We use EVOLVE-FLOW, a thin film thermal flow process simulator, to simulate the thermal flow of borophosphosilicate glass (BPSG) and phosphosilicate glass (PSG) thin films in trenches on patterned wafers, where two-dimensional surface evolution is appropriate. For a fixed anneal time, two parameters control the thin film flow processes : surface tension and viscosity. We demonstrate our simulator by reproducing flowed BPSG film profiles after a 15 min furnace annealing cycle. Digitized as-deposited BPSG film and substrate profiles from experimental scanning electron micrographs are used as initial profiles in the simulations. We then predict flowed phosphosilicate film profiles, and combine experiments and simulations to estimate the surface tension of PSG films in a rapid thermal annealing process. The viscosity of the PSG films is modeled as a function of temperature and phosphorus pentoxide concentration using an Arrhenius type equation. The estimated values of the viscosity and surface tension are used to simulate the PSG film profiles at different annealing times. The simulation results are in good agreement with experimental observations.