Solution:

Locos:

Thermal-grown oxide is mainly used as isolation material in semiconductor fabrication. Local Oxidation of Silicon (LoCos) is the traditional isolation technique used for the isolation of neighboring mos transistors.

At first, a very thin silicon dioxide layer is grown on the wafer, the so-called pad oxide.

Then a layer of silicon nitride is deposited which is used as an oxide barrier. The pattern transfer is performed by photolithography.

After lithography, the pattern is etched into the oxide nitride. The result is the nitride mask as shown in fig. which defines the active areas for the oxidation process.

The next step is the main part of the Locos process i.e. the growth of Thermal oxide. After the oxidation process is finished, the last step is the removal of the nitride layer.

The main drawback of this technique is the so-called Bird's beak effect and the surface area which is lost due to this encroachment.

The advantages are simple to process flow and high oxide quality. because the Whole Locos structure is thermally grown.

STI: Shallow Trench Isolation:

Shallow trench isolation is the preferred isolation technique for the sub $0.5 \mathrm{~mm}$ technology because It completely avoids b the Bird's beak shape characteristics.

With its zero oxide field encroach STI is more suitable for the increased density requirements because it allows to the formation of smaller isolation regions The STI process starts in the same way as the Locos process.

The first price bor saving space with STI is larger no. of different- process steps.