Octadecyltrichlorosilane (OTS), self-assembled-monolayer (SAM) grown on SiO2 in the submonolayer region is investigated by atomic force microscope (AFM), which is further applied to SiO2 characterizations technique. OTS-SAM forms characteristic dendrite-shaped islands in its submonolayer region, whose shape and size significantly depend on the surface roughness of SiO2 formed at different temperatures in the range of 700-1100 degrees C. Moreover, OTS-SAM islands have practical usefulness as a self-patterned-mask for HF etching. When an oxidized Si wafer covered by OTS-SAM islands is dipped into HF, SiO2 in the area uncovered by the islands is selectively removed. This technique is successfully applied for the precise SiO2 thickness measurement in ultrathin (<50 Angstrom) regions by AFM. In addition, this technique enables a simultaneous observation of the morphologies of SiO2 surface and SiO2/Si interface. The result shows, for the first time, the continuity of the steps ion SiO2 surface and SiO2/Si interface, indicating no lateral step motion of Si (111) surface during oxidation.