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Nuclear microanalysis: Recent applications in surface science
Surface Science 162 (1985) 865-870 North-Holland, Amsterdam
NUCLEAR MICROANALYSIS: RECENT APPLICATIONS SCIENCE
865
IN S U R F A C E
R. C A P L A I N , S. F E R J A N I , D. D A V I D and G. B E R A N G E R Univer.~ity of Technologr'. Laboratory of Metallurgy. UA 849 CNRS. BP 233. F-60206 C'ornplbgne C~,dex. F'rance
Received 1 April 1985; accepted for publication 20 May 1985
After recalling the characteristics of nuclear microanalysis, recent improvements are described, which are applicable to surface studies. New elements can be analyzed, such as hydrogen, phosphorus and sulfur, but only under delicate experimental conditions. Microbeams enable point measurements to be obtained, and sample cross sections to be scanned, Finally, X-ray emissions induced by ion beams fill the gap between the ranges of nuclear microanalysis and elastic backscattering. Applications on thin film studies are very convenient, because the energy loss of particles through the matter may be neglected. The application of these improvements with the same machine transforms it into a powerful analytical tool.
1. Introduction N u c l e a r m i c r o a n a l y s i s is now a well-known method, but the l a b o r a t o r i e s which p e r f o r m it are still u n c o m m o n . In fact, an i m p o r t a n t a p p a r a t u s is necessary, with a Van de G r a a f f accelerator, several e x p e r i m e n t a l c h a m b e r s u n d e r v a c u u m and a m e a s u r e m e n t line, often including a c o m p u t e r . So, each a p p a r a t u s is original. Let us recall the principle of the m e t h o d [9]. T h e accelerator provides m o n o e n e r g e t i c particles, which p e n e t r a t e the material of the s a m p l e and react with the nucleii of the light elements present. T h e r e is then a particle emission of a different n a t u r e and energy from that of the incident beam. Some of these particles exit from the specimen a n d contain the useful i n f o r m a t i o n . All the stages of the process are k n o w n quantitatively. F o r each value of particle energy, there is a c o r r e s p o n d i n g d e p t h of reaction, and the energy s p e c t r u m is therefore a unique function of the c o n c e n t r a t i o n profile of the element exa m i n e d . F o r d e p t h s lower than a few t h o u s a n d ,~, calculations b e c o m e simplified, because the energy loss of particles through the m a t t e r m a y be neglected. N u m e r o u s elements, with an a t o m i c weight inferior or equal to 32, can be studied with this procedure. The isotopes of a same element are also detectable, b u t not the states of chemical c o m b i n a t i o n of the atoms. 0 0 3 9 - 6 0 2 8 / 8 5 / $ 0 3 . 3 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
866
R. Caplain et al. / Nuclear microanalvsis
Important improvements have been performed during the last years by several laboratories, working on different ways. So these improvements are dispersed, and there does not exist a single apparatus with the totality of them. The purpose of this paper is to give a synthetic view of these new analytical possibilities: increase of the atomic weight range to hydrogen, sulfur and phosphorus, nuclear microprobe and Particle Induced X-ray Emission (PIXE).
2. Microanalysis of hydrogen, sulfur and phosphorus The nuclear microanalysis of hydrogen is a recent development due to its special characteristics [1]. The nucleus of the hydrogen atom being reduced to a single proton, it is only susceptible to react with more complex nucleii. So, a beam of heavy ions is necessary. In addition, the specimens stability in time is not assured, particularly during bombardment. These are not ideal conditions, but the analysis of hydrogen has a great practical importance. Two nuclear reactions are used. The reaction 1H(~ B, a)aa requires energies lower than 2 MeV, and can therefore be carried out on most accelerators [2]. The other reaction used is IH(ISN, a)~2C, with bombarding energies above 6 MeV [3]. It is only possible to apply it on the tandem Van de Graaff accelerators, where the energy can reach up to 14 MeV if nitrogen ions are double ionised. These reactions have several sharp resonances, which permits a selective exploration of the specimen in depth. The interpretation of the measurements requires many precautions. The molecules of hydrocarbons from the vacuum p u m p can effectively contaminate the surface of the specimen and induce a parasite count (as for carbon). The residual pressure and the vacuum quality are therefore important parameters. A displacement of the hydrogen atoms is also possible, and the measurement of a concentration profile should therefore be corrected as a function of the duration and intensity of the bombardment [3]. The analysis of sulphur is performed with the nuclear reaction ~2S(d, p)33S, which supplies 7 different peaks from the excitated states number 1, 2, 3, 4, 7, 17 and 32 (fig. 1). These peaks are well isolated, with a good signal/noise ratio, but only two of them have a differential cross section giving a significant counting rate. In most cases, it is useful to observe the 32nd excited state peak, with an incident beam of 1440 keV. If it is not possible because interfering reactions on others elements, the 7th excited state peak is convenient, with an incident beam of 1380 keV. These energy values are in connection with a plateau of each differential cross section curve [4]. As for sulfur, phosphorus is one of the heaviest elements giving low energy nuclear reactions. Important progress have recently been made by using the reaction 31p(p, ot)2sSi" This only needs a proton beam of an energy inferior to 2 MeV, which is relatively easy to attain. Nevertheless, because of the low energy
R. Caplain et al. / Nuclear microanalysis
p )z
p 17
40 h V / chaNel
"I1 !!i
]z S (d,p z)" $
:, iiI +, O
867
SO
IO0
lj!
