Feature Article - Thermal Processing - Issues and Effects, Part 2 - By Christopher Henderson

There are three major factors that affect the availability of the oxidizing species. They include diffusivity, solubility and pressure. In terms of diffusivity, oxygen diffuses much faster through silicon dioxide than water molecules. The water molecule, with its angled shape, occupies more volume than the oxygen molecule. This makes it more difficult for the water molecule to move through the silicon dioxide structure. On the other hand, water is approximately 600 times more soluble in silicon dioxide than in oxygen. Silicon dioxide readily traps and holds water, while oxygen tends to diffuse away quickly. Pressure is the third factor affecting oxidation. Increasing pressure from the outside environment increases the concentration of the species in the reaction zone.

This graph shows the effects of pressure on oxide growth in pyrogenic steam. The graph shows the oxide thickness as a function of oxidation time for several different pressures. Notice that the oxide growth rate increases as the ambient pressure is increased.

There are several factors that change the surface potential of the silicon surface. These are crystal orientation, silicon doping concentration, and surface treatment. The <111> plane oxidizes the fastest while the <100> plane oxidizes the slowest. The bonds coming out of the <111> plane more easily accept oxygen. Higher surface doping concentrations give higher oxidation rates. The silicon to silicon bonds are compressed and/or stretched by substitutional impurities. These bonds are more easily broken to accept oxygen than silicon to silicon bonds in an area where the lattice is free from impurities. Finally, surface treatments such as a hydrochloric acid treatment can increase the growth rate. The acid oxidizes the surface, allowing oxygen to bond more easily.

This graph shows the doping effects on oxidation. Notice that as the concentration of phosphorus increases, the growth rate of the oxide increases. The effect is more pronounced at lower temperatures. At higher temperatures, one can see that the oxidation lines are closely packed together.

This graph shows the effects of two different pre-treatments, and no pre-treatment of the silicon surface. The sulfuric acid - hydrogen peroxide clean oxidizes the surface, allowing faster oxide growth, while the ammonium hydroxide - hydrogen peroxide - water clean makes the surface more alkaline, reducing the growth rate.

This graph shows the effects of oxide growth when the silicon surface is exposed to hydrochloric acid. Researchers have studied the effects of chlorine gas on silicon dioxide growth for a number of years, since chlorine can reduce fixed and mobile charge in the silicon dioxide, increase the lifetime of the minority carriers, and reduce the density of oxidation-induced stacking faults in the silicon below. Chlorine can also cause the oxide reaction rate to increase, but the reasons for this are not well understood. Researchers have also observed the buildup of chlorine at the silicon/silicon dioxide interface.