Thin Film Magnetic Recording Heads Research Articles

(Invited) New Developments in Magnetic Inductors for On-Chip Power Conversion, Including Fabrication

Integrated dc-dc power converters have been the subject of intense research interest over the past 10 – 12 years because they offer so many potential benefits, not just in promising savings of up 20% of board space, but also enabling control of individual chip voltages and delivering power at high voltage and low current, thus reducing power train I2R loss. Among the various power converter options, the inductive Buck converter is the most attractive because it can deliver a wide output voltage range with an expected high efficiency (≥ 90%). However, for on-chip applications, the inductive Buck converter is the least developed power converter, there being at present no such inductor ready for use in a standard CMOS back-end-of-line (BEOL) fabrication facility. Major strides towards the realization of on-chip power conversion have been made by IBM in case of its POWER8TM processor (1), and by Intel in case of its 4th generation (Haswell) processor (2), which uses near-chip- integrated power converters; although neither company’s new power converter used an integrated magnetic inductor. After highlighting the important role that magnetic inductors are expected to play in on-chip power conversion, this talk will describe new developments in magnetic materials for inductors, especially those employed at IBM, and the fabrication challenges faced in integrating these materials in inductors. Yoke material and thickness (typically 1 - 5 microns) can be tailored to reach desired inductance values (i.e., 1 – 30 nH) in thin-film ferromagnetic inductors, while endeavoring to maintain a high enough operating frequency (i.e., 20 – 200 MHz). Electrodeposition was an enabling technology for the thin-film magnetic recording head, and was thus used for yoke fabrication of our earlier inductors (3, 4). IBM and Columbia University recently demonstrated an efficiency of 82% in a World's first 3D-integrated inductor/voltage regulator that employed thru-silicon via technology and electrodeposited Ni45Fe55 yokes (5). Nevertheless, significant advancements in eddy current suppression through electrodeposition of either sufficiently resistive or insulating-layer-laminated magnetic layers have proved elusive. Thus, considerable efforts have gone into developing new, higher resistivity, sputtered magnetic alloys (> 100 µΩcm), usually with insulating lamination layers. A notable example produced by sputtering is amorphous Co91.5Zr4Ta4.5 (CZT), which has, for example, been used as the yoke material in inductors fabricated on top of 90 nm CMOS structures by Gardner et al. (6). We chose to explore sputtered, laminated, amorphous Co20Fe60B20 magnetic films due to their extremely soft magnetic soft properties (Hc ≤ 0.1 Oe) and low, magnetic-permeability-related, loss tangent (≤ 0.1) for a wide range of frequencies, and thermal stability up to 350oC. This required development of a new fabrication scheme to integrate the blanket-deposited, sputtered magnetic layers with 200 mm CMOS substrates. We employed ion beam etching (IBE) patterning methods since no other method, e.g., wet etching, could anisotropically etch all of the inductor yoke stack materials. Peak quality factors (Q) ≥ 12 (at 120MHz) were demonstrated on magnetic inductors containing laminated Co20Fe60B20 magnetic films as shown in Fig. 1, double the Q values of previous inductors that had electrodeposited Ni45Fe55. All inductor devices functioned across a 200 mm wafer, the consistent device performance observed indicative of a reliable, high-yield fabrication process. Fig. 1. Left: Comparison of Q values for inductors with sputtered Co20Fe60B20 materials and electrodeposited Ni45Fe55 (for the latter, see ref. 3). Right: Summary of Qpeak values for single-turn inductors on a 200 mm wafer. [1] Z. Toprak-Deniz et al., ISSCC Digest, 112 (2014). [2] E. A. Burton, et al., Proc. Applied Power Electronics and Expositions (IEEE-APEC), p. 432-439 (2014) [3] N. Wang et al., J. Appl. Phys., 111, 07E732 (2012). [4] N. Sturcken et al., ISSCC, 48, 244 (2013). [5] K. Tien et al., in VLSI Technology (VLSI technology), 2015 Symposium on, Kyoto, 2015, pp.C192-C193. [6] D. S. Gardner et al., J. Appl. Phys., 103, 07E927 (2008). Figure 1

