- Email: [email protected]
The new time-of-flight ERDA setup at the HMI-Berlin
Nuclear Instruments and Methods in Physics Research B 139 (1998) 219±224
The new time-of-¯ight ERDA setup at the HMI-Berlin W. Bohne, J. R ohrich *, G. R oschert Hahn-Meitner-Institut Berlin GmbH, Bereich Festk orperphysik, Glienicker Str. 100, D-14109 Berlin, Germany
Abstract The new time-of-¯ight ERDA (TOF ERDA) spectrometer of the Hahn-Meitner-Institut Berlin is presented. It is located at a high-energy target position of the ion-beam laboratory (ISL). The great variety of ions from helium to xenon with variable energies up to several MeV/amu allows the determination of the distribution of all elements in the samples up to a depth of some micrometer. The measurement of the hydrogen concentration is possible with high eciency. With the relatively large solid angle of 1.57 msr fast measurements with low ion beam currents are possible. The long ¯ight path of 123 cm and a time resolution of about 180 ps enable a good mass and depth resolution. Ó 1998 Elsevier Science B.V. PACS: 29,30 Ep; 82,80 Ms; 82,80 Yc; 68.55 Keywords: High-energy heavy-ion beam analysis; ERDA; TOF-mass spectrometer; Hydrogen
1. Introduction Along with developments in material science and technology the analysis of novel materials and structures becomes more and more important. For a long time RBS is successfully employed for depth pro®ling of especially heavy elements in solids. Another ion beam based method, ERDA, ®rst only used for the detection of hydrogen [1], can give information on the distribution of all elements even for complicated samples. For the particle identi®cation various set-ups using the DE±E technique [2], the ``pulse shape'' method [3] or the time-of-¯ight ERDA (TOF-ERDA) are applica-
* Corresponding author. Tel.: +49-30-80622191; fax: +4930-80622293; e-mail: [email protected].
ble. As shown earlier, high mass and energy resolution for the TOF-ERDA are possible [4]. In contrast to exclusive energy measurements with silicon detectors, corrections for the ``pulse height defect'' [5] are not necessary, because the energy of all recoils is calculated from the measured TOF and the corresponding masses. Here we present the new TOF-ERDA set-up at the high-energy beamline of the ISL at the Hahn-Meitner-Institut Berlin. It is conceived as a supplement of the RBS system at ISL with its high mass resolution due to the use of projectiles with masses below 25 and energies of typically 1 MeV/amu [6]. A re®ned detection system, consisting of large area channel-plate detectors and silicon strip detectors for TOF and energy measurements, respectively, overcomes the problem of a small solid angle which is connected with a long measuring time
0168-583X/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 8 - 5 8 3 X ( 9 7 ) 0 1 0 0 0 - 8
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W. Bohne et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 219±224
and a high ¯uence. Therefore, a precise determination not only of the stoichiometry of bulk samples but also of the distribution of light impurities in thin layers or at interfaces is possible almost without distortion due to radiation damage. 2. Experimental setup In the following we will give only a short description of the setup, which is discussed in more detail elsewhere [7]. The high-energy heavy ion beam is focused with a typical spot size of 1 ´ 1 mm2 onto the sample in the centre of a target chamber with a diameter of 40 cm. With the great variety of ions from helium to xenon with variable energies up to several MeV/amu the determination of the stoichiometry or the content even of heavier impurities of the samples up to a depth of some micrometer is possible. The intensity of the beam is continuously controlled and recorded by means of a transmission beam-current monitor [8] in front of the scattering chamber. The detection angle can be varied between 0° and 45°. For future two-beam experiments one exit of the chamber can be connected to the low-energy beam line. The angle of the sample surface to the beam axis is variable and is normally chosen to half of the detection angle. The vacuum in the system is kept in the lPa range. The whole sample holder can be exchanged by a fast load±lock system. The ``long'' TOF energy telescope consists of two channel-plate timing detectors followed by a silicon energy detector. The start detector is a conventional channel-plate detector with an electron emitting foil (30 lg/cm2 polypropylene covered with 2.5 lg/cm2 C and 3 lg/cm2 LiF) tilted by 45° relative to the direction of the recoiling ions in a distance of 18 cm to the sample. The stop detector, a so-called electrostatic-mirror channelplate [9] has a total transmission of 80% due to the acceleration grids and a usable area of 9.2 ´ 7.5 cm2 , which corresponds to a total solid angle of 2.5 msr. The distance between both timing detectors is 123 cm. The energy measurement for the particle identi®cation is done with a rectangular ion-implanted silicon detector placed 2 cm behind the stop detector. Its active area of 72 ´ 60
