(Nanowerk Spotlight) Ferroelectric memory, a type of nonvolatile memory, offers significant advantages in terms of speed, power efficiency, and durability, making it a promising candidate for next-generation memory technology. To meet the growing demand for higher data storage density in ferroelectric memories, researchers have developed a novel approach inspired by scroll-like storage methods.
Traditional methods of increasing storage density involve reducing the size of memory cells or creating 3D stacks of cells. However, reducing memory cell size faces physical limitations, and integrating single-crystalline ferroelectric oxide films in 3D stacks remains challenging. To overcome these limitations, researchers have introduced a method that allows single-crystalline ferroelectric oxides to self-roll-up into scroll-like 3D memory structures.
Self-rolling-up enabled high-density information storage in ferroic oxide membranes. a) Optical image of the flat and pre-patterned thin films with QR codes. b) Optical image of the self-rolling-up process (the black arrow indicates the rolling-up direction). c) Optical image of the scroll in comparison with human hair. d) The front (left) and back (right) sides of the scroll. (Reprinted with permission by Wiley-VCH Verlag)
In the study, the researchers chose a prototype film made of PbZr0.3Ti0.7O3 (PZT), a type of ferroelectric oxide. This material was grown on a stressor layer made of another oxide, which had a slight lattice mismatch, meaning the atoms in the crystal structures of the two materials did not line up perfectly. The PZT/stressor structure was then detached from the substrate on which it was grown.
This release process is where the magic happened: the internal stress caused by the lattice mismatch between the two materials made the PZT/stressor film roll up on its own, like a scroll. This phenomenon, called “self-rolling-up,” transformed the flat membrane into a 3D structure.
Next, the researchers applied piezoelectric force microscopy, a method that uses the mechanical stress to switch the polarization in the ferroelectric material. They used this method to write high-density information onto the flat PZT/stressor membranes before the self-rolling-up occurred. In the rolled-up state, the membranes showed an impressive enhancement in information density — up to 45 times greater than before.
This self-rolling-up behavior is not just an experimental curiosity; it has significant theoretical underpinnings as well. The freestanding PZT/stressor membranes have a strong intrinsic tendency to roll up, driven by the mismatch in their atomic structures. This tendency can result in an area ratio enhancement of 100-450 times, which translates into an ultrahigh-density information storage capacity of 100 Tbit/In2.
Schematics of self-rolling-up of freestanding membrane with a cross-bar structure. It includes epitaxial thin film deposition, pattern bottom single-crystalline oxide electrode, epitaxial growth of ferroelectric oxide film, pattern top metallic electrode, dissolve the sacrificial layer to obtain scrolls, and write and read information. (Reprinted with permission by Wiley-VCH Verlag)
In conclusion, the researchers developed a new method for information storage that capitalizes on the self-rolling-up properties of ferroic oxide films. This method involves writing information onto a nanofilm and then allowing the film to roll up and store the data in a 3D scroll form.
This approach has been demonstrated to increase storage density by approximately 45.7 times, compared to traditional planar structures. Theoretically, the self-rolling-up method can achieve an ultrahigh-density information storage of 100 Tbit/In2.
In addition to its impressive storage density, this method also has several other advantages. It employs a simple preparation process, allows for strong heterogeneous integration, and is compatible with mass production techniques.
The authors conclude that “this method can be applied to any bilayer ferroic oxide film structure with internal stress, which provides a new path for the development of high-density information storage.”
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