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Investigation of Cr segregation within rf-sputtered CoCr films
Journal of Magnetism and Magnetic Materials 62 (1986) 359-366 North-Holland, Amsterdam
359
INVESTIGATION OF Cr SEGREGATION WITHIN rf-SPUTFERED CoCr FILMS J.N. CHAPMAN, I.R. McFADYEN and J.P.C. BERNARDS f Department of Natural Philosophy, University of Glasgow, Glasgow G12, UK t Philips Research Laboratories, P.O. Box 80.000, 5600 J/l Eindhoven, The Netherlands Received 2 June 1986 Composition variations in thin transverse sections of if-sputtered Co7s.sCr2t.5 films have been studied by X-ray microanalysis using a scanning transmission electron microscope. Analyses with a spatial resolution sub.~tantially better than 10 nm have shown that Cr rich regions exist in the boundary layers between 50-100 nm diameter colunms which dominate the microstructure of such films. The extent of the measured Cr enrichment is ~ 1.5 at%, but when beam broadening effects are taken into consideration, the true segregation is found to be much greater. The implications of these results on the magnetic properties of the films are discussed.
1. Introduction
Thin films of Cot00_xCr; (x ~ 20) are of considerable interest as magnetic recording media capable of storing data at very high densities [1-3]. For the films to have properties suitable for this purpose they must be grown in such a way that a substantial perpendicular magnetic anisotropy is developed. This requires a pronounced crystallographic texture with the hexagonal c-axis parallel to the film normal. In practice the microstructure generally shows a columnar structure with the column axes parallel to the film normal and previous studies [4,5] have confirmed that the c-axis texture is well developed when the columnar microstructure is present through a large proportion of the film thickness. In this paper we are concerned with the further characterisation of the microstructure of such fiLms and, in particular, with investigating any segregation of Cr at the column boundaries. This segregation has been suggested by earlier magnetic measurements [6]. As the magnetic properties of CoCr alloys change rapidly in the vicinity of the composition of interest [7,8], knowledge of the presence and extent of any segregation is important if the response of the medium to a magnetic field, and hence its recording properties, are to be understood. Since the columns themselves are only ~-50-100 nm in diameter the technique used to
investigate Cr segregation must have very high spatial resolution as well as adequate sensitivity to permit changes ~-1 at% to be detected. X-ray microanalysis of thin specimens in a scanning transmission electron microscope (STEM) seems best suited for this purpose and was the technique used throughout this work. Experimental details are given in section 2 following a description of specimen preparation. Thereafter, in section 3 analyses of a number of column boundaries and adjacent regions within the columns are presented. Finally, the paper concludes with a discussion of the relationship between the volume of sample excited by the incident electron probe and the volume occupied by a column boundary. Once this is known imph.'cations for the magnetic properties of the medium are drawn from the results of section 3.
2. Experimental details
The thin films were iprepared by rf sputtering using 8" Co78.5Cr21.5 (at%) alloy targets. Polyester foils, which had been midly sputter-etched and pre-heated, were used as substrates. Full details of preparation conditions are given in a previous paper [3]. In some cases the CoCr was deposited directly onto the polyester whilst in others a layer of soft magnetic material (e.g. NiFe) followed by a
0304-8853/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
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J.N. Chapman et aL / Cr segregation in rf-sputtered CoCr films
very thin layer of Ti were deposited initially; in all cases a columnar structure with hexagonal crystallites was present in the CoCr films [3-5]. To support perpendicular magnetisation, and hence achieve the desired recording properties, the CoCr layer must exceed a critical thickness. The thickness of the CoCr layer grown here was typically 0.4 ~m. Films of such a thickness are unsuitable for direct observation by transmission electron microscopy (TEM) whether or not they are backed by other layers. Two alternative exist: the films can be back-thinned to remove the substrate and a substantial fraction of the CoCr itself leaving only ~- 100 ran, or transverse sections can be prepared of the CoCr, substrate and any intervening layers. As the latter reveal the columnar structure more clearly and are more suitable for TEM investigations of the magnetic domain structure supported by the material (to be reported elsewhere), this approach was adopted. Suitable sections were generally prepared by embedding the composite specimen in resin and cutting thin sections using a microtome. By this technique long ribbons of material suitable for examination by TEM were produced but their thickness was rarely less than 100 urn. To obtain samples thinner than this ion milling was used. The procedure involved using Ar ions to bombard a transverse section cut from a block consisting of the material of interest glued between two pieces of Si. Generally satisfactory results were obtained in this way but the area of specimen available for examination was small and the thickness of the section varied appreciably over that area. The thickness of the sample plays a crucial role in determining the spatial resolution achievable in the analyses and this is discussed in section 4. All the results were obtained using an extended VG HB5 STEM equipped with a range of different imaging detectors together with facilities for microdiffraction, energy dispersive X-ray microanalysis (EDX) and electron energy loss spectroscopy (EELS). Images of the transverse sections of CoCr clearly revealed its columnar structure, and mierodiffraction confirmed the pronounced c-axis texture of the columns. The columns are - 50 mn in diameter and the dispersion in orien-