mm2 [10] reduces the solid angle to 1.57 msr. The detector is cooled to about )10°C to reduce the leakage current. To compensate the strong angle dependence of the kinematic factor the detector is sub-divided into 24 discrete vertical strips, each 2.9 mm wide. With this additional position information (DH 0.06°) the energy of each detected recoil can be corrected to one scattering angle. The ``short'' TOF telescope for calibration and simple measurements consists only of a small standard silicon detector at a distance of 48 cm from the sample with a solid angle of 0.39 msr. This detector delivers an energy and stop signal, as well. As start signal we use the RF (radio-frequency) signal of the accelerator supplying the experiment with a pulsed beam with a pulse width of about 0.5 ns and variable pulse separation. All signals are electronically processed with CAMAC devices controlled by our data acquisition system CAMOS9 which uses an auxiliary crate controller running with OS9. The coincident data for energy, TOF (t0 -TOF, t0 includes all signal delays of the whole setup, resolution 8192 channels) and the signals from the silicon strips are transferred via Ethernet to a SUN-workstation where they are stored event-by-event and monitored. 3. Measurements As a basis for a quantitative analysis of samples calibration measurements for energy and time, measurements of the energy loss in the foils, measurements of the energy and time resolution and the determination of the detection eciency of the whole spectrometer were performed. For this purpose dierent a-particle sources and recoils scattered by 120 MeV 86 Kr and/or 250 MeV 129 Xe ions were used. Samples were fabricated by evaporation of dierent metals and oxides onto very thin (4 lg/cm2 ) carbon foils. The energy calibration for the silicon detectors appears to be somewhat dierent for various ions, as expected due to the ``pulse height defect'' in silicon detectors [5]. The energy resolution was determined with aparticles from a ThC' source to about 135 keV. This relatively high value results from the large ca-
W. Bohne et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 219±224
pacitance of the detector. With the very precisely known a-particle energies of this source the length of the ¯ight path and the total delay t0 was determined with an uncertainty in the order of only 10ÿ3 , resulting in an accuracy of the same order for the energy calibration when we calculate the energy for all recoils from the TOF, whereas the energy measured with the silicon detectors have errors up to 2% [5]. Furthermore, the same energy calibration is valid for all detected elements. The relative time calibration is done with a precision time-pulse generator. The time resolution for aparticles amounts to 180 ps. According to our experience for similar systems used at higher energies we can expect a better resolution for heavier ions [11] Assuming, as usual, a 100% detection probability in silicon detectors the eciency for a-particles in the range from 5 to 8.8 MeV is found to be 88% independent of the energy. The eciency for other ions is extracted from the comparison of normalised spectra from the ``long'' and the ``short'' spectrometer. For protons we found an eciency of 40% at an energy of 3 MeV increasing proportionally with the energy loss to a value of 75% at 0.4 MeV. Within the statistical uncertainty of about 2% all heavier ions in the ERDA relevant energy
Fig. 1. Energy vs. (t0 -TOF) spectrum measured at 39° for a healthy part of a human tooth with a beam of 120 MeV 86 Kr.
221
region are detected with a probability of 100%. To prove the measured characteristics of the TOF-spectrometer the results of the analysis of dierent samples are presented. First tests of the applicability of the setup to detect the change in composition of human teeth connected with the formation of caries were carried out. A typical scatterplot (t0 -TOF) versus energy for the enamel of a healthy part of a human tooth [12] is shown in Fig. 1. 86 Kr ions with an energy of 120 MeV were used for this measurement, as well as for all other examples presented. The detection angle always was 39°. In spite of the problems with the not well de®ned untreated surface of the tooth and the precise adjustment in the target holder all constituents are clearly separated. Using the identi®ed mean masses and the time calibration, energy spectra for each element were calculated from the raw data. The spectra for calcium, phosphorous and oxygen are shown in Fig. 2. The composition was extracted by comparison with calculated spectra obtained with the simulation code SIMNRA [13] (open symbols). The calculations included the energy loss of the recoils in the detector foil and the energy straggling due to statistical processes.
