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
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