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Intel, Numonyx to describe stackable phase-change memory array

Eliminating a diode from the cell, researchers develop a memory that lives entirely in the interconnect stack.

By Ron Wilson, Executive Editor -- EDN, 10/28/2009

Intel and Numonyx have produced a test chip carrying a new type of PCM (phase-change memory) that incorporates all the components of the memory cell in between two metal layers in the interconnect stack, freeing the silicon below for other uses and raising the possibility of stacking multiple memory arrays one above the other on a single die.

If the technique proves reliable and manufacturable, it could boost PCM technology to several times the density of NAND flash technology at the same process node, and might permit very large, fast, low-latency non-volatile memories to be built directly above logic circuits on an SOC and linked directly into them by extremely wide interfaces.

The innovation occurred in the design of the memory cell. In previous designs, a PCM cell contained both a phase-change resistor element—which stores a bit of data represented by either a high- or a low-resistance phase of its glass-like material—and a selector element—generally a silicon diode. Each cell sits at the crossing of an X-direction row line and a Y-direction column line, with the phase-change element sandwiched between the two lines where they cross and the diode fabricated in the underlying silicon. In operation, the difference between voltages on the row and column lines forward-biases the diode, allowing current to flow through the resistor and then to a sense amp.

In the new design, researchers removed the silicon diode and replaced it with a selector switch formed by a dot of another Chalcogenide material closely related to the material used for the phase-change resistor. Numonyx Senior Technology Fellow Greg Atwood explained that the materials are chosen and the cell designed so that the PCM resistive element can change phase in response to a programming pulse, switching between amorphous and crystalline states, which have very different resistances. The selector-switch material, in contrast, stays in its amorphous state and functions electrically like a diode: It conducts in one direction and not the other.

The new cell, then, is a stack comprising a row line, a layer of the phase-change material, a layer of the switching material, and a column line. The area of the cell need be no larger than the cross-section area of the phase-change resistor. It's not quite that simple, Atwood explained, as there are barrier layers at each interface between materials in the stack. But the result is a cell entirely contained between two metal layers in the IC's interconnect structure and, in the test chip, barely thicker than a normal inter-layer dielectric.

Atwood did not discuss the electrical characteristics of the switch other than to say that its behavior was basically diode-like. But he and Intel Fellow Al Fazio said that the cell can scale right along with NAND flash cells, and then continue scaling when NAND finally can shrink no more. "Research has demonstrated that the PCM cell can still be stable at 5 nm," Atwood said.

The test chip, which the Intel and Numonyx researchers will describe in a paper at the International Electron Devices Meeting in December, is a full 64-Mbit memory device, with the memory array fabricated using the new cell design, and the row, column, and sense electronics implemented in the underlying silicon. The chip employs only one layer of PCM array, so the concept of a multi-layer stack of PCM arrays on top of an IC is still somewhat theoretical. But the ability to eliminate the selector diode from the cell and to add only a material closely related to the existing Chalcogenide appears to be a clear step forward for PCM architectures.