Non-volatile memories based on scanning probes offer very high data densities, but existing approaches require the probe to be heated, which increases the energy expenditure and complexity of fabrication. Here, we demonstrate the writing, reading and erasure of an ultrahigh-density array of nanoscopic indentations without heating either the scanning probe tip or the substrate. An atomic force microscope tip causes microphase transitions of the polystyrene-block-poly(n-pentyl methacrylate) of a block copolymer to occur at room temperature by application of pressure alone. We demonstrate a data storage density of 1 Tb in^-2, which is limited only by the size of the tip. This demonstration of a pressure-based phase-change memory at room temperature may expedite the development of next-generation ultrahigh-density data storage media.
The Korean team has shown that the tip of an atomic force microscope (AFM) can etch the kind of tiny pits that store data in millipede-like systems simply by pressing on the new material. Lighter pressure can be used to feel for and read out the pits without altering them. It solves one issue but raises another. “The forces needed are relatively high, and this is likely to lead to tip wear issues,” he says. Even systems that use heat suffer such problems, and they would be worse if more force was being used, he says.
“The key development on the polymer side is new bilayer materials,” he says. These combine a hard polymer skin just a few nanometres thick with a softer layer – for example polystyrene – beneath. “It combines the softness you want [to avoid damaging the probe tip and for fast writing speeds] with the thermal stability necessary for long data lifetimes.”
Nanopattern formation on various polymer films by the indentation of AFM tip
We used other polymer films for the fabrication of nanopatterns by the indentation of AFM tip at room temperatures. Here, we employed three different homopolymers: PS, PnPMA, and PMMA. All nanopatterns were prepared by using the same AFM tip (AR5-NCHR) by varying the indentation force from 200 to 1400 nN at room temperature.
The shape of the generated nanopatterns and the depth were maintained for 5 months at room temperature.
The depth of each written nanopattern is almost the same (~ 7.5 nm). Once this patterned film was placed for 2 s onto a heating plate maintained at 120 oC, the nanopatterns were completely erased and a flat film was observed. We did not see any evidence of polymer degradation during repeated writing and erasing. Although we did not carry out the experiments for very long cycles (say more than 104), we consider that the cyclability would be still good even after long write-erase cycles, because no degradation of polymer films is observed during repeated writing and erasing.
The recording time for the fabrication of a pattern with a 5 nm depth is calculated to 5 ms at a down/up speed of 2μm/s, and it would decrease as the down/up speed increases.