Fabrication and characterization of suspended beam structures for SiO 2 photonic MEMS

This paper proposes a microfabrication process for the reliable release of SiO2 beam structures. These structures are intended to be utilized in SiO2 photonic MEMS. A major fabrication challenge is the release of thick (>10 μm) SiO2 structures with high yield. A single mask process is developed based on temporary reinforcement of the SiO2 structure. A supporting layer of Si functions as a reinforcing layer during etching and release, thereby enabling a high fabrication yield. Furthermore, the process allows to create structures of which the final Si support thickness is configurable from tens of micrometers to zero, thereby providing additional design freedom.

The fabrication process is tested on a silicon wafer with a  ∼15 μm thick thermal oxide layer. The obtained suspended structures are mechanically characterized. Two deformation effects can be distinguished: a curvature of the beam and a slope at the base of the beam. These effects are caused by the compressive mean stress and the gradient stress in the thermal SiO2. The curvature of the SiO2–Si beams corresponds to a concave downward profile while the SiO2 beams without supporting Si reveal a small curvature in the opposite direction (concave upward). The slope at the base is approximately $-{{0.5}^{{}^\circ}}$ for the SiO2 beams and between $-{{0.5}^{{}^\circ}}$ and 0° for the SiO2–Si beams. The acquired bending stiffness of long SiO2 beams is in the newton per meter range (e.g. 0.8 N m−1 for a cantilever measuring 1000 μm in length and 13 μm in width).