The application of uranium fission track autoradiography as a method of investigating the problem of soft errors in VLSI silicon memory devices

Soft errors in VLSI random access dynamic memory devices are caused by the passage of ionising alpha-particles, which temporarily alter the contents of a bit location. These alpha-particles are emitted by minute amounts of naturally occurring uranium and thorium, which may be present as trace impurities. A method of fission track autoradiography has been developed for detecting and measuring the presence of uranium in the various component materials used in the memory device. The technique is described, and involves the irradiation of the specimens with thermal neutrons, and the detection of the fission particles produced from the fission of the 0.72% abundant 235U. A specially prepared film of polyimide is used as the SSNTD, and the fission tracks are observed by optical microscopy and counted on the Quantimet 720 Image Analyser. Extremely small amounts of uranium can be measured, equivalent to an alpha-particle flux of 0.001 α/cm2/hour, arising from the 238U and 234U isotopes in the natural uranium present. The results obtained on a variety of specimens, from standards consisting of ion implanted 235U in silicon wafers, and standard NBS glasses are evaluated. (author)