Chemistry Reference
In-Depth Information
this layer is not soluble in the HF solution. This layer is, however, soluble in
solutions, which have strong oxidizing power. Thus, for an oxide that contains nonsat-
urated silicon bonds due to incomplete oxidation, the etch rate is relatively low but can
be increased with addition of oxidation agents such as to remove the partially
oxidized silicon atoms requires that they first be fully oxidized. This mechanism
explains well why is a sensitive etching solution for differences in density,
stoichiometry, bond strain, and impurities in silicon oxides; it etches various types of
glasses and mixed oxide layers much more rapidly than silicon oxide.
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It may also
explain why a layer about 1 nm thick, which has a much lower etch rate than the bulk
oxide, is left on the surface of the silicon substrate at the end of etching in some cir-
cumstances.
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A large fraction of the silicon atoms within a region of a few monolay-
ers at the silicon/oxide interface is known to be only partially oxidized for both thermal
and anodic oxides.
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The structure of silicon oxide, consisting of a three-dimensional network of
constructed from polyhedra of oxygen and silicon ions, may exhibit various crystalline
and vitreous forms depending on the composition and degree of crystallinity. The crys-
talline forms, such as quartz, due to their highly compact structure and uniform Si-O
bond strength, are more resistant to chemical dissolution than the vitreous forms. Dif-
ferent forming conditions and alloy elements affect the etch rate by affecting either the
compactness of the structure or the distribution of the valence electrons which deter-
mines the strength of the bonds and the openness of the structure. For example, a sub-
stitutional impurity with a valence of 3 may create irregularity in the structure and
nonbridging bonds as shown in Fig. 4.41 and change the bond strength of the neigh-
boring Si-O bonds. Also, the presence of an alloying element may enhance or inhibit
the dissolution of the silicon oxide depending on the bond strength of A-O and solu-
bility of the alloying element A in the solution.
