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Cr recording media
Ultramicroscopy 47 (1992) 437-446 North-Holland
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Microstructure and crystallography of textured CoCrTa/Cr recording media T.P. Nolan, R. Sinclair Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA
R. Ranjan and T. Yamashita KOMAG Inc., 591 Yosemite Drive, Milpitas, CA 95035, USA Received at Editorial Office 13 July 1992
Transmission electron microscopy (TEM) has been applied to recording media having a 60 nm CoCrTa film and a 75 nm chromium underlayer, which were DC magnetron-sputtered onto a circumferentially textured NiP/Al substrate. Suitable processing conditions provide in-plane magnetic anisotropy with the preferred direction parallel to the texture lines. This paper focuses on TEM techniques which address the preferred crystallographic orientations purported to be responsible for this important magnetic anisotropy. Elongated probe microdiffraction (EPMD) and standard selected-area diffraction (SAD) patterns from cross-section samples show that the bcc chromium underlayer and the hcp cobalt alloy have strong (110) and (1120) out-of-plane growth directions, respectively, which result in in-plane orientations of the Cr (110) axis and the magnetically easy Co alloy (0001) c-axis. However, the angular spreads of the growth directions are greater perpendicular to the texture grooves than parallel to them, causing similarly greater out-of-plane components of the Cr (110) and Co alloy (0001) axes perpendicular to the texture lines than parallel to them. High-resolution plan-view TEM shows that the Co alloy c-axis has nearly random in-plane orientation. Dark field imaging and EPMD of the plan-view sample show locally altered crystallography near texture lines, but suggest that the change is due in part to the growth orientation tilt, not a dramatic, local, preferred in-plane orientation. Combined, these data suggest that the decreased c-axis in-plane component perpendicular to the texture lines is caused primarily by the (1120) growth direction following the local grooved surface, not the bulk surface, causing the c-axis to be locally tilted out of the bulk film plane near the grooves. This crystallographic difference may have implications upon the observed magnetic anisotropy in these films.
I. Introduction T h i n film m a g n e t i c media, f o r m e d by sputterd e p o s i t i o n of a c h r o m i u m u n d e r l a y e r a n d a C o C r T a alloy m a g n e t i c layer o n t o NiP p l a t e d a l u m i n u m substrates, are widely u s e d in technology. T h e i r high i n - p l a n e coercivity, Hc, a n d relatively low noise are well suited to l o n g i t u d i n a l recording. T h e m a g n e t i c p r o p e r t i e s are, however, strongly d e p e n d e n t u p o n processing p a r a m e t e r s a n d u p o n the resulting m i c r o s t r u c t u r e a n d crystallography of the m e d i u m . M a n y features including grain size a n d shape, precipitates, second p h a s e f o r m a t i o n , alloy s e g r e g a t i o n to grain b o u n d a r i e s , lattice defects, thin-film stresses,
grain isolation a n d crystallographic p r e f e r r e d orie n t a t i o n have b e e n c o n j e c t u r e d to control observed m a g n e t i c properties, m a k i n g it difficult to d e t e r m i n e which ones are i m p o r t a n t . This problem is m a g n i f i e d in complex alloy thin-film m e d i a that have grain sizes a p p r o a c h i n g 10 nm. Complete analysis of the bulk a n d local structure r e q u i r e s the c o m b i n a t i o n of the u n i q u e characterization capabilities of t r a n s m i s s i o n e l e c t r o n microscopy ( T E M ) with bulk analysis t e c h n i q u e s such as X-ray diffraction a n d m a g n e t o m e t r y . T h e p r i m a r y focus here will be o n the use of T E M to analyze m i c r o s t r u c t u r a l f e a t u r e s which may give rise to the m a g n e t i c a n i s o t r o p y in textured media. C i r c u m f e r e n t i a l texture grooves had b e e n ini-
0304-3991/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
T.P. Nolan et aL / Microstructure and crystallography of CoCrTa / C r
438
tially introduced in substrates to minimize the h e a d / d i s k contact area; however, texturing has also allowed significantly increased coercivity in the circumferential r e a d / w r i t e direction over that in the radial direction [1-4]. Despite some empirical success in controlling this anisotropy in coercivity, the associated structural mechanisms have not been clearly determined [5-7]. Analyses of the problem in simpler systems have shown grain shape anisotropy, magnetostriction and crystalline anisotropy to be likely mechanisms [8,9]. Significant magnetic anisotropy due to grain shape requires highly acicular grains, which are not observed in these CoCrTa alloy media. The difference in thermal expansion coefficient between the Cr and Co alloy layers results in thin-film stresses which could be significant if there was anisotropic stress relief; this is difficult to analyze using TEM. A striking difference was, however, observed between the crystallography of the film parallel and perpendicular to the texture lines. The analysis of this feature will be discussed in detail here, to illustrate the difficulty of microstructural analysis of such films and to demonstrate the power of the T E M approach.