150
+
1
Z||
CIANHEL$
Fig. !. Energy spectrum from the 3ZS(d. p)33S reaction, with a 2000 ,k thick sulfur target on nickel substrate ( E d = 1200 keV. 19/~m mylar. 00, b = 150 °)
Ep : 1880 key ",~
13.9 ILoV/cHBnNI
I¢
| Ep=173Z keV Z8. •
31-.
f(p,o<) SO
I¢
| E~=2978 keV ell
10 Ooo041 •
•
•
OtaBImlO4tel
50
•
0040
•
ill
CHANNELS Fig. 2. Energy spectrum from the 3)p(p. a)21
868
R Caplatn et al. / Nuclear microanah'sts
balance of the reaction, the emitted particles have insufficient energy to pass through the screen designed to stop the retrodiffused protons. In the absence of this screen, the measurement system is perturbated by a pile-up effect, giving a peak which can completely mask the a-peak. One solution is to use a d E / d x detector, which completely stops the cK particles, but which is traversed by the protons [5]. Another possibility consists of using a classical E detector, associated with a quick measurement line. This permits us a reduction of the pile-up, but has the disadvantage of reducing the resolution: the width at half the height of the gaussian curve of resolution passes from 40 keV noise, and can be observed at the same time as the protons retrodiffused by the substrate (fig. 2).
3. The nuclear microprobe The impact of the beam of a Van de Graaff accelerator normally has a diameter of between 0.5 and 2 mm, and its shape depends on the focussing conditions. With an intensity of 0.1 to 1 IrA, the current density at the target is therefore of the order of p . A / m m 2. The position of impact is also subject to small fluctuations, and it is therefore difficult to analyze a zone at a predetermined point. The development of very narrow beam devices has nevertheless been undertaken by several laboratories, using two different methods. The microimpacts can be obtained by means of a diaphragm (collimated beams) or by electromagnetic optics (focussed beams). In this way, a nuclear microprobe is constructed, which has many similarities with the electron microprobe [6]. But the optical arrangements which are suitable for electron beams are not sufficiently powerful to deflect ion beams. This problem has been overcome by the use of quadripoles which require, however, delicate correction of aberrations. It is then possible to obtain impacts of a few microns (20 p,m, typically), with intensities of the order of 20 hA. The microbeams have their principal applications in the cartography of specimens. An original application is the production of pictures of the specimen surface either using the resulting X-ray emission, or using the secondary electron emission. This nuclear microprobe is far superior to the electron microprobe for the quantitative analysis of elements of low atomic number ( Z < 16). A number of precautions have to be observed. A vacuum of high quality is necessary to reduce the deviations of ions due to collision with the residual gas atoms. The installation should be insulated from vibrations and corrected for external magnetic field. According to the nature of the specimens, the risk of radiation damage should not be neglected, with current densities of the order of 10 ~ p . A / m m 2. It is therefore sometimes necessary to compromise between beamwidth and intensity.
