Subsections

• 1.2.1 Local Oxidation of Silicon
• 1.2.2 Shallow Trench Isolation

1.2 Isolation Techniques

Thermal grown oxide is mainly used as isolation material in semiconductor fabrication. For the isolation of neighboring MOS transistors there exist two techniques, namely Local Oxidation of Silicon and Shallow Trench Isolation. The differences in their process flow and their final oxide shapes are described in the following.

1.2.1 Local Oxidation of Silicon

Local Oxidation of Silicon (LOCOS) is the traditional isolation technique. At first a very thin silicon oxide 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 nitride. The result is the nitride mask as shown in Fig. 1.1a, which defines the active areas for the oxidation process. The next step is the main part of the LOCOS process, the growth of the 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 to this encroachment. The advantages of LOCOS fabrication are the simple process flow and the high oxide quality, because the whole LOCOS structure is thermally grown.

Figure 1.1: Process sequence for local oxidation of silicon (LOCOS).

1.2.2 Shallow Trench Isolation

The Shallow Trench Isolation (STI) is the preferred isolation technique for the sub-0.5$\mu$m technology, because it completely avoids the bird's beak shape characteristic. With its zero oxide field encroachment STI is more suitable for the increased density requirements, because it allows to form smaller isolation regions. The STI process starts in the same way as the LOCOS process. The first difference compared to LOCOS is that a shallow trench is etched into the silicon substrate, as shown in Fig. 1.2a. After underetching of the oxide pad, also a thermal oxide in the trench is grown, the so-called liner oxide (see Fig. 1.2c). But unlike with LOCOS, the thermal oxidation process is stopped after the formation of a thin oxide layer, and the rest of the trench is filled with a deposited oxide (see Fig. 1.2d). Next, the excessive (deposited) oxide is removed with chemical mechanical planarization. At last the nitride mask is also removed. The price for saving space with STI is the larger number of different process steps.

Figure 1.2: Steps in a typical shallow trench isolation (STI) process flow.