Structural study of thin amorphous SiO2 and Si3N4 films by the grazing incidence X-ray scattering (GIXS) method

A method has been presented for determining a local atomic structure in an amorphous thin film of sub-micron thickness grown on a substrate by the gra zing incidence x-ray scattering (GIXS) method. The capability of this metho d was demonstrated by analyzing amorphous SiO2 and Si3N4 films 200 and 70 n m thick, respectively. A network structure in the amorphous SiO2 film consi sts of SiO4 tetrahedra connecting each other by oxygen atoms at their verti ces. This resembles that in a bulk amorphous SiO2. The local ordering unit structure in the amorphous Si3N4 film was found to be a SiN4 tetrahedron. A significant feature in the present amorphous Si3N4 film is the presence of two types of Si-Si pairs in the near neighbor region while only one type i s present in the alpha-Si3N4 crystal. This indicates that a part of the net work structure formed by the SiN4 tetrahedra is quite different from that i n its crystalline state. According to the coordination number of 3.8 for Si -N pairs, some nitrogen vacancies are quite likely involved in the film. Su ch nitrogen vacancies, then, are responsible for the modified network struc ture in the present amorphous Si3N4 film.