We have investigated the rounding of micron-sized trenches fabricated on Si(001) substrates during annealing in hydrogen ambient in a temperature range of 1000 to 1100 degreesC, especially the effect of hydrogen pressure on the rate of rounding. Observing the profiles of the trenches annealed under hydrogen pressures from 10 up to 760 Torr by scanning electron microscopy, we have found that the rate of corner rounding decreases with increasing hydrogen pressure. It was also found that these rates of corner rounding are smaller than that during annealing in atmospheric argon ambient. This result suggests that adsorbed hydrogen suppresses the surface self-diffusion, by which the corner rounding occurs. We present the contour map of corner curvature in the process parameter space of hydroged-pressure versus annealing temperature for an annealing time of 3 min. (C) 2004 American Vacuum Society.