2. Experimental details The medium considered here was DC magnetron-sputtered under a 12 /xm argon atmosphere using a Varian MDP 350 machine. A 75 nm chromium layer, a 60 nm CoCrTa layer, and a 30 nm carbon overcoat were deposited at 225°C
PERPENDICULARORIENTATION (/)
readhNrite direction
PARALLEL ORIENTATION (//) read/write direction
ample sam direction
electron beam direction
Fig. 1. Schematic diagram showing orientations of TEM specimens used to analyze crystallographic and microstructural features in circumferential and radial directions.
onto a circumferentially textured substrate. The r.m.s, surface roughness measured with a WYCO profilometer was 5.5 nm, with a peak-to-valley height of 60 nm. This film is similar to those used in industrial media, with strong preferred crystallographic orientations in the growth direction and highly anisotropic coercivity. The coercivity measured in the radial direction, H c±, is 970 Oe, while Hcll, the coercivity measured in the circumferential direction, parallel to the texture lines, is significantly greater, being 1310 Oe. These values may be compared to an isotropic coercivity of 1130 Oe observed when the films were deposited under identical conditions on an untextured substrate. In order to analyze the microstructural and crystallographic features corresponding to the magnetic anisotropy, cross-section T E M specimens were prepared in orientations such that the electron beam would be incident parallel and perpendicular to the texture lines, as shown in fig. 1. These two sample orientations elucidate subtle differences in the surface topography and grain growth which may contribute to the observed differences in macroscopic magnetic properties in the circumferential and radial directions on the hard disk. Such cross-sections combined with plan-view samples give invaluable three-dimensional information about the structure of the media, which is either difficult or impossible to obtain by any alternative technique. T E M micrographs from cross-sections of the textured film in the parallel and perpendicular directions, obtained using a Philips EM430ST operating at 300 kV, are shown in fig. 2. The corresponding selected-area diffraction (SAD) patterns were produced by placing the SAD aperture so as to encompass a few ~ m of thin film, and as little substrate as possible. Columnar growth is observed in both the chromium underlayer and the cobalt alloy magnetic layer, although some grains are observed to be overgrown as the chromium thickness increases. Semi-coherent interfaces are observed in high-resolution images such as fig. 3, although several subgrains of cobalt alloy often grow on a single chromium grain. In the parallel orientation (fig. 2a) the texture lines are parallel to the electron beam. Hence,
T.P. Nolan et al. / Microstructure and crystallography of CoCrTa / Cr
they are observed as a groove in the substrate, as labeled. In the perpendicular orientation (fig. 2b), the texture lines are perpendicular to the electron beam and lie in the plane of the photographs. Hence they are not observed in this orientation, and the film is seen to be very flat. The smoother perpendicular orientation shows well defined diffraction spots, corresponding to the chromium {200} planes and cobalt alloy {1120} planes, aligned very precisely with the direction of the columnar growth. The (1120) cobalt alloy and (100) chromium growth directions are thus vertical to within about + 3 °. The c-axis is the magnetic easy axis in HCP cobalt alloy films, hence its orientation strongly affects bulk magnetic properties. Unfortunately, the c-axis orientation is contained in the diffrac-
439
tion spot corresponding to the overlapping Co {0002} basal planes and the Cr {110} planes. This spot is bright and precisely oriented 90° from the growth direction, suggesting that there may be grains with their c-axis in the plane of the film and parallel to the texture lines. However, the observed in-plane diffraction intensity might arise only from Cr {110} planes. SAD cannot deconvolute this information. The parallel orientation (fig. 2a), which shows the grooving, gives rise to diffraction spots corresponding to {200} Cr and {1120} Co planes, which appear as much broader arcs because some grains have growth orientations tilted away from the vertical growth direction. There is a larger variation, about _+10°, in their growth orientations. The in-plane diffraction spot, corresponding to
(b)
Fig. 2. Bright field cross-section T E M micrographs and corresponding selected-area diffraction patterns of textured C o C r T a / C r sample in two orientations. (a) Parallel orientation showing grooves corresponding to texture lines and significant variation in crystallographic orientation of different grains. (b) Perpendicular orientation appearing very flat because thin film does not cross any grooves in this orientation. Crystallography is also precisely defined.
