LETTER pubs.acs.org/NanoLett
Scanning Transmission Electron Microscopy Analysis of Grain Structure in Perpendicular Magnetic Recording Media Faraz Hossein-Babaei,*,† Robert Sinclair,† Kumar Srinivasan,‡ and Gerardo A. Bertero‡ † ‡
Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States Western Digital Corporation, 1710 Automation Parkway, San Jose, California 95131, United States ABSTRACT: The key component of a hard disk medium is a Co-based magnetic layer (ML) grown on a Ru seed layer. The ML nanostructure, composed of less than 10 nm grains, is believed to be controlled by this seed layer. We successfully used scanning transmission electron microscopy energy dispersive spectrometry simultaneous composition-based imaging and Moire pattern analysis for determining the mutual structural and orientation relationship between the two layers revealing a grain-to-grain agreement. The method presented here can be utilized for observing structural correlations between consecutive polycrystalline thin film layers in general. KEYWORDS: PMR medium, Co nanoparticle, seeded growth, multilayer nanostructure, STEM-EDS, Moire pattern, Ru nanoparticle
T
he data storage density of magnetic recording media continues to increase steadily.16 Further progress requires the controlled arrangement of smaller magnetic grains in the recording medium. Obtaining a desirable nanostructure in the magnetic layer (ML), the topmost active layer in a hard disk layer stack (Figure 1), is of critical importance to the performance of the device.7,8 The ML is a continuous nanocomposite thin film of magnetic cobalt-alloy grains isolated from one another by an ideally nonmagnetic intergranular matrix.1,3,9 To enhance the magnetic properties and maintain the required signal-to-noise ratio in the read and write processes, downscaling of the average ML grain size should be accompanied by tightening their size distribution.10 The average grain size in these recording media has reached nanometer scales, about 7 nm at present.3,1114 Cobalt-based perpendicular magnetic recording (PMR) media comprising Co-rich grains of about 7 nm average diameter with standard deviation of ∼1 nm are manufactured by sputter deposition of the different device layers on both sides of a substrate at precisely controlled rates and atmospheric conditions. Different underlayers and seed layers are employed to obtain such grain structures in the ML. Figure 1 shows the layer stacking in a typical device schematically. ML is deposited on a Ru layer which, like ML, has a hexagonal close-packed (hcp) crystal structure. The Ru seed layer is thought to control the ML nanostructure.4,8,15 The grains in both ML and Ru layer exhibit strong textures with their crystallographic c axes aligned perpendicular to the thin film surface.9,16 Understanding the structural correlation between the ML and Ru layer is important for better control of the PMR media nanostructures. Owing to the nanometric feature scales involved and the intended simultaneous observation of the two layers, the transmission electron microscope (TEM) is the most suitable tool. r 2011 American Chemical Society
However, discriminating the ML and Ru layer from one another using phase or diffraction contrast, as used in the broad beam TEM imaging mode, is impeded by the overlapping of the structural features of both layers in the images. Moreover, cross-sectional examination requires specialized and time-consuming specimen preparation, yielding limited thin area for the analysis. Accordingly, here we report a novel use of scanning transmission electron microscope energy dispersive spectrometry (STEM-EDS) to simultaneously observe the nanostructures of the ML and Ru layer. The results show a strong grain-to-grain agreement between the two layer structures. The method used can be utilized for the simultaneous observation and the determination of compositional and orientational correlations among consecutive nanometric layers in general. The disks studied comprise a 1.5 mm thick AlMg substrate with the functional layers shown in Figure 1 sputter deposited on both sides. A 20 μm thick layer of NiP is electrodeposited prior to the sputtering. All the other layers are then grown in a serial arrangement of 20 sputtering chambers. The ML deposited is of CoCrPtTiO2 composition.17,18 Plan-view TEM specimens were prepared using a conventional method:17,19 Each sample was ground and polished from the back side of the sample. After being punched into standard 3 mm diameter TEM specimen disks, the samples were further ground using a Gatan-656 dimple grinder from the back side to a thickness of less than 15 μm at the center. The specimens were then ion milled with Ar+ ions at 45 keV at incident angles of 45° using a Gatan-691 precision ion polishing system. Electron transparent regions near the holes Received: May 25, 2011 Revised: July 28, 2011 Published: August 01, 2011 3751
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Table 1. Measured and Calculated Interplanar Spacings for ML and Ru Layer with Uncertainty of about 1% for the Primary Plane Systems Observed in the SAED Pattern Shown in Figure 2 measured
calculated
hkil
dRu (nm)
dML (nm)
dRu (nm)
dML (nm)
1010
0.235(3)
0.226(3)
0.234
0.223
1120
0.135(2)
0.130(2)
0.135
0.129
2020
0.117(2)
0.113(2)
0.117
0.111
2130
0.089(1)
0.085(1)
0.0884
0.0842
Figure 1. A schematic diagram of layers in a CoPtCrTiO2 alloy-based PMR medium. All layers with the exception of NiP (electrodeposited on the substrate for rigidity) are sputter deposited at precisely controlled conditions onto a 1.5 mm thick AlMg substrate.8
Figure 3. A high-resolution phase contrast TEM image of the ML in a CoCrPtTiO2 PMR medium plan-view specimen showing the {1010} lattice fringes and the Moire pattern resulting from interference of {1120} planes.
