Silicon Wafer Oxidation Techniques

In the semiconductor industry, thermal oxide layers are primarily used as dielectric material in ICs and other integrated MEMS (Micro Electro Mechanical Systems) devices. The most ideal way to produce a thermal oxide layer on Si wafers is by oxidization silicon with oxygen.

What is Thermal Oxide?

In micro-fabrication, the method of producing a thin layer of oxide on the wafer surface is known as thermal oxidation. It is one of the building blocks used in making different semiconductor devices. These have to be grown appropriately, using a high purity, low defect Silicon substrate. These silicon wafers produced using thermal oxidation technique, work as an excellent dielectric material. It process is applied to different materials to produce silicon dioxide.

How this layer of thermal oxide is present in devices?

Field oxide: It is normally found on electrically isolating Polysilicon.

Gate Oxide: This is a very thin, Thermal Oxide Silicon Wafers located over the active or gate region of the transistors.

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Method of Production of Thermal Oxides:

The method of this involves the technique that forces oxidizing agents to diffuse into the wafer at very high temperature. The growth of a silicon oxide layer SiO2 can be carried out via dry and wet oxidation process. In both cases, silicon reacts with oxygen leading to a moving interface towards the substrate.

Wet Thermal Oxide: Wet Oxides are grown pyrogenically using Hydrogen and Oxygen gases in an internal or external torch. It is used to produce a thicker oxide layer of about 2,000-25,000Å. When a thick layer of oxide is produced, it is called field oxide. It is found in device where high dielectric strength is needed. When oxide is present on the Silicon surface, it takes longer time to diffuse through its surface. For instance, it the temperature is 1000°C it will take around 5 hours for growth of wet steam to produce an oxide layer around 10,000Å thick.

Dry Thermal Oxide: Dry oxidation typically takes place at temperatures between 850 and 1200°C. This process allows the production of Si wafers with excellent thickness, uniformity and purity. Therefore, this is the most preferred way to produce high quality thin Thermal oxide Silicon Wafer layers.

These also have higher dielectric strength in comparison to wet dielectric strength.

The most important difference in the growth of dry & wet oxide is the growth rate. The dry oxide grows at a much slower pace in comparison to wet oxide. Due to this dry oxides do not exceed thickness of 1000Å. Also, these are mainly used for thin gate and capacitor oxides where high uniformity and high dielectric strength are needed.

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