Trace contamination in pure high-tech materials

High purity refractory metals and their oxides are widely used in advanced technology, for example in microelectronics or parts of turbines and aircraft. The purity requirements are extremely high, as mobile ions may alter the materials' mechanical and electrical properties to a large extent. Additionally, α emitters such as Th and U can produce defects in data storage cells. Control of these impurities in raw materials as well as during the production process and in the final product requires the greatest sensitivity and the virtual absence of blanks in the analytical process. Neutron activation analysis (NAA) is a direct, non-destructive method with extraordinary sensitivity for some of the important elements in high purity materials. If, after irradiation, the samples are removed from the irradiation containers prior to counting, the method can be considered as basically blank free. The power of determination can be additionally increased by post-irradiation chemistry, where the concentration of the radionuclide produced during irradiation and used for counting will not be altered by the addition of the stable isotopes of its parent element. This makes radiochemical neutron activation analysis (RNAA) an ideal candidate for the sensitive quantification of trace contamination in high purity refractory metals and metal oxides. Silicon nitride has become one of the most important nonoxide ceramic materials for use in high temperature parts of engines and gas turbines. In microelectronics, ultra-pure silicon nitride is used as a substrate and package material for integrated circuits. It was found that the presence of oxides of alkali and alkaline earth elements may lead to fracture and subcritical cracking at high temperatures. Other trace elements were investigated with regard to their influence on the dielectric properties. Comprehensive characterization of the starting materials as well as of the final product is needed, and instrumental neutron activation analysis (INAA) and RNAA proved to be the methods of choice for this delicate analytical challenge. Using optimized irradiation, decay and counting conditions, and owing to the fact that Si and N produce only short lived radioisotopes, up to 55 elements in several high purity Si3N4 materials could be determined quantitatively by INAA. NAA has proved to be a very suitable method for the determination of ultra-traces of Th and U via their daughter nuclides 233Pa and 239Np. Aluminium oxide and aluminium nitride, the most important raw materials for advanced ceramics used in microelectronics and bioceramic endoprotheses, must be of the highest possible purity to ensure their mechanical and insulating properties. Oxides of Cr, Fe, Mg, Na and Ti affect the electrical resistivity of ultra-pure Al2O3, whereas the oxides of Ca, Fe, Mg, Na, Si and Zr reduce the thermal stability and shock resistance. Because of the refractory character of the two materials, direct determination methods using solid samples are of great interest for their analysis. A combination of INAA and RNAA was developed for the characterization of 56 elements in raw and high purity aluminium oxide and nitride powders