K. O. Aung

Patterned media with composite structure for writability at high areal recording density

Writability in bit-patterned media (BPM) is a critical issue for high areal densities. In this study, magnetization reversal for multilayer of (Co/Pd) nanodots was investigated using magnetic force microscopy. We observed an increase of more than 15 times in switching field (Hsw) in BPM over that of continuous films. An exchange coupled structure made of a thin Co layer with in-plane magnetization and high perpendicular anisotropy layer of (Co/Pd) multilayer to reduce the switching field is proposed. When the Co layer is thinner than 2 nm, its magnetization is aligned perpendicular to the film plane due to the exchange coupling with the 15 nm thick (Co/Pd) multilayer. The thin Co layer helps in reducing the Hsw of (Co/Pd) by almost 50% and also its distribution by 57% as measured from remanence magnetization.

Effect of film texture on magnetization reversal and switching field in continuous and patterned (Co/Pd) multilayers

We studied the reversal properties of perpendicular anisotropy (Co/Pd) multilayers with different crystallographic textures. In case of continuous films, an increase in the coercivity and reduction in the switching field distribution (SFD) were observed as the growth is improved. From magnetic force microscopy, a stripe-type domain configuration was observed in films deposited at low gas pressure while a bubble-type domain was observed in high pressure deposited films. In patterned films, the SFD did not vary significantly for samples with different textures although a 2 kOe increase in the switching field was measured. In patterned structures, the controllability of SFD may not be related to the improvement of film crystallographic growth as was observed for unpatterned films. The results from this study indicate that local variation in the intrinsic film properties plays a major role in the SFD.

Antiferromagnetically Coupled Patterned Media and Control of Switching Field Distribution

Switching Field distribution (SFD) is one of the critical issues for writing in bit patterned media (BPM) for high areal densities. It is believed that the magnetostatic interaction is one of the several factors that contribute to the SFD. With the antiferromagnetically coupled (AFC) structure, the magnetostatic interaction can be tailored to understand/reduce SFD. In this study, AFC patterned media is studied with emphasis placed on the effect of the top layer coercivity, which will determine the Mr and hence the magnetostatic interaction. For this study, nanodots with a size and space of 60 and 40 nm respectively were fabricated with electron beam lithography (EBL). Remanent hysteresis curves for the nanodot arrays were obtained by counting the number of reversed dots in magnetic force microscopy (MFM) images at remanent state. The narrowest SFD at a pressure of 1 Pa for top layer was observed possibly because of good crystaline texture and reduced magnetostatic interaction.

Antiferromagnetically coupled patterned media

Magnetostatic interactions in patterned media are responsible for increasing the switching field distribution and reducing the thermal stability. Micromagnetic simulations indicate that a reduced saturation magnetization (Ms) and increased anisotropy constant (Ku) lead to better thermal stability. However, writability will be a concern with this approach. Patterned media with an antiferromagnetically coupled (AFC) recording media structure is studied to obtain high thermal stability even with a larger Ms and larger Ku. With the AFC structure, the reduced remanence magnetization (Mr) leads to a reduced magnetostatic interaction, even though the Ms is larger. Magnetic force microscopy measurements indicate that the AFC media show a narrower switching field distribution.

Nanoimprint mold fabrication and duplication for embedded servo and discrete track recording media

A master mold for nanoimprint lithography was fabricated for discrete track recording (DTR) media using electron beam lithography and conventional etching techniques. The DTR pattern, containing 167 tracks of 120 nm pitch (60 nm land and groove widths) and embedded servo information, was automatically generated using an in-house developed program and was optimized for faster electron beam writing on an x-y stage. A daughter mold was duplicated from the master mold by nanoimprinting, using UV-curable resist and an intermediate polymer stamp technique. Scanning electron microscope images showed that the daughter mold was accurately and completely reproduced from the master mold.

