Committed to memory - Semiconductors

Electronic News, Feb 11, 2002 by Harry Yeates

"Memories are made of this," sang Dean Martin, Rat Pack lynchpin, in 1955. Made of what exactly, Dean? I'm sorry, did you say germanium chalcogenide?

Deano's unlikely prediction could soon prove to be dead on, if work by Arizona company Axon Technologies comes to fruition. With many claiming DRAM will struggle to meet future demands, several new memory technologies are occupying research budgets around the world. These include polymer ferroelectric memory, magnetic RAM (MRAM), FeRAM and the ungainly named Ovonic unified memory (OUM). Axon's entry to this crowded arena exploits expertise in the relatively new field of solid state ionics. In its programmable metallisation cell memory (PMCm), films tens of nanometres thick of a solid, silver atom-laden chalcogenide electrolyte are placed between two electrodes.

Applying a voltage across the electrodes in one direction causes a region of silver metal to accumulate between them, creating a conductive link. Reversing the bias breaks down the silver and restores the initial, resistive 'off' state.

"You can potentially get huge densities with PMCm as it can be physically scaled to below 10nm, we believe," said Professor Michael Kozicki, Axon's chief technical officer and co-founder. "It operates at very low voltage and doesn't require high current, so conductor spacing and size can be minimized and heat generation is low."

Last week Axon licensed PMCm to U.S. DRAM specialist Micron Technology and is in active talks with six more companies -- including one in Europe. Kozicki said costs should be comparable with DRAM, thanks to a combination of relatively low material costs, simple device structures and the use of existing deposition and patterning techniques.

Micron has been working on PMCm under an exclusive development license since March 2000. It cooperated with Axon, which was formed in 1996, through links at Arizona State University, where Kozicki heads the Center for Solid State Electronics Research.

"The main aims of the project were to demonstrate the technology was much more than a laboratory curiosity," he said, "and that devices based on it could be made in a production environment."

The PMC electrolyte is a chalcogenide glass. Chalcogenides are a group of compounds containing the Group VI elements sulphur, selenium and tellurium that exhibit a number of interesting characteristics. "They can undergo significant structural and electrical changes when optical, thermal, or electrical energy is applied," Kozicki explained.

Harry Yeates is an editor with Electronics Weekly, a sister publication of Electronic News.