Use of the focused ion beam to locate failure sites within electrically erasable read only memory microcircuits

Abstract

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.

Focused Ion Beam System—a Multifunctional Tool for Nanotechnology

“…The source and drain lay on opposite ends of the gate, which sat on top of the gate oxide layer, all of which resided on a silicon substrate. TEM samples of the failure site (<100 nm in diameter) were One floating gate appears bright, indicating that it is grounded and therefore the tunnel or gate oxide has failed [25]. desired for investigation on the mechanics behind the problem, but the area of failure had to first be located.…”

“…Then, as an added benefit, the FIB was able to identify weak storage transistors on the memory device, which if not taken care of could soon lead to other failures. Finally, TEM samples were created using the FIB system, a process which will be discussed in depth in a later section [25].…”