Intel and Numonyx have taken the first step to building a stackable memory based on phase-change materials.
The team from the two companies has, so far, implemented only a single-layer 64Mb test chip but claimed the structure they have developed could lead to devices with many stacked layers, potentially satisfying demand for non-volatile memories with much higher densities than are available today. Greg Atwood, senior fellow at Numonyx, said the phase-change technology could potentially scale down to 5nm, whereas flash memory tends to run into problems at around 20nm.
A move into the third dimension could provide much higher capacities simply by layering slices of memory array on top of each other. “We can stack as high as we choose,” said Atwood. “But each layer will need more processing and the more processing you do the more risk you have of defects. So there will be a practical limit to the number of layers you can stack.”
Al Fazio, Intel fellow and technology and manufacturing group director of memory technology development, said the main requirement for a stackable memory is a crosspoint structure with a selection switch to ensure only the bit where two read lines cross is read or written. This can be achieved using silicon transistors or diodes but “it’s hard to stack a silicon diode”.
The team decided to use a material from the same family used in the phase-change memory element itself to act as a switch. When a voltage and a certain amount of current are applied, the material temporarily changes state and allows current through into the memory element itself. The resistance of the memory element determines whether the bit read is a one or zero.
The team has based its switch on a concept devised by the licensor of the original technology, Energy Conversion Devices, called the Ovonic threshold switch (OTS); the term is derived from the name of inventor Stanford Ovshinsky. The threshold switch uses a different mechanism to pass current to the memory devices, and is based on a different mix of the elements used in phase-change memories, allowing it to act as a temporary switch rather than a memory. However, a spokeswoman for Intel said the formulation of the material in the OTS for the stackable memory is Intel and Numonyx intellectual property.
For the technology to move to commercialisation, Fazio said more work needs to be done to explore how the layers will stack. “We see this work as a milestone towards realising the low-cost potential of this memory,” he said.
The structure will be described in a paper to be presented at the IEDM conference in Baltimore in December.