tation of the c-axis (A0s0) is 7 °. X-ray microanalyses were obtained simply by disabling the raster scan and positioning the stationary probe at colunm boundaries and different locations within the columns. The probe used was = 2 nm diameter and its current was -~ 0.2 nA. Whilst smaller probes are readily attainable on the HB5 there was no incentive to use them in this work since beam broadening, as the electron pass through the specimen, increases the effective diameter of the area analysed to a value beyond that set by the probe. Details of this are discussed in section 4. Also, a further decrease in probe size would be accompanied b3r a reduction in probe current. With the selected operating conditions a convenient spectrum acquisition time was = 60 s. In this time no detectable specimen drift was observed and the number of Cr K a X-rays detected was typically between 400 and 1000, dependent on the local thickness. As the generation of X-rays is a random process obeying Poisson statistics the error in such signals is between 5 and 3%. No larger error could be tolerated if Cr segregation at the expected level was to be detected. It should be noted that as the ratio of Co to Cr present is approximately 4 : 1 and as cross-sections for X-ray generation in the two element are comparable, the overall uncertainty in the analyses is only slightly worse than the error in the Cr K a signal. To determine whether or not Cr segregation was present in the thin sections (X-ray absorption and fluorescence effects being negligible) it was only necessary to note the variation in the ratio of the number of Co K a (Nco) to the number of Cr K a (Ncr) photons detected. However, to quantify the observed differences a knowledge of the appropriate X-ray generation cross sections was also required. The ratio of the number of atoms of Co (nco) to the number of atoms of Cr (ncr) in the irradiated volume is given by n cJn
Cr =
Nco/Ncr"OCr/OCo,
(1)
where OCr/OCo is the ratio of cross sections for the generation of K a photons in the two elements. Using the data of Chapman et al. [9], the value for this is 1.07 with an associated uncertainty of < 3%. Thus, x (the atomic percentage of Cr present) can
J.N. Chapman et al. / Cr segregation in rf-sputtered CoCr films
be written as x = lOOncr/(nco +
nor )
= l O 0 ( 1 . 0 7 ( N c o / N c r ) + 1} -1,
(2)
EELS provides a very suitable method for measuring the local section thickness at the positions where the X-ray spectra were recorded. The microscope operating conditions for the two techniques are the same and it is possible to record the two kinds of spectra simultaneously. The thickness (t) is determined [10] using t = A i ln(1T/lo),
(3)
where 13. and 10 are respectively the total number of electrons passing through the spectrometer and the number of those which have not been scattered inelastically. A i is the mean free path for inelastic scattering which was estimated [10] to the 47 nm for 100 keV electrons incident on CoCr. Thicknesses determined in this way are believed to be accurate to between 10 and 15%. Support for this has been obtained by using the technique to measure the thickness of a film of CoCr grown sufficiently thin for direct examination in the microscope without the need for sectioning. A thickness
361
value of 110 nm was determined using eq. (3) which agrees within the expected uncertainties with the figure of 120 nm determined from the deposition rate. This, in turn, was calibrated by sputtering several l a y e r s and measuring their thickness with the Talleysurf stylus technique.
3. Results Fig. 1 shows images of a typical transverse section cut from a film grown on a polyester substrate. The images were recorded in the STEM using the probe described above. The columnar structure of the CoCr is evident in both the bright field (B1z) and the annular dark fidd (ADF) images and we have found the latter particularly useful for locating column boundaries suitably oriented for X-ray microanalysis. That most boundaries will not be so oriented is shown schematically in fig. 2 which represents the result of taking a section through a film with a somewhat irregular columnar structure, the thickness of the section and the mean column diameter being comparable. Whilst a number of boundary regions (e.g. A, B, C) extend throughout the section thickness, few, if
Fig. 1. (a) BF and (b) ADF images of a 130 nm thick transverse section through a CoCr film grown on a polyestersubstrate.
J.N. Chapman et al. / Cr segregation in rf-sputtered CoCr films
362
300 Eounfs
bodo%o
i" CoKo~
20( ErK,~
~'obe
7i ,.i
5.0 b]
Fig. 2. (a) Schematic representation of taking a thin section from a film with a columnar microstructure; (b) correct orientation of the thin section for an electron probe to traverse the whole of the boundary layer at A. t denotes the thickness of the section.
any, will lie parallel to the electron propagation direction. However, by tilting the section in the microscope it is possible to bring individual boundaries into the correct orientation (e.g. A in fig. 2b) so that if the probe is positioned as shown (and b e a m broadening is not too severe) the X-ray spectrum obtained will pertain substantially to the boundary layer. The practical advantage offered by the A D F image for determining when a boundary is correctly oriented is associated with the absence of a substantial amount of confusing diffraction contrast from within the columns together with a sharp contrast change across a well oriented boundary. In fig. 1 the boundaries marked P and Q are examples of ones suitable for analysis. Fig. 3 shows a pair of typical spectra, one obtained with the probe centred at a point on P and the other when the probe was moved a perpendicular distance ~ 20 n m from it. The spectra have been scaled so that the Co K a signal is the same in the two cases and the increase in the Cr K a signal from the boundary region is apparent. To confirm this result 9 further spectra were recorded from the boundary P and from 9 points within the columns on either side of it. Analysis of these spectra showed that N c o / N c r had a mean value of 3.15 when the probe was centred along P and a
5.8
6.6
7
x i03 eV Photon Energy
Fig. 3. X-ray spectra, scaled so that the counts in the Co Ka peaks are equal, taken with the probe centred at a point on P (dashed line) and in the colmrm adjacent to P (solid line). One spectrum has been slightly offset with respect to the other for clarity.
significantly higher value of 3.51 when located in the columns. The standard deviation of both distributions was --0.20 and the standard error in the means was 0.07. N o significant difference was detected in the composition of the columns on either side of P. Figs. 4 and 5 show the results of analysing a number of boundaries from two transverse sections cut from samples prepared at different times. The results shown in fig. 4 came from the section shown in fig. 1 (TS1) whose thickness was estimated, using eq. (3), to be 130 rim. Those in fig. 5 came from a 100 n m thick section (TS2) cut from a composite C o C r / T i / N i F e sample. In both instances it is clear that the boundary regions have
r I-
2.8
I I L_..I
3.2
3.6
~'0 N[o/NCr
Fig. 4. Histograms of the ratios of Nco/NCr with the probe centred on the boundaries between columns (dashed line) and within the columns (solid line). The arrows denote the means of the two distributions. The sample was a 130 nm thick transverse section.
