Relation Between Morphology, Etch Rate, Surface Wetting, and Electrochemical Characteristics for Micromachined Silicon Subject to Galvanic Corrosion

Immersion of silicon in HF-based solutions is utilized in microsystems fabrication to render freestanding mechanical structures. Such etching, however, creates a galvanic couple between silicon and a metallic layer (e.g., gold), resulting in corrosion damage. Morphology, resistive probe, surface wetting, and electrochemical characterization (vs Cu/CuF) of single- and polycrystalline silicon subjected to galvanic corrosion in three HF-based solutions (undiluted 48% HF, UDHF:H2O, and UDHF:Triton X-100) are used here to formally substantiate the results of a previous mechanical (microtensile) study. Porosity estimated from the morphology of corroded samples allows the corrosion current density to be determined from etch-rate measurements, according to Faraday's law. Resistive probe structures are used to simulate the microtensile specimens, thereby characterizing corrosion current as a function of time, etchant type, illumination, and the amount of metal utilized. The measured current density of micromachined silicon is compared against (100) wafer specimens using polarization characterization, identifying the porous Si formation regime. Using sessile drop measurements, the three etchants are further distinguished based on their surface-wetting characteristics. The chronopotentiometry, resistive probe, and microtensile characterizations all identify the behavior regimes of rapid initiation, subsequent steady-state corrosion, and the final catastrophic failure of the microtensile specimens. (c) 2008 The Electrochemical Society. [DOI: 10.1149/1.2996249] All rights reserved.