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Probing ReRAMs: Forming scaling, quantised conductance

Posted: 11 Apr 2016 ?? ?Print Version ?Bookmark and Share

Keywords:ReRAMs? memory? filament? atomic force microscope? AFM?

This year at MRS 2016 in Arizona, United States, Umberto Celano and colleagues from IMEC, Belgium and Department of Physics and Astronomy, KU Leuven Belgium [Ref1] gave attendees a view of what to expect if and when ReRAMs finally fulfil the claims that they will be able to create memories with nanometre sized filaments.

Their work highlighted how the atomic force microscope (AFM) will be an essential tool in bringing any of the emerging memory technologies to commercial success. Especially in its Scalpel Probe Microscopy (SCM) role acting as a Conductance-AFM (C-AFM) for measuring electrical performance and topology at nanometric dimensions.

The IMEC team have formed and characterized the electrical conductance and topology of some of the smallest ReRAM filaments ever reported with constrictions of 3 x 3nm and exposed some very important consideration for those trying to move ReRAMs in that direction. While they have proved the devices still operate with an active volume of that size it is not all good news or in the cautionary words of Umberto Celano:

"Conducting filament stability in ultra-scaled devices will depend on the maximisation of the device volume effectively converted into an active filament volume (i.e. defects rich) and may mean there is a minimum size required in order to provide sufficient defects to guarantee switching functionality...."

The IMEC team achieved the 10nm2 filament area by turning their normal, from the top electrode down Ru/Hf/HfO/TiN memory structure, on its head and used the tip of a CAFM to replace the TiN electrode. This is because the narrowest part of the formed ReRAM occurs closest to the inert electrode and the tip of the CAFM could provide the required electrode dimensions.

They also used the tip of the CAFM as a machine tool to obtain electrical conductivity slices to build a filament tomograph in a manner similar to that described in [Ref 2] for SiOx devices at UCL. The topology of the formed filaments in the HfO devices had many similarities with those obtained for SiOx.

Figure 1: Quantised conductance discontinuities as would be observed as the nanometric sized switching volume (red) increases and decreases its diameter during partial SET and RESET of a ReRAM/RRAM, Inset shows conductance versus voltage steps (Illustrative example).

Their small filament volume devices displayed, as might be expected, quantised conductance. Expected because it is well known that point contacts and filaments with dimensions of the order 1 to 10nm result in quantised conductance and the possibility of of access to a unique type of multi-level cell (MLC) memory based on quantum point contacts (QPC).

Figure 1(a) illustrates the I-V characteristics when filaments of the order 1 to 10nm diameter are formed in the active region of a RRAM/ReRAM. It shows the discontinuities in conductivity that are the signature that quantised conductance is occurring. For computer memory designers those steps in conductance are an alluring feature, suggesting that scaled devices will have an inherent property which would make them ideal candidates for multi-level cell MLC memories. The I-V characteristics illustrated would be observed in a partial SET/RESET operation. The inset in figure 1 illustrates the perfection of the discrete steps when the conductance ratio G/Go is plotted as a function of voltage. In [Ref 3] from statistical analysis of many separate switching events the UCL authors were able to identify at least 10 discrete conductance steps for theirTi/SiOx/Ti structure devices.

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