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A study of preferential sputtering of Ni in Ni-Co alloy films deposited from a single source
Nuclear Instruments North-Holland
and Methods
in Physics
Research
B54 (1991) 513-517
513
A study of preferential sputtering of Ni in Ni-Co alloy films deposited from a single source Anantha
R. Sethuraman
Center for Applied Energy Research, 3572, Iron Works Pike, Lexington, Received
30 July 1990 and in revised form 29 October
KY 4051 I-8433,
USA
1990
Ni-Co alloy films are currently being considered as an improved magnetic storage media and are also presently employed in a variety of catalytic applications. A proper understanding of the surface properties of the Ni-Co films is necessary to enhance the productivity in these applications. In the present investigation the preferential sputtering of Ni on exposure to an ion beam as well as the segregation of Ni is studied using Auger electron spectroscopy (AES). Three compositions of Ni-Co (75% Ni-25% Co, 50% Ni-50% Co and 25% Ni-75% Co by weight) were deposited on silicon substrates from a single source by evaporation. The results of
AES depth profiling indicate constant film composition. It is concluded that no preferential sputtering of Ni occurs in the Ni-Co alloy thin film. The Ni-Co system is proposed as a suitable standard for thin film surface chemical analysis by AES.
1. Introduction The study of structural, surface/interface and electrical properties of alloy thin films has attracted attention in the recent years owing to their widespread applications in the fields of catalysis and microelectronics. In addition to the investigation of the above properties, the segregation characteristics of individual components in an alloy film have also been of importance [l-3]. Cherepin et al. [4] used AES to study Ni-Co alloy surfaces by examining the ratio between the high-energy non-overlapping LMM Auger peaks for Co (656 eV) and Ni (850 eV). However, there is no mention of how they were able to get quantitative measurements by using these peaks. Several atomic per cent of Ni enrichment was noticed on the surface of the alloy and attributed to possible segregation of Ni. Tanaka et al. [5] introduced a new calibration method to study Ni-Co alloy surface composition by AES. The samples studied were polycrystalline buttons of Co-Ni containing 23.7 or 50.4 at.% Co, prepared by electric-arc melting using pure Ni (99.9%) and Co (99.9%). The buttons were homogenized at 1200 K for 24 h, coldrolled and annealed in vacuum at 1200 K for 48 h. Subsequently, the samples were polished with emery cloth (1000 #), annealed and electropolished (1 A cmd2) in perchloric acid solution. For the purpose of quantitation, Ni (859 eV) and Co (656 eV) LMM Auger peaks were measured using a co-axial electron gun with a CMA at 5 keV. The AES spectra were recorded after sputter cleaning with Ar+ at 5 keV. The experimental 0168-583X/91/$03.50
0 1991 - Elsevier Science Publishers
data were compared with the computer simulated spectra and the agreement was very good. The concentrations of Ni and Co from the spectra were found to be 25.2% and 49.8% in comparison to the bulk compositions of 23.7% Ni and 50.4% Co. Hajcsar et al. [6] have published data on Ni-Co alloy preferential sputtering and surface segregation. The results and the conclusions that have evolved from this study are quite contradictory. Six alloys covering the whole range of solid solutions were annealed at 853 K and quenched to give equilibrium segregated surfaces. Computer simulation methods were employed to analyze the spectra obtained from both the high and low energy ranges of Ni and Co. The authors claim that their results have explained previous discrepancies that existed regarding preferential sputtering and surface segregation of Ni-Co alloy. The results from the study lead them to conclude that there was preferential sputtering of Ni over the whole range of compositions studied and the extent was well predicted by the ratio of pure metal sputter yield. They also mentioned that the effect is more noticeable in the low-energy range (in fact, since Ni/Co are ferromagnetic, the effect seen could also be due to stray magnetic fields). Kurokawa et al. [7] studied the surface composition of Ni-Co films under argon ion bombardment using AES and XPS. The LMM and MVV Auger signals were monitored using 5 keV electron beam with a CMA. It was concluded that no apparent surface segregation could be observed by Auger analysis with low and high energy Auger signals. They found that the accuracy with the low-energy Auger signal was not good enough
B.V. (North-Holland)
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A.R. Sethuraman / Ni-Co alloy preferential sputtering
