Structural and optical properties o plasma-deposited a-C:H:Si:O:N films^a

Abstract

Thin a-C:H:Si:O:N films were deposited from plasmas fed hexamethyldisiloxane, oxygen and nitrogen, and characterized as a function of the partial pressure of oxygen in the feed, R_ox. Deposition rates varied from 10 to 27 nm min^-1. Surface roughness was independent of R_ox, being around 10 nm. The films contain C=C and C=O, and also Si-C and Si-O-Si groups. Lower [C] and [N] but greater [O] and [Si] were measured in the films as R_ox was increased. Refractive indices of ~ 1.5 and optical energy gaps which fell from ~ 3.3 to ~2.3 eV were observed with increasing R_ox. The Urbach energy fell with increasing optical gap, which is characteristic of amorphous materials. Such materials have potential as transparent barrier coatings.

Keywords:
plasma enhanced chemical vapor deposition; optical band gap; urbach energy

1. Introduction

Thin films of a-C:H:Si:N^[1-8], a-C:H:Si:O^[9-20], and a-C:H:Si:O:N^[21-24], where a designates amorphous, have been produced by plasma deposition from diverse monomers and comonomers. For example, a-C:H:Si:N has been produced, among others, in plasmas fed hexamethyldisilazane and nitrogen^[1], diethylsilane and ammonia^[3], and methane, silane and nitrogen^[5]. Similarly, a-C:H:Si:O has been produced from plasmas of hexamethyldisiloxane (HMDSO)^[9], trimethylmethoxysilane and argon^[10], and HMDSO and argon^[12]. Films of a-C:H:Si:O:N have been deposited from plasmas fed HMDSO, acetylene and nitrogen^[22], and from tetramethlydisilazane, oxygen and nitrogen^[23]. Applications of a-C:H:Si:N include electrical insulation, protective layers^[1], and electronic passivation of n-type c-silicon^[25]. Amorphous C:H:Si:O films show promise as hydrophobic and corrosion protection layers^[13,18].

In the plasma enhanced chemical vapor deposition (PECVD) of HMDSO/O₂ mixtures film precursors of mass 148, such as Me₃-Si-O-Si=O-Me, where Me represents methyl, and related Si-O, Si-OH and C=O have been detected^[16]. Species present in the plasma phase include H, CH and CO^[26]. At high proportions of O₂ in the feed, O, CO, OH and H are observed, along with CO⁺ and CO₂^+[27]. Another known feature of HMDSO-O₂ plasmas is the production of SiO_x powder^[28]. There are few studies, however, dealing directly with a-C:H:Si:O:N films. An exception is the study of films produced from HMDSO-N₂ mixtures at atmospheric pressure in a dielectric barrier discharge^[29]. At high values of [N₂]/([Ar + N₂]), Si-N groups were detected in the films. Amine and silazane moieties were present.

The present study focuses on plasma-deposited a-C:H:Si:O:N produced from HMDSO with O₂ and N₂. There are few extant studies of such material. The following characterizations of the films allow a composite picture of key properties for possible applications to be mapped. Therefore, morphological, chemical, structural and compositional features, together with optical properties were obtained. Surface roughness and morphology were studied using profilometry and scanning electron microscopy (SEM) respectively. Surface contact angles were measured using goniometry. Chemical structure and composition were probed using infrared reflection-absorption spectroscopy (IRRAS) and energy dispersive X-ray spectroscopy (EDS). The refractive index, optical energy gap, and Urbach energy of the films were calculated from film thickness and ultraviolet-visible near infrared spectroscopy (UVS) data. Subsequently, this suite of properties was used to suggest possible applications.

2. Materials and Methods

The deposition system is shown in Figure 1.

Figure 1
Schematic of the PECVD system.

