Precision standard reference targets for microanalysis by nuclear reactions

Nuclear Instruments and Methods in Physics Research 218 (1983) 177 182 North-Holland, Amsterdam

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P R E C I S I O N S T A N D A R D R E F E R E N C E T A R G E T S F O R M I C R O A N A L Y S I S BY N U C L E A R REACTIONS G. AMSEL Groupe de Physique des Solides de l'Ecole Normale Supbrieure, Universitd Paris VII, Tour 23, 2, Place Jussieu, 75251 Paris Cedex 05, France

J.A. D A V I E S Chemist O' and Materials" Division, Atomic Energy of Canada Limited. Research Company, Chalk River Nuclear lxlboratories, Chalk River, Ontario, Canada KOJ 1JO

This paper is based on the discussions that took place in the Ion beam analysis Conference Workshop devoted to nuclear reaction microanalysis. It attempts to summarize the present status of the field, with special emphasis on the production and calibration of suitable reference targets.

1. Introduction The development of reliable, calibrated reference targets holds the key to widespread acceptance of quantitative ion beam techniques in analysing the nearsurface region of solids. In Rutherford backscattering (RBS) analysis, one reference target containing a precisely known amount of a well chosen high Z nucleus per cm 2 suffices to calibrate all RBS spectra in absolute terms. For the various Z 2 values of interest, appropriate scaling factors may be obtained from the differential scattering cross-sections % ( E ) which in turn may be calculated via well known analytical formulae. Note that electron screening effects must be included; these reduce the cross-sections slightly below the Rutherford values [1-3]. For very low-Z nuclei another type of deviation from Rutherford scattering may occur, due to nuclear interactions that arise when the Coulomb barrier penetrabilities become sizeable. Such effects were shown to be negligible for 4 He scattering on Si, A1 or oxygen at energies up to 2.4 MeV [4,5]. In nuclear reaction analysis (NRA), on the other hand, simple scaling factors are unavailable, as the reaction cross-sections % ( E ) vary strongly with E and 0: also, they differ by orders of magnitude for different isotopes of the same element. Thus, N R A requires precisely known reference targets of each isotope under study, either for direct analytical use or at least for initial determination of accurate cross-section ratios. Already, an analytical accuracy of +3% is achievable with the following commonly used nuclear reactions: 160(d, p), 14N(d, p), a4N(d, a), 12C(d, p) and 0167-5087/83/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

2H(3He, p). The main purpose of this Workshop session has been to discuss the various calibration procedures and other factors involved in achieving such accuracy, to summarize the available information on cross sections, reference standards, etc., and to explore how best to calibrate other nuclear reactions of interest. For technical details the reader will be referred to previously published work, wherever possible.

2. Preparation of standard targets In standard targets for N R A , the low-Z nuclei may be embedded in any high-Z matrix film or supported by a high-Z backing (in contrast to standards for PIXE), since the elastically backscattered beam particles are stopped in an appropriate absorber [6] which is sufficiently thin that the reaction products of interest do not undergo self-absorption. The low-Z impurity content of the backing must be sufficiently small that the corresponding nuclear reactions do not interfere with the one of interest. G o o d reference target films must meet the following requirements: (1) The layer containing the nuclei of interest should be sufficiently thin that the energy loss of the beam in traversing this layer is small enough [5] not to induce a significant variation of % ( E ) . On the other hand, it should be sufficiently thick to yield reasonable counting rates. (2) The film should exhibit a high lateral uniformity over an area of at least 1 cm 2, so as to justify the assumption that its global content is identical to the local content at the (typically, 1 mm 2) beam spot. This also

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makes available several fresh impact points, thus lengthening the lifetime of the reference targets. Uniformity should be better than _+1%, measured on the appropriate " b e a m area" scale (i.e. ordinarily - 1 mm e. but much smaller for microbeam work). (3) Ideally, the films should be amorphous. Polycrystalline texture (if present) should be sufficiently isotropic to reduce channeling effects below the 1% level. (4) The targets should have long term stability in the atmosphere (no corrosion or isotopic exchange effects) and also under ion bombardment (no desorption of the nuclei of interest in the vacuum, no diffusion into the substrate or chemical reaction with it). The latter condition is usually not too difficult to fulfill at moderate beam currents, since the low-Z beam particles ordinarily * encountered in nuclear microanalysis protons, deuterons, He ions - do not create either strong recoil mixing or sputtering. (5) The fabrication procedure should produce highly reproducible targets and, if possible, be easily duplicated in other laboratories. 2.1. Thin sofid films

