Cobalt Metal ALD: Understanding the Mechanism and Role of Zinc Alkyl Precursors as Reductants for Low-Resistivity Co Thin Films

Abstract

In this work, we report a new and promising approach toward the atomic layer deposition (ALD) of metallic Co thin films. Utilizing the simple and known CoCl2(TMEDA) (TMEDA = N,N,N′,N′-tetramethylethylenediamine) precursor in combination with the intramolecularly stabilized Zn aminoalkyl compound Zn­(DMP)2 (DMP = dimethylaminopropyl) as an auxiliary reducing agent, a thermal ALD process is developed that enables the deposition of Zn-free Co thin films. ALD studies demonstrate the saturation behavior of both precursors and linearity depending on the applied number of cycles as well as temperature dependency of film growth in a regime of 140–215 °C. While the process optimization is carried out on Si with native oxide, additional growth studies are conducted on Au and Pt substrates. This study is complemented by initial reactivity and suitability tests of several potential Zn alkyl-reducing agents. For the CoCl2(TMEDA)–Zn­(DMP)2 combination, these findings allow us to propose a series of elemental reaction steps hypothetically leading to pure Co film formation in the ALD process whose feasibility is probed by a set of density functional theory (DFT) calculations. The DFT results show that for reactions of the precursors in the gas phase and on Co(111) substrate surfaces, a pathway involving C–C coupling and diamine formation through reductive elimination of an intermediate Co­(II) alkyl species is preferred. Co thin films with an average thickness of 10–25 nm obtained from the process are subjected to thorough analysis comprising atomic force microscopy, scanning electron microscopy, and Rutherford backscattering spectrometry/nuclear reaction analysis as well as depth profiling X-ray photoemission spectroscopy (XPS). From XPS analysis, it was found that graphitic and carbidic carbon coexist in the Co metal film bulk. Despite carbon concentrations of ∼20 at. % in the Co thin film bulk, resistivity measurements for ∼22 nm thick films grown on a defined SiO2 insulator layer yield highly promising values in a range of 15–20 μΩ cm without any postgrowth treatment.

Co(II) Amide, Pyrrolate, and Aminopyridinate Complexes: Assessment of their Manifold Structural Chemistry and Thermal Properties**

“…Compounds 3-6 demonstrated solution effective magnetic moments μ eff of 4.26 μ B to 4.75 μ B (Table 1) which is in a range often observed for [Co(TMSA) 2 L] type complexes. [13,16,18] When 2 was synthesized and DAD added in situ, the unexpected ligand-separated salt [Li(DAD) 2 ][Co(TMADS) 3 ] resulted (7). It is a rare example of an ion separated Co(II) cobaltate salt with silylamide ligation and Li + counter cation (Scheme 3) and was obtained by crystallization in a yield of 58 %.…”

“…With 2.6133(5) Å, the Co•••Co interatomic distance is slightly elongated in comparison to 1 (2.5864(5) Å). The bridging and terminal CoÀ N binding lengths were slightly shorter with 1.9957(8) Å and 1.8907 (7) Å respectively (Table 3). Consideration of the Co 2 N 2 core structures revealed a pronounced difference between 1 and 2.…”

“…Likewise, no alternation of the Lewis base backbone in 4 was observed. With Σ 0 = 359.88(6) for 3 and Σ 0 = 359.31 (7) for 4, the two Lewis adduct complexes exhibit an expected trigonal planar geometry at their Co(II) ions. Interestingly, the N1À Co1À N3 and N2À Co1À N3 angles which span between the TMSA ligands and the Lewis base are nearly the same for 3 (115.500(3)°and 114.561(29)°) while they are considerably different for 4 (129.118(586)°and 98.406(646)°).…”

“…Selected interatomic distances and angles of complexes 1, [13] character of the two DAD ligands. For the Co(TMADS) 3 À anion, the three ligands spanned around the Co(II) central ion with Σ 0 = 359.97 (7). The average CoÀ N bond length of 1.94 Å was intermediate compared to 2 for terminally coordinating (1.8907(7) Å) and bridging (1.9957(8) Å) TMADS ligands.…”

“…A comprehensive study was eventually motivated by our recent interest in the chemistry of Co as a potential interconnect material in microelectronics. [6,7] Even though operational modes and conditions of CVD and ALD processes differ, their complementary and precise applicability for the deposition of a broad range of thin film materials have made them industrially indispensable in fields like microelectronics, optoelectronics and photovoltaics. [8] Choosing the proper precursor is of crucial importance hereby as it requires specific thermal behavior: The precursor needs to meet demanding requirements such as high thermal stability, sufficient volatility, as well as high but controllable reactivity and stability at a surface.…”