translated from Poroshkovaya Metallurgiya, No. 10 (34),

pp. 67-70, October, 1965

Original article submitted November 22, 1964

A. S. M f t r o f a n o v

The kinetics of formation and growth of various chemical compounds, including silicides of refractory metals,

constitute a complex physicochemical process. Chemical reaction is preceded by chemisorptton, which is difficult to separate directly from the former. This is followed by the growth of a layer of chemical reaction products, pro-

vided the latter are nonvolatile. The chemfsorption and init ial layer growth period is characterized by an absence

of equilibrium at the phase boundaries, so that the concentrations of the reacting materials are variable. This cir- cumstance, of course, influences the layer growth kinetics.

In the case of sufficiently thick layers, the kinetics of layer growth during the nonequtl ibrium period may

obey a linear law with respect to t ime. In the course of t ime, as a result of the increased layer thickness, the dif-

fusion of atoms of one of the components, rather than the boundary process, begins to determine the rate in the case of dense layers. Consequently, the linear law of layer growth becomes transformed into a parabolic one. Strictly

speaking, the parabolic law becomes effective at the instant when a constant concentration of a component, present in the form of a chemical compound, is established at the phase boundaries.

This picture of the growth of chemical compound layers may become much more involved when a slight

amount of a third element participates in the reaction or when the system of the resultant layers proves to be a

multiphase one [ 1- 5].

K i n e t i c s of V a c u u m S i l i c o n i z i n g of M o l y b d e n u m i n t h e P r e s e n c e

of a S m a l l A m o u n t of A l u m i n u m

Silicon, as well as molybdenum, tungsten, and tantalum, almost invariably contains impurities which exert

a certain influence on the vacuum siliconizfng process. In particular, a luminum may constitute such an impurity.

Figure 1 shows the kinetics of growth of a molybdenum disflicide layer in the presence of 1.0-1.2 wt. % aluminum dispersed in silicon (curve 1), as well as si l iconizing in chemical ly-pure silicon (curve 2). It can be seen from a

comparison of these curves that, for sufficiently thick layers, the growth law is linear in the former case and para- bolic in the latter. This indicates that, in the former case, varying conditions prevail at the layer interface as

regards the concentrations of the reacting components.

At a given temperature, a luminum has a much higher vapor pressure than silicon and, consequently, inter- feres with the supply of the latter to the reacting surface. On the other hand, as a result of the removal of a lumi- num impurities under vacuum siliconizing conditions, its vapor concentration above the reacting surface constantly

decreases. Thus, during sil iconizing in a silicon powder slightly contaminated with aluminum, the a luminum con-

centration on the reacting surface decreases in the course of t ime, while the silicon concentration increases. Under these conditions, the thickness of the layer within a certain thickness range exerts no influence on the casing-growth

kinetics, since, compared with the parabolic law, the layer grows in this case with a certain acceleration. In the case of sufficiently thick layers, this leads to a l inear si l iconizing law.

In the course of t ime, much of the a luminum is e l iminated from the powder, and the linear law changes into a parabolic one, which indicates that constant concentrations of free and, consequently, combined silicon have be- come established at the phase boundaries.

It is characteristic that a luminum impurities exert a significant influence on the silfconizing process not only

with regard to its kinetics, by decreasing the rate of growth of the casing and changing the growth law frorri parabolic

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200 -

150

= 100

50

~a

Fig, !.

f ~ . - - 2 ..-"i

f . t -

J

I - " 7 1

0 tO 20 a0 40 so ~0 70

Time, h

Effect of t ime on MoSi2 layer thickness at si l icontzing temperature of 1250~C: 1) alu- minum content 1-1.2 wt.~ 2) s i l iconizing in chemica l ly pure silicon.

to l inear, but also with regard to the structure of the resulting sil icides. Molybdenum dis i l ic ide usually has a tetragonal struc- ture. The si l iconizing of molybdenum in the presence of alu- minum results in the formation of a MoSt 2 phase which has a hexagonal structure. Figure 2 shows a photomicrograph of a molybdenum specimen coated with hexagonal MoSt 2.

Thus, slight amounts of aluminum present in the form of vapors above the react ion surface markedly affect the kinetics of s i l iconizing. In addition, by reducing the supply of sil icon and entering into the molybdenum disi l tc ide la t t ice , they de- crease the absolute rate of growth of the latter.

