Molecule synthesis in an Ar–CH4–O2–N2microwave plasma

The formation of new molecules in a microwave plasma, created from a mixture of Ar, CH 4 , N 2 and O 2 , is investigated by means of an in-depth study of the molecular abundance in the plasma. The molecules are detected by means of tunable diode laser absorption spectroscopy and by absolute mass spectrometry. Three groups of molecules can be discerned in terms of molecular abundance: CO is predominantly formed, together with H 2 O, N 2 and H 2 . The molecules CH 4 and O 2 are significantly depleted, but still abundant in a finite quantity. The third group is formed by several other species like NH 3 , NO, HCN etc. This tendency is expected to occur in every low temperature plasma containing C, O, H and N atoms. Furthermore, the combination of both techniques also allows us to make a clear distinction between the etching mode and deposition mode of the microwave reactor. Etching mainly occurs when the ratio of admixed gas flows Φ(O 2 )/Φ(CH 4 ) > 0.5.