Nano-Tribology of Thin Film Magnetic Recording Media

Chapter 22

NANO-TRIBOLOGY OF THIN FILM MAGNETIC RECORDING MEDIA T. E. Karis IBM Research Division Almaden Research Center

San Jose, CA 95120 Abstract

The magnetic recording industry is projected to continue growing into the foreseeable future. This growth is fueled by increasing data storage density through advances in channel and read/write head integration, tracking servo mechanisms, higher speed spindle motors, chemical integration, and nanotribology. The rotation rate is approaching 20,000 rpm, and the spacing between the read/write head and the disk is approaching molecular dimensions. The ceramic head rails and the disk magnetic layer are currently protected by a complex, yet robust, system comprising 5- I 0 nm thick carbon overcoats and a 1-2 nm thick perfluoropolyether lubricant mm. The mm surface energy and lubricant mobility, which control the surface diffusion, are determined by the mm thickness and chemisorption of polar lubricant end groups on the carbon overcoat. Intermittent contacts between the head and the disk incrementally remove lubricant from asperities. Lubricant diffuses from the surrounding surface to restore the film thickness on the asperities. In this paper, the principles of magnetic recording disk lubrication are reviewed and summarized in terms of a lubrication system comprising lubricant removal, reflow, and chemisorption. New test results are presented to illustrate the lubrication system components. The general principles of the magnetic recording lubrication system should also apply to lubrication of micro- and nanoscale devices.

1 INTRODUCTION The low cost per bit of data storage, along with the reasonably high data rate, short access time, and high reliability are the most salient features of magnetic recording hard disk drives. Higher areal density enables increasing capacity. Recent trends suggest that soon the areal density will surpass 100 Gbits/in 2, the average OEM cost will be about a one cent USD per megabyte, and the data rate will exceed 200 MBytes/sec (Grochowski, 2000). Increases in areal density have been realized by advancing from thin film inductive heads to magnetoresistive and, more recently, to giant S. M. Hsu et al. (eds.), Nanotribology © Kluwer Academic Publishers 2003

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magnetoresistive read heads. Improved servo technology provides a higher track pitch, and faster channels enable higher data rates. Advanced coding schemes ameliorate signal deterioration by thermal asperities. The rapid advancement of magnetic recording throughout the past decade has built upon the skills of manufacturing engineers who refine quality control to provide the nanoscale tolerances which are reduced in every new product. This has been accomplished with essentially the same set of materials, carbon overcoated thin film media, overcoated heads, and functional lubricant. These advances are accomplished while maintaining high reliability. The magnetic recording head (slider) is said to be flying when it is supported over the spinning disk surface by an air bearing formed in the hydrodynamic boundary layer. Presently the physical spacing between the slider and disk (flying height) is about 20 nm, and it will be closer to 10 nm when the areal density is 100 Gbits/in 2 • While the spacing between the slider and the disk is decreasing, the disk rotation rate is increasing. High end disk drives are now commonly running at 10,000 rpm, and 20,000 rpm is on the horizon. The focus of thin film magnetic recording media nano-tribology is in the gap between the head and the disk, which is approaching molecular dimensions. In this paper, the structure and surface chemistry of the lubricant film and the carbon overcoat are described. The tribochemistry and physics during controlled tribological testing is reviewed. A general nanolubrication system model is proposed. New tribological test results are presented to illustrate the components of the model. Areas of nanolubrication which should be the focus of renewed industrial and academic research are identified. From this viewpoint, we see that the principles of the magnetic recording media nano-Iubrication system may also be applied to micro- and nano-scale devices.

2 DISK AND SLIDER FABRICATION The magnetic recording medium is deposited on an aluminum magnesium alloy or, more recently, glass substrate. The magnetic recording layer is typically Co alloy on a Cr underlayer, with 5 to 10 nm of amorphous carbon sputter deposited on top of the magnetic layer as a protective overcoat. After removal from the vacuum deposition system, the disks are stored, polished with a fine abrasive grit, and washed. Perfluoropolyether lubricant is deposited by dip coating from dilute solution in a fluorinated solvent (Scarati and Caporiccio, 1987). Alternatively, lubricant may be deposited within the vacuum system to control the chemisorbed fraction and to avoid the use of fluorinated solvents (Coffey et al., 1994; DeKoven,

