Boron nitride (BN) films with high cubic content were deposited using pulsed dc sputtering of a B4C target and. rf biasing of the substrate. The film deposition was characterized using ultraviolet-extended real-time spectroscopic ellipsometry (UV-RTSE). A three-layer optical model with constrained evolution of the layer thicknesses was used to characterize the growth of the BN films on c-Si substrates. This model was chosen for consistency with previous research and includes (i) an initial hexagonal BN (hBN) layer; (ii) a hexagonal + cubic BN (hBN+cBN) mixed-phase transition layer; and (iii) a topmost cBN layer. The evolution of the layer thicknesses was obtained as a function of the substrate bias (V-avg) used to deposit the BN films under static conditions. These results showed a minimum in the thicknesses of the hBN and mixed-phase layers, thus identifying an optimum Vavg for cBN nucleation and growth from the hBN phase under the deposition conditions used here. Complementary ex situ infrared transmission spectroscopy confirmed that UV-RTSE provides high sensitivity for resolving the crystallographic phase of BN due to differences in the hBN and cBN optical functions. The model, with constrained evolution of thicknesses was also applied to determine the phase evolution of BN under dynamic conditions in which the substrate bias magnitude \V-avg\ was decreased in a stepwise manner during deposition. As a result, the critical substrate bias voltage V-cm needed to sustain pure cBN growth was determined. It was found that for \V-avg\ < \Y-cm\ a second hBN + cBN mixed-phase layer evolves and that upon further reduction in \V-avg\ an onset for the formation of a pure hBN layer appears. The effects of plasma additions of hydrogen and oxygen on the BN phase evolution and the cBN optical properties were documented in dynamic \V-avg\ step-down experiments. It was found that any additions of hydrogen or oxygen gas are detrimental when used under otherwise optimum cBN growth conditions, meaning that these additions promote the growth of hBN.