Structure and Strength of Multilayers
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Experimental Techniques Growth Superlattice thin films are readily deposited by vapor-phase techniques such as sputter deposition, evaporation, and chemical vapor deposition, as well as by electrochemical deposition. Superlattice deposition Systems are similar to conventional film deposition Systems, except
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for the provision to modulate the fluxes and thereby produce alternating super lattice layers. For the metallic and nitride superlattices discussed here, sputter de position and evaporation have been the primary methods used. Modulation of the source fluxes has been achieved either by movable shutters placed between the sputter sources and Substrate, or by a Substrate that moves between the two sources. The fixed-substrate approach has the disadvantage that the superlattice layer thicknesses are nonuniform when deposited over large-area Sub strates,1,2 but has the advantage that it is easier to heat and bias the Substrate. For nitrides, reactive sputtering of metallic targets in nitrogen is commonly used be cause it provides stable, relatively high deposition rates a n d because readily available elemental-metal sputtering targets are used. Textured polycrystalline or epitaxial metal multilayers can be grown by appropriate choice of Substrate, seed layers, a n d g r o w t h conditions. 3 , 4 A typical growth sequence for an epitaxial multi layer would be growth of a seed layer onto a single-crystal oxide Substrate at relative high temperature, for example, Pt on sapphire (0001) at 600°C, followed by the growth of the superlattice at lower temperatures to reduce interdiffusion. A textured polycrystalline metal multilayer can be grown with a similar sequence, only grown on an amorphous Substrate such as oxidized Si or glass, with a seed layer sometimes used to induce the desired growth texture. 4
Mechanical Properties Investigation of the surface hardness was performed using an indentation load-depth sensing apparatus commercially available as a Nanoindenter™. 5 " This instrument directly measures the
load on a triangulär pyramid diamond indenter tip as a function of displacement from the surface. Hardness is determined from the load data using the relation H = L(h)/A(h),
(1)
where L(h) is the measured load and A(h) is the projected area of the indent as a function of the plastic depth h. The area function is determined by an iterative process involving indents into materials of known isotropic properties. 6 Typically, measurements are made under a constant load rate of about 20 mN/s to a nominal depth of about 1/10 of the film thickness/ several indents are made on each sample, and the data is averaged. As discussed by Doerner and Nix,8 the actual depth of the indent includes the plastic depth, as well as the elastic recovery of the material as the indenter is removed. Following their analysis, the unloading portion of the curve was used to estimate the elastic contribution to the total displacement. This analysis assumes a homogeneous material rather than a layered surface structure, but the limitati
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