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Micro/Nano Lithography

Patterned media could enable next-generation hard-disk drives

Step and Flash Imprint Lithography for producing patterned media will require infrastructure components capable of high-volume imprinting.
13 March 2009, SPIE Newsroom. DOI: 10.1117/2.1200902.1514

Since magnetic-recording technology was first applied to the fabrication of hard-disk drives in 1956, their data-storage density has increased by eight orders of magnitude.1,2 As a result, the use of hard drives is ubiquitous, including in servers, desktop computers, laptops, and game consoles.

Advances in the coating properties of thin magnetic films with grain sizes as small as 7nm have enabled this remarkable progress. However, smaller grains eventually become magnetically unstable and the likelihood of a grain spontaneously flipping polarity increases, resulting in the loss of stored data. The practical limitations of this superparamagnetic effect can be avoided by patterning the boundaries between the magnetic domains.3,4 The introduction of such patterned-media technology into manufacturing is expected to enable next-generation hard-disk drives with storage densities exceeding 1Tb/inch2. Step and Flash Imprint Lithography (S-FIL) has been used to pattern hard-disk substrates.5 Here, we discuss the infrastructure required to enable S-FIL in high-volume manufacturing.

Master-template fabrication

In addition to patterned discrete-track recording and bit-patterned media (BPM), servo patterns are included on hard disks to enable the head element to read and write data at precise locations. Servo-pattern design plays a key role in disk-drive performance. Servo patterns, which are typically proprietary to each manufacturer, are placed at regular angular intervals around the disk, dividing the disk surface into a large number of wedge-shaped sectors.

Fabrication of a master template with this type of pattern requires an electron-beam writing system with a rotating stage. This configuration is well-suited for defining the concentric layouts required for patterned-media applications. A template blank (typically fused silica), coated with an electron-beam resist, is fixed on a rotary stage that turns at speeds of between 100 and 4000 revolutions per minute. Patterning is performed by deflecting the electron beam both radially and tangentially to get concentric patterning of discontinuous structures, with minimal beam blanking. An example of a patterned template is shown in Figure 1(a).

Figure 1. (a) Bit-patterned media master template with 50nm pitch. (b) Replicated pattern in a working template.
Template replication

Industry forecasts put market demand for hard-disk recording media at 1 billion units in the next few years. Fabrication of patterned media to meet this demand will require a large supply of imprint templates. The lifetime of a single template is anticipated to be approximately 10,000 imprints, suggesting that at least 100,000 templates will be required. It is not feasible to use electron-beam patterning directly to create this number of templates. Instead, a master template—created by directly patterning with an electron-beam tool—is replicated many times to produce the required supply of working templates for patterning disk media.

Replication of templates can be accomplished using a process analogous to the imprinting of disks (detailed below). After imprinting, a plasma-based etch process is employed to transfer the relief pattern into the fused silica. Figure 1(b) depicts a reverse-tone working template with a BPM pattern.

Imprint process

Patterning of a hard disk can be performed using single-step Drop-on-Demand S-FIL (see Figure 2). First, the liquid imprint resist is deposited by a multinozzle inkjet head across the active surface of the disk substrate. The template is lowered until contact is made with the resist. Capillary action induces the liquid to completely fill the region between the substrate and the topography of the imprint template. The imprint material is then photopolymerized using UV illumination through the fused-silica template.

Figure 2. Step and Flash Imprint Lithography for imprinting a hard disk.

The inkjet-based Drop-on-Demand approach to dispensing imprint material provides several significant advantages over a traditional spin-coating approach. First, the elimination of expensive two-sided coat-and-bake systems provides a compelling cost advantage. Because the imprint liquid is dispensed with picoliter-level precision to match the local volume required by the template patterns, there is no waste stream of excess resist material or rinse solvent. Drop-on-Demand technology also allows the imprint tool to selectively place imprint resist to match the local pattern density of the template. The precision of the dispense technique makes it possible to compensate for localized variations in pattern density across the template, and thus maintain a highly uniform residual layer across the entire substrate surface (see Figure 3). Finally, it eliminates the waste-disposal problems associated with the bulk of the resist being spun off the substrate.

Figure 3. Uniform residual layers are obtained across the entire substrate, independently of pattern density and feature-size variations.
Imprint tool

Two-sided imprinting of disk substrates was performed with an automated UV-nanoimprint lithography tool designed for patterned media applications: see Figure 4(a). The tool provides the high patterning fidelity characteristic of UV-nanoimprint lithography, with automated double-sided disk patterning capability and throughput of 180 disks per hour. Patterned-media applications typically require a modest level of alignment (tens of microns) to assure that the patterns are concentric wih respect to the spindle axis of the disk drive. The UV-nanoimprint-lithography tool provides alignment of the template pattern to the disk substrate to within 10 microns. An example of a disk imprint is shown in Figure 4(b).

Figure 4. (a) Automated UV-nanoimprint-lithography dual-sided imprinting system. (b) An imprinted hard disk.

The patterning of magnetic domains requires industrial-scale lithography at unprecedented levels of feature resolution, pattern precision, and cost efficiency. S-FIL has been demonstrated to meet these requirements. Further refinements in template mastering and replication, together with imprinting tools with increased throughput, are expected to facilitate fabrication of patterned media for the next generation of hard-disk drives with storage densities exceeding one terabit per square inch.

Douglas Resnick, Cynthia Brooks, Gerard Schmid, Mike Miller 
Molecular Imprints Inc.
Austin, TX

Doug Resnick received his PhD in solid-state physics in 1981 from Ohio State University. He is vice president of template technology. He has authored or coauthored more than 150 technical publications and holds 21 US patents.