Laser smoothing of binary gratings and multilevel etched structures in fused silica

We describe a promising approach to the processing of micro-optical components, where CO(2) laser irradiation in raster scan is used to generate localized surface melting of binary or multilevel structures on silica, fabricated by conventional reactive-ion etching. The technique is shown to provide well-controlled local smoothing of step features by viscous flow under surface tension forces, relaxing the scale length of etch steps controllably between 1 and 30 microm. Uniform treatment of extended areas is obtained by raster scanning with a power stabilized, Gaussian beam profile in the 0.5 to 1 mm diameter range. For step heights of 1 microm or less, the laser-induced relaxation is symmetric, giving softening of just the upper and lower corners at a threshold power of 4.7 W, extending to symmetric long scale relaxation at 7.9 W, with the upper limit set by the onset of significant vaporization. Some asymmetry of the relaxation is observed for 3 microm high steps. Also, undercut steps or troughs produced by photolithography and etching of a deep 64 level multistep surface are found to have a polarization-dependent distortion after laser smoothing. The laser reflow process may be useful for improving the diffraction efficiency by suppressing high orders in binary diffractive optical elements, or for converting multilevel etched structures in fused silica into smoothed refractive surfaces in, for example, custom microlens arrays.