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T.P. Nolan et al. / Microstructure and crystallography of CoCrTa / C r
Fig. 3. High-resolution cross-section TEM micrograph of interface between Cr and Co alloy grains showing grain-to-grain coherence.
the overlapping Co {0002} and Cr {110} planes, shows a similar spread into a more diffuse arc. This suggests that there are fewer grains with their c-axis precisely in-plane and perpendicular
to the texture lines than in-plane and parallel to the texture lines. Bulk diffraction techniques including selectedarea diffraction suffer from an inability to separate information from overlapping diffraction peaks, while local techniques such as microdiffraction do not allow easy statistical analysis. This gap can be bridged by elongated probe microdiffraction (EPMD) [10], which can be used to test whether the Co {0002} and Cr {110} planes independently show the suggested preferred orientations. EPMD uses an astigmatic probe, 2 p~m long by 50 nm wide (fig. 4), which can be placed independently on the Cr and Co layers to collect information from many grains within a single thin-film layer. The size and shape of the probe may be adjusted using the spot size, condenser aperture and condenser stigmator controls. The minimum width is limited only by the minimum spot size of the microscope. Such an elongated probe may be formed by setting one condenser stigmator far from standard alignment so that two axes have distinctly different focal points. When the beam is condensed to the focal point of one axis, the other axis is still defocused and the probe is elongated. A 90 ° rotation of the elongated beam may be made by condensing to the focal point of the other axis. Smaller rotations of
Fig. 4. Double-exposure bright field image showing probe used in elongated probe microdiffraction experiments. (a) Probe analyzing Co alloy layer. (b) Probe analyzing Cr layer.
T.P. Nolan et al. / Microstructure and crystallography of CoCrTa / Cr
the probe may be obtained by adjusting the second condenser stigmator and refocusing the beam, allowing crystallographic depth profiling of thin films in any orientation. The resulting diffraction patterns from the two samples are shown in fig. 5. The relative brightness of the Co alloy and Cr patterns was comparable to that observed in fig.
441
5a, showing that most of the information in the SAD pattern came from the Cr underlayer, not the magnetic layer. E P M D also clearly shows that both the Cr and Co layers independently have the described in-plane and growth orientations. The difference between the spots observed in the perpendicular and parallel orientations of the same
Fig. 5. Elongated probe microdiffractionpatterns from chromium and cobalt alloy layers. (a) Sample in perpendicular orientation. (b) Sample in parallel orientation.
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T.P. Nolan et al. / Microstructure and crystallography of CoCrTa / Cr
cobalt alloy layer is thus directly attributable to a greater c-axis out-of-plane component perpendicular to the texture lines than parallel to them. Important crystallographic information may also be obtained from through-foil samples. One might expect that in-plane orientation would be easiest to determine via selected-area diffraction of these samples. Unfortunately, any possible asymmetry in these diffraction patterns due to preferred in-plane orientation is overwhelmed by the large asymmetry observed when the specimen is even slightly tilted away from the vertical growth
orientation, as explained in ref. [11]. Nevertheless, detailed information about the in-plane orientation of the c-axis of the magnetic layer may be obtained from through-foil samples using high-resolution images such as shown in fig. 6. When the electron beam is parallel to the (1120) growth direction of a grain, lattice cross-fringes are observed, which determine crystal orientation. This type of image is also best for analyzing nanoscale features such as grain separation and second phases such as the FCC crystallite indicated. Furthermore, the ability of the plan-view,
Fig. 6. High-resolutionplan-viewTEM micrograph showing c-axisorientations labeled by arrows.
T.P. Nolan et aL / Microstructure and crystallography of CoCrTa / Cr
high-resolution image to determine the c-axis orientation extends beyond those grains under perfect high-resolution conditions. Although grains slightly misaligned from the electron beam do not show lattice fringes, moir6 fringes and stacking faults are still observed, which are shown by microdiffraction and optical diffraction of many such grains to be always parallel to the basal planes. A map of the c-axis orientation may then be made as shown. This image and several similar ones thus confirm that nearly all of the grains are oriented with a vertical (1120) axis, and show that the c-axis is nearly randomly oriented in the plane of the film. Despite the random c-axis orientation observed in high-resolution imaging, the texture lines affect the local crystallography as seen in the lower-magnification dark field image obtained from the inner triple diffraction ring, which contains the in-plane orientation information given in fig. 7. The bright and dark stripes are parallel to the texture lines, showing that the crystallographic orientation at texture lines is different from the orientation 100 nm away from
443
them. E P M D is also a valuable technique when applied to through-foil samples. The local crystallographic orientation can be analyzed in more detail by placing the elongated probe directly on a texture line, as shown in fig. 8a. The resulting diffraction pattern (fig. 8b) may be compared to a similar one obtained after moving the probe about 100 nm away from the texture line (fig. 8c). fig. 8b shows strong apparent local preferred orientation of the Co c-axis and Cr (110) axes parallel to the texture lines. However, there are many less intense, but still visible, spots perpendicular to the texture lines, which correspond to Co c-axes and Cr (110) axes perpendicular to the texture lines, but tilted slightly out of the film plane, hence away from strong diffracting conditions. As the probe is moved away from the texture line, the grains with c-axes oriented perpendicular to the texture lines return to strong diffracting conditions, as shown in fig. 8c. This suggests that much of the apparent local in-plane preferred orientation of the c-axis is due to the small out-of-plane tilt. The combination of the TEM results implies
Fig. 7. Dark field TEM micrograph showing local alteration of crystallographic orientation near texture lines.