Thin films of Be, B, C, AI and Si may be deposited directly by vacuum or ion-beam deposition. Films of the other low-Z nuclei however, can only be deposited in the form of a well-chosen compound. The following techniques may be used for this purpose (except with He and Ne): 1) vacuum deposition of a suitable compound, such as LiF or C a ~ [7]; 2) reactive cathodic sputtering: for example, 14N or 15N deposited as NbN [8] or TaN [9]; 3) ion beam and plasma deposition (currently under development); 4) chemically reactive vapour deposition: 5) chemical and electrochemical techniques. In the latter two techniques, the substrate (or a thin metallic film deposited on a well-chosen inert substrate) is allowed to react either chemically at high temperature with a gas or electrochemically at room temperature with a liquid containing the nuclei of interest. These techniques have been particularly successful for preparing oxygen standards. Anodic oxidation of tantalum is known to proceed under well chosen experimental conditions through an ionic transport process, the electronic current being below 1%; the resulting compound is a highly uniform [10] oxide film with stoichiometry Ta205 [11]. The oxygen content of such oxide films may

* A noticeable exception is hydrogen profiling, which is usually determined via the I H( 15N, ay )12C reaction, using a 6.5 MeV nitrogen beam.

be deduced with accuracy either from Faraday's law [11] or from the final voltage across the anodic film [12]; an absolute accuracy of _+3% is readily achieved. This procedure has also been used to produce high precision 170 and 1~O standards [12,13]. Similarly, thermal oxidation of thin tantalum films was shown to produce stoichiometric Ta205, if correctly carried out; standards, produced in this way and calibrated through their tantalum content using RBS. were found to agree with the anodic standards to a high degree of accuracy [4,111. Such chemical or electrochemical techniques should also be feasible for producing hydrogen or deuterium targets by hydriding thin metallic films. Sufficiently stable hydride standards are, however, not easy to make and a procedure for constructing high quality, reliable hydrogen or deuterium thin films standards is still to be found. Thus, there are many different techniques for constructing thin film reference targets suitable for absolute calibration. Each element or isotope is a particular case and must be treated separately. For most [ow-Z nuclei, satisfactory solutions exist; further progress is still needed, however, for certain elements such as hydrogen or sodium (the commonly used NaC1 or N a O H targets are hygroscopic and hence not stable enough). For isotopically enriched targets, similar methods apply, except that the technical details must be adapted to handle minute amounts of enriched substances, due to their high price and scarcity. Thus, anodic oxidation processes require a very small, specially designed cell [12,13]. Thermal treatments (such as oxidation) require a high quality furnace with very low out-gassing rate, since it must be operated at drastically reduced gas flow rates; similar conditions must also be met for reactive sputtering [8]. In all cases, isotopic exchange with the wall constituents themselves, or with previously adsorbed gases of natural (or different) isotopic composition, must be minimized. 2.2. Ion -implanted targets

For the noble gases, such as 3He or 2°Ne, ion implantation is the only technique that can be used to produce reference targets. The choice of substrate is crucial, as diffusion induced by the probing beam threatens the stability of such targets. Single crystal silicon has often been used, but higher-Z substrates (Nb, Ta, W) would present fewer problems with interfering reactions. For other low-Z nuclei, implantation presents no special advantages except ease of target production. In such targets the nuclei of interest are embedded (typically a few hundred A) beneath the surface and, for resonance reactions, a beam-energy correction term may sometimes be necessary. The resulting targets are highly

G. Amsel, J.A. Davies / Precision standard reference targets

reproducible, uniform and stable. Their interest for absolute calibration purposes is discussed in section 4.4. .2.3. Frozen gas targets