Films were deposited in a cylindrical stainless-steel chamber, containing two circular, horizontal, parallel-plate electrodes. Radiofrequency (13.56 MHz) power was fed from a supply (Tokyo Hy-Power, MB-300) at 70 W, via a matching network used to minimize the reflected power, to the lower electrode, while the upper electrode was grounded. Pressure was monitored using a Pirani gauge (Agilent, PCG-750). The base pressure was ~ 0.7 Pa. Nitrogen and oxygen gases (99.95% pure, White Martins, Brazil) were admitted to the chamber from cylinders via precision leak valves (Edwards, LV10-K). For HMDSO, vapor was introduced via a leak valve from a vial of the liquid reagent. For depositions, the chamber was evacuated continuously using a rotary vane pump (Edwards, E2M18). A HMDSO partial pressure of 8.0 Pa was maintained for all depositions. The stoichiometry of the films was altered by varying the partial pressures of oxygen and nitrogen, while maintaining a total pressure of these two gases at 8.0 Pa. All depositions lasted 30 min.

Substrates, placed on the lower electrode, were of glass, polished stainless steel, and quartz. The glass substrates were used for films examined by profilometry, and goniometry. Polished stainless steel substrates were used for films examined by IRRAS, SEM and EDS. Quartz substrates were employed for films examined by UVS.

Film deposition rates were calculated by dividing the film thickness, h, by the deposition time, t. Film thickness was obtained from a film deposited on a glass slide partially covered by an adhesive tape. Subsequent removal of the tape produced a well-defined film edge, which was measured using profilometry (Veeco DekTak 150). Surface roughness, R_a, was calculated using the arithmetic method from film surface profiles, where R_a is the average of the sum of the absolute values of the deviations from the mean height^[30].

Surface morphology was examined using SEM (Jeol JSM-6010LA) and elemental analysis carried out using an EDS accessory (Dry SD Hyper (EX 94410T1L11)). Surface contact angle measurements were made with drops of distilled deionized water using a goniometer (Krüss DSA25E). Ten measurements were made on each of three drops placed at different positions on the film.

Infrared reflection absorption spectroscopy (IRRAS) was used to accumulate spectra with a Jasco FTIR-410 instrument over the wavenumber range from 400 to 4000 cm^-1. Each spectrum was obtained using 128 scans at a resolution of 4 cm^-1.

A Perkin Elmer Lambda 750 ultraviolet-visible near infrared spectrometer was used to collect spectra in the wavelength interval of 300 to 2500 nm. For most of the spectra it was possible to determine the refractive index, n, of the film using a method given by Cisneros et al.^[31]. When interference maxima and minima are present in a transmittance spectrum, each extremum has an associated integer, m, given by:

$$m\approx {\lambda }_{m-1}/\left({\lambda }_{m-1}–{\lambda }_m\right)$$ (1)

Employing this relation and

$$m=4nh/{\lambda }_m$$ (2)

n may be calculated. The absorption coefficient, α(E), can also be calculated as a function of the photon energy (E). When Tauc plots exhibit a linear region, extrapolation to the x-axis (which shows the variable E) reveals the optical energy gap^[32], which we designate as the Tauc gap.

3. Results and Discussion

Figure 2 shows the film deposition rate as a function of R_ox. Oxygen is very reactive and fragmentation of HMDSO increases rapidly as R_ox is increased, roughly doubling the deposition rate for intermediate values of R_ox. Film deposition, however, is reduced by etching by atomic O. Thus, for R_ox of 4.0 Pa the deposition rate is decreased to close to its value in the absence of oxygen in the feed. For greater R_ox, etching exceeds film growth and no film is produced.

Figure 2
Deposition rate (mean and standard deviation of 6 measurements) and roughness (mean and standard deviation of 6 measurements) as a function of R_ox. Films were deposited on glass substrates.

Groups such as CH_x (x = 1 to 3) and silicon-containing molecular fragments are film precursors. As proposed by Balu et al.^[33] hydrocarbon groups may also be etched via reactions of the form:

$$R+O\to R•+OH$$ (3)

$$R+OH\to R•+H_{2}O$$ (4)

$$R•+O\to RO´+CO+C{O}_2$$ (5)

where R is the polymer backbone and O represents radical or excited oxygen produced in the plasma. The volatile species are lost to the pumping system. In addition, to such species, ions also play a role in film deposition. For example, ions such as O₂⁺ may create surface active sites, remove reaction products held on the film surface, supply energy to drive surface reactions or remove material by direct reactive ion etching^[34].

