Phase formation sequence induced by deposition temperatures in Nb/Si multilayers
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Phase formation sequence induced by deposition temperatures in NbySi multilayers Ming Zhang Department of Mechanic Engineering, Tsinghua University, Beijing 100083 and Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China
W. K. Wang Institute of Physics, Chinese Academy of Sciences, Beijing 100080 China (Received 7 February 1997; accepted 2 December 1997)
The phase formation sequence in NbySi multilayers formed at different deposition temperatures was investigated by x-ray diffraction (XRD) and transmission electron microscopy (TEM). The amorphous phases were found to form in NbySi multilayers deposited at room temperature and 560 ±C, but the compositions of these two amorphous phases were different. The crystalline Nb3 Si and Nb5 Si3 were formed in NbySi multilayers deposited at 180–500 ±C. The interfacial energy and modified heat of formation are adopted to explain our obtained results. The occurrence of crystalline Nb5 Si3 , NbSi2 , and amorphous silicide phase was found when the NbySi multilayers with Nb3 Si phase were annealed at 550 ±C, while only NbSi2 was found to form when annealing this sample at 700 ±C. The mobility of Si takes an important role in phase formation in NbySi multilayers. I. INTRODUCTION
In recent years silicide has attracted much attention due to its practical applications in microelectronics devices as well as in basic research. At present, one of the standing problems in the interfacial reactions of thinfilm couples is the identification of the first nucleated phase and subsequent phase sequence.1–5 During the past decade, phase evolution induced by low-temperature annealing in many melts/silicon systems has been researched extensively.6–9 However, the kinetic and thermodynamic constraints on multilayer structure and phase formation sequence during the nonequilibrium deposition have not been understood well. Recently, the effects of Ar pressure and temperature during deposition process on the structure of multilayers have been investigated.10–12 Like many metal/Si multilayer systems,13–15 the amorphous interlayers in the NbySi system were generally formed prior to the formation of crystalline silicide because of the negative heat of formation from two elemental phases; the compositional ratio of Nb to Si in amorphous interlayer was found to be about 1 : 2.16 Despite other compounds in the equilibrium phase diagram, only one crystalline phase, NbSi2 , has been reported so far to form when as-deposited NbySi thin films are annealed at very high annealing temperature,17,18 which can be explained by the theory that the growth of the silicide is a kinetic phenomenon.19,20 However, in a more recent paper,21–24 Nb3 Si was identified as the first-nucleated phase at the annealing temperature of 550 ±C and multiphases of Nb3 Si, Nb5 Si3 , and NbSi2 were observed to form simultaneously at the annealing temperature of J. Mater. Res., Vol. 13, No. 5, May 1998
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650 ±C, while only
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