Elsevier

Applied Surface Science

Volume 515, 15 June 2020, 145987
Applied Surface Science

Short Communication
Aluminium oxide formation via atomic layer deposition using a polymer brush mediated selective infiltration approach

https://doi.org/10.1016/j.apsusc.2020.145987Get rights and content

Highlights

Abstract

Area selective deposition (ASD) is an emerging method for the patterning of electronic devices as it can significantly reduce processing steps in the industry. A potential ASD methodology uses infiltration of metal precursors into patterned polymer materials. The work presented within demonstrates this potential by examining hydroxy terminated poly(2-vinylpyridine) (P2VP-OH) as the ‘receiving’ polymer and trimethylaluminium (TMA) and H2O as the material precursors in a conventional atomic layer deposition (ALD) process. Fundamental understanding of the surface process was achieved using X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDX) mapping via transmission electron microscopy (TEM). The resulting analysis confirms aluminium inclusion within the polymer film. Spectroscopic and microscopic characterisation show metal infiltration throughout the polymer to the underlying silicon dioxide interface. Exposing the infiltrated film to an oxygen plasma results in the removal of the organic component and resultant fabrication of a sub 5 nm aluminium oxide layer.

Introduction

Research into ‘bottom up’ lithography methods as an alternative to conventional ‘top down’ patterning techniques for next-generation electronic devices has led to major efforts in identifying suitable polymers for block copolymer (BCP) lithography [1], [2], [3], and, more recently, area selective deposition [4], [5]. The study of polymers that are receptive to infiltration of subsequently deposited materials, and that demonstrate the capacity for use in BCP or selective area type patterns, is a growing area of research [6], [7]. Poly(2-vinylpyridine) (P2VP) is an attractive polymer having properties that make it prototypical for patterning and infiltration processes [7], [8], [9]. These advantages are apparent as pyridine-based polymers are ideal for infiltration because of the nitrogen lone pair and it should be noted that systems such as polystyrene-poly(vinylpyridine) are capable of patterning at extreme dimensions (<10 nm) [10]. However, the potential of PVP systems as an infiltration media is much less studied than the intensively studied PMMA (Poly(methyl methacrylate)) [10]. Because of the interest in PVP as a potential acceptor, recent work has shown progress in its use via rapid, high-quality grafting of the polymer as a brush layer, and its infiltration with Al through a liquid phase approach [11]. Our previous studies have concentrated on the surface characterisation of these P2VP-OH brushes [12], and the capacity that these films have in facilitating infiltration via a Cu salt (liquid phase) inclusion process [13].

While a liquid phase infiltration approach into brush films has many benefits, vapor phase area selective infiltration techniques such as ALD have not yet been demonstrated using these brush type films. The focus of this work is on developing an area selective, ALD compatible deposition process using a covalently grafted P2VP-OH brush which could be patterned via conventional lithographic masking or by tailoring selective polymer brush end groups to bind to site specific wafer regions. This approach may have significant relevance for silicon device technologies, particularly in monolithic integration due to the low temperature ceiling (<500 °C) required for complementary metal–oxide–semiconductor (CMOS) compatibility [14].

The vapor phase ALD technique, in which chemical precursors and reactant gases are sequentially admitted to a deposition chamber under vacuum, is a film growth method that is regarded as highly conformal and controllable [15], with applications in a wide range of nanopatterning fields including ASD and BCPs [16]. Traditional ALD methods were based on depositing a material on top of a substrate, however it is important to note - that for ASD and BCP ALD approaches - infiltration into the polymer film is often desired. Numerous studies have been performed on blanket polymer films and BCPs in order to produce metal oxide films and nanopatterns respectively, with several alterations from the standard ALD cycle growth used in achieving infiltration. A review by C. Leng and M. Losego [17] outlined three of these alternative techniques for polymer infiltration – semi-static sequential infiltration synthesis (SIS) [18], [19], [20], flow mode SIS [21] and sequential vapor infiltration (SVI) [22]. Despite operational differences, all three operate by means of holding precursors and/or co-reactants in the chamber for a designated amount of time – unlike in the non-stagnated approach in conventional ALD.

While these methods have achieved positive results, conventional ALD cycle growth has also been extensively reported to achieve infiltration into polymer films [23], [24], [25]. The work presented here demonstrates complete Al infiltration into a P2VP-OH film, with the avoidance of the longer cycle time experienced in a SIS approach. Aluminium was chosen as the material for infiltration, as there is extensive knowledge in the fabrication of Al2O3 thin films via ALD, both through deposition on surfaces and through infiltration into various polymers [26], [27]. Precursor infiltration and metal coordination to the polymer is expected due to the strong chemical interaction between Al and pyridine [28], [29]. Alumina is a high-dielectric constant (high-k) material that excels as a diffusion barrier with good thermal stability, resulting in it having a wide-range of uses in the semiconductor industry [30], [31]. EDX mapping confirms the ability of P2VP to facilitate metal incorporation with this technique, and the XPS analysis allows for a detailed study of the environment of the surface after infiltration. An in-situ oxygen plasma step was performed post ALD infiltration, to produce and study the resultant Al-oxide layer on the Si oxide surface after polymer removal.

Section snippets

Experimental

P2VP-OH (P7544-2VPOH) (PDI: 1.05) with a molecular weight of 6 kg mol−1 was purchased from Polymer Source. The P2VP-OH was dissolved in tetrahydrofuran (THF) and subsequently spin-coated onto Si substrates in accordance with reference [11] to yield approximately 4 nm thick polymer films. Aluminium infiltration into the P2VP-OH film via ALD was performed in a commercial reactor (Picosun Sunale ALD R200 Advanced reactor), using TMA and H2O as precursors and N2 as the carrier gas. The substrate

XPS

Photoemission analysis was performed on the P2VP-OH films pre- and post-ALD deposition, as well as after oxygen plasma processing to examine the effects of polymer removal and metal oxidation. The XPS intensity of the relevant core levels at each stage of the fabrication process (sample of P2VP as received, sample of P2VP after ALD, sample after oxygen plasma) are shown in Fig. 1. XPS spectra of the N 1s and C 1s (Figs. S1 and S2 respectively, supporting information) reveal the presence of the

Conclusion

By optimizing a conventional, relatively quick ALD process, thin layers of Al oxide have been successfully grown through the infiltration of a P2VP film, without the need for precursor hold times found in SIS-like approaches. Clear evidence of the brush layer being receptive to aluminium via a TMA and H2O recipe has been reported for the first time. Further work is now required to optimize the reported process for maximum precursor infiltration into the polymer. XPS and EDX-TEM cross-sectioning

CRediT authorship contribution statement

M. Snelgrove: Conceptualization, Methodology, Formal analysis, Investigation, Writing - original draft, Visualization. C. McFeely: Formal analysis, Investigation, Writing - review & editing. P.G. Mani-Gonzalez: Validation, Formal analysis, Writing - review & editing. K. Lahtonen: Methodology, Investigation, Writing - review & editing. R. Lundy: Conceptualization, Methodology, Investigation, Writing - review & editing. G. Hughes: Conceptualization, Resources, Writing - review & editing,

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work has been facilitated with the financial support of Science Foundation Ireland (SFI) under Grant No. 12/RC/2278 and 16/SP/3809, and from the World Technology Universities Network (WTUN, under the WTUN Exchanges Programme). J.S. was supported by the Vilho, Yrjö and Kalle Väisälä Foundation of the Finnish Academy of Science and Letters. The research done in the Surface Science Group was supported by the Academy of Finland Flagship Programme, Photonics Research and Innovation (PREIN)

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