Magnetoresistive Heads Research Articles

E-beam writing: a next-generation lithography approach for thin-film head critical features

Magnetic recording areal densities using magnetoresistive (MR) head technology are increasing at annual rates in excess of 60% per year and in some applications as great as 100% per year. Today, the critical features of the thin-film head can be patterned with the same optical lithography techniques used by the semiconductor industry. However, with the assumption that areal density increases in magnetic recording are maintained, within the next rive years, thin-film head lithography requirements will exceed semiconductor roadmap projections. This paper describes the use of e-beam lithography as an alternative to optical lithography for the fabrication of critical features in the thin-film head. In particular, the unique geometry of the head structure, the low density of critical head features on the wafer, and the relatively low wafer volume requirements for thin-film heads allow e-beam lithography to become a viable manufacturing alternative for thin-film head fabrication. More important, e-beam lithography is ideally suited for producing thin-film head structures at a development level prior to the optimization of manufacturable optical processing.

Current progress of single-pole-type GMR heads for perpendicular recording

We studied several proposals to address the problems encountered when single-pole-type giant magnetoresistive (SPT-GMR) heads are applied to an actual recording system. The first area of problems was regarding side erasure when there is a head skew. We fabricated a trapezoid-shaped main pole using a focus-ion-beam trimming or using a new wafer-processing technique and examined their write performances and their effects on reducing side erasure. The second area of problems concerned the stray field robustness of SPT-GMR heads when combined with a double-layered perpendicular recording medium. We tried to optimize the robustness and write performance by changing the width of a yoke in the main pole or by positioning a shield above the main pole. The stray field robustness and write performance were adequate in both cases, but a tradeoff relationship exists between both performances.

Fabrication and electrical properties of lapped type of TMR heads for ∼50 Gb/in/sup 2/ and beyond

Tunnel giant magnetoresistance (TMR) heads at /spl sim/50 Gb/in/sup 2/ have been fabricated using improved lapping process. Together with writer of 2.0 T pole materials, recording performance has been demonstrated at magnetic write width of 0.28 /spl mu/m and magnetic read width of 0.18 /spl mu/m. The resistance area product of final wafer data is around 5 /spl Omega//spl middot//spl mu/m/sup 2/, with lead and contact resistance included, resulting in a final head resistance of around 200 /spl Omega/. The output voltage achieved for 1 mA bias current is 42 mV//spl mu/m, and the isolated pulses are stable. With a discrete preamplifier, the track density as measured by 747 curve is 94 kTPI at a bit-error rate of 10/sup -4/ and the linear density is 508 kBPI, achieving an areal density of 48 Gb/in/sup 2/. The noise analysis reveals that the noises come mainly from media and shot noise.

11.5 Gb/in/sup 2/ recording using spin-valve heads in tape systems

We have investigated recording at densities higher than 4.5 Gb/in/sup 2/ in tape systems using spin-valve heads. We report the signal-to-noise ratio at areal recording densities of 4.5 Gb/in/sup 2/ and 11.5 Gb/in/sup 2/ on low-noise obliquely metal evaporated (ME) tapes. Also, we studied an electrostatic discharge failure of giant magnetoresistive heads on ME tape. The results indicate the feasibility of an areal density of 11.5 Gb/in/sup 2/ in a helical-scan tape system.

Sensitivity and rejection capability of thermal asperities in a hard disk drive

With the use of giant magneto-resistive (GMR) head, thermal asperity (TA) has been a big concern in drive industry. In this study, we investigated several heads and disk related factors that affect the TA sensitivity of the drive. TA experiments were conducted by introducing the particles in the drives using a particle injection chamber. It is found that the slider ABS shape can help to reduce TA or contamination at the head/media interface. However, TA sensitivity of the drive mainly depended on the intrinsic property of GMR sensor. TA sensitivity of head increases as the bias current increases. GMR head is much less sensitive to TA compared with MR head. However, if similar bias current is applied to both MR and GMR heads, TA sensitivity of GMR head became almost identical to that of MR head. TA sensitivity of different types of disks was also studied and the dominant factor affecting the TA sensitivity is observed to be the scratch resistance of the carbon overcoat layer.

