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Investigation of hydrogen depletion of organic materials upon ion beam irradiation by simultaneous micro-RBS and micro-ERDA techniques
Nuclear Instruments and Methods in Physics Research B 268 (2010) 2197–2201
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Investigation of hydrogen depletion of organic materials upon ion beam irradiation by simultaneous micro-RBS and micro-ERDA techniques A. Simon *, R. Huszank, M. Novák, Z. Pintye Institute of Nuclear Research of the Hungarian Academy of Sciences, H-4001 Debrecen, P.O. Box 51, Hungary
a r t i c l e
i n f o
Article history: Available online 25 February 2010 Keywords: Polymer degradation Radiation stability Ion beam modification Focussed ion beam Hydrogen depletion Micro-RBS Micro-ERDA
a b s t r a c t Radiation stability investigations in a form of hydrogen depletion of organic materials induced by a focussed 1.6 MeV He+ scanning ion beam within a beam intensity range of 10–50 pA/lm2 is reported. The hydrogen depletion was measured by applying Elastic Recoil Detection Analysis (ERDA) and Rutherford Backscattering Spectrometry (RBS) techniques, simultaneously. H-count rates were measured and further processed as a function of the deposited ion beam fluence. Our results show, that within the deposited fluence range of 1014–1015 ions/cm2 both the Kapton-foil and porphyrin complex (Fe(III)TPP) do not suffer from hydrogen depletion process, indicating only minor chemical structural changes. There is a slight decrease of the hydrogen content of the Mylar-foil while a remarkable loss of hydrogen occurs in case of poly(dimethyl-siloxane) (PDMS) suggesting the degradation of the polymer chain. A full description of our versatile and complex particle detection system for nuclear microprobe applications is also given in the paper. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction The fundamental processes in ion beam interaction with polymers are of high interest and are being actively studied [1,2]. Ion irradiation of polymers is accompanied by radiation-induced effects changing the structure and properties of the irradiated material. The ion irradiation leads to the degradation of polymeric chains, chemical bond cleavage, forming of free radicals and release of gaseous degradation products. The type and degree of the degradation strongly depend on the chemical groups attached to the main chain of the polymer. These processes are also relevant to the development of several applications, such as ion beam lithography, improvement of polymer adhesion, polymer doping by ion implantation, etc. The industrial use of polymers ranges across a broad field of structural, mechanical, electrical and optical applications in microelectronics, opto-electronics and in medicine, as well. Moreover, polymer films, especially Mylar-foil and Kapton-foil, are widely used as H-standards in Elastic Recoil Detection Analysis (ERDA) [3–6]. However, the decomposition of polymer materials when irradiated with a milli- and microbeam also reported in several papers [7–11]. As we also experienced the ion beam damaging effect at least in the form of darkening of the polymers, we have
* Corresponding author. Address: Institute of Nuclear Research of the Hungarian Academy of Sciences (ATOMKI), H-4001, P.O. Box 51, Debrecen, Hungary. Tel.: +36 52 509 211; fax: +36 52 416 181. E-mail address: [email protected] (A. Simon). 0168-583X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2010.02.087
decided to investigate in more details the possible hydrogen depletion of Kapton and Mylar foils for focussed microbeam analyses. These polymers are mainly applied as H-standards but they are used e.g. as backing film for atmospheric aerosols, as well. The investigation also included another two organic materials which are applied widely both in research and industrial fields. Recently, the application of organic dyes, for example different porphyrin derivatives as light absorbing materials in organic solar cells has gained considerable interest as they exhibit very high absorption coefficients [12]. Furthermore, they can be functionalized to obtain specific optical or electrical properties utilised e.g. in solar cells [13]. Poly(dimethyl-siloxane) (PDMS) is a silicon based flexible polymer, base material for many applications nowadays. It has advantageous properties, such as low surface energy, high electrical resistance, constant and high ductility over a wide range of temperatures, etc. It has been widely used to fabricate Micro-ElectroMechanical Systems (MEMS), microfluidic devices [14], biosensors, biochips or creating micro-stamps [15]. 2. Experimental details 2.1. Materials used for the analyses In this paper we present the analyses of four kinds of organic materials (a) poly(ethylene terephthalate) (PET, Mylar, C10H8O4); (b) poly(4,40 -oxydiphenylene-pyromellitimide) (PI, Kapton, C22H10N2O5) both from DuPont; (c) poly(dimethyl-siloxane) (PDMS,
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Ion beam
Optical microscope
Sample De t. C
BNC adaptors
et .B D
BNC adaptors
Det. A
Faraday-cup
Det.E Det. D
±180°
Fig. 1. Layout of the Atomki nuclear microprobe chamber for simultaneous charged particle detection. Dets. A-D: Ortec ULTRA™ Ion-Implanted Si detectors, Det. E: Hamamatsu S5821 Si PIN diode. The rotatable detector holder at transmission geometry also includes a Faraday-cup used for low beam current measurements and settings.