as compared to that possible with the high-energy signals. This was attributed to the fact that stray magnetic fields may affect such measurements since both materials investigated are ferromagnetic. It was also proposed that the Ni-Co alloy surface could be used as a standard for surface-chemical quantification using AES. Kurokawa et al. [8] extended their earlier investigation on Ni-Co alloy surfaces using both ISS and AES. The surface concentration was estimated from the ISS data using spectrum-synthesis method. The results indicated that the composition of the Ni-Co alloy surface under Ar+ ion bombardment was the same as the bulk composition; ion bombardment did not cause surface compositional change in the Ni-Co alloy. Sethuraman et al. [9] investigated deposition of alloy films from a single source using a Ni-Co model alloy system. The Ni-Co evaporation source was synthesized by electro-discharge compaction (EDC) and the films of the Ni-Co alloy with various compositions were deposited on a silicon substrate at room temperature. The films were analyzed using AES, XPS, XRD and TEM. The results from the AES study showed surface chemical uniformity and homogeneity. The XRD results indicated that the structure of the alloy films was fee; TEM results confirmed this observation. 2. Experimental procedure Pure Ni (99.99%) and Co (99.99%) powders of approximately equal size distribution (- 100 mesh) were obtained from Johnson-Mathey. Three compositions, 25% Ni-75% Co, 50% Ni-50% Co and 75% Ni-25% Co (wt.%), were weighed using a digital microbalance. The weighed powders were mixed in a mortar and pestle by a volume mixing method to ensure homogenous mixing. The details of the synthesis of the evaporation source can be found elsewhere [9,23]. The deposition of thin films from a single source was accomplished by physical vapor deposition using a Veeco VE-7700 metal evaporator. The evaporator was equipped with a diffusion pump backed by a mechanical pump and a liquid nitrogen trap to remove unwanted vapors. A thickness monitor (Kronos QM-311) attached to the evaporator was essentially a quartz microbalance that was capable of measuring thickness within l-10 A. The sources were evaporated from a tungsten alumina crucible made by GTE Sylvania Emissive Products Division (CS-1008 series). These tungsten crucibles are coated with non-reactive alumina, thereby minimizing contamination. The crucibles were vacuum outgassed before every deposition cycle for 20 minutes at 1450 o C in a vacuum of about 1 x lo-’ Torr. The substrates used were float-zone silicon with a (111) orientation obtained from Wacker Siltronics. The silicon wafers were initially etched with HF to remove the native oxide, rinsed in de-ionized water, dried, and
introduced into the evaporation chamber within about a minute to avoid prolonged exposure to atmosphere. There will be, however, a thin oxide layer that has formed due to atmospheric exposure between cleaning and introduction into the evaporator. The source to substrate distance was 18 cm and the deposition was carried out on a line-of-sight method. The chamber was evacuated to 1 x lo-’ Torr and held there for about 15 min to remove any residual contaminants. The liquidnitrogen trap was used in the pumping system to ensure removal of oil fumes that might emanate from the diffusion pump. The crucible was slowly heated to the required temperature so that the source melted and thin films were deposited on the silicon substrates to a thickness of about 700 A. After the required thickness was deposited, the crucible was slowly cooled down. The wafers were in vacuum for at least 30 min before they were removed from the chamber. A fresh source was used each time to avoid the oxide contamination that would be introduced if the same shot was used again. The surface and interface analysis of the thin films was carried out using a Kratos XSAMSOO multi-technique surface analysis system capable of AES, XPS, ISS and SIMS with a single-channel detector. The instrument was calibrated using a Ag standard by monitoring the Ag 3d signal (368.2 eV binding energy) using Mg Ko: (1253.6 eV) to a FWHM of 0.98 eV. The alloy thin films were studied by AES sputter depth profiling using an Ar+ beam to etch the surface. AES was carried out with 30’ incident angle electron excitation at 5 keV. The ion gun was positioned at an angle of 45’ to the sample surface and was differentially pumped by a 50 l/s turbomolecular pump. A gas handling system that was controlled by the acquisition software enabled the admission of the required amount of argon to etch the surface and prevented unnecessary argon build up in the ion pump. The ion beam spot and the electron beam spot were aligned before each experiment and was rastered to ensure a wider area of etching than data acquisition. The etching beam was operated at 3.5 keV. The etching rate was calibrated using a standard silicon oxide sample of known thickness. The thickness of the oxide layer divided by the time taken to sputter the oxide layer completely was taken as the etching rate. The etching rate for the alloy films was calculated by sputtering a film of known thickness as measured by the thickness monitor in the evaporator using the same conditions as the standard silicon oxide sample. Since measurement of sputter yields for materials sputtered at high fluence ion beams has still not been standardized, the procedure followed in the estimation of etching rate in this investigation can be taken as a reasonable method with minimum error not considering factors such as surface roughness, substrate effects and shadowing.
