Metastable Phase Formation in Thin Films and Multilayers
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LETIN/FEBRUARY1990
electronic, mechanical, and structural properties of metal-metal interfaces. Metal-Metal S y s t e m s Solid-State Reactions Since the pioneering work of Schwarz and Johnson, 1 amorphous métal alloys hâve been formed by solid-state amorphization reactions (SSAR) in m a n y alloy Systems. 2 In this process, thin films consisting of alternate layers of crystalline elemental constituents are transformed to an amorphous alloy during a solid-state reaction (see Figure 2). It is driven by a large négative heat of mixing between the constituents, which results in the mixed amorphous alloy having a lower free energy than that of the two-phase elemental structure. The lowest free energy state consists of one or more crystalline phases, but thèse are not formed due to kinetic limitations. Several aspects of the reaction hâve been illuminated. Early work revealed that the reaction was proceeded by a diffusion limited layer growth process. 4 This was confirmed by transmission
électron microscopy (TEM)5 and differential scanning calorimetry. 6 A second important aspect of the SSAR process concerns the détails of the diffusive movement of the species. Kirkendall voids observed in the TEM investigations suggested that the late transition métal was the dominant diffuser in the amorphous phase. 5 This was confirmed by Rutherford backscattering marker movement studies in Ni/Zr. 7 This observation explains why the atomic mobility evidenced by growth of 1,000 Â of amorphous phase does not resuit in nucleation of a crystalline phase. Formation of a critical nucleus of an intermetallic compound requires the collective motion of both species. The absence of mobility of the slow diffusing species suppresses the formation of intermetallic nuclei while the fast diffusing species promote intermixing to form an a m o r p h o u s phase. 8 Nucleation sites for the amorphous phase are provided by the disorder found in as-deposited interfaces3'9'10 and grain boundaries. 11 ' 12 It is interesting to note that both fast diffusion13 and disordered interfaces14'15 hâve been correlated with a large size différence between the constituents. The diffusion-controlled growth rate of the amorphous phase will decrease with time as the thickness of the growing amorphous layer increases. Structural relaxation in the growing amorphous phase will exacerbate the decrease in growth rate. 3 This places a constraint on the thickness of the starting crystalline layers which can be completely converted to amorphous phase. Meng et al." pointed out that as the growth rate of the amorphous phase decreases, nucleation of the more thermodynamically stable crystalline phase will eventually occur on a time scale comparable to further growth of the amorphous phase. The thickness of a m o r p h o u s p h a s e which can be grown before crystalline phase nucleation occurs is typically
\\\\\\s Thin Film Media
15|j.rn JL
N S G -0.3 (xm 3 C
Amorphous Nickel-Phosphorus
- J Magnetic Head
« Carbon Coating
J_^ Co-Cr Magnetic Thin Film (40 nm)
(25 nm)
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