Ruthenium metal has a low bulk resistivity (7.1 μΩ cm), a high work function (4.7 eV), and a low solid solubility with strong adhesion to Cu, and is etchable, making Ru an attractive barrier metal or seed layer for Cu electroplating.
1 For these applications, conformal deposition on the high-aspect ratio structures encountered in the back end of line (BEOL) of ultra largescale integration (ULSI) circuits is required. Atomic layer deposition (ALD), based on purge-separated dosing of precursors and self-saturating surface reactions, allows for atomic scale control and inherent high conformality. Because of these advantages, ALD processes for Ru are strongly desired. Although a number of ALD processes for Ru have been reported, currently the most widely reported Ru ALD precursor, Ru(EtCp)
2, as well as other non-zero oxidation state precursors show long nucleation delays (~ 200 ALD cycles) prior to film growth. As nucleation delays can result in island growth and increased roughness and Ru films used in ULSI applications are generally only a few to tens of nanometers thick, nucleation delay is a critical growth parameter. More recent work has shown that zero oxidation state Ru precursors can significantly reduce nucleation delays for O
2-based thermal ALD processes. However, the resistivities of these films are generally 18-40 µ-ohm cm, well above the bulk value. Very recently, a zero oxidation state ALD Ru process based on η
4-2,3- dimethylbutadiene ruthenium tricarbonyl [Ru(DMBD)(CO)
3] and O
2, showed zero nucleation delay, a surface roughness of 0.6 nm RMS, and a resistivity of 14 µΩ-cm.
2 Although the resistivity of films deposited using this Ru(DMBD)(CO)
3 / O
2 process is among the lowest values reported for ALD Ru, it still larger than the resistivity of pure ruthenium. It was speculated that residual oxygen resident in the film was the cause of the higher than bulk resistivity. In this work, we investigate the impact of annealing on the structure and electronic properties of ALD Ru.
ALD Ru was deposited on either Si or Si/SiO2 substrates in a Picosun R-200 SunALE reactor at 260˚C using pure separated pulses of Ru(DMBD)(CO)3 & O2. Ru films were annealed in a reducing forming gas (4% H2 in N2) environment as well as an inert pure N2 environment at 400, 450, and 500C for various times. Resistivity is characterized via 4-pt probe, roughness via atomic force microscopy (AFM, crystallite size via x-ray diffraction (XRD), thickness and density via x-ray reflectivity (XRR), and composition via x-ray photoelectron spectroscopy (XPS). In addition, ruthenium-insulator-semiconductor (MOS) capacitors were used to extract the effective work function of the Ru.
We find that post-deposition annealing reduces resistivity. For 500 C annealing in a 4% H2 / N2 ambient, resistivity was reduced from an as-deposited value of ~15 µΩ-cm to less than 10 µΩ-cm after 30 minutes and ~9 µΩ-cm after 60 mi, closely approaching the bulk value. These results indicate that ALD of Ru using Ru(DMBD)(CO)3 & O2 appears very promising for microelectronics applications.
- R. W. Powell, R. P. Tye, and J. Woodman Margaret, Platin. Met. Rev., 6, 138–143 (1962).
- D. Z. Austin et al., Chem. Mater., 29, 1107–1115 (2017).