Magnetic Inductors and Their Increasing Importance in the Development of on-Chip Power Conversion

Major strides in on-chip power conversion have been made by IBM in case of their POWER8TM processor (1), and by Intel in case of their 4th generation (Haswell) processor (2), although neither company’s new on-chip voltage regulator (VR) uses an integrated inductor. After outlining why on-chip power conversion is so desirable, this talk will highlight the important role of magnetic inductors in power conversion, and will review recent developments in, and prospects for, magnetic materials for inductors, especially those formed by plating methods.Granular DC-DC power delivery, consisting of fast dynamic voltage scaling for each load through use of a dedicated VR, can significantly improve energy efficiency. Traditionally, the large size of the inductor component has impeded efforts to fabricate the VR in one module. The preferred approach to shrinking the inductor is to increase the switching frequency. The downside to higher frequency operation is reduced efficiency and increased heat dissipation. Thus, shrinking the inductor involves a compromise of space vs. performance, and placing the VRM with an integrated inductor on the processor using thin-film fabrication processes is a natural, if challenging, progression.In thin-film ferromagnetic inductors, yoke material and thicknesses (typically 1 - 3 microns) can be tailored to reach desired inductance values (e.g., 10 – 40 nH), while endeavoring to maintain high enough operating frequency (e.g., 50 – 100 MHz). Considerable efforts have gone into developing new magnetic alloys with higher resistivity (> 100 µΩcm) to reduce yoke eddy currents. A notable example produced by sputtering is amorphous Co91.5Zr4Ta4.5 (CZT) [3, 4], which seems to have become the standard against which other materials are compared. This has been used as the yoke material in inductors fabricated on top of 90 nm CMOS structures by Gardner et al. [5], for example.Electroplating has been a standard technique for the deposition of thick metal films due to its high deposition rate, conformal coverage and low cost. It was an enabling technology for the thin-film magnetic recording head, and was thus used for yoke fabrication for our inductors (6, 7). For the latter, Ni45Fe55 was chosen over Ni80Fe20 for its higher magnetic moment (1.6T), high anisotropy field, and higher electrical resistivity (40 µΩcm). Plated Co-based materials are attractive alternatives to Ni-Fe as yoke materials, e.g. due to their higher moment, especially if their resistivity can be made to approach or exceed 100 µΩcm. We are exploring the use of electrolessly-plated, Co-W-P films for inductor applications (8). The electroless Co-W-P films show excellent magnetic properties, with good magnetic anisotropy, and coercivity of less than 0.1 Oe (Fig. 1). The resistivity of the films is about 90-100 µΩcm, which is close to that of most amorphous Co-based alloys. [1] Z. Toprak-Deniz et al., ISSCC Digest, 112 (2014).[2] E. A. Burton, et al., Proc. Applied Power Electronics and Expositions (IEEE-APEC), p. 432-439 (2014)[3] K. Hayashi et al., J. Appl. Phys., 61, 2983 (1987).[4] D. S. Gardner et al., IEEE Trans. Magn. 43, 2615 (2007).[5] D. S. Gardner et al., J. Appl. Phys., 103, 07E927 (2008).[6] N. Wang et al., J. Appl. Phys., 111, 07E732 (2012).[7] N. Sturcken et al., ISSCC, 48, 244 (2013).[8] N. Wang et al., MMM-Intermag, paper HG-11, 2013 Fig. 1. Hysteresis loop (left) and SEM Xsection (right) of an electrolessly-plated Co-W-P films.This work was supported in part by Lawrence Livermore National Laboratory subcontract No. B601996 under prime contract DE-AC52-07NA27344 from the U.S. Government. Inductor fabrication was carried out in the Microelectronics Research Laboratory (MRL) at IBM’s T. J. Watson Research Center.