A.R. Sethuraman
/ Ni-Co
alloy preferential
3. Results and discussion The results of the AES depth profiling studies of the Ni-Co alloy thin films are shown in figs. 1 to 3. The signals monitored were NiLMM (855 eV), CoLMM (775 eV), OKLL (504 eV) and SiLMM (92 eV). The contribution from the secondary peaks of the NiLMM transitions on the CoLMM transitions were corrected for by comparing with spectra obtained from standard Ni and Co and quantification was carried out by the same method used by Kurokawa et al. [8]. It may be seen from the results that the films are of uniform chemical composition starting from the surface into the bulk of the film. There is about 2 to 3% oxygen contamination on the surface of the film due to exposure to atmosphere. The increase of oxygen at the interface is caused by the presence of the native silicon oxide on the silicon substrate [9]. Fig. 1 shows the AES depth profile obtained by sputtering a 75% Ni-25% Co alloy film. It is seen that the surface as well as the bulk of the film is very uniform in composition. There are no indications of preferential sputtering of Ni or Co. The oxygen signal shows a mild increase on the surface of the film and reduces to zero. This is due to the surface oxidation of the film owing to its exposure to atmosphere. An increase in the subst!ate signal is seen after sputtering through about 550 A of the overlayer film accompanied by a corresponding decrease in the Ni and Co signals. As expected, there is a slight increase in the oxygen signal as we approach the silicon substrate caused by the native silicon oxide. After about a depth of 800 A, the Auger signal from Ni or Co is zero and the silicon signal reaches a value of = 100%. The experiment was
515
repeated and was found to yield very consistent values within a scatter of 2-5%. The AES depth profile of a 50% Ni-50% Co alloy film is given in fig. 2. The oxygen contamination on the surface of this film is negligible. The relative atomic concentrations of Ni and Co remain constant around 50% and start decreasing after a depth of = 500 A into the bulk of the film. There is a corresponding increase in the oxygen and silicon signals which is indicative of the proximity to the substrate. The silicon signals reach a maximum value and the metallic signal are zero around 900 A. The profile confirms a film composition of 50% Ni-50% Co. The AES depth profile of 75% Co-25% Ni film is shown in fig. 3. The surface oxygen contamination is minimal. The metal signal characteristics resemble those of fig. 1 (75%Ni-258Co). Further, the interface region also bears similarity to that of fig. 1. This may be indicative of majority species influence in the interfacial reaction zone. The broad interface between the Ni/Co alloy film and the Si substrate is believed to be due the influence of the sputter beam effects. Nevertheless, it is not expected to degrade the quality of information obtained. There are several aspects of the overlayer that need to be addressed. It has been reported by Hajcsar et al. [6] that Ni preferentially sputters in a Ni-Co alloy film. However, data in figs. l-3 do not show such preferential sputtering. Furthermore, there is evidence in the literature to support the observation that preferential sputtering of Ni does not occur [l-13,16-23]. If preferential sputtering of Ni had occurred in the samples studied, a rapid reduction in the atomic concentration values of Ni would have been noticed in the AES depth
400 Depth
sputtering
600 bwstromsl
Fig. 1. AES depth profile of 75% Ni-25% Co alloy film on silicon.
516
A.R. Sethuraman
/ Ni-Co
alloy preferential
sputtering
500 Depth
(Angstroms)
Fig. 2. AES depth profile of 50% Ni-50% Co alloy film on silicon.
profiles. Possibilities of interferences due to the ionetching processes are very limited since the size of the electron beam is well within the envelope of the etching beam. Preferential sputtering of Co cannot be envisaged since the sputter yield of Co at 1 keV argon ion beam is less than that of Ni [12]. In the study of Hajcsar et al. [6], six compositions covering the entire range of the Ni-Co alloy system have been studied. Their results on preferential sputtering studies indicate that Ni sputters preferentially over the whole composition range. However, there is no mention of how they arrived at the estimate of sputter yield of Ni and Co when etched with a 3 keV Ar+ ion beam.
