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Russia, U.S. get closer to universal memory

Technology News |
By Julien Happich

Researchers worldwide are seeking a “universal” memory to replace DRAM, SRAM, flash and spinning-disks. Of the many proposals being researched, this one has the advantage of being fabricated on a silicon substrate with conventional tools; it’s also potentially fast, dense and nonvolatile, plus could be adapted to operate like an artificial neuromorphic synapse, too—a memristor—making it suitable for both conventional computers and the cognitive computers—cognizers—of the future.

The material is an ultra-thin (2.5-nanometer) polycrystalline ferroelectric film on silicon invented through a collaboration among the MIPT, the University of Nebraska and the University of Lausanne (Switzerland).

Fig. 1: A silicon-based ferroelectric tunnel junction is written by applying an external electric field, which changes the direction of the ferroelectric’s polarization vector and thus the shape of the potential barrier. (Source: Moscow Institute of Physics and Technology (MIPT))

“The difference between our approach and other attempts to grow ultra-thin ferroelectric films, particularly, on silicon, is that we grow polycrystalline (rather than epitaxial) alloyed hafnium-zirco­nium [Hf-Zr] oxide films, which retain their ferroelectric properties down to thicknesses of under three nanometers,” said Andrei Zenkevich, head of MIPT’s Laboratory of Functional Materials and Devices for Nanoelectronics, told EE Times.


Making this ultra-thin ferroelectric material compatible with silicon substrates allows the vast CMOS fabrication facilities already in place to easily switch over to the hafnium-zirco­nium oxide-material. The plan is to build ferroelectric tunnel junctions using the material, according to the researchers.

“We employ Atomic Layer Deposition technique and use alternate cycles of Hf and Zr precursors combined with that for O (H2O) to grow initially amorphous mixed Hf-Zr oxide with a pre-defined composition. The oxide film is further crystallized during ALD growth of the capping TiN layer,” Zenkevich told EE Times.

So far the researchers have merely demonstrated the fabrication and characterization of the material itself, next they plan on building prototypes to prove the tunnelling effect can be used for real memory chips, although the theory is already well worked out. The 1s and 0s are stored by virtue of a reverse in polarization across the hafnium-zirco­nium-oxide layer (see figure one), which could be performed in the manner of a neuromorphic memristor—merely passing current through in the right direction.

“The work to demonstrate the so called tunnelling electro-resistance effect in a prototypic memory device is under way now,” Zenkevich told us. “Judging from pulsed measurements of the polarization reversal, the prospective write time is within the nanosecond range. The reading of the information occurs non-destructively by measuring the (tunnelling) current through the junction and access time should mainly depend on the electronic circuitry.”

Fig. 2: The cross section of the non-volatile memory structure shows a polycrystalline fused film of hafnium and zirconium oxides grown on a highly doped silicon substrate (upper electrode, titanium nitride). (Source: Moscow Institute of Physics and Technology (MIPT))

The reason ferroelectric tunnel junctions might lead to a universal memory type is that they are very small and yet can retain their values indefinitely without the need to consume power. The best part, of course, is that they can be fabricated in conventional CMOS fabs and appear to be scalable like other CMOS components.

It should take several years to confirm all these favourable processes, by which time we could be well into the cognitive computer era, where the hafnium-zirco­nium-oxide could act as the memory element in electronics brains—namely the neuromorphic synapses or memristors.

Funding was provided by the Russian Science Foundation and MIPT’s Centre for the Collective Use of Unique Scientific Equipment in the Field of Nanotechnology.

Get all the details in Ultrathin HfZrO Ferroelectric Films on Si in ASC’s Applied Materials, where the process is fully described.

About the author:

R. Colin Johnson is Advanced Technology Editor at EE Times

 

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