Crystallization of Amorphous Tungsten Disilicide: Stacking Faults and Resistivity
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F.K. LeGoues, F.M. d'Heurle, R. Joshi IBM T.J. Watson Research Center, PO 218, Yorktown Heights, N.Y. 10598 and Ilka Suni Technical Research Center of Finland, Semiconductor Laboratory Otakaari 7B, SF-02150, Fspoo, Finland
ABSTRACT Films of WSi, have been investigated by electron microscopy after annealing at different temperatures. It is shown that films annealed at 550'C contain a high density of stacking faults. The crystalline identity of these stacking faults has been established. Undoubtedly the high density of stacking faults is the cause of the resistivity maximum observed in films annealed at 550'C.
INTRODUCTION Regardless of their mode of preparation, films of the refractory metal disilicides from TiSi, to WSi, are amorphous when deposited on substrates at low temperatures 11-31. The exception of silicides formed by ion implantation [4,51 need not concern us here. The resistivity of these silicides in the as-deposited condition varies, depending on the specific silicide, about a mean value of 500 pf9cm. Upon heating, crystallization occurs in a range of temperature from about 400'C to 500'C. A relatively sharp drop in resistivity, from the value characteristic of the amorphous state to a value approaching that of the well-annealed crystals (below 50 gffcm) is observed at a temperature sonic 50' above the temperature where x-ray diffraction peaks become observable. It is quite likely that the sharp resistivity drop is delayed with respect to the initial crystallization temperature because a low resistivity requires not only the crystalline state but also long range crystalline order, which is disturbed for example when the grain size is too small. One notes that all the disilicides which are of concern here share the same basic atomic structure [6]: they are constituted of stoichiometric planes of hexagonally packed atoms in which each metal atom is surrounded by six Si atoms. These planes can be stacked in different order, ABA in the normal tetragonal phase of WSi, and MoSi,, ABCA in CrSi, or TaSi,, and ABCDA only in TiSi_. WSi, and MoSi, alone can exist in two forms, hexagonal and tetragonal [7-101. The coordination number is only 10 rather than 12 as in many simple metals; the metal atoms are entirely surrounded by Si atoms so that there are no metal nearest neighbors (a condition which is similar to that of the Au atoms in ordered CuAu). When WSi, crystallizes it does so at low temperatures in the hexagonal form, then it changes to the usual tetragonal form above a transition temperature of about 550'C [7,81. It has been observed [7] that when WSi, is annealed its resistivity, measured at room temperature, does not decrease continuously as a function of the annealing temperature as, for example, for TaSi, I 11. Rather, the already high resistivity of the as-deposited films initially undergoes a measurable increase, and passes through a maximum for an annealing temperature corresponding roughly to the transition from the hexagonal to the tetragonal phase. The similarity between the transformations in
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