Figure 2. A BF diffraction contrast TEM image of the ML (a) in a CoPtCrTiO2 PMR medium plan-view specimen showing the Co alloy grain structure and Moire fringes in each grain. Both the Ru layer and ML are present in this region producing the Moire fringes. The associated SAED pattern (b) is produced from the contribution of plane systems in the [0001] zones of both the ML and the Ru layer.
perforated after ion milling in the 1030 nm specimen thickness range were used for TEM studies. These regions included the MLRu layer interface. The TEM instrument used for the analysis was FEI G2 F20 Tecnai operated at 200 kV in both TEM and STEM modes. Figure 2 shows a bright field (BF) TEM image along with the selected area electron diffraction (SAED) pattern of the observed region. The hcp crystal structure is confirmed for both ML and Ru layer from the diffraction pattern. Owing to the vertical orientation of the c axes in all grains (i.e., the c axes are parallel to the electron beam), only the plane systems in the [0001] zone, i.e., {hki0}, contribute diffraction rings. Table 1 presents the interplanar spacings of the primary plane systems determined from the SAED pattern along with those calculated based on
alloy data.20,21 For calculated values, pure Ru for Ru layer and the nominal ML composition (excluding Ti and O which are largely segregated to the intergranular phase)17 for ML grains were assumed. The lattice constant, a, of the CoCrPt alloy in the ML is smaller than that of Ru, and the slightly larger radius rings in the SAED patterns are, therefore, due to the diffraction from the ML. The lattice mismatch between the ML and Ru layer depends on the ML composition. For the specified sample, the lattice constants were calculated from the SAED pattern to be 0.271 ( 0.003 nm for the Ru layer grains (compared with 0.270 nm of bulk pure Ru20) and 0.261 ( 0.003 nm for the ML grains (compared with 0.257 nm linearly calculated for the nominal CoPtCr composition of ML grains from the available CoPt and CoCr data21) after calibration with a standard Au nanoparticles specimen. The small differences between the measured and calculated lattice constants are attributed to the incorporation of oxygen and Ti species in ML and Ru layer grains during their sputter deposition, which was not considered in the calculations. The BF TEM image in Figure 2a shows the nanostructure of the specimen, where both the ML and Ru layer are present, comprising nanosized crystalline grains embedded in a lighter shaded amorphous intergranular matrix.22 The large period striations within the grains are due to the Moire effect which arises from the difference in d-spacings of the respective planes in the Ru layer and ML grains. The Moire fringes are consistent with the in-plane polycrystalline orientations and show that both thin films are contained in the viewing area. By averaging the data obtained from 60 different grains observed in BF TEM images, the Moire pattern period, λM, was measured to be 3.1 ( 0.3 nm. 3752
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Figure 4. Long irradiation time (30 s) and wide area (800 nm diameter irradiating beam diameter) EDS spectrum obtained from the specimen comprising the carbon overcoat, ML, Ru seed layer, and the NiW layer.