A frozen gas target is extremely useful for determining, with high precision, the cross-section ratios for different nuclei bombarded simultaneously under well-defined conditions [14,15]. It allows several nuclear reactions to be calibrated against a single reference target. The basic premise is that a frozen film of known stoichiometry may be obtained by condensing a suitable high-purity gas such as CO 2, D20, N20 or NO. Since accurate primary standards of known oxygen content are easy to construct, this frozen-gas technique has permitted various reference targets to be calibrated directly against the 160(d, p) reaction cross section. Isotopically enriched reference targets can be easily produced by freezing enriched gases of known isotopic composition. Various frozen gas studies [16,17] have shown that prolonged bombardment by MeV protons or deuterons causes significant erosion to occur; hence, suitable precautions should always be taken to ensure that stoichiometry changes during bombardment are negligible. Typical erosion rates are - 1 - 2 m o l e c u l e s / M e V ion, for which bombardment doses up to - 100/~C on a 3 mm 2 spot can be quite acceptable. Note, however~ that certain gases (for example, N20 ) exhibit much higher erosion than expected [15].

3. Absolute calibration

Using accurately known thin-film reference targets or frozen-gas targets, absolute cross-sections may be determined with precision. This may be achieved either (1) by measuring all relevant experimental parameters (detector solid angle, integrated beam current, etc.) or (2) by calibrating the system with a high-Z RBS reference target of known cross section in exactly the same geometry as that used for the nuclear reaction analysis. The latter approach avoids having to calibrate in absolute terms the current integrator and the detector solid angle. A reliable set of such calibrated high-Z reference targets for RBS already exists: namely, the Bi-implanted silicon wafers (Series I and II) prepared at Harwell. The Series I samples were initially distributed throughout the Ion Beam Analysis community (as part of John Baglin's round-robin standards project) shortly before the Karlsruhe conference in 1975. Their reproducibility and uniformity is of the order of _+1% and their Bi content has been measured independently to better than _+3% by several different laboratories [18]. In a separate contribution to these proceedings [19], a cross-calibra-

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tion between the absolute yield technique of L'Ecuyer et al. [18] and the direct weighing technique of Rigo et al. [4] shows excellent agreement (1-2%) between the two sets of data, with the current " b e s t " value for the Series I standard being (4.83 _+ 0.05) × 1015 Bi cm 2. Mitchell and Eschbach [20] are planning to calibrate the Series II samples with comparable accuracy and make them available. Once absolute nuclear reaction cross-sections are available for well defined values of the bombarding energy E and detection angle 0, accurate analyses (nuclei per cm 2) may be performed in any laboratory, in principle even without reference targets, provided the experimental system has been calibrated in absolute terms. While such absolute yield measurements are certainly feasible in laboratories having the same quality of equipment and expertise as that in which the absolute cross-sections have been measured, it was the consensus of the Workshop participants that a much better procedure (for general use) is always to have a suitably calibrated reference target available in each laboratory. 3.1. Experimental requirements

Even when such a reference target is used to calibrate the detector solid angle and the current integration, high-precision N R A still requires careful consideration of the following factors; 1) An extremely stable current integrator. 2) A pure He + beam without any He 2+ or He ° contamination due to charge exchange after beam analysis, i.e. a very good vacuum all along the beam line, when the RBS calibration of the set-up is carried out, or when a 3He probing beam is used. Note that charge exchange is negligible for MeV protons or deuterons. 3) A fully efficient secondary electron suppression system, for electrons originating in the target and in the beam defining collimators. 4) A well calibrated accelerator energy scale, E. 5) An accurately known detection angle, 0. 6) An accurate dead-time correction (including loss induced by the pulse pile-up rejection system). Furthermore, it is rather difficult to carry out the RBS calibration with the same detector as that used for nuclear reaction analysis (the solid angles for the latter may be very large); hence, different detector geometries might be used in the two experiments and the ratio of the corresponding solid angles must be determined accurately. To summarize, absolute cross-section data may suffice for calibrating nuclear reaction analysis systems in those laboratories which are already suitably equipped for high precision yield measurements. For routine work, however, it is safer to use at least one calibrated thin film reference target (the easiest to make being 160),

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plus secondary targets which have been calibrated against this reference target by using the corresponding published cross-section ratios at well defined values of E and 0.