Fig. 2. Energy spectra, calculated from the measured TOF, for calcium, phosphorous and oxygen of the same sample used for Fig. 1. The results of the corresponding simulation calculations are shown for comparison (open circles).
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W. Bohne et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 219±224
The tooth enamel is assumed to consist mainly of hydroxyapatite Ca10 (PO4 )6 (OH)2 . To achieve an agreement between the experimental and calculated spectra one has to assume a surface layer of about 1018 atoms/cm2 with a P content varying from 8% at the surface up to 23%. Whereas the oxygen content decreases from 67% to 53% the Ca content stays constant at a value of 19% found also for the bulk. The relative uncertainties for the concentration amount to about 4%. In the bulk the concentrations for all components stay constant. The calculated oxygen content of 66% is essentially higher than the stoichiometric value of 59%. This cannot be explained with additional water in the samples, because the hydrogen content found of 1.5% is even smaller than the expected value of 4%. Taking into account the excess of oxygen the calculated contents of 12% for phosphorus and 19% for calcium correspond within the given uncertainties to the stoichiometric values. The bulk concentrations of 0.8% for C, 0.2% for N, 0.5% for Na and 0.1% for F match the values reported by other authors [14]. The detected variations for the dierent layers show that a change of composition in the surface enamel for
Fig. 3. Energy spectra, calculated from the measured TOF at 39° with a beam of 120 MeV 86 Kr, for the constituents of a CuInS2 layer (kinematics not corrected, see text). The results of the corresponding simulation calculations are shown for comparison (open circles).
example due to caries attack can be examined. Because no standard samples are needed and also light elements including hydrogen can be detected the ERDA is a outstanding supplement to PIXE and NRA, which are often used for trace element detection. Ion beam analysis is an important tool in the ®eld of the development of novel materials and devices. Besides the determination of stoichiometry, homogeneity and thickness of ®lms, grown by various methods, dopants or impurities with concentrations in the ppm region can be detected. As an example a 2 lm thick layer of CuInS2 (a promising absorber material for thin ®lm solar cells) on molybdenum coated soda±lime glass [15] was irradiated. The sample was fabricated by double-source evaporation of copper and indium and subsequent sulfurization of this precursor in an H2 S containing atmosphere. In Fig. 3 (kinematic energy broadening due to the large opening angle of 2.9° not corrected) the energy spectra of all components, together with the results of the simulation (open circles) are depicted. Due to the thickness and the high energy loss in this layer recoils from the substrate could not be detected. This allows the conclusion that the substrate is completely covered by CuInS2 . The bulk is nearly stoichiometric (Cu: 25.8 0.5%; In: 24.0 0.5%; S: 49.9 0.5%; Na: 0.20 0.05%; O: 0.10 0.05%) but on top of the sample the O- and Na-content is much higher. In a layer with a thickness of about 70 20 nm the concentrations of both elements decrease from the surface to the bulk with averaged values of 8 1% and 7 1%, respectively. From SIMS measurements of Cu(In,Ga)Se2 , sodium is known as a fast diusing element, which shows an enrichment at the surface and at the interface [16]. The reported correlation to the oxygen content was also observed for our sample. The high concentration of oxygen in the surface layer arises probably from oxide formation during air storage of the precursor. Because no hydrogen signal was visible an upper limit of 0.005% for the concentration in the bulk can be estimated. For simple samples, in particular if mainly the hydrogen content or only the stoichiometry is of interest, already measurements with the ``short'' telescope are adequate. In this case no further cal-
W. Bohne et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 219±224
ibration for the hydrogen signal is needed, due to the 100% eciency. Such measurement were performed with a series of thin silicon nitride ®lms to quantify the stoichiometry and the hydrogen content. The ®lms were produced by ECR±CVD deposition on standard silicon wafers using nitrogen and silane as precursor gases with post-deposition rapid thermal annealing [17]. Apart from silicon and nitrogen a large amount of hydrogen and some oxygen, especially at the surface, were detected, and well resolved. In Fig. 4 the element yields, transformed to the depth scale, for a sample grown with a N2 /SiH4 gas ratio of 1.6 followed by subsequent annealing at 700°C for 30 s are shown. As a main result the stoichiometry for Si3 N4 is con®rmed within an uncertainty of 4%. The thickness is estimated to 236 5 nm, assuming a density of 3.44 g cmÿ3 of Si3 N4 , which agrees well with the value of 220 nm measured with a Dektak surface pro®le measuring system. The layer is very homogeneous and smooth. The later can be concluded from the steep low energy edge of the energy spectra. Only the transformation of the narrow energy spectrum of hydrogen to a depth scale is dicult because of the large energy step per channel necessary for the measurement of the high energetic heavy ions. Nevertheless it can be concluded that the hydrogen in the silicon nitride layer is distributed relative homogeneously with
223
an atomic concentration of 13.9 0.5%. The small hydrogen content below the interface may be attributed to diusion into silicon and is under further investigation. The oxygen on the surface can be attributed to a thin oxide layer or water. In addition a weak carbon contribution is measured in the order of 1014 atoms/cm2 , most probably due to carbon deposition during the analysis.