J.N. Chapman et al. / Cr segregation in rf-sputtered CoCr films
363
NCo/NCr
m
IF-'•
4.0
I
3.6
I
F-I I
I I
m
3!2
I
3.6
3.2
fq
~.0 Nco/NCr
i
I
25nm
I
Fig. 5. As fig. 4 but for a 100 um thick section. 3.6
an enhanced Cr content. A summary of the results of the analyses is given in table 1. The mean value of Nco/Ncr in the boundary region is lower than that in the columns themselves by, on average, between 8 and 10%. This figure should be compared with the accuracy with which the mean values were determined which, based on the standard error in the means, was = 1%. It should be emphasized that m u c h of the spread in the individual distributions is attributable to the statistical errors in the Cr K a peak and that examination of a large number of spectra,, as was done here, was necessary not only to ensure that any results quoted were typical of the samples as a whole but also to reduce the statistical error to an acceptable level. Use of eq. (2) enables the mean Nco/Nc~ ratios to be converted to atomic compositions. The mean values of x for the columns in TS1 and TS2 were 20.9 and 20.3, respectively, whilst at the boundaries the corresponding figures were 22.3 and 21.9. These values are in good agreement with the known overall composition (x = 21.5) bearing in nfmd the 3% uncertainty in OcJOco Detailed interpretation of these figures to yield the true composition of the boundary layer requires both a knowledge of the width of the layer and the extent of the beam broadening. The latter Table 1 Summary of the X-ray microanalysis results
3.2 Distance
Fig. 6. Variation of the ratio of Nco/Ncr in 1.2 nm steps across the colunm boundaries in a 60 nm thick transverse section.
is appreciable in foils of 100 and 130 nm thickness and before discussing its magnitude in the following section it is useful ,to present the limited results obtained from the few suitable column boundaries in an ion milled specimen. Two such boundaries were investigated in a section (TS3) whose local thickness was 60 nm. This time, rather than taking analyses either directly on a boundary or well into the surrounding columns, analyses were made with the position of the probe stepped at --1.2 nm intervals from one side of the boundary to the other. The results of this are shown in fig. 6. Despite the rather poor statistics in the individual spectra the position of the boundary is clear in both scans, the ratio of NCo/NCr there being depressed by 10% relative to that in the columns. It is ~also of interest to note the distance over which the value of the ratio is low, this being < 5 n m in both instances. Although beam broadening prevents this figure being equated directly with the width of the boundary layer it does at least give an upper limit to the extent of the Cr rich region.
Sample
Thickness (rim)
Nco//Ncr (columns)
Nco//NCr (boundaries)
4. Discussion
TS1 TS2
130 100
3.52+0.04 3.65 + 0.03
3.24+0.04 3.32 + 0.03
In the preceding section evidence was presented of Cr segregation at the boundaries between col-
364
J.N. Chapman et al. / Cr segregation in rf-sputtered CoCr films
umns in rf sputtered CoCr films. This is not the first time that this phenomenon has been observed, both Hoffman et al. [11] and Jhingan [12] having reported on it previously. The former concluded, from an examination of replicas of the surfaces of CoCr films which had been treated with a Co preferential etch, that a Cr rich layer existed at the boundaries. They were, however, unable to estimate the extent of the enrichment. Jhingan used the same technique as that reported here, but unlike our results, he found that whilst some boundaries showed an increased Cr content others showed increased Co. This we never observed and we can offer no explanation for the discrepancy. As described above, analysis of the X-ray spectra from our thin transverse CoCr sections suggested that there was a Cr enrichment of --1.5 at% in the vicinity of the boundaries between columns. This figure, however, should be regarded as a lower bound to the true segregation which has taken place as, inevitably, even with the probe precisely centred on a perfectly oriented boundary, the volume excited by the probe will be part boundary layer and part column. To proceed further then it is necessary to know the volume excited by the probe in the transverse sections examined and the geometry of the boundary layer. The former may be estimated from a consideration of beam broadening as the electrons pass through the specimen. Reed [13] has derived an analytical expression for the diameter of the circle on the bottom surface of the specimen which contains 90% of the electron trajectories. By studying how this varies with specimen thickness and making allowance for the finite size of the probe, the volume excited (V) can be calculated for specimens of the thickness used. Details of this are given in fig. 7a and in table 2. Table 2 also shows the diameter (d) which a cylinder extending through the specimen thickness would have if its volume were to equal V. We suggest that this figure represents a reasonable measure of the spatial resolution attained in the analyses and, indeed, its value for the 60 nm thick section closely resembles the apparent boundary width observed in fig. 6. Whilst all the above calculations were based on an analytical model for beam broad-
a)
T
60nm
10!nm 13 m
l t 25nm,
b)
A 100nm T Fig. 7. The distribution of electron trajectories in traversing a thin CoCr specimen. (a) Shows how the area containing 90% of the trajectories varies with depth according to an analytical model [13]; (b) shows the results of a Monte Carlo calculation.
ening we note that close agreement exists between the predictions of the Reed model and the results of Monte Carlo calculations [14]. An example of the latter showing the trajectories through a CoCr section of thickness 100 nm is shown in fig. 7b. More serious problems exist in estimating the extent and shape of the boundary layer. The traces Table 2 Relationship between volume analysed and film thickness Sample TS1 TS2 TS3
Thickness
V
d
(rim)
(rim3)
(rim)
130 100 60
22500 8000
15 10 5
Ii00
J.N. Chapman et al. / Cr segregation in rf-sputtered CoCr films
in fig. 6 show that it has an upper bound to its width of 5 nm whilst examination of the sharpness of the interfaces between columns in the A D F images suggest that a value of ~--2 rim, which is the probe diameter, might be appropriate. Neither of these, however, can provide information on the detailed shape of the boundary layer which in any case seems likely to vary depending on the precise morphology and crystal structure of individual columns. To proceed further we make the simplifying assumption that the boundary layers are uniform with width w and extend through the section thickness. It is then straightforward to calculate for a given w and t what fractions ( f ) of the volume excited by the electrons is occupied by the boundary layer. Finally, denoting the measured Cr concentration when the probe is centred on the columns by x c and when it is centred on the boundaries by x ' it is possible to deduce the concentration in the boundary layer (Xb) by x b = x'/f-
xc(1 - f ) / f .