As observed in Figure 2a, this Moire fringe periodicity is quite common among the grains of this sample. Only a few percent of grains examined demonstrate Moire patterns of significantly smaller periods (not seen in Figure 2a), which presumably arise due to strong diffraction from higher order planes in these crystals as the c axis crystallographic texture is not exact.9 Figure 3 shows the high-resolution phase contrast TEM image of a grain exhibiting the typical Moire pattern period. The dominant lattice fringes show {1010} planes. In such phase contrast images, the lattice fringes from the crystalline grains extend a short distance beyond the grains and overlap in the intergranular regions, making the matrix phase appear crystalline. This artifact arises owing to the delocalization effect caused by the spherical aberration of the objective lens of the TEM.23 The orientational relationship between the Moire and lattice fringes reveals that the Moire pattern arises from the interference of {1120} planes. Denoting the {1120} interplanar spacings of Ru layer and ML grains by d1 and d2, respectively, the resulting Moire interference pattern period, λM, for a parallel plane system such as that assumed for the ML grains and their respective seed grains at the Ru layer, is24,25 λM ¼
1 d1 d2 ¼ g1 g2 d2 d1
ð1Þ
The reciprocal lattice vectors g1 and g2 have magnitudes equivalent to inverses of the respective interplanar spacings. Using the {1120} d spacings resulted from the SAED patterns of ML and the Ru layer (see Table 1), the Moire pattern period is calculated as 3.5 ( 0.3 nm similar to the value directly measured from the TEM images. Presumably Moire fringes from {1010} planes are too widely separated (6.1 nm) to be seen clearly within 7 nm grains. Energy dispersive spectrometry (EDS) spectra were obtained from regions of specimens thick enough to contain both the ML and Ru layer such as that shown in Figure 2. In these experiments the specimen was tilted +15° toward the EDS detector to enhance the detection efficiency of the X-rays emitted. Figure 4 shows an EDS spectrum obtained from the specimen over a large area (800 nm across) and X-ray collection time of 30 s at a count rate of ∼3000 s1. All elements in the sputtered layers present in the region analyzed show X-ray peaks in the spectrum. Maps were produced by rastering the electron beam over the specimen and collecting X-ray signals over the dwelling time for each pixel point of the map. The parameters of the STEM-EDS analysis software (gun lens current, beam spot size, pixel size, analysis area, and pixel dwelling time) were selected to obtain EDS elemental maps with a 1 nm resolution. Short dwelling times of 2 s per pixel point, imposed by the nanoscale sample drifts, and the low brightness of the 1 nm diameter probe limited the number of pixels in each map to 20 30. The collected EDS spectra of the specimen were used for constructing two-dimensional elemental
Figure 5. The BF TEM image (a) and the elemental maps obtained simultaneously for Co (b), Pt (c), Ti (d), Ru (e), and O (f) from a 21 nm wide region of a specimen comprising the ML and Ru layers. Note the relative displacement of the Co grains to the left relative to the Ru seed grains in the Co and Ru maps shown in (g) and (h) at higher magnification.
X-ray maps based on each element’s KR peak filtered from the spectra. An energy window was manually selected around a characteristic peak of an element after its identification in the reference spectrum collected over a long period of 30 s; for example, the 6.87.1 keV energy window was used to identify Co. The maps were produced by measuring the relative concentration of an element at a pixel point by integrating the spectrum intensity curve over the energy window specified. The brightness of each pixel on the map represents the relative concentration of the element within that map. The obtained STEM-EDS data were analyzed using the software TEM Imaging and Analysis 2. The elemental maps extracted from a 21 13 nm2 area on the sample are given in Figure 5. Nanometer-scale features are noticeable. In the ML, Co and Pt are more concentrated in the grains while Ti and O are segregated into the intergranular matrix as described by others.17 In the Ru layer map, likewise, the grains appear brighter than the grain boundaries owing to the formation of voids at grain boundaries due to the high ambient pressure used during the sputter deposition of this layer.26 The agreement in the nanostructures of the two layers is Ru grain to Co alloy grain. This was confirmed in three different regions examined. 3753
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Figure 6. Co and Ru maps obtained at different sample tilt angles from the same sample region demonstrate different lateral displacements which are indicated with the relative positions of the bright vertical lines. The actual tilt angles were determined from matching the measured and calculated displacements.
Close inspection shows that there is a small displacement between the corresponding features in the Co and Ru elemental maps, which was measured to be about 2.5 nm for all the grains examined in this case. This is not due to a displacement of the Ru and Co alloy grains but rather the TEM specimen tilt. The observed lateral displacement is consistent with the specimen tilt angle, and the layer thicknesses where examined. Denoting the distance between centers of the two layers as t, the displacement between their features in the elemental maps would be t sin(θ), where θ is the actual tilt angle of the specimen examined. The predicted displacements of 1.4, +0.7, and +3.9 nm matched the measured values (see Figure 6) at actual tilt angles of 9°, +6°, and +26°, respectively, with t being 9 nm. The total specimen thickness based on this value is 23 nm equivalent to the thicknesses of the carbon overcoat, the ML, and part of the Ru layer needed to examine the structures of the two layers simultaneously. We reported observing a grain-to-grain structural agreement between the ML and the Ru seed layer in a PMR medium and concluded that, at the utilized fabrication conditions, each grain in the Ru layer acts as seed for the growth of a single Co grain in the ML. The grain structural correlation was observed by the simultaneous mapping of the two layers using a novel STEMEDS technique to distinguish the two structures based on their different chemical compositions. SAED and Moire pattern analyses revealed a close crystallographic relationship between the corresponding grains in ML and Ru layer. The method presented is applicable for the simultaneous nanostructural analyses of multilayer thin film stacks.