4. Calibration procedures for reference targets This question was already partially discussed in the preceding section, but a critical review of the various calibration possibilities is given here. 4.1. Direct weighing Low-Z nuclei are usually too light for accurate weighing. If, however, a compound of known stoichiometry is deposited, weighing may yield excellent precision, provided all the usual precautions are followed (see, for example, ref. 4). Care must always be taken to correct for those nuclei already present in the surface region of the substrate prior to film deposition. This is especially important for oxygen where the amount of 160 in the surface oxide of the substrate must be added to the measured film content. The amount of this inescapable contamination may be obtained by nuclear microanalysis: i.e. by comparing the yields from a bare substrate and from the completed target. 4.2. R B S measurement of the high-Z film component This is just another method for utilizing the high-Z component of a deposited compound of very well-known stoichiometry. The precision is set by that of the primary RBS standard and of the assumed stoichiometry. This method has been used to calibrate the oxygen targets obtained by chemical means (thermal oxidation of Ta thin films) mentioned in section 2.1 [11].

calibrated by (p, c~) reactions, which yield no counts from 12C. A similar method may be used for 19F. One serious question is whether 4He scattering on 14N, 15N, 19F, etc. is really Rutherford. As mentioned in the introduction, the validity of 4He RBS cross-sections has already been demonstrated up to 2.4 MeV [4,5] for 160, 27A1 and 28Si; such evidence, however. cannot be extrapolated to other low-Znuclei. A consensus was reached during the Workshop discussions that the most convincing evidence for Rutherford behaviour would be to show that the backscattering yield follows the predicted E 2 and s i n - 4 0 / 2 dependence over a wide range of E and 0. Another test would be to compare the backscattering yields of 4He and 3 He from the same low-Z target. In the case of 14N, an additional cross-check could be made by comparing the RBS result with that obtained by the 14N(d, p) and 14N(d, o~) reactions, both of which have recently been calibrated by the frozen target technique [15] mentioned in sections 2.3 and 4.5. A similar cross-check could also be made for ~5N, with 15NO being used as the frozen gas target. 4.4. lon implantation fluence measurements This method may be of some use, but it is doubtful whether sufficiently high absolute accuracy can be achieved. Even assuming that the charge state of the incoming beam is well defined and that secondary electron suppression and current integration are carried out with precision, significant corrections are still needed for sputtering and ion-reflection effects. For low-Z implants in high-Z targets, the latter effect can be especially significant [21] and careful study would be needed to estimate accurately its magnitude. Implantation is of course an excellent technique for preparing a series of secondary reference targets, with a reproducibility and uniformity of _+ 1% or better, for subsequent calibration against one of the primary reference standards.

4.3. R B S measurement of the low-Z film component 4. 5. Cross-section ratios with frozen targets Provided that 4He backscattering for the low-Z nuclei of interest still follows the (corrected) Rutherford law, RBS may be used to calibrate directly a reference target in which the film to be measured is deposited on a very low-Z backing (beryllium or vitreous carbon, for example). A secondary target, containing the same film deposited on the normal high-Z backing, may then be cross-calibrated by comparing the nuclear reaction yields of the two targets. This comparison must be carried out under specially chosen experimental conditions in order to minimize the interference from the low-Z backing: for example, it may be necessary to choose a nuclear reaction other than the one routinely used. An example may be the case of 14N and 15N, for which reference targets might be constructed in this way, using vitreous carbon as a backing. These nuclei may be cross-

The technique has been described in detail in reference [14]; its recent application in calibrating the (d, p) and (d, c~) reactions on 14N appears in the present Proceedings [15]. The resulting cross section ratios for various reactions, relative to the 160(d, Pl) reaction at 972 keV and 150 ° , are summarized in table 1. With these published ratios, plus one of the readily available absolute standards for 160 (sections 2.1 and 4.6), it is then a simple matter for each laboratory to develop its own set of secondary standards for 160, 14N, 12C, 3He and 2 H. As far as precision is concerned, it may be advantageous to select a calibration energy E at the peak of a broad resonance in the reference reaction, as in refs. 14 and 15 where the broad 160(d, pl)170 reaction reso-

G. A msel, J.A. Davies / Precision standard reference targets Table 1 Cross section ratios a) relative to 160(d. Pl ) 170 (using 972 keV deuterons, 750 keV 3He, and 0 = 150"). Isotope

Nuclear reaction group

Cross section ratio

160 14N [15]

d. d.