4. Summary It is demonstrated that our TOF setup with its large solid angle is very capable for material analysis. The values achieved for energy and time resolution allow the precise detection of all elements, including hydrogen. From a comparison of the experimental and simulated energy spectra the absolute concentration as well as the depth pro®les for all constituents can be determined. For simple cases, i.e. samples which contain only elements separated by many mass units, TOF measurements with a standard silicon detector using the RF from the accelerator as start signal are suf®cient. An advantage of the system is the variable detection range for the TOF. When using large TOF ranges the minimum energy-detection threshold can be kept very small. Therefore, it is possible for special cases to increase the sensitivity of the telescope using ion beams at much lower incident energies. On the other hand, the use of heavier ion beams at higher energies will improve the mass resolution. Because all these ions are available at the ISL accelerator, optimised measurements for various problems are possible. For the future, we will improve the mass and energy resolution with suitable modi®cations of the detector system. With a larger energy detector and timing detectors with higher transmission the solid angle could be enlarged almost by a factor of 2.
References Fig. 4. Element yields transformed to depth scale for a Si3 N4 : H layer obtained at 39° with a beam of 120 MeV 86 Kr.
[1] J. L'Ecuyer et al., J. Appl. Phys. 47 (1976) 381. [2] J.P. Stoquert, G. Guillaume, M. Hage, J.J. Grob, C. Ganter, P. Siert, Nucl. Instr. and Meth. B 44 (1989) 184.
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[3] G. Pausch, W. Bohne, D. Hilscher, Nucl. Instr. and Meth. A 337 (1994) 573; G. Pausch, W. Bohne, D. Hilscher, H.G. Ortlepp, D. Polster, Nucl. Instr. and Meth. A 349 (1994) 281. [4] P. Goppelt, B. Gebauer, D. Fink, M. Wilpert, Th. Wilpert, W. Bohne, Nucl. Instr. and Meth. B 68 (1992) 235. [5] J.B. Moulton, J.E. Stephenson, R.P. Schmitt, G.J. Wozniak, Nucl. Instr. and Meth 157 (1978) 325. [6] Annual report of the Hahn-Meitner-Institut Berlin 1994, HMI-B 526, 1995, 91. [7] W. Bohne, J. R ohrich, G. R oschert, Proc. IBA-13 Nucl. Instr. and Meth. B 136±138 (1998) 633. [8] W. Bohne, S. Hessler, G. R oschert, Nucl. Instr. and Meth. B 113 (1996) 78. [9] F. Busch, W. Pfeer, B. Kohlmeyer, D. Sch ull, F. P uhlhofer, Nucl. Instr. and Meth. 171 (1980) 71. [10] Eurysis Mesures, F-78067 St. Quentin Yvelines, France.
[11] W. Bohne, W. Galster, K. Grabisch, H. Morgenstern, Nucl. Instr.and Meth. A 240 (1985) 145. [12] C. Finke, Abteilung f ur Kinderzahnmedizin der Charite Berlin, Private communication. [13] M. Mayer, SIMNRA Users Guide, Report IPP 9/113, Max-Planck-Institut f ur Plasmaphysik, Garching, 1997. [14] H.E. Schroeder, Orale Strukturbiologie, G. Thieme Verlag, Stuttgart, 1992. [15] M. Gossla, Hahn-Meitner-Institut Berlin, Private communication. [16] M. Bodeg ard, J. Hedstr om, K. Granath, A. Rockett, L. Stolt, Proceedings of the Thirteenth European Photovoltaic Solar Energy Conference, H.S. Stephens & Associates, Bedford, 1995, pp. 2080. [17] G. Gonz ales, Facultad de Ciencias Fõsicas, Fõsica Aplicada III, Universidad Complutense de Madrid, Private communication.
