Deposition of Large-Grain Polycrystalline Aluminum Nitride at Low Temperature Via Bias-Enhanced Atomic Layer Annealing

Abstract

Three-dimensional integration of microelectronic devices and high-power radio frequency devices requires the integration of electrically insulating heat spreaders. Aluminum nitride is one potential material, as it can be deposited within backend thermal budgets and conducts heat isotopically, as opposed to diamond which requires deposition temperatures in excess of 900 °C and hexagonal boron nitride which only conducts heat laterally. Growth of crystalline aluminum nitride on non-lattice matched substrates is possible using bias-enhanced atomic layer annealing, in which a plasma treatment step with controlled ion energy follows each cycle of traditional atomic layer deposition precursor dosing. Tris(dimehtylamido) aluminum was used as the aluminum precursor to minimize carbon contamination, and anhydrous hydrazine was used to prevent oxygen contamination. Plasma treatment with neon, krypton, and argon at substrate biases of -10V, -25V, and -40V were investigated. Varying the gas identity and substrate bias provides control over the momentum and kinetic energy of ions bombarding the surface, resulting in selective control over the crystalline orientation. Across all gas and bias combinations ion flux is held constant by adjusting the power applied to the plasma. In-situ x-ray photoelectron spectroscopy (XPS) is used to study the elemental composition of the samples. Films deposited with this technique routinely show carbon and oxygen content less than 3% by XPS. Ex-situ grazing-incidence X-ray diffraction (GI-XRD) and x-ray reflectivity (XRR) are used to analyze the crystallinity, thickness, and density of the deposited material. 40 nm thick films were deposited onto HF-etched Si(111) and SiC substrates. GI-XRD scans of each condition are shown in Figure 1. Atomic layer annealing on Si(111) substrates with neon plasma produced films with poor crystallinity and preferential (002) orientation. For these films, the AlN (002) peak position is shifted relative to the peak position in films treated with argon and krypton, indicating the deposition of strained films. This may be due to embedding of neon ions, causing relatively small crystallite sizes of 5 nm to 9 nm. Films deposited with krypton plasma treatment also showed preferential AlN (002) deposition, though crystallite sizes are large and range from 9 nm to 11 nm. Optimal crystallization occurred when using argon plasma treatment with a -25V substrate bias, producing a film with selective (200) orientation and an average crystallite size of 27 nm. This crystallinity resulting from this condition may be the result of enhancing the preferential (200) crystallinity afforded by the purely thermal atomic layer deposition of the same precursors. Accordingly, the impinging argon ions would have significant momentum to increase surface adatom mobility, but kinetic energy sufficiently low as to not change the preferential orientation. Figure 1