Patterned media

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Patterned media is a potential future hard disk drive technology to record data in magnetic islands (one bit per island), as opposed to current hard disk drive technology where each bit is stored in 20-30 magnetic grains within a continuous magnetic film. The islands would be patterned from a precursor magnetic film using nanolithography. It is one of the proposed technologies to succeed perpendicular recording due to the greater storage densities it would enable.

Comparison with existing HDD technology[edit]

In existing hard disk drives, data are stored in a thin magnetic film. This film is deposited so that it consists of isolated (weakly exchange coupled) grains of material of around 8 nm diameter.[1] One bit of data consists of around 20-30 grains that are magnetized in the same direction (either 'up' or 'down' with respect to the plane of the disc). One method of increasing storage density has been to reduce the average grain volume. However the energy barrier for thermal switching is proportional to the grain volume. With existing materials, further reductions in the grain volume would result in data loss occurring spontaneously due to superparamagnetism.

In patterned media, the thin magnetic film is first deposited so that there is strong exchange coupling between the grains. Using nanolithography it is then patterned into magnetic islands. The strong exchange coupling means that the energy barrier is now proportional to the island volume, rather than the volume of individual grains within the island. Therefore storage density increases can be achieved by patterning islands of increasingly small diameter, whilst maintaining thermal stability.[2] Patterned media is predicted to enable areal densities up to 20-300 Tb/in2 as opposed to the 1 Tb/in2 limit that exists with current HDD technology.[3]

Methods of patterned media fabrication[edit]

In research, one of the processes in creating prototypes is by ion beam proximity lithography. Ion beam proximity lithography uses stencil masks to produce patterns in ion-sensitive material (resist) which is transferred to magnetic material.[4] The stencil mask contains a thin free standing silicon nitride membrane in which openings are formed. The pattern to be generated is first formed on a substrate that contains a photo-resist using electron beam lithography. Next the substrate is used to transfer the given pattern onto the nitride membrane (stencil mask) using the process of plasma etching. To create sufficient substrates is to maintain size uniformity of the openings which is transferred to the mask during the fabrication process (etching). Many factors contribute to the achievement and maintenance of size uniformity in the mask, such as: pressure, temperature, energy (amount of voltage), and power used when etching. To optimize the process of etching uniform patterns correctly under these parameters, the substrate can be used as a template to fabricate stencil masks of silicone nitride through the process of ion proximity beam lithography. The stencil mask can then be used as a prototype to create pattern media.

References[edit]

  1. ^ Weller et al., L10 FePtX–Y media for heat-assisted magnetic recording, (2013), 210, 7, 1245-1260, Physica Status Solidi A
  2. ^ Ross, C.A.: "Patterned magnetic recording media"p 203-235. Annual Review of Materials Science, 2001: 31.
  3. ^ Griffiths et al., Directed self-assembly of block copolymers for use in bit patterned media fabrication, (2013), 46, 503001, Journal of Physics D: Applied Physics
  4. ^ Wolfe et al.: A proximity ion beam lithography process for high density nanostructures.,(1996):.14, 3896-3899. Journal of Vacuum Science and Technology B