The new time-of-¯ight ERDA setup at the HMI-Berlin W. Bohne, J. R ohrich *, G. R oschert Hahn-Meitner-Institut Berlin GmbH, Bereich Festk orperphysik, Glienicker Str. 100, D-14109 Berlin, Germany
Abstract The new time-of-¯ight ERDA (TOF ERDA) spectrometer of the Hahn-Meitner-Institut Berlin is presented. It is located at a high-energy target position of the ion-beam laboratory (ISL). The great variety of ions from helium to xenon with variable energies up to several MeV/amu allows the determination of the distribution of all elements in the samples up to a depth of some micrometer. The measurement of the hydrogen concentration is possible with high eciency. With the relatively large solid angle of 1.57 msr fast measurements with low ion beam currents are possible. The long ¯ight path of 123 cm and a time resolution of about 180 ps enable a good mass and depth resolution. Ó 1998 Elsevier Science B.V. PACS: 29,30 Ep; 82,80 Ms; 82,80 Yc; 68.55 Keywords: High-energy heavy-ion beam analysis; ERDA; TOF-mass spectrometer; Hydrogen
1. Introduction Along with developments in material science and technology the analysis of novel materials and structures becomes more and more important. For a long time RBS is successfully employed for depth pro®ling of especially heavy elements in solids. Another ion beam based method, ERDA, ®rst only used for the detection of hydrogen [1], can give information on the distribution of all elements even for complicated samples. For the particle identi®cation various set-ups using the DE±E technique [2], the ``pulse shape'' method [3] or the time-of-¯ight ERDA (TOF-ERDA) are applica-
* Corresponding author. Tel.: +49-30-80622191; fax: +4930-80622293; e-mail: [email protected].
ble. As shown earlier, high mass and energy resolution for the TOF-ERDA are possible [4]. In contrast to exclusive energy measurements with silicon detectors, corrections for the ``pulse height defect'' [5] are not necessary, because the energy of all recoils is calculated from the measured TOF and the corresponding masses. Here we present the new TOF-ERDA set-up at the high-energy beamline of the ISL at the Hahn-Meitner-Institut Berlin. It is conceived as a supplement of the RBS system at ISL with its high mass resolution due to the use of projectiles with masses below 25 and energies of typically 1 MeV/amu [6]. A re®ned detection system, consisting of large area channel-plate detectors and silicon strip detectors for TOF and energy measurements, respectively, overcomes the problem of a small solid angle which is connected with a long measuring time
0168-583X/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 8 - 5 8 3 X ( 9 7 ) 0 1 0 0 0 - 8
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W. Bohne et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 219±224
and a high ¯uence. Therefore, a precise determination not only of the stoichiometry of bulk samples but also of the distribution of light impurities in thin layers or at interfaces is possible almost without distortion due to radiation damage. 2. Experimental setup In the following we will give only a short description of the setup, which is discussed in more detail elsewhere [7]. The high-energy heavy ion beam is focused with a typical spot size of 1 ´ 1 mm2 onto the sample in the centre of a target chamber with a diameter of 40 cm. With the great variety of ions from helium to xenon with variable energies up to several MeV/amu the determination of the stoichiometry or the content even of heavier impurities of the samples up to a depth of some micrometer is possible. The intensity of the beam is continuously controlled and recorded by means of a transmission beam-current monitor [8] in front of the scattering chamber. The detection angle can be varied between 0° and 45°. For future two-beam experiments one exit of the chamber can be connected to the low-energy beam line. The angle of the sample surface to the beam axis is variable and is normally chosen to half of the detection angle. The vacuum in the system is kept in the lPa range. The whole sample holder can be exchanged by a fast load±lock system. The ``long'' TOF energy telescope consists of two channel-plate timing detectors followed by a silicon energy detector. The start detector is a conventional channel-plate detector with an electron emitting foil (30 lg/cm2 polypropylene covered with 2.5 lg/cm2 C and 3 lg/cm2 LiF) tilted by 45° relative to the direction of the recoiling ions in a distance of 18 cm to the sample. The stop detector, a so-called electrostatic-mirror channelplate [9] has a total transmission of 80% due to the acceleration grids and a usable area of 9.2 ´ 7.5 cm2 , which corresponds to a total solid angle of 2.5 msr. The distance between both timing detectors is 123 cm. The energy measurement for the particle identi®cation is done with a rectangular ion-implanted silicon detector placed 2 cm behind the stop detector. Its active area of 72 ´ 60