Development of gold to gold interconnection flip chip bonding for chip on suspension assemblies

Gold to gold interconnection (GGI) flip chip bonding technology has been developed to bond the drive IC chip on the integrated circuit suspension used in hard disk drives. GGI is a lead free process where the Au bumps and Au bond pads are joined together by heat and ultrasonic power under a pressure head. The use of GGI flip chip assembly process will help to eliminate equipment parts and processing steps of the traditional flip chip C4 process and hence shortens the overall cycle time. With the integrated circuit suspension design, it becomes possible to assemble the drive IC chip close next to the magneto-resistive head slider on the suspension. This paper describes a flip chip bonding method joining the drive IC chip on integrated circuit suspension with GGI bonding. The reliability evaluations are concentrated on thermo-mechanical analysis, robustness and functional performance of the final assembly. GGI bonding for chip on suspension application is still relatively new and has not been achieved for volume use. Work is still being done to establish and extend the limits of the technology with regard to long term reliability.

Dependence of electromigration-induced failure lifetimes on NiFe thin-film thickness in giant magnetoresistive spin-valve read heads

The physical mechanisms responsible for the dependence of electromigration-induced failure lifetimes on NiFe thin film thickness in giant magnetoresistive spin-valve read heads have been studied to determine the maximum allowable current density. Based on measured median-times-to-failure, a maximum current density of about 1–2×108 A/cm2 was found to be safely used in 3–5 nm thick NiFe films. Grain size analyses using a transmission electron microscopy suggests that the longer lifetimes of thinner films are mainly due to the smaller number of grain boundaries and fewer triple points which result in less atomic flux divergence.

The effect of Pd buffer layers on magnetic exchange energy of Co/Pd multilayered films for perpendicular magnetic recording

A Co/Pd multilayered film is one of the promising perpendicular magnetic recording (PMR) media with high magnetic squareness, due to interfacial magnetic anisotropy energy. However, the lack of a magnetic decoupling factor in the Co/Pd film, unlike a compositionally decoupled CoCr based alloy film, possesses large magnetic exchange energy which results in large magnetic domain sizes and high noise source. In this research, we investigate a domain isolation parameter ( α) to control the magnetic exchange energy and domain sizes in terms of the deposition pressures of a Co/Pd recording multilayer and a Pd buffer layer in a PMR disk. The high pressure in the Pd buffer layer is effective to isolate and reduce the Co/Pd domain sizes with a low α value. The observations with a magnetic force microscope (MFM) and a cross sectional transmission electron microscope (TEM) prove that the physically decoupling effect originated from the surface roughness of the Pd buffer layer, plays a major role in creating a fine domain pattern in the Co/Pd multilayer. The dynamic read/write performances of the Co/Pd multilayered disk by a merged giant magneto resistive (GMR) head show that the low α value disk with magnetically decoupled grains has a low medium noise level, compared with the high α value disk.

Estimation of the Head Sensitivity Function in Scanning Magnetoresistance Microscopy

We apply Bayesian image analysis techniques to a problem in a newly developed scanned probe technology that uses commercial magnetoresistive (MR) record and playback heads as probes to sense magnetic fields. This technology can be used for magnetic imaging and for evaluating playback and record processes in magnetic recording. In MR microscopy, an MR head is raster scanned while in physical contact with a magnetic sample (e.g., hard disk media, tape, or fine magnetic particles). By plotting the MR resistance as a function of position, a very high resolution (on the order of .1 × 1.0μm) magnetic image of the sample is constructed. This case study focuses on characterizing the head sensitivity function (HSF), which depends on the physical dimensions and the magnetic properties of the MR head. These sensitivity functions are of great practical interest because they ultimately relate to the head's performance in a high-density data storage environment. Estimating the HSF requires a deconvolution that has features that prevent the problem from being straightforward: both the HSF and the source being scanned are unknown, and there is a substantial amount of correlated noise in the scanned image. We take a Bayesian approach to model and estimate the HSF, while accounting for noise and other nuisance effects such as thermal drift. Besides yielding a point estimate, which is a fairly difficult task here, this approach also quantifies uncertainty so we can assess whether certain features of the estimated head sensitivity function appear to be genuine.

Electromigration-induced failure of single layered NiFe Permalloy thin films for a giant magnetoresistive read head

Electromigration-induced failures have been investigated in single layered NiFe Permalloy thin films, which are used in giant magnetoresistance (GMR) read heads in high-density magnetic recording disk drives. The time-to-failure and median-time-to-failure (MTTF, t50) of NiFe thin films were found to be strongly dependent on the applied current density, film thickness, and ambient temperature. The activation energy (ΔQ) of NiFe films derived from the “Black equation” was found to be 0.8 eV. The MTTF and standard deviation (σ) of a NiFe thin film electromigration test stripe with 10 nm thickness, 5 μm width, and 20 μm length were 3.74 and 1.74 h, respectively, at 160 °C ambient temperature. Typical electromigration failures such as “voids and cracks” were observed at the cathode or center region of NiFe film test stripes. These results suggest that the electromigration-induced failures are one of the crucial factors in determining the reliability of GMR spin-valve read heads used in high-density magnetic recording disk drives.

