Resolving self-limiting growth in silicon nitride plasma enhanced atomic layer deposition with tris-dimethylamino silane precursor

Self-limiting character of the involved surface reactions is essential for highly uniform and conformal growth in atomic layer deposition (ALD). However, the poor conformal coverage (<75%) that is often reported with silicon nitride (SiN_x) plasma enhanced ALD (PEALD) processes using metalorganic Si-precursors is confounding. In this article, we report our study of the SiN_x PEALD process using the tris-dimethylamino silane (3DMAS) precursor and forming gas (5% H₂–95% N₂) reactant plasma. For the substrate temperature (T_sub) range of 50 °C ≤ T_sub ≤ 150 °C, growth per cycle (GPC) for SiN_x deposition was found to approach saturation at 0.034 ± 0.001 nm/cycle though higher T_sub required longer 3DMAS exposures (t_A) for saturation. However, for T_sub > 150 °C, SiN_x GPC was seen to increase with t_A, indicating nonself-limiting growth from potential chemical vapor deposition-like side reactions emerging at higher temperatures. The refractive index (n) of 2.097 ± 0.003 at 2 eV with an optical bandgap of ∼1.7 eV determined from in situ spectroscopic ellipsometry measurements, and peaks s1 and n1 with ΔBE = 295.42 eV in Si2p and N1s XPS spectra measured on the capped SiN_x sample were found to agree with the optical constants and chemical characteristics reported for the silicon nitride material. SiN_x films deposited at T_sub = 250 °C (nonself-limiting) were found to be more resistant to ambient oxidation as compared to SiN_x PEALD films grown at T_sub = 100 °C. Although an entire 30 nm thick SiN_x PEALD film was oxidized after an unavoidable long ambient exposure, a cross-sectional transmission electron microscope image showed a conformal coverage of 95%–98% in a 3D trench structure with an aspect ratio of 4.5. Furthermore, higher resistance to ambient oxidation in plasma treated of SiN_x PEALD films demonstrates a potential of postgrowth treatments to improve desirable material properties without resorting to high-temperature processes.