Nano-fabrication of Magnetic Recording Media

1. Nano-fabrication of Magnetic Recording Media Wesley Tennyson Engineering Physics Ph.D. Candidate Homer L. Dodge Dept. of Physics and Astronomy at The University of Oklahoma Presented for—Fundamentals of Nanotechnology: From Synthesis to Self-Assembly

2. Outline • Motivation • Nano-Fabrication Essentials • High density dots are not enough • Current Technology • Perpendicular media • Patterned Creation • Lithography • Guided self-assembly • Imprint lithography • Langmuir-Blodgett • Aperture array lithography • Summary areal density = bit density x track density J. Phys. D: Appl. Phys. 35 (2002) R157-R161.

3. Motivation 40% growth rate of areal density  ~700 Gbits / in2 by 2011 Superparamagnetic Effect limits continued reduction of grain size below d ~ 20nm. Patterned nanoparticles or patterned media (PM) avoids this problem. PM can have higher track and linear densities. Nanoparticles typically have only one magnetic domain  better signal to noise With patterned media 1 Tbit/in2 may be achieved. (Left) AFM image of a typical Fe dot array fabricated using alumina mask anodized at 40 V. The standard deviation of the dot height is about 4 nm. Chang-Peng Li et. al., Appl. Phys. 100, (2006) 074318 (Right) a Typical SEM image of Fe dot array fabricated using alumina mask anodized at 40 V with average diameter and periodicity of 67 and 104 nm, respectively; b typical SEM image of Fe dot array fabricated using alumina mask anodized at 25 V with average diameter and periodicity of 32 and 63 nm, respectively.

4. Nanofabrication Essentials • Bit feature fidelity (uniform diameter) • Incredibly high density (> 40 nm period) • Uniform coverage over a large area Additionally mechanical requirements: • Arranged in circular array • Long range order!! Cheaper M. Geissler and Y. Xia, Adv. Mat. 16 (2004) 1249. A. Moser. et.al., J. Phys. D: Appl. Phys. 35 (2002) R157-R167. J. Phys. D: Appl. Phys. 35 (2002) R157-R161.

5. Current Technology: Perpendicular Media • Thermally stable at smaller sizes • Easy-axis oriented out-of plane deposited on soft underlayer • Higher signal to noise • Increased read back signal • Underlayer coupling increased Other recent advances • TAC– Thermally assisted recording • AFC– antiferromagnetically coupled media (Above) Schematic representation of a magnetic transition in AFC media. J. Phys. D: Appl. Phys. 35 (2002) R157-R161.

6. Pattern Creation: Lithography • Interference lithography—feature size down to 100 nm • Interference Patterned defined by lithography • Pattern fully transferred after reactive ion etching • Feature sizes are too large for discrete bits C.A. Ross, J. Appl. Phys. 91, (2002) 6848.

7. Pattern Creation: Guided self-assembly • Block copolymers have good short range order but lack long range order Solution— • Interference lithography defines trenches, ensuring long range order • Block copolymer is deposited by spin casting into shallow grooves • Reactive Ion Etching completes the pattern transfer Appl. Phys. Lett. 81, (2002) 3657. J. Phys. D: Appl. Phys. 38 (2005) R199-R222.

8. Pattern Creation: Imprint Lithography • A stamp defines the pattern • Typical material polydimethysiloxane (PDMS): low adhesion and high elasticity • But PDMS is not rigid enough for nano-scale Solution: use PDMS as an anti-adhesion layer on a rigid substrate • Immune to most resolution limits • Feature Sizes on the order of ~100nm J. Vac. Sci. Technol. B. 15(6) (1997) 2897. Adv. Mater. 18 (2006) 3115-3119.

9. Pattern Creation: Langmuir-Blodgett • Layer-by-layer technique • Single or sub-monolayers can be deposited one at a time • Deposition occurs as the substrate is drawn through the film on liquid • Mono-dispersed spheres were transferred to PDMS stamps via LB • Short range order is still problematic (left) TEM of Langmuir-Blodgett film (right) SEM of patterned μ-dot arrays (below) AFM of μ-dot arrays J. Am. Chem. Soc. 125, (2003) 630-631.

10. Pattern Creation: Aperture Array Lithography J. Membrane Sci. 249, (2005) 193 – 206.

11. Summary • Superparamagnetism places a lower limits on the thin film bit size • Areal densities larger than 1 Tbit per inch2 will be in hard drives only if: • The manufacturing requirements can be met: bit feature fidelity, incredibly high density (> 40 nm period), uniform density over a large area, long range order and arranged in circular array • New techniques cost less than the established • Nano-patterning of nanoparticles may be the solution <http://www.hitachigst.com/hdd/research/ recording_head/pr/PerpendicularAnimation.html> (or search for get perpendicular) Outlook • As of Oct. 17, 2007 – Maximum areal density achieved by Western Digital with 520 Gbits per inch2. Followed by Seagate with 421Gbits per inch2 (as of Sept. 18 2006). • Typical Hard drives have 200 Gbits per in2, as featured in WD's 250 GB WD (available since May 2006)

14. Additional Notes AVS 54th International Symposium    Nanomanufacturing Topical Conference Wednesday Sessions       Session NM-WeM Invited Paper NM-WeM11 Nano-fabrication of Patterned Media Wednesday, October 17, 2007, 11:20 am, Room 615 Session: Nanomanufacturing for Information Technologies Presenter: T.-W. Wu, Hitachi Global Storage Technologies The outlook of magnetic storage technology predicts that, with current 40% growth rate, the recording areal density will hit ~700 Gbits/in2 in 2011. However, the magnetic recording physics also predicts that perpendicular magnetic recording (PMR) media will hit the thermal instability limit as the grain size of the magnetic coating scaled down below ~5nm in diameter. Because patterned media (PM) leverages the geometric decoupling magnetic exchange, a magnetic material even with ultra-small (e.g. d<5nm) but strong magnetically coupled grains can still be utilized to constitute the required recording bit (d=10~15nm) and avoid the thermal instability. Furthermore, because of its geometrically defined bit border, PM can achieve both higher track and linear densities than does the continuous media and hence boost the aerial density. As a disruptive magnetic recording technology, PM is viewed as one of the most promising routes to extending magnetic data recording to densities of 1 Tbit/in2 and beyond. The fabrication of PM disk starts with the imprint master mold creation followed by pattern replication by nano-imprinting, pattern transfer by reactive ion etch and finished with blank deposition of a magnetic coating. The key challenges in the PM substrate fabrication are how to create those nano-scaled features (e.g. pillars with 20nm in diameter) with acceptable fidelity? How to create them with an incredibly high density (e.g. a square lattice with less than 40nm in period) in a very large area (e.g. ~2 square inches) and also within a reasonable time frame? How to inspect them with a reasonable statistics basis? In addition, those features need to be arranged in a circular array and have a very stringent long range order as well. Although the physical feasibility at each critical stage has been demonstrated to a degree in the recent years, to ensure a manufacturing feasibility for the production of patterned disk substrates, the process robustness and reliability, parts longevity, high throughput tooling and low cost operation, etc. are still far from completion and remain as immense challenges. In order to achieve the goal of PM hard disk drive (HDD) production in 2011 time frame, many scientific innovations and technology advances, such as the r-θ e-beam machine, guided self-assembly patterning, double-side high throughput imprinting and RIE, etc. are critically needed.

15. Nano-Fabrication Essentials: Extras J. Phys. D: Appl. Phys. 38 (2005) R199 – R222. B D Terris and T Thomson J. Physics D: Applied Physics 38 (2005) R199-R222.