|Jo et. al. introduce a novel concept for the formation of nanopatterns on a polymer film at room temperature.|
Reviewed by Jeff Morse, PhD, National Nanomanufacturing Network
Jo A, Joo W, Jin WH, Nam H, and Kim JK. Ultrahigh-density Phase-change Data Storage Without the Use of Heating. Nature Nanotechnology. Advanced Online Publication 13 September 2009. DOI: 10.1038/nnano.2009.260
Industry roadmaps for high density data storage media are presently targeting >1 Terabit per square inch in the next few years via competitive approaches to pattern data bits using techniques such as electron beam lithography, nanoimprint lithography, or directed self assembly of block copolymer (BCP) systems. Alternative methods for data storage at the nanoscale have incorporated the use of scanning probe techniques wherein an atomic force microscope (AFM) -like tip is used to create nanopatterned data bits by indenting the surface of a polymer film. However, these techniques require the AFM tip to be heated to ~350°C. In addition, as a result of inefficient transfer of heat to the film (<1%) through the AFM tip, these techniques are very power intensive.
Recently, Jo et. al. introduced a novel concept for the formation of nanopatterns on a polymer film using an AFM tip without the need for heating. The authors investigated the polystyrene-block-poly(n-pentyl methacrylate) copolymer system (PS-b-PnPMA) for its baroplastic properties that enable a transition from phase segregated microdomains to controllably disordered structures upon the application of pressure at room temperature. While the pressure applied to the AFM tip is very small (~nN), the local force on the polymer film is large due to the small diameter of the AFM tip. The nanopattern dimensions are ultimately limited by the tip diameter, and the indentation depth by the force and length of time that pressure is applied to the tip.
In their approach, the authors first coated a substrate with a PS-b-PnPMA film exhibiting lamellar microdomains oriented parallel to the surface. Then, utilizing a sharpened silicon AFM tip, the authors created patterns of 1 nm depth and 25 nm spacing—equivalent to a density of 1.03 Tb/in^2. Importantly, the authors generated these nanopatterns at room temperature. Since the glass transtion temperature of the BCP system is ~70°C, the patterns remain stable at room temperature and can be erased by thermal cycling the substrate to 120°C for 2 seconds.
In order to confirm the nanopatterns were formed via a transition from ordered microdomain to disordered structure, the authors conducted a cross-sectional transmission electron micrograph (TEM) study of the BCP film before and after patterning. The study images show that the BCP contained phase segregated microdomains parallel to the substrate before indentation. After sufficient AFM pressure is applied to overcome the interface surface forces, a highly disordered region is formed resulting in an indentation in the BCP film. After thermal annealing of the nanopatterned film, the film transitions back from disordered regions to ordered microdomains where the AFM pressure was applied. The authors also demonstrated the ability to transform the nanopatterns to electrical signals by integrating a piezotransformer with the AFM tip.
Thus a versatile, room temperature method for generating nanopatterns in BCP films has been demonstrated with possible implications for next generation ultrahigh-density data storage applications.
Image reprinted by permission from Macmillan Publishers Ltd: Nature Nanotechnology advanced online publication 13 September 2009 (DOI: 10.1038/nnano.2009.260).