Indeed, smashing all prior research claims on FeRAM and scalable to geometries an order of magnitude smaller than today’s 130nm FeRAM commercial offerings, the results are so promising that they are being included in the current version of the International Technology Roadmap for Semiconductors (ITRS).
A result of a sub-project called 'Cool Memory’ at Saxonys’ cluster Cool Silicon, the technology relies on newly found ferroelectric effects in doped Hafnium oxide (HfO2). Considering that Hafnium oxide is already commonly used as a high-k gate dielectric in CMOS transistors, the processes are pretty much already in place for its ferroelectric variant, readily scalable with CMOS transistors.
So why look at doped Hafnium oxide in the first place? We asked Dr. Thomas Mikolajiick, Professor for Nanoelectronic Materials and Director of the NaMLab at TU Dresden, coordinator for Cool Silicon.
“This research goes back to 2007 at DRAM maker Qimonda, when a PhD candidate Tim Böscke was doing research to improve HfO2 as a high-k dielectric for capacitors in dynamic random access memories, using dopants to stabilise the material”, explained Mikolajiick. “At certain dopant concentrations and under specific treatments, Böscke noticed that strange peaks occurred in the CV characteristic of the material, and that it behaved as a ferroelectric. This was totally unexpected!”
At that time, Qimonda’s resources were already shrinking (the company went out of business in 2009), but further investigation was performed at NaMLab, historically created as a joint venture between Qimonda and the University of Technology of Dresden (TU Dresden), do development work on FeRAM.
Back in 2009, there was still a lot of work to do, notably to make sure that the effects being observed were not just parasitics.
“We’ve spent the last four to five years characterising the material’s properties and tuning its parameters to make it applicable to FeRAM devices” told us Mikolajiick. “The ferroelectric effects in doped orthorhombic HfO2were further corroborated through computational simulation at imec, among other labs”.
“The next step was to convince GlobalFoundries to integrate FE-HfO2 in its CMOS process, and the first samples we have already outperformed all other FeRAM technologies and other non-volatile memories at a comparable node”.
Fig. 1: Comparing gate-stack structures at the 28nm node – a perovskite-type FeFET, a HfO2-based FeFET and a FinFET cell design.
So far, FeRAM manufacturers such as TI, Ramtron (recently acquired by Cypress) and Fujitsu are all using lead zirconate titanate (PZT) as the ferroelectric material in one-transistor one-capacitor memory cells. But none of them have been successful in scaling PZT beyond 130 nm, because the perovskite-type material is notoriously difficult to deposit and its FE-properties degrade at reduced thickness.