Article pubs.acs.org/cm
Atomic Layer Deposition of Ruthenium and Ruthenium Oxide Thin Films from a Zero-Valent (1,5-Hexadiene)(1-isopropyl-4methylbenzene)ruthenium Complex and O2 Hyo Jun Jung,†,‡,∥ Jeong Hwan Han,†,∥ Eun Ae Jung,†,§ Bo Keun Park,† Jin-Ha Hwang,‡ Seung Uk Son,§ Chang Gyoun Kim,† Taek-Mo Chung,*,† and Ki-Seok An*,† †
Division of Advanced Materials, Korea Research Institute of Chemical Technology (KRICT), 141 Gajeong-Ro, Yuseong-Gu, Daejeon 305-600, Republic of Korea ‡ Department of Materials Science and Engineering, Hongik University, 72-1 Sangsu-Dong, Mapo-Gu, Seoul 121-791, Korea § Department of Chemistry, Sungkyunkwan University, Suwon 440-746, Korea S Supporting Information *
ABSTRACT: Ruthenium (Ru) and ruthenium oxide (RuO2) thin films were grown by atomic layer deposition (ALD) using a novel zerovalent (1,5hexadiene)(1-isopropyl-4-methylbenzene)Ru complex and O2 as the Ru precursor and oxidant, respectively. The self-limiting growth mode for the Ru and RuO2 ALD processes was achieved while varying the Ru precursor and O2 feeding time. Metallic Ru films were deposited at growth temperatures of 230−350 °C, while the temperature window for the growth of the RuO2 film was limited to 300 cycles on SiO2 and 500 cycles on TiN when Ru(thd)3 and O2 were employed) at temperatures of 275−400 °C. This poor nucleation behavior hinders the practical use of the Ru precursors listed above for microelectronics applications because a long incubation time leads to a rough surface morphology and undesired oxidization of the sublayer during nongrowing cycles due to prolonged exposure to the O2 reactant. For mass production applications, moreover, it is necessary to minimize unproductive time because of the high cost of Ru. Hence, to improve the nucleation properties at the early growth stage, a great deal of research has been performed via two primary ways: (1) substrate modification by pretreatment or coating a nucleation enhancing layer and (2) synthesis of a novel Ru precursor which exhibits enhanced chemisorption Received: September 26, 2014 Revised: November 20, 2014 Published: December 2, 2014 7083
dx.doi.org/10.1021/cm5035485 | Chem. Mater. 2014, 26, 7083−7090
Chemistry of Materials
Article
Scheme 1. Synthesis of (1,5-Hexadiene)(1-isopropyl-4-methylbenzene)Ru Complex
at 25 °C. Then, 1,5-hexadiene (10.7 g, 0.13 mol) was added to the solution followed by refluxing for 15 h. After cooling to room temperature, the volatiles were removed in vacuum, and the residue was extracted into hexane. After removing solvent, the crude product was then distilled (100 °C/10−1 Torr) to obtain the pure product as yellowish brown liquid (16 g, 80%). The final complex is a heteroleptic compound containing η4-1,5-hexadiene and η6-1-isopropyl-4-methylbenzene. Scheme 1 exhibits the synthetic procedure of the Ru precursor. 1H NMR (C6D6, 300 MHz) δ 1.12 (d, 6H), 1.37 (d, 2H), 1.51 (d, 2H), 1.83 (s, 3H), 2.00 (m, 1H), 3.45 (m, 2H), 4.34 (q, 2H), 4.50 (q, 4H), 4.66 (q, 2H). Elemental analysis: Calcd for C16H24Ru: C, 60.54; H, 7.62. Found: C, 60.91; H, 8.08. 13C NMR (C6D6, 75.5 MHz) δ 21.6, 23.6, 35.8, 39.6, 61.7, 78.1, 79.3, 80.8, 83.3, 85.7. The TGA/DTA characteristics of the synthesized Ru complex were examined under atmospheric pressure as shown in Figure 1. The TGA
probability on a heterogeneous substrate. It appears that substrate modification by pretreatment or coating an additional layer is not applicable to state of the art techniques in highly scaled DRAM technology applications. Therefore, the development of a novel Ru precursor has been emphasized for the actual use of Ru-based thin films in mass production applications. Recently, it was documented that Ru ALD processes using zerovalent Ru precursors exhibited better initial growth properties compared to the use of a di/trivalent Ru precursors.7,13−15 For instance, the incubation time of Ru ALD from (ethylbenzene)(1-ethyl-1,4-cyclohexadiene)Ru(0), (ethylbenzene)(1,3-cyclohexadiene)Ru(0), and (1-isopropyl-4methylbenzene)(1,3-cyclohexadiene)Ru(0) precursors with O2 have been reported to be as short as 2 s should be introduced to obtain the saturated growth mode. Here, in the case of Ru ALD, the determination of self-limited condition with respect to the O2 pulse time should be performed very carefully because the phase change from Ru to RuOx can occur when excessive O2 is applied in the reactor. Therefore, the change of the resistivity as a function of the O2 pulse length was measured (Supporting Information Figure S1). A saturated resistivity of about 40 μΩ· cm implies that no phase transition occurs in the O2 pulse time range of 1−6 s. The impact of the purge time on the film thickness was also investigated where it was concluded that a N2 pulse of 10 s is sufficient for ALD growth (Supporting Information Figure S3). From the above experiments, the ALD sequence for the Ru film was fixed at 9 s (Ru precursor) - 10 s (N2) - 2 s (O2, 200 sccm) - 10 s (N2). Figure 3(a) shows the change of the Ru film thickness grown on SiO2 at a deposition temperature of 270 °C as a function of the number of Ru ALD cycles. The calculated GPC of Ru ALD is 0.076 nm/cycle, which is comparatively higher than that from tmhd- or Cp-containing Ru precursors.8,10 Considering the highly scaled device size of future
Figure 3. (a) Variation of the Ru film thickness grown on SiO2 as a function of the number of ALD cycles. (b) Variation of the Ru layer density as a function of the number of ALD cycles on SiO2, TiN, Al2O3, and Pt. The inset shows an enlarged graph of the data shown in part (b).
microelectronics where a Ru ALD film can potentially be adopted as a bottom and/or top electrode for a DRAM capacitor or as a Cu seed layer for interconnect technology, the 7085
dx.doi.org/10.1021/cm5035485 | Chem. Mater. 2014, 26, 7083−7090
Chemistry of Materials
Article
negligible nucleation retardation, the minimum thickness for a continuous and uniform Ru film can be estimated from steady state GPC to be approximately 3.8 nm. After 300 cycles, the SEM image of the 23 nm thick Ru film showed densely packed small grains and a smooth surface morphology. The AFM measurements revealed that the deposited Ru films for 300 cycles on SiO2 at the temperatures of 180−350 oC show a very smooth surface with an rms roughness of 0.18−0.64 nm (Supporting Information Figure S4). The influence of the growth temperature on the film density and resistivity was examined in the temperature range of 180− 350 °C, as shown in Figure 5(a). The densities of the deposited
achieved GPC appeared to be sufficiently high. As mentioned in the Introduction, meanwhile, another research interest of the Ru ALD process is understanding of the initial growth behavior. This is because ultrathin (
