Summary

Sacrificial oxide is a common method in silicon microfabrication technology. This layer is created in order to release different MEMS structure or in integrated circuits in order to improve the device performance. This layer is etched away later in the process. SiO2 is used as a sacrificial layer because of its availability in silicon microfabrication processes and etch selectivity against silicon. It has stable and reproducible properties, and is compatible to high temperature during polycrystalline silicon deposition. Different varieties of oxides are used as sacrificial oxide. They can be thermally grown or CVD deposited. Thin oxides are usually grown in dry ambient condition (pure oxygen). Wet oxidation (water vapor) is used for thick oxide. Silicon is consumed when oxide is grown. Deal-Grove model explains and predicts the oxidation kinetic. However, this model does not fully explain the oxidation of shaped surface, oxidation kinetics in mixed ambient and for very thin oxide. Many models have been developed to predict the oxidation behavior for thin oxides, all motivated by improvement over Deal-Grove model, but none has gained widespread acceptance. Most of the models today use Deal-Grove model as a beginning equation and then add to or modify it. Silicon dioxides are also deposited using CVD techniques. Deposition usually occurs at lower temperature, and an oxide film can be deposited on top of any surface where silicon is not available for oxide growth. Silane and oxygen are the common gas sources used for CVD deposition. Tetraethooxysilane (TEOS) can also be used as a source of silicon for oxide deposition. Silicon-on-insulator is also used as a starting material and the buried oxide as a sacrificial oxide. These fabricated oxides are etched away sometimes during the process. In many cases, sacrificial oxide is a buried layer and there is a small opening that allows the etchants to reach the oxide surface as well as allows the etch products to come out of the cavity. Therefore, modeling the etch kinetics of every design needs to be considered differently. The etchants used in sacrificial oxide etching should have high selectivity against the device structure material that is typically silicon. Hydrofluoric acid based compounds are good for sacrificial oxide etching. Wet etching may result in stiction failure that is the permanent stiction of device structure to the substrate due to the residue products from wet etching. Various techniques have been developed to alleviate this problem. One is using anhydrous HF and CH3OH in gas phase to achieve dry release of device structure. Application of sacrificial oxide includes MEMS as well as IC fabrication. Few examples are: micromotors, sensors, actuators and accelerometers as well as immunoisolating biocapsules, drug delivery devices, microneedles, DNA electrophoretic devices. Silicon dioxide sacrificial layer is used in IC fabrication in order to capture defects in the silicon surface before gate oxide growth.

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