The l inear law of growth of the s i l ic ide layer can readi ly be interpreted mathemati .cal ly by assummg the absence of equi- l ibrium on the phase boundary, In the case of the parabolic law,

~he rate of growth of the layers depends in the first approximat ion on two factors, Ac and D, which are incorporated

in the rate constant k of the parabol ic law y2 = kt. The absence of equil ibr ium on the phase boundaries in the case of isothermal process conditions (when D = const) indicates a change of the boundary concentrations and, eonse- que~!tly, Ac, Thus, under nonequil ibrium conditions, k in fact constitutes a var iable parameter, which l inearly de- pends on &c,

The si l icon concentrat ion at the s i l ic ide layer boundary adjoining molybdenum may in our case be regarded

as constant, Then, Ac will be determined by the si l icon concentrat ion at the inner boundary layer, Cboun d.

At a low-percentage aluminum content of silicon, the change of a luminum concentrat ion with t ime at con- stant temperature may he taken to be l inear. This means that Cboun d and, consequently, k will also be l inear func- tions of t ime, i ,e , , k = k ' t . Hence,

te"t y~ = k t = k ' t ~, y = � 9

and the law of growth of the layer thickness is expressed by a straight l ine.

Naturally, this simple ma thema t i ca l interpretat ion of the complex phenomenon of s i l tcide layer growth under nonequil ibt ium conditions does not aspire to provide a full explanat ion of the kinetic curves of stlicontztng,

Fig, 2. Photomicrograph of molybdenum dis i l ic ide layer with hexagonal structure on molybdenum. From top to bottom: MoSi 2, MosSi 3, Mo. X840.

836

,of ~1,6 -/ g'...2

0,8 - ~ "~

300 400 0 100 2'00

Time, rain

Fig. 3. Ini t ial stage of sil iconizing of molyb- denum at temperatures of (deg C): 1) 1200;

2) 1250; 3) 1300.

~O -<

oa ~o

5 0,5

�9

2jO

,5/ 3

lJ 2

_ . - - <

10o 200 300 Time, rain

Fig. 4. Ini t ial stage of siliconiztng of tanta- lum at temperatures of (deg C): 1) 1200;

2) 1250; 3) 1300.

K i n e t i c s of G r o w t h o f M u l t i p h a s e S i l t c i d e L a y e r s

A linear law of growth may apply not only in the case of dense, sufficiently thick single-phase layers, as was

demonstrated above, but ,also in the case of multiphase layers. Once again, this is due to the absence of equilibrium

on the phase boundaries. Figures 3 and 4 show the kinetics of growth of silicide phases in the systems Mo- S i and

T a - S i during the ini t ia l period of siliconizing. As can be seen from these figures, the rate of growth of the whole

layer increases in the course of t ime, which may be attributed to the increase of the parameters Ac and D on tran-

sition from the phase MoaSi to the phase M%Si a, etc. In fact, each of fl~ese kinetic curves represents a set of para- bolas characterizing the kinetics of growth of each phase separately.

It is always possible to select the siltconizing temperature in such a manner that, within a certain range of

layer thicknesses, the rate of growth of the whole layer will obey a linear Iaw with respect to t ime, constituting the envelope of a family of several parabolas:

g2 _ k t -t- A,

where A is a variable parameter characterizing the nucleation t ime of a phase.

Naturally, with increasing thickness of each phase and of the whole layer in the course of t ime, equilibrium

is established on the phase boundaries, and the layer growth law changes from linear to parabolic, since diffusion

becomes the sole ra te-determining factor.

S U M M A R Y

It is shown that, during the vacuum siliconiztng of refractory metals in the absence of an equil ibrium silicon

concentrat ion at the phase boundaries, the growth of the silicide layer as a function of t ime may obey. a linear law. This is observed during the ini t ial period of siliconizing, when new phases are formed, as well as in the presence of

impurities whose vapor pressure is much higher than that of silicon.

1.

2.

3.

4. 5.

L I T E R A T U R E C I T E D

K. Hauffe, Reactions in Solid Bodies and on Their Surfaces[Russian translation], Moscow, IL (1963), Vol. 2.

V. E. Ivanov, E. P. Nechiporenko, V. M. Krivoruchko, and A. S. Mttrofanov, Fiz. Metal. i Metalloved., 17, 6 (1964). K. Hauffe and H. Pfeiffer, Zs. Metallkunde, Bd.44, H. 1 (1953).

A. R. Cox and R. Brown, J. Less-Common Metals, 6, No. 1 (t964).

The Use of Vacuum in Metallurgy [in Russian], Moscow, Acad. Sci. USSR Press (1963), p. 168.

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