1. Field of the Invention
This invention relates generally to bit-patterned media (BPM) magnetic recording disks made with an imprint template, and to an imprint template made using directed self-assembly (DSA) of block copolymers.
2. Description of the Related Art
Magnetic recording hard disk drives with patterned magnetic recording media have been proposed to increase data density. In patterned media, also called bit-patterned media (BPM), the magnetic recording layer on the disk is patterned into small isolated data islands or “dots” arranged in data regions in radially-spaced data tracks, with each dot capable of storing an individual bit. Like conventional non-patterned disks, BPM disks are also required to have nondata servo regions or sectors that are used for read/write head positioning. The nondata servo sectors contain servo “burst” fields that generate readback signals that provide radial and circumferential position information about the read/write head. This information in turn is used by the servo system in moving the read/write head to the desired data track and maintaining it on track. In BPM disks the servo burst fields typically contain patterned servo blocks separated by nonmagnetic spaces. However, the complexity of typical servo patterns and the requirement for their precise alignment with the patterned data tracks make the use of conventional servo patterns difficult in BPM disk drives.
One proposed method for fabricating BPM disks is by imprinting with a template, sometimes also called a “stamper” or “mold”, that has a topographic surface pattern. In this method the magnetic recording disk substrate with a polymer film on its surface is pressed against the template. The patterns on the template may be protrusions (pillars), or recesses (holes), and the type of the pattern is referred to as the polarity of the template. The polymer film receives the reverse image of the template pattern and then, depending on the polarity of the template, either becomes a mask for subsequent etching of the disk substrate if the template is hole-type, or becomes a sacrificial layer for a tone reversal process such as liftoff followed by etching of the disk substrate, if the template is pillar type, to form pillars on the disk in both cases. In one type of BPM, the magnetic layers and other layers needed for the magnetic recording disk are first deposited on the flat disk substrate. The polymer film used with imprinting is then pressed on top of these layers. The polymer film receives the reverse image of the template pattern and then becomes a mask, or sacrificial layer for tone reversal, for subsequent milling, etching or ion-bombarding the underlying layers.
The template may be a master template for directly imprinting the disks. However, the more likely approach is to fabricate a master template with a pattern of pillars corresponding to the pattern of pillars desired for the disks and to use this master template to fabricate replica templates using imprint lithography. The replica templates may have the opposite or same polarity of the master template. The replica templates are then used to directly imprint the disks.
Directed self-assembly (DSA) of block copolymers (BCPs) has been proposed for making the template and is believed capable of achieving areal bit densities of greater than 1 Terabits/in2. DSA is a technique that involves forming a periodic pattern utilizing the self-assembly of BCPs. An underlayer having optical contrast that has a preferential affinity for one of the copolymers controls the pattern sequence. It has been reported that when DSA is used, it is possible to form a high-precision dot pattern having a pitch of 10 nm or less. However, BCPs have a property whereby the volume thereof is minimized when they self assemble, and therefore although it is easy to form a pattern with equal spacing such as in the data regions, it is difficult to form the complex servo patterns in the servo sectors. A separate process is therefore required to form the servo patterns. However, it is difficult to precisely position the data regions and the servo sectors relative to one another if the two regions are produced in separate processes. Another problem with DSA for making the template is that because the thickness of the BCP layer cannot be precisely controlled, there can be a large variation in the diameter of the dots, i.e., a broad dot diameter distribution.
What is needed is an imprint template made with DSA of BCPs that allows the data regions and servo sectors to be formed simultaneously, and that produces a template with a narrow dot diameter distribution.