Isolation Techniques with reference to VLSI circuits.
Answers
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
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.