Chapter 25
Nanotribology of Polymer Surfaces for Disk Drive Applications C. Mathew Mate and Junhua Wu IBM Research Division, Almaden Research Center, San Jose, CA 95120
In this paper, we discuss the importance the polymer layers to the protection of disk surfaces inside of disk drives by first providing a set of criteria that can be used for developing a disk lubricant system or other polymer protective overlayers. Next, we discuss how contact angle measurements can be used to provide insights into how the polymer lubricants behave on disk surfaces. Finally, we discuss a new type polymer layer that we have developed to supplement traditional disk overcoats. This polymer overlayer is formed by vapor depositing a cyanate ester monomer then polymerizing the film with UV irradiation.
In disk drives, as illustrated in Figure 1, bits of information are written and read from magnetic domains in magnetic layers on disk surfaces using recording heads attached to the back of sliders. When a disk is spinning at normal operating speeds, a thin air bearing film, 30-50 nm in thickness, keeps the slider from physically contacting the disk surface. When the drive is powered on and off, however, the sliders in most of today's disk drives must take off and land on the disks, generating many slider-disk contacts. This is most severe for disk drives in laptop computers, which are frequently powered off to extend the battery life of the computer. The magnetic layer and the information stored in it are protected during these slider-disk contacts by several protective layers: an overcoat, usually consisting of amorphous carbon, and a thin film of polymer lubricant. To achieve higher recording densities in future disk drives, the spacing between the magnetic layer on the disk and the recording head will need to be considerably reduced in order to decrease the size of the magnetic domains in the disk magnetic layer. This spacing reduction will lead to severe reductions both in the thickness of die air bearing film, resulting in morefrequentslider-disk contacts, and in the spacing available for the layers of protective materials. Consequently,
© 2000 American Chemical Society
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Figure 1. Schematic of the inside of a disk drive. Enlarged section shows the details of a typical slider-disk interface in current disk drives.
407 these protective materials such as the overcoat and lubricant will have to provide more wear and corrosion protection with less material. Nanotribology. Nanotribology is the study of how friction, lubrication, and wear occur at the nanometer and sub-nanometer scale, i.e., at the atomic and molecular level Since the lubricant films currently in disk drives are only about 1 nm in thickness and, within a few years, the carbon overcoats will only be a few nanometers in thickness, improved understanding of how these materials work at the atomic and molecular level to provide tribological protection is critically needed to develop dramatically more effective materials. In this paper, we discuss three different topics on the application of polymers for improving the nanotribology of the disks used in disk drives. First, we discuss some of the criteria for a good lubricant system, which provides a general overview of what is desired for protective polymers on disk surfaces. Next, we discuss the how contact angle measurements can be used to gain insights into how lubricating disk surfaces with perfluoropolyether polymers can help satisfy these criteria and how these polymers interact with a disk surface and with certain types of contaminants found in disk drives. Finally, we present a novel type of polymer overlayer that we have developed that resides between the carbon overcoat and the lubricant film to help smooth out die final disk surface. This polymer overlayer is formed by vapor depositing a cyanate ester monomer onto disk surfaces then polymerizing the layer with UV irradiation. Criteria For Good Polymer Layers on Disk Surfaces A number of criteria are used within the disk drive industry for selecting a polymer lubricant material (J). Since these criteria are often contradictory, the principal challenge is finding a lubricant system that provides the best compromise for the particular slider-disk interface under development. Other polymers developed as a protective material, either as overcoat or overlayer as discussed below, should also satisfy these criteria. A good lubricant system or protective polymer layer should provide: 1. Good boundary lubrication. • Provides adequate wear protection so that the slider-disk interface lasts the expected life of the disk drive product. • Ensures against the hazards of flying in close proximity for long periods of time. 2. Mobility ofpolymer lubricant • Improves long term durability (i.e., polymer molecules should have enough mobility to move across the disk surface to replenish lubricant loss during slider-disk contacts). 3. Good adhesion ofpolymer lubricant or polymer protective layer to the surface. • Provides long term durability by preventing polymer molecules from being displaced during slider-disk contacts.
