ALD/ALE 2022 Session AF1-MoA: New Precursors and Processes II

[1] Keiko et al. U. S. Patent Appl. Publ. 2011. US20130068953A1

[2] Popov et al. Chem. Mater. 2019, 31 (3), 1101-1109.

Thin films of cobalt metal are subject of high research and economic interest owing to their potential to facilitate performance increases in integrated circuitry (IC) devices at continuously shrinking dimensions. Its good conductivity and stability are among the reasons cobalt is handled as a replacement to copper. To implement cobalt thin films, atomic layer deposition (ALD) is a favorable fabrication technique allowing precise device architectures to be coated uniformly with sub nanometer thickness control.^[1]

In ALD, the choice of precursor is of crucial importance because its thermal behavior and reactivity are paramount for successful deposition of a desired thin film material. Revisiting the library of Co precursors, all nitrogen coordinated precursor complexes are underexplored; surprisingly as Co(II) diazadienyls have successfully ventured into the field.^[2]

Motivated by this premise, we studied a variety of new nitrogen coordinated Co complexes and their potential for ALD. They have been synthesized and thoroughly characterized by EA, EI-MS, SC-XRD, TGA and vapor pressure estimations.^[3]

An entry point to this study was found in the Co(II) bis(trimethysilyl)amide dimer 1. This very compound was reacted with a selection of amides, imides, amide-adducts, pyrroles, and amido-amines (Fig. 1). With structural and chemical ligand variations in the form of aromatic moieties, varying ring size, substitution patterns, bonding character and strategic incorporation of silylamide functionalities, a variety of cobalt compounds was successfully obtained (Fig. 1). The modifications were performed to gain a broader understanding of influences on reactivity, stability, and volatility with the aim of tailoring the compounds suitable for ALD.

Whereas some of the forwarded complexes followed the expected structural trends, others adopted distinctive and unexpected coordination motives (Fig. 2). This was reflected by their thermal behavior and underlined that gradual ligand alteration can have a vast impact Additionally, the assessment of volatility also shows distinct differences between the employed ligands (Fig. 3). Especially volatile with high stability were the Co(II) iminopyrrolate 9 and the silylamido-amine 11. Those two newly uncovered precursors were found to be especially promising candidates for application in ALD process development.

[1] A. E. Kaloyeros, et al., ECS J. Solid State Sci. Technol.2019, 8, 119–152.

[2] J. P. Klesko, et al., Chem. Mater.2016, 28, 700–703.

[3] D. Zanders, J. Obenlüneschloß, et al., Eur. J. Inorg. Chem.2021, 5119–5136.

Tin(II) oxide is one of the few p-type semiconducting materials that possesses excellent electrical and optical properties. Hence this interesting material system is very appealing for applications in chemical sensing, as anode material for Li-ion batteries or as a transparent conductive oxide (TCO) in thin film transistor (TFT’s), particularly due to the high hole mobility. Nevertheless, the deposition of stoichiometric SnO thin films is a challenging task. Atomic layer deposition (ALD) is a preferred process for thin film deposition as it is capable of yielding high quality, dense and uniform layers with an excellent control over film thickness as well as the ability to control the stoichiometry. Sn(II) oxide readily oxidizes to Sn(IV) and there are not many ALD processes known for obtaining highly stoichiometric SnO films. Thus, care must be taken with respect to process parameter selection and precursor choice.

Herein, we report the successful deposition of stoichiometric SnO thin films in a simple water based ALD process, employing the homoleptic, liquid tin(II) formamidinate precursor [Sn(ⁱPr₂fAMD)₂] that was recently reported for ALD of SnS.[1]. Based on the promising thermal properties (Figure 1), good thermal stability and high reactivity towards water, we employed this precursor for ALD of SnO. The newly developed ALD process yielded SnO layers in the temperature range of 120 °C to 180 °C on Si substrates. The formation of tetragonal romachite-type SnO films was confirmed by Grazing Incidence X-ray Diffraction (GI-XRD) while surface and bulk composition were verified by means of X-ray Photoelectron Spectroscopy (XPS) and Rutherford Backscattering Spectroscopy (RBS) / Nuclear Reaction Analysis (NRA) (Figure 2). The films were found to exhibit the desired stoichiometry alongside high purity. Optical bandgaps of as-deposited and annealed films were determined via Tauc-plotting from UV-Vis measurements. This is a very promising ALD process development where the role of precursor and co-reactant choice enabled the phase control of SnO films.