14N [15]

d,

lac [14] 2H [14]

d, P0 3He, P0

--- 1.000 0.032 0.078 0.085 0.456 0.057 0.0080 0.119 1.91 4.08

Pl P0 (Pl + P2 ) P4 P5 P6 a0

aj Estimated precision: _+3%.

nance at 972 keV was chosen. By taking measurements at several energies on each side of the resonance, this procedure provides an internal energy calibration for the accelerator; furthermore, since d o o ( E ) / d E is now zero, a small error in E has little effect on the precision, at least for the reference reaction. In some cases, it might be advantageous to choose E and 0 in such a way as to minimize d % ( E ) / d O too. The frozen gas technique might be usefully applied to measure the 13C/160, 15N/160 and 1 9 F / 1 6 0 crosssection ratios, since no precise absolute reference targets of 13C, 15N or 19F are yet available.

4.6. Electrochemical techniques Coulometric or anodic voltage measurements, as quoted in section 2.1, are the most precise, rapid and easily reproducible procedure for producing calibrated standard targets. Unfortunately, the only known case at present is that of oxygen [10-12]. The anodic Ta205 targets so obtained have been adopted as the primary reference standards in the cross-section ratio technique.

4. 7. Calibrating isotopically enriched reference targets Such standards are of basic importance in isotopic tracing experiments; they are also often required in order to take advantage of a more convenient nuclear resonance for depth profiling [6]. The most frequently encountered enriched isotope pairs are 6Li/VLi, l ° B / l a B , 12C/13C, 14N/15N, and 160/180. The calibration techniques quoted above may be used in this case in the following way. For those methods based on the known stoichiometry of the film, its isotopic composition should be the same as that of the enriched

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material used to grow the film, since (except perhaps for deuterium) isotope effects are small. Thus, near equilibrium, the resulting isotopic composition of films obtained by vacuum deposition, chemical or electrochemical methods, reactive sputtering, freezing, etc. should not differ from that of the medium from which they were grown by more than a fraction of a percent. Consequently, if the enrichment of the medium is well known, the isotope content of the film may be deduced from the overall thickness calibration. Thus, for example, RBS measurement of the high-Z component of the film yields the overall number of elemental atoms of interest in the film; the isotopic content stems then from the isotopic composition of the medium [12]. The same holds true for frozen targets or for those produced electrochemically. Here again, as in Section 4.1, care must be taken to correct for those nuclei already present prior to film formation. Unfortunately, the original isotopic composition of the enriched medium is not always known with high precision; furthermore, in spite of all the precautions discussed in section 2.1 (see ref. 8, for example), the enrichment sometimes decreases in an uncontrolled way during the target preparation process. This difficulty may be overcome by applying the so-called " m a j o r natural isotope defect" method [13]. Consider the case of an 1SO reference target, where the natural isotopic abundance is 0.204%. We prepare two targets consisting of oxide films which are atomicall)' identical (i.e. the same stoichiometry and the same high-Z content, hence the same total oxygen content), but one of the films is grown in a natural isotopic medium and the other in a highly ISO-enriched medium. For example, the two films can be grown anodically with the same coulometry, or by thermal oxidation of identical tantalum films deposited on an inert backing. If n~ and n 2 are the 160 contents of the natural oxide and labeled films, respectively, then the ~SO content of the enriched film is just the missing quantity of oxygen, i.e. n I - n 2 , plus a small correction for the naturally occurring 180. Hence, the 1SO content of the enriched film may be determined by measuring only the 160 content of the two films, via the 160(d, pl)lVo reaction. A detailed discussion of this technique may be found in ref. 13. The technique can be readily extended to films of unequal (but well-measured) thicknesses, provided they have identical stoichiometry. It may be applied to any element in which only one isotope is dominant in nature; it may also be used for frozen targets, with the thicknesses of the two films being measured through their non-labelled component (for example, by freezing 13CO2 or 15NO and measuring the oxygen content). The interesting point is that no a priori knowledge of the enrichment of the separated isotope is required. However, the precision is high only when the enrichment is

