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ATOM PROBE CHARACTERISATION OF Co/TRANSITION-METAL MULTILAYER
STRUCTURES
A. Cerezo, M. Hetherington, A. Petford-Long
To cite this version:
A. Cerezo, M. Hetherington, A. Petford-Long. ATOM PROBE CHARACTERISATION OF
Co/TRANSITION-METAL MULTILAYER STRUCTURES. Journal de Physique Colloques, 1989,
50 (C8), pp.C8-349-C8-354. �10.1051/jphyscol:1989859�. �jpa-00229957�
COLLOQUE DE PHYSIQUE
Colloque C8, Supplkment au n0ll, Tome 50, novembre 1989
ATOM PROBE CHARACTERISATION OF Co/TRANSITION-METAL MULTILAYER STRUCTURES
A. CEREZO, M.G. HETHERINGTON and A.K. PETFORD-LONG
Department of Metallurgy and Science of Materials, University of Oxford, Parks Road, GB-Oxford OX1 3PH, Great-Britain
Abstract Cobaltltmnsition-metal multilayer structures grown on CO field-ion specimens have been analysed by both conventional atom probe (AP) and the position-sensitive atom probe (POSAP). The combination of the two techniques allows the chemical abruptness and the roughness of the interfaces to be studied independently, and can be supported by high resolution transmission electron microscope (HREM) studies of multilayers deposited simultaneously on flat substrates. Layered structures can be formed with sharp interfaces, with epitaxial growth having been observed, even when using only a simple thermal evaporation source. However, growth by this method seems to be somewhat inconsistent, with carbon contamination leading to the formation of interdiffused, and in the extreme even amorphous layers. With the use of electron beam sources in place of filament evaporators, this technique should provide a powerful complement to the more conventional characterisation methods.
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INTRODUCTIONThe ability to tailor electronic properties in semiconductor multiple quantum well structures is well known. In metallic multilayer structures, with periods down to atomic dimensions, there exists the possibility of controlling magnetic and mechanical properties by suitable selection of the layer widths [l]. Of particular interest are the composite magnetic materials made up of Cdtransition-metal multilayers, which have potential applications in the field of data storage and recording. Due to the size of these structures, chemical and morphological effects on the atomic scale are likely to be of great importance in determining the magnetic properties which are observed. While structural and magnetic information on a fine scale can be obtained using electron microscopy techniques, the high-resolution analytical capability of the atom probe provides a means to gain unambiguous information on interface chemical abruptness (as distinct from any roughness that might exist) and possible grain boundary segregation effects.
Metallic multilayers composed of up to hundreds of layers are typically grown using either electron beam evaporation sources, or by sputter deposition. Whilst the use of electron beam evaporators in our experiments is planned, we wished to carry out a preliminary study using simpler thermal evaporation sources. Cobalt field-ion specimens prepared by electropolishing in standard perchloric acid solutions were imaged in a VG FIMlOO atom probe and field evaporated to give a smooth endform. After imaging, each specimen was withdrawn to the auxiliary chamber of the instrument, and allowed to warm to room temperature before deposition. In order to reduce possible sources of contamination, the specimens were allowed only the minimum warm-up time necessary, found to be 15 minutes, before the growth of the multilayers. Deposition was from conventional tungsten evaporator filaments loaded with metal in the form of wire (Fe, Ni, CO) or flakes (Cr) and was monitored usinga quartz crystal oscillator. The filament assembly in the auxiliary chamber was placed below and somewhat in front of the specimen (which was rotated during the deposition) and at a distance of IOcm. Base pressure in the chamber was <10-6Pa (<l0-8mbar) prior to evaporation, and around 10-5Pa (10-7mbar) during deposition. The rate of evaporation was typically 0.05-0.2 nm/s (0.5-2 u s ) , with the thickness of each deposited layer being 1-2 nm (10-20
A),
and any single multilayer structure having 3-7 layers. In some cases, a flat oxidised silicon substrate, cleaned by heating in vacuum to r20°C for 20mins., was placed in the chamber next to the field-ion specimen. This allowed subsequent study of the multilayer by electron microscopy techniques. After deposition, the field-ion specimens were returned to the main chamber of the instrument for analysis either by AP or, in the case of CoJCr multilayers, POSAP.Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989859
Ni ions
CO layer I Ni layer I CO substrate
I l
Figure 1. Cumulative plot from the analysis of a clean Ni,Co bi-layer deposited on a CO field-ion specimen.