408 • Reduces lubricant loss due to spin-off by reducing mobility. 4. Low volatility. • Reduces loss of polymer molecules due to evaporation. 5. Low surface energy ofthefinallubricated disk surface. • Lowers adhesive and friction forces. • Minimizes the amount of contamination adsorbing onto disk surfaces. 6. Thermal and chemical stability. • Stable towards catalytic decomposition. • Stable towards formation offrictionalpolymers. • Compatible with other disk drive components. 7. Corrosion resistance. • Water, oxygen, and other contaminants should not be able to penetrate the overcoat and protective layers and to corrode the underlying magnetic layer. Perfluoropolyethers. Historically, perfluoropolyether polymers have been preferred as disk lubricants since they are liquid at disk drive operating temperatures with low surface tensions and low volatility, as well as being chemically inert and stable at high temperatures, satisfying many of the criteria enumerated above (2). In particular, the oxygen ether linkages in the perfluoropolyether polymer provide the polymer backbone with sufficient flexibility for the material to be liquid over a wide temperature range. Often the perfluoropolyether polymer backbones are terminated by functional end groups designed to attach the lubricant molecules to the overcoat surface, so as to prevent them from being displaced from the surface during slider-disk contacts. In order to meet the increasing severe tribological demands placed on the disk protective layers, new type polymer materials beyond the traditional perfluoropolyether lubricants will most like have to be developed for the disk drive applications and new ways of bonding or adhering the polymers to the disk surface will have to be utilized. Contact Angle Measurements One of the above criteria is that thefinisheddisk surface should have a low surface energy. This is beneficial because sliding surfaces with low surface energies tend to have low friction and adhesion forces acting between them. Also, low surface energies help to rninimize the adsorption of contaminants that are always present in disk drives, which can lead to a wide variety of tribological failures of the slider-disk interface. The low surface tensions of perfluoropolyether lubricants can help lower disk surface energies. From the Young equation or die Girifalco-Good-FowkesYoung equation, high contact angles imply low surface energies; so contact angle studies can provide insights of the surface energies of lubricated disks. In addition, contact angle measurements can provide information into how the lubricant polymers interact with disk surfaces and how easily they are displaced by certain types of contaminants. Contact angle measurements are frequently used in the magnetic storage industry to gain insights into the chemical nature of surfaces inside disk drives (3-7). One can use this technique to monitor manufacturing processes for finishing the
409 surface of components, that is, if the contact angle on a test sample is out of specification, then the manufacturing process is adjusted until the component surface has the desired contact angle. The question naturally arises as to what is a desirable contact angle for these surfaces and what surface properties influence the contact angle. Obviously, a better understanding of the physical and chemical factors that influence contact angles would be of great value to the storage industry, as well as other industries that make use of this common technique. Contact Angle Results. We have recently investigated the contact angle of various liquids on disks similar to those found in disk drives (8). Figure 2 shows the cosine of the contact angle of alkane droplets on a disk with an amorphous hydrogenated carbon overcoat (CHx) lubricated with different thicknesses of the perfluoropolyether [ Z-Dol-4700: H O C H 2 C F 2 ( O C F 2 Ch )w (OC$ \ OCE CH OH with a molecular weight of 4700 amu]. Contact angle measurements were performed by releasing a small liquid droplet onto the freshly lubricated surfaces and measuring the advancing contact angle when the speed of the contact line is near zero. The Young-Dupré equation AfF=y/(l+cos0)
(1)
relates the cosine of the contact angle θ to the work of adhesion Δ FF between the alkane and the disk surface. The work of adhesion for alkanes on these lubricated surfaces can be separated into a term that describes the direct interaction with the lubricant material and a term that describes how the interaction with the substrate is mediated by the intervening lubricant layer: AIF-ai +
ftie-™-
(2)
which can also be written as ^-1=οο80 =α + M ~ ^
E
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ο
where L is the lubricant layer thickness and L the screening distance of the substrate interaction through the lubricant layer. The solid lines in Figure 2 show that the fit of this model to the data is very good. The screening distances L are very short (5-7Â) and comparable to the 6 Â chain diameter of the Fomblin Z-Dol-4700 polymer. So, within a few layers of lubricant molecules, the alkanes are effectively interacting with a bulk-like lubricant film. The screening distances L may also be a measure of how well the lubricants cover the surface in presence of the alkane liquid with the short screening length indicating good coverage and little displacement of the lubricant moleculesfromthe surface of the disk by the alkane liquids. Water contact angle measurements reveal to what degree the lubricated surfaces are hydrophobic or hydrophilic. Figure 3 shows how the cosine of the water contact angle changes with increasing Z-Dol lubricant thickness on different types of a
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Copyright 1998 American Chemical Society.)