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high [13], typically a b o v e - 70%, so that n 1 >> n 2. It should be n o t e d that RBS m e a s u r e m e n t o f the l o w - Z film c o m p o n e n t s (section 4.3), w h e n feasible, yields i n d e p e n d e n t l y the c o n t e n t o f each isotopic species in a n e n r i c h e d target. N o t e too that ion i m p l a n t a t i o n is a n excellent t e c h n i q u e for p r o d u c i n g isotopically pure targets. T h e a u t h o r s wish to t h a n k M. H a u t a l a for e n l i g h t e n ing d i s c u s s i o n s while p r e p a r i n g the W o r k s h o p , a n d the c o n f e r e n c e c h a i r m e n for giving t h e m time for writing this p a p e r after the c o n f e r e n c e .

References [1] J. L'Ecuyer, J.A. Davies and N. Matsunami, Nucl. Instr. and Meth. 160 (1979) 337. [2] H.H. Andersen, F. Besenbacher, P. Loftager and W. M611er, Phys. Rev. A 21 (1980) 1891. [3] M. Hautala and M. Luomaj~rvi, Rad. Eft. 45 (1980) 159. [4] S. Rigo, C. Cohen, A. L'Hoir and E. Backelandt, Nucl. Instr. and Meth. 149 (1978) 721. [5] J.R. MacDonald, J.A. Davies, T.E. Jackman and L.C. Feldman, J. Appl. Phys. 54 (1983) 1800. [6] G. Amsel, J.P. Nadai, E. d'Artemare, D. David, E. Girard and J. Moulin, Nucl. Instr. and Meth. 92 (1971) 481.

[7] D. Dieumegard, B. Maurel and G. Amsel, Nucl. Instr. and Meth. 168 (1980) 93. [8] B. Maurel and G. Amsel, these Proceedings (IBA-6), p. 159. [9] G. Amsel and D. David, Rev. Phys. Appl. 4 (1969) 383. [10] J.P.S. Prigle, J. Electrochem. Soc. 119 (1972) 482. [11] G. Amsel, J.P. Nadai, C. Ortega, S. Rigo and J. Siejka, Nucl. Instr. and Meth. 149 (1978) 705. [12] D. Phillips and J.P.S. Pringle, Nucl. Instr. and Meth. 135 (1976) 389. [13] G. Amsel, J.P. Nadai, C. Ortega and J. Siejka, Nucl. Instr. and Meth. 149 (1978) 713. [14] J.A. Davies and P.R. Norton, Nucl. Instr. and Meth. 168 (1980) 611. [15] J.A. Davies, T.E. Jackman, H.H. Plattner and I. Bubb, these Proceedings (IBA-6), p. 141. [16] W.L. Brown, L.J. Lanzerotti, J.M. Poate and W.M. Augustnyniak, Phys. Rev. Left. 40 (1978) 1027. [17] R.W. Ollerhead, J. Bottiger, J.A. Davies, J. L'Ecuyer, H.K. Haugen and N. Matsunami, Rad. Eft. 49 (1980) 203. [18] J. L'Ecuyer, J.A. Davies and N. Matsunami, Nucl. Instr. and Meth. 160 (1979) 337. [19] C. Cohen, J.A. Davies, A.V. Drigo' and T.E. Jackman, these Proceedings (IBA-6), p. 147. [20] I.V. Mitchell, H.L. Eschbach, L. Avaldi and W. Dobma, these Proceedings (IBA-6), p. 91. [21] J. Bottiger and K.B. Winterbon, Rad. Eft. 20 (1973) 65.