INFLUENCE OF GAS RESIDENCE TIME ON THE DEPOSITION OF NITROGEN-RICH AMORPHOUS-SILICON NITRIDE

The importance of gas phase reactions in plasmas containing ammonia and silane used to deposit amorphous silicon nitride was first demonstrated by Smith et al. in 1990 [J. Electrochem. Sec. 37 (1990) 614]. These workers concluded that, at high NH3/SiH4 ratios (R = 25) and rf power densities (> 60 mW cm(-2)), it was possible to fully aminate all of the available silane in the plasma to produce tri- and tetra-aminosilanes as the dominant plasma species. Nitride films grown from such an aminosilane plasma regime were nitrogen-rich and contained no measurable Si-II. bonding (by FT-IR). In this work, optical emission spectroscopy (OES) and as-deposited film bonding data have been used to establish deposition conditions consistent with such an aminosilane plasma regime, The gas residence time, tau, was then varied by altering the total gas flow to the reactor while keeping pressure, NH3/SiH4 ratio, rf power and substrate temperatures constant. A gradual shift from an aminosilane to a silane dominated plasma, as the residence time is decreased, was observed (via OES of the SiH A(2) Delta emission at 414.2 nm). In addition, at low tau, the re-appearance of Si-H bonding in the as-grown films indicates a significant decrease in the degree of amination of the available silane in the plasma, consistent with the OES results. Conversely, it is shown that full amination of the silane is possible even with a lower NH3/SiH4 ratio and rf power density than those used by Smith et al. [J. Electrochem. Sec. 37 (1990) 614], provided the residence time is sufficiently long.