Surface Energies of Magnetic Recording Head Components

The rate of material removal during fixed abrasive lapping is a function of friction coefficient, the surface tension of the lubricant and of the substrate, and the contact angles between the interfaces. In this study, the authors measured the surface energies of materials typically found in thin film magnetic recording heads using contact angle measurements and the Lifshitz–van der Waals acid/base approach. The different materials tested were NixFey, Al2O3, and Al2O3-TiC. Sample preparation procedures were also considered. The chemical used to wash the surface was observed to affect the measured substrate surface energies. Surface energy values for samples washed with either acetone or hexane showed comparable results. The NixFey gave the highest measured surface energy (46.3–48.8 mJ m−2) followed by Al2O3 (44.1–45.3 mJ m−2) and Al2O3-TiC (43.3–45.3 mJ m−2). In contrast, the oil-washed samples measured generally lower surface energy values. The study characterized the interaction of two lubricant types against the three materials. The oil-based lubricant spreads completely on oil-washed samples mainly because of the low surface tension of the oil (22.0 mJ m−2) and did not show measurable contact angles. In comparison, the water-soluble lubricant ethylene glycol, due to its higher surface tension (48.0 mJ m−2), formed higher contact angles ranging from 47.2 to 59.6° on the different substrates.

Self-heating in thin-film magnetic recording heads due to write currents

Self-heating due to write currents is a critical concern in thin-film magnetic recording heads; the reliability of the head-media interface and magnetoresistive sensors can be degraded by excessive temperature rise during writing operations. A combined experimental and theoretical study shows that the magnetic saturation of yoke structures and the heat transfer across the air bearing strongly influence the magnitude and spatial distribution of the temperature rise. This paper describes three-dimensional numerical heat transfer simulations that can accurately model self-heating as a function of the amplitude and frequency of write current pulses.

Micro- and nanoscale thermal phenomena in thin-film magnetic recording heads

As the magnetic data storage industry continues to pursue aggressive miniaturization of sensors and write elements in thin-film recording heads, it faces serious thermal management challenges. The present work develops theoretical models of heat conduction and power dissipation in current-in-plane and current-perpendicular-to-plane sensors to highlight key physics and help examine the influence of various parameters. Fundamental physics of heat conduction in thin films and across interfaces is discussed to facilitate systematic exploration of novel high thermal conductivity materials and methods to minimize the thermal interface resistance. Experimental techniques for thermal characterization of recording heads and constituent thin films are also presented.

On the advanced lapping process in the precision finishing of thin-film magnetic recording heads for rigid disc drives

Precision abrasive finishing is a key technology in the manufacture of thin-film magnetic heads (TFH) for rigid disc drives. The read and write microdevices of the recording heads are fabricated on ceramic substrates (“wafers”) using thin-film processing technology. After wafer processing, the substrates are sliced to individual bars (containing 46 heads per bar) using a diamond dicing wheel. They are then finished using an advanced lapping process with individual head close-loop control (sensor height control for the control of the material removal as well as for the end point detection). Finally, the bars are diced into individual heads (called “sliders”) using a diamond dicing wheel. The slider abrasive finishing process critically affects the magnetic, electrical and mechanical performance of the recording heads. This paper presents the results of some experimental studies based on the state-of-the-art abrasive finishing of recording heads, taking into account the critical challenges involved, namely the sensor height control, pole-tip recession (PTR) (metal dishing), alumina recession (AluR) (oxide erosion), trailing-edge profile, polishing uniformity, smearing, surface roughness (scratches) and air-bearing surface (ABS) flatness. The relative advantages of the fixed- versus free-abrasive processes are also discussed. The results of chemical-mechanical fixed-abrasive lapping/nanogrinding to achieve near-zero PTR (to minimize magnetic space loss) and low AluR (to reduce flying height variation) to meet the high areal density head requirements are also presented.

