Researchers from A*STAR’s Institute of Materials Research and Engineering (IMRE) and the National University of Singapore (NUS) have discovered that an ultra-smooth surface is the key factor for “self-assembly” – a cheap, high-volume, high-density patterning technique.
This allows manufacturers to use the method on a variety of different surfaces. This discovery paves the way for the development of next generation data storage devices, with capacities of up to 10 Terabits/in^2 which could lead to significantly greater storage on much smaller data devices.
Common hard disk drives today have a density of about 600 Gigabits/in2. The current industry goal is to increase this by more than 15 times to 10 Terabits/in2. The lucrative disk storage systems market posted $8.1 billion in sales for Q2, 2012. The disk storage systems market posted US$8.1 billion in sales for Q2, 2012 (according to the International Data Corporation’s press release in September 2012).
SEM and AFM images of the magnetic media with and without the TranSpin layer.
The “self-assembly” technique is one of the simplest and cheapest high-volume methods for creating uniform, densely-packed nanostructures that could potentially help store data. Self-assembly is one of the leading candidates for large scale nanofabrication at very high pattern densities. One of its most obvious applications will be in the field of bit patterned media, or the hard disk industry. It is widely used in research and is gaining acceptance in industry as a practical lithographic tool for sub-100 nm, low-cost, large area patterning. However, attempts to employ self-assembly on different surface types, such as magnetic media used for data storage, have shown varying and erratic results to date. This phenomenon has continued to puzzle industry researchers and scientists globally.
2. Researchers from A*STAR’s IMRE and NUS have now solved this mystery and identified that the smoother the surface, the more efficient the self-assembly of nanostructures will be. This breakthrough allows the method to be used on more surfaces and reduce the number of defects in an industrial setting. The more densely packed the structures are in a given area, the higher the amount of data that can be stored.
3. “A height close to 10 atoms, or 10 angstroms in technical terms, is all it takes to make or break self-assembly,” explained Dr MSM Saifullah, one of the key researchers from A*STAR’s IMRE who made the discovery. This is based on a root mean squared surface roughness of 5 angstrom. The team discovered that this was the limit of surface roughness allowed for the successful self-assembly of dots, which could eventually be used in making high-density data storage. “If we want large scale, large area nanopatterning using very affordable self-assembly, the surface needs to be extremely smooth so that we can achieve efficient, successful self-assembly and with lower incidences of defects.”
Nanofabrication by conventional techniques such as optical, electron and nanoimprint lithographies show good reliability and reproducibility. The former two, due to the availability of good depth of focus and ability to pattern thick resists, are quite immune to the presence of surface roughness. On the other hand, nanoimprint lithography, especially using soft molds, can be used to imprint rough and uneven surfaces. Self-assembly, unlike conventional lithography methods, enables the creation of nanopatterns through physical movement of the polymer chains in a block copolymer. Our work has clearly shown that such a movement is sensitive to angstrom-scale surface roughness and its increase may result in lowering of the reliability and reproducibility of self-assembly as a nanofabrication technique. Furthermore, a systematic investigation of the effect of surface roughness on self-assembly may be essential before it is employed as a reliable high density nanofabrication method. For example, in the case of BPM, ~20 nm features as obtained by self-assembly of PS-b-PDMS block copolymer will provide an areal density of ~400 Gb per square inch on a surface with Rrms less than 5 Å. To achieve areal densities of 1 Tb per square inch and beyond in BPM, the bit size has to shrink to less than 12 nm. If self-assembly of block copolymers is employed to achieve such high areal densities and small bit sizes, we speculate that tighter control over the surface roughness of magnetic media will become necessary.
Schematic representation of different layers of (a) continuous CoCrPt-SiO2, (b) granular CoCrPt-SiO2, (c) granular FePt-C-Cu and (d) granular FePt-C magnetic media.
ABSTRACT – Self-assembly of block copolymers has been identified as a potential candidate for high density fabrication of nanostructures. However, the factors affecting its reliability and reproducibility as a patterning technique on various kinds of surfaces are not well-established. Studies pertaining to block copolymer self-assembly have been confined to ultra-flat substrates without taking into consideration the effect of surface roughness. Here, we show that a slight change in the angstrom-scale roughness arising from the surface of a material creates a profound effect on the self-assembly of polystyrene-polydimethylsiloxane block copolymer. Its self-assembly was found to be dependent on both the root mean square roughness (Rrms) of the surface and the type of solvent annealing system used. It was observed that surface with Rrms less than 5.0 Å showed self-assembly. Above this value, the kinetic hindrance posed by the surface roughness on the block copolymer leads to its conforming to the surface without observable phase separation.
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