Connections between morphological and mechanical evolution during galvanic corrosion of micromachined polycrystalline and monocrystalline silicon

Many microsystems fabrication technologies currently employ a metallic overlayer, such as gold, in electrical contact with silicon structural layers. During postprocessing in hydrofluoric-based acid solutions, a galvanic cell is created between the silicon and the metallic layer. Micromachined tensile specimens reveal that such etching in the presence of a galvanic cell can cause a catastrophic reduction in the tensile strength and apparent modulus of silicon. Detailed failure analysis was also used to compare fractured corroded Si to otherwise identical reference specimens via surface based (electron and scanning probe) microscopy as well as cross-section based structural- and composition-characterization techniques. For both polycrystalline and single-crystal silicon, galvanic corrosion can result in a thick corroded surface layer created via porous silicon formation, and/or generalized material removal depending on the etch chemistry and conditions. Under certain etching conditions, the porous silicon formation process results in cavity formation as well as preferential grain-boundary attack leading to intergranular fracture. The nature and severity of corrosion damage are shown to be influenced by the surface wetting characteristics of the etch chemistry, with poor wetting resulting in localized attack facilitated by the microstructure and good wetting resulting in generalized attack. The measured stiffness of the tensile specimens can be used to determine the effective modulus and porosity of the corroded surface layer. Extending beyond previous investigations, the present work examines the quantitative connection between the choice of chemical etchant, the corresponding damage morphology, and the resulting degradation in strength and apparent modulus. The present work also uniquely identifies important differences in polycrystalline and single-crystal Si based on their disparate damage evolution and related mechanical performance. (c) 2008 American Institute of Physics.