Engineered thin films consisting of periodic arrays of silicon microcrystallites in a hydrogenated amorphous silicon host matrix have been prepared by plasma-enhanced chemical vapor deposition where the hydrogen dilution of silane is modulated in multiple cycles. These types of films have been guided by a phase evolution diagram, depicting the deposition conditions and film thickness at which the material exhibits amorphous, microcrystalline, or mixed-phase (amorphous+microcrystalline) characteristics, developed for intrinsic Si: H prepared with varying H-2 dilution on unhydrogenated a-Si: H. Real time spectroscopic ellipsometry (RTSE) has been used in situ to noninvasively determine the phase evolution of the resulting hydrogenated mixed-phase (amorphous+microcrystalline) silicon thin films and corroborated with dark-field transmission electron microscopy. Such tailored microstructures are of growing interest as components of thin film photovoltaic devices, and RTSE is shown to be a key technique for structure verification. (C) 2009 American Vacuum Society. [DOI: 10.1116/1.3212893]