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Figure 3. Cosine of water contact angle vs. lubricant thickness. The solid lines show the fit of the data to the empirical equation: cosfl = a + (ufy+y (Reproduced from reference 8. Copyright 1998 American Chemical Society.) wafgr
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411 surfaces. Lubricant thickness for ail type of surfaces were measured by ellipsometry, which was calibrated using X-Ray Reflectivity (9). For all surfaces, we observed that the cosine monotonically decreases, indicating that all the surfaces become more hydrophobic with increasing lubricant thickness. The lines in Figure 3 show fits to the experimental data using the following empirical expression: COS θ water
=
Gw +
(4)
which mainly serves as guides to the eye as scatter in the data prevents confident fits. From Figure 3 it is apparent that, for very thick films, the cosine of the water contact angle is quite different for each substrate, even for the same lubricant. This indicates that water still interacts, to varying degrees, with the underlying solid surface even in presence of a thick lubricant film. For example, the water contact angle on silicon wafers only increases slightly with the addition of the Z-Dol-4700 lubricant. A possible explanation for this effect could be that water much more easily displaces the Z-Dol molecules from the very hydrophilic SiO surface in comparison the carbon overcoats. This phenomena should be contrasted with that for alkanes droplets on very thick lubricant films where the contact angle is independent of the type of substrate. The screening length for water, represented by in the empirical expression, ranges from 20-47 A , which is considerably longer than the screen length L (5-7 Â) observed for alkanes and may be further evidence that water more easily displaces the lubricant. The importance of these contact angle results to the disk drive industry can be summarized as follows: Since disks in disk drives are typically lubricated with 5-20 Â of perfluoropolyether lubricant, their surfaces should be reasonably well screened against the adsorption of alkanes and other hydrocarbons, particularly at the upper range of this thickness range, but only slightly screened against the adsorption of water, which may lead to problems for drives operating in high humidity environments. While perfluoropolyether lubricants lower the surface energies of disks, the thickness needed to reach lowest surface energy depends strongly on the type of substrate, lubricant, and die test liquid used for the contact angle measurement. x
a
Polycyanate Ester Overlayer for Disks In addition to the liquid polymers used as lubricant, solid polymer films have been frequently investigated as overcoats and overlayers on disk surfaces. For example, bonding and crosslinking of perfluoropolyether lubricants to disk surfaces using ultraviolet irradiation has been reported by several groups (10-11). Solid fluorocarbon films have also been made on disk surfaces using sputtering, plasma, and ion beam deposition methods (12-14). Most of these past efforts have focused on developing solid lubricant films. Here, we report how a solid polymer film can be used to help smooth out or planarize the disk surface. The polymer film that we
412 have developed consists of polyeyanate esters that reside between the carbon overcoat and lubricant film. Cyanate esters are a well developed group of high temperature, thermosetting polymers (15). Polyeyanate ester polymers are known as coating materials, primarily as matrix resins for composites (e.g., boards, cards, etc.) or for dielectric coatings for semiconductor devices. These coatings are typically several microns to millimeters thick and are polymerized by thermal curing of a cyanate ester monomer, which has been coated by spinning onto the device substrates or by dip-coating onto fibers. For these coatings, some of the favorable properties obtained by using polyeyanate esters are low dielectric loss, high temperature stability, excellent adhesion capacity, and good mechanical properties. Preparation. In this communication, we focus on preparing ultra-thin polymer overlayers on disks starting with the cyanate ester monomer, 2,2'-bis(4-cyanatophenyl)isopropylidene (BCPP), supplied by Ciba Specialty Chemicals. Figure 4 shows the chemical structure of the BCPP monomer and the ring forming cyclotrimerization reaction that occurs when these monomers are heated in the presence of a suitable catalyst. Our method for forming the polyeyanate ester overlayer on disk surfaces is illustrated in Figure 5. This method consists of two steps: Deposition. (Figure 5a) A source containing the monomer is heated to 110°C. At this temperature, BCPP sublimes at a sufficient rate through air to be vapor deposited onto an unlubricated disk placed