444
T.P. Nolan et al. / Microstructure and crystallography of CoCrTa / C r
Fig. 8. Elongated probe microdiffraction of plan-view sample. (a) Double-exposure bright field image of probe placed directly on texture line. (b) Diffraction pattern with probe directly on texture line. (c) Diffraction pattern with probe 100 nm away from texture line.
T.P. Nolan et al. / Microstructure and crystallography of CoCrTa / C r
Fig. 9. High-resolution cross-section TEM micrograph of texture line, showing dependence of crystallography on local substrate normal.
that important changes in crystallography are controlled by the local tilt of the surface. The growth orientations appear to follow the local surface normal, as opposed to the bulk substrate normal. Hence, grooves such as the texture lines shown in fig. 2a cause tilting of the growth axes of individual grains away from the film normal. High-resolution cross-section micrographs clearly show this effect. As an example, fig. 9 is a highresolution image of such a groove. It is typical of a few grooves which have been examined, but this is not yet a complete statistical sample. The 20 ° change in the substrate surface normal is followed by a roughly 20° change in the Cr (100) growth direction. This information is then propagated to the Co alloy through the grain-to-grain growth observed at the interface between the Cr underlayer and the Co alloy magnetic layer. 3. Discussion
The bulk magnetic properties of thin-film magnetic media are strongly dependent upon the microstructure and crystallography resulting from thin-film growth. Hence, analysis of these media requires three-dimensional information about the
445
local crystallography on a nanometer scale. A combination of cross-section and through-foil TEM techniques can provide this detailed information. Bright field, dark field, and high-resolution images of through-foil samples are the best techniques for a general description of the microstructure. When oriented correctly, high-resolution images are particularly powerful because they contain detailed information about many features conjectured to control magnetic properties, including second phases, stacking faults and grain isolation, as well as the in-plane orientations described. A good specimen orientation may be obtained, recalling that specimen tilts make the diffraction pattern highly asymmetric, by simply tilting the specimen until the diffraction pattern becomes bright and symmetric. Cross-section samples are required for more detailed information about the topography and crystallography of the film. Preparation of samples in more than one orientation corresponding to magnetic properties of interest is time-consuming, but provides an invaluable three-dimensional description of the film. Cross-section, bright field images are best for analyses of the topography and grain parameters over several micrometers of film. High-resolution imaging is useful for analyses of interfaces and precise orientations of selected grains. Large-scale crystallographic determinations such as described for through-foil samples are, however, not obtainable because the nearly random in-plane orientation makes very few grains oriented for high-resolution imaging at any one time. On the other hand, the vertical growth direction does not interfere with analysis of diffraction patterns in cross-section samples. Elongated probe microdiffraction is a particularly powerful technique, when applied to multiple layers of thin films. It can give a crystallographic depth profile not available by any other technique. The resolution of the depth profile is only limited by the probe size of the microscope.
4. Conclusion
Circumferentially textured C o C r T a / C r media are observed to have anisotropic coercivity. Corn-
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T.P. Nolan et al. / Microstructure and crystallography of CoCrTa / C r
pared to an identical film deposited on an untextured substrate, H c has been significantly enhanced parallel to the texture lines and similarly decreased perpendicular to them. TEM has been used to analyze subtle differences in the crystallography of the textured medium parallel and perpendicular to the texture lines, which appear to correspond to the observed anisotropy in magnetic properties. The Cr underlayer is observed to grow in a columnar fashion with a (100) growth orientation. The CoCrTa alloy magnetic layer grows grain-to-grain, semi-coherently, with a (117-0) growth orientation, which requires the c-axis to lie in the plane of the film. The precision of the growth and c-axis orientations depend upon the substrate topography. The textured film is less flat perpendicular to the texture lines than parallel to them, resulting in greater fluctuation in crystallographic orientation. The resulting decreased in-plane orientation of the magnetically easy c-axis perpendicular to the texture lines may affect the observed magnetic anisotropy. A combination of T E M techniques, including high-resolution imaging and elongated probe microdiffract~on is valuable in determining the microstructure.
Acknowledgment The authors would like to acknowledge Dick Lavine of Intevac Inc. for assistance in sample preparation.
