Perpendicular patterned media for high density magnetic storage

Citation

Wong, Joyce Y. (2000) Perpendicular patterned media for high density magnetic storage. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/WEP6-MN90. https://resolver.caltech.edu/CaltechETD:etd-05032007-080831

Abstract

Current longitudinal thin-film media in magnetic hard-disk drives are facing an oncoming limit caused by the superparamagnetic effect, in which the individual grains in the medium become so small that they are no longer stable against thermal fluctuation. This situation is undesirable as the stored information may be lost within an unexpectedly short time frame. There have been several proposed solutions in addressing the superparamagnetic limit, and one of them is perpendicular patterned media. In this approach, a periodic array of magnetic pillars is defined lithographically on a non-magnetic substrate. Binary data of "1" or "0" can be stored in each of these elements, which have two possible magnetization directions perpendicular to the plane of the medium.

In our perpendicular patterned media design, Ni columns of 150-230nm diameter with a 6:1 aspect ratio are embedded in an (AlGa)2O3/GaAs substrate. The fabrication procedure uses a combination of high resolution electron beam lithography, dry etching, and electroplating. The high aspect ratio in the column is achieved by taking advantage of the high etching rate and selectivity of AlGaAs/GaAs over (AlGa)203 in the Cl2 chemically assisted ion beam etching process. In addition to being a robust etching mask, the (AlGa)2O3 layer also plays an important role in the chemical mechanical polishing procedure to remove the overplated Ni mushrooms.

Once the Ni columns are fabricated, magnetic characterization is performed using magnetic force microscopy and scanning magnetoresistance microscopy. The former measurement confirms that the electroplated Ni columns are magnetic while the latter determines whether the individual columns are stable enough to retain the recorded information. We have successfully demonstrated recording in our 170nm diameter Ni column array arranged in a square format using a commercial read/write head. This is the first demonstration of single magnetic column per bit data storage in a prototype perpendicular patterned medium. Furthermore, we have recorded in higher density Ni column arrays in the form of tracks, corresponding to 1.3 and 2.6Gbits/in.2. Even though we are limited by the spatial resolution of the magnetoresistive read sensor, we have continued to pursue higher density structures up to an areal density of 350Gbits/in.2. Consideration of the issues in high resolution patterning and the magnetic stability of the individual columns have prompted researchers in the magnetic recording industry to anticipate the ultimate storage limit of perpendicular patterned media to be around 1Tbits/in.2

Thesis Files