Device isolation
- 1. Device isolation techniques By- Neha Sharma Assistant Professor Department of Electronics and Communication Govt. Engineering College, Jhalawar
- 2. Isolation Techniques • Thermally 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.
- 4. • 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, 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.
- 5. • Bird's beak • The LOCOS process utilizes the different rates of oxidation of silicon and silicon nitride, which is used for local masking. • The silicon nitride masks regions where no oxidation should occur, the oxide only growths on the bare silicon. Since silicon and silicon nitride have different coefficients of thermal expansion, a thin oxide layer - the pad oxide - is deposited between the silicon and the silicon nitride to prevent strain due to temperature changes.
- 7. • For lateral isolation of transistors, a so-called field oxide (FOX) is deposited on the bare silicon surface. While the oxidation on the bare silicon takes place, the pad oxide causes a lateral diffusion of oxide beneath the silicon nitride and thus a slight growth of oxide at the edge of the nitride mask. This extension has the shape of a bird's beak whose length depends on the length of the oxidation process and the thickness of the pad oxide and the nitride as well.
- 9. • The Shallow Trench Isolation (STI) is the preferred isolation technique for the sub-0.5m 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.