Ni multilayered thin-film materials

Nuclear Instruments and Methods in Physics Research B67 (1992) 590-594 North-H011and

Nuclear Instruments & Methods in Physics Research Sechon 13

Microtopography and interface width of sputter profiled Cr/Ni multilayered thin-film materials N. T a n o v i 6 a, L. T a n o v i 6 b, j . F i n e c, B. G a k o v i 6 h p. P a n j a n a a n d N. P o p o v i 6 e , Department of Physics, Unit'ersity of Sarajet'o, 71000 Sarajet'o, Yugoslat'ia h Electrical Engineering Faculty, Unh'ersity of Sarajet'o, 71000 Sarajet'o, Yugoslat'ia c Surface attd Microanalysis Science Dit.ision, National bzstitute of Standards and Technology, Gaithersburg, MD 20899, USA d "'Joker Stefan'" Institute, 61000 Ljuhljana, ~tgosl,tt'ia " "Boris i~'drid'" Institute, 11000 Belgrade, Yugoslat'ia

interfilce widths of sputter profiled Cr/Ni multilayers have been found to depend on the specific sputter deposition technique used in specimen fabrication. Scanning electron microscopy and a sensitive stylus instrument were used to show that these interface widths are related to the microtopography of the profiled surface. We have found that differences in sputter induced microtopography can be attributed to differences in crystallite orientation in the thin-fihn layers and that these differences in crystallite structure result from the sputter technique employed.

1. Introduction Compositional profiles of elemental concentration as a function of distance beneath a surface can be obtained, often with nanometer depth resolution, by a variety of techniques. Many of these depth profiling techniques combine ion bombardment induced sputtering for surface layer removal with a surface-sensitive analytica~ method lbr monitoring the composition of the exposed surface. Measuremeuts of this type produce a sputter depth profile: the surface composition is monitored as a function of sputtering time. One very widely used method for monitoring surface composition is Auger electron spectroscopy (AES); it is very well suited for elemental depih profile analysis due to the small size of the analyzed region (typically a micrometer in diameter). Because of this high lateral resolution and due to the relative ease of application and data interpretation, Auger sputter depth profile analysis has become a generally accepted technique in many areas of research and technology [1,2]. Ion bombardment induced surface erosion (i.e., sputtering) is now widely used for atom layer removal in surface analysis. Because this erosion occurs on an atomic scale, the resolved depth in sputter depth profiling should, in principle, also be of atomic dimensions. There are, however, a number of factors which can affect the measured depth resolution and the accuracy of sputter depth profiles. Several physical processes that occur during ion bombardment which can significantly alter the original compositional depth distribution are atomic mixing, preferential sputtering,

radiation enhanced diffusion, radiation induced segregation and Gibbsian adsorption. Although many of these processes are not completely understood, we believe that it is possible to obtain sputter depth profiles for specific metal/metal interfaces which do accurately reflect the structure and composition of the unaltered interface. One such interface system is a thin-film multilayered assembly of alternating layers of Ni and Cr. In the past decade there has appeared a number of papers concerned with the depth resolution at C r / N i interfaces of multilayered thin films and how the parameters associated with the sputter profiling influence the interface profile shape. The first systematic study of the multilayered C r / N i interfaces by AES sputter depth profiling was made by Hofmann et al. in 1977 [3]. Based on these results, Fine and Navingek selected this same pair of metals for the development of a C r / N i thin-film multilayered standard reference materials (SRM) for sputter depth profile calibration in 1982 [4,5]. Since then, numerous investigations have been made to characterize such C r / N i interfaces: the width of these sputter profiled interfaces and their width dependence on sputtered depth, ion beam energy, ion current density, ion beam angle of incidence, and on alignment of ion and probe beams [6-9], have been measured. The influence of the above analysis parameters seems to be reasonably well understood. In recent years, two new methods have been developed for reducing the effect of bombardment generated surface roughness on interface resolution: (1) sample rotation during ion bombardment [10] and (2)

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N. Tanoci6 et al. / microtopography and interface width of Cr / Ni the use of two ion beams simultaneously t! 1,12]. Both methods have been shown to improve deptn resolution so that very precise interface measurements can now be made. Careful control and optimization of instrumental parameters does make it possible to obtain sputter profiles that are both reproduc'b!e and accurate. As a result of our present Auger profiling techniques and i,lterface fitting software [13], interface profile measurements can be obtained that are sensitive to nanometer variations in sputtered depm. Using these profiling and fitting techniques, interface profile widths were measured as a function of sputtered depth for two separate sets of C r / N i multilayered thin-film materials. We wished to compare the profiled interface widths of some prototype C r / N i materials (referred to as Material B) to our well characterized NIST SRM 2135 (Material A). These two sets of materials should be very similar: the films were of the same purity, about the same thickness, the same substrate smoothness, and both sets of films had been fabricated by sputter deposition but using somewhat different deposition techniques. Interface width measurements made on samples taken from either of the two sets were reproducible and consistent. We found, however, that there was a significant 30% difference in interface widths between t.. two sets. Since the instrumental measurement parameters were held constant for all of our sputter profiling, it seemed reasonable that these observed differences in width could be related to structural differences in the two sets of materials. Anaiysis of the microtopography of the ion bombarded surfaces of the Ni and Cr layers indicated that the surface roughness was different for the two sets of materials. Investigation by X-ray spectroscopy showed that the crystalline orientation of individual layers of the C r / N i structures were different and could be correlated with the bombardment induced microtopography.