SiOC2H6) from Dow Corning as Sylgard 184 elastomer kit and (d) Fe(III) meso-tetraphenyl porphyrin (Fe(III)TPP, C44H28ClFeN4) from Aldrich. PDMS polymer samples were prepared by mixing the liquid silicone pre-polymer and the curing agent in the recommended ratio of 10:1. Then the pre-polymer was cured in a glass Petri-dish for 30 min. at a temperature of 125 °C and was left to finish curing overnight at a room temperature. The porphyrin thin film was spin-coated from a solution of 49.4 mg of Fe(III)TPP in 5 ml of chlorophorm (CH3Cl) onto a silicon substrate at 1000 rpm for 30 s. After the spin-coating the film was allowed to dry for overnight at room temperature in a desiccator. The uniformity of the samples was checked with an AMBIOS XP-I type profilometer before the IBA analyses and it was found to be uniform enough within the range of the scan size of the measurement. 2.2. Experimental setup Rutherford Backscattering Spectrometry (RBS) and Elastic Recoil Detection Analysis (ERDA) were implemented simultaneously with a focussed 1600 keV He+ ion beam on the Oxfordtype nuclear microprobe facility at HAS-ATOMKI, Debrecen, Hungary [16]. The beam was focussed to a 2 2 lm2 spot while the scan size was set to 500 500 lm2 – except for the Mylar-foil where 625 625 lm2 area was scanned. Data were collected in 256 256 pixel and the scanning speed was set to 10 ls/tick. The sample chamber is equipped with a High Precision Vacuum Generators
5-axes goniometer, which makes available both the high depthresolution micro-RBS and micro-ERDA measurements. The applied tilt angle was 80°, thus, the effective scan size was 2879 500 lm2 (3599 625 lm2 for the Mylar-foil). For the calculation of the total deposited fluence the effective scan size was taken. The layout of the microbeam chamber for particle detection measurements is shown in Fig. 1. Dets. A-D: Ortec ULTRA™ IonImplanted Si detectors (50 mm2 sensitive area and 18 keV system energy resolution). Dets. A&B are mainly used for micro-RBS measurements [17–19]. Using Det. A simultaneous Particle Induced X-ray Emission (micro-PIXE) and micro-RBS analysis can be implemented [20,21] even combined with micro-ERDA [22,23], too. Detectors setup: Det. A: H = 165° at Cornell geometry, O = 27.1 msrad; Det. B: H = 135° at IBM-geometry, O = 55.8 msrad. Det. C is used for ERDA: H = 30° at IBM-geometry, O = 15.1 msrad There is a 1.1 mm wide aperture on its front surface covered with a 9 lm thick Mylar-foil. There is a rotatable detector holder at transmission geometry equipped with Dets. D&E. A similar rotatable detector holder was used before at the University of Oxford for Scanning Transmission Ion Microscopy (STIM) [24], a Transmission Channelling version of STIM [25] is still in use at University of Surrey Ion Beam Centre [26,27]. In our case Det. E is a Hamamatsu S5821 Si PIN diode. A carbon Faraday-cup is also installed onto this rotatable flange and especially used for low beam current measurements and beam settings. Beside STIM [28,29] we widely use this rotatable detector holder for Ion Beam Induced Current (IBIC) investigations [30–32] and for Proton Beam Writing [33].
A. Simon et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 2197–2201
2.3. Spectrum and data evaluation Data were collected by the OM_DAQ 2004 system [34] in listmode. Micro-RBS and micro-ERDA spectra were evaluated with the RBX computer code [35] version 5.17. The RBX code was developed to evaluate precisely RBS and channelling measurements of simple 1D structured samples, including all the main energy spread contributions calculated by DEPTH [36]. To get the hydrogen-loss data the listmode files were further processed. At first, the RBS and ERDA count numbers were extracted from each data blocks together with the measurement parameters like charge and elapsed time. Then, the count rates of the ERDA and RBS signals were calculated using the time-stamps of each data blocks. The charge was collected on the sample but absolute total deposited charge values were determined by fitting the RBS spectra. In order to be independent of any possible beam current fluctuation the accumulated charge rate distribution was also calculated using the time-stamps in each data blocks. Finally, the calculated ERDA and RBS count rate distributions were normalised using the calculated charge rate distributions.
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or activated by the ion beam-, can be dislocated or leave the sample completely. As stated already in Section 1, the damage increases if micrometer sized beams with high areal beam densities are used for ion beam analysis. A sensitive analysis of concentration profiles becomes impossible if the profile itself is altered by the irradiation before sufficient amount of data is collected. Both displacement collisions induced by nuclear stopping and electronic stopping effects, like excitation and ionization play role in the damage effects and have to be considered. We report the micro-RBS and micro-ERDA spectra of Fe(III)TPP and PDMS here. The surface peaks of the constituting elements and spectra fits are shown in Figs. 2 and 3. We think that it is very useful that the investigated porphyrin molecule includes an iron metal centre which can be a marker element together with the low energy constituents for an accurate RBS energy calibration. In addition, this standard can be prepared as a thin layer on a substrate, thus, the accuracy of the ERDA spectrum evaluation can be increased, as well. All in all, this material can be ideal as an ERDA calibration standard.
