New Phase-Change Memory (PCRAM) That Could Serve As In-Between Technology Between Slower Nonvolatile Memory and DRAM Discovered

A new device, a type of phase change memory (PCRAM), has been discovered. The discovery could be useful in future compute-in-memory schemes.

New Nanocomposite PCRAM Discovered

In a new study, researchers in Taiwan and the United States claim to have proven a novel and promising approach to nonvolatile memory that could improve the performance of the next CPUs since it is tiny enough, energy-efficient, and operates at a low enough voltage.

This device belongs to a class of memory known as phase change memory, which can change resistance- a form of information- by melting and reconstructing its crystal structure. The crystal in question, known as a nanocomposite superlattice, improves the power required to write a bit by an order of magnitude.

This type of phase-change memory (PCRAM), according to the experts, will be especially helpful in upcoming compute-in-memory techniques that reduce energy consumption in machine learning by moving less data between memory and processor.

Although PCRAM has already reached the market, it only represents a small portion. It is a transitional technology since it is faster and nonvolatile than flash memory. However, it is slower than DRAM, a computer's volatile primary memory. A single phase-change device, however, can store more data than a single device of any other kind.

One of the issues impeding PCRAM's progress is the excessive current required to switch between states. However, compromises have been made to address this, including drifting resistance values.

The team's affiliate at Stanford University stabilized resistance and lowered current in previous studies. Their solution consisted of repeating nanometer-scale layers of two distinct crystal materials named superlattices. Less recent is required to heat and change the phase of such a structure because atomic-scale gaps between the layers hinder heat transfer.

What is GST467?

GST467 contains tellurium, antimony, and germanium in a 4:6:7 ratio. The University of Maryland researchers found GST467; they then worked with Stanford and TSMC researchers to employ it in superlattice PCRAM. The novel material's nanoscale crystal facets qualify it as a nanocomposite.

"These can act as a crystallization template," said Xiangjin Wu, a doctoral researcher in the laboratory of Eric Pop at Stanford.

When a new bit is written, such templates facilitate the device's ability to restore its crystal structure. A 40-nanometer device that operates at 0.7 volts and switches in roughly 40 nanoseconds while utilizing less than 1.5 picojoules with layers of GST467 and antimony telluride alternating in a superlattice has been achieved by Stanford post-doctoral researcher Asir Intisar Khan, Wu, and their team. It could store data in eight separate resistance states for multi-bit storage per device or use in analog machine-learning circuits. It also exhibited low resistance drift and withstood around 200 million switching cycles.

"With switching that low, logic and memory integration are possible," Khan said.

Rather than employing larger devices intended for I/O, as they currently are, the memory cells can be operated using regular logic transistors. The researchers plan to investigate the potential system-level benefits of incorporating the new PCRAM onto logic chips and enhance the device's endurance at higher temperatures.

Specifically, it might be helpful in experimental 3D chips that are manufactured from the bottom up instead of properly connected stacks of silicon integrated circuits (SICs), as some modern advanced CPUs and GPUs do. Since high temperatures during the device's construction would damage layers beneath it, the new PCRAM might be a good fit for integration on top of silicon or other layers.

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