Invited Paper PS1-ThM7
Growth Precursor Measurements and Study of Plasma Chemistry by Means of Mass Spectrometry

Thursday, October 18, 2007, 10:00 am, Room 606

The knowledge of absolute fluxes of reactive species such as radicals or energetic ions to the surface is crucial in understanding the growth or etching of thin films. These species have due to their high reactivity very low densities and their detection is therefore a challenging task. Mass spectrometry (MS) is an ultra sensitive technique and it will be demonstrated in this talk that it is an optimal choice for identification of growth precursors and for the study of plasma chemistry in general. MS measures the plasma composition directly at the surface, it is not limited by (non)existence of accessible optical transitions, as is for example laser spectroscopy, and when properly designed and carefully calibrated, it provides absolute densities of measured species. Two examples of application of MS will be presented. First, the composition of remote argon/acetylene expanding thermal plasma at the position of the substrate has been analyzed by means of molecular beam threshold ionization MS. More than twenty species have been detected including radicals with densities as low as 10¹⁰ cm^-3. Resonantly stabilized radicals, with C₃ being the most important one, have been identified as growth precursors of hard hydrogenated amorphous carbon films and the plasma chemistry pathway leading to their formation has been understood. In the second example, a temporal evolution of neutral species densities during initial stage of dust particle formation in a low pressure acetylene discharge has been measured. Mass spectra with time resolution of 100 ms have been obtained using a step-scan approach and they have been decomposed and quantitatively and qualitatively analyzed using Bayes statistics and calibration measurements. Based on a comparison of our results with in literature available positive and negative ion mass spectra measurements and plasma chemistry modeling of comparable plasma, the electron attachment to larger C_nH₂ species is proposed as an initial step in dust particle formation. Additionally, the analyses indicates the involvement of vinylidene, isomer of acetylene, or vinylidene anion in formation of first aromatic ring and it shows that surface reactions are a significant source of aromatic compounds.