(4)
Table 3 shows the results obtained for TS1 and TS2 assuming that w = 2 nm. If the width of the boundary layer is less than 2 nm x b will be greater whilst, if 2 mn represents an underestimate, values of x b closer to x ' will be obtained. Nonetheless, bearing in mind how rapidly the magnetic properties of Col00_xCrx change as x increases above 20 [8] it is clear that the local properties of the boundary region will differ significantly from those of a film with homogeneous composition. Indeed, if the Cr content rises to > 25% there will be non-ferromagnetic regions between the columns. Examination of table 3 suggests that this is a distinct possibility. If much of the boundary region is non-magnetic the coupling between adjacent columns will be essentially dipolar rather than exchange and as such will be much weaker. As exchange energy accounts for a large fraction of the total energy of domain wails Table 3 Determination of boundary composition assuming w = 2 n m Sample
xc
x'
f
xb
TS1 TS2
20.9 20.3
22.3 21.9
0.14 0.21
31 28
365
in homogeneous films, this may then affect the domain structure within the film. Even if the Cr segregation is not sufficient to increase x to > 25 the results here show conclusively that Cr enrichment in excess of 1.5% takes place at the boundaries. We may conclude, therefore, that the coupling between columns, whether exchange or dipolar, is reduce from the value it would have in a film of uniform composition. There are three possible mechanisms which may be the cause for the Cr segregation: segregation of a new phase, segregation of oxidized Cr or movement of Cr atoms towards column boundaries for thermodynamic reasons. The first possibility, segregation of a new phase, can be rejected because the temperatures involved are too high [6]. To investigate the second possibility the oxygen content in the films was measured with AES. The oxygen content is beneath the detection limit of AES, which is 0.5 at%. If we assume that we need at least as much oxygen as chromium to cause chromium oxide to segregate, then 1.5 at% oxygen is needed, so this possibility can be excluded too. Thus we are left with the third possibility, segregation for thermodynamic reasons. If we consider surface bond breaking and bulk elastic strain energy as the main factors contributing to the driving force for segregation [15], then CoCr indeed belongs to the group of alloys in which the minority component segregates.
Acknowledgement W e are grateful to Professor R.P. Ferrier for many useful discussions, to Drs. C.P.G. Schrauwen and Ing. R.W.J. Gcuskens for sputtering the CoCr films, to Ing. B.H. Koek for preparing the ion milled transverse section, to Dr. G.W. Lorimcr for making available the Monte Carlo programme used in this work and to the S E R C for provision of equipment and a studcntship for one of us 0RM).
References [1] Y. N a k a m u r a and S. Iwasaki, in J A R E C T vol. 15. Recent Magnetics for Electronics, ed. Y. Sukurai ( O H M S H A and
North-Holland, 1984) chap. i.i.
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J.N. Chapman et al. / Cr segregation in rf-sputtered CoCr films
[2] K. Ouchi and S. Iwasaki, in JARECT vol. 15. Recent Magnetics for Electronics, ed. Y. Sukurai (OHMSHA and North-HoUand, 1984) chap. 1.5. [3] S.B. Luitjens, C.P.G. Schrauwen, J.P.C. Bernards and V. Zieren, IEEE Trans. Magn. MAG-21 (1985) 1438. [4] J.W. Smits, S.B. Luitjens, F.J.A. den Broeder and A.G. Dirks, J. Magn. Magn. Mat. 31-34 (1983) 920. [5] M. Futamoto, Y. Honda, H. Kakibayashi and K. Yoshida. Japan. J. Appl. Phys. 24 (1985) L460. [6] J.W. Smits, S.B. Luitjens and F.J.A. den Broeder, J. Appl. Phys. 55 (1984) 2260. [7] K. Kobayashi and G. Ishida, J. Appl. Phpys. 52 (1981) 2453. [8] P.L Grundy and M. Ali, J. Magn. Magn. Mat. 40 (1983) 154. [9] LN. Chapman, W.A.P. Nicholson and P.A. Crozier, J. Microsc. 136 (1984) 179.
[10] C. Colliex and C. Mory, in: Quantitative Electron Microscopy, eds. J.N. Chapman and A.J. Craven (Scottish Universities Summer School in Physics 1983, Edinburgh University Press) chap. 5. [11] H. Hoffman, H. Mandl and Th. Schurmann. ICM 1985, San Francisco, paper 5ph6 (not published in proceedings). [12] A.K. Jhingan, J. Magn. Magn. Mat. 54-57 (1986) 1685. [13] J.I. Goldstein, J.L. Costley, G.W. Lorimer and S.J.B. Reed in SEM 1977 ed. O. Johari (IITRI, Chicago, 1977) p. 315. [14] D.F. Kyser, in: Introduction to Analytical Electron Microscopy, eds. J. Hren, J.I. Goldstein and D.C. Joy (Plenum Press, New York, 1979) chap. 6. [15] F.F. Abraham and C.R. Brundle, J. Vac. Sci. Technol. 18 (2) (1981) 506.
