Improvement of EEPROM cell reliability by optimization of signal programming

Deterioration of stressed oxides: Application to the prediction of a non-volatile memory cell endurance

“…We had first to determine which programming voltages must be applied on an EEPROM cellÕs electrodes in order to get a rectangular voltage drop, V tun , across the tunnel oxide in write mode and in erase mode. Former studies [13] showed that an optimized signal with adequate V min and V max , as shown in Fig. 3, are suitable.…”

“…Many studies [12] have investigated the conduction properties of thin oxides undergoing dynamic stress as it is the case for the tunnel oxide of an EEPROM cell. In particular, EEPROM cell endurance can be improved [13] by using an optimized signal which diminishes the peak electric field across the tunnel oxide in write and erase modes.…”

Non-volatile memory cell design: Coupling ratio impact on tunnel oxide reliability

“…A positive or a negative charge may be at two different magnitudes, V T-HIGH , V T-LOW which correspond, respectively, to erase and write mode [4]. We define the programmed window, DV T , as the difference between these two threshold voltages (2).…”

“…This test consists in measuring the programming window changes versus the write/erase cycling number up to one million cycles. A closure of threshold voltage window reveals the tunnel oxide degradation [4]. The earlier the closure, the greater the degradation.…”

“…Generally, designers increase coupling ratio to decrease the applied voltage [2]. In a previous work [3], we have shown that the coupling ratio affects the electric field through the tunnel oxide, and changes the reliability of the cell [4]. The electric field through the tunnel oxide increases with the coupling ratio, it depends on tunnel oxide area.…”