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2000). After lubrication, the disks are glide burnished with a special air bearing slider to remove asperities below the specified flying height. The finished disks undergo final surface analysis and testing, and they are mounted between spacer rings on a spindle motor and assembled into the disk drive. The slider substrate is a TiCAl 20 3 ceramic wafer. Magnetic elements are sputter deposited onto the wafer, and it is sliced into thin rows (approximately 0.5x2x50 mm). One face of the row is polished by lapping and a 5-10-nm-thick carbon overcoat is deposited. The rows are photolithographically patterned with air bearing rails, and diced into sliders (~1 mm wide). The sliders are adhesively bonded to a gimbal on a stainless steel suspension (Qian et al., 1999). The spring suspension provides a normal force equal to the air bearing lift force at the operating linear velocity to set the slider flying height. The head gimbal assemblies are mounted on a comb which attaches to the servo actuator, and assembled into the disk drive.

3 OVERCOATS Strong adhesive forces are inherent between un lubricated ceramic and metal. In direct ceramic-metal contact, adhesion outweighs cohesion in the metal, and metal is transferred to the ceramic (Miyoshi, 1990). The carbon overcoats on the ceramic slider and the magnetic recording layer enable sliding between the two surfaces without cohesive failure in the magnetic layer. Addition of the carbon overcoat on the slider improves the wear durability (Wang et al., 1994). Carbon overcoats also inhibit corrosion of the magnetic layer in aqueous solutions, although micro-scratches made during sliding can provide corrosion sites (Suzuki and Kennedy, 1989). Carbon overcoats are sputtered with Ar inert gas and usually hydrogen or nitrogen to make hydrogenated CHx or nitrogenated CNx carbon, respectively (Anoikin et al., I 998a,b). The properties and characterization of thin carbon overcoats were reviewed by Tsai and Bogy, 1987. Lower density of sputter deposited carbon is due in part to the presence of a network of voids, and decreasing the partial pressure or the rf power increases the film density (Tsai and Bogy, 1987). Carbon overcoats with a more uniform grain size distribution, more Sp3 bonding, and a more homogeneous work function distribution had the best durability (Khan et al., 1988). A heterogeneous morphology for sputtered carbon was recently proposed by Kasai et al., 1999. Surface roughness measurements on typical sputtered carbon were consistent with a granular structure of closely packed spheroids, 2.7 nm in diameter. The granular nature of the overcoat is

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consistent with the effects of oxygen, water, and alcohols on the dangling bond signal as measured by electron spin resonance (ESR) spectroscopy. The CH x overcoat surface contains unsaturated carbon-carbon bonds (Kasai et ai., 1999), and oxides (hydroxyl, carboxyl, and carbonyl, Wang et ai., 1996), while the CN x surface contains imines and carbonitrile (Waltman et at., 1999). Overcoats other than sputtered carbon have been considered. The durability of carbon deposited with plasma assisted chemical vapor deposition approaches that of sputtered carbon (Koishi et ai., 1993). An ion beam deposited fluorinated carbon overlayer exhibited good durability (Karis et al., 1998). Recently a vapor deposited and UV polymerized cyanate ester overlayer was deposited on carbon overcoats to improve their interaction with lubricants and to provide corrosion resistance (Mate and Wu, 2000). Durability and corrosion resistance was provided by a 10 nm thick nickel oxide film and a perfluoropolyether lubricant on top of the magnetic layer (Yanagisawa et al., 1989).

4 LUBRICANTS The lubricants used for particulate magnetic recording disks throughout the 1970's and most of the 1980's were perfluoropolyethers with nonpolar CF 3- end groups. These nonpolar perfluoropolyethers physisorb on surfaces, primarily through dispersion interaction force. The carbon overcoats of modern thin film magnetic recording media are lubricated by perfluoropolyethers with polar end groups that enhance their attachment to the surface and limit lubricant spinoff induced by centrifugal force and air shear (Merchant et al., 1990; Ruhe et al. 1994; Mate and Wilson, 2000). Lubricants with polar end groups also have better durability and static friction properties than those with nonpolar end groups (Scarati and Caporiccio, 1987; Bhushan, 1990). The molecular structures of the commercial perfluoropolyethers referred to here are shown in Table 1. The Fomblin Z type pefluoropolyethers are random copolymers of perfluoro methylene oxide and ethylene oxide with 0.8 < min < 1.6. The Z type is available with several different end groups e.g., a nonpolar Z03 with two fluoromethyl end groups, polar Zdiac with two carboxylic acid end groups, and polar Zdol with two hydroxyl end groups. The Demnum type perfluoropolyethers are homopolymers of perfluoro n-propylene oxide. The Krytox type perfluoropolyethers are homopolymers of perfluoro isopropylene oxide. The Y perfluoropolyethers are similar to Krytox with about 3% perfluoro methylene oxide randomly copolymerized into the chain, and nonpolar fluoromethyl end groups. Fomblin, Demnum, and Krytox are the registered trade name of commercial


295

Table 1. Perfluoropolyether lubricant molecular structures. The degree of polymerization is the sum of the alphabetic sUbscripts plus 2 (taking into account the end groups).

