Super-Dense Computer Memory
A novel type of computer memory could, in theory, let you store tens or even hundreds of times as much data on your smartphone. Researchers at Rice University have demonstrated a more practical way to manufacture it.
The type of memory in question, resistive random access memory (RRAM), is being developed by several companies, but fabrication usually requires high-temperatures or voltages, making production difficult and expensive. The Rice researchers have shown a way to make RRAM at room temperature and with far lower voltages.
Like flash memory, RRAM can store data without a constant supply of power. Whereas flash memory stores bits of information in the form of charge in transistors, RRAM stores bits using resistance. Each bit requires less space, increasing the amount of information that can be stored in a given area.
What’s more, it should be easier to stack up layers of RRAM, helping to further increase the amount of information that can be packed onto a single chip. RRAM can also operate a hundred times faster than flash. Some prototypes can store data densely enough to enable a terabyte chip the size of a postage stamp.
“Why don’t you have all the movies you would like on your iPhone? It’s not because you wouldn’t like to, it’s because you don’t have room,” says James Tour, a professor of materials science at Rice University who led the work.
Several companies are making progress towards commercializing RRAM. A startup called Crossbar plans to release its first product, for embedded chips—the type found in car dashboards and coffee makers—by the end of the year (see “Denser, Faster Memory Challenges Both DRAM and Flash”). Tour says he expects to conclude a licensing deal with an unnamed memory manufacturer within two weeks.
Tour’s process starts with a layer of silicon dioxide riddled with tiny holes—each five nanometers wide. This porous layer is sandwiched between two very thin layers of metal, which serve as electrodes. A voltage is applied, causing the metal to migrate into the holes, forming an electrical connection between the electrodes.
Finally, the researchers apply another voltage, causing a tiny break to form in the metal inside the pores, and silicon to form in this gap.
Bits can be stored by changing the conductivity of that silicon with a low-voltage pulse. The RRAM retains its set state until another pulse is used to rewrite the bit.
The new design requires lower voltages than previous designs. This prevents damage during manufacturing, and it means the memory can theoretically be switched hundreds of thousands of times, 100 times more than previous versions. The memory can also be made at room temperature, so it should be easier to integrate the memory storage with other electronics on a chip.
The new RRAM should also be more amenable to stacking. Some manufacturers are just starting to introduce flash memory with multiple layers. Samsung, for instance, is making a version that could eventually have as many as 24 layers. But the individual memory units on a flash chip require three connections, which makes forming multiple layers of memory difficult and expensive. The new RRAM design only requires two connections. In theory, the Rice researchers say, you could make hundreds of layers, each one so thin that the memory chip could still easily fit inside portable electronic products.
The new work is a “major step forward,” says Wei Lu, a professor of electrical engineering and computer science at the University of Michigan, and co-founder of Crossbar. But he notes there are several options for next-generation memory chips, and that getting advances to market is challenging. “While you can get many materials to switch,” Lu says, “making a product is a completely different story.”
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