Oxygen ions, such as O₂⁺ may be produced in the plasma by reactions^[34] such as those given in Equations 6 to 9.

$$e+O_2\to O_2{}^{*}+e$$ (6)

$$\to 2O+e$$ (7)

$$\to O^{*}+O+e$$ (8)

$$\to O_2{}^{+}+2e$$ (9)

Subsequently, carbon may be etched, for example, via reactions^[34] indicated in Equations 10 and 11.

$$2\left(n+1\right)C+O_2{}^{+}+n{O}_2\to 2\left(n+1\right)CO$$ (10)

$$2\left(m+1\right)C+O_2{}^{+}+m{O}_2\to \left(m+1\right)C{O}_2$$ (11)

Figure 2 also shows the film surface roughness as a function of R_ox. While no systematic variation is observed, the absolute values are all small, ~10 nm, and the error bars of the measurements overlap. Thus, under the conditions used, etching does not significantly alter the surface roughness.

Scanning electron micrographs of the surface of the films grown at different R_ox are shown in Figure 3.

Figure 3
Scanning electron micrographs of the surface of the films deposited at (a) R_ox = 0 Pa; (b) R_ox = 0.8 Pa; (c) R_ox = 1.6 Pa; (d) R_ox = 2.4 Pa; (e) R_ox = 3.2 Pa; (f) R_ox = 4.0 Pa. Films were deposited onto glass substrates.

A few, roughly spherical features, of diameter up to ~1 µm are seen in the micrographs of the films deposited at R_ox of 0.8 to 2.4 Pa. In cold plasmas containing HMDSO particles often form^[26], and when sufficiently large become negatively charged, which keeps them suspended. As the plasma is rich in hydrogen, the particles heat-up owing to surface recombination, which is exothermic. This, in turn, increases the oxidation rate of organic matter.

Water contact angle, θ, is shown as a function of R_ox in Figure 4. There is a fall in θ from ~90 º for the film deposited without oxygen in the feed to ~73º for the film produced at an oxygen partial pressure of 3.2 Pa. Increased film oxygen content may be responsible for this.

Figure 4
Water surface contact angles (mean and standard deviation of 30 measurements) as a function of R_ox. Films were deposited on glass substrates.

Figure 5 presents IRRAS spectra for the films deposited at different R_ox. A peak centered at ~3400 cm^-1 shifts to ~3600 cm^-1 as R_ox increases. This absorption is attributed to hydroxyl groups and, at higher R_ox, specifically to OH ν in free SiOH. Absorptions at 2960 and 2900 cm^-1 are seen in the spectra of the films deposited at low R_ox. These are attributed to C-H stretching in CH₃ and CH₂, respectively. Each spectrum exhibits an absorption at ~840 cm^-1 caused by –CH₃ ρ in Si(CH₃)₃^[19,35,36].

Figure 5
IRRAS spectra of films deposited at different values of R_ox. Film thicknesses were as follows: R_ox = 0 Pa (286 nm); R_ox = 0.8 Pa (805 nm); R_ox = 1.6 Pa (652 nm); R_ox = 2.4 Pa (667 nm); R_ox = 3.2 Pa (617 nm); R_ox = 4.0 Pa (307 nm). Films were deposited onto polished stainless-steel substrates.

Although oxygen is present in the HMDSO molecule, as residual gas in the deposition chamber, and it is also known that post-deposition reactions may occur between free radicals in plasma polymers and ambient oxygen and

water^[37,38], no significant absorptions attributable to hydroxyl groups are present in the spectrum of the film deposited at zero R_ox. For all non-zero R_ox, however, hydroxyl groups are present in the film. This suggests that the incorporation of hydroxyl groups is mainly dependent on oxygen deliberately introduced into the chamber feed.

Silicon is not bonded to hydrogen in HMDSO but an absorption at ~2100 cm^-1, attributed to SiH, is present in the spectrum of the film grown at zero R_ox. Overlapping absorptions occur near 1000 cm^-1, are attributed to Si-O-Si and Si-O-C. Ethylene and methylene groups are expected to be produced by moderate fragmentation of the HMDSO molecule. The Si-O-Si structure is also found in the monomer. No Si-O-C structures are present in the monomer. A small shoulder is apparent at ~1160 cm^-1 in the spectra of films grown at greater R_ox, and this is attributed to Si-CH₂-CH₂-Si and is indicative of a loss of hydrogen in relation to the HMDSO molecule. As nitrogen concentrations in the films are only a few at.% (as discussed below in relation to the EDS data) there are no well-defined absorptions caused by nitrogen-containing groups.

