This article reports that chemically active sites on SiO2 surfaces can be either passivated or introduced intentionally by treating them in a proper chlorosilane gas, SiHnCI4-n (n = 0,1,2). Out experiments of Si chemical vapor deposition on SiO2-covered Si have shown that Si deposition is suppressed on SiCl4- and SiHCl3-treated samples, while an SiH2Cl2 treatment drastically enhances Si nucleation. Thus, the chlorosilane treatment is a unique way for us to control the interface bonds between the SiO2 surface and the Si deposits on it. We also demonstrate resistless selective-area deposition using a SiHCl3-treated ultrathin SiO2 mask layer. Patterns are defined on the mask surface by direct electron-beam irradiation which induces Cl desorption thereby forming chemically reactive surface defects.