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Kondo, H.; et al. “Design and Construction of Magnetic Storage Devices” Handbook of Micro/Nanotribology. Ed. Bharat Bhushan Boca Raton: CRC Press LLC, 1999 © 1999 by CRC Press LLC Part II Applications © 1999 by CRC Press LLC 12 Design and Construction of Magnetic Storage Devices Hirofumi Kondo, Hiroshi Takino, Hiroyuki Osaki, Norio Saito, and Hiroshi Kano 12.1 Introduction 12.2 Hard Disk Files Heads • Construction of the Magnetoresistive Head • The Disk • The Head-Disk Interface 12.3 Tape Systems The Recording Head • Magnetic Tapes • The Head–Tape Interface 12.4 Floppy Disk Files Floppy Disk Heads • Floppy Disks • High-Storage-Capacity Floppy Disks • Head–Floppy Disk Interface References 12.1 Introduction Magnetic recording is the most common technology used to store many different types of signals. Analog recording of sound was the first and is still a major application. Digital recording of encoded computer data on disk and tape recorders has evolved as another major use. Hard disk drives use high signal frequencies coupled with high medium speeds, and emphasize small access times together with high reliability. A third large application area is video recording for professional or consumer use. The high video frequencies are normally recorded using rotatory-head drums. Despite the availability of other methods of storing data, such as optical recording and semiconductor devices, magnetic recording media has the following advantages: (1) inexpensive media, (2) stable storage, (3) relatively high data rate, (4) high volumetric density. In principle, a magnetic recording medium consists of a permanent magnet and a pattern of remanent magnetization can be formed along the length of a single track, or a number of parallel tracks on its surface. Magnetic recording is accomplished by relative motion between a magnetic medium (tape or © 1999 by CRC Press LLC FIGURE 12.1 (a) Illustration of the recording and reproducing process. (b) Schematic of cross-sectional view showing the magnetic field at the gap. disk) against a stationary or rotatory read/write head. The one track example is given in Figure 12.1a. The medium is in the form of a magnetic layer supported on a nonmagnetic substrate. The recording or the reproducing head is a ring-shaped electromagnet with a gap at the surface facing the medium. When the head is fed with a current representing the signal to be recorded, the fringing field from the gap magnetizes the medium as shown in Figure 12.1b. For a constant medium velocity, the spatial variations in remanent magnetization along the length of the medium reflect the temporal variations in the head current, and constitute a recording of the signal. The recording magnetization creates a pattern of external and internal fields, in the simplest case, to a series of contiguous bar magnets. When the recorded medium is passed over the same head, or a reproducing head of similar construction, the flux emanating from the medium surface is intercepted by the head core, and a voltage is induced in the coil proportional to the rate of change of this flux. The voltage is not an exact replica of the recording signal, but it constitutes a reproduction of it in that information describing the recording signal can be obtained from this voltage by appropriate electrical processing. The combination of a ring head and a medium having longitudinal anisotropy tends to produce a recorded magnetization. This combination has been the one used traditionally, and it still © 1999 by CRC Press LLC FIGURE 12.2 Areal density migration of magnetic recording media. Optical media shown for comparison. dominates all major analog and digital applications. Ideally, the pattern of magnetization created by a square-wave recording signal would be like that shown in Figure 12.1a. In between recording and reproduction, the recorded signal can be stored indefinitely even if the medium is not exposed to magnetic fields comparable in strength to those used in recording. Whenever recording is no longer required, it can be erased by means of a strong field applied by the same head as that used for recording or by a separate erase head. After erasure, the medium is ready for a new recording. Overwriting an old signal with a new one, without a separate erase step, is available for writing. Figure 12.2 shows a road map of magnetic storage devices including hard disks (fixed and removable), magnetic tapes, and floppy disks and of optical storage device. The recording density has been increasing continuously over the years and a plot of logarithm of the areal density vs. year almost gives a straight line. The areal density of the hard disk is almost the same as the optical medium. For high areal recording density, the linear flux density and the track density should be as high as possible. Reproduced signal amplitude decreases rapidly with a decrease in the recording wavelength and track width. The signal loss is a function of magnetic properties and thickness of the magnetic coating, read gap length, and head–medium spacing. For high recording densities, high magnetic flux density and coercivity of a medium are needed. Regarding the materials, metal