A simple and accurate method to extract neutral threshold voltage of floating gate flash devices and its application to flash reliability characterization

A commonly used method to determine the neutral threshold voltage (Vtn) of floating gate device is by measuring the Vt of the device after a UV erasure treatment. However, such a UV erasure treatment is no more feasible for advanced technologies with Cu backend, because of limited UV transparency of the dielectric stack. A fully electrical method is needed for the determination of such a Vtn. Based on the Fowler-Nordheim (FN) tunneling formula, and by measuring the FN programming and erasure speed characteristics, a perfect straight line can be obtained if I/Eox versus ln(t) is plotted. From the fitting parameters of such a straight line, the Vtn value can be obtained. This method, that is based on the physics of FN tunneling, has been used to derive the neutral threshold voltage (Vtn) of the 90 nm 2T-FNFN embedded flash device before and after P/E cycling. The results give us a good insight in device degradation speed.