The microstructure in fluorite-structure oxide-based ferroelectric thin films, especially when on standard semiconductor manufacturing platforms, is poly-/nano-crystalline, which controls the functionality, performance, and reliability of the device technologies based on them. Understanding the relationships between microstructure, process, and performance for this class of materials has remained challenging. Here, a systematic approach is presented for analyzing and visualizing grains, their size distributions, and interlayer templating effects in ferroelectric thin film systems by utilizing an advanced microscopy technique, namely nanobeam electron diffraction, coupled with dark-field transmission electron microscopy and atomic resolution scanning transmission electron microscopy. A 10 nm TiN/10 nm Hf0.5Zr0.5O2 (HZO)/10 nm TiN ferroelectric heterostructure is probed. A geometric mean of the grain size in HZO of 26.8 nm ranging from 5 to 95 nm with top and bottom TiN layers having a much smaller grain size of approximately 6.8 nm ranging from 3 to 17 nm is observed. Furthermore, there is evidence of templating effects between HZO and TiN grain and domain boundaries showing [111] and [001] growth directions locally for HZO and TiN, respectively.
Discovery of ferroelectricity in HfO2 has
sparked a
lot of interest in its use in memory and logic due to its CMOS compatibility
and scalability. Devices that use ferroelectric HfO2 are
being investigated; for example, the ferroelectric field-effect transistor
(FEFET) is one of the leading candidates for next generation memory
technology, due to its area, energy efficiency and fast operation.
In an FEFET, a ferroelectric layer is deposited on Si, with an SiO2 layer of ∼1 nm thickness inevitably forming at the
interface. This interfacial layer (IL) increases the gate voltage
required to switch the polarization and write into the memory device,
thereby increasing the energy required to operate FEFETs, and makes
the technology incompatible with logic circuits. In this work, it
is shown that a Pt/Ti/thin TiN gate electrode in a ferroelectric Hf0.5Zr0.5O2 based metal-oxide-semiconductor
(MOS) structure can remotely scavenge oxygen from the IL, thinning
it down to ∼0.5 nm. This IL reduction significantly reduces
the ferroelectric polarization switching voltage with a ∼2×
concomitant increase in the remnant polarization and a ∼3×
increase in the abruptness of polarization switching consistent with
density functional theory (DFT) calculations modeling the role of
the IL layer in the gate stack electrostatics. The large increase
in remnant polarization and abruptness of polarization switching are
consistent with the oxygen diffusion in the scavenging process reducing
oxygen vacancies in the HZO layer, thereby depinning the polarization
of some of the HZO grains.
Nanoscale polycrystalline thin-film heterostructures
are central
to microelectronics, for example, metals used as interconnects and
high-K oxides used in dynamic random-access memories (DRAMs). The
polycrystalline microstructure and overall functional response therein
are often dominated by the underlying substrate or layer, which, however,
is poorly understood due to the difficulty of characterizing microstructural
correlations at a statistically meaningful scale. Here, an automated,
high-throughput method, based on the nanobeam electron diffraction
technique, is introduced to investigate orientational relations and
correlations between crystallinity of materials in polycrystalline
heterostructures over a length scale of microns, containing several
hundred individual grains. This technique is employed to perform an
atomic-scale investigation of the prevalent near-coincident site epitaxy
in nanocrystalline ZrO2 heterostructures, the workhorse
system in DRAM technology. The power of this analysis is demonstrated
by answering a puzzling question: why does polycrystalline ZrO2 transform dramatically from being antiferroelectric on polycrystalline
TiN/Si to ferroelectric on amorphous SiO2/Si?
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