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080505: When is a Memristor a ReRAM?
Ed’s Threads 080505
Musings 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?

—E.K.

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080505: When is a Memristor a ReRAM?

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11 Comments:

Anonymous Anonymous said...

Maybe Stan (now a Bay Area radio star) should have read reference 4 in his own paper. It contains stuff like, oh, results and facts on a whole bunch of devices that walk and quack like "memristors". Other references in his paper and the Nature Materials review suggest that there are also previous models that allowed some fairly sophisticated circuit design. So, what did HP "invent", like, exactly? I think this big PR over-the-top-fest will come back to haunt the good Dr. Williams and his otherwise competent team.

Tue May 06, 11:01:00 PM PDT  
Anonymous Tarun Kansal said...

ReRAM seems to be a grear technology for future. But I m wondering what are its advantages comparing to our ongoing memory technologies??

Wed May 07, 01:15:00 AM PDT  
Blogger SST's Ed's Threads said...

In response to the comment by "anonymous," I can only say that I suspect that this PR is part of “building the new blueprint for corporate research” as directed by new HP Labs head Prith Banerjee, to convert “scientific discoveries into the marketplace.”

Wed May 07, 01:03:00 PM PDT  
Anonymous Anonymous said...

Memristor is NOT ReRAM. Typically a resistance-change memory device changes its state only above a certain threshold voltage. However, according to the equations of memristor in HP's paper, the memristance M is a function of q, which is the integral of idt. So applying low voltage is also gradually changing the state, and the resistance state should be continuous.

Wed May 07, 01:29:00 PM PDT  
Blogger SST's Ed's Threads said...

A memristor may technically never "be" a ReRAM, yet essentially identical engineered materials are used for both devices. Theoretically an analog memristor should indeed demonstrate continuous change in resistance, while a ReRAM is intended to store digital information as two or more discrete resistance levels. The control circuitry must be completely different between the two, yet the engineered oxide which changes resistance may be identical.

Wed May 07, 03:41:00 PM PDT  
Anonymous Anonymous said...

Hello anonymous - hey, same name as me (maybe we even work for the same Higher Power). The threshold thing may be a bit of a red herring as the key to memristor action is "history dependent resistance" and all of the other referenced devices possess this. But you do bring up a very good point - how can you build a memory cell, let alone an array of them, if you are using a device that does not have some kind of threshold voltage? A threshold-less device would not be a memory cell, more of a read-disturbistor. In any case, the "real" TiO2 device shown in the HP paper does have a threshold (somewhere between 0.5 and 1 volt). So I ask again, what exactly did HP invent here? A really bad memory cell or a device that has been in the literature for decades?

Wed May 07, 09:44:00 PM PDT  
Blogger SST's Ed's Threads said...

Think of it this way: how can a capacitor be the memory element of a DRAM cell? In the same manner a memristor can be the memory element in a ReRAM cell (only the capacitor leaks and so is volatile, while the memristor retains resistance and so is non-volatile). ReRAM uses the voltage-induced switching effect between high- and low-resistance states, which can be read as the 1s and 0s of digital memory.

Thu May 08, 01:11:00 PM PDT  
Blogger Marcelo said...

I found it quite unfortunate
that S. Williams et al. in their recent Nature paper have simply ignored our recent work in theoretical modeling of
the non-volatile resistive switching effect in MIM structures that use transition metal oxide dielectrics.

Our first work appeared in 2004 in Physical Review Letters, and subsequent work appeared in PRL and APL.

Contrary to their claim in the opening paragraph of the Nature paper
"... until now no one has presented either a useful physical model or
an example of a memristor.", our 2004 paper does introduce a model,
which certainly seems to have been useful as demonstrated by the over 100 citations it has received so far.

M. J. Rozenberg, I. H. Inoue and M. J. Sanchez, Phys. Rev. Lett. 92, 178302 (2004).

M. Rozenberg
[email protected]

Sat May 10, 04:14:00 AM PDT  
Anonymous Krieger said...

