This paper reviews the state of the head-disk interface (HDI) technology, and more particularly the head-medium spacing (HMS), for today’s and future hard-disk drives. Current storage areal density on a disk surface is fast approaching the one terabit per square inch mark, although the compound annual growth rate has reduced considerably from ~100%/annum in the late 1990s to 20–30% today. This rate is now lower than the historical, Moore’s law equivalent of ~40%/annum. A necessary enabler to a high areal density is the HMS, or the distance from the bottom of the read sensor on the flying head to the top of the magnetic medium on the rotating disk. This paper describes the various components of the HMS and various scenarios and challenges on how to achieve a goal of 4.0–4.5？nm for the 4？Tbit/in2 density point. Special considerations will also be given to the implication of disruptive technologies such as sealing the drive in an inert atmosphere and novel recording schemes such as bit patterned media and heat assisted magnetic recording. 1. Introduction As the areal density of commercial hard disk drives is quickly approaching the terabit per square inch milestone [1–5] (Figure 1), the need to improve the reliability of the head-disk interface (HDI) and to further decrease the head-medium spacing (HMS) is becoming eversmore critical [3, 6, 7]. Low HMS is a necessary enabler to good writability as well as strong read-back signal integrity [8, 9]. It is estimated that the HMS will soon need to cross the 7？nm mark in order to reach this terabit per square inch density point [2, 6]. It is remarkable to realize that the error rate of the stored digital signal that is being read back improves approximately by about 2x for every 0.3–0.5 nanometer of decreased HMS. In addition to relentless demand for novel, ultrathin protecting films of overcoat and lubricant, and subnanometer air gap between the disk and the head, alternative recording technologies presently being contemplated involve heating the disk to over 500°C (heat-assisted magnetic recording or HAMR) [10–12] and/or physically isolating magnetic bits on small islands of sub-30？nm in physical dimensions (bit-patterned recording or BPR) [13–16]. Figure 1: Areal density evolution of HDD and flash memory. After Grochowski [ 17], with permission. In this paper, the roadmap to an areal density of 4 terabits per square inches will be discussed. Particular emphasis will be given to the various spacing components that comprise the HMS budgetand their physical limits. The various implications of recording
C. M. Mate, Q. Dai, R. N. Payne, B. E. Knigge, and P. Baumgart, “Will the numbers add up for sub-7-nm magnetic spacings? Future metrology issues for disk drive lubricants, overcoats, and topographies,” IEEE Transactions on Magnetics, vol. 41, no. 2, pp. 626–631, 2005.
X. Yang, S. Xiao, W. Wu et al., “Challenges in 1 Teradotin. 2 dot patterning using electron beam lithography for bit-patterned media,” Journal of Vacuum Science & Technology B, vol. 25, no. 6, pp. 2202–2209, 2007.
M. Suk, K. Miyake, M. Kurita, H. Tanaka, S. Saegusa, and N. Robertson, “Verification of thermally induced nanometer actuation of magnetic recording transducer to overcome mechanical and magnetic spacing challenges,” IEEE Transactions on Magnetics, vol. 41, no. 11, pp. 4350–4352, 2005.
K. Miyake, T. Shiramatsu, M. Kurita, H. Tanaka, M. Suk, and S. Saegusa, “Optimized design of heaters for flying height adjustment to preserve performance and reliability,” IEEE Transactions on Magnetics, vol. 43, no. 6, pp. 2235–2237, 2007.
T. Shiramatsu, M. Kurita, K. Miyake, et al., “Drive-integration of active flying-height control slider with micro thermal actuator,” IEEE Transactions on Magnetics, vol. 42, no. 10, pp. 2513–2515, 2006.
J. Itoh, Y. Sasaki, K. Higashi, H. Takami, and T. Shikanai, “An experimental investigation for continuous contact recording technology,” IEEE Transactions on Magnetics, vol. 37, no. 4, pp. 1806–1808, 2001.
L. Wu and F. E. Talke, “Modeling laser induced lubricant depletion in heat-assisted-magnetic recording systems using a multiple-layered disk structure,” Microsystem Technologies, vol. 17, no. 5-7, pp. 1109–1114, 2011.
Y. S. Ma, L. Gonzaga, C. W. An, and B. Liu, “Effect of laser heating duration on lubricant depletion in heat assisted magnetic recording,” IEEE Transactions on Magnetics, vol. 47, no. 10, pp. 3445–3448, 2011.