400
Depth
The conclusions of Hajcsar et al. [6] are based on the assumption that the sputter-yield ratio of Ni and Co does not vary at higher ion beam energies since there is no data available for the sputter yield of pure metals at ion beam energies of about 3 keV. This is not acceptable since measurement of pure-element sputter yields at higher ion beam energies has not yet been standardized [11,12]. Seah [12] has reported corrected sputter yields of pure elements at 500 eV and 1 keV Ar+ beams. An attempt was made to extrapolate the sputter yields of pure metals at higher energies. The curve is not a straight line. Further, it was pointed out that the accuracy of the measurements is within a scatter of 20%.
600
IAngstroma)
Fig. 3. AES depth profile of 25% Ni-75% Co alloy film on silicon. The failure to see. the Si signal reach 100% in this plot is only due to data processing software limitations.
A.R. Sethuraman
/ Ni-Co
According to this, the data presented by Hajcsar et al. [6] suggesting that Ni sputters preferentially varying about 2% from the expected bulk compositions do not seem valid. Thus, the results from the AES depth profiles of the present study and the conclusions by Kurokawa et al. [7,8] and Tanaka et al. [5] and Sethuraman et al. [9] are valid. The Ni-Co system poses interesting problems to scientists attempting to explain surface segregation. The current theories of segregation can be broken into four groups [6,23] as follows: bond-breaking, surface-tension, continuum elasticity and electronic. A number of controversies exist regarding this aspect of the Ni-Co alloy system. Hajcsar et al. [6] and Cherepin et al. [4] reported that Ni segregates in the Ni-Co system with very slight indications from AES studies. Hajcsar et al. [6] suggested that segregation occurs based on their studies of the low-energy Auger peaks of Ni and Co. This is not a very reliable method since the low energy peaks could be influenced by stray magnetic fields. Cherepin et al. [6] reported that several atomic percent enrichment of Ni on the surface was noticed using AES, but there is no mention of how they obtained the quantitative values. On the other hand, Tanaka et al. [5] and Kurokawa et al. [7,8] have reported that segregation does not occur in Ni-Co alloy system. In the present investigation, the films were studied at room temperature and no annealing was performed. In this condition, equilibration does not occur and indications of surface segregation are very difficult to assess. More studies are necessary to determine the effect of alloying on the surface segregation of these alloy films. An interesting aspect of the study is the effect of sputtering on the alloy films. Hajcsar et al. [6] have suggested that preferential sputtering of Ni occurs in the films. However, as reported by Tanaka et al. [5] and Kurokawa et al. [7,8] and from the results of AES from this investigation, preferential sputtering does not occur and even so the amounts are so small that they pose no restriction on using the Ni-Co alloy film as a standard for AES depth profiling.
4. Conclusions The Ni-Co alloy film system is very stable and Ni does not preferentially sputter when etched by an ion beam. The Ni-Co alloy film does not undergo any ion beam induced damage and is proposed as a suitable standard for surface chemical analysis by AES depth profiling.
alloy preferential
sputtering
517
Acknowledgements
The financial support for the author provided by University of Kentucky Graduate School is gratefully acknowledged.
References PI D. Briggs and M.P. Seah, Eds., Practical Surface Analysis by Auger and X-Ray Photoelectron Spectroscopy (Wiley, Chichester, 1983). A. Fahlman, R. Nordberg, PI K. Siegbahn, C.N. Nordling, K. Hamrin, J. Hedman, G. Johansson, T. Bermark, S.E. Karlsson, I. Lindgren and B. Lindgren, ESCA - Atomic, molecular and solid state structure studied by means of electron spectroscopy, Nova ACTA Regiae Sot. Upasalienus, Ser. IV, 20, Uppsale (1967). [31 M.P. Seah, Surf. Sci., 32 (1972) 703. Phys. 141 V.T. Cherepin, M.A. Vasilev and O.A. Yakubtsov, Status Solidi A53 (1979) K207. M. Takemori and T. Homma, J. Electron 151 A. Tanaka, Spectrosc. Relat. Phenom 32 (1983) 277. P.T. Dawson and W.W. Smeltzer, Surf. WI E.E. Hajcsar, Interf. Anal. 10 (1987) 343. M. Tezuka, K. Takegoshi, M. Kudo and R. I71 A. Kurokawa, Shimizu, Nucl. Instr. and Meth. B39 (1989) 57. R. Shimizu, Y. Kubota and H.J. Kang, PI A. Kurokawa, Surf. Interf. Anal., 14 (1989) 388. P.J. Reucroft, R.J. De Angelis, D.K. PI A.R. Sethuraman, Kim and K. Okazaki, J. Vat. Sci. Techn. A8 (1990) 2255. K. Okazaki, D.K. Kim and H.R. Pak, Proc. 2nd Int. Conf. m on Rapidly Solidified Materials - Properties and Processing, San Diego, CA, ASM International (1988) p. 183. 1111 P. Sigmund, Phys. Rev. 184 (1969) 383. WI M.P. Seah, Thin Solid Films 81 (1981) 279. 1131 M.P. Seah and C. Lea, Thin Solid Films 81 (1981) 257. [I41 R. Shimizu, Nucl. Instr. and Meth. B18 (1987) 486. [151 P. Sigmund, Nucl. Instr. and Meth. B18 (1987) 375. Mater. Sci. and Eng. 69 Ml R. Kelly and D.E. Harrison, (1985) 449. u71 G. Betz, Surf. Sci. 92 (1980) 283. WI R. Kelly, Nucl. Instr. and Meth. 149 (1978) 553. [191 R. Kelly and A. Oliva, Nucl. Instr. and Meth. B13 (1986) 283. P. Sigmund, A. Oliva and G. Falcone, Nucl. Instr. and PI Meth., 194 (1982) 541. P. Sigmund, J. Vat. Sci. Tech. 17 (1980). PI P21 P. Sigmund, J. Appl. Phys. 50 (1979). Sethuraman, PhD Dissertation, University of 1231 A.R. Kentucky (1990).
