Synthesizing CFx Films by Reactive High Power Impulse Magnetron Sputtering of Carbon in Argon/TetraFluoromethane (Ar/CF4) and Argon/Octafluorocyclobutane (Ar/c-C4F8) Atmosphere

The atmospheres of octafluorocyclobutane and tetrafluoromethane were used to demonstrate the synthesis of amorphous CF_x thin films by reactive high power impulse magnetron sputtering (rHiPIMS). The characterization of plasma and other depositions using positive ion mass spectrometry, with Hiden EQP 1000, was achieved in an industrial coater at a process pressure of 400 mPa, and at a substrate temperature of 110ºC. The composition of the CF_x film is estimated by the analysis of elastic recoil detection, within the range of 0.15 < x < 0.35, by maintaining the partial pressure of F-containing reactive gases from 0 mPa to 110 mPa.

Using Positive Ion Mass Spectrometry for Plasma Characterization

The results obtained from the plasma and process characterization were correlated to the properties of CF_x thin films and ab initio calculations. The calculations of DFT predicted that the precursor species most important are CF, CF₂, CF₃, C_2, and F for film growth in mixtures of Ar/CF₄. For discharges of carbon present in mixtures of Ar/C₄F₈, C₂F₂ was also predicted to be significant. The results of the average time of positive ion mass spectrometry are in agreement with the theoretical calculations. The cations of C⁺, Ar⁺, CF⁺, CF³⁺,CF₂+ and F+ were found to be in abundance (Figure 1a and Figure 1b).

Figure 1. a) and b): Relative ion fluxes as a function of the reactive gas partial pressure for discharges in a) Ar/C₄F₈ and b) Ar/CF₄ extracted from time averaged IEDFs of corresponding processes.

The process characterization of rHiPIMS displayed two routines of deposition based on the partial pressure of F-containing reactive gas; an ionization cascade progresses as the partial pressure rises above 42 mPa in Ar/CF₄ plasmas, this results in a rising peak target current and the formation of CF₄ fragments increasing, especially CF₃. In C₄F₈ plasmas the ionization cascade onset for partial pressures was observed above 11 mPa, accompanied with an increased production of CF.

The two routines are identical in the thin film properties, predominantly in hardness, elastic modulus, and structure of their chemical bonds. The mechanical reaction of the CF_x films can even be correlated to the precursor ions that are abundantly present in the plasma. In C₄F₈ discharges, next to C⁺ and Ar⁺, CF⁺, species are found in abundant quantities. CF and CF⁺ have three hanging bonds that help to build strong cross links within the carbon matrix. Film deposits of CF_x in C₄F₈ display higher levels of hardness, over a broader range, with incorporated F (Figure 2).

Figure 2. Hardness over the fluorine content for CF_x films grown in Ar/C₄F₈ (black squares) and Ar/CF₄ (red circles).

On the other hand, CF³⁺ discharged by Ar/CF₄ was determined to be the species with very high abundance. There is one bond dangling in CF³⁺ and its neutral counterpart, so it does not contribute significantly to the growing film cross linking.

Conclusion

C₄F₈ has advantages with regards to the controllability of the film properties, and CF₄ covers a wide range of the applicable process window (thin film deposition and etching). The dissociation of CF₄ into primarily CF₃ and F can be used for surface treatments and surface termination, leading to low surface energies. As a result, the processes of rHiPIMS in CF₄ or C₄F₈ provide a tool that is versatile in the functionalization of carbon, as well as carbon-based surfaces and thin films.

This information has been sourced, reviewed and adapted from materials provided by Hiden Analytical.

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