a few centimeters above the heated source, as illustrated in Figure 5a. The disk is kept at room temperature during vapor deposition, and the BCPP is uniformly deposited at the rate of a few  per minute. For the overlayers, we typically deposited 5-15  of monomer as measured by ellipsometry. For thicknesses greater than 15 Â, we usually observed dewetting of the final polymer film. Polymerization. (Figure 5b) We found that heating the ultra-thin cyanate monomer films vapor deposited onto disks caused evaporation rather than polymerization. Consequently, we had to used a different method, UV initiated polymerization, to convert the soft monomer film into a robust polymer film. For the UV curing, the monomer films were placed under a UV lamp (185-254 nm, intensity ~ 5 milliwatts/cm ), while oxygen and other reactive gases were excludedfromthe chamber by flushing with the inert gas, nitrogen. A few minutes of UV irradiation was sufficient to covert a 1 nm thick film of monomer completely into polymer. 2
Polymerization was checked several ways: Solubility. Solvents that could easily remove the BCPP monomer, like methyl-ethyl-ketone (MEK), were unable to dissolve the UV cured product.
413
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Figure 4. The chemical structure of the BCPP monomer and how the cyanate end groups react in the bulk with heating to form a cross-linked ring network.
Figure 5. Schematic of the preparation technique for forming the polyeyanate ester overlayer on disk surfaces.
414 Heating. The UV cured films were stable upon heating to 220°C for 10 minutes, while the monomer films quickly evaporated at this temperature. FTIR. Figure 6 compares the spectra of the monomer and the UV cured polymer obtained from grazing angle Fourier Transform Infrared (FTIR) spectroscopy. For the monomer spectrum, the vibrational doublet centered around 2250 cm" characterizes the O N cyanate groups in the monomer, and the disappearance of these peaks in the cured spectrum is consistent with the cyanate groups reacting to form crosslinked networks. Also, new broad peaks emerge in the range 900-1800 cm' in the UV cured spectrum characteristic of polymer formation. 1
1
Results and Discussion. The UV polymerization process demonstrated here has several advantages over other methods for forming polymer films such as plasma and chemical vapor deposition or curing with heat and electron beams. Photo-polymerization using ultraviolet light is a clean, efficient, and compact process. Further, the polymerization reaction can be instantly started and stop and potentially reach 100% polymerization in a short period of time. The process is also environmentallyfriendlyin that it does not require the use any solvents or noxious gases. Polymerization Mechanism. The polymerization reaction that occurs by heating the BCPP monomer is thought to involve the reaction of the O N end groups to form s-triazine rings as shown in Figure 4. However, the spectrum in Figure 6 for a UV cured polymer does not exhibit the expected vibrational peaks for the s-triazine ring indicating that, when UV irradiation is used to initiate polymerization, the reaction may follow a different mechanism such as cyclodimerization and ring expansion. Further work is needed to elucidate the polymerization mechanism that occurs during UV irradiation. Topography. Atomic force microscopy (AFM) was used to measure how the topography of silicon wafers and disk surfaces change with the addition of the polyeyanate ester overlayer. Figure 7 shows an AFM image of a bare silicon wafer and of a 10 Â film of the polyeyanate ester overlayer. Even though the image of the polyeyanate ester overlayer has a finer texture than the silicon wafer, the rms roughness remains unchanged, 1.3 Â for the overlayer vs. 1.1 Â for the silicon wafer, demonstrating that the polyeyanate overlayer does not increase the surface roughness so long as the film thickness is kept below 15 Â. (For films thicker than 15 Â, dewetting of the polymer film is often observed.) When the polyeyanate ester film is formed on disk surfaces, which are much rougher than the silicon wafers, the AFM images show that the polymer fills in the deep valleys and pinhole defects on the surface, demonstrating the ability of the polyeyanate ester overlayer to planarize surfaces. For example, the root-mean-square roughness for one type of disk surface studied was found to decrease from 16 Â to 8 Â over a ΙμηιχΑμτη area with the addition a 10 Â thick polyeyanate ester overlayer. As discussed earlier, the goal of developing new polymer lubricants and overlayers is to satisfy the numerous demands placed on these very thinfilmswhile
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Figure 6. Vibrational spectra collected using FTIR of a thick monomer film before and after polymerization with UV irradiation.