References [1] E. Teng and N. Ballard, IEEE Trans. Magn. 22 (1986) 579. [2] S. Uchinami, F. Beppu, S. Ito, N. Tokubuchi, K. Noda, Y. Notohara and K. Kanai, IEEE Trans. Magn. 23 (1987) 3408. [3] E.M. Simpson, P.B. Narayan, G.T.K. Swami and J.L. Chao, IEEE Trans. Magn. 23 (1987) 3405. [4] D.J. Seagle, N.C. Fernelius and M.R. Khan, J. Appl. Phys. 61 (1987) 4025. [5] T.C. Arnoldussen, E.M. Rossi, A. Ting, A. Brunsch, J. Schneide and G. Trippel, IEEE Trans. Magn. 20 (1984) 821. [6] R. Nishikawa, T. Hikosawa, K. Igarashi and M. Kanamaru, IEEE Trans. Magn. 25 (1989) 3890. [7] T. Lin, R. Alani and D.N. Lambeth, J. Magn. Magn. Mater. 78 (1989) 213. [8] S. Chikazumi and S.H. Charap, Physics of Magnetism (Krieger, Malabar, 1964) pp. 128-302. [9] M.P. Sharrock, MRS Bull. 15 (3) (1990) 53. [10] M.A. Parker, K.E. Johnson, C. Hwang and A. Bermea, in: Proc. 49th Annual EMSA Meeting, Eds. G.W. Bailey and E.L. Hall (San Francisco Press, San Francisco, 1991) p. 762. [11] P.B. Hirsch, A. Howie, R.B. Nicholson, D.W. Pashley and M.J. Whelan, Electron Microscopy of Thin Crystals (Butterworths, London, 1965) p. 116.
lli.h~i"liil'l'tl.~ilul
Microstructure and crystallography of textured CoCrTa/Cr recording media T.P. Nolan, R. Sinclair Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA
R. Ranjan and T. Yamashita KOMAG Inc., 591 Yosemite Drive, Milpitas, CA 95035, USA Received at Editorial Office 13 July 1992
Transmission electron microscopy (TEM) has been applied to recording media having a 60 nm CoCrTa film and a 75 nm chromium underlayer, which were DC magnetron-sputtered onto a circumferentially textured NiP/Al substrate. Suitable processing conditions provide in-plane magnetic anisotropy with the preferred direction parallel to the texture lines. This paper focuses on TEM techniques which address the preferred crystallographic orientations purported to be responsible for this important magnetic anisotropy. Elongated probe microdiffraction (EPMD) and standard selected-area diffraction (SAD) patterns from cross-section samples show that the bcc chromium underlayer and the hcp cobalt alloy have strong (110) and (1120) out-of-plane growth directions, respectively, which result in in-plane orientations of the Cr (110) axis and the magnetically easy Co alloy (0001) c-axis. However, the angular spreads of the growth directions are greater perpendicular to the texture grooves than parallel to them, causing similarly greater out-of-plane components of the Cr (110) and Co alloy (0001) axes perpendicular to the texture lines than parallel to them. High-resolution plan-view TEM shows that the Co alloy c-axis has nearly random in-plane orientation. Dark field imaging and EPMD of the plan-view sample show locally altered crystallography near texture lines, but suggest that the change is due in part to the growth orientation tilt, not a dramatic, local, preferred in-plane orientation. Combined, these data suggest that the decreased c-axis in-plane component perpendicular to the texture lines is caused primarily by the (1120) growth direction following the local grooved surface, not the bulk surface, causing the c-axis to be locally tilted out of the bulk film plane near the grooves. This crystallographic difference may have implications upon the observed magnetic anisotropy in these films.
I. Introduction T h i n film m a g n e t i c media, f o r m e d by sputterd e p o s i t i o n of a c h r o m i u m u n d e r l a y e r a n d a C o C r T a alloy m a g n e t i c layer o n t o NiP p l a t e d a l u m i n u m substrates, are widely u s e d in technology. T h e i r high i n - p l a n e coercivity, Hc, a n d relatively low noise are well suited to l o n g i t u d i n a l recording. T h e m a g n e t i c p r o p e r t i e s are, however, strongly d e p e n d e n t u p o n processing p a r a m e t e r s a n d u p o n the resulting m i c r o s t r u c t u r e a n d crystallography of the m e d i u m . M a n y features including grain size a n d shape, precipitates, second p h a s e f o r m a t i o n , alloy s e g r e g a t i o n to grain b o u n d a r i e s , lattice defects, thin-film stresses,
grain isolation a n d crystallographic p r e f e r r e d orie n t a t i o n have b e e n c o n j e c t u r e d to control observed m a g n e t i c properties, m a k i n g it difficult to d e t e r m i n e which ones are i m p o r t a n t . This problem is m a g n i f i e d in complex alloy thin-film m e d i a that have grain sizes a p p r o a c h i n g 10 nm. Complete analysis of the bulk a n d local structure r e q u i r e s the c o m b i n a t i o n of the u n i q u e characterization capabilities of t r a n s m i s s i o n e l e c t r o n microscopy ( T E M ) with bulk analysis t e c h n i q u e s such as X-ray diffraction a n d m a g n e t o m e t r y . T h e p r i m a r y focus here will be o n the use of T E M to analyze m i c r o s t r u c t u r a l f e a t u r e s which may give rise to the m a g n e t i c a n i s o t r o p y in textured media. C i r c u m f e r e n t i a l texture grooves had b e e n ini-
0304-3991/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
T.P. Nolan et aL / Microstructure and crystallography of CoCrTa / C r
438
tially introduced in substrates to minimize the h e a d / d i s k contact area; however, texturing has also allowed significantly increased coercivity in the circumferential r e a d / w r i t e direction over that in the radial direction [1-4]. Despite some empirical success in controlling this anisotropy in coercivity, the associated structural mechanisms have not been clearly determined [5-7]. Analyses of the problem in simpler systems have shown grain shape anisotropy, magnetostriction and crystalline anisotropy to be likely mechanisms [8,9]. Significant magnetic anisotropy due to grain shape requires highly acicular grains, which are not observed in these CoCrTa alloy media. The difference in thermal expansion coefficient between the Cr and Co alloy layers results in thin-film stresses which could be significant if there was anisotropic stress relief; this is difficult to analyze using TEM. A striking difference was, however, observed between the crystallography of the film parallel and perpendicular to the texture lines. The analysis of this feature will be discussed in detail here, to illustrate the difficulty of microstructural analysis of such films and to demonstrate the power of the T E M approach.
