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Advantages of resistive RAM for next-gen NVM

Posted: 09 Apr 2012 ?? ?Print Version ?Bookmark and Share

Keywords:NAND flash? Resistive RAM? hafnium-oxide?

Switching speed: To measure switching speed, we used a pulsed operation mode, exemplified here for a Reset switching. Thus, stimuli were applied on the sourceline (SL) and wordline (WL) of the serial 1T1R (1-transistor, 1-resistor) test vehicle, while the response was monitored with a digital oscilloscope on a small series resistance attached to the cell bitline (BL; figure 2a). The switching was time-confined to a maximum duration given by the width of the SL pulse, i.e. 10 ns. We carefully designed the experimental setup to minimize the impact of the parasitic elements (e.g. capacitances), having a reasonably short system time constant. The resistive element, initially in the on-state, switches to the off-state quickly, leading to a decrease of the signal within just 3 to 4 ns (figure 2b). When taking into account the impact of the testing environment on the collected waveform, the observed transition time duration gives a higher margin for the intrinsic switching time, which can be shorter.

On/off window & operating voltages: The on/off window easily exceeds a factor of 10, with modest (figure 3a), which may open up paths for multilevel operation.

Figure 3: Typical on/off window (expressed in read-out cell current) achievable with sub-3-V pulsed operation, with verify (a) and Reset pulse amplitude-duration voltage-time tradeoff, showing no significant degradation when scaling cell size from 1 um2 down to 10 x 10 nm2 (b). Data are for an oxide film thickness of 10 nm. The dashed lines are guide for the eye. Similar conclusions hold for Set switching (not shown).

The voltage-time dilemma is a popular term used to express the limited ability of RRAM to display nonlinearity. This is however not specific to RRAM, but present in virtually all memory structures, which ideally need to on one hand allow for indefinitely long stability under no or low-electrical stimuli (for retention, read-out and disturbs immunity), while on the other hand providing fast change of state under operating stimuli (for P/E or S/R).

The S/R voltages required to operate these cells thus display the usual tradeoff with time. Nevertheless, the pulse amplitude-time dependence shows that the cells can still be operated with voltages well below 3 V, even for pulses as short as 10 ns. Furthermore, in a comparison of large area cells (in the order of 1 um2) with smallest-size cells (of 10 x 10 nm2), the voltage-time characteristics maintain similarity (figure 3b), which shows that we should expect no considerable performance degradation when considering aggressively scaled structures. Compared to NAND flash, RRAM has the benefits of low-operating voltages.

Reliability: retention & endurance: The usual 10 year requirement for NVM retention is met by most of the RRAM cells, with a median cell reaching this limit at an extrapolated temperature of around 100C. As expected, retention turns out to be most critical for the on-state, where retention loss is attributed to filament dissolution. Retention improvement is possible through material optimization and careful sealing of the active region with oxygen-free layers.

Endurance tests performed on unoptimized samples showed cycling of at least 10 Mcycles in a single shot. The failure at the end of the tests was, however, recoverable with a stronger stimulus to "unlock" them from the stuck state, and cycled again with adjusted slightly stronger conditions. These facts suggest that careful balancing of the S/R test conditions, next to process improvement, may allow superior reliability and extended device lifetime, which can well exceed billion cycles, even on the smallest device sizes. This figure is far above the conventional requirement for flash memory, pinpointed to 100 kcycles, although reference value for data storage flash is commonly lowered down 10 kcycles, on arguments of practical, as well as economical nature associated with a commodity product.

Scalability, energy consumption, cell array issues
The data discussed so far provide evidence of RRAM operation on an effective area of nearly 10-x-10-nm2 without compromising any of the major performance or reliability figures. This size is the smallest reported to date, for HfO2-based RRAM cells and demonstrate cell scalability in the nanometer range, which is beyond scaling limits of NAND flash. Filament formation has been observed experimentally, for instance on TEM pictures, for metallic filaments, such as those formed in nitrous oxide RRAM.

Figure 4: Extracted filament size for 10-x-10-nm2 cells, operated with 10-ns pulse duration. The filament was asssumed cylindrical, with a saturated sub-stochiometric hafnia resistivity.8

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