Mass spectrometer analysis of plasma containing easily dissociated species: Absolute concentrations of the main free radicals Si(CH3)1,2,3, and H2Si(CH3) produced in a microwave discharge sustained in an Ar–Si(CH3)4 gas mixture

This paper deals with the experimental methods developed using mass spectrometer to analyse the gas composition in plasma containing molecular species with a low dissociation threshold. These methods are developed to study mainly the case of a plasma produced in an Ar–Si(CH3)4 gas mixture. The Si(CH3)4 tetramethylsilane molecule is interesting because it is easily dissociated within the plasma. However, this molecule and the radicals produced in the plasma are also easily dissociated into the mass spectrometer ionization chamber. Thus, special investigation methods, taking into account the various dissociation processes of the different species, is required to correct the mass spectrometer signal intensity. Classical first correction methods are used, and concern the transmission factor depending on parameters like the m/z ratio and the emission electron current intensity into the mass spectrometer ionization chamber. We also develop a new method with the aim of correcting the dissociative ionization part of the molecular precursor involved in the signal intensity. This method is first tested in the simpler case of an Ar–CH4 mixture containing plasma, then is applied to plasma sustained in an Ar–Si(CH3)4 gas mixture. We discuss the influence of the different dissociative ionization processes to the signal intensity corresponding to the main Si(CH3)1,2,3 and H2Si(CH3) radicals observed at m/z = 73, 58, 45, and 43, at electron energy lower than 30 eV. We show that in that case, the signal intensity of these Si(CH3)1,2,3+, H2Si(CH3)+ ions is mainly due to the direct ionization of the parent radicals.