2. Experimental details The medium considered here was DC magnetron-sputtered under a 12 /xm argon atmosphere using a Varian MDP 350 machine. A 75 nm chromium layer, a 60 nm CoCrTa layer, and a 30 nm carbon overcoat were deposited at 225°C
PERPENDICULARORIENTATION (/)
readhNrite direction
PARALLEL ORIENTATION (//) read/write direction
ample sam direction
electron beam direction
Fig. 1. Schematic diagram showing orientations of TEM specimens used to analyze crystallographic and microstructural features in circumferential and radial directions.
onto a circumferentially textured substrate. The r.m.s, surface roughness measured with a WYCO profilometer was 5.5 nm, with a peak-to-valley height of 60 nm. This film is similar to those used in industrial media, with strong preferred crystallographic orientations in the growth direction and highly anisotropic coercivity. The coercivity measured in the radial direction, H c±, is 970 Oe, while Hcll, the coercivity measured in the circumferential direction, parallel to the texture lines, is significantly greater, being 1310 Oe. These values may be compared to an isotropic coercivity of 1130 Oe observed when the films were deposited under identical conditions on an untextured substrate. In order to analyze the microstructural and crystallographic features corresponding to the magnetic anisotropy, cross-section T E M specimens were prepared in orientations such that the electron beam would be incident parallel and perpendicular to the texture lines, as shown in fig. 1. These two sample orientations elucidate subtle differences in the surface topography and grain growth which may contribute to the observed differences in macroscopic magnetic properties in the circumferential and radial directions on the hard disk. Such cross-sections combined with plan-view samples give invaluable three-dimensional information about the structure of the media, which is either difficult or impossible to obtain by any alternative technique. T E M micrographs from cross-sections of the textured film in the parallel and perpendicular directions, obtained using a Philips EM430ST operating at 300 kV, are shown in fig. 2. The corresponding selected-area diffraction (SAD) patterns were produced by placing the SAD aperture so as to encompass a few ~ m of thin film, and as little substrate as possible. Columnar growth is observed in both the chromium underlayer and the cobalt alloy magnetic layer, although some grains are observed to be overgrown as the chromium thickness increases. Semi-coherent interfaces are observed in high-resolution images such as fig. 3, although several subgrains of cobalt alloy often grow on a single chromium grain. In the parallel orientation (fig. 2a) the texture lines are parallel to the electron beam. Hence,
T.P. Nolan et al. / Microstructure and crystallography of CoCrTa / Cr
they are observed as a groove in the substrate, as labeled. In the perpendicular orientation (fig. 2b), the texture lines are perpendicular to the electron beam and lie in the plane of the photographs. Hence they are not observed in this orientation, and the film is seen to be very flat. The smoother perpendicular orientation shows well defined diffraction spots, corresponding to the chromium {200} planes and cobalt alloy {1120} planes, aligned very precisely with the direction of the columnar growth. The (1120) cobalt alloy and (100) chromium growth directions are thus vertical to within about + 3 °. The c-axis is the magnetic easy axis in HCP cobalt alloy films, hence its orientation strongly affects bulk magnetic properties. Unfortunately, the c-axis orientation is contained in the diffrac-
439
tion spot corresponding to the overlapping Co {0002} basal planes and the Cr {110} planes. This spot is bright and precisely oriented 90° from the growth direction, suggesting that there may be grains with their c-axis in the plane of the film and parallel to the texture lines. However, the observed in-plane diffraction intensity might arise only from Cr {110} planes. SAD cannot deconvolute this information. The parallel orientation (fig. 2a), which shows the grooving, gives rise to diffraction spots corresponding to {200} Cr and {1120} Co planes, which appear as much broader arcs because some grains have growth orientations tilted away from the vertical growth direction. There is a larger variation, about _+10°, in their growth orientations. The in-plane diffraction spot, corresponding to
(b)
Fig. 2. Bright field cross-section T E M micrographs and corresponding selected-area diffraction patterns of textured C o C r T a / C r sample in two orientations. (a) Parallel orientation showing grooves corresponding to texture lines and significant variation in crystallographic orientation of different grains. (b) Perpendicular orientation appearing very flat because thin film does not cross any grooves in this orientation. Crystallography is also precisely defined.
