Patterned Magnetic Media Made by Self-Assembled Block-Copolymer Lithography
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Introduction Patterned media, in which each bit of data is stored according to the magnetization direction of a discrete magnetic nanostructure, are believed to offer the possibility of higher data densities than conventional thin-film hard-disk media.1,2 In a thin-film medium, each bit is stored in a region of the film containing on the order of 100 grains, each of which is ~8 nm across. Increases in data density require the grain size to be reduced in order to maintain a high enough number of grains per bit to ensure a good signal-to-noise ratio. However, reductions in grain volume make the medium vulnerable to superparamagnetism, in which thermal excitations cause spontaneous reversal of the magnetization direction of the grains, erasing the written data. The superparamagnetic effect becomes significant when KuV ≈ 60kBT, where Ku is the magnetic anisotropy of the material, V is the grain volume, kB is the Boltzmann constant, and T is the temperature.3 One way to offset the effects of superparamagnetism until
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higher data densities is to store each bit of data in one exchange-coupled magnetic nanostructure, rather than in a collection of exchange-decoupled grains; superparamagnetic effects still exist, but now the relevant V is the volume of the magnetic nanostructure, instead of the grain volume of the thin film. This leads to the concept of replacing the thin-film medium with an array of discrete magnetic nanostructures, as illustrated in Figure 1. Although attractive in terms of data density, the use of patterned media presents several difficulties in implementation. For data densities on the order of 1 terabit (Tb)/in.2, higher than that believed achievable with conventional media, the patterned structure requires a periodicity of ~25 nm and a feature size of about onehalf the period, formed at low cost over the disk surface. This presents a major challenge in fabrication technology. Second, the read/write head must be able to address a track pitch of ~25 nm, requiring the fabrication of ultranarrow read and
write elements and major improvements in servo technology. Finally, the write process must synchronize with the physical location of the bits on the disk, unlike the case of conventional media, where the magnetic film is continuous. Despite these obstacles, a considerable amount of progress has been made on the fabrication and testing of patterned media. In this article, we focus on a fabrication method based on the self-assembly of a block copolymer (BCP) to obtain large-area, small-period nanostructures in an economical process.
BCP Lithography Conventional optical lithography involves exposure of a resist layer to photons, followed by a development step. The feature size is therefore limited by diffraction, making the generation of sub-50-nm features difficult. To create smaller features, electron-beam lithography is commonly used, but the serial nature of the process makes it expensive for patterning large areas. The self-assembling nature of BCPs makes them ideal for the generation of periodic
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