Ferroelectric random access memory (FRAM) is a form of non-volatile memory that is now used widely in applications from identity cards to portable medical devices. Despite its attractiveness for use as a low-power memory device, however, its future use in electronic devices hinges on whether the technology can be miniaturized to the nanometer-scale dimensions anticipated for the next generation of electronics. A team of researchers from Korea led by Jin Kon Kim from the Pohang University of Science and Technology (POSTECH) have now shown that the maximum data storage density of FRAM could be higher than previously thought.1

The ‘bit’ that records data in FRAM is essentially a capacitor formed by sandwiching a ferroelectric — a material that stays electrically polarized even in the absence of an electric field. This behavior allows the capacitor to remain charged, and therefore store data, even when the power source is disconnected.

Fig. 1: Lead titanate nanoislands can be assembled into high-density arrays by polymer self-assembly. The ferroelectric material is first encapsulated in polymer micelles (left), which assemble into an array and once heated leaves behind an array of nanoislands (right).

To explore the size limits of ferroelectrics, Kim’s team fabricated large arrays of ferroelectric nanoislands just a few tens of nanometers in size and tested their properties. The researchers assembled high-density arrays of the ferroelectric material lead titanate by polymer self-assembly, which involved encapsulating the ferroelectric in polymer spheres (Fig. 1) and then allowing these ‘micelles’ to settle out of solution onto a substrate. The micelles self-assembled into closely packed arrays that, after heating to burn off the polymer, gave dense arrays of ferroelectric islands. Kim’s team chose a substrate that is both conducting and has a structure similar to that of lead titanate, which helped the nanoislands to crystallize on heating.

The ferroelectric islands were about 22 nm wide and 7 nm high, dimensions smaller than scientists believed could support ferroelectric behavior. Yet Kim found clear evidence that the islands displayed good ferroelectric properties. “We think that the very high quality of the lead titanate crystal structure is the main reason that the islands showed ferroelectricity, even though the islands themsleves were so small,” says Kim. The arrays correspond to a bit density of about 0.2 terrabytes per square inch — high density by today’s standards. Kim believes that using micelles with a thinner shell structure will allow his team to reach even higher densities.