Tribological issues in perpendicular recording media - Springer Link

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Tribology Letters, Vol. 26, No. 1, April 2007 (Ó 2006) DOI: 10.1007/s11249-006-9174-9

Tribological issues in perpendicular recording media Q. Daia, U. Nayaka, D. Marguliesa, B. Marchona,*, R. Waltmanb, K. Takanob and J. Wangb a

San Jose Research Center, Hitachi Global Storage Technologies, 650 Harry Road, San Jose, CA 95120, USA b Hitachi Global Storage Technologies, 5600 Cottle Road, San Jose, CA 95193, USA

Received 3 August 2006; accepted 2 November 2006; published online 23 December 2006

As the hard disk drive industry is transitioning from longitudinal (LMR) to perpendicular (PMR) recording, a new set of reliability challenges had to be overcome. In particular, the magnetic media structure, which relies on well-segregated grains of 6–10 nm diameter, can exhibit a rough structure, with peak-to-mean amplitude of 3–6 nm. In this paper, we will discuss how this topography could affect the overall reliability of the head–disk interface. In the first part, we will illustrate the loss of adequate coverage from the overcoat on PMR media, compared to the smoother LMR media, and we will attempt to quantify the topography in terms of its deviation from a Gaussian height distribution. Particular emphasis will be given to surface outliers and their removal during the burnishing process. The second part will be devoted to the lubricant preferential migration to the grain boundaries, driven by surface tension. It will be shown by an EELS line scan that the lubricant film is indeed thicker in the valleys between the grains, in agreement with surface tension driven redistribution. Finally, we will demonstrate that the Touchdown Height (TDH) of a PMR disk is 0.5 nm higher than its LMR counterpart, owing to its enhanced nano-roughness. Once recognized, these challenges can be overcome through a careful and thorough optimization of the various processing parameters, eventually leading to an overall reliability level equal or better than LMR media. KEY WORDS: magnetic data storage, magnetic data disks, nanotribology, corrosion

1. Introduction As the quest for higher areal density in rigid disk drives continues, Perpendicular Magnetic Recording (PMR) is becoming the industry’s next focus [1]. In contrast to its Longitudinal Magnetic Recording (LMR) counterpart, PMR media technologies reported thus far generally relies on a media with small, well-segregated grains connected by amorphous grain boundaries [2]. This grain structure may introduce a surface roughness of several nanometers amplitude [3], which is not negligible in the scale of the required magnetic spacing [4]. The finished media roughness is affected by a large number of variables, and the factors that help produce a smooth media may cause poor magnetic performance. It is not the intention of the paper to discuss the details on how to minimize the roughness of the finished media while maintaining optimum magnetic performance, as this topic was discussed elsewhere [2]. Hence, we will only discuss issues associated with somewhat ‘‘rough’’ PMR media, without implying that it is a necessary condition to proper magnetic performance. While overcoat coverage challenge is anticipated with increasing media roughness [5–7], we will also report that the problem is compounded by susceptibility to wear during disk burnishing [8]. It will be shown that the surface asperity height distribution can sometimes *To whom correspondence should be addressed. E-mail: [email protected]

deviate from a normal (Gaussian) distribution, leading to preferential overcoat reduction when the topography outliers are removed. In this article, we will attempt to characterize the fine nanostructure of PMR media using Atomic Force Microscopy (AFM) and Scanning Transmission Electron Microscopy (STEM), and we will study its effect on the tribological properties of the interface. In particular, we will quantify the coverage ability of the protective overcoat, and we will study the surface tension-driven migration of the lubricant to the grain boundaries. The removal of surface outliers during burnishing will also be presented. Finally, the lowest safe flying height before contact (touchdown height or TDH) will be measured and compared to LMR media.

2. Experimental 2.1. Samples All samples investigated for this article were thin film media sputtered on glass substrate. PMR media with a rough nanostructure were fabricated with a recording layer composed of CoPtCr with addition of SiOx to facilitate grain segregation [2], whereas the LMR media were prepared with a CoPtCrX alloy. Overcoat material was nitrogen-doped ion beam deposited carbon (IBD). Overcoat thickness was controlled within ±0.2 nm using ellipsometry, and calibrated against X-ray 1023-8883/07/0400–0001/0 Ó 2006 Springer Science+Business Media, LLC

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Dai et al./Tribological issues in perpendicular recording media

Reflectometry (XRR). Unless otherwise noted, all media samples were coated with about 1 nm of Ztetraol lubricant (2500 Da molecular weight) made by SolvaySolexis (Thorofare, NJ). Post-sputter burnishing was performed using an abrasive tape (0.5 lm diameter alumina particles) brought into contact with the rotating media surface.