First of all I would like to remind that to the moment of publication of L. Chaua’s paper a lot of papers about switching resistance of thin oxide as well organic films were already published. (Discussion of L. Chaua’s paper is separate story).
It was suggested many mechanism of switching and mechanism memory in thin films. By this time are published about 1000 papers about switching and memory phenomena in different types of thin films and theoretical papers explained there phenomena.

According L. Chaua’s definition a memrestor is charge- controlled device, its resistance (conductance dependents of the complete past story (injected charges (g) or integral of memristor current).

Resiatance of HP’s device is determined by changing distribution of charges (oxygen vacancies – oxygen ions) within TiOx film. By program, one part of the TiOx film receives oxygen ions (TiOx film is doped by oxygen ions) other part of the TiOx film loses oxygen ions (TiOx film is undoped). The balance of a charge in HP’s memristor cell does not change. It is necessary to distinguish a flow of electron charges and flow of oxygen ion charges through cell. Only ionic charges can change memristor resistance.

I agree with Anonymous, Memristor is not ReRAM. HP’s memristor don’t have threshold voltage. It is mean memristor cell can be program or erased by read voltage.
Basically, it is possible to use memristor as memory cell, but reading is destructive, like DRAM. It is mean after every reading (pulse at least after every 100 or 1000 reading pulse it is necessary again reprogram the memristor cell without threshold voltage.

There are a lot of resistance memory cells which have threshold voltage and use doped and undoed phenomena in thin films (conjugated polymer and related organic materials, inclusion compounds like WO3 and semiconductor). These memory cells use additional superionic layer as source of different type of ions. (See for example Spansion and AMD patents. There are about 30 patents).

From point of view circuit designer, it is more useful and simple do use switchable diode with memory which also have threshold voltage and use doped and undoed phenomena in thin films. (See for example Spansion and AMD patents).

Switchable diodes with memory allow ease to create a passive memory array without additional diode (in this case with the memristor cell, it is desirable to use Zener diode).
I guess, switchable diode with memory can be called as “memdiode”. It is joke.

Sun May 18, 09:28:00 PM PDT  
Anonymous Anonymous said...

The HP result uses Pt/TiO2/Pt which has been used as a ReRAM or RRAM. The big question is when they apply 1 Volt across a 5 nm layer, that gives a 2 MV/cm electric field. Probably the less conductive part has even higher electric field. Why doesn't the layer breakdown under this condition?

Tue May 20, 11:39:00 PM PDT  
Anonymous Anonymous said...

Krieger (mein Bruder): HP gets around the destructive read problem by using Alternating Current.

Tue Jun 10, 11:49:00 AM PDT  

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080407: CNT and graphene dreams may be real
Ed’s Threads 080407
Musings by Ed Korczynski on April 7, 2008

CNT and graphene dreams may be real
Carbon nano-tubes (CNT) are the only viable (pun-intended) new materials being developed to replace copper as the electrical interconnects for future ICs. There are no known room-temperature superconductors, and optical interconnects require relatively slow and expensive lasers and detectors, and CNTs are the future. The theory and practice of growing CNTs was thoroughly reviewed at this spring’s Materials Research Society (MRS) meeting, and the applications as electronic IC interconnects will be seen at the International Interconnect Technology Conference (IITC) to be held in Burlingame, California in June. The deadline for submitting late news to IITC is this Friday.

Carbon can form an amazing variety of stable crystals and molecules based on different bond energies and angles between atoms. In crystalline form, sp2 electron orbitals can form 2D planes of graphite or sp3 electron orbitals can form 3D tetrahedral of diamond. The 2D form of solid carbon shows very interesting properties when reduced down to less than a few atomic layers.

Graphene is one or two atomic layers only, which results in geometrically induced electron energy-band modification and the ability to form semiconducting devices. Graphene is a great potential “long-shot” technology first reported in January 2006 Solid State Technology…sure to generate many Ph.D. theses and likely to benefit DARPA programs…but still quite a way away from proven as commercially manufacturable. As Gordon Moore reminds us in this recent interview, “The actual idea of an MOS transistor was patented in the mid-'20s,” though it was not until over 40 years later that Intel started making a business out of it.

