ALD/ALE 2024 Session AF1-TuM: Precursors and Chemistry: Precursor Design, New Precursors, Process Development II

In ALD, metal complexes (MCs) are mostly used as precursors. Because a sufficient concentration of the precursor is necessary for saturated adsorption, MCs with high vapor pressures are required. Therefore, a method to accurately predict vapor pressure is required when designing new precursors for ALD. In this study, the vapor pressure prediction of MC using COnductor-like Screening MOdel Segment Activity Coefficient (COSMO-SAC) method¹ was investigated.

Vapor pressure calculations using COSMO-SAC method were performed using Amsterdam Modeling Suite². The existing COSMO-SAC method can predict the vapor pressure of MCs with CO or cyclopentadienyl groups with double or half the accuracy of COSMO-SAC method, which is sufficient for precursor selection. However, the vapor pressures of MCs with β-diketonate, amidinate, and alkoxide groups were underestimated by a factor of 100–1000. Therefore, the conventional COSMO-SAC method cannot be applied to a wide range of precursor selection methods.

To improve the accuracy of vapor pressure estimation, we added three procedures to the conventional COSMO-SAC method. Figure 1 shows a schematic of the conventional COSMO-SAC method (black arrows) and improved COSMO-SAC method (colored arrows). As there are no ε_dsp (empirical parameters to calculate dispersive interactions) data for metal atoms, we investigated the non-regressive method to calculate ε_dsp (blue arrows in Fig 1). Because the dispersion force is known to be correlated with polarizability, we predicted that ε_dsp would also be correlated with polarizability. We found a correlation between ε_dsp and polarizability, expressed by the following equation:

ε_dsp = 1035×α^1.34,

where α is the polarizability of the atoms in the molecule. By calculating the α of the metal atom in the MC and substituting it into this equation, we estimated the ε_dsp of the respective metal atoms. Then, we modified the activity coefficient due to dispersion interactions (γ_dsp) based on the polarizability of molecules (red arrows in Fig 1) and branching in the molecules (green arrows in Fig 1).

Figure 2 shows the measured vapor pressure of titanium tetraisopropoxide (TTIP) and those estimated using our method and the conventional method. Our method reproduced the experimental values more accurately than the conventional method did.

The majority of the developments in elemental ALD have involved transition metals. However, interest in main group ALD has increased in recent years due to potential applications in a variety of technologies.^1-3 Research in the Emslie group is focused on the design, synthesis, and characterization of new precursor molecules, and investigations into their use as thermal ALD precursors through a combination of solution-state reactivity studies and ALD reactor studies. Through modification of the precursor design of a previously reported elemental antimony ALD process,⁴ we recently developed the first example of room-temperature thermal ALD of a pure element, using Sb(SiMe₃)₃ in combination with SbCl₃ to grow pure antimony films on H-terminated Si and SiO₂ substrates.⁵ Efforts to expand this research to elemental boron ALD will be described, including synthesis of the potential precursors, (R₃E)₂B(NR'₂) (E = Si or Ge), spectroscopic and crystallographic characterization of these compounds, and attempted boron deposition using a boron trihalide (BX₃; X = Cl, Br) co-reactant; anticipated byproducts in this reactivity are R₃EX and B(NR'₂)X₂, where the formation of strong E–X bonds provides a thermodynamic driving force. Development of a thermal ALD process for elemental germanium will also be described, achieving self-limiting growth at relatively low temperatures, and exhibiting substrate selectivity for deposition on H-terminated Si rather than SiO₂.

References

1. Salinga, M; Kersting, B.; Ronneberger, I.; Jonnalagadda, V. P.; Vu, X. T.; Le Gallo, M.; Giannopoulos, I.; Cojocaru-Mirédin, O.; Mazzarello, R.; Sebastian, A. Monatomic Phase Change Memory. Nat. Mater. 2018, 17, 681-685.
2. Glavin, N. R.; Rao, R.; Varshney, V.; Biano, E.; Apte, A.; Roy, A.; Ringe, E.; Ajayan, M. Emerging Applications of Elemental 2D Materials. Adv. Mater. 2020, 32, 1904302.
3. Pillarisetty, R. Academic and Industry Research Progress in Germanium Nanodevices. Nature 2011, 479, 324-328.
4. Pore, V.; Knapas, K.; Hatanpää, T.; Sarnet, T.; Kemell, M.; Ritala, M.; Leskelä, M.; Mizohata, K. Atomic Layer Deposition of Antimony and its Compounds Using Dechlorosilylation Reactions of Tris(triethylsilyl)antimony. Chem. Mater. 2011, 23, 247-254.
5. Al Hareri, M.; Emslie, D. J. H. Room-Temperature Atomic Layer Deposition of Elemental Antimony. Chem. Mater. 2022, 34, 2400-2409.

Research on ALD of metal tellurides has been focused on phase-change memory materials. Owing to their electrical switching behaviour, binary and ternary compounds and alloys of germanium, antimony and tellurium have become an important area of development for memory and selector devices. Elemental tellurium has also garnered interest in materials research due to its thickness-dependent electrical properties, air stability and high hole mobility. Thus, it has been found to be a potential candidate to ensure continuing the miniaturization of transistors. However, process studies for depositing tellurium films are quite scarce. This study aims to illuminate a new ALD chemistry route for elemental Te films by combining an alkylsilyl telluride precursor with tellurium dichloride. In addition to elemental Tefilms,the new ALD process can be used for tuning the composition of metal telluride compoundssuch as GeTe.