Substrates for these SRMs were cut from Si(100)wafers specially polished to ensure minimal surface defects. The Cr and Ni deposition target purity was better than 99.99% and deposition rates were monitored by a quartz crystal microbalance during fabrication. Samples from this set of thin-film structures are referred to as Material A. Material B consisted of a set of prototype C r / N i materials that were being evaluated to determine if they would be suitable as a replacement materials for SRM 2135. These thin-film strcctures had been fabricated using magnetron sputter deposition techniques. Although the film purity, thickness, and structure of Material B were very similar to Material A, data was needed of the sputter profiled interface widths.

3. Measurements and results Samples of both Materials A and B were sputter profiled using the same experimental conditions. A low current density 1 keV argon ion beam was rastered to ensure uniform erosion; the current density of the static beam was about 10 mA/cm2; it was very stable ( + 2 % ) for the long sputtering times used (10-20 h). Beam optimization and positioning was done using ion imaging techniques [14]. Elemental Auger analysis was carried out while the specimen was being continuously

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2. Materials fabrication The C r / N i NIST standard reference material (SRM 2135) was developed primarily for calibrating sputtered depth scales and erosion rates in surface analysis [4,5]. It consists of nine alternating metal thin-film layers, five layers of pure chromium and four of pure nickel, on a polished silicon (100) substrate. The individual layers have thicknesses that are nominally 53 nm for the Cr and 64 nm for the Ni one. This thin-film structure was produced by sputter deposition using a plasma beam-sputter deposition technique; all the layers were deposited in situ. First, an amorphous 40 nm silicon layer was deposited onto the silicon substrate; alternating layers of Cr and Ni were then deposited on top of the a-Si with Cr forming the outermost layer.

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1O0 200 300 400 500 SPU'FFERED DEPTH (aml Fig. 1, Interface resolution (defined here as the sputter depth (nm) required for the Auger signal intensity to change from 90% to 10% of its steady state value prior to the interface) of Cr/Ni multilayered, for Materials A (*, o) and B (*, A ) as a function of sputtered depth. Depth profiles were obtained with I keV Ar ion beam at an incidence angle of 50° at beam currents of 80 nA on 2 x 2 ram 2 surface. VIII. SPUTTERING, DESORPTION

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N. Tano~.i~ et al, / Microtopography attd interface width o f Cr / Ni

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MATERIAL B Fig. 2. Typical displacement profiles obtained with a stylus instrument of sputtered regions on Materials A and B. Sputter profiling was done using a rastered 1 keV argon ion beam (rastered area was 2 × 2 mm2). Note the four rougher Ni layers on Material B.

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Fig. 3. SEM micrographs of the sputter profiled surface of Material B at various magnifications: (a) the overall sputtered regton shows fou~ layers of Ni (lighter colored), 66 x ; (b) surface topography, 400 X; (c) surface roughness of the Ni layer, 800 x ; (d) the same Ni surface roughness, 2000 x .

N. TanoviCet al. / microtopography and interface width of Cr / Ni

sputter eroded. The Auger lines of Ni (860 eV) and Cr (525 eV) were measured with a high resolution singlepass cylindrical mirror analyzer (with a concentric electron gun) using 100 MHz pulse counting techniques. A 2.5 keV primary electron beam (50-70 hA) was used to excite the spectra which were acquired on a multichannei analyzer and subsequently transferred to a computer for analysis. Auger ~;,utter depth profile measurements of both A and B Materials consistently indicate that the interface widths obtained on Material B are about 30% greater than those of Material A. These interface widths as a function of sputtered depth are shown in fig. 1; the similar depth dependence for both sets of materials is indicative of an increasing surface microroughness which develops with sputtered depth and has been shown to dominate the sputtered width of C r / N i interfaces measured at room temperature [6,15].