magnetic powder (MP) and a monolithic cobalt alloy thin film of higher magnetic saturation and coercivity have been launched in recent media. So as to a magnetic head, higher frequency response and sensitivity are required. It is known that the signal loss as a result of spacing can be reduced exponentially by reducing the separation between the head and medium. A physical contact between the medium and the head occurs during starting and stopping operation and a load-carrying air film is developed at the interface in the relative motion. Closer flying heights lead to undesirable collision of asperities and increased wear so that this air film should be thick enough to mitigate any asperity contacts; on the contrary it must be thin enough to attain a large reproduced signal. Thus, the head–medium interface should be designed with optimum conditions. The achievement of higher recording densities requires smoother surfaces. The ultimate objective is to use two smooth surfaces in contact for recording provided the tribological issues can be resolved. Smooth surfaces lead to an increase in adhesion, friction, and interface temperatures. Friction and wear issues are resolved by appropriate selection of interface materials and lubricants, by controlling the © 1999 by CRC Press LLC FIGURE 12.3 Schematic diagram of hard disk drive. dynamics of the head and medium, and the environment. A fundamental understanding of the tribology of the magnetic head–medium interface becomes crucial for the continuous growth of the magnetic storage industry. In this chapter materials and construction used in the modern media and heads are reviewed. Selected interesting fabrication processes of these devices are also described. 12.2 Hard Disk Files Magnetic heads for rigid disk drives are discussed in this section. Figure 12.3 shows the schematic of the rigid disk drive. A 3.5-in.-diameter disk is widely used and two to three disks are typically stacked in one hard disk drive. For very high storage density drives, up to about ten disks are stacked. Writing and reading are done with magnetic heads attached to a spring suspension. The slider surface (air-bearing surface) is designed to develop a hydrodynamic force to maintain an adequate spacing (~50 nm) between a head slider and a disk surface. The magnetic head assembly is actuated by a stepper motor or voice coil motor to access the data on the disk. The magnetic head-suspension assembly is high, and the fast access speed can be achieved. From these characteristics, hard disk drives have an advantage of fast access speed and high storage density. 12.2.1 Heads The areal density of the rigid disk drives have been increasing 60% per year; the magnetic recording head performance must be improved continuously to maintain this high growth rate of the areal recording density. The track width of the recording head must be narrower and narrower and the transfer rate © 1999 by CRC Press LLC FIGURE 12.4 The schematic diagram of the ferrite monolithic head. FIGURE 12.5 The schematic diagram of the composite head. FIGURE 12.6 The schematic diagram of the thin-film head. becomes higher and higher. The ferrite bulk head (monolithic head, Figure 12.4) and the composite MIG head (metal-in-gap head Figure 12.5) were widely used for the rigid disk drives. Since these two types of bulk recording heads are fabricated mainly by conventional machining processes, it is difficult to control a narrow track width down to 10 µm. On the other hand, thin-film inductive heads are fabricated by using the same photolithography processes that are used for semiconductor devices, which allows control of a narrow track width. The coil inductance must be reduced for the high transfer rate application. The yoke size of the monolithic head is almost the same as that of the MIG head shown in Figures 12.4 and 12.5 (Jones, 1980). Figure 12.6a shows the eight-turn thin-film inductive head and Figure 12.6b shows the slider with a thin-film head. Minimizing the total magnetic ring yoke size of the film head, the coil inductance of the thin-film head can be reduced. Film heads have an advantage of the © 1999 by CRC Press LLC FIGURE 12.7 FIGURE 12.8 SEM image of the thin-film inductive head. The schematics of the slider fabrication process. high-frequency response and reduced inductance due to a small volume of magnetic yoke and allows higher transfer rate. Very high recording density drives require the use of a magnetoresistive (MR) head which will be described later. 