Deposition technology for thin film magnetic recording heads reader fabrication

We review vacuum deposition technology for the fabrication of readers of thin film magnetic heads. We consider three different parts of the deposition, namely the magnetoresistive material, the longitudinal bias and metallization, and the gap dielectrics. The magnetoresistive materials are used to sense the magnetic flux from the media. Typical stacks (‘spin-valves’) are built of 10 or more individual layers with individual layer thicknesses down to less than 1 nm. We discuss the aspects of thickness and microstructure control. Magnetic stabilization and lead metallurgy is used to define the sensor laterally, both magnetically and electrically. We discuss the morphology and material property control. Gap dielectrics are thin Al 2O 3 films which provide dielectric separation between metallic layers. These films are deposited using ion beam deposition (IBD) and physical vapor deposition (PVD) techniques. We discuss the three respective applications from the point of view of device manufacturing in an industrial context.

Resistance as a reliability indicator in lifetime testing of thin film magnetic recording heads

It is demonstrated that the resistance rise observed during lifetime testing of magnetoresistive heads is primarily due to interlayer diffusion of Ta into the Au, NiFe, and NiFeRh layers, not electromigration damage to the sensor. 3.2 Gb/in.2 production heads which were preannealed at 275 °C for 168 h without bias current prior to lifetime testing showed no increase in resistance, or evidence of electromigration, after 200 h at 150 °C with a current density of 7.6×107 A/cm2. In contrast, identical heads, which were not preannealed, showed an average 14% resistance rise during the same lifetime test. Step annealing of heads and sheet films of Cr/CoCrPt/Ta/Au/Ta and NiFeRh/Ta/NiFe/Ta showed that the resistance rise was linear in t1/2, where t is the annealing time, in the earliest stages of the anneal cycle. This behavior is indicative of diffusion processes in the heads.

Wear coatings for magnetic thin film magnetic recording heads

Many coatings are in use today for the wear protection of thin film heads. With the trend towards smaller head-media distances, the thickness of the coating has to diminish. In this paper some of these coatings: zirconia, chromium nitride, chromium oxide and diamond (+diamond-like carbon) will be discussed. It will be shown that the film hardness is influenced by the microcrystalline structure, the texture and the film stress. The film hardness shows a good correlation with the reciprocal wear coefficient. The wear resistance of the materials discussed increases in the order: zirconia, chromium nitride, chromium oxide, diamond.

Micromechanical and tribological characterization of hard amorphous carbon coatings as thin as 5 nm for magnetic recording heads

Amorphous carbon coatings are employed to protect magnetic coatings in thin-film magnetic recording heads against wear and corrosion. These coatings have to be thin (5–20 nm) in order to minimize loss of read back signal amplitude. Micromechanical and tribological characterization of ultra thin amorphous carbon coatings of 5, 10 and 20 nm thickness, deposited by ion beam deposition technique on Al2O3TiC substrate was conducted. Hardness, elastic modulus and micro/nano-scratch resistance of these coatings were measured using a nanoindenter. Friction and wear properties were measured using an accelerated ball-on-flat tribometer. The protection mechanisms of the coatings to the Al2O3TiC substrate are discussed. These coatings were found to be very wear and scratch resistant and should find use in magnetic recording heads.

Electrochemical process for advanced package fabrication

Interconnections for high-end applications are essentially low-resistance transmission-line structures with precisely controlled cross-sectional shapes and dimensions. The relatively thick copper conductors-typically 6 µm or more-combined with the stringent control required on the 10-20-µm-wide cross sections stretches the capabilities of the subtractive etch and lift-off processes that are typically used in semiconductor fabrication to pattern evaporated and sputtered metal films. Electroplating through a photoresist mask, which has proven itself to be a highly effective, precision manufacturing process for thin-film magnetic recording heads, is, however, capable of meeting and far exceeding the requirements of package fabrication. This paper describes the fabrication of a package structure that integrates traditional dry-process technologies with electrolytic copper plating to form the conductors, polyimide backfill and planarization steps to form the dielectric, and electroless deposition to selectively clad the copper lines to prevent adverse reaction of the copper with water generated during the polyimide cure. The discussion highlights salient issues which are pertinent to the compatibility of the individual process steps and to the extension of the technology to more demanding packaging structures and to other applications.

