Determination of uranium, thorium and α emitters in semiconductor materials

It has recently pointed out that some errors in semiconductor memory storage are attributable to alpha particles from trace amounts of uranium, thorium or α emitters contained in the semiconductor material. The concentration of such uranium, thorium and α emitters can be as high as several ppb as the capacity of memory storage increases. It is very difficult to carry out analysis and evaluation satisfactorily. The present article reviews recent studies that seek to develop analysis and evaluation methods. The alpha particle flux density in semiconductor material can be determined by direct measurement of the particles or by making analysis of uranium and thorium, which are the major sources of alpha particles. An effective direct measuring method for ppb-level or sub-ppb level analysis is to perform alpha-particle counting by using a 2π-type gas flow proportional counter. Methods available for microanalysis of uranium and thorium include solid fluorophotometry, fission tracking, secondary ion mass spectrometry, glow discharge mass spectrometry, electric heat evaporation/ICP emission spectrometry, ICP mass spectrometry and neutron activation analysis. (Nogami, K.)