(December 20, 2010) — At IEEE’s International Electron Device Meeting (IEDM 2010) this December, Imec researchers demonstrated the competition effect in the switching of several conductive filaments in resistive RAM (RRAM) (Figure 1) using a small-size FUSI gate NMOS transistor. The work was laid out in IEDM 2010 paper #28.4 ("Generic learning of TDDB applied to RRAM for improved understanding of conduction and switching mechanism through multiple filaments").

Figure 1. 3D stackable RRAM array where the diode is used as a selector. SOURCE: Imec

The group was also able to establish the theory for predicting the maximum applicable set voltage and expand the post-BD conduction model revealing that the reset operation corresponds to a pinch-off of the filament at its narrowest point (Figure 2).

Figure 2. The filament in a resistive element of an RRAM can be modeled as a narrow constriction in an insulator barrier. The energy band diagram with a parabolic potential well illustrates this constriction. SOURCE: Imec

Imec program manager, Malgorzata Jurczak, told ElectroIQ that RRAMs are a significant departure from conventional flash memory. "Conventional flash is based on charge storage, while RRAM is based on resistive change of the memory element," said Jurczak. "Another major difference is that in Flash, addressing (WL) and reading (BL) occur in one single device, whereas in RRAM, the resistive element is a two-terminal device that will require an additional selector device to address it." She noted that currently, it is not yet clear what type of selector device this will be, e.g., BJT, diode, or MOSFET. In the paper, the researchers note that many stacks of combinations of materials need a forming step to create a small conductive filament connecting the electrodes in the device.

Thomas Hoffmann, director, FEOL program, Imec, discussed the research in a podcast interview with Debra Vogler, senior technical editor, at IEDM.

Listen to Hoffman’s IEDM talk: Download (for iPhone/iPod users) or Play Now

This work was aimed at demonstrating the viability of RRAM in order to go to the next step, which would be mass production, he said. The consortium was able to leverage its know-how in reliability characterization because the switching mechanism used in RRAM is very similar to the breakdown mechanism seen in memories. "We see a lot of commonalities in turning on and off the resistive elements," noted Hoffmann.

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