and Methods
in Physics
Research
B54 (1991) 513-517
513
A study of preferential sputtering of Ni in Ni-Co alloy films deposited from a single source Anantha
R. Sethuraman
Center for Applied Energy Research, 3572, Iron Works Pike, Lexington, Received
30 July 1990 and in revised form 29 October
KY 4051 I-8433,
USA
1990
Ni-Co alloy films are currently being considered as an improved magnetic storage media and are also presently employed in a variety of catalytic applications. A proper understanding of the surface properties of the Ni-Co films is necessary to enhance the productivity in these applications. In the present investigation the preferential sputtering of Ni on exposure to an ion beam as well as the segregation of Ni is studied using Auger electron spectroscopy (AES). Three compositions of Ni-Co (75% Ni-25% Co, 50% Ni-50% Co and 25% Ni-75% Co by weight) were deposited on silicon substrates from a single source by evaporation. The results of
AES depth profiling indicate constant film composition. It is concluded that no preferential sputtering of Ni occurs in the Ni-Co alloy thin film. The Ni-Co system is proposed as a suitable standard for thin film surface chemical analysis by AES.
1. Introduction The study of structural, surface/interface and electrical properties of alloy thin films has attracted attention in the recent years owing to their widespread applications in the fields of catalysis and microelectronics. In addition to the investigation of the above properties, the segregation characteristics of individual components in an alloy film have also been of importance [l-3]. Cherepin et al. [4] used AES to study Ni-Co alloy surfaces by examining the ratio between the high-energy non-overlapping LMM Auger peaks for Co (656 eV) and Ni (850 eV). However, there is no mention of how they were able to get quantitative measurements by using these peaks. Several atomic per cent of Ni enrichment was noticed on the surface of the alloy and attributed to possible segregation of Ni. Tanaka et al. [5] introduced a new calibration method to study Ni-Co alloy surface composition by AES. The samples studied were polycrystalline buttons of Co-Ni containing 23.7 or 50.4 at.% Co, prepared by electric-arc melting using pure Ni (99.9%) and Co (99.9%). The buttons were homogenized at 1200 K for 24 h, coldrolled and annealed in vacuum at 1200 K for 48 h. Subsequently, the samples were polished with emery cloth (1000 #), annealed and electropolished (1 A cmd2) in perchloric acid solution. For the purpose of quantitation, Ni (859 eV) and Co (656 eV) LMM Auger peaks were measured using a co-axial electron gun with a CMA at 5 keV. The AES spectra were recorded after sputter cleaning with Ar+ at 5 keV. The experimental 0168-583X/91/$03.50
0 1991 - Elsevier Science Publishers
data were compared with the computer simulated spectra and the agreement was very good. The concentrations of Ni and Co from the spectra were found to be 25.2% and 49.8% in comparison to the bulk compositions of 23.7% Ni and 50.4% Co. Hajcsar et al. [6] have published data on Ni-Co alloy preferential sputtering and surface segregation. The results and the conclusions that have evolved from this study are quite contradictory. Six alloys covering the whole range of solid solutions were annealed at 853 K and quenched to give equilibrium segregated surfaces. Computer simulation methods were employed to analyze the spectra obtained from both the high and low energy ranges of Ni and Co. The authors claim that their results have explained previous discrepancies that existed regarding preferential sputtering and surface segregation of Ni-Co alloy. The results from the study lead them to conclude that there was preferential sputtering of Ni over the whole range of compositions studied and the extent was well predicted by the ratio of pure metal sputter yield. They also mentioned that the effect is more noticeable in the low-energy range (in fact, since Ni/Co are ferromagnetic, the effect seen could also be due to stray magnetic fields). Kurokawa et al. [7] studied the surface composition of Ni-Co films under argon ion bombardment using AES and XPS. The LMM and MVV Auger signals were monitored using 5 keV electron beam with a CMA. It was concluded that no apparent surface segregation could be observed by Auger analysis with low and high energy Auger signals. They found that the accuracy with the low-energy Auger signal was not good enough
