Deposition of organosilicon coatings from trimethylsilyl acetate and oxygen gases in capacitively coupled RF glow discharge
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–CH and Si–O bonds occurring in commonly used precursors, the structure of TMSAc described by linear formula 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/O 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 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 (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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