Shape transformation of silicon trenches during annealing at high temperatures in a hydrogen ambient was investigated using scanning electron microscopy (SEM) and atomic force microscopy (AFM). By SEM observation of the trench profiles, we found that the rate of shape transformation increases with decreasing hydrogen pressure. Performing the simulation based,on a continuum surface model, we show that the shape transformation during annealing in a hydrogen ambient is due to surface self-diffusion. By quantitative comparison of the results between the experiment and simulation, we estimated the diffusion coefficients. The obtained activation energy for surface diffusion under a hydrogen pressure of 40 Torr was much higher than that measured under ultrahigh-vacuum conditions. Furthermore, it was found by AFM observation of the trench sidewall surfaces that, during the thermal treatment, the large roughness of the as-etched trench sidewall surface decreased significantly due to surface self-diffusion of silicon atoms, resulting structures with atomically flat terraces and steps. (C) 2003 American Vacuum Society.