ANALYTICAL SCHEME FOR TRACING SOURCES OF CONTAMINATION DURING PROCESSING OF SI3N4 IN CLEAN ROOMS

This work investigates the uptake of impurities during processing of S i3N4 and describes an analytical scheme for detecting sources of conta mination. For this purpose a process as simple and short as possible w as chosen, using commercial starting materials with a high standard of purity and reproducibility. The uptake of non-metallic and metallic c ontaminants was investigated by choosing elements which were specific for individual processing steps. This was difficult in the determinati on of metallic impurities in a powder consisting of Si3N4 with Y2O3/Al 2O3 additives, because the powder mixture and the sources of contamina tion (milling balls, attritor disk, wall materials) were similar in co mposition and the available analytical methods were not precise enough to detect the small increase in concentration that occurred. Therefor e pure Si3N4 powders were milled in order to get an indication of the kind and concentration of impurity introduced by the individual millin g materials and steps. These elements can then be used as monitor elem ents to trace sources of contamination and to optimize processing para meters. Experience with the processing of Si3N4 with Y2O3/Al2O3 additi ves by cyclic milling, spray drying, burn-out and isopressing are repo rted. Contamination by carbon is unavoidable. Its concentration during the process is relatively high, as it is added in the form of process ing aids (deflocculants, binders), but temporary, as it can be complet ely burned out. Oxygen is predominantly taken up during milling. Good deflocculation reduces the milling time and thus limits the uptake of oxygen. As a consequence of these findings the processing parameters c ould be optimized. Thus the uptake of metallic impurities, e.g. Fe cou ld be limited to 10 mug/g and the uptake of oxygen was found to be les s than 0.2 wt%.