LOCOS - Function of Layers and Structures

Function of Layers and Structures

The silicon wafer (layer 1) is used as a basis for building electronic structures (such as MOS transistors).

To perform local oxidation, the areas not meant to be oxidized will be coated in a material that does not permit the diffusion of oxygen at high temperatures (thermal oxidation is performed in temperatures between 800 and 1200°C), such as silicon nitride (layer 3, step III).

During the growth of the immersed insulating thermal oxide structures (steps V and VI), the silicon nitride layer (layer 3) is pushed upwards. Without the buffer oxide (layer 2, also known as pad oxide), this would create too much tension in the Si substrate (layer 1), the plastic deformation would occur and the electronic devices would be damaged.

Therefore a buffer oxide (layer 2) is deposed by the CVD (step II) between the Si substrate (layer 1) and the silicon nitride (layer 3). At high temperatures, the viscosity of silicon oxide decreases and the stress created between the silicon substrate (layer 1) and nitride layer (layer 3), by the growth of the thermal oxide (steps V and VI), is relieved.

The insulating structures (structure 4) are formed by thermal oxidation of silicon. During this process, the silicon wafer is "consumed" and "replaced" by silicon oxide. The volume of silicon oxide to silicon is about 2.4:1, which explains the growth of the insulation structures and the created tension.

The disadvantage of this technology is that the insulating structures are rather large, and therefore, fewer MOS transistors can be formed on one wafer.

Reduction of dimensions of insulating structures is solved by the STI (Shallow Trench Isolation, also known as Box Isolation Technique). In this process, trenches are formed and silicon dioxide is deposed inside. The LOCOS technology can't be used in this way, because of the change of the volume during the thermal oxidation, which would induce too much stress in the trenches.