FTIR reflectance spectroscopy of SICxNyOz(H) ceramic coatings. Structure interpretation by optical modelling

SiCxNyOz(H) ceramic coatings become increasingly attractive because of their superior properties as hardness, wear resistance, and chemical stability. The layers are deposited on steel substrates by plasma enhanced chemical vapour deposition (PE-CVD) using Hexamethyldilazane as precursor. The influence of RF power (3W - 90W) on structure and optical properties of the coatings has been investigated.On a first glance the composition of the deposited material seems to be strongly dependent on deposition conditions, e.g. on RF power of the plasma. But because of the appearance of intense interference fringes a straightforward interpretation of the reflectance spectra is virtuallyimpossible. To support the interpretation, the spectra have to be transformed into absorption spectra or into the dielectic function. A specially designed spectra fit procedure has been developed which is based on a Lorentzian shaped set of oscillators to model the dielectric function and a multilayer model which c overs a wide range of different optical configurations. The powerful GSA (generalised simulated annealing) algorithm has been adapted to fit a set (three or more) of measured reflectance spectra with differential angle of incidence and the two polarisations to the established optical model. The GSA procedure has been applied to both fitting the originally measured reflectance spectra and for fitting their derivatives. The latter attempt has been found more convenient in regions of low absorptivity (n > 1800cm-1). The imaginary part of the resulting dielectic function indicates the existence of different functional groups within the network structure of the layer: Si-H (2150 cm-1, 2020 cm-1), C-H (2890 cm-1), N-H (3350 cm-1), SiC (820 cm-1), Si-N (870 cm-1, 460 cm-1), Si-NH-Si (1550 cm-1) and Si-O (1100 cm-1).Surprisingly, the network structure does not significantly depend on the RF power applied during deposition. Only small changes of the oscillator strength has been detected. It is concluded that the extremely strong influence of the deposition conditions on the reflectance spectra is mainly an optical effect. It is not the change of chemical structure which caused the observed effect but the dependence of layer thickness on RF power and, to a smaller degree, the change of the (high frequency) refractive index.