Bit Patterned Media Fabrication Research Articles

In situ grazing-incidence small-angle X-ray scattering observation of block-copolymer templated formation of magnetic nanodot arrays and their magnetic properties

The fabrication of bit-patterned media (BPM) is crucial for new types of hard disk drives. The development of methods for the production of BPM is progressing rapidly. Conventional lithography reaches the limit regarding lateral resolution, and new routes are needed. In this study, we mainly focus on the dependence of the size and shape of magnetic nanodots on the Ar+-ion etching duration, using silica dots as masks. Two-dimensional (2D) arrays of magnetic nanostructures are created using silica-filled diblock-copolymer micelles as templates. After the self-assembly of the micelles into 2D hexagonal arrays, the polymer shell is removed, and the SiO2 cores are utilized to transform the morphology into a (Co/Pt)2-multilayer via ion etching under normal incidence. The number of preparation steps is kept as low as possible to simplify the formation of the nanostructure arrays. High-resolution in situ grazing-incidence small-angle X-ray scattering (GISAXS) investigations are performed during the Ar+-ion etching to monitor and control the fabrication process. The in situ investigation provides information on how the etching conditions can be improved for further ex situ experiments. The GISAXS patterns are compared with simulations. We observe that the dots change in shape from cylindrical to conical during the etching process. The magnetic behavior is studied by utilizing the magneto-optic Kerr effect. The Co/Pt dots exhibit different magnetic behaviors depending on their size, interparticle distance, and etching time. They show ferromagnetism with an easy axis of magnetization perpendicular to the film. A systematic dependence of the coercivity on the dot size is observed.

Directed self-assembly of high-chi block copolymer for nano fabrication of bit patterned media via solvent annealing

We report the formation of nanoimprint master templates that can be used for the fabrication of bit patterned media (BPM). The template was formed by directed self-assembly, with solvent annealing, of a symmetric ABA triblock copolymer to form perpendicularly oriented lamellae on chemical patterns. We used a high-χ block copolymer, poly(2-vinyl pyridine)-block-polystyrene-block-poly(2-vinyl pyridine) to achieve smaller feature sizes than are possible with polystyrene-block-poly(methyl methacrylate). The work shows that triblock copolymers can provide a large processing window in terms of pitch commensurability. Using block-selective infiltration (atomic layer deposition with sequential long soaking/purge cycles), an alumina composite with high etch resistance was specifically incorporated into the polar and hydrophilic P2VP domains. Subsequently, the surface pattern was successfully transferred into underlying Si substrates by etching with a fluorine-containing plasma to create a nanoimprint master. The line/space pattern of the nanoimprint master met the BPM fabrication requirement of defectivity <10−3. For demonstration purposes, the nanoimprint master was used to imprint a replica pattern of photoresist on a quartz wafer.

Bit-Patterned Magnetic Recording: Theory, Media Fabrication, and Recording Performance

Bit Patterned Media (BPM) for magnetic recording provide a route to densities $>1 Tb/in^2$ and circumvents many of the challenges associated with conventional granular media technology. Instead of recording a bit on an ensemble of random grains, BPM uses an array of lithographically defined isolated magnetic islands, each of which stores one bit. Fabrication of BPM is viewed as the greatest challenge for its commercialization. In this article we describe a BPM fabrication method which combines e-beam lithography, directed self-assembly of block copolymers, self-aligned double patterning, nanoimprint lithography, and ion milling to generate BPM based on CoCrPt alloys. This combination of fabrication technologies achieves feature sizes of $<10 nm$, significantly smaller than what conventional semiconductor nanofabrication methods can achieve. In contrast to earlier work which used hexagonal close-packed arrays of round islands, our latest approach creates BPM with rectangular bitcells, which are advantageous for integration with existing hard disk drive technology. The advantages of rectangular bits are analyzed from a theoretical and modeling point of view, and system integration requirements such as servo patterns, implementation of write synchronization, and providing for a stable head-disk interface are addressed in the context of experimental results. Optimization of magnetic alloy materials for thermal stability, writeability, and switching field distribution is discussed, and a new method for growing BPM islands on a patterned template is presented. New recording results at $1.6 Td/in^2$ (teradot/inch${}^2$, roughly equivalent to $1.3 Tb/in^2$) demonstrate a raw error rate $<10^{-2}$, which is consistent with the recording system requirements of modern hard drives. Extendibility of BPM to higher densities, and its eventual combination with energy assisted recording are explored.