Each layer was formed by deposition of 1.5nm of metal, as measured by a crystal oscillator thickness monitor.
The atom probe analysis shows that the interfaces in the multilayer are effectively abrupt on the atomic scale.
0 50 100 150 200 250 300 Total ions Fe ions
Substrate
300
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Figure 2. Atom probe analysis of a poorly defined Fe/Co/Fe multilayer (1.5nm period).
The carbon contamination observed (measured to be about 8 at%, averaged throughout the layers) appears to have generated a high degree of intermixing.
0 100 300
CO ions 2 - RESULTS
Figure 1 shows a cumulative plot obtained from the AP analysis of a CdNi bi-layer (1.5nm layers) evaporated onto a cobalt field-ion specimen which had been imaged to 8 kV before deposition. The layers are found to be very clean, with a total carbon level of only about 2 at%. From this ladder diagram, the interfaces are seen to be abrupt on the atomic scale, with the extent of the 'mixed' region between the layers being only a single monolayer thick. This shows that it is indeed possible to produce high quality metallic multilayers with a simple arrangement using only filament evaporation sources.
The difficulty we have found, however, is in the consistent production o f layers of the quality o f that shown in figure 1. Often, the layers were observed to be highly inter-mixed and found to contain significant levels of carbon contamination, as shown in the analysis of a F d C d F e multilayer (1.5nm layers again) of figure 2. This contamination may have been due to a small air leak present in the auxiliary chamber, or generated by accumulated deposits around the filament assembly, which would naturally act as a getter and then out-gas during the heating of the filaments. Despite this, the layers were still found to be growing epitaxially, as shown in the sequence of field-ion images of figure 3.
The micrograph taken during the analysis of the layers, figure 3b, shows that the crystallography of the contaminated
Figure 3. Sequence of field-ion images from the analysis of the Fe/Co/Fe' multilayer shown in figure 2: a) image from substrate before deposition;
b) FIM image of layer during analysis (position shown in figure 2); c) CO substrate after analysis.
Figure 4. HREM image of a metallic layer deposited as a 7 period multilayer of 1.5nm spacing. The resulting layer is totally intermixed, but is nonetheless ply-crystalline
multilayer matches perfectly with the underlying Co. In some cases the degree of intermixing was even greater, producing essentially a single mixed layer with little or no indication of the expected variation of composition. It should be stressed that the layers were deposited onto specimens at room temperature, at a reasonable distance (10 cm minimum) from the filaments, so that heating effects should have been negligible. In addition, high resolution electron microscopy examination of layers deposited simultaneously on a silicon substrate in these cases also show a mixed poly-crystalline layer, so that these mixing effects are not produced by the geometry of the specimen. Figure 4 shows an HREM image from a layer which was deposited as a C d C r multilayer consisting of 7 layers, 1.5nm thick. No composition modulation is visible in the KREM micrograph of this film
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and the atom probe analysis of the layer deposited onto the CO field-ion specimen showed it to be to totally intermixed.The combination of quantitative chemical analysis and spatial information available from the POSAP makes this a powerful instrument in the study of metallic multilayers, since it is able to yield the morphology of the interfaces between layers separately from any intermixing which might be present [2]. However, there is some limitation in the systems which can be studied due to the limited mass resolution of the technique. In some cases, such as Fe/Co, the mass separation is sufficient that the interface structure will be observable, although exact data on any interdiffusion will be complicated by a slight overlap between the 56Fe2+ and 59C02+ mass peaks a t m/n = 28.0 and 29.5 respectively. In cases such as C K o , the separation between peaks in the spectrum is sufficiently large for no overlap to occur, as shown in figure 5, and should therefore allow quantitative information to be obtained on both the chemical diffuseness and the morphology of the interface.
In figure 6, cross-sectional views are shown from the POSAP analysis of a CKo/Cr/CdCr multilayer of 2nm period, using a local-area dependent depth calibration described elsewhere [3]. The interfaces are seen to be regular and relatively flat, also appearing chemically abrupt. It should be noted that since this represents a cross-sectional view, it is the projection through about 15 nm of material, as for a TEM micrograph, and so care must be taken in interpreting any single image in terms o f diffuse or rough interfaces. However, the POSAP data also cames information of variations in depth, and this can be used to derive, for example, the topology of the interface. Alternatively, by taking a selected area around the centre of the POSAP image approximately equal in size to the probe hole in the conventional atom probe, it is possible to construct a ladder diagram, equivalent to a conventional AP analysis (figure 7).
