Aging Effects on the Durability of Tantalum Nitride Films
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at. Res. Soc. Symp. Proc. Vol. 505 ©1998 Materials Research Society
determined using a tip shape factor verified by direct imaging and used with the slopes from the unloading curves to determine stiffness and elastic modulus following the method of Oliver and Pharr. [6] The resistance to interfacial fracture was determined on the Nano Indenter IITm configured to control normal loads and lateral indenter displacements. These tests employed a conical diamond indenter with a nominal 1pnm tip radius and a 90* included angle that was simultaneously driven into the films at a rate of 500 pN/s and across the films at a rate of 0.5 pm/s until a portion of the film spalled from the substrate. During each test, the normal and tangential loads, the indenter depth, and the lateral position were continuously monitored. Residual stresses were determined using x-ray diffraction. Strain energy release rates at fracture were estimated using the circular blister analysis of Hutchinson and Suo. [8] RESULTS AND DISCUSSION The microstructure of both the polycrystalline aluminum nitride substrates and the tantalum nitride films were examined using SEM and TEM techniques. Both techniques revealed yttriumrich phases embedded in the aluminum nitride substrates as shown in Figure la. This result was expected as yttrium oxide is commonly added to aluminum nitride to improve both processing and heat transfer properties. The yttrium oxide can react to form several different yttrium aluminates. Our analyses have identified the presence of at least Y 2 0 3 "AI20 3 (yttrium aluminum Perovskite or YAP). There was no discernible effect of these second phases on the structure, composition, or thickness of the tantalum nitride films. [9,10] The structure of all films consisted of a 200-nm-thick amorphous layer near the interface followed by columnar grains extending to the surface. (Fig. lb) The columnar grains consisted of hexagonal close-packed tantalum atoms with interstitially distributed nitrogen. These grains formed with a [00021 crystal orientation parallel to the substrate normal. Some fine-scale porosity was also identified along the columnar grain boundaries. [9] Electron diffraction measurements identified the polycrystalline portions of the films in this study as Ta 2N in the trigonal form (space group P3 lm; lattice parameters: a70.5255 nm and c=0.4919 nm). The compositions of the films maintained a two-to-one ratio between tantalum and nitrogen. In both amorphous and crystalline microstructures.
(b)
(a)
Figure 1. (a) The AIN substrates consist of yttrium-rich phases embedded in the aluminum nitride. Nevertheless, the films were remarkably uniform in thickness. (b) The structure of all films consisted of a 200-nm-thick amorphous layer near the interface followed by columnar grains extending to the surface.
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The sputter deposition did produce a high structural defect content and high compressive residual stresses which can change with time and adversely affect reliability. Following .the approach of Sun, Tisone, and Cruzan, [11]
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