LETTER
(6) Li, W. M.; Chen, Y. J.; Huang, T. L.; Xue, J. M.; Ding, J. J. Appl. Phys. 2011, 109, 07B758. (7) Bertram, H. N.; Zhou, H.; Gustafson, R. IEEE Trans. Magn. 1998, 34, 1845–1847. (8) Judy, J. H. J. Magn. Magn. Mater. 2005, 287, 16–26. (9) Kwon, U.; Sinclair, R.; Velu, E. M. T.; Malhotra, S.; Bertero, G. IEEE Trans. Magn. 2005, 41, 3193–3195. (10) Weller, D.; Moser, A. IEEE Trans. Magn. 1999, 35, 4423–4439. (11) Srinivasan, K.; Piramanayagam, S. N.; Chantrell, R. W.; Kay, Y. S. J. Magn. Magn. Mater. 2008, 320, 3036–3040. (12) Lister, S. J.; Thomson, T.; Kohlbrecher, J.; Takano, K.; Venkataramana, V.; Ray, S. J.; Wismayer, M. P.; de Vries, M. A.; Do, H.; Ikeda, Y.; Lee, S. L. Appl. Phys. Lett. 2010, 97, 112503. (13) Jung, H. S.; Kwon, U.; Kuo, M.; Velu, E. M. T.; Malhotra, S. S.; Jiang, W.; Bertero, G. IEEE Trans. Magn. 2007, 43, 615–620. (14) Wang, J.-P.; Shen, W.; Hong, S. IEEE Trans. Magn. 2007, 43, 682–686. (15) Bertero, G. A.; Wachenschwanz, D.; Malhotra, S.; Velu, S.; Bian, B.; Stafford, D.; Wu, Y.; Yamashita, T.; Wang, S. X. IEEE Trans. Magn. 2002, 38, 1627–1631. (16) Hirayama, Y.; Honda, Y.; Kikukawa, A.; Futamoto, M. J. Appl. Phys. 2000, 87, 6890–6892. (17) Risner, J. D.; Nolan, T. P.; Bentley, J.; Girt, E.; Harkness, S. D., IV; Sinclair, R. Microsc. Microanal. 2007, 13, 70–79. (18) Chen, C.; Sakurai, R.; Hashimoto, M.; Shi, J.; Nakamura, Y. Thin Solid Films 2004, 459, 200–202. (19) Risner, J. D.; Sinclair, R.; Bentley, J. J. Appl. Phys. 2006, 99, 033905. (20) Pearson’s Handbook of Crystallographic Data for Intermetallic Phases, 2nd ed.; Villars, P., Calvert, L. D., Pearson, W. B., Eds.; ASM International: Materials Park, OH, 1991. (21) Buschow, K. H. J.; van Engen, P. G.; Jongebreur, R. J. Magn. Magn. Mater. 1983, 38, 1–22. (22) Nolan, T. P.; Risner, J. D.; Harkness, S. D., IV; Girt, E.; Wu, S. Z.; Ju, G.; Sinclair, R. IEEE Trans. Magn. 2007, 43, 639–644. (23) Erni, R. Aberration-Corrected Imaging in Transmission Electron Microscopy: An Introduction; Imperial College Press: London, 2010; pp 7377. (24) Williams, D. B.; Carter, C. B. Transmission Electron Microscopy: A Textbook for Materials Science; Williams, D. B., Carter, C. B., Eds.; Springer: New York, 1996; Vol. 3, pp 445448. (25) Creath, K.; Schmit, J.; Wyant, J. C. Optical Metrology of Diffuse Surfaces. In Optical Shop Testing, 3rd ed.; Malacara, D., Ed.; Wiley: Hoboken, NJ, 2007; pp 757762. (26) Thornton, J. A. J. Vac. Sci. Technol., A 1986, 4, 3059–3065.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected],
[email protected].
’ REFERENCES (1) Piramanayagam, S. N.; Srinivasan, K. J. Magn. Magn. Mater. 2009, 321, 485–494. (2) Judy, J. H. J. Magn. Magn. Mater. 2001, 235, 235–240. (3) Piramanayagam, S. N. J. Appl. Phys. 2007, 102, 011301. (4) Weller, D.; McDaniel, T. Media for Extremely High Density Recording. In Advanced Magnetic Nanostructures; Sellmyer, D. J., Skomski, R., Eds.; Springer: New York, 2006; pp 295324. (5) Piramanayagam, S. N.; Tan, H. K.; Ranjbar, M.; Wong, S. K.; Sbiaa, R.; Chong, T. C. Appl. Phys. Lett. 2011, 98, 152504. 3754
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