359
INVESTIGATION OF Cr SEGREGATION WITHIN rf-SPUTFERED CoCr FILMS J.N. CHAPMAN, I.R. McFADYEN and J.P.C. BERNARDS f Department of Natural Philosophy, University of Glasgow, Glasgow G12, UK t Philips Research Laboratories, P.O. Box 80.000, 5600 J/l Eindhoven, The Netherlands Received 2 June 1986 Composition variations in thin transverse sections of if-sputtered Co7s.sCr2t.5 films have been studied by X-ray microanalysis using a scanning transmission electron microscope. Analyses with a spatial resolution sub.~tantially better than 10 nm have shown that Cr rich regions exist in the boundary layers between 50-100 nm diameter colunms which dominate the microstructure of such films. The extent of the measured Cr enrichment is ~ 1.5 at%, but when beam broadening effects are taken into consideration, the true segregation is found to be much greater. The implications of these results on the magnetic properties of the films are discussed.
1. Introduction
Thin films of Cot00_xCr; (x ~ 20) are of considerable interest as magnetic recording media capable of storing data at very high densities [1-3]. For the films to have properties suitable for this purpose they must be grown in such a way that a substantial perpendicular magnetic anisotropy is developed. This requires a pronounced crystallographic texture with the hexagonal c-axis parallel to the film normal. In practice the microstructure generally shows a columnar structure with the column axes parallel to the film normal and previous studies [4,5] have confirmed that the c-axis texture is well developed when the columnar microstructure is present through a large proportion of the film thickness. In this paper we are concerned with the further characterisation of the microstructure of such fiLms and, in particular, with investigating any segregation of Cr at the column boundaries. This segregation has been suggested by earlier magnetic measurements [6]. As the magnetic properties of CoCr alloys change rapidly in the vicinity of the composition of interest [7,8], knowledge of the presence and extent of any segregation is important if the response of the medium to a magnetic field, and hence its recording properties, are to be understood. Since the columns themselves are only ~-50-100 nm in diameter the technique used to
investigate Cr segregation must have very high spatial resolution as well as adequate sensitivity to permit changes ~-1 at% to be detected. X-ray microanalysis of thin specimens in a scanning transmission electron microscope (STEM) seems best suited for this purpose and was the technique used throughout this work. Experimental details are given in section 2 following a description of specimen preparation. Thereafter, in section 3 analyses of a number of column boundaries and adjacent regions within the columns are presented. Finally, the paper concludes with a discussion of the relationship between the volume of sample excited by the incident electron probe and the volume occupied by a column boundary. Once this is known imph.'cations for the magnetic properties of the medium are drawn from the results of section 3.
2. Experimental details
The thin films were iprepared by rf sputtering using 8" Co78.5Cr21.5 (at%) alloy targets. Polyester foils, which had been midly sputter-etched and pre-heated, were used as substrates. Full details of preparation conditions are given in a previous paper [3]. In some cases the CoCr was deposited directly onto the polyester whilst in others a layer of soft magnetic material (e.g. NiFe) followed by a
0304-8853/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
360
J.N. Chapman et aL / Cr segregation in rf-sputtered CoCr films
very thin layer of Ti were deposited initially; in all cases a columnar structure with hexagonal crystallites was present in the CoCr films [3-5]. To support perpendicular magnetisation, and hence achieve the desired recording properties, the CoCr layer must exceed a critical thickness. The thickness of the CoCr layer grown here was typically 0.4 ~m. Films of such a thickness are unsuitable for direct observation by transmission electron microscopy (TEM) whether or not they are backed by other layers. Two alternative exist: the films can be back-thinned to remove the substrate and a substantial fraction of the CoCr itself leaving only ~- 100 ran, or transverse sections can be prepared of the CoCr, substrate and any intervening layers. As the latter reveal the columnar structure more clearly and are more suitable for TEM investigations of the magnetic domain structure supported by the material (to be reported elsewhere), this approach was adopted. Suitable sections were generally prepared by embedding the composite specimen in resin and cutting thin sections using a microtome. By this technique long ribbons of material suitable for examination by TEM were produced but their thickness was rarely less than 100 urn. To obtain samples thinner than this ion milling was used. The procedure involved using Ar ions to bombard a transverse section cut from a block consisting of the material of interest glued between two pieces of Si. Generally satisfactory results were obtained in this way but the area of specimen available for examination was small and the thickness of the section varied appreciably over that area. The thickness of the sample plays a crucial role in determining the spatial resolution achievable in the analyses and this is discussed in section 4. All the results were obtained using an extended VG HB5 STEM equipped with a range of different imaging detectors together with facilities for microdiffraction, energy dispersive X-ray microanalysis (EDX) and electron energy loss spectroscopy (EELS). Images of the transverse sections of CoCr clearly revealed its columnar structure, and mierodiffraction confirmed the pronounced c-axis texture of the columns. The columns are - 50 mn in diameter and the dispersion in orien-
