Patterned Media Recording: the future technology for magnetic storage industry Xiaojun Zhang (Mechanical Engineering) Jie Wu (Physics) Final project of EE 235 course

Information Storage Hard disk drive (HDD) is one of the most important data storage media for electronics. Main advantage: 1.Big capacity. 2.Economical in terms of cost per bit. 3.Permanent (no power consuming to maintain data). HDD is NOT replaceable in modern life.

In 1999, sales of hard drives reaches US $32 billion. In 2007, the 2 biggest HD oems - Seagate and Western Digital collectively reported an annual HD revenue of nearly $17 billion. An a substantial improvement of magnetic storage could enable entirely new computing applications, with spillovers across the computer industry and every industry that uses magnetic recording to store data. Example of how fast a new technology is adopted by this industry: The market of HDD

Time: September 4, 1956 Name: IBM 305 RAMAC Capacity:5 million 8-bit characters. Size:fifty 24-inch diameter disks Areal density: 2,000 bit/in 2 In 2005, the areal density is Commercial HD: 100~150 Gbit/in 2 Toshiba (perpendicular recording): 179 Gbit/in 2 Toshiba's experimental systems: 277 Gbit/in 2 Seagate Technology demonstrates: 421 Gbit/in 2 Maximum of perpendicular recording technology: 1 Tbit/in 2 The dramatic advance of HDD

Moore’s law

The price/performance ratio in terms of cost per bit: 1965: $10,000/MB 1989: $36/MB 1994: $1/MB 2000: 2¢/MB 2004: 0.1¢/MB 2009: 0.01 ¢/MB Price of HDD, DRAM and Flash

Trends dominated by technology

The future of record media Our focus today

Hard Disk Drive

Magnetic Recording Fundamentals Magnetic field

Two Problems in Magnetic Recording Thermal Stability Issue Transition Jitter Noise

Average Thermal Energy is k B T (K B is the Boltzmann’s constant) T normally is room temperature, ~ 300K Energy barrier to switch a domain is K u V (V is the volume of the domain; K u is the anisotropy constant of the material. Higher K u means higher writing magnetic field) K u V/k B T demtermines the thermal stability. Normally it should be larger than 60. Thermal Stability Issue Magnetic Domain Thermal Fluctuation Induced Magnetization Switch

Transition Jitter Noise Transitions meanders between random grains. This transition jitter causes noise. More grains at the boundary can make the transition smoother, and thus reduce noise. Normally, for each bit cell, there must be 100 or more grains to get good signal-to- noise ratio (SNR). Transition boundary

Longitudinal Media Recording Before 2005, HDDs were made by longitudinal recording. However, as the bit size becomes smaller and smaller, thermal instability becomes a problem. (K u V/k B T) The magnetization of each bit is directed along the disk surface. This head-to-head or tail-to-tail structure makes them unstable against thermal fluctuation. Since it uses fringing field, which is normally smaller than gap field. Materials with high K u can not be used.

Perpendicular Media Recording The first commercially available disk drive with a diameter of 1.8" was produced by Toshiba in Soon after that in January 2006, Seagate Technology began its first laptop sized 2.5-inch hard drive. Most recently in February 2009 Seagate Technology announced the first 7200 rpm 2.0 Terabyte Hard Drive using PMR technology.

Perpendicular Media Recording The bits in perpendicular recording are magnetized up or down perpendicular to the disk surface. With the combination of soft magnetic underneath, perpendicular recording technology realized the use of gap field. Materials with higher K u can be used to circumvent the thermal instability problem. (K u V/k B T) Recording Layer Soft Magnetic Layer

Patterned Media Recording Comparison of conventional media recording with patterned media recording (from Hitachi)

Patterned Media Recording In conventional recording techniques, if we increase grain volume V, the noise due to transition jitter will increase accordingly. In patterned media recording, the magnetic bits are perfectly patterned and isolated from each other. Therefore the jitter problem can be reduced. Each island is a single magnetic domain. Patterned media is therefore thermally stable. Since we only need one grain for each bit instead of 100 grains, the areal density can be increased roughly by 100 times with the same thermal stability.

Patterned Media Recording For the same areal density, we can get better thermal stability with patterned media recording. Typical design for a patterned media recording

Nano-fabrication approaches Optical lithography poor spatial resolution Focused Iron Beam not suitable for massive production E-beam lithography low throughput Block copolymer lithography not uniform in big scale

A candidate approach: E-beam lithography +block copolymer assembly

Encouraging result: improved uniformity & 4 times density multiplication

A key step towards massive production A. Cr patterns B. Si patterns Patterns obtained after pattern transfer

Summary HDD experience dramatic development for the last 50 years and will keep this trend as Moore’s law requires. The development of HDD is generated by the emerging and adoption of new technologies. Patterned Media Record is the future technique to replace longitudinal and perpendicular media recording. Nano-fabrication technique is the key to realize our goal.