The Si/SiO2 interface morphology is observed with subnanometer resolution by an ultrahigh vacuum scanning tunneling microscope (STM). We analyze the roughness of the Si/SiO2 interface for a chemical oxide film formed by a wet chemical process (NH4OH/H2O2/H2O treatment). The oxide film is selectively removed by irradiating a field emission electron beam extracted from a STM tip at a temperature of 300-350 degrees C. We find that during the chemical process the roughness of the Si/SiO2 interface increases with the treatment time.