tation of the c-axis (A0s0) is 7 °. X-ray microanalyses were obtained simply by disabling the raster scan and positioning the stationary probe at colunm boundaries and different locations within the columns. The probe used was = 2 nm diameter and its current was -~ 0.2 nA. Whilst smaller probes are readily attainable on the HB5 there was no incentive to use them in this work since beam broadening, as the electron pass through the specimen, increases the effective diameter of the area analysed to a value beyond that set by the probe. Details of this are discussed in section 4. Also, a further decrease in probe size would be accompanied b3r a reduction in probe current. With the selected operating conditions a convenient spectrum acquisition time was = 60 s. In this time no detectable specimen drift was observed and the number of Cr K a X-rays detected was typically between 400 and 1000, dependent on the local thickness. As the generation of X-rays is a random process obeying Poisson statistics the error in such signals is between 5 and 3%. No larger error could be tolerated if Cr segregation at the expected level was to be detected. It should be noted that as the ratio of Co to Cr present is approximately 4 : 1 and as cross-sections for X-ray generation in the two element are comparable, the overall uncertainty in the analyses is only slightly worse than the error in the Cr K a signal. To determine whether or not Cr segregation was present in the thin sections (X-ray absorption and fluorescence effects being negligible) it was only necessary to note the variation in the ratio of the number of Co K a (Nco) to the number of Cr K a (Ncr) photons detected. However, to quantify the observed differences a knowledge of the appropriate X-ray generation cross sections was also required. The ratio of the number of atoms of Co (nco) to the number of atoms of Cr (ncr) in the irradiated volume is given by n cJn
Cr =
Nco/Ncr"OCr/OCo,
(1)
where OCr/OCo is the ratio of cross sections for the generation of K a photons in the two elements. Using the data of Chapman et al. [9], the value for this is 1.07 with an associated uncertainty of < 3%. Thus, x (the atomic percentage of Cr present) can
J.N. Chapman et al. / Cr segregation in rf-sputtered CoCr films
be written as x = lOOncr/(nco +
nor )
= l O 0 ( 1 . 0 7 ( N c o / N c r ) + 1} -1,
(2)
EELS provides a very suitable method for measuring the local section thickness at the positions where the X-ray spectra were recorded. The microscope operating conditions for the two techniques are the same and it is possible to record the two kinds of spectra simultaneously. The thickness (t) is determined [10] using t = A i ln(1T/lo),
(3)
where 13. and 10 are respectively the total number of electrons passing through the spectrometer and the number of those which have not been scattered inelastically. A i is the mean free path for inelastic scattering which was estimated [10] to the 47 nm for 100 keV electrons incident on CoCr. Thicknesses determined in this way are believed to be accurate to between 10 and 15%. Support for this has been obtained by using the technique to measure the thickness of a film of CoCr grown sufficiently thin for direct examination in the microscope without the need for sectioning. A thickness
361
value of 110 nm was determined using eq. (3) which agrees within the expected uncertainties with the figure of 120 nm determined from the deposition rate. This, in turn, was calibrated by sputtering several l a y e r s and measuring their thickness with the Talleysurf stylus technique.
3. Results Fig. 1 shows images of a typical transverse section cut from a film grown on a polyester substrate. The images were recorded in the STEM using the probe described above. The columnar structure of the CoCr is evident in both the bright field (B1z) and the annular dark fidd (ADF) images and we have found the latter particularly useful for locating column boundaries suitably oriented for X-ray microanalysis. That most boundaries will not be so oriented is shown schematically in fig. 2 which represents the result of taking a section through a film with a somewhat irregular columnar structure, the thickness of the section and the mean column diameter being comparable. Whilst a number of boundary regions (e.g. A, B, C) extend throughout the section thickness, few, if
Fig. 1. (a) BF and (b) ADF images of a 130 nm thick transverse section through a CoCr film grown on a polyestersubstrate.
J.N. Chapman et al. / Cr segregation in rf-sputtered CoCr films
362
300 Eounfs
bodo%o
i" CoKo~
20( ErK,~
~'obe
7i ,.i
5.0 b]
Fig. 2. (a) Schematic representation of taking a thin section from a film with a columnar microstructure; (b) correct orientation of the thin section for an electron probe to traverse the whole of the boundary layer at A. t denotes the thickness of the section.
any, will lie parallel to the electron propagation direction. However, by tilting the section in the microscope it is possible to bring individual boundaries into the correct orientation (e.g. A in fig. 2b) so that if the probe is positioned as shown (and b e a m broadening is not too severe) the X-ray spectrum obtained will pertain substantially to the boundary layer. The practical advantage offered by the A D F image for determining when a boundary is correctly oriented is associated with the absence of a substantial amount of confusing diffraction contrast from within the columns together with a sharp contrast change across a well oriented boundary. In fig. 1 the boundaries marked P and Q are examples of ones suitable for analysis. Fig. 3 shows a pair of typical spectra, one obtained with the probe centred at a point on P and the other when the probe was moved a perpendicular distance ~ 20 n m from it. The spectra have been scaled so that the Co K a signal is the same in the two cases and the increase in the Cr K a signal from the boundary region is apparent. To confirm this result 9 further spectra were recorded from the boundary P and from 9 points within the columns on either side of it. Analysis of these spectra showed that N c o / N c r had a mean value of 3.15 when the probe was centred along P and a
5.8
6.6
7
x i03 eV Photon Energy
Fig. 3. X-ray spectra, scaled so that the counts in the Co Ka peaks are equal, taken with the probe centred at a point on P (dashed line) and in the colmrm adjacent to P (solid line). One spectrum has been slightly offset with respect to the other for clarity.
significantly higher value of 3.51 when located in the columns. The standard deviation of both distributions was --0.20 and the standard error in the means was 0.07. N o significant difference was detected in the composition of the columns on either side of P. Figs. 4 and 5 show the results of analysing a number of boundaries from two transverse sections cut from samples prepared at different times. The results shown in fig. 4 came from the section shown in fig. 1 (TS1) whose thickness was estimated, using eq. (3), to be 130 rim. Those in fig. 5 came from a 100 n m thick section (TS2) cut from a composite C o C r / T i / N i F e sample. In both instances it is clear that the boundary regions have
r I-
2.8
I I L_..I
3.2
3.6
~'0 N[o/NCr
Fig. 4. Histograms of the ratios of Nco/NCr with the probe centred on the boundaries between columns (dashed line) and within the columns (solid line). The arrows denote the means of the two distributions. The sample was a 130 nm thick transverse section.
