We use synchrotron x-ray diffraction to characterize the roughness of
the buried Si(001)/SiO2 interface, for three types of oxide, without m
odification of the.interface. We show that the thermal oxide interface
is 0.5 +/- 0.1 times as rough as the native oxide interface, suggesti
ng that the oxide growth decreases the roughness slightly. We also mea
sure the roughness of a chemically grown oxide interface.