mm2 [10] reduces the solid angle to 1.57 msr. The detector is cooled to about )10°C to reduce the leakage current. To compensate the strong angle dependence of the kinematic factor the detector is sub-divided into 24 discrete vertical strips, each 2.9 mm wide. With this additional position information (DH 0.06°) the energy of each detected recoil can be corrected to one scattering angle. The ``short'' TOF telescope for calibration and simple measurements consists only of a small standard silicon detector at a distance of 48 cm from the sample with a solid angle of 0.39 msr. This detector delivers an energy and stop signal, as well. As start signal we use the RF (radio-frequency) signal of the accelerator supplying the experiment with a pulsed beam with a pulse width of about 0.5 ns and variable pulse separation. All signals are electronically processed with CAMAC devices controlled by our data acquisition system CAMOS9 which uses an auxiliary crate controller running with OS9. The coincident data for energy, TOF (t0 -TOF, t0 includes all signal delays of the whole setup, resolution 8192 channels) and the signals from the silicon strips are transferred via Ethernet to a SUN-workstation where they are stored event-by-event and monitored. 3. Measurements As a basis for a quantitative analysis of samples calibration measurements for energy and time, measurements of the energy loss in the foils, measurements of the energy and time resolution and the determination of the detection eciency of the whole spectrometer were performed. For this purpose dierent a-particle sources and recoils scattered by 120 MeV 86 Kr and/or 250 MeV 129 Xe ions were used. Samples were fabricated by evaporation of dierent metals and oxides onto very thin (4 lg/cm2 ) carbon foils. The energy calibration for the silicon detectors appears to be somewhat dierent for various ions, as expected due to the ``pulse height defect'' in silicon detectors [5]. The energy resolution was determined with aparticles from a ThC' source to about 135 keV. This relatively high value results from the large ca-
W. Bohne et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 219±224
pacitance of the detector. With the very precisely known a-particle energies of this source the length of the ¯ight path and the total delay t0 was determined with an uncertainty in the order of only 10ÿ3 , resulting in an accuracy of the same order for the energy calibration when we calculate the energy for all recoils from the TOF, whereas the energy measured with the silicon detectors have errors up to 2% [5]. Furthermore, the same energy calibration is valid for all detected elements. The relative time calibration is done with a precision time-pulse generator. The time resolution for aparticles amounts to 180 ps. According to our experience for similar systems used at higher energies we can expect a better resolution for heavier ions [11] Assuming, as usual, a 100% detection probability in silicon detectors the eciency for a-particles in the range from 5 to 8.8 MeV is found to be 88% independent of the energy. The eciency for other ions is extracted from the comparison of normalised spectra from the ``long'' and the ``short'' spectrometer. For protons we found an eciency of 40% at an energy of 3 MeV increasing proportionally with the energy loss to a value of 75% at 0.4 MeV. Within the statistical uncertainty of about 2% all heavier ions in the ERDA relevant energy
Fig. 1. Energy vs. (t0 -TOF) spectrum measured at 39° for a healthy part of a human tooth with a beam of 120 MeV 86 Kr.
221
region are detected with a probability of 100%. To prove the measured characteristics of the TOF-spectrometer the results of the analysis of dierent samples are presented. First tests of the applicability of the setup to detect the change in composition of human teeth connected with the formation of caries were carried out. A typical scatterplot (t0 -TOF) versus energy for the enamel of a healthy part of a human tooth [12] is shown in Fig. 1. 86 Kr ions with an energy of 120 MeV were used for this measurement, as well as for all other examples presented. The detection angle always was 39°. In spite of the problems with the not well de®ned untreated surface of the tooth and the precise adjustment in the target holder all constituents are clearly separated. Using the identi®ed mean masses and the time calibration, energy spectra for each element were calculated from the raw data. The spectra for calcium, phosphorous and oxygen are shown in Fig. 2. The composition was extracted by comparison with calculated spectra obtained with the simulation code SIMNRA [13] (open symbols). The calculations included the energy loss of the recoils in the detector foil and the energy straggling due to statistical processes.
Fig. 2. Energy spectra, calculated from the measured TOF, for calcium, phosphorous and oxygen of the same sample used for Fig. 1. The results of the corresponding simulation calculations are shown for comparison (open circles).
