Iron impurities in Si₃N₄ processing

The atomic environment of iron impurities is investigated during the processing cycle of reaction-bonding silicon nitride (RBSN). Several analysis techniques are utilized, including X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine structure (EXAFS), and electron spin resonance (ESR), to examine iron impurities in the starting silicon powder, in sintered silicon compacts, and in RBSN materials. Results indicate that iron impurities in as-received metallurgical grade silicon powder are incorporated in the silicon bulk as a highly distorted FeSi₂ compound. No surface iron or iron-based particulate is observed in the starting material. Upon sintering, the iron environment becomes an ordered FeSi₂ structure. In the RBNS material, the FeSi₂ structure is again distorted, as observed by both EXAFS and ESR.