Take 60 carbon atoms and you can coax them together into a cage-like spheroid called a “buckyball” or fullerene (C60)—initially predicted by R. Buckminster Fuller based on the potential for stable bond-angles in regular polyhedra—which has the same 2D form as graphene. Larger and more complex carbon cage molecules can be formed, and seem to be formed naturally by stars in space. Take a continuous supply of carbon atoms and you can coax them together using a catalyst particle into growing as a nano-tube with that same basic 2D form. You can grow both single-walled CNT (SWCNT) and multi-walled CNT (MWCNT). Both grow off of metal catalyst particles, which must somehow first be deposited in the bottom of vias to form interconnects between lines; making the connection on the top side seems like it will be inherently a bit tricky.

At IITC this year, researchers from MIRAI-Selete and Waseda University (Japan) will show actual integration results for CNT in 160nm diameter vias at temperatures as low as 365°C. The team will report that the CNT fabrication process didn’t degrade a fragile low-k (2.6) dielectric and that the vias sustained a current density as high as 5.0 MA/cm2 at 105°C for 100 hours with no deterioration.

SEM cross-sections of 160nm-diameter CNT vias fabricated with growth temperatures of (a) 450°C and (b) 400°C (IITC2008 Paper #12.4, “Robustness of CNT Via Interconnect Fabricated by Low Temperature Process over a High-Density Current,” A. Kawabata et al.)

One of the reasons that MRS meetings are exciting for materials scientists and engineers is that truly leading results are shown. Oleg Kuznetsov et al.—from Honda Research Institute in Columbus OH (USA) and Goteborg University (Sweden) and Duke University (USA)—presented information on the size-dependence peculiarities of small catalyst clusters and their effect on SWCNT growth. Though exact mechanisms are not fully understood yet, we know that nano-scale catalysts particles play key roles in growth, and that sizes alter growth properties. The general background assumption is a vapor-liquid-solid (VLS) model for growth: carbon in the vapor phase is absorbed into the catalyst particle as a liquid from which solid SWCNT grows out. An observed ‘paradox’ is that with decrease of catalyst size from 3nm to 1nm the required minimum temperature for SWCNT growth increases. Molecular dynamics simulations revealed that reducing the catalyst particle size reduces its solubility of carbon atoms and thereby requires higher temperature for SWCNT growth.

Since the researchers used Fe as the catalyst for SWCNT growth, their rigorous modeling work included a re-working of the classic Fe-C phase diagram where they showed that SWCNTs grow in a liquidous region above the Eutectic point. The Fe-C phase diagram is arguably the foundation of modern materials engineering, since it shows how to make the varieties of steel which are the physical backbone of construction in our age, and is taught in all undergraduate materials science courses. While I haven’t been looking very hard, but this is the first time I’ve seen something new in a Fe-C phase diagram since I left MIT in 1984.

—E.K.

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posted by [email protected]
080407: CNT and graphene dreams may be real

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2 Comments:

Anonymous Joel Cook said...

I had always understood that the discoverers of the fullerenes (Curl, Kroto and Smalley) named C60 "Buckminsterfullerene" since the structure they elucidated resembled one of his geodesic domes. I had never understood that Buckminster Fuller had predicted the C60 allotrope of carbon a priori as you state.

Wed Apr 09, 08:06:00 AM PDT  
Blogger SST's Ed's Threads said...

Hi Joel: While I cannot comment on what the discoverers of the fullerenes knew of Fuller's work (so they may have only known of geodesic domes), Fuller predicted the 60-atom structure would be a stable molecule based on first principles of what he called "synergetics" (http://www.bfi.org/our_programs/who_is_buckminster_fuller/synergetics) without predicting that carbon would be the first element shown in this form. Of course, the geodesic dome was first shown only because Fuller had used synergetics principles...he did not discover the geodesic dome first and then derive an explanation for how it could be stable...he conceived of a stable structure from first principles and then showed it.

Wed Apr 09, 12:07:00 PM PDT  

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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.