Subsurface reactions of silicon nitride in a highly selective etching process of silicon oxide over silicon nitride

In situ subsurface reaction measurements of silicon nitride in a selective etching process of silicon oxide over silicon nitride using an x-ray photoelectron spectroscopy (XPS) and a Fourier transform infrared reflection absorption spectroscopy (FTIR RAS) have been carried out. Under low selectivity etching conditions using an electron cyclotron resonance plasma employing a pure octafluorocyclobutane (C4F8) gas, a clear difference has been observed between time-evolution spectra of FTIR RAS and those of XPS on the etched silicon nitride films. From these results it has been found that the etching reaction layer is thicker than that under highly selective etching conditions and that SiF3 bonds are located in the deeper region rather than in the shallow region of the reaction layer. On the other hand, under highly selective etching conditions employing C4F8 gas diluted by Ar gas, it has been observed that the shallow region of the reaction layer in the etched silicon nitride films becomes C–C cross-linking-rich and CN sp2 bond-rich. From these results, it is concluded that the C–C cross linking prevents CN sp2 bonds from reacting with F atoms, resulting in the suppression of the etch by-products such as FCN.