ALD/ALE 2025 Session AS-TuA: Area Selective Deposition I

Area-selective atomic layer deposition (AS-ALD) methods promise bottom-up approaches for device fabrication, yet it remains difficult to achieve high selectivity with subtle surface chemical differences. In this work, we demonstrate dopant-selective ALD (DS-ALD) for the patterning of differently doped silicon surfaces. Our approach enables orthogonal patterning which can direct ALD to either heavily or lightly doped silicon. As a result, we can fabricate a metal oxide semiconductor field effect transistor (MOSFET) gate [1] on lightly doped silicon and a semiconductor-metal contact [2] on heavily doped silicon. In each of these patterning approaches, an organic surface-initiated mask is grafted from the surfaces of dopant modulated silicon structures. An etchant (aqueous KOH) then diffuses through the mask and selectively attacks the underlying silicon surface, thus degrafting the mask only in targeted regions. This process yields features to which ALD chemistries can deposit compatible metal or dielectric thin films. We characterize the process through a combination of X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and Current-voltage (IV) measurements.

To direct ALD of gate dielectric and metal to the lightly doped silicon typically in the channel of a MOSFET, a hydrosilylation and subsequent polymerization results in the appropriate pattern after applying the selective etchant. We characterize the doping contrast required to enable this approach as well as the minimum achievable feature sizes. Notably, we observe an increase in etching selectivity as etching progresses which mimics the mechanism of aqueous KOH etching of bare silicon. ALD of the gate dielectric and metal results in working field effect devices.

To direct ALD of contact metal towards heavily doped silicon, as might be needed for the source/drain regions of a MOSFET, we leverage undecylenic acid’s bifunctional nature which results in different attachment chemistry on differently doped silicon surfaces. Treatment with KOH removes the mask from the heavily doped silicon surface allowing the deposition of Pt metal via ALD on that same region. Current-voltage measurements demonstrate a metal semiconductor silicide contact.

In summary, by combining different organic mask materials with differently doped semiconductor surfaces, we can direct patterning and the subsequent AS-ALD. Our findings establish guidelines for novel approaches to nanoscale patterning and set the stage for fabricating a variety of high-performance electronic devices.

Presently, research on area selective atomic layer deposition (AS-ALD) is attracting strong interest. AS-ALD simplifies the process of deposition on patterned substrates and high aspect ratio (AR>70) structures, holding promise for various applications in semiconductor manufacturing. Inhibitors such as self-assembled monolayers (SAMs) and (dimethylamino)trimethylsilane (DMA-TMS) are typically used to passivate non-growth areas.¹ However, the deterioration of selectivity at temperatures exceeding 500℃ limits their applications in high temperature deposition processes. Fluorination is regarded as a candidate for overcoming these problems.² In this study, we present a novel approach to achieve high selectivity in high temperature AS-ALD on SiN versus SiO₂ through effective surface-selective fluorination without damage to the substrates.

Experiments were conducted with a batch furnace capable of treating over one hundred 300 mm wafers simultaneously. A wide range of substrates were utilized including 300mm wafers with blanket SiN and SiO₂ films and various nanostructured samples. Prior to inhibition, samples were precleaned by dilute HF to remove native oxides. An HF gas passivation was employed at 630℃ for 10 minutes to selectively inhibit the SiO₂ surface. ALD SiN was deposited with dichlorosilane (SiH₂Cl₂) as the precursor and NH₃ as the co-reactant at 630 ℃.

The initial AS-ALD sequence involved performing ALD cycles following HF passivation, resulting in AS-ALD of up to 6 nm SiN on SiN while maintaining good within-wafer non-uniformity (Win Unif.) of <3%. We then developed the area-selective-deposition-Loop (ASD-Loop) technique, consisting of alternating 10-minute HF passivation with ALD cycles, as shown in Figure 1. We achieved an AS-ALD of 28 nm on SiN blanket wafers with high selectivity while keeping excellent Win Unif. of <2% and high film quality, as confirmed by the wet etching rate. The ASD-Loop has also been successfully applied to SiN/SiO stripe-patterned substrates (Figure 2), resulting in a maximum ASD thickness of 21 nm; and to high-AR holes on SiO₂/SiN stacks (Figure 3), resulting in AS-ALD with high conformality of >90%. Thus, we have demonstrated high-temperature AS-ALD on 300 mm wafers and complex nanostructures with high selectivity, superior uniformity, and reliable quality. This offers a new pathway for the integration of AS-ALD into nanofabrication schemes, showing significant potential for advancing high-performance semiconductor applications including DRAM and 3D NAND manufacturing.

