Full Length ArticleHydrogen plasma etching mechanism at the a-C:H/a-SiCx:H interface: A key factor for a-C:H adhesion
Graphical abstract
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 CC 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.
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