2.2. Roughness measurements Media roughness was measured with Atomic Force Microscopy (Nanoscope), using conventional probe tips (Nanosensor). The scan size was fixed to 11 lm in order to better resolve the grain structure. For overcoat coverage study, roughness measurements were performed on unburnished disks. For the study of wear susceptibility, roughness and distributions were measured both before and after post-sputter burnishing.

2.3. Corrosion Overcoat coverage was evaluated through electrochemical polarization measurements [9] (EG&E 283), using a AgCl/KCl reference electrode, and de-ionized water as an electrolyte. Solution ionic resistivity was negligible due to the close proximity of the two electrodes [10]. The electrode diameter was fixed at 5 mm. Because the surface area was identical for all measurements, only exchange current instead of exchange current density was reported. Corrosion susceptibility of the finished media was tested by exposing the media to a 65 °C/90% temperature/humidity environment for 7 days. After exposure, the disks were allowed to cool, followed by examination with Candela Optical Analyzer (6100) in the scatter light mode.

2.4. Lubricant titration The perfluoropolyether liquid used for the titration work was monodispersed Zdol (Mn = 4010 Da, Mw/ Mn < 1.08, 99.9% OH functionality). The liquid was applied to the carbon surfaces from a solution (hydrofluoroether solvent HFE-7100) using a standard dip-coating methodology, with a typical solution concentration of 0.8 g/l and a typical disk withdrawal rate of several mm/s. Specular reflection FTIR (Nicolet Magna Model 560) was used to quantify the film thickness of the perfluoropolyether film on the disk surface. A standard specular reflection adapter (Harrick, New York) was used to maintain the proper angle of incidence to the sample. The changes in the film thickness were quantified by relating the perfluoropolyether absorption band at 1282 cm)1, due to the combination of C–F and C–O stretching vibrations of the main chain, to film thickness previously calibrated via X-Ray

Photoelectron Spectroscopy (XPS). Titrated thickness was defined as the maximum bonded lubricant thickness after thermal treatment and solvent rinse, as defined by Waltman et al. [11].

2.5. STEM The samples were prepared by conventional ion beam techniques. The surface of interest was protected by a low-melting-point wax. An ultra-sonic drill cut a 3-mm disk from sample. This disk was ground to less than 100lm thickness from the substrate side. It was then mechanically dimpled using a diamond polish of various grits, until the center was 10-lm thick. The protective wax was then removed by dissolving in an acetone bath. Finally, the disk was Argon ion-milled to electron transparency. For the EELS line scans, the beam diameter was about 0.5 nm, with a current of 275 pA. The beam was stepped 0.25 nm along the line. Each spectra was acquired in 2 s. The HAADF (High Angle Annular Dark Field) STEM (Scanning Transmission Electron Microscopy) image was acquired with an inner detector angle of 40 mrad.

2.6. Touchdown height (TDH) For this study, a novel, fully calibrated, method to measure the TDH of both perpendicular and longitudinal media was developed. The method uses a slider with a small pad that can be made to protrude from the Air-Bearing Surface (ABS) by resistively heating the write coil element, or by an independent heater at the trailing edge near the read–write element [12]. This actuation scheme can be adjusted with 0.1-nm precision. The slider ABS itself is especially designed so that its flying height can also be adjusted by varying its velocity. To calibrate the slider flying height, a special piezoelectric (PZT) sensor is mounted on the slider, and flown on a specially made disk with bumps of known heights [13] By controlling the maximum interference with the bumps to be less than 2 nm during calibration of the flying height, we get repeatability to 0.1 nm indicating that bump wear or pad wear during calibration is small. The lowest bump is 5 nm tall and the maximum protrusion is 7 nm, so that flying heights below 5 nm can be spanned with thermal actuation until contact is made with the media.

3. Results and discussion 3.1. Roughness Figure 1a shows a 1 1 lm AFM image of an unburnished LMR media deposited on a glass substrate. The image is featureless at a 5 nm vertical scale, with the exception of a few texture lines from the underlying

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Figure 1. 1 1 lm AFM image of (a) LMR and (b) PMR media.

