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Monthly Archives: January 2016

3D XPoint uses PCM Material in ReRAM Device

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IM Flash pre-announced “3D XPoint”(TM) memory for release later this year, and lack of details has led to widespread confusion regarding what it is. EETimes has reported that, “Chalcogenide material and an Ovonyx switch are magic parts of this technology with the original work starting back in the 1960’s,” said Guy Blalock, co-CEO of IM Flash at the 2016 Industry Strategy Symposium hosted by the SEMI trade group. However, contradicting industry terminology conventions, in another article EETimes reported that a spokesperson for Intel has said that, “3D XPoint should not be described as ReRAM.”
First promoted by the master of materials solutions-looking-for-problems Sanford Ovshinsky under the name “Ovonic” trademark, chalcogenide materials form glassy structures with meta-stable properties. With proper application of heat and electrical current, chalcogenides can be made to switch between low-resistivity crystalline and high-resistivity amorphous phases to create Phase-Change Memory (PCM) arrays in silicon circuit architectures. Chalcogenides can also function as the matrix for the diffusion of silver ions in a cross-point device architecture to create a digital “Resistive RAM” (or “ReRAM” or “RRAM”), or create an analog memristor for neuromorphic applications as explored by Prof. Kris Campbell of Boise State in collaboration with Knowm.

Hitachi and Renesas Technology developed Phase-Change Memory (PCM) cell technology employing Ta2O5 interfacial layer to enable low-power operation. (Source: Hitachi)

Hitachi and Renesas Technology developed Phase-Change Memory (PCM) cell technology employing Ta2O5 interfacial layer to enable low-power operation. (Source: Hitachi)

The Figure shows a schematic cross-section of a typical PCM cell. From a scientific perspective, we could say that any memory cell that relies upon a change in material phase to encode digital data should be termed a PCM. However, due to the history of this specific type of PCM device being the only architecture explored for decades (and commercialized for limited niche sub-markets), and due to the fundamentally different circuit architectures, it is reasonable to categorically deny that any cross-point device is a “PCM.”
However, any cross-point memory device based on a resistance change has to be a ReRAM regardless of the switching phenomenon:  phase-change, filament-growth, ion-diffusion, etc. So we could say that this new chip uses PCM material in a ReRAM device.
—E.K.

Controlling Polymers to Tune TFTs

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Thin-film transistors (TFT) created using only additive process steps could create new low-cost ICs with functionalities beyond silicon, but only if we understand how to control structures at the molecular level. Thin films of conjugated polymers such as poly(3-hexylthiophene) (P3HT) can provide useful conductivity when the electron mobilities are controlled within as well as between molecules. In producing TFTs using such organic macromolecules, we must rigorously control the deposition and annealing processes so that the right molecules line up in the right order.
Peter F. Green, Professor of Chemical Engineering, Macromolecular Science and Engineering at the University of Michigan, and his team fabricated ~55 nm thin films of P3HT using resonant-infrared matrix-assisted pulsed laser evaporation (RIR-MAPLE), as well as conventional spin-casting. The films produced by MAPLE show a higher degree of structural disorder, with localized trap sites that reduce mobility out-of-plane by an order of magnitude compared to spin-cast films.

(Source: Peter Green, University of Michigan)

(Source: Peter Green, University of Michigan)

The Figure shows that despite the disorder of MAPLE-deposited P3HT, enhanced carrier density at the dielectric interface allows TFTs to exhibit similar in-plane mobilities to those built using conventionally spin-coated films. TFTs were top-contact, bottom-gate designs on 300nm thermal oxide on highly doped silicon. In-plane carrier mobilities of MAPLE-deposited versus spin-cast films were 8.3 versus 5.5 (×10 -3 cm2/V/s). In principle, the ability to independently control in- and out-of-plane mobilities allows for the fine tuning of TFT parameters for different applications.
—E.K.