High-density Bit-patterned Media Research Articles

An Iterative TMR Mitigation Method Based on Readback Signal for Bit-Patterned Media Recording

Off-track condition or track misregistration (TMR) is one of the most significant problems in the extremely high-density bit-patterned media recording (BPMR) system, since a track pitch becomes narrower. Typically, the TMR can be detected and handled by a servo system; however, it requires some special data to be inserted in the tracks so as to estimate the amount of head offset. Nonetheless, this paper proposes an iterative TMR mitigation method for BPMR systems based on the readback signals. First, we design several pairs of the 2-D asymmetric target and its corresponding 2-D equalizer that match the BPMR channel for each TMR level. Then, we exploit the three adjacent data tracks obtained from the low-density parity-check decoders to estimate the TMR level. Last, a pair of the 2-D asymmetric target and its corresponding 2-D equalizer that is best fit to the estimated TMR level will be used to alleviate the TMR effect in the readback signal for the next global iteration. Simulation results indicate that the proposed system can effectively estimate the TMR level and performs better than the conventional system without a TMR mitigation method, especially when the TMR level is high and/or the position jitter is large.

Integration of nanoimprint lithography with block copolymer directed self-assembly for fabrication of a sub-20 nm template for bit-patterned media

We propose a novel strategy to integrate the nanoimprint lithography (NIL) technique with directed self-assembly (DSA) of block copolymer (BCP) for providing a robust, high-yield, and low-defect-density path to sub-20 nm dense patterning. Through this new NIL-DSA method, UV nanoimprint resist is used as the DSA copolymer pre-pattern to expedite the DSA process. This method was successfully used to fabricate a 1.0 Td in−2 servo-integrated nanoimprint template for bit-patterned media (BPM) application. The fabricated template was used for UV-cure NIL on a 2.5-inch disk. The imprint resist patterns were further transferred into the underlying CoCrPt magnetic layer through a carbon hard mask using ion beam etching. The successful integration of the NIL technique with the DSA process provides us with a new route to BPM nanofabrication, which includes the following three major advantages: (1) a simpler and faster way to implement DSA for high-density BPM patterning; (2) a novel method for fabricating a high-quality dot pattern template through an iterative imprint-DSA-template procedure; and (3) an uncomplicated integration scheme for implementing non-periodic servo features with BCP patterns, thus accelerating the transition of moving the DSA technique from laboratory research to the BPM manufacturing environment.

Investigation of Graph-Based Detection in Iterative Decoding for Bit-Patterned Media Recording

High-density bit-patterned media recording (BPMR) can be obtained by reducing the spacing between data bitislands in both the along-and across-track directions, thus leading to severe intersymbol interference (ISI) and intertrack interference (ITI) because of small bit and track pitches, respectively. Here, we propose to use the graph-based detector, instead of the trellis-based detector, in iterative decoding to combat the ISI and the ITI for a multi-head multi-track BPMR system. Specifically, the readback signal is sent to the graph-based detector before iteratively exchanging the soft information with a decoder. Experimental results indicate that at low to moderate complexity, the proposed scheme outperforms the existing schemes, especially at high recording density.

Orientation and Position Control of Self-Assembled Polymer Pattern for Bit-Patterned Media

Directed self-assembly (DSA) is expected to be a solution for the fabrication process of high-density bit-patterned media. A DSA pattern of polystyrene-b-polydimethylsiloxane diblock copolymer with 20 nm pitch was fabricated on a 2.5 in disk substrate using a postguide. Uniformity of the dot alignment as well as dot-pitch fluctuation and linearity of pseudodot tracks are estimated using image analyses of SEM photographs. Uniformity along the circumferential direction was confirmed. All the SEM images at eight different angles at r=29.4 mm showed single domain. Pitch distribution and linearity error were estimated to be 14% and 7.8%, respectively. Uniformity along the radius direction was estimated with the moire method. The single-domain region is expected to be 5 mm in width. Margin of the postguide pitch for the single-domain formation is revealed to be more than 15% of the self-assembled dot pitch.

Performance of the Contraction Mapping-Based Iterative Two-Dimensional Equalizer for Bit-Patterned Media

Bit-patterned media (BPM) storage is one of the promising technologies to overcome the limitations of the conventional magnetic recording. However, a high-density BPM storage has intertrack interference, intersymbol interference and noise, which severely degrade the system performance. In this paper, we present a simple iterative two-dimensional (2-D) equalizer based on the contraction mapping theorem to mitigate these adverse effects. In the simulation, we demonstrate the bit separation characteristics of the one-dimensional (1-D) and proposed 2-D equalizers and evaluate the bit-error-rate (BER) performance of the proposed equalizer comparing with the conventional equalizers. According to the simulation results, the proposed equalizer is a promising candidate with maintaining proper complexity for a high-density BPM storage.

Effect of inter-bit material on the performance of directly deposited bit patterned media

We evaluated the effects of inter-bit material on the switching performance of bit patterned media (BPM) fabricated by direct deposition of magnetic material onto pre-patterned substrates. We performed a controlled experiment to vary the sidewall thickness and symmetry in bits with nominally identical size and pitch. Thick, asymmetric sidewalls resulted in significant broadening of the switching field distribution to 14%–20% compared to 10%–11% for bits with thin, symmetric sidewalls. These differences were attributed to changes in the intrinsic properties and dipolar interactions as supported by micromagnetic simulations. Our results highlight the importance of controlling inter-bit material to achieve high-density BPM.

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.

Two-Dimensional Pulse Response and Media Noise Modeling for Bit-Patterned Media

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <?Pub Dtl=""?>In high density bit-patterned media (BPM), bits or magnetic islands are very close leading to significant inter-track interference (ITI) in addition to inter-symbol interference (ISI). Also, in BPM, major sources of media noise are the fluctuations in the location of the magnetic islands and fluctuations in the sizes of the magnetic islands. The location fluctuations can happen in both along-the-track and in cross-track directions. The size fluctuations affect both ISI and ITI. In order to obtain a more realistic channel model that includes the 2-D interference and the 2-D impact of media noise, we have developed a 2-D pulse response simulator for BPM. Utilizing the 2-D pulse response, we model the ITI and the media noise. Simulation results show that to achieve an acceptable bit error rate (BER), the location and size fluctuations should be smaller than 8% of the bit period at a density of 1.5 <formula formulatype="inline"><tex Notation="TeX">${\rm Tb/in}^{2}$</tex> </formula>. </para>