A facile approach for screening isolated nanomagnetic behavior for bit-patterned media

Bit-patterned media (BPM) fabricated by the direct deposition of magnetic material onto prepatterned arrays of nanopillars is a promising approach for increasing magnetic recording of areal density. One of the key challenges of this approach is to identify and control the magnetic interaction between the bits (on top of the nanopillars) and the trench material between the pillars. Using independent techniques, including magnetic force microscopy, the variable-angle magneto-optic Kerr effect, and remanence curves, we were able to determine the presence and relative intensities of exchange and dipolar interactions in Co-Pd multilayer-based BPM fabricated by direct deposition. We found that for pitches of 30 nm or less, there were negligible exchange interactions, and the bits were found to be magnetically isolated. As we move to higher densities, the absence of exchange interactions indicates that direct deposition is a promising approach to BPM fabrication.

Ion Implantation Challenges for Patterned Media at Areal Densities Over 5 Tbpsi

Ion implantation of lighter (4He <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ) and heavier ion species (121Sb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ) was studied to investigate their suitability in fabricating bit patterned media (BPM) for areal densities ≥ 5 Tbpsi. Conventional CoCrPt- SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and the next generation high-anisotropy L1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> FePt media were employed to understand the mass-dependent lateral straggle of the ions. First-order reversal curve measurement for CoCrPt- SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> media revealed an increase in exchange interaction in the implanted films with the increasing fluence. Interestingly, there was an order-disorder transformation of the phase for L1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> FePt media upon implantation. Implantation using lighter ions resulted in a larger lateral straggle. This lateral movement of ions from the unmasked to masked regions during BPM fabrication will, furthermore, impede the process of bit isolation. In contrast, lateral straggle was reduced for heavier ions. However, this was achieved at the expense of the diffusion of host atoms displaced from their lattice positions which caused a reduction of anisotropy even in the unimplanted regions. Therefore, the requirement to strike a balance between the lateral straggle of the implanted species and the movement of host atoms to accomplish ultra-high densities in implantation-assisted-BPM recording is reported as a challenging problem.

Determination of Position Jitter and Dot-Size Fluctuations in Patterned Arrays Fabricated by the Directed Self-Assembly of Gold Nanoparticles

The promise of magnetic bit-patterned media (BPM) in the hard disk drive industry hinges on its capability to extend the data storage density beyond that achievable by conventional continuous media. Its success, however, depends strongly on meeting the jitter and throughput requirements of BPM with a suitable fabrication process. In this paper, we report on the directed self-assembly of gold nanoparticles using a topographical template as an approach to fulfill the BPM fabrication requirements. The effects of position jitter and dot-size fluctuations are examined by performing image analysis on scanning electron microscopies of samples fabricated with areal densities of 4.4 Tdot/in2. For comparison, we considered three different cases: electron-beam lithography-defined templates, monolayer films of self-assembled nanoparticles on an unpatterned substrate, and nanoparticles directed to assemble within a template. Our analysis provides evidence for the improvements in position jitter of the directed assembly of nanoparticles over those that were left to assemble without a template.

Lateral displacement induced disorder in L10-FePt nanostructures by ion-implantation

Ion implantation is a promising technique for fabricating high density bit patterned media (BPM) as it may eliminate the requirement of disk planarization. However, there has not been any notable study on the impact of implantation on BPM fabrication of FePt, particularly at nano-scale, where the lateral straggle of implanted ions may become comparable to the feature size. In this work, implantation of antimony ions in patterned and unpatterned L10-FePt thin films has been investigated. Unpatterned films implanted with high fluence of antimony exhibited reduced out-of-plane coercivity and change of magnetic anisotropy from perpendicular direction to film-plane. Interestingly, for samples implanted through patterned masks, the perpendicular anisotropy in the unimplanted region was also lost. This noteworthy observation can be attributed to the displacement of Fe and Pt atoms from the implantation sites to the unimplanted areas, thereby causing a phase disorder transformation from L10 to A1 FePt.