The increased roughness of the PMR media is detrimental to the proper coverage of the protective overcoat. To quantify this, we used electrochemical polarization measurements in the anodic branch [10,14]. If a media surface has no overcoat, the anodic current is expected to be high, because the entire electrode of the exposed cobalt alloy can participate freely in the following anodic reaction: Co ! Co2þ þ 2e . If the media is fully covered with an overcoat, the cathodic reaction can still occur on the top of the overcoat surface, if this overcoat is adequately conductive and active [10]. The anodic reaction, however, can only occur in pores or defects through the overcoat [10,14]. Anodic current measurement at a fixed potential is therefore a good estimate of the coverage property of the overcoat. Figure 2 shows a set of polarization curves in the range Table 1. Roughness parameters on pre-burnish samples, obtained from the AFM pictures

LMR PMR

Rp (nm)

Rv (nm)

Rq (nm)

1.1 2.7

1.3 2.0

0.25 0.5

-5.0 -5.5 log10[Current (A)]

3.2. Intrinsic coverage properties

of 0.5 to )0.5 V, obtained on three freshly deposited, unburnished PMR media surfaces with overcoat thickness ranging from (a) 0 nm, (b) 1.2 nm, and (c) 4.5 nm. Indeed, one finds that the anodic current decreases with increasing overcoat thickness, as expected. To quantitatively compare the coverage limit, we have chosen to plot the anodic current measured at a fixed potential, E = 0.6 Voc (with respect to the open circuit potential), as a function of the overcoat thickness. The result in figure 3 shows that there is a critical,

-6.0

0 nm

-6.5

1.2 nm

-7.0 -7.5 -8.0

4.5 nm

-8.5 -9.0 -0.5

0 0.5 Potential (V) Figure 2. Polarization curve of PMR media for various overcoat thicknesses.

7.0

Anodic Current at E=0.6Voc

substrate. In comparison, figure 1b obtained on a PMR media shows a highly granular structure, with measured grain sizes in the range of 6–10 nm, depending on the AFM tip radius. The actual roughness of a given PMR media will strongly depend on the deposition condition, alloy type/thickness, and the underlying substrate topography. Details on how these processes affect the media roughness and magnetic performances are beyond the scope of this discussion. Table 1 shows typical roughness values obtained from these images. Compared with LMR media, this PMR media under investigation has over 1 nm higher Rp (peak to mean) and Rv (valley to mean), and the root-mean-square roughness Rq is approximately twice as high. These AFM observations are in agreement with earlier studies, obtained from TEM cross-section [2].

6.0 5.0 4.0 3.0 2.0 1.0 0.0 0

1

2

3

4

5

Overcoat Thickness (nm) Figure 3. Anodic current at 0.6 Voc, as a function of overcoat thickness. Diamonds: PMR; squares: LMR. The solid lines are power fits to the data.

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minimum thickness above which the overcoat fully protects the cobalt alloy [15]. However, this thickness threshold is larger in the case of a rough PMR media (3.5 nm), compared to its LMR counterpart (2.5 nm). Since the alloy and the overcoat material were chemically similar for the two types of media, the loss of coverage associated with this particular PMR media sample is largely attributed to its increased roughness [5–7]. It is worth noticing that this increased coverage threshold is of the same order of magnitude as the differences in roughness peak (Rp) and valley and (Rv) values between LMR and PMR, as seen in table 1. These electrochemical results are in good agreement with lubricant titration measurements performed on similar samples. This titration scheme is a very sensitive probe to the proper coverage of the overcoat over the cobalt alloy surface, owing to the great difference in affinity of the lubricant molecule to both surfaces [11]. As illustrated in figure 4, a sudden rise in titrated lubricant thickness is observed under a critical overcoat thickness value, corresponding to the onset of full coverage. Similar to the polarization results, PMR media exhibit 1 nm higher coverage threshold compared to the LMR samples (4.5 vs. 3.5 nm, respectively). Also, it is worth pointing out that lubricant titration seems to be slightly more sensitive than electrochemical measurements, as coverage threshold with this technique are about 1 nm higher than the ones observed using electrochemical means (figure 3).

