Journal of Vacuum Science & Technology A, Vol.22, No.3, 902-907, 2004
Use of the focused ion beam to locate failure sites within electrically erasable read only memory microcircuits
Occasional electronically erasable programmable read only memory retention failures occurred after thousands of read write cycles. The physical sites of the failed cells were known, but not their location within the individual memory bits. Memory storage transistors have normal gate oxides and thinner tunneling oxide regions for programming. Focused ion beam (FIB) images are brighter when samples are grounded due to a passive voltage contrast mechanism. Thus, using precision etching and polishing to expose memory cells, transistors with leaky (grounded) gate-to-channel characteristics stand out. FIB examination successfully identified the failed transistors and additionally highlighted previously undetected leaky but still functional cells. FIB etching was next used to physically isolate the normal gate and tunneling oxide regions of failed transistors. This proved that the leakage only occurred within the tunnel oxide regions. Further analysis using precision focused ion beam/transmission electron microscopy cross sectioning and atomic force microscopy, identified local thickness reductions in the tunnel oxide which occurred during manufacture. Gate dielectric breakdown failures increase very rapidly with reduced thickness. Calculations showed that unacceptable gate oxide leakage would develop in a time comparable with the observed memory loss failures. (C) 2004 American Vacuum Society.