Effect of Oxide Structure

The many etch rate equations described above are empirical in nature even though mechanistic arguments are made in each specific case. One important omission in these quantitative formulations on the etching kinetics is the lack of consideration of the effect of the structure of silicon oxides. As shown in Fig. 4.40, etch rate can vary over more than three orders of magnitude for different types of oxides. It increases with increasing disorder of the oxide structure with the most ordered oxide, that is, quartz, having the lowest etch rate. The structural disorder of the silicon oxide can be due to impurities, partial oxidation of the silicon atoms, and degree of crystallinity. 

FIGURE 4.40. Etch rate of different oxides in 1% HF. The disorder in composition and structure increases from quartz to anodic oxide. 

Monk et attributed the role of doping elements in the oxide to the electronic characteristics of the elements. Whether the doped atom is electrophilic or nucleophilic toward the Si atom governs how easily HFJ is coordinated with the Si atom. If the oxide film has an electrophilic dopant, the etch rate increases, and if it is nucleophilic, the etch rate decreases, hi a PSG film the doped P atom, which has a valence of 5, has one valence electron more than Si. This means that a doped oxide can provide the electron to oxygen more easily than can a nondoped one. Therefore, the silicon-oxygen bond in P-doped oxide can be broken more easily. 

Nielsen and Hackleman ° found that the dissolution of thermal S1O2 in BHF solutions is impeded by the application of a cathodic potential of 2-AV across the oxide film and postulated that a layer of partially reduced oxide is formed on the surface and 

FIGURE 4.41. Disruption of silicon oxide structure due to the presence of an impurity element, (a) Structure without impurity (b) structure with impurity. 

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