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

Abstract

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 (SiNx) plasma enhanced ALD (PEALD) processes using metalorganic Si-precursors is confounding. In this article, we report our study of the SiNx PEALD process using the tris-dimethylamino silane (3DMAS) precursor and forming gas (5% H2–95% N2) reactant plasma. For the substrate temperature (Tsub) range of 50 °C ≤ Tsub ≤ 150 °C, growth per cycle (GPC) for SiNx deposition was found to approach saturation at 0.034 ± 0.001 nm/cycle though higher Tsub required longer 3DMAS exposures (tA) for saturation. However, for Tsub > 150 °C, SiNx GPC was seen to increase with tA, 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 SiNx sample were found to agree with the optical constants and chemical characteristics reported for the silicon nitride material. SiNx films deposited at Tsub = 250 °C (nonself-limiting) were found to be more resistant to ambient oxidation as compared to SiNx PEALD films grown at Tsub = 100 °C. Although an entire 30 nm thick SiNx 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 SiNx PEALD films demonstrates a potential of postgrowth treatments to improve desirable material properties without resorting to high-temperature processes.