[1]Kim et al., ACS Appl. Mater. Interfaces, 2019, 11, 45892−45902

Zinc oxide draws attention for its relatively low cost, the wide bandgap of 3.37 eV,[1] and for its electrical and optical properties for applications such as gas sensors, solar cells, thin-film transistors and light-emitting diodes.[2,3] ZnO thin films can be obtained by different deposition methods, from sol-gel approach to sputtering, and gas-phases based approaches such as molecular beam epitaxy (MBE), chemical vapour deposition (CVD), atomic layer deposition (ALD).

Within the materials deposition techniques, Atmospheric Pressure Spatial ALD (AP-SALD) is gaining momentum since it is a high throughput and low-cost alternative to conventional ALD. AP-SALD is based on a physical separation (rather than temporal separation, as is the case in conventional ALD) of gas-diluted reactants over the surface of the substrate by a region containing an inert gas.[4]

In this presentation, we will show that a volatile shape-directing agent can be used to tune the texture and growth rate of ZnO thin films deposited by AP-SALD. While the use of such agents is common in liquid-based synthesis approaches, their use in the AP-SALD system has never been used to the best of our knowledge. (002)-oriented hexagonal wurtzite-type ZnO films were obtained at 200 °C, without any post-deposition treatment.

[1]Velázquez-Nevárez GA, Vargas-García JR, Aguilar-Hernández J, Vega-Becerra OE, Chen F, Shen Q, et al. Optical and Electrical Properties of (002)-Oriented ZnO Films Prepared on Amorphous Substrates by Sol-Gel Spin-Coating. Mater Res 2016;19:113–7. doi:10.1590/1980-5373-mr-2016-0808.

[2]Zhao M-J, Sun Z-T, Hsu C-H, Huang P-H, Zhang X-Y, Wu W-Y, et al. Zinc Oxide Films with High Transparency and Crystallinity Prepared by a Low Temperature Spatial Atomic Layer Deposition Process. Nanomaterials 2020;10:459. doi:10.3390/nano10030459.

[3]Dong J, Han D, Li H, Yu W, Zhang S, Zhang X, et al. Effect of Al doping on performance of ZnO thin film transistors. Appl Surf Sci 2018;433:836–9. doi:10.1016/j.apsusc.2017.10.071.

[4] Muñoz-Rojas D, Huong Nguyen V, Masse de la Huerta C, Jiménez C, Bellet D. Spatial Atomic Layer Deposition. Chem. Vap. Depos. Nanotechnol., IntechOpen; 2019. doi:10.5772/intechopen.82439.

Rare earth (RE) containing thin films are indispensable for several different applications owning to their broad number of intrinsic functional properties. Especially, RE-oxides and -sulfides are of high interest for various applications such as high-κ dielectrics (Y₂O₃, CeO₂), optical fibers in lasers (Yb₂O₃), spintronics (EuS) or photovoltaics (Nd₂S₃). However, to gain fundamental insights and accelerate the development of these devices, reliable synthesis routes that yield high-quality material are required.[1] Vapor phase deposition methods such as atomic layer deposition (ALD) can meet these demands, but to develop promising processes for the formation of the RE-containing materials, precursors with certain pre-requisites are of utmost importance.[2] Besides sufficient volatility and thermal stability at the operating conditions, the precursors need to exhibit a high reactivity towards the employed co-reactant.

Commonly used ligands with oxygen-metal bonds suffer from a low reactivity towards the co–reactants due to strong RE–O bonds, which limits their applicability in ALD processes. On the other hand, precursors with guanidinate (dmg), amidinate (amd) and formamidinate (famd) ligand moieties have gained prominence since their all-nitrogen coordinated ligands ensure high reactivity toward mildly reactive co-reactants such as H₂O, while retaining volatility and thermal stability.[2,5]

Consequently, a series of RE containing precursors, with the formula [M(L)₃] (M = Y, Ce, Eu, Yb; L = dpdmg, dpamd, dpfamd) were synthesized (Figure 1), structurally analyzed, and their thermal properties evaluated.[1-5] Furthermore, the tunability of the thermal properties via a variation of the ligand side chains was demonstrated for Y formamidinates [3] as well as for amidinates and guanidinates of Ce and Yb [2] with systematic thermogravimetric analysis studies (TGA).

