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Qimonda’s late legacy: 28nm FeRAM By Julien happich CMOS-compatible 28nm FeRAM could become commercially available within three to five years, according to research from a collaborative project between NaMLab at TU Dresden, the Fraunhofer Institute for Photonic Micro Systems (IPMS) and GlobalFoundries. 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, 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 stabilize 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 Comparing gate-stack structures at the 28nm node – a perovskite-type FeFET, a HfO2-based FeFET and a FinFET cell design. 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), to 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 characterizing the material’s properties and tuning its parameters to make it applicable to FeRAM devices” told us Mikolajiick. “The ferroelectric effects in doped orthorhombic HfO2 were 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”. 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 130nm, because the perovskite-type material is notoriously difficult to deposit and its FE-properties degrade at reduced thickness. In contrast, the researchers have shown that FE-HfO2 exhibits stable ferroelectric properties at film thicknesses in the nanometer range (5 to 30nm), which could make ferroelectric field-effect transistors (FeFET) a suitable alternative for non-volatile memory (1T FRAM) in highly integrated 2D or even 3D CMOS designs. Mikolajiick expects the technology to displace NOR-Flash in embedded memory applications, highlighting that the integration of such FeFETs RAMs is much simpler, requiring only 3 extra steps versus 7 to 10 extra layers for floating-gate based NOR-flash devices. And again, hafnium oxide is readily available as high-k material in today’s CMOS processes, so it is only a matter of adding another gate oxide layer, albeit a ferroelectric one. In prior research, the NaMLab was also able to demonstrate significantly faster operation speed with program and erase times in the nanosecond range and lower voltage operation. “Typical technologies currently used for non-volatile memory are based on the principle of chargestorage,” Mikolajick says. “This A micrograph of the Fe-HfO2 structure. has several disadvantages. Writing, for instance, requires high voltage and is very energy intensive. Due to the high voltage, certain circuit parts for controlling memory cannot be reduced to desired sizes which renders such memory inefficient for small www.electronics-eetimes.com Electronic Engineering Times Europe January 2015 27


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