ALD2023 Session AS1-TuM: Surfaces and ASD

Since the initial adoption of atomic layer deposition in capacitor dielectrics for DRAM, and for gate dielectrics in logic at 45 nm, ALD has played a transformative role in enabling a wide variety of integration, without which the industry would have been unable to scale.However, over the last 15 year the number of steps required to process a chip has dramatically increased and integration has become increasingly complicated.

Selective deposition can offer enabling and/or simplified integration approaches, and while there has been strong adoption of some selective processes (e.g. selective epitaxy of Si and selective capping of copper with CVD Co), there remains a tremendous opportunity for the industry to solve many integration problems with selective ALD processes.

This talk will focus on several high value problems that can be addressed by selective ALD, and the additional steps required to enable these selective processes.A specific case study on leveraging selective ALD to enable higher chip speeds through reducing RC delays by leveraging an integration scheme enabling selective ALD TaN will be examined.In this particular case, the selective barrier can reduce via resistance by >50% with no reliability degradation.

Finally, the lessons learned in enabling the selective ALD TaN process will be highlighted as they relate to enabling the many other selective ALD applications that promise to continue enabling device manufacturers roadmaps.

Controlling the deposition of metal films grown on oxides by atomic layer deposition (ALD) requires stringent control over the surface nucleation sites, typically hydroxyl groups. The hypothesis driving this project is that preliminary surface modification steps may be used to alter the hydroxyl surface density and with that, the properties of the ALD metal films. In the case of area-selective ALD, for example, the use of surface modifiers such as silylation agents has been implemented to deactivate all silanol sites on the non-growth surface in order to spatially direct film deposition onto the growth surface. The extent of silylation may also be controlled to tune the ALD in terms of the film density. Partial silylation can help deposit metal nanoparticles of varying sizes by immobilizing them thereby preventing their aggregation and sintering at high temperatures.

The medium in which surface modification reactions, such as silylation, are carried out in is vital to achieve control over the efficiency of the process of blocking nucleation sites. A liquid-phase environment affords little control over the surface modification process and may result in the formation of defects within the organosilane layer. These drawbacks may be eliminated by conducting the modification step in the gas phase, as was tested in this work.

In our XPS study, Si-OH sites in silica surfaces were blocked in a controlled fashion via gas-phase silylation using N,N-dimethyltrimethylsilylamine. The silylation efficacy was assessed by evaluating the kinetics of metal oxide ALD on the resulting substrates. The results from gas-phase silylation were compared to a set of experiments in which the modification step was performed in the liquid-phase. In both cases, the SiO_x/Si(100) surface becomes fairly unreactive during the initial stages of TiO₂ or HfO₂ ALD and remains passivated up to 5 TiO₂ cycles; the passivation is less effective with HfO₂. Nucleation sites are eventually re-activated with increasing cycles, but partial surface passivation is maintained up to 50 cycles. These results suggest that the metal precursor can eventually chemisorb onto defect sites in the silylated layer after sufficient interaction with the surface, propagating film growth. Moreover, as the results obtained with gas- vs. liquid-phase silylation are comparable in terms of ALD passivation, it is concluded that there is an intrinsic limitation to the degree of passivation achievable using this surface modification scheme. Finally, preliminary NMR results with porous silica indicate that the extent of silylation can be tuned to modify the ratio of free vs. capped nucleation sites.

Area-selective atomic layer deposition (AS-ALD) has attracted tremendous interest as an alternative bottom-up patterning process to implement versatile fabrication of selectively formed thin films in both vertical and lateral direction in extremely downscaled 3D semiconductor devices. In this study, we report a methodology for achieving inherently selective deposition of Hf_1-xZr_xO₂(HZO) thin films by catalytic local activation not only on noble metal surfaces like Ru and Pt, but also on the TiN surfaces. For achieving selective deposition on metal surfaces, O₂ gas was utilized as a mildly oxidizing co-reactant and cyclopentadienyl-tris(dimethylamido)hafnium(zirconium) precursors (Hf(Zr)[Cp(NMe₂)₃]) which require strong oxidizing agents were used to deposit HZO thin films. As a results of catalytic dissociation of O₂ molecules, we successfully achieved inherent selectivity greater than ~7 nm on both blanket Ru substrate and Pt/Si patterned substrates. Furthermore, it was demonstrated that the anti-ferroelectric HZO thin films with high dielectric constants of 31 and 34 can be fabricated selectively on TiN and Ru substrate, respectively, through the inherent AS-ALD method using catalytic local activation and ozone post-treatment. It should be noted that this intriguing approach for achieving inherent selectivity expands the potential utility of bottom-up nanopatterning processes for next-generation nanoelectric applications.

Distinct reactivity of unique atomic arrangements (i.e. defects) on material surfaces allows for selective surface chemistry exclusively at those sites. We present an atomic layer deposition (ALD)-based technique of site-selective ALD (SS-ALD) targeting undesirable defect sites. Defects on the TiO₂ and other oxidized surfaces affect the electronic properties, interfaces, and performance of devices utilizing those interfaces. We present first principles calculations to predict the difference in hydration/hydroxylation of pristine TiO₂ terraces and minority atomic configurations including step edges and oxygen vacancies. In situ ellipsometry reveals the nucleation behavior of SS-ALD at process conditions precisely tuned for selective hydroxylation of surface defects. An island growth model for nucleation and atomic force microscopy (AFM) imaging are consistent with a site-selective growth mechanism that depends on defect density.

Iridium (Ir) has low figure of merit (ρ₀λ) and high melting temperature properties, so it has been recently spotlighted as a very important copper (Cu) alternative interconnect material in next-generation semiconductor devices. In particular, in the case of depositing a thin film using atomic layer deposition (ALD) technology, it can be expected to deposit extremely thin film with conformal, uniform and excellent step coverage characteristics even in a very complex structure or a trench structure of several nm dimension due to the inherent self-limiting characteristic of ALD. In this regard, ALD-Ir is considered as one of the most suitable metallization process for the application of advanced semiconductor interconnects. Therefore, considerable efforts have been conducted to develop a reliable ALD-Ir process having improved film qualities. And, ALD-Ir process with excellent thin film properties such as low electrical resistivity and negligible oxygen impurities was recently reported using Tricarbonyl (1,2,3-η)-1,2,3-tri(tert-butyl)-cyclopropenyl iridium (C₁₈H₂₇IrO_3, TICP) precursor and oxygen [1]. However, in the case of this TICP precursor, it was difficult to deposit extremely thin and continuous Ir films on the hydroxyl-terminated oxide layer due to its long incubation delay.

Therefore, in this study, a method for depositing a very thin, uniform and continuous ALD-Ir thin film with low resistivity even on oxide materials was explored by reducing the incubation period and promoting nucleation using various surface pretreatment conditions. In addition, the nucleation behavior as well as film properties of ALD-Ir on the oxide material were systemically compared and analyzed according to the surface pretreatment conditions, and finally, ALD-Ir thin film with excellent properties on the oxide surface was obtained.

References

1. Park, Na-Yeon, et al. Chemistry of Materials 34.4 (2022): 1533-1543.