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W. Bohne et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 219±224
The tooth enamel is assumed to consist mainly of hydroxyapatite Ca10 (PO4 )6 (OH)2 . To achieve an agreement between the experimental and calculated spectra one has to assume a surface layer of about 1018 atoms/cm2 with a P content varying from 8% at the surface up to 23%. Whereas the oxygen content decreases from 67% to 53% the Ca content stays constant at a value of 19% found also for the bulk. The relative uncertainties for the concentration amount to about 4%. In the bulk the concentrations for all components stay constant. The calculated oxygen content of 66% is essentially higher than the stoichiometric value of 59%. This cannot be explained with additional water in the samples, because the hydrogen content found of 1.5% is even smaller than the expected value of 4%. Taking into account the excess of oxygen the calculated contents of 12% for phosphorus and 19% for calcium correspond within the given uncertainties to the stoichiometric values. The bulk concentrations of 0.8% for C, 0.2% for N, 0.5% for Na and 0.1% for F match the values reported by other authors [14]. The detected variations for the dierent layers show that a change of composition in the surface enamel for
Fig. 3. Energy spectra, calculated from the measured TOF at 39° with a beam of 120 MeV 86 Kr, for the constituents of a CuInS2 layer (kinematics not corrected, see text). The results of the corresponding simulation calculations are shown for comparison (open circles).
example due to caries attack can be examined. Because no standard samples are needed and also light elements including hydrogen can be detected the ERDA is a outstanding supplement to PIXE and NRA, which are often used for trace element detection. Ion beam analysis is an important tool in the ®eld of the development of novel materials and devices. Besides the determination of stoichiometry, homogeneity and thickness of ®lms, grown by various methods, dopants or impurities with concentrations in the ppm region can be detected. As an example a 2 lm thick layer of CuInS2 (a promising absorber material for thin ®lm solar cells) on molybdenum coated soda±lime glass [15] was irradiated. The sample was fabricated by double-source evaporation of copper and indium and subsequent sulfurization of this precursor in an H2 S containing atmosphere. In Fig. 3 (kinematic energy broadening due to the large opening angle of 2.9° not corrected) the energy spectra of all components, together with the results of the simulation (open circles) are depicted. Due to the thickness and the high energy loss in this layer recoils from the substrate could not be detected. This allows the conclusion that the substrate is completely covered by CuInS2 . The bulk is nearly stoichiometric (Cu: 25.8 0.5%; In: 24.0 0.5%; S: 49.9 0.5%; Na: 0.20 0.05%; O: 0.10 0.05%) but on top of the sample the O- and Na-content is much higher. In a layer with a thickness of about 70 20 nm the concentrations of both elements decrease from the surface to the bulk with averaged values of 8 1% and 7 1%, respectively. From SIMS measurements of Cu(In,Ga)Se2 , sodium is known as a fast diusing element, which shows an enrichment at the surface and at the interface [16]. The reported correlation to the oxygen content was also observed for our sample. The high concentration of oxygen in the surface layer arises probably from oxide formation during air storage of the precursor. Because no hydrogen signal was visible an upper limit of 0.005% for the concentration in the bulk can be estimated. For simple samples, in particular if mainly the hydrogen content or only the stoichiometry is of interest, already measurements with the ``short'' telescope are adequate. In this case no further cal-
W. Bohne et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 219±224
ibration for the hydrogen signal is needed, due to the 100% eciency. Such measurement were performed with a series of thin silicon nitride ®lms to quantify the stoichiometry and the hydrogen content. The ®lms were produced by ECR±CVD deposition on standard silicon wafers using nitrogen and silane as precursor gases with post-deposition rapid thermal annealing [17]. Apart from silicon and nitrogen a large amount of hydrogen and some oxygen, especially at the surface, were detected, and well resolved. In Fig. 4 the element yields, transformed to the depth scale, for a sample grown with a N2 /SiH4 gas ratio of 1.6 followed by subsequent annealing at 700°C for 30 s are shown. As a main result the stoichiometry for Si3 N4 is con®rmed within an uncertainty of 4%. The thickness is estimated to 236 5 nm, assuming a density of 3.44 g cmÿ3 of Si3 N4 , which agrees well with the value of 220 nm measured with a Dektak surface pro®le measuring system. The layer is very homogeneous and smooth. The later can be concluded from the steep low energy edge of the energy spectra. Only the transformation of the narrow energy spectrum of hydrogen to a depth scale is dicult because of the large energy step per channel necessary for the measurement of the high energetic heavy ions. Nevertheless it can be concluded that the hydrogen in the silicon nitride layer is distributed relative homogeneously with
223
an atomic concentration of 13.9 0.5%. The small hydrogen content below the interface may be attributed to diusion into silicon and is under further investigation. The oxygen on the surface can be attributed to a thin oxide layer or water. In addition a weak carbon contribution is measured in the order of 1014 atoms/cm2 , most probably due to carbon deposition during the analysis.