Perfluoropolyether

Molecular Structure

Zdol

HOCH 2CF2 (OCF 2)m (OCF 2CF 2)n OCF 2CH2OH

Zdiac

HOOCCF 2 (OCF 2)m (OCF 2CF 2)n OCF 2COOH

Z03

CF3 (OCF 2 )m (OCF 2CF 2 )n OCF 3

Demnum S100

CF 3CF2CF2 (OCF 2CF 2CF2 )xo OCF2CF3

Krytox 143AD

CF3CF2CF2 (OCF(CF 3)CF2 )xo OCF 2CF3

Y

CF3CF2CF2 (OCF(CF 3)CF2 )a (OCF 2 )b OCF2CF3

samples produced by Ausimont S.p.A (Milan, Italy), Daikin (Osaka, Japan), and Du Pont (Wilmington, DE), respectively. Novel lubricants have also been prepared and tested. For example, the octadecylamine salt, the amide, and the octadecanol ester with perfluorodecanoic acid, which form self assembled mono layers, were tested on carbon overcoats (Seki and Kondo, 1991). Low friction and chemisorption were observed with an oriented film of stearyl amine and a dialkyl carboxylic acid on carbon overcoats (Sano et al., 1994). Hydrocarbons have not been widely considered as lubricants for rigid magnetic recording media. Although stearic acid has been used as a lubricant in some disk drive products (Gregory et al., 1988), and virtually all floppy disks are lubricated with tridecyl stearate (Gini et al., 1981).

4.1 Chemisorption Fomblin Zdol is the most widely used thin film magnetic recording disk lubricant. Zdol both physisorbs and chemisorbs on carbon overcoats. Chemisorbed (often referred to in the literature as bonded) Zdol is defined as lubricant which remains attached to the overcoat after washing with a nonpolar solvent such as perfluorohexane (Merchant et al., 1990). There are two leading mechanisms which have been proposed to account for chemisorption: (1) proton transfer from the hydroxyl end group to dangling bonds in the overcoat (Yanagisawa, 1994; Kasai et al., 1999), and (2) hydrogen bonding between the hydroxyl end group and carboxylic acids on CHx and imines on CN x (Waltman et al., 1999).

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Chemisorption mechanism (1) is supported by the observation that Zdol chemisorption, or oxygen, quenches the ESR signal, and that the chemisorbed fraction is higher when Zdol is applied directly to the overcoat before exposure to atmospheric oxygen (DeKoven, 2000). There are both weak and strong chemisorption sites. Table 2 shows the lubricant thickness before and after washing with nonpolar solvent perfluorohexane followed by washing with polar solvent trifluoroethanol. After washing with perfluorohexane, about 0.1 nm of more weakly chemisorbed Zdol was removed during the subsequent wash with trifluoroethanol. The weakly chemisorbed, portion of the chemisorbed Zdol is probably that which is attached by hydrogen bonding. The total amount of weakly chemisorbed Zdol was about the same for both low and high chemisorbed fraction, so that the weakly chemisorbed Zdol was 21 % of the total chemisorbed lubricant at the low level of chemisorption and 12% of the total chemisorbed lubricant at the high level of chemisorption. Table 2. The total and chemisorbed lubricant amounts on CN x disks dip lubricated with 1 nm of Zdol 4000 from the test series shown in Fig. 1. The low chemisorbed fraction was at 65°C and 40% RH, and the high chemisorbed fraction was in ambient air at .65°C and 5% RH, both for 630 hours after lubrication. The nonpolar solvent is perfluorohexane, and the polar solvent is trifluoroethanol.

