Properties and Fracture Resistance of Thin Polycrystalline Aluminum Films on Sapphire Substrates
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at. Res. Soc. Symp. Proc. Vol. 403 01996 Materials Research Society
A120 400 nm
5 nm
(a)
(b)
Figure 1. (a) The films were polycrystalline with a log normal grain size distribution and mean grain sizes of 163 nm for the 25"C film and 160 nm for the 250"C film. (b) Each thin film system consists of 90-nm-thick (111) aluminum on a 5-nm-thick amorphous layer of oxide on (0001) sapphire substrates. The films shown were deposited at 25"C. and across the films at a rate of 0.5 gm/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. The fracture energy and interfacial fracture toughness values then were determined from the data using the elastic approach of Venkataraman et al. (6,7) RESULTS AND DISCUSSION The microstructures of the films deposited at 25"C and 250"C were essentially identical. Planview TEM (Figure I a) showed that the films were polycrystalline. Measurement of 600 grains in each film showed that there was a log normal distribution of grain sizes with a mean grain size of 163 nm for the film deposited at 25°C and 160 nm for the film deposited at 250"C. These results are consistent with earlier work by Attardo et al. (8) showing that grain size increases in evaporated aluminum films only when deposition occurs above 300'C. (8) Cross-sectional TEM further revealed that the aluminum films were deposited on 5 nm thick surface layers of amorphous aluminum oxide (Figure lb) that were present on all (0001) sapphire substrates used in this study. Although these films were deposited on amorphous layers, the (111) planes of the aluminum films were parallel to the (0001) planes of the sapphire substrates. There were no in-plane preferred orientations. Nanoindentation showed that the elastic moduli and hardness values of the two films superimposed but exhibited very different behaviors with indenter depth. As shown in Figure 2, the measured elastic modulus of each film increased from a value of 50 GPa near the film surfaces to values approaching 500 GPa at depths into the sapphire substrate. The values at the surface are slightly lower than single crystal aluminum measurements while those at depths beyond the nominal depth of the interface approach those for (0001) sapphire substrates. The gradient in measured modulus values from bulk aluminum to single crystal sapphire values also agrees well with the elastic modulus measurements in numerous thin film systems and represents an increasing contribution of the sapphire substrate properties to the measured values. Figure 3 shows that the hardness values for each film also superimposed. However, instead of exhibiting a gradient from single crystal aluminum values near the surface to sapphire values beyond the interface, they exhibited a value of 3 GPa that was between bulk aluminum and sapphire hardness values. The measured values were also independent of film depth. This is in direct contrast with behavior observed in many thin film systems
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