Fabrication of High Anisotropy Nanoscale Patterned Magnetic Recording Medium for Data Storage Applications

Abstract

Conventional magnetic recording systems based on continuous medium recording are rapidly approaching their superparamagnetic limit. A shift to patterned media, where the data are stored in arrays of discrete nanomagnets, will help extend the areal bit densities due to a significant increase in the thermal activation volume. One of the key challenges is the development of a cost-effective strategy for media manufacturing. In this work, we present ion beam proximity lithography (IBPL) as a low cost tool for media patterning.(Co/Pd) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</sub> magnetic mutlilayers were used as a patterned medium material. Such magnetic multilayers exhibit very large and easily tunable vertical magnetic anisotropy, which makes them suitable for ultra-high density magnetic recording applications. The magnitude of the anisotropy can be varied by controlling the quality of the interfaces and/or by changing the thicknesses of the individual layers in the Co/Pd bi-layer stack. Also, an appropriate choice of a buffer/seed layer can help promote enhanced intergranular exchange coupling, an essential attribute of patterned medium materials. Magnetic films were deposited by magnetron sputtering in 2.5mTorr Ar pressure at room temperature on silicon wafers coated with a 0.5mum thermal oxide. A 5nm Ta seed was used to promote exchange-coupled films. The deposition conditions and the thicknesses of individual Co (5.2 Aring) and Pd (6.6 Aring) layers were optimized to achieve the largest vertical anisotropy, smallest coercivity (to minimized domain wall pinning), and the remnant squareness of one. X-ray diffraction was used as a benchmarking tool to precisely gauge the period of the (Co/Pd) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</sub> superlattices and the thicknesses of individual Co and Pd layers. Optimized films had a surface roughness of less than lnm. Medium patterning was accomplished using IBPL, a high-throughput direct write lithography where a large array of ion beamlets shaped by a stencil mask is used to write an arbitrary device pattern. In IBPL system used in this work, helium ions are extracted from a duo-plasmatron ion source and are then accelerated through a constant gradient tube towards a mask (silicon nitride stencil membrane). A 30 keV He+ ion-beam with an ion current density of 140nA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> was used. HSQ, a high resolution negative tone resist, was used for patterning. The sample was developed in 0.24N TMAH and the pattern was transferred into the multilayers using HSQ as the hard mask. Reactive ion etching (RIE) with CHF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> was used to remove HSQ. SEM micrograph of a patterned medium prototype with 43nm features on a 135nm pitch and the vertical M-H loops for the continuous and patterned medium are shown. A 15x coercivity increase as a result of patterning can be observed.