0.90

Nonpolar Solvent Washed (nm) 0.42

Polar Solvent Washed (nm) 0.33

0.97

0.88

0.77

Chemisorbed Fraction

Before Wash (nm)

Low High

After initially depositing the Zdol film on the overcoat by dip coating from solution, the chemisorbed fraction increases with time. The chemisorption reaction is suppressed by high relative humidity (RH) (Karis, 2000). The chemisorption reaction is reversible. Completely chemisorbed Zdol returned to equilibrium with 20-30% physisorbed over several weeks time at ambient temperature and RH on a CHx overcoat (Karis et al., 2001). Temperature and RH influence the chemisorption reaction through water adsorption on dangling bonds and surface oxides (Zhao et al., 1999). The typical chemisorption rate of Zdol on a CNx overcoat with time after dip coating is shown in Fig. 1. The rate of chemisorption was highest at 65°C and 5% RH. The rate decreased when the RH was decreased from 5% to 0% at 65°C, even though more surface sites should be available at the very low RH. This decrease in the chemisorption rate between low and very low RH is attributed to the decrease in Zdol mobility at very low RH as

297


0.8

.65 C, O%RH 065 C, 5%RH .65 C,40%RH 0 AmbientT and RH

.......o=

o

C.I

E

~

0.6 -

o

"CS ~

,.Q

'o"'

·s fI'J

0.4

-= U ~

~

•.. .j•.'

.........

.................................... .........

0.2

o I.E+OO

l.E+Ol

l.E+02 Time (hrs)

l.E+03

l.E+04

Figure 1. Chemisorption of initially 1 nm thick Zdol 4000 with OIC ratio 0.67 on CN x overcoat as a function of time showing the effect of high and low relative humidity (RH).

observed by Min et al., 1995. The chemisorption at 65°C was significantly decreased by increasing the RH to 40%. The chemisorption at ambient temperature and RH was higher than at 65°C and 40% RH. Ambient temperature was between 20 and 25°C, and the ambient RH was between 30 at 50%. These results suggest that the chemisorption of Zdol on CNx overcoat is a strong function of both RH and temperature. The chemisorption rate is also influenced by the Zdol polymer chain relaxation times, which govern the mobility, hence the rate at which reactive end groups diffuse to active surface sites (Oshanin et al., 1998; Karis, 2000). The relaxation times for bulk perfluoropolyethers calculated from rheological measurements of the storage and loss shear modulus master curves are listed in Table 3. The relaxation time Tc for Zdol increased by a about factor of 2 as the ole ratio decreased from about 0.69 to 0.65. Assuming that the chain relaxation time on the surface scales with the bulk relaxation time, the increase in Tc should decrease the chemisorption rate at a given time after dip coating, and increase the time to the transition from end

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Table 3. The relaxation time Tc for bulk, unfractionated, commercial perfluoropolyethers at 60°C. The number average molecular weight Mil, and the oxygen to carbon ratio ole were measured by nmr spectroscopy The relaxation times were calculated from rheologically measured dynamic shear modulus master curves using time-temperature superposition.

Perfluoropolyether

Mn (amu)

ole

Zdol4000

3,600

0.693

2.40

Zdol4000

3,600

0.666

2.40

Zdol4000

4,300

0.658

3.50

Zdol4000

3,900

0.650

5.20

Z03

7,600

0.646

3.28

Demnum S100

5,000

0.333

12.50

Krytox 143AD

6,600

0.333

23.00

Tc

(I-"s)

bead to segmental diffusion limited chemisorption. These trends are reflected in the experimental dependence of the chemisorption kinetics on Zdol chain composition in Waltman et al., 2000.

4.2 Diffusion The disjoining pressure gradient arises from the thickness dependence of the surface energy, and it provides the driving force for surface diffusion (Tyndall et al., 1999; Karis and Tyndall, 1999). Physisorbed lubricant diffuses toward regions of higher disjoining pressure. The total surface energy of Zdol lubricated overcoats decays non-monotonically toward its bulk value with increasing Zdol film thickness due to molecular orientation induced by polar interactions with the surface (Tyndall and Waltman, 1998; Tyndall et al., 1998a, b; Karis, 2000). The typical surface energy, and the underlying film structure, as a function of Zdol thickness for low and high chemisorbed fractions are shown in Fig. 2. At the lower cllemisorbed fraction, the hydroxyl end groups are exposed in some regions of film thickness, giving rise to local maxima in the polar component of the surface energy. The diffusion velocity increases rapidly with decreasing film thickness near monolayer coverage (van der Waals chain diameter ~ 0.4 nm) as the surface energy sharply increases toward that of the un lubricated carbon overcoat with decreasing film thickness. Since the disjoining pressure is the negative gradient of the surface free energy with respect to film thickness, regions of stable film thickness are those in which the surface energy decreases with increasing film thickness.

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