4. Summary It is demonstrated that our TOF setup with its large solid angle is very capable for material analysis. The values achieved for energy and time resolution allow the precise detection of all elements, including hydrogen. From a comparison of the experimental and simulated energy spectra the absolute concentration as well as the depth pro®les for all constituents can be determined. For simple cases, i.e. samples which contain only elements separated by many mass units, TOF measurements with a standard silicon detector using the RF from the accelerator as start signal are suf®cient. An advantage of the system is the variable detection range for the TOF. When using large TOF ranges the minimum energy-detection threshold can be kept very small. Therefore, it is possible for special cases to increase the sensitivity of the telescope using ion beams at much lower incident energies. On the other hand, the use of heavier ion beams at higher energies will improve the mass resolution. Because all these ions are available at the ISL accelerator, optimised measurements for various problems are possible. For the future, we will improve the mass and energy resolution with suitable modi®cations of the detector system. With a larger energy detector and timing detectors with higher transmission the solid angle could be enlarged almost by a factor of 2.
References Fig. 4. Element yields transformed to depth scale for a Si3 N4 : H layer obtained at 39° with a beam of 120 MeV 86 Kr.
[1] J. L'Ecuyer et al., J. Appl. Phys. 47 (1976) 381. [2] J.P. Stoquert, G. Guillaume, M. Hage, J.J. Grob, C. Ganter, P. Siert, Nucl. Instr. and Meth. B 44 (1989) 184.
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W. Bohne et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 219±224
[3] G. Pausch, W. Bohne, D. Hilscher, Nucl. Instr. and Meth. A 337 (1994) 573; G. Pausch, W. Bohne, D. Hilscher, H.G. Ortlepp, D. Polster, Nucl. Instr. and Meth. A 349 (1994) 281. [4] P. Goppelt, B. Gebauer, D. Fink, M. Wilpert, Th. Wilpert, W. Bohne, Nucl. Instr. and Meth. B 68 (1992) 235. [5] J.B. Moulton, J.E. Stephenson, R.P. Schmitt, G.J. Wozniak, Nucl. Instr. and Meth 157 (1978) 325. [6] Annual report of the Hahn-Meitner-Institut Berlin 1994, HMI-B 526, 1995, 91. [7] W. Bohne, J. R ohrich, G. R oschert, Proc. IBA-13 Nucl. Instr. and Meth. B 136±138 (1998) 633. [8] W. Bohne, S. Hessler, G. R oschert, Nucl. Instr. and Meth. B 113 (1996) 78. [9] F. Busch, W. Pfeer, B. Kohlmeyer, D. Sch ull, F. P uhlhofer, Nucl. Instr. and Meth. 171 (1980) 71. [10] Eurysis Mesures, F-78067 St. Quentin Yvelines, France.
[11] W. Bohne, W. Galster, K. Grabisch, H. Morgenstern, Nucl. Instr.and Meth. A 240 (1985) 145. [12] C. Finke, Abteilung f ur Kinderzahnmedizin der Charite Berlin, Private communication. [13] M. Mayer, SIMNRA Users Guide, Report IPP 9/113, Max-Planck-Institut f ur Plasmaphysik, Garching, 1997. [14] H.E. Schroeder, Orale Strukturbiologie, G. Thieme Verlag, Stuttgart, 1992. [15] M. Gossla, Hahn-Meitner-Institut Berlin, Private communication. [16] M. Bodeg ard, J. Hedstr om, K. Granath, A. Rockett, L. Stolt, Proceedings of the Thirteenth European Photovoltaic Solar Energy Conference, H.S. Stephens & Associates, Bedford, 1995, pp. 2080. [17] G. Gonz ales, Facultad de Ciencias Fõsicas, Fõsica Aplicada III, Universidad Complutense de Madrid, Private communication.