Density functional theory (DFT) calculations of the reaction between selected Y, Yb and Ce precursors with H₂O revealed a pronounced reactivity towards mild oxidizing agents and underlined their suitability for ALD processes.[2,3] Actual ALD experiments of Y formamidinate- and Ce guanidinate-based precursors with H₂O as the co-reactant confirmed the results from DFT and yielded high-quality thin films of Y₂O₃ [3,4] (Figure 2) and CeO₂ [2], respectively. A high reactivity of the [Eu(L)₃] (L = dpdmg, dpamd, dpfamd) towards O₂ and S₂ was obtained from DFT studies, further underlining the suitability of these ligand motifs in precursors for vapor phase deposition fabrication methods.[1]

The continuous miniaturization of semiconductor devices has triggered growing interests in new strategy for depositing conformal coatings on complex geometry which are critical for device reliability. Despite ALD being a mainstream technique for conformal coatings, features with increasing aspect ratio have rendered this conventional ALD increasingly challenging to achieve the desired step coverage.

Various approaches have been conducted to improve film conformality and film quality on high aspect ratio (HAR) trenches such as screening of molecules with potential chemistry as inhibitor and tuning of ALD process parameters. In this work, we have narrowed down our approach by successfully identifying molecules with suitable chemistry (alkyl-halides) that could act as growth inhibitor in the first pulse step of ALD. This inhibitor enables the control of inherent kinetics of gas transport within the trenches by reducing the surface reactive sites for precursors on the top region while allowing more precursors to diffuse deeper into the trenches, thus improving film conformality in high aspect ratio trenches. Besides growth inhibition, the selected inhibitor could function as film quality enhancer and could also be easily removed during reactant pulse step without contaminating the films.

By leveraging the selected inhibitor chemistry and ALD deposition characteristics, our approach has demonstrated significant improvement in TiN film conformality to 90% in HAR trench features, as determined by the ratio of bottom side film thickness to top side film thickness [1]. In terms of film resistivity, an observable 50% improvement was obtained owing partially to large decrease in Cl impurity as determined by SIMS depth profiles [1]. The introduction of inhibitor has also exhibited promising benefits in the ALD of high-k dielectric film, HfO₂. Improvement in HfO₂ film conformality to 97% in HAR trench features as well as significant improvement of 95% in leakage current due to enhancement in film quality were simultaneously observed [2].

Based on these experimental evidences, our approach has been proven its promising potential as an innovative solution to address the challenges of depositing highly conformal films of excellent quality on high aspect ratio features such as DRAM trench capacitor and 3D NAND.

References:

1. C. Yeon, J. Jung, H. Byun, K.C. Tan, T. Song, S. Kim, J.H. Kim, S.J. Lee and Y. Park, AIP Advances 11, 015218 (2021).
2. K.C. Tan, J. Jung, S. Kim, J. Kim, S.J. Lee and Y. Park, AIP Advances 11, 075008 (2021).

We have proposed and demonstrated a novel thin film deposition technique by transferring the principles of atomic layer deposition (ALD), known with gaseous precursors, towards precursors dissolved in a liquid. The technique can also be considered as a generalization of already established methods such as the ‘layer by layer’ growth or the ‘successive ion layer adsorption and reaction’ (SILAR). 'Solution ALD' (sALD) shares the fundamental properties of standard ‘gas ALD’ (gALD), specially the self-limiting growth and the ability to coat conformally deep pores. It has been already shown that it is possible to transfer standard reactions from gALD to sALD such as TiO₂ deposition . However, sALD also offers novel opportunities such as overcoming the need for volatile and thermally robust precursors.

To illustrate this, we establish sALD procedures for depositing films of non oxides layers such as Pb-compound deposition, MOF, selenides and sulfides. Those examples highlight how ionic, polar, or high-molecular weight precursors that only exist in the condensed phase are now rendered amenable to being utilized in surface-controlled thin film formation by sALD for depositing materials that would otherwise be more difficult or more expensive to achieve by gALD or hazardous. The deposition was achieved on small (2 cm*1 cm) and large samples (up to 10 cm*10 cm). The ALD behavior has been shown. The influence of the deposition parameters on the morphology, the crystalline structure and the chemical composition has been investigated by scanning electron microscopy, atomic force microscopy, grazing incidence x-ray diffraction and x-ray photoelectron spectroscopy. Additionally, the nucleation is studied in function of chemical pretreatment to control the growth and allow selective deposition of SnS, SnSe and Sb₂Se₃.

This particular ALD process offer the growth of high quality and crystalline layers in middle conditions with the use of low cost and environmental friendly precursors.

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