Microstructure and magnetic properties of a Co/Pd multilayer on a controlled Pd/Si seed layer for double-layered perpendicular magnetic recording media

An approach to fabrication of a patterned magnetic recording medium for next generation data storage systems is presented. (Co/Pd)n magnetic multilayers are evaluated as candidates for patterned medium materials for their high and easily controllable magnetic anisotropy. The multilayer films deposited on a Ta seed layer enable high intergranular exchange coupling—an essential feature of a patterned magnetic recording medium. The quality of (Co/Pd)n superlattices was optimized via deposition conditions and monitored using low-angle x-ray diffraction. An estimated in-plane (hard-axis) magnetization saturation field in excess of 40 000 Oe was observed. Vertical (easy-axis) hysteresis loops for as-deposited continuous magnetic multilayers exhibited a low coercivity of 930 Oe, indicating highly uniform (magnetically) films with weak domain wall pinning. Ion-beam proximity lithography was used to pattern magnetic multilayers into 43 nm islands on a 135 nm pitch. Following patterning, easy-axis coercivity increased nearly 15-fold to 12.7 kOe.

͑Co/ Pd͒ N multilayers exhibit high vertical magnetic anisotropy and have been extensively explored as recording medium candidates for high density magnetic recording applications. In this work, ͑Co/ Pd͒ N multilayers are deposited by magnetron sputtering and patterned into large periodic arrays of 200 nm islands to enable controlled domain wall injection for quantitative comparison of magnetic anisotropy energies. Magnetic properties are correlated with x-ray photoelectron spectroscopy data, an approach commonly used to probe the binding energies and valence band positions. Confirming theoretical predictions, it is demonstrated that the degree of d-shell hybridization at Co/Pd interfaces directly correlated with the magnitude of magnetic anisotropy.

Ion irradiation of continuous and patterned (Co∕Pd)n magnetic multilayer films has been studied as a mean to control magnetic anisotropy as well as to evaluate possible ion irradiation damage involved in ion-beam proximity lithography patterning. The coercivity of patterned medium was found to decrease from 11kOe for as patterned samples to 0.3kOe for samples with 800μC∕cm2 ion irradiation. Remnant squareness of the patterned samples remained essentially unchanged. As the number of bilayers increases in the sample, the effects vary, suggesting that several mechanisms of damage occur. Significantly, for typical irradiation doses used in ion-beam proximity lithography, no measurable alteration of magnetic properties was observed.

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In ion beam proximity lithography, ions that are incident on the nominally opaque regions of a stencil mask can scatter into the open windows and escape, exposing a wide area of the substrate. Since these ions can lose much of their initial energy in the mask, the scattered particle exposure is concentrated near the resist surface. The resulting loss of contrast can be mitigated to some extent by using aperture array lithography ͑AAL͒ where a mask of reduced density minimizes the number of windows from which a scattered ion can escape. Even so, the problem worsens as the pitch of an array, printed by multiple, offset exposures of the AAL mask, shrinks below about 250 nm. The only solution is to increase the mask thickness, hence the window aspect ratio, to reduce the escape angles of the scattered particles. In this article, the authors characterize an effective background dose in the first 75 nm of poly͑methylmethacrylate͒ resist for 30 keV He + ion exposures of 0.6 m thick masks with 45, 80, and 110 nm circular windows on 150, 300, and 400 nm pitches, respectively. They project that would be ͑6.8± 0.8͒ % ͑͒ of the primary ion dose for the printing of dense arrays, with period equal to twice the window diameter, over this range of feature sizes.

Conventional magnetic recording systems based on continuous medium recording are rapidly approaching their superparamagnetic limit. 1 A shift to patterned media 2 , 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. 3 One of the key challenges is the development of a costeffective strategy for media manufacturing. In this work, we present ion beam proximity lithography (IBPL) as a low cost tool for media patterning. (Co/Pd) n magnetic mutlilayers were used as a patterned medium material. Such magnetic multilayers exhibit very large and easily tunable vertical magnetic anisotropy 4 , which makes them suitable for ultra-high density magnetic recording applications. 5 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. 6 Magnetic films were deposited by magnetron sputtering in 2.5mTorr Ar pressure at room temperature on silicon wafers coated with a 0.5µm thermal oxide. A 5nm Ta seed was used to promote exchangecoupled films. The deposition conditions and the thicknesses of individual Co (5.2Å) and Pd (6.6Å) 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) n superlattices and the thicknesses of individual Co and Pd layers (See Figure 1). Optimized films had a surface roughness of less than 1nm. Medium patterning was accomplished using IBPL 7 , 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) 8 as illustrated in Figure 2. A 30 keV He+ ion-beam with an ion current density of 140nA/cm 2 was used. HSQ, a high resolution negative tone resist 9 , 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 3 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 in Figure 3. A 15x coercivity increase as a result of patterning can be observed. 1. H. N. Figure 3: (a) SEM micrograph of patterned medium sample and (b) Vertical M-H loops for continuous and patterned medium.Figure 1. (a) High angle x-ray measurements of multilayers with varying Co layer thickness in the (Co/Pd) n structure....

We describe a self-limiting, low-energy argon-ion-milling process that enables noncircular device patterns, such as squares or hexagons, to be formed using precursor arrays of uniform circular openings in poly(methyl methacrylate) defined using electron beam lithography. The proposed patterning technique is of particular interest for bit-patterned magnetic recording medium fabrication, where square magnetic bits result in improved recording system performance. Bit-patterned magnetic medium is among the primary candidates for the next generation magnetic recording technologies and is expected to extend the areal bit density limits far beyond 1 Tbit/in(2). The proposed patterning technology can be applied either for direct medium prototyping or for manufacturing of nanoimprint lithography templates or ion beam lithography stencil masks that can be utilized in mass production.