A stroboscopic scanning solid immersion lens microscope

A high resolution scanning optical microscope for stroboscopic time-resolved studies is presented. High optical efficiency is the prime motivation in selecting a solid immersion lens to surpass conventional diffraction limited resolution. A microscope design including details concerning the fabrication, mounting, and operation of both the hemispherical and the truncated-sphere solid immersion lenses is discussed. A thin-film magnetic recording head is used as a convenient, planar test specimen with well-defined optically reflective and magnetic features on a submicrometer length scale that allows characterization of the lateral spatial resolution of the instrument. In addition, the versatility of the microscope is demonstrated through examples of time-resolved magneto-optic imaging of the head. Further enhancement of the spatial resolution through the incorporation of confocal imaging is also described. The tradeoffs governing the selection of a particular style of microscope for specific circumstances are outlined.

Methods for measurement of development parameters in the manufacturing line for use in photolithography modeling

One of the outstanding problems of photolithography is the measurement of development parameters, particularly in one's own manufacturing line. We present an inexpensive practical method for obtaining values of development parameters for use in photolithography modeling. The method utilizes the actual production equipment used in the fab, as well as the computer modeling package for which the development parameters are being collected. In addressing experimental geometries with which one may carry out such data collection, we discuss the critical issue of standing waves and their impact on measurement accuracy. Two techniques are presented for solving the problem of standing waves and their extreme impact on the quality of collected data. One involves creating a photoactive component latent image in which there are no standing waves. The other is a means by which development parameters may still be obtained for situations such as photolithography of thin film magnetic recording heads, where the standing waves may not necessarily be deconvolved from the development process or data collection. Development parameters using 1.15 /spl mu/m of AZ 4110 resist on a NiFe substrate were measured using the latter technique.

Superparamagnetic magnetic force microscopy tips

We report on magnetic force microscopy (MFM) images of a thin-film magnetic recording head taken using batch micromachined silicon tips coated with nanocomposite Fe60(SiO2)40 and Fe70(SiO2)30 films. The small Fe grain size (<10 nm) and dilute Fe volume fraction (0.29–0.4) of these granular films produce tip coatings of low remanence and essentially zero coercivity, reduced by the superparamagnetic properties of these films. We have used these tips to obtain MFM images of the write field of the head with high spatial and magnetic-field resolution. In comparison to images taken using commercial Co85Cr15-coated tips, these MFM images show reduced tip memory effects and clearly delineate the gap field from the pole pieces.

Influence of Composition and Oxygen Partial Pressure in Sintering Atmosphere on Microstructure and Magnetic Properties of Ni–Zn Ferrite for Thin Film Magnetic Recording Heads

Ni–Zn ferrite is a promising material as the substrate for thin film magnetic recording heads. The influence of specimen composition and partial oxygen pressure (PO2) in sintering atmosphere (N2+O2) on the microstructure and magnetic properties of Ni–Zn ferrite has been investigated by SEM and XPS. The results obtained are summarized as follows.(1) ZnO precipitates were observed at the grain boundaries, especially at the triple points of the Ni–Zn ferrite specimen sintered at 1453 K for 10.8 ks under 20% PO2. Such precipitates, however, were not detected in the specimen sintered in the atmosphere whose PO2 was 25%.(2) Although the permeability (μ) and the resistivity (ρ) of the Ni–Zn specimens sintered at 1453 K for 10.8 ks under 10 to 100% PO2 increased with increasing PO2, both the loss coefficient (Q) and the relative loss coefficient (tan δ/μ) showed an optimum value under 20% PO2.(3) The 0.3 mol% increment of ZnO and the 0.3 mol% decrement of NiO than the conventional composition (Fe2O3: 49.7 mol%, ZnO: 32.6 mol%, NiO: 17.7 mol%) prevented ZnO precipitation on the Ni–Zn ferrite specimen sintered at 1453 K for 10.8 ks in air.(4) The HIP’ed specimen after sintered at 1423 K, 30 K lower than the conventional sintering temperature of 1453 K, in air showed neither ZnO precipitation nor pores.

