Micromachined microbeams made from porous silicon for dynamic and static mode sensing

Through a controlled variation of the applied current during porous silicon formation, newly developed processes enable previously unattainable structural integrity of all-mesoporous silicon microelectromechanical systems (MEMS) structures. Such structures are desirable for applications such as sensing where the large surface area and low Young's modulus of the high porosity layer enable ultra-high sensitivity detection of adsorbed species. In this work, micromachined all-mesoporous silicon microbeams were released, allowing both the dynamic and static sensing modes to be studied using such porous structures. Resonant frequencies (50–250 kHz) of released doubly clamped porous silicon microbeams were measured, allowing mechanical properties to be extracted. Static mode sensing of vapour at the 1100 ppm level was also performed, with the released porous silicon cantilevers showing a significant 6.5 μm (3.7% of a 175 μm beam length) and repeatable deflection after exposure.