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This type of depth-dependent increase in surface microtopography (due to sputtering) can be associated with the mieroerystalline structure of the Cr and Ni thin films [16]. Our observed differences in the interface widths of Materials A and B, therefore, suggested that there were significant differences in the thin-film crystal structure of these two sets of materials. A systematic study of the microtopography of the ion bombarded regions produced during the Auger sputter depth profile measurements was made using a sensitive stylus instrument. Typical stylus displacements profiles measured on both Materials A and B are shown in fig. 2. The stylus profile of Material B, in contrast to that of Material A, shows some unusual topography associated with the four Ni layers. Examination of these ion bombarded regions on Material B by scanning electron microscopy (SEM) indicated that the exposed Ni areas were quite rough and that this

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IV. Tanot'iC et al. / Microtopography and hzterface width of Cr / Ni

microtopography was visible, even at relatively low magnification, as can be seen in fig. 3. Althlough it is well known that surface topography, either pre-existing or bombardment induced can affect the width of sputter profiled interfaces [15-17], the measured interface widths for Material B, in fig. 1, appear to be representative of the type of surface roughness which is increasing monotonically with sputtered depth - not at all like roughening observed only for the Ni layers. In order to determine whether morphologiical differences in the film structure of Materials A and B could be responsible for the observed microtopography present on the Ni layers, X-ray analysis was made of the thin film layers to identify their crystal orientation. These X-ray analyses, shown in fig. 4 for both Materials A and B, indicate that the Cr and Ni layers are crystallinelike having low-index directions oriented normal to the specimen surface. For Material A, the outer Cr and Ni layers are (100) while the subsequent Ni and Cr layers are (200); the deeper layers are of a higher index orientation and are less transparent to the ion beam. Material B, however, is composed of a number of differently oriented crystallites, as seen in fig. 4, that include Ni(ll 1) or Cr(ll0). Since ion bombardment induced microtopography measurements on single crystal surfaces [18] indicate that the less transparent crystal orientations develop rougher surfaces than do the more transparent ones, we would expect that the crystalline structure of Material B is responsible for the broader interfaces observed during sputter profiling.

4. Summary and conclusions Differences in sputter profiled interface widths obtained on two sets of C r / N i muitilayered thin-film structures can be attributed to differences in surface roughness that resulted from the sputter profiling process. Although both sets of C r / N i structures were fabricated by sputter deposition, the deposition technique used for each set was different and resulted in thin film morphologies that were not the same. We found that the thin film layers of Material B were composed of crystallites with higher Miller indices than those of Material A. Because of this difference in crystal transparency, the surface microroughness developed on Material A is less than that of Material B and is consistent with the broader interfaces measured on Material B.

The enhanced roughness observed on sputtered Ni layers of Material B, but not on Cr layers, will be the subject of further study.

Acimowledgements This work was partially supported by the EC Commission for Scientific and Technological Cooperation through funds made available in a joint research project CI1"-CT90-0767. Two of the authors (N.T. and L.T.) would like to express their gratitude to the Surface and Microanalysis Science Division at NIST, Gaithersburg, MD 20899, USA for the opportunity of spending a year there as visiting scientists.

References [1] Practical Surface Analysis with AES and XPS, eds. D. Briggs and M.P. Seah (Wiley, Chichester, 1983). [2] Thin Film and Depth Profile Analysis, ed. H. Oechsner, Topics in Current Physics, no. 37 (Springer, Berlin, 19841. [3] S. Hofmann, J. Erlewein and A. Zalar, Thin Solid Films, 43 (19771 275. [4] J. Fine, B. Navin~ek, F. Devarya and T.D. Andreadis, J. Vac. Sci. Technol. 20 (1982) 449. [5] J. Fine and B. Navingek, J. Vac. Sei. Technol. A3 (1985) 1408. [6] J. Fine, P.A. Lindfors, M.E. Gorman, R.L. Gerlach, B. Navingek, D.F. Mitchell and G.P. Chambers, J. Vac. Sci. Teehnol. A3 (1985) 1413. [7] F. Davaria, M.L. Roush, J. Fine, T.D. Andreadis and O.F. Goktepe, J. Vac. Sci. Technol. A I / 2 (1983) 467. [8] A. Zalar, P. Panjan and S. Hofmann, Thin Solid Films 181 (1989) 277. [9] A. Zalar, E.W. Seibt and P. Panjan, Vacuum 40 (1990) 71. [10] A. Zalar, Thin Solid Films 124 (1985) 223. [11] D.E. Sykes, D.D. Hall, R.E. Thurstans and J.M. Walls, Appl. Surf. Sci. 5 (1980) 1(13. [12] A. Barna, P.B. Barna and A. Zalar, Vacuum 40 0990) 115. [13] W.H. Kirchhoff, G.P. Chambers and J. Fine, J. Vac. Sei. Technol. A3 (1986) 1666. [14] J. Fine and L. Gordon Jr., J. Appl. Phys. 49 (19781 1236. [15] D. Marton and J. Fine, Thin Solid Films 185 (19901 79. [16] D. Marton and J. Fine, Thin Solid Films 151 (1987) 433. [17] L. Tanovi6, N. Tanovi~ and A. Zalar, in: Erosion and Growth of Solids Stimulated by Atom and Ion Beams, eds. G. Kiriakidis, G. Carter and J.L. Whitton, NATO AS! Series E: Appl. Sci. 112 (1986) 174. [18] L. Tanovi~ and N. Tanovi~, Nucl. Instr. and Meth. BI8 (! 987) 538.