Characterization of moving bits (MBs) and QBD in wet/dry tunnel oxides for floating gate type nonvolatile memory (FG-NVM) applications

Abstract

A critical problem of floating gate type nonvolatile memories (FG-NVMs) used in flash memories or EEPROMs is anomalous charge loss which leads to threshold voltage ( V t) shifts on a time scale of months or years at room temperature. The number of these moving bits (MB's) is greatly affected by the nature and properties of the tunnel oxide that is used in the NVM technology. In this paper we compare different types of tunnel oxides and determine the impact on the MB issue with different measurement techniques. The appearance of MB is demonstrated by a low electric field test. This test is a close match to real lifetime conditions and as such gives a good indication of the MB performance of the device. Nevertheless since this test is very time consuming it is not feasible to use it for monitoring during manufacturing. It is demonstrated that a fast QBD measurement correlates very well with MB test itself. As such the QBD test is a valid tool for fast NVM reliability evaluation. Oxide leakage measurements, on the other hand, do not give any indication of MB characteristics. Wet oxide grown at 750 °C proves to be far superior in terms of MB performance compared to the dry oxides grown at 900 or 960 °C. The initial MB measurements before write–erase cycling show a strong impact of the thickness and the type of oxide. This is still valid after 10K write–erase cycles: increasing the thickness of the tunnel oxide from 7.5 to 10 nm decreases the number of MB's by at least three orders of magnitude. After 10K cycles, one order of magnitude less MB's are observed for the wet oxide versus the dry oxide.