R. Caplain et al. / Nuclear rnicroanalysis
869
4. The PIXE method Spectometry using the X-rays emitted by an excitated material has been used for physico-chemical analysis for a long time, the excitation usually being provided by photon or electron bombardment. Excitation by ion beams is more recent, and its development has been permitted by the production of semiconductor detectors with high resolution. The simultaneous observation of the lines of several elements has thus become possible with a sensitivity comparable to that of X-ray fluorescence. The analysis of most metals is possible, K radiations covering the range from AI to Zr, and L radiations that from Br to U. With special equipment (windowless detectors), it is possible to descend in atomic number to C and B (K radiation). Beams of low energy (10 to 100 keV) have been used to study the surface, using different ions. At these energies, the variations of effective section are large and create tresholds. Applications are therefore limited to thin superficial layers. The PIXE method has different disadvantages, mainly the lack of resolution between peaks and the poor sensitivity. Its principal advantage is economic, because of the rapidity of the analysis [7]. An interesting feature of this method is the possibility of working without using a vacuum chamber, for specimens having a high vapour pressure. The proton beam exits through a window made of nickel sheet of 2 m g / c m 2, then penetrates an analysis chamber filled with helium and containing the specimen and the detector. This permits a greater flexibility in the choice of the specimens. 5. Conclusion
This paper has been limited to more recent developments of true nuclear microanalysis on surfaces studies, the criteria being the use of simultaneous bombardment and measurement and the different nature of the incident and emitted particles. The study could be extended to elastic backscattering and channeling, because the apparatus required is the same. Numerous light elements can now be analyzed by nuclear reactions and, for some of them, the existence of narrow resonances permits the depth resolution to be improved considerably. For the heavier atomic elements, elastic retrodiffusion is possible. The space between the parts of the periodic table covered by these techniques is amply filled by the PIXE technique, which makes possible to cover all the whole range with a single nuclear analysis installation. Thus, the association of a Van de Graaff accelerator, several specialized chambers and a measurement line can be considered as a powerful and polyvalent tool, capable of rivaling most commercial physico-chemical analyzers [8].
870
R. Caplam et al. / Nuclear ,,croanalysts
References [11 [2] [3] [4] [5] [6] [7] [8] [91
J.F. Ziegler and 26 others, Nucl. Inst. Methods 149 (1978) 19. J.P. Bugeat and E. Ligeon, Nucl. Inst. Methods 159 (1979) 117. J.P. Thomas, M. Fallavier and J. Tousset, Nucl. Inst. Methods 187 (1981) 573. R. Caplain, D. David and G. Beranger, Rev. Physique Appl. 17 (1982) 441. E. Ligeon, M. Bruel, A. Bontemps, G. Chambert and J. Monnier, J. Radioanal. Chem. 16 (1973) 537. J.A. Cookson, Nucl. Inst. Methods 165 (1979) 477. G. Deconninck, G. Demortier and F. Bodart, At. Energy Rev. 13 (1975) 367. D. David and G. Beranger, to be published. G. Amsel, J.-P. Nadai, E. D'Artemare, D. David, E. Girard and J. Moulin. Nucl. Instr. Methods 92 (1971) 481.
NUCLEAR MICROANALYSIS: RECENT APPLICATIONS SCIENCE
865
IN S U R F A C E
R. C A P L A I N , S. F E R J A N I , D. D A V I D and G. B E R A N G E R Univer.~ity of Technologr'. Laboratory of Metallurgy. UA 849 CNRS. BP 233. F-60206 C'ornplbgne C~,dex. F'rance
Received 1 April 1985; accepted for publication 20 May 1985
After recalling the characteristics of nuclear microanalysis, recent improvements are described, which are applicable to surface studies. New elements can be analyzed, such as hydrogen, phosphorus and sulfur, but only under delicate experimental conditions. Microbeams enable point measurements to be obtained, and sample cross sections to be scanned, Finally, X-ray emissions induced by ion beams fill the gap between the ranges of nuclear microanalysis and elastic backscattering. Applications on thin film studies are very convenient, because the energy loss of particles through the matter may be neglected. The application of these improvements with the same machine transforms it into a powerful analytical tool.
1. Introduction N u c l e a r m i c r o a n a l y s i s is now a well-known method, but the l a b o r a t o r i e s which p e r f o r m it are still u n c o m m o n . In fact, an i m p o r t a n t a p p a r a t u s is necessary, with a Van de G r a a f f accelerator, several e x p e r i m e n t a l c h a m b e r s u n d e r v a c u u m and a m e a s u r e m e n t line, often including a c o m p u t e r . So, each a p p a r a t u s is original. Let us recall the principle of the m e t h o d [9]. T h e accelerator provides m o n o e n e r g e t i c particles, which p e n e t r a t e the material of the s a m p l e and react with the nucleii of the light elements present. T h e r e is then a particle emission of a different n a t u r e and energy from that of the incident beam. Some of these particles exit from the specimen a n d contain the useful i n f o r m a t i o n . All the stages of the process are k n o w n quantitatively. F o r each value of particle energy, there is a c o r r e s p o n d i n g d e p t h of reaction, and the energy s p e c t r u m is therefore a unique function of the c o n c e n t r a t i o n profile of the element exa m i n e d . F o r d e p t h s lower than a few t h o u s a n d ,~, calculations b e c o m e simplified, because the energy loss of particles through the m a t t e r m a y be neglected. N u m e r o u s elements, with an a t o m i c weight inferior or equal to 32, can be studied with this procedure. The isotopes of a same element are also detectable, b u t not the states of chemical c o m b i n a t i o n of the atoms. 0 0 3 9 - 6 0 2 8 / 8 5 / $ 0 3 . 3 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
866
R. Caplain et al. / Nuclear microanalvsis
Important improvements have been performed during the last years by several laboratories, working on different ways. So these improvements are dispersed, and there does not exist a single apparatus with the totality of them. The purpose of this paper is to give a synthetic view of these new analytical possibilities: increase of the atomic weight range to hydrogen, sulfur and phosphorus, nuclear microprobe and Particle Induced X-ray Emission (PIXE).