B.V. (North-Holland)
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A.R. Sethuraman / Ni-Co alloy preferential sputtering
as compared to that possible with the high-energy signals. This was attributed to the fact that stray magnetic fields may affect such measurements since both materials investigated are ferromagnetic. It was also proposed that the Ni-Co alloy surface could be used as a standard for surface-chemical quantification using AES. Kurokawa et al. [8] extended their earlier investigation on Ni-Co alloy surfaces using both ISS and AES. The surface concentration was estimated from the ISS data using spectrum-synthesis method. The results indicated that the composition of the Ni-Co alloy surface under Ar+ ion bombardment was the same as the bulk composition; ion bombardment did not cause surface compositional change in the Ni-Co alloy. Sethuraman et al. [9] investigated deposition of alloy films from a single source using a Ni-Co model alloy system. The Ni-Co evaporation source was synthesized by electro-discharge compaction (EDC) and the films of the Ni-Co alloy with various compositions were deposited on a silicon substrate at room temperature. The films were analyzed using AES, XPS, XRD and TEM. The results from the AES study showed surface chemical uniformity and homogeneity. The XRD results indicated that the structure of the alloy films was fee; TEM results confirmed this observation. 2. Experimental procedure Pure Ni (99.99%) and Co (99.99%) powders of approximately equal size distribution (- 100 mesh) were obtained from Johnson-Mathey. Three compositions, 25% Ni-75% Co, 50% Ni-50% Co and 75% Ni-25% Co (wt.%), were weighed using a digital microbalance. The weighed powders were mixed in a mortar and pestle by a volume mixing method to ensure homogenous mixing. The details of the synthesis of the evaporation source can be found elsewhere [9,23]. The deposition of thin films from a single source was accomplished by physical vapor deposition using a Veeco VE-7700 metal evaporator. The evaporator was equipped with a diffusion pump backed by a mechanical pump and a liquid nitrogen trap to remove unwanted vapors. A thickness monitor (Kronos QM-311) attached to the evaporator was essentially a quartz microbalance that was capable of measuring thickness within l-10 A. The sources were evaporated from a tungsten alumina crucible made by GTE Sylvania Emissive Products Division (CS-1008 series). These tungsten crucibles are coated with non-reactive alumina, thereby minimizing contamination. The crucibles were vacuum outgassed before every deposition cycle for 20 minutes at 1450 o C in a vacuum of about 1 x lo-’ Torr. The substrates used were float-zone silicon with a (111) orientation obtained from Wacker Siltronics. The silicon wafers were initially etched with HF to remove the native oxide, rinsed in de-ionized water, dried, and
introduced into the evaporation chamber within about a minute to avoid prolonged exposure to atmosphere. There will be, however, a thin oxide layer that has formed due to atmospheric exposure between cleaning and introduction into the evaporator. The source to substrate distance was 18 cm and the deposition was carried out on a line-of-sight method. The chamber was evacuated to 1 x lo-’ Torr and held there for about 15 min to remove any residual contaminants. The liquidnitrogen trap was used in the pumping system to ensure removal of oil fumes that might emanate from the diffusion pump. The crucible was slowly heated to the required temperature so that the source melted and thin films were deposited on the silicon substrates to a thickness of about 700 A. After the required thickness was deposited, the crucible was slowly cooled down. The wafers were in vacuum for at least 30 min before they were removed from the chamber. A fresh source was used each time to avoid the oxide contamination that would be introduced if the same shot was used again. The surface and interface analysis of the thin films was carried out using a Kratos XSAMSOO multi-technique surface analysis system capable of AES, XPS, ISS and SIMS with a single-channel detector. The instrument was calibrated using a Ag standard by monitoring the Ag 3d signal (368.2 eV binding energy) using Mg Ko: (1253.6 eV) to a FWHM of 0.98 eV. The alloy thin films were studied by AES sputter depth profiling using an Ar+ beam to etch the surface. AES was carried out with 30’ incident angle electron excitation at 5 keV. The ion gun was positioned at an angle of 45’ to the sample surface and was differentially pumped by a 50 l/s turbomolecular pump. A gas handling system that was controlled by the acquisition software enabled the admission of the required amount of argon to etch the surface and prevented unnecessary argon build up in the ion pump. The ion beam spot and the electron beam spot were aligned before each experiment and was rastered to ensure a wider area of etching than data acquisition. The etching beam was operated at 3.5 keV. The etching rate was calibrated using a standard silicon oxide sample of known thickness. The thickness of the oxide layer divided by the time taken to sputter the oxide layer completely was taken as the etching rate. The etching rate for the alloy films was calculated by sputtering a film of known thickness as measured by the thickness monitor in the evaporator using the same conditions as the standard silicon oxide sample. Since measurement of sputter yields for materials sputtered at high fluence ion beams has still not been standardized, the procedure followed in the estimation of etching rate in this investigation can be taken as a reasonable method with minimum error not considering factors such as surface roughness, substrate effects and shadowing.
A.R. Sethuraman
/ Ni-Co
alloy preferential
3. Results and discussion The results of the AES depth profiling studies of the Ni-Co alloy thin films are shown in figs. 1 to 3. The signals monitored were NiLMM (855 eV), CoLMM (775 eV), OKLL (504 eV) and SiLMM (92 eV). The contribution from the secondary peaks of the NiLMM transitions on the CoLMM transitions were corrected for by comparing with spectra obtained from standard Ni and Co and quantification was carried out by the same method used by Kurokawa et al. [8]. It may be seen from the results that the films are of uniform chemical composition starting from the surface into the bulk of the film. There is about 2 to 3% oxygen contamination on the surface of the film due to exposure to atmosphere. The increase of oxygen at the interface is caused by the presence of the native silicon oxide on the silicon substrate [9]. Fig. 1 shows the AES depth profile obtained by sputtering a 75% Ni-25% Co alloy film. It is seen that the surface as well as the bulk of the film is very uniform in composition. There are no indications of preferential sputtering of Ni or Co. The oxygen signal shows a mild increase on the surface of the film and reduces to zero. This is due to the surface oxidation of the film owing to its exposure to atmosphere. An increase in the subst!ate signal is seen after sputtering through about 550 A of the overlayer film accompanied by a corresponding decrease in the Ni and Co signals. As expected, there is a slight increase in the oxygen signal as we approach the silicon substrate caused by the native silicon oxide. After about a depth of 800 A, the Auger signal from Ni or Co is zero and the silicon signal reaches a value of = 100%. The experiment was
515
repeated and was found to yield very consistent values within a scatter of 2-5%. The AES depth profile of a 50% Ni-50% Co alloy film is given in fig. 2. The oxygen contamination on the surface of this film is negligible. The relative atomic concentrations of Ni and Co remain constant around 50% and start decreasing after a depth of = 500 A into the bulk of the film. There is a corresponding increase in the oxygen and silicon signals which is indicative of the proximity to the substrate. The silicon signals reach a maximum value and the metallic signal are zero around 900 A. The profile confirms a film composition of 50% Ni-50% Co. The AES depth profile of 75% Co-25% Ni film is shown in fig. 3. The surface oxygen contamination is minimal. The metal signal characteristics resemble those of fig. 1 (75%Ni-258Co). Further, the interface region also bears similarity to that of fig. 1. This may be indicative of majority species influence in the interfacial reaction zone. The broad interface between the Ni/Co alloy film and the Si substrate is believed to be due the influence of the sputter beam effects. Nevertheless, it is not expected to degrade the quality of information obtained. There are several aspects of the overlayer that need to be addressed. It has been reported by Hajcsar et al. [6] that Ni preferentially sputters in a Ni-Co alloy film. However, data in figs. l-3 do not show such preferential sputtering. Furthermore, there is evidence in the literature to support the observation that preferential sputtering of Ni does not occur [l-13,16-23]. If preferential sputtering of Ni had occurred in the samples studied, a rapid reduction in the atomic concentration values of Ni would have been noticed in the AES depth
400 Depth
sputtering
600 bwstromsl
Fig. 1. AES depth profile of 75% Ni-25% Co alloy film on silicon.