References

1. R. Khan et al., Chem. Mater. 2018, 30, 7603.

2. H. Oh et al., Adv. Funct. Mater. 2024, 34, 2316872.

With the scaling of semiconductor devices, the increase in via resistance, which constitutes a significant portion of the Cu interconnect resistance, has emerged as a critical issue. To reduce via resistance, studies [1, 2] have been conducted on forming bottomless barriers using area selective deposition (ASD), where a high-resistivity barrier layer is selectively deposited only on the via/trench sidewalls, SiO₂-based dielectrics without being deposited on the via bottom, Cu. In these studies, self-assembled monolayers (SAMs) with long alkyl chains were used as inhibitors for preventing the adsorption of the precursor on metallic surfaces. However, SAMs are typically adsorbed onto surfaces through solution-based processes, which pose compatibility issues with semiconductor processes and, due to their relatively long molecular structure, cannot form a uniform inhibitor layer within 3D nanostructures. To prevent these issues, this study used a small molecule inhibitor (SMI) to form a bottomless barrier. Additionally, based on previously reported studies [1], the Ru/ZnO bilayer, with ZnO for the bottomless barrier and Ru for the liner and seed layer, was applied for the sustainable Cu interconnects [Figure 1]. The ZnO ASD process is carried out using an amine-based SMI that selectively absorbs on Cu, with diethylzinc and H₂O used for the ZnO ALD process. After the ZnO ASD process and the removal of the inhibitor, the Ru film was deposited using an ALD process with tricarbonyl(trimethylenemethane)ruthenium and O₂. By controlling the conditions of the ZnO ASD process, we confirmed and analyzed the selective deposition of ZnO on the SiO₂-based dielectric substrate not on Cu one using TEM, XPS, and XRD etc. The detailed results will be presented in the conference.

References
[1]Mori, Yuki, et al. Small 19.34 (2023): 2300290.
[2]You, Shi, et al. "Selective Barrier for Cu Interconnect Extension in 3nm Node and Beyond", IITC (2018)

Acknowledgements
This work was supported by the Technology Innovation Program (Public-private joint investment semiconductor R&D program (K-CHIPS) to foster high-quality human resources) (RS-2023-00236667, High-performance Ru-TiN interconnects via high-temperature atomic layer deposition (ALD) and development on new interconnect materials based on ALD) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea) (1415187401). This work was also supported by SK hynix semiconductor through the cluster research between UNIST and SK hynix semiconductor.

Over the past decade, research into the area-selective behavior of M-(^tbu₂DAD)₂ compounds has highlighted the significant potential of these precursors. [1,2] The low-temperature AS-ALD deposition of metallic nickel and cobalt films on various metallic surfaces has demonstrated its technological relevance. However, while Co(^tbu₂DAD)₂ continues to be the focus of many studies, [3][4] Ni(^tbu₂DAD)₂ has seemingly received less attention. Despite their similar thermodynamic properties, handling Ni(^tbu₂DAD)₂ has proven more challenging than its cobalt counterpart.

In this study, we further investigate the delivery characteristics of Ni(^tbu₂DAD)₂ and expand the range of growth and non-growth materials for its selective deposition. We also examine the loss of selectivity on silicon-based surfaces, which are typical non-growth areas. This undesirable material nucleation can arise from several factors, but in the case of Ni(^tbu₂DAD)₂, it is especially pronounced on hydroxyl groups of silicon oxide substrates. After identifying the nature of these nucleation sites, we developed a method to deliberately create growth regions using focused ion beam (FIB) irradiation. This approach enables direct patterning of growth areas on non-growth substrates, such as SiO₂. Our findings demonstrate that both induced selectivity (achieved through FIB area activation) and the inherent selectivity of Ni(^tbu₂DAD)₂ for metallic surfaces can coexist, offering new potential strategies for advanced nanofabrication.

References:

[1]Kerrigan, Marissa M., et al. "Low temperature, selective atomic layer deposition of nickel metal thin films." ACS applied materials & interfaces 10.16 (2018): 14200-14208.

[2]Klesko, Joseph P., Marissa M. Kerrigan, and Charles H. Winter. "Low temperature thermal atomic layer deposition of cobalt metal films." Chemistry of Materials 28.3 (2016): 700-703.

[3]Breeden, Michael, et al. "Proximity effects of the selective atomic layer deposition of cobalt on the nanoscale: implications for interconnects." ACS Applied Nano Materials 4.8 (2021): 8447-8454.

[4] Ashburn, Nickolas, et al. "Density functional theory study on reaction mechanisms of Co (tbu2DAD) 2 for area selective-atomic layer deposition of Co films on metal surfaces." Journal of Vacuum Science & Technology A 41.5 (2023).