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MOBILITY KEY TO SILICON ETCHING
Surface diffusion and dynamics play key role in semiconductor processing
MITCH JACOBY
Bucking conventional wisdom, researchers at the University of North Carolina, Chapel Hill, report that surface diffusion plays a key role in silicon etching. The finding contradicts current models that describe halogen etching of semiconductors as a purely kinetic process.
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MOBILE Nanometer-scale scanning tunneling micrographs recorded in quick succession reveal dynamics of defect diffusion during bromine (black in schematic) etching of silicon (yellow). A string of silicon dimer vacancies (DV string) rearranges to add to the length of single vacancy strings (SV strings), thereby minimizing repulsion (red zigzag) between bromine atoms. For reference, the white arrow marks the same position in each image. |
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Etching silicon with chlorine or bromine is a standard procedure for preparing the semiconductor for applications in microelectronics. The process has been well studied, and molecular mechanisms have been developed to describe the reaction's elemental steps.
Although successful in some regards, today's models fall short when it comes to providing a detailed account of certain common etching scenarios. For example, bromine etching of the (100) crystal face of silicon is known to result in two types of surface morphologies, depending on reaction temperature. Typically, scientists have invoked separate mechanisms to rationalize each type of morphology.
Now, based on real-time scanning tunneling microscopy analysis of etching, chemistry professor John J. Boland and graduate student Cari F. Herrmann assert that a single mechanism governs the process. But unlike traditional views that hold that surface defects formed during etching are immobile, the Chapel Hill team demonstrates that vacancies (lattice positions with missing atoms) are relatively free to move around. The group reports that products of etch reactions that form kinetically at low temperatures are transformed thermodynamically into more stable features at higher temperatures [Phys. Rev. Lett., 87, 115503 (2001)].
"Surface etching has traditionally been viewed as a chemical chiseling process in which holes are made and elongated by gradual chipping away," Boland remarks. "We've shown that etch vacancies are mobile and that the observed etch morphologies are the result of a coalescence process. Mobility is key, and the extent of thermodynamic relaxation determines the final surface morphology."
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