Poster session II

Size dependent phenomena during the formation of Gd and Fe silicide thin films

G.L. MolnBr - G. Pet6 - Z.E. HorvBth - E. Zsoldos - N.Q. KhBnh KFKI Research Institute for Materials Science, H-1525 Budapest, P.O.Box 49 (Hungay)

Tel. : +36 I 3959045 -Fax : +36 I 3959284 -E-mail : molnar.g@ra.atki.yki.hu

Thin films of metal silicides have attracted attention because of their scientific curiosity and technical importance. The applicability in very-large scale integrated electronic circuits as interconnects, contacts and electrodes motivate the studies of the characteristics and properties of different type silicides. With the continuous scaling down of device dimensions. the silicide film thickness decreases, entering a border, where size dependent phenomena occur during the solid phase reaction of metal film and Si substrate.

The purpose of this work is to study the effect of film thickness on the evolution of reaction product in case of two different type metals (gadolinium and iron) silicide, in order to develop a kind of phase formation engineering.

Gd-silicide belongs to the rare-earth-metal silicide films, which are attractive materials since they form the lowest Schottky barrier on n-type silicon, and are potential materials for infrared sensors. The P-FeSi2 has a semiconductive direct band gap of 0.87 eV, for this reason it is a potential material of optoelectronic applications in silicon integrated technology.

The formation of phases was investigated during the solid phase reaction of Gd thin film with (1 1 1) and (100) oriented Si substrate and Fe thin films with Si( 11 1) substrate as a function of thickness and annealing by X-ray diffraction, Rutherford backscattering and transmission electron microscopy.

For Gd films under 30 nm thickness, the phase formation was affected by the substrate orientation, The first phase was amorphous on Si( 100). At higher temperatures epitaxial hexagonal GdSil was found on Si( 11 l), while on Si( 100) epitaxial orthorhombic GdSi2 was formed. For thicker gadolinium films on Si( 11 1) a conventional diffusion-reaction process appeared. The hexagonal GdSil phase formed first and then transformed to the second phase (orthorhombic GdSiz). The ratio of these phases as a function of thickness could be described by a model. On Si(100) substrate at each thickness and annealing only orthorhombic GdSi- phase was formed.

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Another type of phase evolution could be experienced in case of Fe-Si solid phase reaction at constant annealing temperature and time (700°C, 7 minutes) as a function of the initial iron film thickness (from 5 nm to 27.5 nm in 2.5 nm steps).

After annealing FeSi phase was detected in the thinner samples. Samples with Fe layer thicker than 12.5 nm contained a P-FeSi:! phase formed by nucleation controlled mechanism. This special phase sequence was explained with the help of a model, based on the critical radius of nuclei of the new phase.

The phase formation depended on the time and temperature of the annealing and even on the initial metal film thickness and substrate orientation. The advantages of using the film thickness as a variable during investigation of solid phase thin film reactions are discussed particularly.

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