Lead structures for thin film magnetic recording heads

Inductive coupling of lead structures in thin film magnetic recording heads causes crosstalk noise in multi-track or read-after-write systems. We analyze various lead structures for multi-element thin film head devices by computing mutual inductance. We find that a lead structure in which one thin film lead is placed on top of the other has superior performance with mutual coupling several orders of magnitude less for the modeled geometries.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The role of disk carbon and slider in water adsorption

Water adsorption was measured on carbon overcoated, thin film magnetic recording disks and magnetic recording heads. Measurements were made using an ellipsometer with a humidity chamber mounted on the sample stage. Water adsorption is dependent primarily on the hydrogen content of the carbon overcoat, for both lubed and unlubed disks. Water adsorption is independent of the lube type and thickness, except when the carbon overcoat has a low hydrogen content. In the latter case, water adsorption depends on the functional endgroup of the lube and slightly on lube thickness. Water adsorption on magnetic recording heads depends both on the head composition and surface roughness. >

Determination of the narrow read width of thin film magnetic recording heads using an error rate model

A method to determine the magnetic-recording characteristics of a thin film head has been developed using error rate sampling. By combining results from a typical '747' curve and a modified '747' curve, called adjacent data curve, parameters such as the write width, the read width, and the side fringing erased band of a thin film head can be directly measured. In addition, the signal to noise ratio and the noise intensity level in the side fringing erased bands, normalized to signal strength, can also be determined, These measured parameters can be used to calculate the track profile width. When comparing the calculated profile widths with those obtained from the directly measured track profile curve, excellent agreement has been found, thus validating the proposed method.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

ＣｏＦｅ電着薄膜の磁気特性

Films with higher saturation magnetization are desired for improving the recording density of thin film magnetic recording heads. A study has been conducted on the effects of the magnetic properties of organic additives on the plating baths from which cobalt-iron films with a Bs greater than 1.5T were obtained. Film with a coercivity lower than 1 Oe was obtained from a bath containing 0.02mL/L of 2-propyn-1-ol (PPO) and 60g/L of naphthalene-1, 3, 6-trisulfonic trisodiumsalt (NTS). Internal stress decreased with the amount of NTS, and the sulfur content of the film correlated with internal stress and with coercivity.The effects of heat treatment on the soft magnetic properties of the films were also investigated. The anisotropic field Hk can be changed by the repeated heat treatment in a magnetic field directed along the hard axis. Films with a small anisitropy field (Hk 2000).Under this type of heat treatment, Hk was affected by direction of magnetic fields at three stages: (a) during deposition, (b) during initial heat treatment, and (c) during final heat treatment.

Microstructural Control of Ni&amp;ndash;Zn Ferrites for Thin Film Heads

The microstructural changes during sintering of Ni-Zn ferrites used as the substrate for thin film magnetic recording heads have been studied by SEM, XRD and XPS. Any precipitates were not observed in the specimen air-sintered and furnace-cooled from 1423 K. ZnO precipitates, however, were observed at the grain boundaries, especially at the grain boundary triple points of the specimen furnace-cooled from 1453 K. While the above-mentioned ZnO precipitates were not observed on the specimen sintered in oxygen, the grain coarsening was observed in this case, with residual pores being trapped inside the grains. The existence of Ni 3+ ion was detected by XPS on the specimen prepared by HIP process, where the ratio of Ni 3+ /(Ni 2+ +Ni 3+ ) was confirmed to be about 3%...