The researchers have shown that they can build stackable memory based on “ionic memory technology,” which could make them ideal candidates for storage cells in high-density memory. Best of all, the new method uses well-known electronics materials.
“This opens the door to inexpensive, high-density data storage by ‘stacking’ memory layers on top one another inside a single chip,” Kozicki said. “This could lead to hard drive data storage capacity on a chip, which enables portable systems that are smaller, more rugged and able to go longer between battery charges.”
Kozicki said stacking memory cells has not been achieved before because the cells could not be isolated. Each memory cell has a storage element and an access device; the latter allowing you to read, write or erase each storage cell individually.
“Before, if you joined several memory cells together you wouldn’t be able to access one without accessing all of the others because they were all wired together,” Kozicki said. “What we did was put in an access, or isolation device, that electrically splits all of them into individual cells.”
Up until now, people built these access elements into the silicon substrate.
“But if you do that for one layer of memory and then you build another layer, where will you put the access device,” Kozicki asked. “You already used up the silicon on the first layer and it’s a single crystal, it is very difficult to have multiple layers of single crystal material.”
The new approach does use silicon, but not single crystal silicon, which can be deposited in layers as part of the three-dimensional memory fabrication process. Kozicki said his team was wrestling with how to find a way to build an electrical element, called a diode, into the memory cell. The diode would isolate the cells.
Kozicki said this idea usually involves several additional layers and processing steps when making the circuit, but his team found an elegant way of achieving diode capability by substituting one known material for another, in this case replacing a layer of metal with doped silicon.
“We can actually use a number of different types of silicon that can be layered,” he said. “We get away from using the substrate altogether for controlling the memory cells and put these access devices in the layers of memory above the silicon substrate.”
“Rather than having one transistor in the substrate controlling each memory cell, we have a memory cell with a built-in diode (access device) and since it is built into the cell, it will allow us to put in as many layers as we can squeeze in there,” Kozicki said. “We’ve shown that by replacing the bottom electrode with silicon it is feasible to go any number of layers above it.
With each layer applied, memory capacity significantly expands.
“It turns out to be a ridiculously simple idea, but it works,” Kozicki said of his stackable memory advance. “It works better than the complicated ideas work.”
“The key was the diodes, and making a diode that was simple and essentially integrated in with the memory cell. Once you do that, the rest is pretty straightforward.”