3500
B-10 Momer
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Figure 7. Tapping mode A F M image of a bare Si(100) wafer with 15 Â of native SiOx and a wafer covered with 10 Â of B-10 monomer then cured with UV irradiation.
417 maintaining good magnetic recording performance. To handle the static friction forces occurring at the slider-disk interface when the drives are powered up, most current disk drive products have either a dedicated, roughened landing zone or use a load-unload mechanism. In the data zone, the magnetic layer is also formed on a slightly roughened or textured substrate to achieve the best magnetic recording characteristics. However, for the best durability and corrosion protection, the smoothest surface is desired for the data zone. Since sputter-deposited carbon overcoats tend to conform to the surface of the rough magnetic layer, the addition of a polyeyanate ester overlayer, to provide a smoother final disk surface, has the potential to improve the tribology of the disk without compromising the magnetic performance, which will be explored further in future work. Acknowledgments We would like to thank J. Lyerla and G. Tyndall for valuable discussions. We would also like to thank J. Hedrick for suggesting using the BCPP cyanate ester for the polymer overlayer and for valuable discussions and advice during this project. Literature Cited 1.Mate, C. M. Tribology Letters 1998, 4, pp 119-123. 2. Homola, A. M . IEEE Trans. Magn. 1996, 32, pp 1812. 3. Bhushan, B. "Tribology and Mechanics of Magnetic Storage Devices", 2nd ed.; Springer, New York, 1996. 4. Lee, J. K.; Smallen, M.; Enguero, J.; Lee, H. J.; Chao, A. IEEE Trans. Magn. 1993, 29, pp 276. 5. Ruhe, J.; Blackman, G.; Novotny, V. J.; Clarke, T.; Street, G. B.; Kaun, S. J. Appl. Polym. Sci. 1994, 53, pp 825. 6. Perry, S. S.; Mate, C. M.; White, R. L.; Somorjai, G. A. IEEE Trans. Magn. 1996, 32, pp 115. 7. Tyndall, G. W.; Leezenberg, P. B.; Waltman, R. J.; Castenada, J. Tribology Letters 1998, 4, pp 103. 8. Wu, J.; Mate, C. M . Langmuir, 1998,14,pp 4929-4934. 9. Toney, M . F.; Mate, C. M.; Pocker, D. J. IEEE Trans. Magn. 1998, 34, pp 1774-1776. 10. Homola, A.M.; Lin, L.J.; Saperstein, U.S. Patent No. 4,960,609. 11.Lee, H.J.; Zubeck, R.; Hollars; Lee, J.K.; Chao, Α.; Smallen, M. J. Vac. Sci. Tech. A 1993, 11, pp 711-714. 12. Harada, K; J. Appl. Polym. Sci. 1981, 26, pp 3707-3718. 13. Koishi, R.; Yamamoto, T.; Shinohara, M . Tribol. Trans. 1993, 36, pp 49-54. 14. Karis, T. E.; Tyndall, G. W.; Crowder, M . S. IEEE Trans. Magn. 1998, 34, pp 1747-1749. 15. Fang, T.; Shimp, D. A. Prog. Polym. Sci. 1995, 20, pp 61-118.