Fabrication and Characterization of FePt Exchange Coupled Composite and Graded Bit Patterned Media

Three methods to fabricate continuous FePt films with graded magnetic anisotropy for bit patterned media (BPM) were evaluated. Continuous FePt films with surface roughness of less than 0.3 nm were achieved in continuous FePt hard magnetic films, FePt/Fe exchange coupled composite (ECC) films and FePt/Fe based graded films. Depositing an Fe-rich film on FePt at high temperature was found to form large grains and cause the film surface very rough for BPM fabrication. Depositing Fe on FePt at room temperature and then annealing it to create graded anisotropy through the layer interdiffusion process was demonstrated to fabricate FePt/Fe based graded BPM. The continuous FePt films with hard layer only, ECC structure and graded magnetic anisotropy were patterned using a di-block copolymer self-assemble hard mask method with 25 nm dot size over 2-inch substrate. The switching field distribution (SFD) broadening and degradation of FePt BPM was studied. The reduction of SFD was achieved using a postannealing process. It was confirmed that the patterned graded BPM sample has smaller switching field and larger thermal energy barrier than the ECC sample.

Integration of Servo and High Bit Aspect Ratio Data Patterns on Nanoimprint Templates for Patterned Media

Magnetic recording bit patterned media (BPM) stands as a promising technology to deliver thermally stable magnetic storage at densities beyond 1 Tb/in2. High throughput BPM fabrication will be enabled by nanoimprint lithography which relies on a high-quality master template to be able to meet pattern fidelity and low defectivity specifications. Master template fabrication for BPM can be done by e-beam lithography alone or by e-beam directed self-assembly of block copolymers. Incorporating servo features in the fabrication of master templates brings numerous nanofabrication challenges that vary depending on which method is used. Here we demonstrate the fabrication of conventional servo features at 200 Gd/in2 using e-beam lithography and we explore some nonconventional servo geometries that are compatible with self-assembly for BPM beyond 1 Td/in2.

Write Margin Measurement of Bit Patterned Media With 20 nm Dots

Bit-patterned media (BPM) with 20 nm diameter dots and a 72 nm dot period were fabricated from hard/soft-stacked base media with a [Co/Pt]-super-lattice hard layer and a Co soft layer. The write margin of synchronous recording was evaluated using a static tester. All recorded dots were in single domain state, as expected from previous experiments. The write margin was about 50 nm, and there was a total margin loss almost corresponding to the dot diameter. According to an analysis of margin loss factors based on the slope recording model, the margin loss factors for compensating effect of multidomain dots decreased drastically. Therefore it was confirmed that the slope recording model well illustrated the recording process of small dots in single-domain states. The margin loss factor due to the dot position jitter was 17.7 nmp-p and became the dominant loss factor. The loss factor attributable to the switching field distribution also increased rapidly to 11.3 nmp-p and was the second largest loss factor. To secure a sufficient write margin for a robust recording system, suppression of these degradation factors in the BPM fabrication process is required.

Comparison of bit-patterned media fabricated by methods of direct deposition and ion-milling of cobalt/palladium multilayers

In the fabrication of bit-patterned media (BPM), two processes are commonly used, i.e., the pattern transfer by ion-milling into an underlying film of magnetic material, and the direct deposition of the magnetic material onto a pre-patterned substrate. We experimentally compared the switching performance of the BPM based on Co/Pd multilayers fabricated using these methods in terms of their switching field distribution (SFD) and physical characteristics of the bits. Our results show that both methods resulted in a narrow (∼15%) SFD at low areal recording densities of ∼0.15 Tdot/in2. However, at higher densities of up to 0.6 Tdot/in2, the SFD of the ion-milled samples detrimentally broadened to ∼30% while the BPM from the direct-deposition method maintained its narrow SFD up to a high bit density of 0.6 Tdot/in2. Our results suggest that in Co/Pd multilayer systems, the direct-deposition method, which produces more uniform bit sizes and profiles especially at high bit densities, is a more promising approach to achieving high-density BPM.

Large area patterning of single magnetic domains with assistance of ion irradiation in ion milling

This study demonstrates a pronounced ion irradiation effect in ion milling of magnetic thin films. In fabrication of bit-patterned media, the ion irradiation could facilitate bit island isolation before complete removal of magnetic materials by ion milling. Combined with block copolymer lithography, sub-20 nm CoPt dots with uniaxial perpendicular anisotropy, resembling Stoner–Wohlfarth-like single domains, were achieved. X-ray diffraction demonstrates that the degradation of the magnetic film by ion irradiation is related to crystal structure damage.