3.3. Wear-induced corrosion Even when the initial overcoat thickness has met the minimum coverage requirement, the media might still corrode if some of the high points of the nanostructure, seen on figure 1b, have been worn off. This wear process can occur during disk burnishing, which is part of the disk manufacturing process aimed at removing defects. This effect is illustrated in figure 5, where part of an

Figure 5. Optical map (scatter mode) of a PMR disk after T&RH exposure. The dotted line indicates the separation between the burnished and unburnished parts of the disk.

experimental disk was subjected to a burnish operation. The disk was then subjected to a corrosion test, after which an optical scanner in a scattered light mode was used to examine the disk surface for corrosion growth. It is apparent that the part of the disk that had been burnished has produced more corrosion spots compared with the part that was untouched. In order to further understand the nature of the wear process of the PMR grain structure, we carried out statistical analysis on the AFM images. Figure 6 shows

LMR 0.999 0.99

PMR LMR

110 100 90 80

PMR

0.90

Probability

Titrated Thickness (Angstrom)

120

0.50 0.25 0.05

70

0.01

60

0.001

50 40

-3

30

-2

-1

0

1

2

3

AFM Height in nm

0

10

30 40 20 50 60 Carbon Thickness (Angstrom)

70

Figure 4. Lubricant titration measurements on LMR and PMR media samples.

Figure 6. Normal probability plots of the AFM height, comparing LMR with PMR. The dotted lines are Gaussian fits to the data. The origin of the abscissa corresponds to the centerline average of the image.

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Dai et al./Tribological issues in perpendicular recording media 14 12

log10(cum. peak dist.)

a normal probability plot of the surface height data obtained from figure 1. The dotted lines are true Gaussian fits and the difference in slope illustrates the difference in standard deviation, or root-mean-square Rq, in the distributions. It appears that on the positive part of the distributions, the LMR media follows a fairly normal distribution, whereas PMR media exhibit a significant deviation from the Gaussian fit, indicating the presence of topography outliers. These features have often been associated with PVD processes, where shadowing effects can enhance the roughness at a length scale comparable to the grain size, leading to abnormally high peaks [16,17]. It is conceivable that these tall peaks could be preferentially removed in a wear process such as during burnishing. Indeed, further AFM statistical analysis confirms this hypothesis: Figure 7 shows the Normal Probability plot obtained from an AFM image (positive height only) of the PMR media before and after burnishing. Compared with the original surface, which exhibited a spiky topography, the media that have been burnished exhibits a height statistics much closer to a normal distribution, with a lower Rp and similar Rq. These measurements were repeated extensively and appear very reproducible. From the understanding presented so far, it is possible to quantitatively estimate the number of peaks that have lost overcoat coverage as a result of burnish damage. One can extrapolate the peak height distribution obtained from a 1 1 lm AFM image to the entire disk surface by multiplying the number of peaks by 109, roughly corresponding to the number of

10 8

1 nm

(a)

6 4

(b) 2 nm

2 0 1

4 6 7 5 Height (nm) Figure 8. Cumulative distribution plot, extrapolated to the whole disk, (a) before and (b) after burnish. 2

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square microns of the disk surface (10 cm2). Such an extrapolation is shown in figure 8 for disks pre- and post-burnish, and fitted with a Gaussian function. Unlike figure 7, figure 8 focuses on the tallest (>1 nm in height), actual grain height, as opposed to a simple pixel height distribution. This extrapolation exercise shows that from the original 1 1 lm AFM image with one peak at 2.7 nm or higher, we get 109 peaks on the entire disk with similar peak heights. Furthermore, we anticipate that on the entire disk there should be 100 peaks with peak height greater than ca. 5.5 nm. These tallest peaks are expected to be more susceptible to burnish removal based on

(a)

Probability

0.999 0.99 0.90 0.75 0.50

0

0.5

1

1.5

2

2.5

3

2

2.5

3

AFM Height in nm

(b)

Probability

0.999 0.99 0.90 0.75 0.50

0

0.5

1

1.5

AFM Height in nm Figure 7. Normal Probability plot of the AFM height (positive heights only), comparing PMR media (a) before and (b) after burnish.