J.N. Chapman et al. / Cr segregation in rf-sputtered CoCr films
363
NCo/NCr
m
IF-'•
4.0
I
3.6
I
F-I I
I I
m
3!2
I
3.6
3.2
fq
~.0 Nco/NCr
i
I
25nm
I
Fig. 5. As fig. 4 but for a 100 um thick section. 3.6
an enhanced Cr content. A summary of the results of the analyses is given in table 1. The mean value of Nco/Ncr in the boundary region is lower than that in the columns themselves by, on average, between 8 and 10%. This figure should be compared with the accuracy with which the mean values were determined which, based on the standard error in the means, was = 1%. It should be emphasized that m u c h of the spread in the individual distributions is attributable to the statistical errors in the Cr K a peak and that examination of a large number of spectra,, as was done here, was necessary not only to ensure that any results quoted were typical of the samples as a whole but also to reduce the statistical error to an acceptable level. Use of eq. (2) enables the mean Nco/Nc~ ratios to be converted to atomic compositions. The mean values of x for the columns in TS1 and TS2 were 20.9 and 20.3, respectively, whilst at the boundaries the corresponding figures were 22.3 and 21.9. These values are in good agreement with the known overall composition (x = 21.5) bearing in nfmd the 3% uncertainty in OcJOco Detailed interpretation of these figures to yield the true composition of the boundary layer requires both a knowledge of the width of the layer and the extent of the beam broadening. The latter Table 1 Summary of the X-ray microanalysis results
3.2 Distance
Fig. 6. Variation of the ratio of Nco/Ncr in 1.2 nm steps across the colunm boundaries in a 60 nm thick transverse section.
is appreciable in foils of 100 and 130 nm thickness and before discussing its magnitude in the following section it is useful ,to present the limited results obtained from the few suitable column boundaries in an ion milled specimen. Two such boundaries were investigated in a section (TS3) whose local thickness was 60 nm. This time, rather than taking analyses either directly on a boundary or well into the surrounding columns, analyses were made with the position of the probe stepped at --1.2 nm intervals from one side of the boundary to the other. The results of this are shown in fig. 6. Despite the rather poor statistics in the individual spectra the position of the boundary is clear in both scans, the ratio of NCo/NCr there being depressed by 10% relative to that in the columns. It is ~also of interest to note the distance over which the value of the ratio is low, this being < 5 n m in both instances. Although beam broadening prevents this figure being equated directly with the width of the boundary layer it does at least give an upper limit to the extent of the Cr rich region.
Sample
Thickness (rim)
Nco//Ncr (columns)
Nco//NCr (boundaries)
4. Discussion
TS1 TS2
130 100
3.52+0.04 3.65 + 0.03
3.24+0.04 3.32 + 0.03
In the preceding section evidence was presented of Cr segregation at the boundaries between col-
364
J.N. Chapman et al. / Cr segregation in rf-sputtered CoCr films
umns in rf sputtered CoCr films. This is not the first time that this phenomenon has been observed, both Hoffman et al. [11] and Jhingan [12] having reported on it previously. The former concluded, from an examination of replicas of the surfaces of CoCr films which had been treated with a Co preferential etch, that a Cr rich layer existed at the boundaries. They were, however, unable to estimate the extent of the enrichment. Jhingan used the same technique as that reported here, but unlike our results, he found that whilst some boundaries showed an increased Cr content others showed increased Co. This we never observed and we can offer no explanation for the discrepancy. As described above, analysis of the X-ray spectra from our thin transverse CoCr sections suggested that there was a Cr enrichment of --1.5 at% in the vicinity of the boundaries between columns. This figure, however, should be regarded as a lower bound to the true segregation which has taken place as, inevitably, even with the probe precisely centred on a perfectly oriented boundary, the volume excited by the probe will be part boundary layer and part column. To proceed further then it is necessary to know the volume excited by the probe in the transverse sections examined and the geometry of the boundary layer. The former may be estimated from a consideration of beam broadening as the electrons pass through the specimen. Reed [13] has derived an analytical expression for the diameter of the circle on the bottom surface of the specimen which contains 90% of the electron trajectories. By studying how this varies with specimen thickness and making allowance for the finite size of the probe, the volume excited (V) can be calculated for specimens of the thickness used. Details of this are given in fig. 7a and in table 2. Table 2 also shows the diameter (d) which a cylinder extending through the specimen thickness would have if its volume were to equal V. We suggest that this figure represents a reasonable measure of the spatial resolution attained in the analyses and, indeed, its value for the 60 nm thick section closely resembles the apparent boundary width observed in fig. 6. Whilst all the above calculations were based on an analytical model for beam broad-
a)
T
60nm
10!nm 13 m
l t 25nm,
b)
A 100nm T Fig. 7. The distribution of electron trajectories in traversing a thin CoCr specimen. (a) Shows how the area containing 90% of the trajectories varies with depth according to an analytical model [13]; (b) shows the results of a Monte Carlo calculation.
ening we note that close agreement exists between the predictions of the Reed model and the results of Monte Carlo calculations [14]. An example of the latter showing the trajectories through a CoCr section of thickness 100 nm is shown in fig. 7b. More serious problems exist in estimating the extent and shape of the boundary layer. The traces Table 2 Relationship between volume analysed and film thickness Sample TS1 TS2 TS3
Thickness
V
d
(rim)
(rim3)
(rim)
130 100 60
22500 8000
15 10 5
Ii00
J.N. Chapman et al. / Cr segregation in rf-sputtered CoCr films
in fig. 6 show that it has an upper bound to its width of 5 nm whilst examination of the sharpness of the interfaces between columns in the A D F images suggest that a value of ~--2 rim, which is the probe diameter, might be appropriate. Neither of these, however, can provide information on the detailed shape of the boundary layer which in any case seems likely to vary depending on the precise morphology and crystal structure of individual columns. To proceed further we make the simplifying assumption that the boundary layers are uniform with width w and extend through the section thickness. It is then straightforward to calculate for a given w and t what fractions ( f ) of the volume excited by the electrons is occupied by the boundary layer. Finally, denoting the measured Cr concentration when the probe is centred on the columns by x c and when it is centred on the boundaries by x ' it is possible to deduce the concentration in the boundary layer (Xb) by x b = x'/f-
xc(1 - f ) / f .
