Argonne National Laboratory Center for Nanoscale Materials U.S. Department of Energy

Atomic Scale Modeling and Simulation of Silicon Anisotropic Etching

N. Moldovan

A well-known process in microelectromechanical systems (MEMS) technology involves etching silicon in alkaline solutions, which produces accurate 3-D silicon structures, sometimes with atomic smoothness, by taking advantage of the strong dependence of the etching rate on crystal orientation. Significant experimental effort has been made to characterize this anisotropy (polar etching rate diagrams, temperature dependencies, roughness measurements, in situ STM records during etching, electrochemistry studies etc.). The experimental results were used in complex simulations that successfully predicted the evolution of the 3-D geometry, starting from the experimentally measured etching diagrams.

However, until recently, there was only a poor understanding of the causes of the variation in etching rate with crystal orientation. Recently, two types of atomic-scale simulations were performed: atom-by-atom removal algorithms and bond-breaking algorithms. The latter produced simulated etching diagrams that are very similar to the experimental ones, assuming that the local interatomic Si-Si bond strength depends on the presence and type of second-order neighbor atoms. This is especially important at the crystal surface. The theory permits a genuine parameterization for describing etching behavior and presents a nice example of explaining macroscopic properties of matter starting from atomic level properties.

Experimental polar etching diagram for Si in KOH solution (hk0) family of planes

Simulation with the bond-breaking algorithm of the polar etching diagram for the (hk0) family of planes at different temperatures of the solution. The temperature increases from the center (58°C) toward the outside (70°C, 80°C, 95°C).

Simulated polar etching diagrams for a specific point in the parameter space,
for the (hk0) planes (left) and (hkk) planes (right). Bond-breaking algorithm.

January 23, 2002

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