A small band, whose center lies in the 1630 to 1700 cm^-1 region, depending on R_ox, may be caused by stretching in C=C and C=O, respectively. Neither of these structures (nor SiH) is present in the monomer molecule, indicating that multiple-step reactions occur in their formation.

Figure 6 shows the relative concentration, calculated using the method of Lanford and Rand^[39], of Si-O bonded to Si or C, and of OH as a function of R_ox. As Si-O-Si is the central structure of the HMDSO molecule, it is retained at high concentrations despite considerable variation in [O] and [C] as discussed below in relation to EDS analyses. The steep rise in [OH] with increasing R_ox clearly indicates the strong incorporation of oxygen as a hydroxide.

Figure 6
Relative concentration of Si-O-Si, Si-O-C, Si-CH₃, CH, and OH as a function of R_ox. These values were calculated from the area of the relevant infrared absorption divided by the film thickness, and were normalized to the maximum obtained for each species.

As also shown in Figure 6, [Si(CH)₃] and [CH] fall with increasing R_ox and rise only for R_ox > 3.2 Pa. This reflects the loss of hydrogen and carbon with increasing R_ox.

Figure 7 shows the concentration (at.%) of the elements C, O, Si and N as determined by EDS as a function of R_ox (the hydrogen content cannot be measured and has been ignored). In the HMDSO molecule the number O, Si and C atoms are, 11%, 22% and 67%, respectively. Compared to this, [O] and [Si] increase, while [C] decreases, in the film deposited without oxygen in the feed.

Figure 7
Atomic concentration of C, O, Si and N in the films determined by EDS as a function of R_ox.

The concentration of carbon, [C], falls with increasing R_ox, except beyond a R_ox of 3.2 Pa, where it rises. Both [O] and [Si] rise with increasing R_ox, except beyond a R_ox of 3.2 Pa, where the concentrations of both elements fall. These decreases in [O] and [Si] at high R_ox may be caused by synergistic effects between O and O₂⁺, producing etching and sputtering. For R_ox in the interval from 0.8 to 3.2 Pa, θ falls and [O] rises (Figure 4 and Figure 7). For R_ox greater than 3.2 Pa, θ rises and [O] falls. Thus, greater values of [O] are associated with smaller contact angles. Inspection of Figure 7 also shows that [N] falls from ~6 at.% for zero R_ox to ~0 at.% when R_ox is increased to 4.0 Pa. This reflects the diminishing supply of nitrogen in the plasma feed as R_ox is increased.

Figure 8 shows the transmittance spectra of the films grown at different R_ox in the ultraviolet-visible near infrared region. Multiple interference extrema are observed for wavelengths above about 500 nm. The inset of Figure 8 shows the refractive index of the deposited material as a function of R_ox. Typical values of n are ~1.5, lower than those found for films deposited from diethylsilane, ammonia and nitrogen, which are typically 1.7 to 1.8, depending on the deposition temperature^[2]. The films of the present study, however, differ in having low nitrogen contents. As the refractive index depends on the effective polarizability, the molar mass and the density^[40], the relative constancy of n implies that the effects in any changes in these parameters tend to cancel out. Values of n close to 1.5 were also found by Mota et al.^[19] for a-C:H:Si:O films deposited from HMDSO. Similarly, the values reported here are consistent with those of 1.55 reported by Amri et al.^[24] for films deposited from HMDSO and nitrogen at low power (20 W).

Figure 8
Transmittance spectra for films deposited at different R_ox. Films were deposited onto quartz substrates. Inset shows the refractive index of the deposited material as a function of R_ox.

Tauc plots of the transmission spectra of the films, shown in Figure 9, allow the determination of the optical gap. The optical gap, shown as a function of R_ox in Figure 10, falls from ~ 3.3 to ~2.3 eV as R_ox is increased. This interval overlaps with that of 1.87 and 2.7 eV reported by Swatowska^[8] for a-C:H:Si:N films deposited from CH₄, SiH₄ e NH₃. Greater nitrogen or carbon contents tend to increase the gap^[8]. Thus, the decline in [C] and [N] with increasing R_ox may account, at least in part, for the observed fall in the gap. The decline also depends, however, on the densities and types of dangling, single and multiple bonds. The (unknown) hydrogen content probably accompanies the fall in [C]. Thus, the density, for example, of Si-O probably increases while that of Si-C decreases. As bond lengths of Si-O and Si-C are about 163 pm^[41] and 185 pm^[42], respectively, such changes are expected to modify the optical transmittance and hence the gap.

