Focused Ion Beam (fib) for the Fabrication of Grating Couplers in Si3n4 Waveguides

Diffraction gratings are very convenient elements for coupling out-of-plane light in planar waveguides of sub-micronic thickness. They have been investigated in dielectric [1] and highindex waveguides [2], and a platform for modeling has been created [3], but technology should still be improved for giving gratings efficient and simple as modeled. Standard methods like reactive ion etching through a photoresist mask, done with electron beam lithography or optical holography, give errors due to the initial surface roughness and to inaccurate corrugation control, and can not be used on micromechanical delicate structures. Focused Ion Beam (FIB) has been shown as a versatile powerful direct-write nanotechnology technique able to solve many of these problems [4]. It allows depositing and milling a variety of materials with nanometer resolution without masks nor sacrificial photosensitive layers. So, an important advantage of this technique is the ability of making sub-micron structures as the diffraction grating couplers onto chips with fragile elements, thus completing the technology.