Ed’s Threads 080519Musings 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.”
Labels: Axon, memory, memristor, ReRAM, solid-state electrolyte
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080519: Resistive memory resists definition
Ed’s Threads 080505Musings by Ed Korczynski on May 5, 2008
When is a Memristor a ReRAM?
HP published that they are the first
to have fabricated a novel circuit element first predicted in 1971 called the “memristor.
” The HP authors claim that, “until now no one has presented either a useful physical model or an example of a memristor.” HP is certainly leading the world, but as one of many companies working on this technology for resistance-change random-access memory (ReRAM) applications. This spring’s Materials Research Society meeting featured an afternoon session on ReRAM
with presentations by HP as well as Fujitsu, FZ Jülich, IMEC, Panasonic, and Samsung.
Antique circuit theories are rarely invoked at MRS meetings, so the focus of the ReRAM session was all about how you engineer complex atomic-layer oxide elements. Another sub-session covered organic switching elements for printable ultra-dense memories in the far future. In other memory technology, the usual suspects are still doing the same tap-dances about FeRAM and MRAM, but PRAM seems to have new momentum
due to investments by Intel and ST in Numonyx
and so may take over some of the mainstream.
Robert Muller of IMEC presented fundamentals of ReRAM cells based on Cu+ and Ag+ charge-transfer complexes for memory applications. Using Ag/CuTCNQ/Al structures, Cu+TCNQ- is a solid ionic conductor, and so a potential can reduce alumina to aluminum along with a corresponding oxidation of the “noble” metal on the other side. The main resistance change is expected as an interfacial effect within a few nm gap between the solid ionic conductor and the aluminum electrode, where Cu filaments form as conductors. IMEC has seen retention time of up to 60 hours so far, but theoretically this can be much higher. The integration problem is that TCNQ begins to degrade at 200°C, so another material may be needed for dense IC memories.
Z. Wei et al. of Panasonic talked about FeOx ReRAM, as first presented by S. Muraoka et al. at IEDM 2007. Fe3O4 reduces to higher-resistance Fe2O3. Both bipolar and unipolar transitions are possible, however, the bipolar high-resistance state (HRS) degrades in only ~100 hours at 85°C, while the unipolar transition retains high resistance to >1000 hours. Interestingly, the low-resistance state (LRS) of the unipolar mode shows metallic (instead of semiconducting) dependence of resistivity to temperature. Both fast switching and long retention may be achieved by combining bipolar (<100ns>1000 hours @85°C) modes.
Herbert Schroeder et al. of Jülich Forshlungszentrum (“FZ Jülich”) showed a simple stack geometry using 100nm thick Pt top and bottom electrodes with a central TiO2 layer 27-250nm thick. As produced, Pt/TiO2/Pt is insulating (in the MΩ to GΩ range) so that “electroforming” is needed. Up to 30mA is needed for the reset current with simple unipolar stacks, though HRS/LRS is ~1000 which is excellent and has been shown with read-out voltages of 0.3V over up to 80 cycles. Bipolar switching has a HRS/LRS of only ~5, but the reset current is merely 1mA and so applicable to real-world circuits. Room-temperature reactive sputtering of Ti results in polycrystalline TiO2 with columnar grains of 5-20nm dia. The possible mechanism of “forming” is the electro-reduction of TiO2 into TiO or Ti which creates oxygen ions to drift to the anode and appear as voids.
H. Kawano et al. of Fujitsu Labs (along with the Nagoya Institute of Technology) explained some of the inherent trade-offs in device properties depending upon the top electrode used with Pr0.7Ca0.3MnO3 bipolar switching material. The mechanism for bipolar switching is more complex and the switching speed strongly depends on the electrode material; using Ag or Au as the top electrode results in 100-150ns, while an easily oxidized metal such as Al or Ti results in ~1ms. Ta forms a thinner oxide which allows 100ns switching with HRS:LRS of 10 at 7V, and this ratio was maintained up to 10,000 cycles. With Pt as both electrodes they saw no ReRAM effects.
Julien Borghetti of HP Information and Quantum System Lab (IQSL) said that they use a TiO2 target to sputter ~30nm TiOx and after a forming step the HRS:LRS ratio is 1000-10,000 for bipolar switching. After formation, the HRS shows essentially no temperature dependence on the conduction, which implies that tunneling current must be responsible for the conduction. From IV curves at different temperatures and biases, it seems that most of the TiOx has parallel degenerate or metallic states which account for ~200Ω resistance which is present in both the HRS and LRS. Then there is a tunneling gap which accounts for the difference between the two states, and it seems to be <3 nm thick and consists of some defects which assist in the tunneling. Cryogenic tests down to 3°K show resonant tunneling through a degenerate gas of electrons.
More details on the HP ReRAM manufacturing process can be found in my recent SST article, “Imprint litho forms arrays for new fault-tolerant nanoscale circuits”
(Solid State Technology, April 2008) which summarizes the main information the company has presented at IEDM, SPIE, and MRS conferences in the last half-year. HP has shown how cross-bar circuits built with ReRAM switches can function both as interconnects and as logic elements. The titania/platinum materials set which can provide reversible ReRAM is not ready for production, but alumina/aluminum is ready to go and can provide irreversible effects. HP Corvalis in Oregon, with its old subtractive Al metal fab, has all the processing capability needed to integrate alumina/aluminum ReRAM with traditional CMOS circuitry for FPGA applications.
Does calling the fundamental switching element in a ReRAM a “memristor” make it switch any faster or retain a state any longer? HP’s labs and fabs do great work and deserve recognition, but unless HP plans to use memristors as novel circuit elements it’s confusing to use the term for ReRAM memory arrays. One blogging circuit designer has already imaged the possibility of building large-scale analog neural networks out of memristor arrays
. Now that we’ve discovered that our ReRAMs could be memristors, the next question is: what do we do with them?
Labels: FRAM, memory, memristor, MRAM, MRS, PRAM, ReRAM, semiconductor
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080505: When is a Memristor a ReRAM?