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previous discussions, and this is indeed observed from the distribution obtained after burnishing, where extrapolation suggests that no peak greater than 3.7 nm would be expected. These two distributions also allow an estimate of how many peaks have lost how much overcoat through burnishing. For example, about 108 peaks have sustained about 1 nm overcoat reduction, whereas less than 100 peaks have lost 2 nm. Going even further, one can easily compute the estimate of the number of peaks with a given amount of overcoat thickness left, based on the parabolic fits of figure 8, and given an initial thickness of 4 nm. This estimate is reproduced on figure 9. Depending on the intrinsic coverage threshold of the overcoat as discussed above, this curve allows a rough estimate of the number of ‘‘weak’’ or easily corrodible surface sites on the disk surface. It also highlights how critical overcoat thickness control is for such a structure.

3.4. Lubricant segregation Thin liquid films tend to segregate to the pores of rough surfaces, even on molecularly thin layers, because of the competition between disjoining and the capillary pressures [18,19]. As a result, one expected aspect of the PMR media nanoroughness would be its ability to redistribute the lubricant thickness from the peaks (top of the grains) to the valleys (grain boundary). To probe this, we carried out high-resolution STEM measurements, coupled with Electron Energy Loss Spectroscopy (EELS). Figure 10 shows a 70 nm wide TEM image obtained from the CoPtCr(SiOx) magnetic layer [2]. It clearly shows the granular structure discussed above. The 6–10 nm wide grains correspond to the small AFM features shown in figure 1. Figure 11 is an EELS scan across two adjacent grains shown on figure 10, displaying atomic distribution of the Chrome (Cr), Cobalt (Co), Oxygen (O), and Fluorine (F) elements. Cr, Co, and O signals are from the magnetic structure, whereas fluorine is the signature of the perfluoropolyether

Figure 10. High angle annular dark field STEM image showing the grains and the location of the EELS line scan. HDF-STEM is known as a z-contrast image.

lubricant [20]. As expected, there is enrichment in oxygen, concurrent with depletion of chrome and cobalt between the grains, owing to the poor solubility of these metallic oxides in their parent metal lattice [2]. The fluorine signal, however, shows enhancement in the grain boundary, a clear sign of surface tension driven lubricant segregation. This effect could actually be beneficial for the overall reliability of the head/disk interface, as it provides some sort of a tightly held lubricant reservoir for in situ replenishment, after possible intermittent contact-induced damage [21].

3.5. TDH measurements TDH measurements were performed using an airbearing surface composed of a separated 30 28 lm

Relative Elemental Concentration (Arbitrary Units)

log10(Cum. Number of Peaks)

50

8 7 6 5 4 3 2 1 0 1.8

2

2.2

2.4

2.6

2.8

Carbon Thickness on Peak (nm)

Figure 9. Estimated cumulative distribution of carbon thickness after burnish, on top of the magnetic grains.

40

Co Cr O F

30

20

10

0 0

2

4

6

Line scan position (nm) Figure 11. Results of EELS line-scan.

8

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Figure 12. TDH measuring slider has a separated pad for sensing contact with the calibration bumps and the disk surface during TDH measurements.

Figure 13. PZT sensor signal for various interference levels (a) decreasing disk velocity, (b) increasing heater protrusion.

we move the slider over to the ‘‘5 nm’’ bump and thermally protrude the pad until contact is made. The heater power is then reduced to the reference value, and the slider velocity is reduced until contact with the bump. The bump calibration disk is then substituted by the disk whose TDH we want to measure. With the slider at the same velocity, the heater power is increased until contact with the disk surface is seen (figure 16). Using this process, TDH measurements for experimental LMR and PMR media made on the same

25

"Flying Height" nm

contact pad (figure 12), allowing for large thermal actuation with more than 95% of the actuation going into reducing the clearance between head and disk. The small size of the contact pad also reduces the effect of slider roll and slider roughness on the TDH measurement. The piezoelectric sensor is mounted over the entire back surface, and senses the second and third bending mode vibrations of the structure during asperity contact [13]. The high contact sensitivity of the sensor is shown by varying interference with a 10-nm-tall calibration bump (figure 13) by changing flying height or by changing heater power. The contact is easily discernable with only 0.2 nm of interference. By measuring the contact with bumps with varying heater power, we get the calibration curves (figure 14) interpolated between the highest calibration bump (20 nm) and the lowest calibration bump (10 nm). The offset in flying height between the curves is due to the increasing protrusion of the pad with increasing heater power. The offsets in flying height can be referenced to a ‘‘zero’’ of mechanical protrusion of the pad relative to the trailing edge center of the lift pad (point A in figure 12). The offsets then give the protrusion relative to this reference point as a function of heater power (figure 15). We can see that the trailing edge pad is recessed away from the disk (relative to point A in figure 12) in the flying state until 22 mw of power is applied. Using all the measurements of the bump contact velocities at various heater powers, one obtains an ‘‘effective bump height’’ seen in table 2. We now have all the elements for making the TDH measurement: the flying height of the slider for any given protrusion (i.e. heater power), the effective bump heights, and the ability to protrude the pad down to the TDH of the disk surface. The procedure is as follows: after choosing a reference heater power (>22 mw to compensate for ‘‘recession’’) and making contact with the 10 nm bump,

