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 CFx 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 CFx 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 CFx thin films and ab initio calculations. The calculations of DFT predicted that the precursor species most important are CF, CF2, CF3, C2, 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).

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.

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 CFx 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 CFx in C4F8 display higher levels of hardness, over a broader range, with incorporated F (Figure 2).

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

Figure 2. Hardness over the fluorine content for CFx 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.

For more information on this source, please visit Hiden Analytical.

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