Comparison with the cumulative plot of figure 1 shows a similar quality of layer growth, and confirms that the interface is indeed abrupt on an atomic scale. The small amount of roughness apparent in the cross-sectional images of these layers is on the scale of 2-3 monolayers and may be indicative of atomic facets at the interfaces between the layers, as would be expected from the relative orientations of CO and Cr layers in such a structure 141.
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D I S C U S S I O NPreliminary experiments have shown the capability of atom probe and POSAP techniques in the study of metallic multilayers, where layers are grown within the vacuum system of the microscope. Currently these structures are being investigated primarily by X-ray diffraction, which is sensitive to the composition grading at interfaces, and TEM techniques, which give a projection through the thickness of the specimen (5-10 nm) [S]. Structural features on a smaller scale are obviously very difficult to observe in the TEM (with the exception of interface steps at highly planar interfaces [6]) but will give the appearance of intermixing with large area sampling techniques like X-ray diffraction.
The 1-2nm lateral resolution (and atomic depth resolution) of the AP provides a means of obtaining accurate data on interface abruptness which is not affected by roughness on this scale, whilst the sub-nanometre imaging of the POSAP allows direct observation of the interface morphology. These techniques could therefore yield much important information to complement the more conventional analytical tools. Conversely, correlation between POSAP analyses and data from both X-ray and TEM techniques will help to resolve the difficulties of interpretation of POSAP images caused by irregularities in field evaporation from thin layers.
Whilst we have found that high quality layers can be grown with simple deposition apparatus, using only heated filament evaporators, it seems that cleanliness of the vacuum chamber and evaporator assembly is paramount in the production of consistently good multilayers. Once regular results can be acheived, we hope to study the interface abruptness and morphology on the atomic scale, as a function of deposition conditions and subsequent annealing, and investigate the r6le of these effects on the magnetic properties of multilayer structures.
Figure 6. CO and Cr element distributions (seen in cross section) generated from the POSAP analysis of a Cr/CoJCr/Co/Cr multilayer structure (2nm period) deposited on a CO specimen. The interfaces are seen to be relatively flat, with a roughness equivalent to only 2-3 monolayers, and appear chemically abrupt. Note that the calculated depth scale agrees well with the film thickness measurements using a crystal oscillator monitor.
400
200
0
Cr ions
I
Figure 5. Mass spectrum o b tained from a POSAP analysis of a Co/Cr multilayer structure. There is very little overlap between the main Cr and CO peaks, allowing both chemical diffuseness and interface morphology i n these structures t o be m e a s U r e d quantitatively.
0 200 400 600
Total ions
Figure 7. Cumulative plot constructed from a selected area analysis of the POSAP data shown in figure 5. The size of the selected area used is roughly equal to the size of the probe hole in a conventional atom probe.
20 30 40
Mass-to-charge
Acknowledaements
The authors would like to thank Professor Sir Peter Hirsch FRS, Dr G D W Smith and Dr C R M Grovenor for the provision of experimental facilities. We are also grateful to the following for financial support in the form of research fellowships: The Royal Society (AC), Science and Engineering Research Council (MGH and AKP-L) and Wolfson College, Oxford (AC). The Oxford atom probe facility is funded by the Science and Engineering Research Council.
References
[ l ] S. Iwasaki and Y. Nakamura, IEEE Trans. Mag. MAG- 13 (1 977) 1272.
[2] A. Cerezo, T.J. Godfrey, C.R.M. Grovenor, M.G. Hetherington, R.M. Hoyle, J.P. Jakubovics, J.A. Liddle, G.D.W. Smith and G. M. Worrall, I. Microscopy 154 (1989) 215.
131 A. Cerezo and M.G. Hetherington, these Proceedings.
[4] M.B. Steams, Phys. Rev.B 38 (1988)8109.
[S] M. B. Steams, C. H. Lee, C-H. Chang and A. K. Petford-Long, in Metallic multilayersystems(eds. M. Hong, D. V. Gubser and S. A. Wolf), Metallurgical Society, Warrendale, PA, 1988, p.55.
161 C. D'Anterroches, I. Microsc. Spectrosc. Electron. 9 (1 984) 147.