(4)
Table 3 shows the results obtained for TS1 and TS2 assuming that w = 2 nm. If the width of the boundary layer is less than 2 nm x b will be greater whilst, if 2 mn represents an underestimate, values of x b closer to x ' will be obtained. Nonetheless, bearing in mind how rapidly the magnetic properties of Col00_xCrx change as x increases above 20 [8] it is clear that the local properties of the boundary region will differ significantly from those of a film with homogeneous composition. Indeed, if the Cr content rises to > 25% there will be non-ferromagnetic regions between the columns. Examination of table 3 suggests that this is a distinct possibility. If much of the boundary region is non-magnetic the coupling between adjacent columns will be essentially dipolar rather than exchange and as such will be much weaker. As exchange energy accounts for a large fraction of the total energy of domain wails Table 3 Determination of boundary composition assuming w = 2 n m Sample
xc
x'
f
xb
TS1 TS2
20.9 20.3
22.3 21.9
0.14 0.21
31 28
365
in homogeneous films, this may then affect the domain structure within the film. Even if the Cr segregation is not sufficient to increase x to > 25 the results here show conclusively that Cr enrichment in excess of 1.5% takes place at the boundaries. We may conclude, therefore, that the coupling between columns, whether exchange or dipolar, is reduce from the value it would have in a film of uniform composition. There are three possible mechanisms which may be the cause for the Cr segregation: segregation of a new phase, segregation of oxidized Cr or movement of Cr atoms towards column boundaries for thermodynamic reasons. The first possibility, segregation of a new phase, can be rejected because the temperatures involved are too high [6]. To investigate the second possibility the oxygen content in the films was measured with AES. The oxygen content is beneath the detection limit of AES, which is 0.5 at%. If we assume that we need at least as much oxygen as chromium to cause chromium oxide to segregate, then 1.5 at% oxygen is needed, so this possibility can be excluded too. Thus we are left with the third possibility, segregation for thermodynamic reasons. If we consider surface bond breaking and bulk elastic strain energy as the main factors contributing to the driving force for segregation [15], then CoCr indeed belongs to the group of alloys in which the minority component segregates.
Acknowledgement W e are grateful to Professor R.P. Ferrier for many useful discussions, to Drs. C.P.G. Schrauwen and Ing. R.W.J. Gcuskens for sputtering the CoCr films, to Ing. B.H. Koek for preparing the ion milled transverse section, to Dr. G.W. Lorimcr for making available the Monte Carlo programme used in this work and to the S E R C for provision of equipment and a studcntship for one of us 0RM).
References [1] Y. N a k a m u r a and S. Iwasaki, in J A R E C T vol. 15. Recent Magnetics for Electronics, ed. Y. Sukurai ( O H M S H A and
North-Holland, 1984) chap. i.i.
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J.N. Chapman et al. / Cr segregation in rf-sputtered CoCr films
[2] K. Ouchi and S. Iwasaki, in JARECT vol. 15. Recent Magnetics for Electronics, ed. Y. Sukurai (OHMSHA and North-HoUand, 1984) chap. 1.5. [3] S.B. Luitjens, C.P.G. Schrauwen, J.P.C. Bernards and V. Zieren, IEEE Trans. Magn. MAG-21 (1985) 1438. [4] J.W. Smits, S.B. Luitjens, F.J.A. den Broeder and A.G. Dirks, J. Magn. Magn. Mat. 31-34 (1983) 920. [5] M. Futamoto, Y. Honda, H. Kakibayashi and K. Yoshida. Japan. J. Appl. Phys. 24 (1985) L460. [6] J.W. Smits, S.B. Luitjens and F.J.A. den Broeder, J. Appl. Phys. 55 (1984) 2260. [7] K. Kobayashi and G. Ishida, J. Appl. Phpys. 52 (1981) 2453. [8] P.L Grundy and M. Ali, J. Magn. Magn. Mat. 40 (1983) 154. [9] LN. Chapman, W.A.P. Nicholson and P.A. Crozier, J. Microsc. 136 (1984) 179.
[10] C. Colliex and C. Mory, in: Quantitative Electron Microscopy, eds. J.N. Chapman and A.J. Craven (Scottish Universities Summer School in Physics 1983, Edinburgh University Press) chap. 5. [11] H. Hoffman, H. Mandl and Th. Schurmann. ICM 1985, San Francisco, paper 5ph6 (not published in proceedings). [12] A.K. Jhingan, J. Magn. Magn. Mat. 54-57 (1986) 1685. [13] J.I. Goldstein, J.L. Costley, G.W. Lorimer and S.J.B. Reed in SEM 1977 ed. O. Johari (IITRI, Chicago, 1977) p. 315. [14] D.F. Kyser, in: Introduction to Analytical Electron Microscopy, eds. J. Hren, J.I. Goldstein and D.C. Joy (Plenum Press, New York, 1979) chap. 6. [15] F.F. Abraham and C.R. Brundle, J. Vac. Sci. Technol. 18 (2) (1981) 506.