Dry etching is widely used for nanofabrication and it requires reliable etching masks. However, hard dry etching masks usually need high-temperature and/or plasma deposition, which may causes damage to temperature-sensitive materials and semiconductor surface. Here, we develop a novel etching mask material, silicon monoxide (SiO), which is thermally evaporated and hence can avoid such drawbacks. The etching selectivity of evaporated SiO is shown to be higher than 50:1, comparable to sputtered SiO2. A nanochannel device, called self-switching diode (SSD), is fabricated to evaluate the mask-deposition-process damage because of its high sensitivity to the process damage. In comparison to commonly used sputtered SiO2 and polymethyl methacrylate (PMMA) mask, the SSD fabricated using evaporated SiO exhibit the highest channel conductance, strongest nonlinearity, and best high-frequency performance. Hall measurements also reveal that the carrier mobility of nanochannels etched with SiO mask is twice that of similar channels with SiO2 mask.