Surface roughening of (100) Si at low temperatures during electron cyclotron resonance hydrogen plasma cleaning is studied in an ultrahigh vacuum environment. The effects of process parameters on surface roughness are quantitatively analyzed by atomic force microscopy besides reflection high energy electron diffraction. Crystalline defect morphology is studied by transmission electron microscopy to understand its role in surface roughness. Surface roughness is strongly related to the nucleation and growth of {111} platelet defects at the Si subsurface region and the preferential etching at positions where {111} platelet defects intersect the Si surface. Hydrogen ion flux and substrate temperature can be successfully controlled to tailor the {111} platelet defects, therefore, surface roughness. ©1995 American Institute of Physics.