Planarization of patterned magnetic recording media to enable head flyability
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- Choi, C., Yoon, Y., Hong, D. et al. Microsyst Technol (2011) 17: 395. doi:10.1007/s00542-011-1222-1
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The fabrication and planarization of patterned magnetic recording media is investigated and the flyability of magnetic recording sliders on a patterned and planarized 65 mm glass disk is investigated a small coupon of patterned media with an array of nano pillars of 40 nm diameter and 60 nm height was first fabricated by e-beam lithography and reactive ion etching (RIE) to investigate the planarization process for patterned media. Since read/write flyability tests require a patterned disk rather than a small coupon area, we have prepared a bit patterned glass disks of 65 mm diameter (2.5 in.) using the so-called “Ag ball-up process” in combination with RIE. This “Ag ball-up process” permits the manufacturing of a nano-sized bit patterns on a large area, i.e., on a disk with 65 mm diameter. Planarization of the patterned area was performed with hydrogen silsesquioxane (HSQ) by spin coating. The HSQ layer was back-etched using RIE, resulting in a smooth surface. “Flyability testing” indicates significantly improved flying stability of typical magnetic recording sliders on the planarized glass disks, with the standard deviation of flying height fluctuations on the order of 0.1 nm. The latter value is comparable to that of “smooth” disks.
Bit patterned recording media (BPM) have received increased attention as potential candidates for areal densities in excess of 1 Tbit/in.2. (Richter et al. 2006; Terris and Thomson 2005). As the areal density in disk drives increases to values near 1 Tbit/in.2, the magnetic energy of a single bit is not sufficient anymore to prevent thermally activated bit reversals, and errors in the read back process are likely to occur. For bit patterned media (BPM), the switching volume is identical to that of an individual bit since the size of the bits is lithographically pre-defined. The size of a typical bit in BPM has linear dimensions as small as 10 nm. This small bit is expected to be stable with respect to thermal fluctuations, have a sharp magnetic transition and exhibit low noise, both along and across a track.
Two major fabrication methods exist for BPM (Thomson et al. 2006; Shaw et al. 2007; Kitade et al. 2004). In the first method, a layer of high coercivity magnetic material is first deposited on a flat substrate. This layer is then patterned into discrete magnetic bits using lithographic techniques such as ion beam milling, reactive ion etching (RIE), or focused ion beam milling (FIB). The main advantage of this method is that no magnetic material is left between neighboring bits to cause magnetic interference. In addition, roughness on the media surface is uniform because the height of the bits is identical to the thickness of the recording layer. However, a main disadvantage of this method is that the magnetic material can easily be damaged by the ion etching patterning process. Furthermore, material removed during ion-etching can be re-deposited on the media during the patterning process, which is undesirable (Shaw et al. 2007; Kitade et al. 2004). In the second method, the substrate is lithographically pre-patterned into islands, or so-called pillars, before the magnetic material is deposited. The advantage of this method is that the magnetic material deposited on the top of the pillars is isolated from adjacent islands without the need for ion milling or other etching methods, i.e., there is no damage of the magnetic material due to processing of the pattern. However, the magnetic material in the areas adjacent to bits (trenches) can introduce undesirable noise during the read/write process and magnetic interactions between the magnetic materials deposited in the trench areas and on the top of the islands.
The RIE to remove the overfilled HSQ resist is performed using oxygen ions. Unlike the Ar atoms used for physical ion milling and removal of magnetic metal layer for patterning, the oxygen ion bombardment is much milder and hence the magnetic recording layer is not damaged. Furthermore, the top layer of the Co/Pd multilayer material to be patterned is the Pd layer, which does not react with oxygen. Therefore, the magnetic property is not affected much during the oxygen RIE process.
In order to minimize magnetic interactions of this latter type, the height of the individual pillars should be large enough so that the magnetic material deposited in the trench areas does not contribute much to the read signal picked up by the flying read/write head. The desirable height would depend on the diameter of the bit pattern as well as the spacing between the adjacent bits. The pillar aspect ratio of 1:1 or higher would be desirable. However, protruding pillars of large height exert a significant influence on the flying characteristics of magnetic heads. In fact, investigations on discrete track media disks have shown that flying of a slider over a discrete track disk may not be possible at all if the flying height loss due to the discrete tracks exceeds the design flying height of the slider on a smooth disk. The loss of flying height of a slider on BPM is anticipated to be more severe with an increase in the height (depth) of the bit pattern. To reduce topographical effects of the bit pattern (pillar height), it is necessary to fill the recessed areas with a nonmagnetic material to create a “smooth” disk. The process of filling the trench regions around protruding bits is called “planarization”. It has been reported by Hattori et al. (Hattori et al. 2004) that deposition of SiO2 by sputtering and removal of the excess SiO2 by chemical mechanical polishing (CMP) is an effective method for planarization of BPM. However, CMP is delicate and time-consuming, and damage of magnetic material could easily occur during the mechanical/chemical polishing process.
In this paper we demonstrate a simple and efficient planarization method for BPM surfaces containing nano-scale surface topography. We use hydrogen silsesquioxane (HSQ), a commonly used negative resist, to fill and planarize the nano-topographically patterned recording media, which on annealing converts to mostly silicon oxide. The HSQ can easily be spin-coated to fill the etched recess areas. In addition, the mechanical properties and thickness of HSQ can be controlled for optimum “slider flyability” by altering and/or controlling the post-deposition baking temperature and plasma treatment. After spin-coating, HSQ is back-etched by RIE to remove most of the excess material beyond the top surface of the magnetic bits. Flyability tests with planarized BPM were performed, which demonstrated excellent flying characteristics.
2 Experimental procedures
The gas flow rate, pressure, and RF power used for the RIE were 50 sccm, 100 mTorr, and 300 W, respectively.
The morphology of the BPM on a Si substrate was visualized using field emission scanning electron microscopy (SEM) and atomic force microscopy (AFM) before and after planarization.
3 Results and discussion
In preliminary tests we observed that HSQ did not provide sufficient mechanical strength to provide an acceptable tribological interface. It has been reported that the chemical structure of HSQ exhibits a ladder or cage-like arrangement which is mechanically “soft”. This structure can be transformed into a mechanically stronger, SiO2-like network structure if HSQ is exposed to oxygen plasma. (Penaud et al. 2006; Kawamori et al. 2006) Therefore, prior to flyability testing, we have applied oxygen plasma irradiation to harden and improve the density of the planarized HSQ. The BPM structure processed in this way exhibited much better flyability.
Flying height on BPM with various recess depths after planarization
Recess depth (nm)
Flying height (nm)
4 Summary and conclusions
We have fabricated nanoscale BPM by e-beam lithography and RIE. Planarization of BPM was performed using spin coating of HSQ. Plasma treatment was used to mechanically strengthen the HSQ resist, and etch back was used to remove excess material. A large diameter (65 mm) disk with nano-topographical features was fabricated using a “ball-up” technique and RIE. The flyability of magnetic recording sliders was then evaluated for planarized disks with HSQ and compared with partially planarized disks. HSQ-planarized disks exhibited significantly improved head flyability, similar to that of smooth glass disk surfaces, indicating that the read/write head flyability problem on nano patterned, high-density recording media can be substantially mitigated.
This research was supported by CMRR (Center for Magnetic Recording Research) at UC San Diego, NSF-CMMI, Nanomanufacturing Division, Grant No. 0856674, CNMT Grant No. 05K1501-01210 under the 21st Century Frontier R&D Programs, Ministry of Science and Technology, Korea, and National Research Foundation (NRF) grant through World Class University Program (R33-2008-000-10025-0).
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