Today's ULSI technology is to a large extent based on the excellent properties of thermally grown silicon dioxide layers. SiO2 is used as gate dielectric in MOS devices, as implantation or doping mask, and for device isolation purposes.
Most of the silicon oxidation processes today are carried out at atmospheric pressure in oxidation furnaces at temperatures between 800 and 1,200°C. The appropriate oxidant gases are dry oxygen, wet oxygen (water steam). For wet oxidation, the water steam can be extracted by influx of gases (oxygen, argon, or nitrogen) into a vessel filled with water. Another possibility is to initiate a reaction of oxygen and hydrogen at the inlet of the reactor tube. The wafers are placed vertically or horizontally in quartz boats and the temperature of the hot zone is controlled with an accuracy of 1°C, in order to grow oxide layers in a reproducible manner.
It has been proven by a number of experiments that thermal oxidation of silicon proceeds by diffusion of the oxidant through the growing oxide. The oxidation reaction itself takes place at the oxide-silicon interface. During oxidation, the oxide-silicon interface moves into the silicon material as the silicon is oxidized. Taking into account the densities of silicon and of SiO2, it can be shown that about 44% of the oxide layer grows into the silicon substrate. The remaining 56% grows on top of the silicon, resulting in a non-planar surface if oxidation is local.
The lateral and vertical isolation of device structures in ULSI determines to a large extent the performance of the technology with respect to packaging density and parasitic effects. Classical local oxidation (LOCOS) has been widely used to laterally isolate the active regions of an integrated circuit. The oxide is grown at those parts of the wafer surface where a Si3N4 masking layer has its openings. Before starting the oxidation there is already a thin oxide layer in between the nitride layer and the silicon substrate. Because of diffusion of the oxidant into the region below the nitride, also the oxide beneath the nitride is growing leading to the characteristic bird's beak shape after completion of the process. This situation is illustrated in the Figure 1, which shows the result of an oxidation in wet oxygen at 950°C for 10 hours(from: C. Claeys, J. Vanhellemont, G. Declerck, J. Van Landuyt, R. Van Overstraeten, S. Amelinckx, VLSI Science and Technology/1984, K.E. Bean, G. Rozgoni, Eds., The Electrochemical Society, Pennington, 1984, p. 272.).