Reactions in the System  O 2 ‐  NO  ‐ SiO2 in a Conventional Furnace: I. Oxygen Exchange Reactions

Abstract

There are many chemical reactions occurring when samples are oxidized in a conventional furnace, both with the sample and within the furnace itself. Many of the reactions are not gas-phase reactions, but take place on a third body (catalyst), which is typically a surface or interface. Thereby the catalytic activity of these third bodies is vital for reaction kinetics. The third body can be both the sample itself and the walls of the furnace. The oxidation mechanisms are, of course, directly related to the reactions taking place on the sample. The reactions in the furnace can result in changes in gas-phase composition and pressure, and thereby change the oxidation conditions. The oxidation kinetics can be extremely sensitive to gas-phase composition and an example is oxidation of silicon where parts per million of water vapor increase the oxidation rate considerably. 1 In order to control the atmosphere in the furnace it is therefore vital to know the chemical reactions that occurs. A chemical reaction usually proceeded in many steps and, to fully understand the reaction, knowledge of all steps and the mechanisms active in each step are required. However, we seldom have this complete understanding of the reactions. Luckily, it is in many cases sufficient to understand the main steps of the reaction and in particular the rate-limiting step. Oxidation of silicon and metals (substrate) involves reactions like bond breaking in the O 2 molecule, charge transfer between O 2 and the substrate, and incorporation of oxygen in the oxide. Apart from the chemical reactions, there are also other processes, like diffusion, which can limit the oxidation kinetics. 2 The formation of a solid oxide will separate the oxygen source from the substrate and for the reaction to proceed the oxidizing species (which can be, for example, O 2Ϫ , O 2 , O, Me, or Me nϩ ) has to be transported through the growing oxide (diffusion). However, even when diffusion is rate limiting, an understanding of the chemical reactions active at the two interfaces (O 2 -oxide and oxide-substrate) is needed, since these reactions set the boundary conditions for diffusion. 3,4 The various oxygen exchange reactions, by using isotopic labeled oxygen, 3,5-7 can be used to study the chemical reactions and the diffusion. There are three stable isotopes of oxygen which can be used, and the natural abundance of oxygen is 99.76% 16 O, 0.04% 17 O, and 0.2% 18 O. All three isotopes are available highly enriched on the commercial market. Today the isotopes 16 O and 18 O are the most frequently used, especially when studying oxidation in pure O 2 .The exchange reactions of most interest in oxidation are the oxygen exchange between oxygen molecules catalyzed by the surface of the oxide (O 2 o O 2 ) and between oxygen molecules and the oxide (O 2 o SiO 2 and O 2 o Me x O y ). Although the rate of the exchange reactions depends on many parameters, like the thickness of the growing oxide, a study of the bulk oxide can give valuable information on the active mechanisms. In princ...