Deposition of organosilicon coatings from trimethylsilyl acetate and oxygen gases in capacitively coupled RF glow discharge

https://doi.org/10.1016/j.porgcoat.2020.105927Get rights and content

Highlights

  • Plasma polymerization of trimethylsilyl acetate (TMSAc) monomer using RF capacitively-coupled discharge.

  • Properties of prepared organosilicon coatings vary with the ratio of monomer in trimethylsilyl acetate/O2 gas mixture.

  • Variation of deposition parameters leads to the formation of hydrophilic SiO2-like coatings to the hydrophobic polymeric structures.

Abstract

The aim of the present work was to prepare SiOxCyHz coatings in capacitively-coupled RF glow discharge from gaseous mixture of trimethylsilyl acetate (TMSAc) monomer and oxygen. The properties of deposited coatings have been investigated using several characterization methods. The presented study proves that properties of TMSAc-based thin films are significantly dependent on the monomer ratio in TMSAc/O2 mixture which varied from 25.0% to 92.3%. Using a low TMSAc ratio (high content of oxygen gas) resulted in organosilicon materials similar to SiO2 in terms of chemical composition and mechanical properties. Increasing the fraction of TMSAc in gaseous mixture led to the formation of more organic polymeric structures enriched with carbon atoms bonded in CHx chains. Values of Martens hardness decreased from 3.97 to 0.76 GPa with increasing TMSAc fraction from 25.0% to 92.3%. The degree of hydrophobicity of prepared coatings evolved in accordance with observed structural changes corresponding to applied monomer ratios. The water contact angle gradually increased from 75.0° to 95.3° with increasing TMSAc fraction. The prepared TMSAc plasma polymers became more hydrophobic with more intensive integration of hydrocarbon species into the surface structure.

Introduction

Organosilicon thin films belong to intensively studied materials because of their great potential to be applied in many industrial fields. In recent years, organosilicon materials prepared via the PECVD method suitable for corrosion protection of metal surfaces and protection of plastic substrates were reported [1], [2], [3], [4], [5], [6], [7], [8], [9]. These types of coatings were examined as well as low-k dielectrics for microelectronics, anti-reflection coatings for solar cells, water-repellent surfaces, oxygen/moisture barrier films, etc. [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. Furthermore, organosilicon compounds prepared by plasma technology have become an integral part of the development of industrial composite materials [21], [22], [23], [24], [25].

Surfaces based on organosilicon precursors prepared by PECVD also exhibit specific properties that make them interesting for medical applications. Organosilicon materials prepared by plasma techniques are promising in this field not only because of satisfactory mechanical and anti-corrosion properties but also in terms of behavior of different types of cells and immobilization of biomolecules [26], [27], [28], [29], [30], [31], [32], [33], [34]. Surfaces modified by depositing organosilicon coating can be used for optimization of cell attachment, cell proliferation and protein adsorption. For this reason, organosilicon-based materials have a great potential in development of implants with high biocompatibility or in modification of cell-culture dishes and biosensors [26], [27], [28], [29]. In recent years, plasma polymerization of organosilicon precursors by remote PECVD has shown its potentiality in BioMEMS microfabrication [30], [31], [32]. On the other hand, a number of scientific groups are engaged in the development of surfaces with a negative effect on adhesion of cells [33], [34], [35], [36], [37], [38], [39], particularly bacterial cells. Although organosilicon coatings themselves have the potential to inhibit the formation of bacterial biofilm [34], most of the reported antibacterial coatings are based on nanocomposite structure including organosilicon matrix and silver or copper nanoparticles [35], [36], [37], [38], [39].

Desired properties of organosilicon material are achieved by optimization of the deposition process, including the choice of proper precursor and deposition parameters [6], [10]. Hexamethyldisiloxane (HMDSO), trimethylsilane (TMS), tetramethyldisiloxane (TMDSO) and tetraethoxysilane (TEOS) are among the most widely used monomers for the preparation of organosilicon materials [10], [17], [2], [4], [32], [34].

The present work discusses the character of thin films based on trimethylsilyl acetate (TMSAc) monomer, which is unique due to its chemical structure [40]. Besides Si–CH3 and Si–O bonds occurring in commonly used precursors, the structure of TMSAc described by linear formula CH3CO2Si(CH3)3 includes the –(C=O)–O– group. The presented ester group could be possibly converted to the carbonyl or carboxyl group during the PECVD process [41]. Thanks to this possibility of integration of these groups into resulting coatings, the research of TMSAc-based coatings is beneficial not only for the development of thin films for industry (e.g. protective coatings, low-k dielectrics, etc.) but also for creation of hydrophilic surfaces for bioapplications doped by C=O/COOH groups, e.g. coatings improving cells adhesion and surfaces suitable for immobilization of biomolecules through presented functional groups [41]. Similarly to other organosilicon monomers (e.g. HMDSO), using TMSAc monomer is advantageous for the formation of protective hydrophobic surfaces for industry [14], [15] and for medical applications as well (e.g. testing surfaces or coatings repulsing biological components suitable for the fabrication of biochips) [27], [42]. This study is focused on organosilicon coatings prepared using TMSAc/O2 plasma of a low pressure capacitively-coupled discharge.

Section snippets

Preparation of TMSAc coatings

Organosilicon coatings were prepared in RF glow capacitively-coupled discharges from 97% TMSAc monomer supplied by Sigma–Aldrich (with vapor pressure p=46.66 hPa at 30 °C) [43] and oxygen in a parallel plate reactor R1 described in more detail in [44], [45]. Silicon and glass substrates were placed on the bottom carbon electrode which was coupled to an RF generator (13.56 MHz) via a blocking capacitor. Thin films were prepared using different flow rates of the used gases, where the total flow

Chemical composition-FTIR

Graphs of relative absorbances divided by thickness Arel/d (Section 2.2, Eq. (1)) in MIR spectral range from 500 cm−1 to 4000 cm−1 (0.062–0.496 eV) shown in Fig. 2 include several absorption peaks characteristic for organosilicon coatings which were identified according to available literature [4], [5], [10], [54], [55], [56], [57].

Medium absorptions presented in measured IR spectra at lower wavenumbers (750 cm−1 to 950 cm−1) including peaks at 800 cm−1, 840 cm−1 and 890 cm−1 are probably

Conclusion

In the present work organosilicon plasma polymers were prepared using TMSAc/O2 plasma of capacitively-coupled RF glow discharge. The variation of TMSAc fraction in a gaseous mixture from 25% to 92.3% resulted in smooth TMSAc-based thin films exhibit mechanical, optical and surface properties in wide range from properties of SiO2-like materials to properties of soft organic plasma polymers.

The increasing ratio of TMSAc monomer in gas mixture leads to the formation of SiOxCyHz materials with a

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgements

The present work was supported by Czech Science Foundation under project GACR 19-15240S and by the project LM2018097 funded by Ministry of Education Youth and Sports of the Czech Republic. Štěpánka Kelarová is Brno Ph.D. Talent Scholarship Holder-Funded by the Brno City Municipality. A part of the present work focused on the study of surface free energy was performed by Michal Kuchařík thanks to the SOČ project of Department of Physical Electronics of Masaryk University supporting the education

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