Tag Archives: RRAM

Replacement for Flash Memory

Today flash memories or thumb drives are commonly used as devices that store information even without power—nonvolatile memory. However, physicists and researchers are of the opinion that flash memory is nearing the end of its size and performance limits. Therefore, the computer industry is in search of a replacement for flash memory. For instance, the National Institute of Technology (NIST) conducted research is suggesting resistive random access memory (RRAM) as a worthy successor for the next generation of nonvolatile computer memory.

RRAM has several advantages over flash. Potentially faster and less energy hungry than flash, it is also able to pack in far more information within a given space. This is because its switches are tiny enough to store a terabyte within a space the size of a postage stamp. So far, technical hurdles have been preventing RRAM from being broadly commercialized.

One such hurdle physicists and researchers are facing is the RRAM variability. To be a practical memory, a switch needs to have two distinct states—representing a digital one or zero, and a predictable way of flipping from one state to the other. Conventional memory switches behave reliably when they receive an electrical pulse and switch states predictably. However, RRAM switches are still not so reliable, and their behavior is unpredictable.

Inside a RRAM switch, an electrical pulse flips it on or off by moving oxygen atoms around, thereby creating or breaking a conductive path through an insulating oxide. When the pulses are short and energetic, they are more effective in moving ions by the right amount for creating distinct on/off states. This potentially minimizes the longstanding problem of overlapping states largely keeping the RRAM in the R&D stage.

According to a guest researcher at NIST, David Nminibapiel, RRAMs are as yet highly unpredictable. The amount of energy required to flip a switch may not be adequate to do the same the next time around. Applying too much energy may cause it to overshoot, and may worsen the variability problem. In addition, even with a successful flip, the two states could overlap, and that makes it unclear whether the switch is actually storing a zero or a one.

Although this randomness takes away from the advantages of the technology, the researcher team at NIST has discovered a potential solution. They have found the energy delivered to the switch may be controlled with several short pulses rather than using one long pulse.

Typically, conventional memory chips work with relatively strong pulses lasting about a nanosecond. However, the NIST team found less energetic pulses of about 100 picoseconds, which were only a tenth of the conventional pulses, worked better with RRAM.  Sending a few of these gentler signals, the team noticed, was more useful not only for flipping the RRAM switches predictably, but also for exploring the behavior of the switches.

That led the team to conclude these shorter signals reduce the variability. Although the issue does not go away totally, but tapping the switch several times with the lighter pulses makes the switch flip gradually, while allowing checking to verify whether the switch did flip successfully.

The Energy Efficient RRAMs

Engineers at Stanford are making 3-D memory chips that can offer faster and more energy efficient solutions for computer memory. These are the Resistive random Access Memory or RRAMs, which are based on a new semiconductor material. It stores data based on temperature and voltage. However, the actual workings of RRAMs continued to be a mystery until a team at Stanford used a new tool for their investigations. They found the optimal temperature range to be lower than they had expected. This could lead to memory that is more efficient.

Conventional computer chips operate on a two dimensional plane. Typically, the CPU and memory communicate with each other through the data bus. While both the CPU and memory components have advanced technically, the data bus has lagged, leading to a slowdown of the entire system when crunching large amounts of data.

The special semiconductor RRAMs can be stacked one on top of the other, creating a 3-D structure. This brings the memory and its logic components closer together. As conventional silicon devices cannot replicate this, the 3-D high-rise chips can work at much higher speeds and be more energy efficient. Not only is this a better solution for tacking the challenges of Big Data, it can also extend the battery life of mobile devices.

The RRAMs work more like a switch. As explained by the Stanford engineers, in their natural state, the RRAM materials behave just as insulators do—resist the flow of electrons. However, when zapped with an electric field, a filament-like path opens up in the material, and electrons can flow through it. A second jolt closes the filament, and the material returns to being the insulator it was. Alternating between the two states generates a binary code with no signal transfer representing a zero and the passage of electrons representing a one.

The temperature rise of the material when subjected to the electric field causes the filament to form, allowing electrons to pass through. So far, the engineers were unable to estimate the exact temperature of the material that caused the switch. They needed much more precise information about the fundamental behavior of the RRAM material before they could hope to produce reliable devices.

As the engineers had no way of measuring the heat produced by a jolt of electricity, they heated the RRAM chips using a hot plate, while not applying any voltage. They then monitored the flow of electrons as filaments began to form. This allowed the team to measure the exact temperature band necessary for the materials to form the filaments. The engineers found the filaments formed between 26.7 and 126.7°C. Therefore, future RRAM devices will require less electricity for generating these temperatures, and that would make them more energy efficient.

Although at this moment, RRAMs are not yet ready to be incorporated into consumer devices, the researchers are confident that the discovery of the temperature range will speed up development work.

According to Ziwen Wang, a member of the team, the voltage and temperature discovered can be the predictive design inputs for enabling the design of a better memory device. The researchers will be presenting their find at the IEEE International Electron Devices Meeting in San Francisco.