Sub-50-nm track pitch mold using electron beam lithography for discrete track recording media

Using electron beam lithography and ZEP 520 resist, molds with a track pitch of 50nm were fabricated on thermal silicon oxide for discrete track recording applications. In this article, the detailed process for patterning a mold with 50nm track pitch and 10nm feature size is described. Although bilayer polymethylmethacrylate of 120nm thickness is able to pattern tracks of micrometer length and pitch down to 70nm, patterned tracks with pitches below 70nm exhibited waviness and edge roughness. In contrast, fine patterns with 50nm pitch were achieved with a single 60nm layer of ZEP 520 resist. Pattern transfer using ion milling followed by reactive ion etching produced 10nm features. High resolution scanning electron microscopy was used to evaluate the size and uniformity of the tracks during each step of the process.

Material and Layer Design to Overcome Writing Challenges in Bit-Patterned Media

In this paper, the problem of writability in bit-patterned media (BPM) for high areal density will be discussed. A new film structure is proposed, made of a composite in-plane and perpendicular anisotropy layers to improve writability and reduce the time of magnetization switching in BPM. To demonstrate the efficiency of an in-plane anisotropy layer in assisting the switching of the magnetization of the high perpendicular anisotropy recording layer, we use micromagnetic simulation to study magnetization reversals in BPM for 5 Tb/in2. Experiments have been carried out on patterned arrays of 60-nm-size dots made of [Co(0.3 nm)/Pd(0.8 nm)]x15 multilayer and Co(2 nm)/[Co(0.3 nm)/Pd(0.8 nm)]x15 composite structure. The mean switching field calculated from remanence magnetization curves shows a reduction of more than 50% from its initial value by adding a 2-nm-thick Co bottom layer with in-plane anisotropy. No difference in switching field distribution was observed in the two structures studied, indicating the merit of assisting the switching of high anisotropy patterned media by exchange coupling to an in-plane anisotropy layer.

Planarization of Patterned Recording Media

The planarization of patterned recording media is essential to overcome the ¿flying height reduction¿ and ¿flying instability¿ of the head-slider. This paper reports two planarization techniques; one by compressing a morphological smooth surface onto a low glass transition temperature Tg/UV curable polymer/material coated patterned sample, while the other employs spin coating to coat patterned disks. A roughness of 0.3 nm was achieved by planarization using compression of smooth Si onto 35 K PMMA (heated above Tg to decrease its viscosity). It was also found that decreasing the spin speed from 5000 rpm to 1000 rpm improves the filling of grooves using spin coating of hydrogen silsesquioxane (HSQ) decreasing the height difference between the filled groove region and the land region from about 6 nm to 4 nm. Initial investigation using compression of smooth Si onto diluted HSQ also shows a decrease of roughness as dilution increases from 50.0% to 66.6%. Both techniques were applied in the planarization of a UV curable resist. The results show that about 1 nm is observed between the patterned and unpatterned regions using only spin coating. However, with compression using a flat mold, no height difference on average is observed.

Achieving High Aspect Ratio of Track Length to Width in Molds for Discrete Track Recording Media

Discrete track media (DTM) fabricated by nanoimprint lithography (NIL) is considered as a potential technology for future hard disk drives (HDD). In the fabrication of a master mold for NIL, patterning the resist tracks with a narrow distribution in the width is the first critical step. This paper reports the challenges involved in the fabrication of high aspect ratio discrete tracks on Polymethylmethacrylate (PMMA) resist by means of electron beam lithography. It was observed that fabrication parameters applied for successful patterning of discrete tracks in nanoscale length were not directly suitable for the patterning of discrete tracks in micron scale. Hence different approaches such as thick layer resist coating, introducing of post exposure baking process, and varying of exposure parameters were used in order to achieve uniform sharp discrete tracks in micron scale length on the resist. The optimal parameters were used to pattern 20 𝜇m long tracks with 70 nm track pitch on the resist.

Novel planarizing scheme for patterned media

A novel planarization scheme, consisting of compression of a low glass transition temperature (Tg) and low viscosity poly(methyl methacrylate) (PMMA) polymer into grooves of patterned recording media using a surface with flat morphology, has been proposed and investigated. Si (100) surface was used as the smooth surface for pressing PMMA onto the patterned media. Patterned samples prepared with such a planarization method showed very smooth topography of roughness as small as 3 A (which is comparable to present hard disk media based on continuous film) by atomic force microscope. A magnetic sacrificial layer is proposed in addition as a solution to etching or polishing issues.