440
T.P. Nolan et al. / Microstructure and crystallography of CoCrTa / C r
Fig. 3. High-resolution cross-section TEM micrograph of interface between Cr and Co alloy grains showing grain-to-grain coherence.
the overlapping Co {0002} and Cr {110} planes, shows a similar spread into a more diffuse arc. This suggests that there are fewer grains with their c-axis precisely in-plane and perpendicular
to the texture lines than in-plane and parallel to the texture lines. Bulk diffraction techniques including selectedarea diffraction suffer from an inability to separate information from overlapping diffraction peaks, while local techniques such as microdiffraction do not allow easy statistical analysis. This gap can be bridged by elongated probe microdiffraction (EPMD) [10], which can be used to test whether the Co {0002} and Cr {110} planes independently show the suggested preferred orientations. EPMD uses an astigmatic probe, 2 p~m long by 50 nm wide (fig. 4), which can be placed independently on the Cr and Co layers to collect information from many grains within a single thin-film layer. The size and shape of the probe may be adjusted using the spot size, condenser aperture and condenser stigmator controls. The minimum width is limited only by the minimum spot size of the microscope. Such an elongated probe may be formed by setting one condenser stigmator far from standard alignment so that two axes have distinctly different focal points. When the beam is condensed to the focal point of one axis, the other axis is still defocused and the probe is elongated. A 90 ° rotation of the elongated beam may be made by condensing to the focal point of the other axis. Smaller rotations of
Fig. 4. Double-exposure bright field image showing probe used in elongated probe microdiffraction experiments. (a) Probe analyzing Co alloy layer. (b) Probe analyzing Cr layer.
T.P. Nolan et al. / Microstructure and crystallography of CoCrTa / Cr
the probe may be obtained by adjusting the second condenser stigmator and refocusing the beam, allowing crystallographic depth profiling of thin films in any orientation. The resulting diffraction patterns from the two samples are shown in fig. 5. The relative brightness of the Co alloy and Cr patterns was comparable to that observed in fig.
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5a, showing that most of the information in the SAD pattern came from the Cr underlayer, not the magnetic layer. E P M D also clearly shows that both the Cr and Co layers independently have the described in-plane and growth orientations. The difference between the spots observed in the perpendicular and parallel orientations of the same
Fig. 5. Elongated probe microdiffractionpatterns from chromium and cobalt alloy layers. (a) Sample in perpendicular orientation. (b) Sample in parallel orientation.
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cobalt alloy layer is thus directly attributable to a greater c-axis out-of-plane component perpendicular to the texture lines than parallel to them. Important crystallographic information may also be obtained from through-foil samples. One might expect that in-plane orientation would be easiest to determine via selected-area diffraction of these samples. Unfortunately, any possible asymmetry in these diffraction patterns due to preferred in-plane orientation is overwhelmed by the large asymmetry observed when the specimen is even slightly tilted away from the vertical growth
orientation, as explained in ref. [11]. Nevertheless, detailed information about the in-plane orientation of the c-axis of the magnetic layer may be obtained from through-foil samples using high-resolution images such as shown in fig. 6. When the electron beam is parallel to the (1120) growth direction of a grain, lattice cross-fringes are observed, which determine crystal orientation. This type of image is also best for analyzing nanoscale features such as grain separation and second phases such as the FCC crystallite indicated. Furthermore, the ability of the plan-view,
Fig. 6. High-resolutionplan-viewTEM micrograph showing c-axisorientations labeled by arrows.
T.P. Nolan et aL / Microstructure and crystallography of CoCrTa / Cr
high-resolution image to determine the c-axis orientation extends beyond those grains under perfect high-resolution conditions. Although grains slightly misaligned from the electron beam do not show lattice fringes, moir6 fringes and stacking faults are still observed, which are shown by microdiffraction and optical diffraction of many such grains to be always parallel to the basal planes. A map of the c-axis orientation may then be made as shown. This image and several similar ones thus confirm that nearly all of the grains are oriented with a vertical (1120) axis, and show that the c-axis is nearly randomly oriented in the plane of the film. Despite the random c-axis orientation observed in high-resolution imaging, the texture lines affect the local crystallography as seen in the lower-magnification dark field image obtained from the inner triple diffraction ring, which contains the in-plane orientation information given in fig. 7. The bright and dark stripes are parallel to the texture lines, showing that the crystallographic orientation at texture lines is different from the orientation 100 nm away from
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them. E P M D is also a valuable technique when applied to through-foil samples. The local crystallographic orientation can be analyzed in more detail by placing the elongated probe directly on a texture line, as shown in fig. 8a. The resulting diffraction pattern (fig. 8b) may be compared to a similar one obtained after moving the probe about 100 nm away from the texture line (fig. 8c). fig. 8b shows strong apparent local preferred orientation of the Co c-axis and Cr (110) axes parallel to the texture lines. However, there are many less intense, but still visible, spots perpendicular to the texture lines, which correspond to Co c-axes and Cr (110) axes perpendicular to the texture lines, but tilted slightly out of the film plane, hence away from strong diffracting conditions. As the probe is moved away from the texture line, the grains with c-axes oriented perpendicular to the texture lines return to strong diffracting conditions, as shown in fig. 8c. This suggests that much of the apparent local in-plane preferred orientation of the c-axis is due to the small out-of-plane tilt. The combination of the TEM results implies
Fig. 7. Dark field TEM micrograph showing local alteration of crystallographic orientation near texture lines.
