Low-Temperature Deposition of Large-Grained Polycrystalline AlN Using Trisdimethylamido Aluminum (TDMAA) and Anhydrous Hydrazine

Abstract

High quality, large-grained AlN films are of interest for use as either heat spreading layers or as a buffer/templating layer for further film growth. As the typical deposition temperature for high-quality crystalline AlN is usually in excess of 800 oC, there is a large amount of thermal strain in the films and it is advantageous to develop processes to lower the deposition temperature so that it is compatible with back end of line (BEOL) processing. In this work, the low-temperature (<400 oC) deposition of polycrystalline AlN films is demonstrated by atomic layer annealing (ALA) which is a variant of ALD that utilizes a third pulse of low-energy inert gas ions in addition to the usual metal and co-reactant pulses [1].Using trimethyl aluminum (TMA) or tris(dimethylamido) aluminum (TDMAA) with the highly reactive nitrogen-containing precursor hydrazine (N2H4), AlN can be deposited at ~200 oC [2]; however, these films are amorphous and would have low thermal conductivity due to a high degree of phonon scattering. Using TDMAA with N2H4 or NH3 at temperatures >350 oC, polycrystalline films can be deposited in a purely thermal process; however, the reported grain sizes are small (<5 nm) or there is a mixture of polycrystalline and amorphous phases [3-4]. ALA has been used to deposit crystalline films such as AlN [1,5] and GaN [6] on a near lattice-matched substrate (sapphire) at low temperatures, but a nitrogen-containing plasma was used.In the present study of AlN ALA on Si (111) (a non-lattice matched substrate), two metal precursors (TMA and TDMAA) were compared using ultra-high purity anhydrous N2H4 as a co-reactant and argon ions with tuned energy for the third pulse. It was found that deposition using TDMAA as the Al precursor resulted in high-quality AlN films with large grain size (>20 nm) and low C/O contamination (<1.5%) whereas films deposited using TMA had much higher carbon content (>5%), owing to its thermal instability at 400 oC. Transmission electron micrographs for a 40 nm film grown using TDMAA and N2H4 show vertical grain structure with grains spanning the entire thickness of the film. As heat spreading layers often need to be in excess of 1 micron thick in order to be relevant for use in high volume manufacturing, it is also demonstrated that these 40 nm films successfully transfer the large grain size of the ALA film to a thick sputtered AlN film (which can be deposited relatively quickly) by acting as a template layer.[1] H-Y. Shih et al, Scientific Reports 7:39717.[2] M. Mizuta et al, Japanese J. Appl. Phys. 25(12), L945-L948 (1986).[3] R. G. Gordon, U. Riaz, and D. M. Hoffman, J. Mater. Res., 7(7) (1992).[4] A. I. Abdulagatov et al Russian Microelec. 47(2), 118-130 (2018).[5] W-C. Kao et al, RSC Adv. 9, 12226-12231 (2019).[6] W-H. Lee et al ACS Sustainable Chem. Eng. 7,1, 487-495 (2019).This work was supported in part by the Semiconductor Research Corporation. Figure 1