12.2.1.1 Structure and Fabrication Process of Thin-Film Inductive Heads Figure 12.6a shows the cross section and the planar view of the thin-film inductive head. The magnetic gap is located on the air-bearing surface (ABS). The track width is defined in the planar view. In order to achieve high magnetic yoke efficiency, the track width should be narrower compared with the width of the recessed yoke area. Figure 12.7 shows the SEM image of a thin-film head. Film heads must be deposited on a substrate for which a hard Al2O3–TiC ceramic is usually employed. With few exceptions, permalloy, which is the alloy of approximately 80 wt% Ni with 20 wt% Fe, is used for the magnetic layer of the film head, because an annealing process is not necessary to obtain the high permeability (1000 to 3000) and the low coercivity (3 to 5 Oe). As indicated in Figure 12.8, the heat-cured photoresist materials are used for insulation layers. After plating the coil layer, the surface of coil layer is not smooth; therefore, the photoresist is coated. The photoresist insulation layer also makes the surface of the upper coil layer smooth. Figure 12.8 shows the fabrication process of a thin-film head element. Several thousands of the head elements are fabricated on the same substrate at the same time. The thin-film heads are fabricated by stacking thin-film layers. First, the magnetic layer is deposited; then the coil layer and the upper magnetic layer are plated subsequently. The passivation layers are also deposited between the coil layer © 1999 by CRC Press LLC FIGURE 12.9 Schematics of the framed permalloy plating: (a) after plating permalloy; (b) after removing photoresist. FIGURE 12.10 SEM image of the photoresist frame for plating permalloy upper yoke. and both the upper and lower permalloy magnetic layers. Finally, the thick protective Al2O3 layer (30 to 50 µm) is sputtered for protecting the head element. Then the wafer is sliced into the head sliders. In a thin-film head fabrication process, permalloy and copper can be deposited by evaporation, sputtering, or plating. In a photolithography process, a deposited film is etched physically or chemically through a patterned photoresist. In a electroplating process, a material is plated only on the conductive layer. Materials cannot be plated where a conductive layer is not exposed. Figure 12.9 shows this electroplating process. First, a conductive layer is deposited on all areas of a wafer and a photoresist is coated and patterned. The patterned photoresist covers a part of a conductive layer. An electroplating material (permalloy or copper) can be plated only on the exposed area. After removing this frame, patterned permalloy or copper can be obtained in Figure 12.9. Figure 12.10 shows the SEM images of the frame of an upper permalloy layer for an electroplating. The copper and the upper permalloy layer is also plated by using a photoresist frame. This frame is patterned on a conductive layer; permalloy is plated only on the exposed area of the under conductive layer. After removing the resist frame, the patterned upper permalloy layer is obtained as shown in Figure 12.7 The top pole width is controlled by the photoresist patterning width and the track width tolerance of the upper permalloy yoke can be reduced. 12.1.1.2 Head Slider Manufacturing Process After finishing the wafer process, the wafer must be sliced into the head sliders. First, a wafer (Figure 12.11a) is sliced to a row of bars (Figure 12.11b). The sliced surface (surface A in Figure 12.11b) is lapped very carefully, because this surface will be an ABS and the head throat height is controlled through this lapping process. The throat height of thin-film head is about 1 µm, the tolerance of this row bar lapping process is required less than 1 µm. The row bar is attached to the toolings for lapping an ABS ( Figure 12.11c) of row bars. This tooling can be bent for obtaining a precise throat height for all head tips in a row bar. After finishing the throat height lapping, many row bars are aligned and the lapped surfaces are etched to make the ABS at the same time (Figure 12.11d). Recently, in order to obtain a constant flying height for all disk radii, a negative pressure air bearing has been widely used. The shape of a negative-pressure air bearing is not simple; an ion-etching process must be used to make a air-bearing © 1999 by CRC Press LLC FIGURE 12.11 The schematics of the slider fabrication process. FIGURE 12.12 head yoke. Typical domain configurations in an inductive film shape. After the ion-etching process, the head slider can be obtained by dividing a row bar into the head sliders (Figure 12.11e). 12.2.1.3 Domain Structure in a Thin-Film Head Magnetic materials are composed of individual domains with local magnetization which is equal to the saturation magnetization of the materials. Magnetic domain structure is defined by minimizing the total magnetic energy of the domain wall, the magnetic anisotropy, and the magnetostriction energy. In general, when the size of the magnetic film is reduced to several hundred microns, the magnetic domain structure becomes clear. Since the size of a magnetic yoke of a film head is almost the same size, domain structure affects the read-and-write characteristics and the stability of the film head. A typical domain structure of the upper magnetic yoke is shown in Figure 12.12. The easy axis is indicated by the arrow direction, and the magnetization direction of most domain patterns is parallel to the easy axis. The domains are separated by a 180° Bloch wall. When the magnetic easy axis is in the x-direction, a large portion of a domain aligns the x-direction. To reduce total magnetic energy, a domain whose magnetization direction aligns in the y-direction appears in the edge region, because domains of the y-direction cancel surface magnetic charges. Also magnetostriction effects must be considered for designing a film © 1999 by CRC Press LLC