Electromagnetic analysis for optical coherence tomography based through silicon vias metrology.

This paper reports on progress in the analysis of time-domain optical coherence tomography (OCT) applied to the dimensional metrology of through-silicon vias (TSVs), which are vertical interconnect accesses in silicon, enabling three-dimensional (3D) integration in microelectronics, and estimates the deviations from earlier, simpler models. The considered TSV structures are 1D trenches and circular holes etched into silicon with a large aspect ratio. As a prerequisite for a realistic modeling, we work with spectra obtained from reference interferograms measured at a planar substrate, which fully includes the dispersion of the OCT apparatus. Applying a rigorous modal approach, we estimate the differences to a pure ray tracing technique. Accelerating our computations, we focus on the relevant fundamental modes and apply a Fabry-Perot model as an efficient approximation. Exploiting our results, we construct and present an iterative procedure based on the minimization of a merit function, which concludes TSV heights reliably, accurately, and rapidly from measured interferograms.

Potential-induced resonant tunneling through a redox metalloprotein investigated by electrochemical scanning probe microscopy.

The redox metalloprotein azurin self-chemisorbed onto Au(1 1 1) substrates has been investigated by electrochemically controlled scanning tunneling (STM) and scanning force/lateral force microscopy (SFM/LFM) and cyclic voltammetry (CV) in aqueous solution. The combined use of STM and SFM/LFM under electrochemical control in the negative side of the azurin redox midpoint (+116 mV vs. SCE) has delivered unique information on the nature of the STM images. While in STM the bright spots, believed to be associated with azurin molecules, are visible only for potential values higher than -125 mV, the concurrent electrochemical SFM results show adsorbed proteins over the whole potential range investigated (from -225 to +75 mV). Stepping the potential back and forth (between -25 and -125 mV) in STM imaging, it has been possible to make bright spots appearing and disappearing repeatedly, indicating that STM image formation arises possibly through resonant tunneling via the redox levels of azurin. These results represent the first clear evidence of potential-dependent tunneling in proteins adsorbed onto a conductive substrate.