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T.P. Nolan et al. / Microstructure and crystallography of CoCrTa / C r
Fig. 8. Elongated probe microdiffraction of plan-view sample. (a) Double-exposure bright field image of probe placed directly on texture line. (b) Diffraction pattern with probe directly on texture line. (c) Diffraction pattern with probe 100 nm away from texture line.
T.P. Nolan et al. / Microstructure and crystallography of CoCrTa / C r
Fig. 9. High-resolution cross-section TEM micrograph of texture line, showing dependence of crystallography on local substrate normal.
that important changes in crystallography are controlled by the local tilt of the surface. The growth orientations appear to follow the local surface normal, as opposed to the bulk substrate normal. Hence, grooves such as the texture lines shown in fig. 2a cause tilting of the growth axes of individual grains away from the film normal. High-resolution cross-section micrographs clearly show this effect. As an example, fig. 9 is a highresolution image of such a groove. It is typical of a few grooves which have been examined, but this is not yet a complete statistical sample. The 20 ° change in the substrate surface normal is followed by a roughly 20° change in the Cr (100) growth direction. This information is then propagated to the Co alloy through the grain-to-grain growth observed at the interface between the Cr underlayer and the Co alloy magnetic layer. 3. Discussion
The bulk magnetic properties of thin-film magnetic media are strongly dependent upon the microstructure and crystallography resulting from thin-film growth. Hence, analysis of these media requires three-dimensional information about the
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local crystallography on a nanometer scale. A combination of cross-section and through-foil TEM techniques can provide this detailed information. Bright field, dark field, and high-resolution images of through-foil samples are the best techniques for a general description of the microstructure. When oriented correctly, high-resolution images are particularly powerful because they contain detailed information about many features conjectured to control magnetic properties, including second phases, stacking faults and grain isolation, as well as the in-plane orientations described. A good specimen orientation may be obtained, recalling that specimen tilts make the diffraction pattern highly asymmetric, by simply tilting the specimen until the diffraction pattern becomes bright and symmetric. Cross-section samples are required for more detailed information about the topography and crystallography of the film. Preparation of samples in more than one orientation corresponding to magnetic properties of interest is time-consuming, but provides an invaluable three-dimensional description of the film. Cross-section, bright field images are best for analyses of the topography and grain parameters over several micrometers of film. High-resolution imaging is useful for analyses of interfaces and precise orientations of selected grains. Large-scale crystallographic determinations such as described for through-foil samples are, however, not obtainable because the nearly random in-plane orientation makes very few grains oriented for high-resolution imaging at any one time. On the other hand, the vertical growth direction does not interfere with analysis of diffraction patterns in cross-section samples. Elongated probe microdiffraction is a particularly powerful technique, when applied to multiple layers of thin films. It can give a crystallographic depth profile not available by any other technique. The resolution of the depth profile is only limited by the probe size of the microscope.
4. Conclusion
Circumferentially textured C o C r T a / C r media are observed to have anisotropic coercivity. Corn-
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T.P. Nolan et al. / Microstructure and crystallography of CoCrTa / C r
pared to an identical film deposited on an untextured substrate, H c has been significantly enhanced parallel to the texture lines and similarly decreased perpendicular to them. TEM has been used to analyze subtle differences in the crystallography of the textured medium parallel and perpendicular to the texture lines, which appear to correspond to the observed anisotropy in magnetic properties. The Cr underlayer is observed to grow in a columnar fashion with a (100) growth orientation. The CoCrTa alloy magnetic layer grows grain-to-grain, semi-coherently, with a (117-0) growth orientation, which requires the c-axis to lie in the plane of the film. The precision of the growth and c-axis orientations depend upon the substrate topography. The textured film is less flat perpendicular to the texture lines than parallel to them, resulting in greater fluctuation in crystallographic orientation. The resulting decreased in-plane orientation of the magnetically easy c-axis perpendicular to the texture lines may affect the observed magnetic anisotropy. A combination of T E M techniques, including high-resolution imaging and elongated probe microdiffract~on is valuable in determining the microstructure.
Acknowledgment The authors would like to acknowledge Dick Lavine of Intevac Inc. for assistance in sample preparation.
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