Figure 9
Tauc plots for films deposited at different R_ox. Inset shows the variation of ln(α(ν)) vs. the photon energy.

Figure 10
Optical gap of the deposited material as a function of R_ox. Inset shows the Urbach energy as a function of the optical gap.

A fall in the band gap of a-C:H:Si:O films at greater applied powers has been attributed to a greater density of dangling bonds, which are responsible for the formation of localized defects and hence of localized states in the band structure^[43]. The reduction in the gap with increasing R_ox is also consistent with the finding that greater Si-O-Si and Si-O-C concentrations in HMDSO plasma films are associated with lower gaps^[19].

The inset of Figure 9 shows plots of ln(α) v E, for each film, from which Urbach energy, E_U, was calculated. The resulting E_U values are shown as a function of the optical gap in Figure 10 (inset). A linear fall is observed, which is consistent with the behavior of other amorphous materials^[44]. Greater values of E_U (found at higher R_ox) are associated with disordering and defect states and thus with a reduction in the optical gap.

4. Conclusions

Smooth, hydrophobic a-C:H:Si:O:N thin films may be produced from cold plasmas fed mixtures of HMDSO-O₂-N₂. The films have a complex structure, containing a network with Si-O-Si and C-C chains, similar to that reported by Hilbert et al.^[45] but with greater Si and O contents. The oxygen content generally rises, and the carbon content falls with increasing R_ox. Silicon contents are around 20 at.% and nitrogen contents a few at.%. At low R_ox there are relatively high concentrations of Si-CH₃ groups, which accounts, at least in part, for the hydrophobicity of these films. As R_ox increases, the incorporation of oxygen and complex structural changes, cause a decrease in the optical gap. Lower Urbach energies are associated with greater optical gaps.

Owing to the combination of hydrophobicity, smoothness, good adhesion to diverse substrates, and high optical transparency, films grown at low R_ox may find application as transparent barrier coatings. Similar films have been investigated as interlayers for SiO_x diffusion barrier coatings on polypropylene, but in this case plasma-polymerized HMDSO offered the advantage of greater deposition rates ^[22]. Films deposited from HMDSO-O₂-Ar mixtures may find application as multilayered organosilicon/silica films for the protection of metal surfaces^[20]. Although there is a report of plasma-deposited nanocomposite hydrogenated silicon oxycarbonitride films obtained from tetramethlydisilazane-O₂-N₂ mixtures^[23], characterizations beyond chemical composition and structure are required to suggest possible applications of this material. The mechanical properties of our films, such as hardness, Young’s modulus and stiffness, are also relevant to possible applications, but require separate study.

5. Acknowledgements

The authors thank Fundação de Amparo à Pesquisa do Estado de São Paulo (2017/15853-0) and Conselho Nacional de Desenvolvimento Científico e Tecnológico for financial support. This study was also financed in part by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil, CAPES - Finance code 001. We thank Prof. José H. Dias da Silva for helpful discussion of the optical data and Jamille Altheman for technical assistance with the SEM/EDS measurements.

• a
  This paper has been partially presented at the 3rd International Conference on Materials Sciences and Nanomaterials, ICSMN 2019, Oxford University, Oxford, UK, 22nd to 24th July 2019.
• How to cite: Lopes, J. F. S. C., Tardelli, J., Rangel, E. C., & Durrant, S. F. (2021). Structural and optical properties of plasma-deposited a-C:H:Si:O:N films. Polímeros: Ciência e Tecnologia, 31(3), e2021035. https://doi.org/10.1590/0104-1428.210043.

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Publication Dates

• Publication in this collection
  07 Jan 2022
• Date of issue
  2021

History

• Received
  01 June 2021
• Reviewed
  06 Oct 2021
• Accepted
  29 Nov 2021