Micromachining of transparent materials with Fresnel diffraction of infrared radiation

A simple and productive micromachining method of a silica glass using a TEA (transversely excited atmospheric) CO₂ laser (10.6 μm) was investigated. A copper grid mask with a thickness of 2 μm, square apertures of 20×20 μm was attached to the silica glass surface, and it was irradiated by the TEA CO₂ laser light. As a result, circular holes were formed on the silica glass surface at the center of each aperture in a spatial resolution down to the sub-wavelength scale. The minimum diameter of the hole was less than 3 μm, which is less than one third of the wavelength of the TEA CO₂ laser. A depth of the holes ranging from 100 nm to 600 nm could be micromachined on silica glass surface. The mechanism of the micromachining was discussed based on electric field distributions of the CO₂ laser light by a 3D full-wave electromagnetic field simulation solver. The simulation results are in good agreement with the experimental results. It was found that the micromachining of the circular holes were caused by Fresnel diffraction of the CO₂ laser light at the apertures.