20

15

10

19.4 mw 34.9 mw

5

55.3 mw 75.3 mw

0 10

15

20

25

30

35

Speed (m/s) Figure 14. The fit to the bump contact velocities interpolated to integral velocities for various heater powers.

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HF PZT Signal Amplitude (V)

FH offset/Protrusion (nm)

-7 -6 -5

y = -0.120x + 2.550 -4 -3 -2 -1 0 0

20

40

60

0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05

80

0 0

90

180

270

360

Degrees

Power (mw) Figure 15. Protrusion referenced to the center trailing edge of the lift pad (point A, figure 12).

substrate type are shown in table 3. The table shows the effective bump height of the ‘‘5 nm’’ bump measured using the protrusion relative to the 10 nm bump, the increased protrusion needed to make contact with the disk, and the ensuing TDH value. Consistent with a rougher structure, it can be seen that PMR media have a slightly higher TDH (2.4 nm), compared to LMR media (1.8 nm). These TDH values are of the same order as the height of the peaks from the AFM data (table 1), suggesting that the media nanostructure is a major part of the TDH budget, perhaps greater than the contribution from the substrate micro-waviness [22].

4. Conclusion We have shown that a PMR media design based on metal oxide grain isolation can lead to a significantly rougher topography on a nanoscale, compared with longitudinal media. The roughness was quantified in terms of peak height, peak depth, and root-meansquare roughness from AFM measurements. Overcoat coverage was assessed using anodic polarization measurement, as well as lubricant titration. Both techniques demonstrate that the higher roughness of the PMR media under investigation required thicker overcoat in order to achieve full coverage, compared

Figure 16. Contact at touchdown height due to increasing power into heater.

Table 3. TDH values for PMR and LMR media

Experimetal PMR Media 1 Experimetal PMR Media 2 Experimetal LMR Media

5 nm bump height (nm)

Protrusion to TDH (nm)

TDH (nm)

6.33, ‘‘0 deg bump’’

3.93

2.40

6.25, ‘‘0 deg bump’’

3.86

2.39

5.32, ‘‘90 deg bump’’

3.56

1.76

with LMR media. For the same reason, the intrinsic TDH of the PMR media is slightly higher than LMR media. In addition, we have also shown that compared with longitudinal media, PMR growth tends to lead to a significant amount of topography outliers, making the media more susceptible to contact damage. High-resolution EELS measurements also suggest that lubricant tends to segregate in the valleys at the grain boundaries, driven by gradients in capillary pressure. The key to a good PMR media reliability therefore resides in a careful optimization of the media structure and processing parameters that alleviate those effects.

Table 2. ‘‘Effective bump heights’’ from contact velocity measurements Bump height (corrected, nm) Voltage (V) 0 deg. ‘‘20 nm’’ 0 deg. ‘‘15 nm’’ 0 deg. ‘‘10 nm’’ 90 deg. ‘‘20 nm’’ 90 deg. ‘‘15 nm’’ 90 deg. ‘‘10 nm’’

0.67

2.50

3.00

4.00

5.00

Avg (nm)

19.50 15.17 9.86 20.84 14.91 10.53

18.90 15.58 9.68 20.73 15.39 10.52

19.37 14.95 9.80 20.84 15.18 10.66

19.61 14.62 9.81 20.90 14.89 10.96

19.09 15.15 9.67 20.95 15.05 10.89

19.29 15.09 9.76 20.85 15.08 10.71

Std. Dev. (nm) 0.26 0.31 0.08 0.07 0.19 0.18


Acknowledgment The authors are indebted to Dr. Philip Rice from IBM Almaden Research Center for help in TEM measurements.

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