516
A.R. Sethuraman
/ Ni-Co
alloy preferential
sputtering
500 Depth
(Angstroms)
Fig. 2. AES depth profile of 50% Ni-50% Co alloy film on silicon.
profiles. Possibilities of interferences due to the ionetching processes are very limited since the size of the electron beam is well within the envelope of the etching beam. Preferential sputtering of Co cannot be envisaged since the sputter yield of Co at 1 keV argon ion beam is less than that of Ni [12]. In the study of Hajcsar et al. [6], six compositions covering the entire range of the Ni-Co alloy system have been studied. Their results on preferential sputtering studies indicate that Ni sputters preferentially over the whole composition range. However, there is no mention of how they arrived at the estimate of sputter yield of Ni and Co when etched with a 3 keV Ar+ ion beam.
400
Depth
The conclusions of Hajcsar et al. [6] are based on the assumption that the sputter-yield ratio of Ni and Co does not vary at higher ion beam energies since there is no data available for the sputter yield of pure metals at ion beam energies of about 3 keV. This is not acceptable since measurement of pure-element sputter yields at higher ion beam energies has not yet been standardized [11,12]. Seah [12] has reported corrected sputter yields of pure elements at 500 eV and 1 keV Ar+ beams. An attempt was made to extrapolate the sputter yields of pure metals at higher energies. The curve is not a straight line. Further, it was pointed out that the accuracy of the measurements is within a scatter of 20%.
600
IAngstroma)
Fig. 3. AES depth profile of 25% Ni-75% Co alloy film on silicon. The failure to see. the Si signal reach 100% in this plot is only due to data processing software limitations.
A.R. Sethuraman
/ Ni-Co
According to this, the data presented by Hajcsar et al. [6] suggesting that Ni sputters preferentially varying about 2% from the expected bulk compositions do not seem valid. Thus, the results from the AES depth profiles of the present study and the conclusions by Kurokawa et al. [7,8] and Tanaka et al. [5] and Sethuraman et al. [9] are valid. The Ni-Co system poses interesting problems to scientists attempting to explain surface segregation. The current theories of segregation can be broken into four groups [6,23] as follows: bond-breaking, surface-tension, continuum elasticity and electronic. A number of controversies exist regarding this aspect of the Ni-Co alloy system. Hajcsar et al. [6] and Cherepin et al. [4] reported that Ni segregates in the Ni-Co system with very slight indications from AES studies. Hajcsar et al. [6] suggested that segregation occurs based on their studies of the low-energy Auger peaks of Ni and Co. This is not a very reliable method since the low energy peaks could be influenced by stray magnetic fields. Cherepin et al. [6] reported that several atomic percent enrichment of Ni on the surface was noticed using AES, but there is no mention of how they obtained the quantitative values. On the other hand, Tanaka et al. [5] and Kurokawa et al. [7,8] have reported that segregation does not occur in Ni-Co alloy system. In the present investigation, the films were studied at room temperature and no annealing was performed. In this condition, equilibration does not occur and indications of surface segregation are very difficult to assess. More studies are necessary to determine the effect of alloying on the surface segregation of these alloy films. An interesting aspect of the study is the effect of sputtering on the alloy films. Hajcsar et al. [6] have suggested that preferential sputtering of Ni occurs in the films. However, as reported by Tanaka et al. [5] and Kurokawa et al. [7,8] and from the results of AES from this investigation, preferential sputtering does not occur and even so the amounts are so small that they pose no restriction on using the Ni-Co alloy film as a standard for AES depth profiling.
4. Conclusions The Ni-Co alloy film system is very stable and Ni does not preferentially sputter when etched by an ion beam. The Ni-Co alloy film does not undergo any ion beam induced damage and is proposed as a suitable standard for surface chemical analysis by AES depth profiling.
alloy preferential
sputtering
517
Acknowledgements
The financial support for the author provided by University of Kentucky Graduate School is gratefully acknowledged.
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