Elsevier

Applied Surface Science

Volume 455, 15 October 2018, Pages 1179-1184
Applied Surface Science

Full Length Article
Hydrogen plasma etching mechanism at the a-C:H/a-SiCx:H interface: A key factor for a-C:H adhesion

https://doi.org/10.1016/j.apsusc.2018.05.203Get rights and content

Highlights

  • Hydrogen etching radically modifies the interlayer thickness and Si/C ratio.

  • Amorphous carbon coatings adhesion depends on Si/C ratio in the interlayer.

  • Etching shows two behaviors based on the non-homogeneity of the interlayer.

  • Hydrogen plasma etching affords good adhesion of amorphous carbon coatings.

Abstract

Hydrogenated amorphous carbon thin films (a-C:H) do not show good adhesion on ferrous alloys. When a silicon-containing intermediate thin film is chosen, deposition temperatures of at least 300 °C are necessary to provide adequate adhesion. However, a new approach is introduced to achieve similar results at lower temperatures: a selective chemical hydrogen plasma etching before the carbonaceous film deposition. The effect is known but its physicochemical mechanism and kinetics remain open. We have investigated the role of hydrogen etching in the modification of the outermost and innermost interfaces of a-SiCx:H interlayers. The results show that hydrogen radically modifies the interlayer thickness in the outermost region and increases the C/Si ratio in the innermost. Moreover, the hydrogen etching allows the formation of more Csingle bondC bonds at the outermost interface, which is correlated to better adhesion. Finally, we proposed a mechanism in order to explain physical and chemical changes caused by hydrogen etching.

Introduction

Interfacial chemical analysis of multi-nanolayers in solid state is a cutting-edge issue due to the very few available techniques to explore the interfacial chemical structure of two solid thin films in a fast and easy way. In particular, this analysis of solid/solid interfaces might provide evidences about the physicochemical origin of adhesion forces of coatings. Although, techniques like electron energy-loss spectroscopy and atom probe tomography can analyze solid interphases and grain boundaries they are time-consuming and not widespread available.

Hydrogenated amorphous carbon (a-C:H) thin films are state-of-the-art coatings with characteristic properties such as ultra-low friction coefficient, chemical inertness and low wear rates [1], [2], [3], [4], [5], [6]. Nevertheless, a-C:H coatings show poor adhesion on ferrous alloys. In order to overcome such a problem, Cr, CrN, Ti, Zr, Mo and W interlayers are used [7], [8], [9]. Silicon-containing interlayers are also used due to the versatility of depositing these thin films in a simple way by a low cost PECVD (plasma-enhanced chemical vapor deposition) technology [10], [11], [12], [13], [14].

Recently, X-ray photoelectron spectroscopy could confirm that oxygen degrades the a-C:H adhesion on ferrous alloys when a silicon-containing interlayer is deposited at temperatures lower than 300 °C [15]. Consequently, the role of interfacial chemistry is crucial to promote adhesion. In the attempt to improve the a-C:H adhesion on ferrous alloys in low temperatures (≤300 °C), new chemical modification strategies at the a-C:H/a-SiCx:H interface must be contemplated.

One current possibility is to use a plasma etching to generate a selective cleaning of the interlayer outermost interface. More dangling bonds can promote better adhesion, based on chemical interactions from both layers at the interface. In fact, more Csingle bondC bonds prompt better adhesion of a-C:H on ferrous alloys intermediated by a silicon-containing interlayer [15]. Moreover, research groups have used different halogenated gases (such as CF4 and SiF4) to promote selective etchings [16], [17], [18] and confirmed a difference between Si and C etching rates depending on the precursor and its interactions.

Hydrogen is a versatile tool for etching pure silicon [19], [20], [21], nitrogen implanted iron alloys substrates [22], [23], silicon carbide films [24], [25] and hydrogenated amorphous silicon films [26], [27]. Under relative low temperatures (such as 70 °C), it is possible to observe high desorption rates of volatile silicon-based compounds from solid silicon, in a passivation-desorption mechanism [19]. Using these experimental facts, this work has used pure hydrogen plasma likewise other experiments recently published in our research group, which report an adhesion improvement in a phenomenological way only [28]. Therefore, the mechanism, kinetics and the hydrogen chemical effect at the a-C:H/a-SiCx:H interface are still not explored and discussed in light of a physicochemical approach.

Consequently, the aim of this work is to investigate the chemical mechanism during the hydrogen etching process that guarantees adhesion of a-C:H on steel by inspecting the chemical structure of the a-C:H/a-SiCx:H solid interface.

Section snippets

Experimental

We produced a set of samples with a “sandwich” structure composed of a-C:H thin film//SiCx:H interlayer//substrate (ferrous alloy) with pulsed-DC PECVD assisted by electrostatic confinement [29]. The hydrogen etching was performed after interlayer deposition, which represents the outermost interface, varying the processing time (0, 1, 2, 4, 6 and 10 min) in order to investigate the kinetics. Following this, a protective thin film of a-C:H was immediately deposited during 1 min to prevent other

Results and discussion

We show four representative chemical profiles obtained by GDOES where it is possible to observe the chemical modifications that were made at different hydrogen plasma processing times (Fig. 1a–d). The space between vertical dashed lines defines the interlayer region due to an established criterion in the first derivate change for carbon and silicon signals as explained elsewhere [12], [13], [14], [15]. The a-C:H thin film region does not show differences in terms of the carbon profile because

Conclusion

Summing up, it was possible to propose a physicochemical mechanism to discuss and understand the hydrogen etching influence in silicon-containing interlayers. The non-homogeneity of the structure plays an important role to distinguish two different regions and, consequently, two different behaviors. The thermodynamic stability of volatile carbon and silicon species controls the chemical kinetic in outermost and innermost regions of the interlayer. Better adhesion is related to the formation of

Acknowledgment

The authors are grateful to UCS, INCT-INES (# 465423/2014-0), and Plasmar Tecnologia Ltda. for financial support. L.M.L., A.E.C., C.D.B., F.G.E. and C.A.F. are CNPq or CAPES fellows. The authors would like to thank LCMIC-UCS for microscopy facilities, C.H. Wanke for helping in FTIR measurements and all Epipolé Research Group colleagues for the support during this work.

References (39)

Cited by (9)

  • Influence of base pressure prior to deposition on the adhesion behaviour of carbon thin films on steel

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    The oxygen presence at the interfaces of the carbon thin film//interlayer//ferrous alloy system comes from either the residual gases absorbed on the walls or the residual air into the chamber during plasma deposition [31]. Different methods to remove or minimise the oxygen content have been proposed, such as, the use of a relatively high deposition temperature or hydrogen etching [32,33]. However, there is a lack of systematic studies to evaluate the influence of the chamber base pressure on the adhesion of carbon thin films on ferrous alloys without any other treatment.

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