Shallow trench isolation

[1] STI is created early during the semiconductor device fabrication process, before transistors are formed.

The key steps of the STI process involve etching a pattern of trenches in the silicon, depositing one or more dielectric materials (such as silicon dioxide) to fill the trenches, and removing the excess dielectric using a technique such as chemical-mechanical planarization.

[2] Certain semiconductor fabrication technologies also include deep trench isolation, a related feature often found in analog integrated circuits.

[4] Basically, due to the electric field enhancement at the edge, it is easier to form a conducting channel (by inversion) at a lower voltage.

[5][6] The main concern for electronic devices is the resulting subthreshold leakage current, which is substantially larger after the threshold voltage reduction.

Scaling of isolation with transistor size. Isolation pitch is the sum of the transistor width and the trench isolation distance. As the isolation pitch shrinks, the narrow channel width effect becomes more apparent.
The shallow trench isolation fabrication process of modern integrated circuits in cross-sections.