Mechanical Effects of Galvanic Corrosion on Structural Polysilicon

The mechanical properties of miniaturized materials depend strongly on their structure, which can be altered by wet chemistry methods common in microsystems postprocessing. In a comprehensive and systematic study, we examine the dissolution of silicon when immersed in various hydrofluoric acid (HF)-based chemistries. Specifically, the frequency of mechanical resonance f_R of microcantilever beams is used as a vehicle to examine the corrosion of polycrystalline silicon (polySi). A decrease in f_R that occurs as a function of immersion time in HF was measured for microcantilevers as well as "comb drives" in contact with a noble metal (gold). Time-dependent variation was also observed in the modulus and hardness measured during indentation testing, sometimes with pronounced difference for specimens contacted to gold. Secondary sources of influence, such as in-plane-oriented residual strain (which remained unchanged), through-thickness-oriented residual strain gradient (increased away from the substrate), and electrical resistance (greatly increased) are examined, but were found not to significantly contribute to the decrease in f_R of the microcantilevers. Morphological characterization identified attack on the surface along with grain delineation for the polySi, with the formation of a nanoscale porous layer at the near surface. The damage to the microcantilevers can be modeled by approximating the beams as a laminated composite structure. Such analysis suggests that damage, induced as the result of galvanic corrosion, results from the decreased stiffness of the near surface porous silicon (PS) layer as well as a change in the effective thickness of the beams. Last, corrosion damage is compared between eight representative HF-based chemistries. The measurements here suggest that the fabrication and postprocessing of microsystems components are important, because they can greatly influence the material properties, design, performance, life- time, tribology, manufacture, and required operating environment of microscale and nanoscale devices