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080519: Resistive memory resists definition
Ed’s Threads 080519
Musings by Ed Korczynski on May 19, 2008 (updated May 21)

Resistive memory resists definition
My recent blog entry about "memristors" and ReRAMs generated a lot of feedback (both on and off the record). The prevailing opinion seems to be that many companies have been working on resistive memory cells for many years, and most of the complex oxide structures at the core of these devices could function as “memristors” if people chose to look at them as analog circuit elements. Another variation on this complex theme was recently announced with Axon Technologies receiving a US patent for a copper-doped silicon oxide materials-system.

Axon, a spin-out from Arizona State University (ASU), is working with several potential partners on commercialization and full production. Dr. Michael Kozicki, founder and president of Axon, explained the science behind the technology in an exclusive interview with WaferNEWS. The company just announced it has been awarded US Patent # 7,372,065 for using copper and silicon dioxide to form a ReRAM they call a "programmable metallization cell" (PMC) memory device -- the 27th US patent issued to Axon relative to this technology since work started 10 years ago at ASU. Kozicki told WaferNEWS that the company has "strong technical and business relations with several companies in the memory and storage industry," including a license of the PMC technology to Micron and Infineon.

With modest voltages and very low current, you can grow a metallic nano-bridge that dramatically lowers resistance. By using common interconnect materials, the cost of integrating a huge range of low-cost discrete and embedded memory cells can be substantially reduced. “We honestly believe that going this way offers almost free memory for any chip," Kozicki said. "In essence you are adding materials and at most two mask steps in BEOL. So the cost of adding just a few kilobytes to a chip could be marginal.”

After first starting with tungsten-oxide as the electrolyte for main-steam IC applications, Axon found limitations of ~10,000 read-write cycles before breakdown. In contrast, “the first copper-oxide cells we built showed 1M read-write cycles,” Kozicki said. A somewhat porous silicon oxide can work. “If you have a very dense thermally grown oxide then ions move very slowly,” he noted. The company experimented with both CVD and PVD oxide, with the most success with PVD seen so far. “We could potentially expand this to spin-on-glasses too,” he added.

Kozicki explained that the fundamental ion transport speed in the oxide controls the inherent tradeoff between switching-speed and stability. Working with “stable” interconnect materials results in switching speed on the order of micro-seconds or hundreds-of-nanoseconds. Access time is determined by the surrounding circuitry, and so should be comparable to DRAM for reads. Thus, for writing, this should still be comparable to flash memory in terms of speed, but with lower current (since there would be no charge pumps) and maybe one extra mask.

ReRAM—such as Axon’s PMC variant—could replace DRAM for many applications and even some NAND flash due to an area advantage. In addition to all of these possible uses of solid electrolytes as digital memory cells, analog functionality along the lines of a “memristor” is also possible. “A solid electrolyte is simply a glass in which ions easily conduct,” explained Kozicki. “At the highest possible level, the number of things you can do with moving ions around goes way beyond memory.”

“The analog-ness of these devices is not in question,” declared Kozicki, adding that working with Polytechnica Milano has shown "some interesting effects.” Programming 10μA you get on the order of 10kΩ, if you program with 100μA then you’re down to ~1000Ω, and 1μA gives you 100kΩ. “These states are quite stable," he said. “The resistance ends up being what is the aggregate current flowing through the device.”


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080519: Resistive memory resists definition

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Ed's Threads is the weekly web-log of SST Sr. Technical Editor Ed Korczynski's musings on the topics of semiconductor manufacturing technology and business. Ed received a degree in materials science and engineering from MIT in 1984, and after process development and integration work in fabs, he held applications, marketing, and business development roles at OEMs. Ed won editorial awards from ASBPE, including interviews with Gordon Moore and Jim Morgan, and is not lacking for opinions.