W. D. Zhou, Y. Zeng, B. Liu, S. K. Yu, W. Hua, and X. Y. Huang, “Evaporation of polydisperse perfluoropolyether lubricants in heat-assisted magnetic recording,” Applied Physics Express, vol. 4, no. 9, Article ID 095201, 3 pages, 2011.
B. E. Knigge, Z. Z. Bandic, and D. Kercher, “Flying characteristics on discrete track and bit-patterned media with a thermal protrusion slider,” IEEE Transactions on Magnetics, vol. 44, no. 11, pp. 3656–3662, 2008.
G. G. Wang, X. P. Kuang, H. Y. Zhang, et al., “Silicon nitride gradient film as the underlayer of ultra-thin tetrahedral amorphous carbon overcoat for magnetic recording slider,” Materials Chemistry and Physics, vol. 131, pp. 127–131, 2011.
C. Y. Chan, K. H. Lai, M. K. Fung, et al., “Deposition and properties of tetrahedral amorphous carbon films prepared on magnetic hard disks,” Journal of Vacuum Science & Technology A, vol. 19, pp. 1606–1610, 2001.
B. K. Yen, R. L. White, R. J. Waltman, C. Mathew Mate, Y. Sonobe, and B. Marchon, “Coverage and properties of a-SiNx hard disk overcoat,” Journal of Applied Physics, vol. 93, no. 10, pp. 8704–8706, 2003.
Y. Hijazi, E. B. Svedberg, T. Heinrich, and S. Khizroev, “Comparative corrosion study of binary oxide and nitride overcoats using in-situ fluid-cell AFM,” Materials Characterization, vol. 62, no. 1, pp. 76–80, 2011.
F. Rose, B. Marchon, V. Rawat, D. Pocker, Q. F. Xiao, and T. Iwasaki, “Ultrathin TiSiN overcoat protection layer for magnetic media,” Journal of Vacuum Science & Technology A, vol. 29, Article ID 051502, 11 pages, 2011.
M. L. Wu, J. D. Kiely, T. Klemmer, Y. T. Hsia, and K. Howard, “Process-property relationship of boron carbide thin films by magnetron sputtering,” Thin Solid Films, vol. 449, no. 1-2, pp. 120–124, 2004.
M. A. Samad, E. Rismani, H. Yang, S. K. Sinha, and C. S. Bhatia, “Overcoat free magnetic media for lower magnetic spacing and improved tribological properties for higher areal densities,” Tribology Letters, vol. 43, pp. 247–256, 2011.
E. Rismani, S. K. Sinha, H. Yang, and C. S. Bhatia, “Effect of pretreatment of Si interlayer by energetic C+ ions on the improved nanotribological properties of magnetic head overcoat,” Journal of Applied Physics, vol. 111, Article ID 084902, 10 pages, 2012.
X. C. Guo, B. Knigge, B. Marchon, R. J. Waltman, M. Carter, and J. Burns, “Multidentate functionalized lubricant for ultralow head/disk spacing in a disk drive,” Journal of Applied Physics, vol. 100, no. 4, Article ID 044306, 2006.
D. Gonzalez, V. Nayak, B. Marchon, R. Payne, D. Crump, and P. Dennig, “The dynamic coupling of the slider to the disk surface and its relevance to take-off height,” IEEE Transactions on Magnetics, vol. 37, no. 4, pp. 1839–1841, 2001.
Z. Jiang, M. M. Yang, M. Sullivan, J. L. Chao, and M. Russak, “Effect of micro-waviness and design of landing zones with a glide avalanche below 0.5 " for conventional pico sliders,” IEEE Transactions on Magnetics, vol. 35, no. 5, pp. 2370–2372, 1999.
N. Tagawa, H. Tani, and K. Ueda, “Experimental investigation of local temperature increase in disk surfaces of hard disk drives due to laser heating during thermally assisted magnetic recording,” Tribology Letters, vol. 44, pp. 81–87, 2011.
N. Tagawa, H. Andoh, and H. Tani, “Study on lubricant depletion induced by laser heating in thermally assisted magnetic recording systems: effect of lubricant thickness and bonding ratio,” Tribology Letters, vol. 37, no. 2, pp. 411–418, 2010.