Fabrication of suspended thermally insulating membranes using frontside micromachining of the Si substrate: characterization of the etching process

We describe a frontside Si micromachining process for the fabrication of suspended silicon oxide or nitride membranes for thermal sensors. Membrane release is achieved by means of lateral nearly isotropic dry etching of the bulk silicon substrate, the etching being optimized for high rates and high selectivity with respect to the photoresist used to protect the device and the membrane material. Lateral Si etch rates of the order of 6–7 μm min−1 have been achieved in a high-density F-based plasma, which permit a reasonable etching time for the release of the membrane and the simultaneous formation of the cavity underneath ensuring thermal isolation of the final device. The proposed process can enhance the flexibility of device design and reduce the complexity of the fabrication process, since it does not require any additional steps other than the photoresist lithography for the protection of the active elements (e.g. polysilicon heaters and catalytic materials) that are formed on top of the membrane, due to the high selectivity of the process for Si etching with respect to the photoresist. We attempt to explain the observed dependencies of etch rates and selectivities on the plasma parameters and the dimensions of the released membranes by means of a simulator of the mechanisms involved in etching of structures.