High-density reproduction characteristics with MR head for rotary drum in helical-scan tape system

A magneto-resistive (MR) head for a rotary drum in a helical-scan tape system has been developed to realize a high-capacity tape storage system with a high data transfer rate that uses a small cassette. Also, various experimental ME (Metal Evaporated) tapes whose magnetic layer thickness is 0.1 /spl mu/m or less (thin-magnetic-layer ME tapes) for the MR head has been fabricated. Reproduction characteristics of the MR head with the ME tapes was evaluated. The output voltage of the MR head was 20 times that of a conventional inductive head. An areal recording density of 1.2 Gbit/inch/sup 2/ was found to be possible combining the MR head with the thin-magnetic-layer ME tapes whose magnetic layer thickness was 0.09 /spl mu/m.

Thermal asperity of TMR heads for removable disk drives

The thermal asperity of the tunneling magnetoresistive (TMR) head was studied using Zip-type flexible media, and compared with that of the anisotropic magnetoresistive (AMR) head. The examined head had a shielded structure with the TMR element close to the air bearing surface. Nevertheless, it generated relatively small thermal asperity even when the mechanical spacing between the head and medium was less than 20 nm, at which large and frequent thermal asperity was observed when the AMR head was used. Such relatively small thermal asperity of the TMR head Is attributed mainly to the small low-temperature coefficient of the electrical resistivity of the TMR element.

Analysis of tunneling magnetoresistance test structures by low energy electron nanoscale-luminescence spectroscopy

The performance of state-of-the-art tunneling magnetoresistive (TMR) heads depends on the thickness of insulating layers less than a few nanometers thick that separate two magnetic films. We have used low energy electron nanoscale-luminescence (LEEN) spectroscopy to observe optical emission from TMR test structures with buried insulating oxides less than a few nanometers thick. TMR structures grown by metal evaporation consisted of a 0.8–3 nm Al oxide layer on a 2.4 nm CoFe alloy (84:16) on a multilayer metal-on-Si substrate, all capped with a 0.4 nm CoFe with Pt overlayer. LEEN studies with excitation energies ranging from 0.5 to 3 keV enabled emissions from the buried oxide layers versus the free surface to be distinguished. We used different compositions, thicknesses, and oxidation exposures to separate Al oxide from transition metal oxide emissions, as well as from the ambient-exposed Pt surface. A broad peak centered at 2.2–2.5 eV increased with O plasma exposure to Al films on the CoFe alloy. Emission from oxidized CoFe without Al consists of a broad emission centered at 2.5 eV. Common to all these spectra is emission at 1.8 eV, which energy-dependent LEEN demonstrates is due to the ambient-exposed Pt. Spectral changes versus oxygen exposure reveal the regime separating oxidation of the magnetic and nonmagnetic layers.

Deposition and properties of tetrahedral amorphous carbon films prepared on magnetic hard disks

The areal density of information stored on the hard disk has doubled every two years. This substantial increase in disk storage has resulted from the application of giant magnetoresistance heads, new thin film media, and better electronic recording channels. However, such an increase cannot be easily attained without reducing the separation between the magnetic read-write head and magnetic recording medium surfaces. This can be achieved by using a thinner protective overcoat. In this study, ultrathin tetrahedral amorphous carbon (ta-C) films were deposited on magnetic hard disks (CoCrTa/Cr/NiP/Al–Mg) by a magnetic filtered cathodic arc deposition under different substrate bias voltages. The obtained films exhibited smoother surfaces than the uncoated disks as indicated by the atomic force microscopic measurement. The Raman spectra acquired showed a single asymmetric Lorentzian curve shape. Tetrahedral amorphous carbon coatings were subjected to an accelerated corrosion test in vapors of concentrated hydrochloric acid for 24 h. The corrosion test demonstrated a reduced density of corrosion sites when compared to conventional diamond-like carbon films. Similarly, the scratch resistance of the ta-C coated disks, investigated by a nanoindenter, showed significant improvement in comparison to uncoated disks.

