ALD/ALE 2021 Session AS2: Selective ALD by Area-Deactivation

Titanium oxide (TiO₂) area-selective deposition (ASD) has been studied for applications such as hard masks for tone inversion, dielectric layers, solar cells, and photocatalysis due to the etch resistance, high dielectric constant, and high refractive index of TiO₂. While these studies¹ have successfully demonstrated several nanometers of selective deposition using various passivation, activation, and defect mitigation techniques, there is still work needed to understand selectivity loss mechanisms in order to further expand selectivity windows to reach commercial requirements. Herein we evaluate selectivity loss mechanisms of TiO₂ deposited on trimethyl-silane passivated SiO₂ substrates from TiCl₄/H₂O.

Dimethylamino-trimethylsilane (DMA-TMS) passivates SiO₂ substrates with a 300 s exposure at 250 °C while Si, TiO₂, TiN, and Ru surfaces remain unaffected.² This DMA-TMS passivated SiO₂ limits TiO₂ surface coverage to ~5% after 100 cycles (3.8 nm on the growth surface) at 150 °C, as characterized by scanning electron microscopy (SEM) and Rutherford backscattering spectrometry (RBS) (Fig. 1).

Nuclei on the non-growth surface were analyzed with SEM and atomic force microscopy (AFM) to gain insight on nucleation mechanism, adsorption kinetics, and possible impact of diffusion (Fig. 2). Particle size distributions (PSDs) reveal a high concentration of small nuclei even for high cycle numbers. This indicates new nuclei generation on the non-growth surface during deposition. Kinetic modeling³ was employed to fit experimental data according to this information using an estimated site generation rate of 4.6x10^-5 sites cm^-2 cycle^-1 and a 30-cycle nucleation delay. Modeling results correspond to a thickness of ≈4 nm TiO₂ while maintaining 90% selectivity (t_S = 0.9 = 4.02 nm).* Furthermore, we compare PSDs at low (150 °C) and high (300 °C) deposition temperatures to evaluate changes in species diffusivity and loss of effectiveness in the passivation layer. Finally, this insight is applied to develop techniques for extending TiO₂ selectivity windows, including repeated passivations and periodic etching to mitigate defects. The improved understanding on TiO₂ selectivity loss identifies effective strategies to increase selectivity to reach advanced technology nodes.

With feature dimensions of state-of-the-art semiconductor devices moving toward the 5 nm node and beyond, device fabrication based on top-down approaches is becoming ever more challenging. Recently, area-selective atomic layer deposition (ALD) has evolved as a promising method to fabricate well-defined patterns from the bottom-up with atomic-scale accuracy. Within the different approaches for area-selective ALD, the preparation of patterned substrates with material-selective organic functionalization offers a promising direction for achieving self-aligned patterns over large areas. In this study, we focus on area-selective ALD of Al₂O₃ on functionalized SiO₂ with small-molecule aminosilanes through the vapor phase. Using in situ optical surface diagnostics, we investigate the atomistic-scale mechanism that leads to the reaction of the ALD precursors with the functionalized SiO₂ surface.