Surface-dependent adsorption and diffusion processes in area-selective deposition of ruthenium
Vapour-phase chemical deposition techniques rely on an interplay of adsorption, diffusion, reaction, and desorption processes, all of which can be surface-dependent. Area-selective deposition (ASD) exploits this surface dependence to deposit material selectively on a target area. Ruthenium receives increasing interest for several applications, including conductors for nanoelectronic interconnects, etch-resistant hardmasks, and heterogeneous catalysts. Ruthenium deposition is typically needed only locally on nanoscale features, and ruthenium is challenging to pattern using conventional top-down approaches, which motivates the use of ASD. However, Ru ASD remains challenging due to our limited understanding of the elementary processes which govern Ru growth on different surfaces and nanoscale patterns. This thesis is therefore devoted to understanding surface-dependent growth behaviour during Ru deposition, influencing this growth behaviour by tailoring the surface, and elucidating Ru ASD mechanisms on nanopatterns.
Ru is a high surface energy metal, which can diffuse and aggregate on low surface energy substrates during deposition to minimise surface energy. Ru deposition processes investigated in this thesis display Ru adspecies diffusion on low surface energy dielectrics. In contrast, no observable diffusion occurs on metals and metal nitrides due to their high surface energy. Consequently, Ru diffuses and aggregates into particles on low surface energy dielectrics during deposition, while film growth is immediately observed on metals and metal nitrides. Two Ru deposition processes are compared in this thesis, (1-ethylbenzyl)(1,4-cyclohexadienyl)-ruthenium (EBECHRu) and O2 atomic layer deposition (ALD) at 325 °C and (carbonyl)-(alkylcyclohexadienyl)Ru/H2 chemical vapour deposition (CVD) at 250 °C. The surface-dependent growth mechanism of both processes is determined through a combined theoretical/experimental investigation, which finds that the two Ru deposition processes display different Ru adatom diffusion lengths λ on a low surface energy dielectric surface.
The Ru ASD mechanism on nanopatterns depends on how λ compares to d, where d is the maximum distance from any point on the non-growth surface to the nearest growth surface. Nanopattern dimensions in this thesis are chosen so that λ ALD < d < λ CVD, and investigates the ASD mechanism in both cases. TiN is chosen as growth surface, because both Ru precursors used in this study adsorb readily on TiN, and because EBECHRu adsorption is enhanced by Ti surface species throughout steady-state film growth. -Si(CH3)x terminated SiO2 is chosen as non-growth surface, because adsorption of both precursors is least favourable and Ru adspecies diffusion length is highest on this surface. A high Ru adspecies diffusion length on the non-growth surface is of interest because it can allow Ru adspecies to migrate from the non-growth to the growth surface.
When λ <d , Ru adspecies remain on the non-growth surface after deposition. These adspecies can be removed by a corrective etch step when the Ru adspecies or film on SiO2 are either smaller or etched sufficiently faster compared to the Ru film on TiN. Surface-dependent adsorption during the initial stages of EBECHRu/O2 ALD is exploited to meet this condition. EBECHRu adsorption is enhanced on TiN and suppressed on -Si(CH3)x terminated SiO2. Moreover, EBECHRu adsorption is inhibited on Ru particles that are too small (<0.85 nm) to catalytically dissociate the O2 coreagent, because EBECHRu adsorption relies on O surface species originating from O2 dissociation. In contrast, EBECHRu adsorption proceeds readily on Ru particles >0.85 nm and Ru films, which catalytically dissociate O2. This size-dependent reactivity limits initial Ru particle growth on SiO2. During the initial stage of the deposition, the sizes of Ru particles on SiO2 are kept below the Ru film thickness on TiN. This allows particles on SiO2 to be removed by a non-selective Ru dry etch while retaining part of the Ru film on TiN. Deposition temperature affects differences in adsorption rates between the two surfaces, which can be used to further enhance size differences between the Ru film on TiN and particles on SiO2.
When λ >d , Ru adspecies can migrate towards the growth surface where they can be incorporated into the growing ASD film. Surface-dependent diffusion during (carbonyl)-(alkylcyclohexadienyl)Ru/H2 CVD is exploited to migrate Ru adspecies from the -Si(CH3)x terminated SiO2 surface area towards the TiN growth surface. Adspecies diffusion enhances the ASD growth rate on TiN, which is a potential advantage of this approach. High temperatures enhance the Ru adspecies diffusion length, which can be used to enhance Ru migration from SiO2 to TiN. The ASD growth rate can vary depending on the area of the non-growth surface surrounding it, which could pose a challenge when a uniform thickness or amount of Ru is desired. Nanopattern topography impacts surface diffusion, as sharp SiO2 corners impede Ru adspecies migration on 3D features.
Dimethylamino-trimethylsilane (DMA-TMS) reacts selectively with SiO2, rendering the SiO2 surface -Si(CH3)3 terminated before ASD. Dielectric surfaces in relevant nanopatterns are often -SiOH terminated, either due to the deposited dielectric or as a result of nanopattern fabrication. DMA-TMS is a small molecule that reacts selectively with -SiOH groups to produce a -Si(CH3)3 terminated surface. DMA-TMS is unreactive towards -SiOSi- groups, and needs a minimum of 2.41 -SiOH nm-2 to achieve the steric maximum density of -Si(CH3)3 groups on the SiO2 surface. DMA-TMS selectively silylates SiO2 to enable ASD of and on several materials. TiO2, TiN, and Ru surfaces remain reactive under DMA-TMS treatment conditions that passivate SiO2. In addition, TiO2, TiN, and Ru can be deposited selectively to DMA-TMS treated SiO2 by TiCl4/H2O, TiCl4/NH3, and EBECHRu/O2 ALD. The selective surface reactions of DMA-TMS can therefore enable ASD of and on TiO2, TiN, and Ru, and potentially other materials. Furthermore, DMA-TMS is compatible with cyclic passivation-deposition-defect removal processes which can enhance ASD selectivity but require the SiO2 surface functionalisation to be selective towards both the growth surface and the ASD-grown material.
This thesis expands our understanding of surface-dependent growth mechanisms, surface functionalisation, and growth behaviour on nanopatterns, which can aid the development of ASD processes for a wider range of materials and applications.