2. Microanalysis of hydrogen, sulfur and phosphorus The nuclear microanalysis of hydrogen is a recent development due to its special characteristics [1]. The nucleus of the hydrogen atom being reduced to a single proton, it is only susceptible to react with more complex nucleii. So, a beam of heavy ions is necessary. In addition, the specimens stability in time is not assured, particularly during bombardment. These are not ideal conditions, but the analysis of hydrogen has a great practical importance. Two nuclear reactions are used. The reaction 1H(~ B, a)aa requires energies lower than 2 MeV, and can therefore be carried out on most accelerators [2]. The other reaction used is IH(ISN, a)~2C, with bombarding energies above 6 MeV [3]. It is only possible to apply it on the tandem Van de Graaff accelerators, where the energy can reach up to 14 MeV if nitrogen ions are double ionised. These reactions have several sharp resonances, which permits a selective exploration of the specimen in depth. The interpretation of the measurements requires many precautions. The molecules of hydrocarbons from the vacuum p u m p can effectively contaminate the surface of the specimen and induce a parasite count (as for carbon). The residual pressure and the vacuum quality are therefore important parameters. A displacement of the hydrogen atoms is also possible, and the measurement of a concentration profile should therefore be corrected as a function of the duration and intensity of the bombardment [3]. The analysis of sulphur is performed with the nuclear reaction ~2S(d, p)33S, which supplies 7 different peaks from the excitated states number 1, 2, 3, 4, 7, 17 and 32 (fig. 1). These peaks are well isolated, with a good signal/noise ratio, but only two of them have a differential cross section giving a significant counting rate. In most cases, it is useful to observe the 32nd excited state peak, with an incident beam of 1440 keV. If it is not possible because interfering reactions on others elements, the 7th excited state peak is convenient, with an incident beam of 1380 keV. These energy values are in connection with a plateau of each differential cross section curve [4]. As for sulfur, phosphorus is one of the heaviest elements giving low energy nuclear reactions. Important progress have recently been made by using the reaction 31p(p, ot)2sSi" This only needs a proton beam of an energy inferior to 2 MeV, which is relatively easy to attain. Nevertheless, because of the low energy
R. Caplain et al. / Nuclear microanalysis
p )z
p 17
40 h V / chaNel
"I1 !!i
]z S (d,p z)" $
:, iiI +, O
867
SO
IO0
lj!
150
+
1
Z||
CIANHEL$
Fig. !. Energy spectrum from the 3ZS(d. p)33S reaction, with a 2000 ,k thick sulfur target on nickel substrate ( E d = 1200 keV. 19/~m mylar. 00, b = 150 °)
Ep : 1880 key ",~
13.9 ILoV/cHBnNI
I¢
| Ep=173Z keV Z8. •
31-.
f(p,o<) SO
I¢
| E~=2978 keV ell
10 Ooo041 •
•
•
OtaBImlO4tel
50
•
0040
•
ill
CHANNELS Fig. 2. Energy spectrum from the 3)p(p. a)21
868
R Caplatn et al. / Nuclear microanah'sts
balance of the reaction, the emitted particles have insufficient energy to pass through the screen designed to stop the retrodiffused protons. In the absence of this screen, the measurement system is perturbated by a pile-up effect, giving a peak which can completely mask the a-peak. One solution is to use a d E / d x detector, which completely stops the cK particles, but which is traversed by the protons [5]. Another possibility consists of using a classical E detector, associated with a quick measurement line. This permits us a reduction of the pile-up, but has the disadvantage of reducing the resolution: the width at half the height of the gaussian curve of resolution passes from 40 keV noise, and can be observed at the same time as the protons retrodiffused by the substrate (fig. 2).