The depositions were done in a commercial flow type ALD reactor using (Et₃Si)₂Te(bis(triethylsilyl) telluride) and TeCl₂ as precursors. The process is expected to involve a dechlorosilylationreaction, which follows the Lewis hard-soft acid-base principle. Film morphology was studied with SEM, film crystallinity and crystallization temperature with XRD, and elemental composition with ToF-ERDA. Tellurium growth was observed at a temperature range of 90-110 °C. Film morphology and growth rate were highly dependent on the substrate. On native silicon oxide, the nucleation was sparse, and morphology of the deposited material initially resembled nanowires that subsequently coalesced with further deposition cycles. On Au the film morphology was smoother and continuous already at low cycle numbers. The considerable difference in the growth rates between native SiO₂ and Au is leveraged to attain area selective growth on Au. The films deposited in mild conditions are crystalline exhibiting hexagonal crystalline structure, and of high purity. A GeTe ALD process was modified with the studied Te process to yield tellurium-rich Ge_xTe_y films with enhanced phase transition temperature.

Atomic Layer Deposition (ALD) is a booming technology to deposit thin films and it has been applied in several fields. This technique is based on surface-chemical reactions, and relies on the gas phase transport of metal containing molecules into a reaction chamber. However, not any molecule is suitable to be used as precursor, as they must be thermally robust while being sufficiently volatile and chemically labile to react with the surface functional groups. Organometallic chemistry offers a nearly infinite set of options to design new efficient precursors, though predicting their volatility and reactivity in the ALD chamber remains a challenge.^[1] Establishing an efficient method to assess the physical and chemical properties of complexes would grant access to new ALD precursors and a better understanding of surface reactions.

This communication focuses on the development of new efficient gallium precursors to be used in the ALD of oxygen-free gallium-containing sulfide thin films.^[2] A series of gallium complexes with chelating nitrogen based ligands (guanidinate, amidinate and triazenides) were synthesized and characterized (NMR, XRD) thanks to modular procedures.^[3,4] To assess the thermal stability of the reagents and shed light on their transport in ALD, thermal analysis (TGA,DSC) were realized under N₂ and vacuum to mimic transport conditions. Finally, reactivity studies in solution of established ALD precursors and synthesized complexes provide an insight of surface reactions, which might take place in an ALD chamber. Having selected the most promising candidates, they were tested for film growth in an ALD apparatus using QCM setup. Interestingly, it was possible to highlight the crucial role of the non-chelating ligands in the film growth. Complexes with methyl ligands lose their surface reactivity after few cycles, whereas their substitution with amino groups can give a constant GPC over the whole range of the study. Gallium sulfide thin film could be prepared and their chemical fine composition was analyzed by XPS.

These results display the challenges that we may encounter in the targeting of new precursors and the importance of establishing a precise method to predict reactivity in ALD conditions.

References:

[1] S. E. Koponen, P. G. Gordon, S. T. Barry, Polyhedron2016, 108, 59–66.

[2] N. Schneider, M. Frégnaux, M. Bouttemy, F. Donsanti, A. Etcheberry, D. Lincot, Materials Today Chemistry2018, 10, 142–152.

[3] A. P. Kenney, G. P. A. Yap, D. S. Richeson, S. T. Barry, Inorganic Chemistry2005, 44, 2926–2933.

[4] S. Dagorne, R. F. Jordan, V. G. Young, Organometallics1999, 18, 4619–4623.

In this work we have deposited cobalt phosphide (Co₂P) by thermal ALD. Co₂P is a metallic material that has potential in a variety of applications such as hydrogen evolution reaction and lithium-ion batteries.

In general, the ALD processes for metal phosphides are scarce and mostly use the highly toxic PH₃ gas as the phosphide source. To our knowledge, only two ALD processes for transition metal phosphides has been reported. CoP_x has been deposited using Co(AMD)₂ with PH₃ plasma and H₂ plasma as precursors.[1] MoP was deposited by using MoCl₅ and PH₃.[2]

In this work Co₂P was deposited by using CoCl₂TMEDA (TMEDA = N,N,N′,N′-tetramethylethylenediamine) and tris(trimethylsilyl)phosphine (P(SiMe₃)₃) as precursors. Alternatively, Co₂P can be deposited by using a three-precursor process: CoCl₂(TMEDA), NH₄F and P(SiMe₃)₃. The advantages of our processes are that we avoid the use of toxic PH₃ but also the possible damage caused by plasma.

Co₂P films were deposited with the CoCl₂(TMEDA) + P(SiMe₃)₃ process at 225–250 °C. The GPC was approximately 0.65 Å at 250 °C. Saturation of the GPC with respect to both precursors was verified at 250 °C. The films consist of orthorhombic Co₂P as measured by XRD, and the Co/P ratio is close to 2 as measured by ToF-ERDA. The main impurity is carbon, decreasing with the decreasing deposition temperature from 9.5 at-% at 250 °C to 6.7 at-% at 225 °C. However, with the three-precursor process lower deposition temperatures can be used and the C content is lower, e.g., ~ 3.3 at-% at 215 °C. In addition, only a small F content (e.g. ~0.4 at-%) was found in the films.

[1] H. Zhang, D. J. Hagen, X. Li, A. Graff, F. Heyroth, B. Fuhrmann, I. Kostanovskiy, S. L. Schweizer, F. Caddeo, A. W. Maijenburg, S. Parkin, R. B. Wehrspohn, Angew. Chem. Int. Ed. 2020, 59, 17172

[2] R. Browning, P. Plachinda, R. Solanki, Appl. Nano 2023, 4, 38–44