In this paper, we extended the silicon (Si) etching behavior using CO 2 laser with glass attached to the Si backside. The CO2 laser with a wavelength of 10.64 um is not absorbed by Si material and cannot do any micromachining for pure Si. However, Si becomes able to be etched by CO2 laser as we put a glass below Si. The etching occurs from the top surface of Si downward to the bottom. If the time is enough long, it will etch through the Si wafer to be a hole structure. The Si etching depth increases with increasing laser power and pass number. The diameters of hole vary with the laser power, pass number and scanning speed. The mechanism may be involved in the interaction between photon, thermal and electron properties of materials to change the Si absorption behavior. It needs further studying in future and will be a promising technology in Si micromachining for the application of microdevices and MEMS.