3. The nuclear microprobe The impact of the beam of a Van de Graaff accelerator normally has a diameter of between 0.5 and 2 mm, and its shape depends on the focussing conditions. With an intensity of 0.1 to 1 IrA, the current density at the target is therefore of the order of p . A / m m 2. The position of impact is also subject to small fluctuations, and it is therefore difficult to analyze a zone at a predetermined point. The development of very narrow beam devices has nevertheless been undertaken by several laboratories, using two different methods. The microimpacts can be obtained by means of a diaphragm (collimated beams) or by electromagnetic optics (focussed beams). In this way, a nuclear microprobe is constructed, which has many similarities with the electron microprobe [6]. But the optical arrangements which are suitable for electron beams are not sufficiently powerful to deflect ion beams. This problem has been overcome by the use of quadripoles which require, however, delicate correction of aberrations. It is then possible to obtain impacts of a few microns (20 p,m, typically), with intensities of the order of 20 hA. The microbeams have their principal applications in the cartography of specimens. An original application is the production of pictures of the specimen surface either using the resulting X-ray emission, or using the secondary electron emission. This nuclear microprobe is far superior to the electron microprobe for the quantitative analysis of elements of low atomic number ( Z < 16). A number of precautions have to be observed. A vacuum of high quality is necessary to reduce the deviations of ions due to collision with the residual gas atoms. The installation should be insulated from vibrations and corrected for external magnetic field. According to the nature of the specimens, the risk of radiation damage should not be neglected, with current densities of the order of 10 ~ p . A / m m 2. It is therefore sometimes necessary to compromise between beamwidth and intensity.
R. Caplain et al. / Nuclear rnicroanalysis
869
4. The PIXE method Spectometry using the X-rays emitted by an excitated material has been used for physico-chemical analysis for a long time, the excitation usually being provided by photon or electron bombardment. Excitation by ion beams is more recent, and its development has been permitted by the production of semiconductor detectors with high resolution. The simultaneous observation of the lines of several elements has thus become possible with a sensitivity comparable to that of X-ray fluorescence. The analysis of most metals is possible, K radiations covering the range from AI to Zr, and L radiations that from Br to U. With special equipment (windowless detectors), it is possible to descend in atomic number to C and B (K radiation). Beams of low energy (10 to 100 keV) have been used to study the surface, using different ions. At these energies, the variations of effective section are large and create tresholds. Applications are therefore limited to thin superficial layers. The PIXE method has different disadvantages, mainly the lack of resolution between peaks and the poor sensitivity. Its principal advantage is economic, because of the rapidity of the analysis [7]. An interesting feature of this method is the possibility of working without using a vacuum chamber, for specimens having a high vapour pressure. The proton beam exits through a window made of nickel sheet of 2 m g / c m 2, then penetrates an analysis chamber filled with helium and containing the specimen and the detector. This permits a greater flexibility in the choice of the specimens. 5. Conclusion
This paper has been limited to more recent developments of true nuclear microanalysis on surfaces studies, the criteria being the use of simultaneous bombardment and measurement and the different nature of the incident and emitted particles. The study could be extended to elastic backscattering and channeling, because the apparatus required is the same. Numerous light elements can now be analyzed by nuclear reactions and, for some of them, the existence of narrow resonances permits the depth resolution to be improved considerably. For the heavier atomic elements, elastic retrodiffusion is possible. The space between the parts of the periodic table covered by these techniques is amply filled by the PIXE technique, which makes possible to cover all the whole range with a single nuclear analysis installation. Thus, the association of a Van de Graaff accelerator, several specialized chambers and a measurement line can be considered as a powerful and polyvalent tool, capable of rivaling most commercial physico-chemical analyzers [8].
870
R. Caplam et al. / Nuclear ,,croanalysts
References [11 [2] [3] [4] [5] [6] [7] [8] [91
J.F. Ziegler and 26 others, Nucl. Inst. Methods 149 (1978) 19. J.P. Bugeat and E. Ligeon, Nucl. Inst. Methods 159 (1979) 117. J.P. Thomas, M. Fallavier and J. Tousset, Nucl. Inst. Methods 187 (1981) 573. R. Caplain, D. David and G. Beranger, Rev. Physique Appl. 17 (1982) 441. E. Ligeon, M. Bruel, A. Bontemps, G. Chambert and J. Monnier, J. Radioanal. Chem. 16 (1973) 537. J.A. Cookson, Nucl. Inst. Methods 165 (1979) 477. G. Deconninck, G. Demortier and F. Bodart, At. Energy Rev. 13 (1975) 367. D. David and G. Beranger, to be published. G. Amsel, J.-P. Nadai, E. D'Artemare, D. David, E. Girard and J. Moulin. Nucl. Instr. Methods 92 (1971) 481.