Layered silicon/silicon nitride structures were fabricated on (100) Si with 0.1 mTorr of N2 background pressure. Evaporated Si (PVD) layers were alternated with layers grown by simultaneous nitrogen ion bombardment and Si evaporation (IBAD). The ion to vapor flux ratios were chosen to yield stoichiometric Si3N4 (XN=0.57) at 150, 500, 1000 and 1500 eV and varied at a constant energy of 500 eV to yield sub- and superstoichiometric Si1-xNx. The microstructures were examined by cross-sectional and plan-view TEM; composition of the layers was deteiTnined by RBS. Films grown at low substrate temperatures were amorphous. Stoichiometric and superstoichiometric compositions showed a cellular (5 nm) structure in the amorphous IBAD layers. In addition, the IBAD Si1-xNx layers showed variations in contrast parallel to the substrate interface, indicative of compositional fluctuations on a scale of <20 nm. The IBAD layers were dense, but the PVD-Si layers contained voids that were aligned with the direction of the impingement of the evaporant on the substrate surface. A high substrate temperature (750°C) yielded polycrystalline Si layers and phase-ordering in the Si1-xNx layers.