Read performance of tunneling magnetoresistive heads

We discuss the read performance, including the off-track performance and noise characteristics, of TMR heads. A vortex-like sense current field in TMR heads creates insensitive zones in the free layer and affects the output voltage and effective read width. The 1/f noise in the MHz-order frequency range was observed. To achieve a low-noise TMR head for high-density recording with small junction area, a high-quality thin barrier is a key to obtaining low 1/f noise, as well as small R/spl middot/A products to reduce the shot noise.

Micro-magnetic and electric analysis on MR head baseline popping and instabilities

A general explanation for four types of instabilities in magneto-resistive (MR) recording heads is given. A micro-magnetic analysis of the root causes of the instabilities shows that baseline popping (BLP), permanent magnet reversal instability (PMRI), spiking noise and writer induced instability have the same root cause as random telegraph noise (RTN) which was reported previously. The details and differences in the formation of each individual phenomenon by the same cause have been investigated in this paper. An MR reader and pre-amplifier electric circuit simulation confirms that baseline popping is due to random telegraph baseline shift passing through a pre-amp DC blocking capacitor which is discharged and recharged at the two levels of the baseline.

Longitudinal bias method using a long distance exchange coupling field in tunnel magnetoresistance junctions

A longitudinal bias method using a long distance exchange coupling field, which was observed between an IrMn layer and a NiFe/CoFe bilayer across a Cu interlayer, has been investigated for magnetic read heads. The exchange coupling field could be changed in the layered film consisting of Ta/NiFe/Cu(0, 1.0, 1.5, 2.0 nm)/IrMn. In order to align the easy axes of the free and pinned ferromagnetic layers perpendicular to each other, a two-step annealing process was carried out to rotate the easy axis of the free layer in the tunnel magnetoresistive read heads. The angle between the two easy axes was 72° in the junction after an optimized two-step annealing. The tunnel magnetoresistive readers, which have a resistance×area product of about 2000 Ω μm2 and a tunnel magnetoresistance ratio of about 15%, showed an output voltage of 9080 μVp−p and 2270 μVp−p/μm, which was comparable to that of a typical giant magnetoresistive reader for a 7.5 Gbits/in.2 hard disk drive.

Scanning magneto-resistance microscopy with FIB trimmed yoke-type magneto-resistive tape heads

Scanning magneto-resistance microscopy has been performed with thin film yoke-type magneto-resistive tape heads possessing eight channels. The read flux guides of these channels have been trimmed down from 24 μm to widths varying between 5.5 μm and 148 nm by focused ion beam milling with Ga + ions. Tracks written on ME tape with an untrimmed write channel have been successfully imaged with all the trimmed channels. A significant attenuation (>5%) of read-back voltage across the MR sensor is only observed for channels possessing flux guides trimmed by 77% to 5.5 μm. A 52% (∼6 dB) drop in read-back voltage is observed for a channel possessing a flux guide trimmed by 99.4% to 148 nm.

Thermal modeling of magnetoresistive heads in three dimensions

We present an analytical model that can describe the dissipation of thermal energy generated by Joule heating in magnetoresistive heads. The model is simple in that it requires no finite-element analysis and can be quickly configured to investigate the effects of varying head characteristics. However, despite its simplicity, the model can quantify heat transfer through all surfaces of the sensor effectively, as demonstrated by comparison with experimental data. With this model, the effects of varying system geometry, materials, bias current, environment, and defects can be investigated, Analysis of these effects identifies parameters to which Joule heating is most sensitive. We discuss promising areas for device improvement, such as increasing the gap thermal conductivity and decreasing contact resistance at interfaces.

Comparisons of standard and modified perpendicular magnetic recording disks with ring head writing

To overcome the superparamagnetic effect in longitudinal magnetic recording (LMR) technology, the combination of perpendicular magnetic recording (PMR) media by writing with an inductive ring head has been suggested as one of the candidates to overwhelm the thermal fluctuation of the recorded magnetization. This has specially attracted attention on its practical application to hard disk drives (HDD), because the conventional LMR head can apply to new PMR disks. In this research, the four types of standard single and double, and modified thin single- and pseudo-double-layered PMR disks are fabricated and evaluated in terms of crystallographic and magnetic properties. Read/write performances of the modified disks by a merged giant magneto resistive (GMR) head show superiority in signal-to-noise ratio (SNR) to those of the standard disks.