In this paper we present primary hexamethyldisiloxane (HMDSO) molecule dissociation paths in a remote argon-fed expanding thermal plasma, where the HMDSO dissociation is initiated by charge exchange reactions with argon ions followed by dissociative recombination reactions with electrons. Investigation of the argon/HMDSO plasma chemistry by means of cavity ring down spectroscopy and threshold ionization mass spectrometry has allowed the detection and identification of radical species, such as CH x , SiC x H y and SiCH x O, and oligomerization products. The charge exchange reaction rate constant between argon ions and HMDSO molecules has been found to be equal to (4 ± 2) × 10 −16 m 3 s −1 . This reaction has a dissociative character, i.e. the dissociation occurring at the Si-C and Si-O bonds accompanied by the abstraction of radicals, e.g. CH 3 and OSi(CH 3 ) 3 . Under the experimental conditions investigated, the ions produced via charge exchange reactions lead to negligible ion-induced oligomerization routes. Instead, they undergo recombination reactions with electrons, leading to Si-O bond dissociation and further abstraction of methyl radicals.
Time-resolved cavity ringdown spectroscopy (τ-CRDS) has been applied to determine the surface reaction probability β of Si and SiH3 radicals during plasma deposition of hydrogenated amorphous silicon (a-Si:H). In an innovative approach, our remote Ar-H2-SiH4 plasma is modulated by applying pulsed rf power to the substrate and the resulting time-dependent radical densities are monitored to yield the radical loss rates. It is demonstrated that the loss rates obtained with this τ-CRDS technique equal the loss rates in the undisturbed plasma and the determination of the gas phase reaction rates of Si and SiH3 as well as their surface reaction probability β is discussed in detail. It is shown that Si is mainly lost in the gas phase to SiH4 [reaction rate kr=(3.0±0.6)×10−16m3s−1], while the probability for Si to react at an a-Si:H surface is 0.95<βSi<1 for a substrate temperature of 200°C. SiH3 is only lost in reactions with the surface and measurements of β of SiH3 for substrate temperatures in the range of 50–450°C show that βSiH3=(0.30±0.03), independent of the substrate temperature. The implications for a-Si:H film growth are discussed.
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