3.2. Hydrogen count rate distributions 3. Results and discussion 3.1. Micro-RBS and micro-ERDA spectra Each ion beam analytical techniques inhere more or less the modification of the original sample. As a result, atoms -scattered
Under ion beam radiation, beside the different physical effects, complex chemical processes can take place in organic materials. For initial steps, loss of the kinetic energy of the penetrating ions occurs by inelastic collisions, resulting in ionization and excitation of the target material. The excited-state molecules may return to
Fig. 2. Micro-RBS spectrum (left) and micro-ERDA spectrum (right) of a PDMS sample. Measurement parameters:1.6 MeV He+ ion beam, 2 2 lm2 beam spot, 2879 500 lm2 scan area, RBS: H = 135°, ERDA: H = 30° tilt angle: 80°, total deposited charge: 1.1 lC.
Fig. 3. Micro-RBS spectrum (left) and micro-ERDA spectrum (right) of a 81 nm thick Fe(III)TPP layer on a Si substrate. Measurement parameters:1.6 MeV He+ ion beam, 2 2 lm2 beam spot, 2879 500 lm2 scan area, RBS: H = 135°, ERDA: H = 30° tilt angle: 80°, total deposited charge: 0.78 lC.
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Fig. 4. Hydrogen count rate distributions as a function of the Deposited Ion Beam Fluence. For the detailed calculation method please see text in Section 2.3. Both the Kaptonfoil and the Fe(III)TPP complex do not suffer from hydrogen depletion process indicating only minor chemical structural changes. There is a slight decrease of the hydrogen content of the Mylar-foil while a remarkable exponential loss of hydrogen occurs in case of PDMS suggesting the degradation of the polymer chain. The residual of the fit is also shown in the graphs.
the ground state through radiationless decay or undergo homolytic dissociation reactions to form free radicals. The free radicals are very reactive and cause a number of chemical reactions in polymers. In order to determine the radiation stability of the H content of the investigated organic materials, H-count rates were extracted from the listmode files. The graphs in Fig. 4 show such distributions. The H-count rate is in a linear correlation with the H concentrations. Please note, that a constant H-rate distribution means same detected count numbers in each time interval. i.e. a constant hydrogen content. The main advantage of our method is that with this unique data processing, the ion beam induced chemical degradations causing irreversible hydrogen depletion can be determined even after a routine ERDA measurement, as well. The hydrogen loss takes place by an irreversible process of homolytic scission of C–H bonds. The degree of the irreversibility mainly depends on the structure of the molecule. It is well known, that molecules containing aromatic functional groups are generally more stable under irradiation because of their ‘‘protection” effect. However, the Mylar-foil contains aromatic groups which provide relatively high radiation stability but it contains ethylene groups too, which can undergo bond scission can be responsible for some H-loss (see Fig. 4a). The short initial exponential part shows that the majority of the reactions causing H-loss take place during the first part of the irradiation period. The Kapton polymer contains almost only aromatic bonds, thus, it has one of the highest radiation stability among the polymers. Our data in Fig. 4b show a constant H-count rate distribution which means there is no irreversible process of homolytic scission of C–H bonds. The PDMS just the opposite type of polymer, it includes only single covalent bonds causing low radiation stability. It is clearly seen in Fig. 4c that the H loss is already significant at relatively low ion beam fluence. The distribution is fitted well with an exponential function which cor-
responds to a first order reaction kinetics. The chemical bonds in the Fe(III)TPP molecule are similar to the Kapton polymer’s one. The porphyrin molecule includes almost only aromatic bonds, so its radiation stability is also very high as it is shown in Fig. 4d. In overall, we can conclude that both data sets can be fitted by an exponential function – at least at the beginning of the damaging process. The results show a very similar trend than that of in [37] where an exponential model of the surface layer destruction was developed by Behrisch et al., This suggests, that in our case especially in case of PDMS and Mylar-foil a similar damaging mechanism occurs; but because of the different experimental conditions and chemical structure of the targets the damaging effect is not that significant than in [37].
4. Conclusions H-count rate as a function of the deposited ion beam fluence shows that within the deposited fluence range of 1014–1015 ions/ cm2 both the Kapton-foil and the Fe(III)TPP complex do not suffer from hydrogen depletion process indicating only minor chemical structural changes. There is a slight decrease of the hydrogen content of the Mylar-foil while a remarkable exponential loss of hydrogen occurs in case of PDMS suggesting the degradation of the polymer chain. The unique data processing used for these evaluations makes possible the determination of ion beam induced chemical degradations causing irreversible hydrogen depletion even following a routine ERDA measurement. The high radiation stability makes the porphyrin complex ideal as a calibration standard. On the other hand, the complex molecule includes an iron metal centre which can be an useful marker for an accurate RBS energy calibration. In addition, this standard can be prepared as thin layer on a substrate, thus, the accuracy of the ERDA spectrum evaluation can be increased, as well.
A. Simon et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 2197–2201
Acknowledgements Discussions with Dr. Edit Szilágyi (RMKI KFKI, Budapest, Hungary) is most appreciated. Authors are thankful to Dr. Attila Csik (HAS-ATOMKI, Debrecen, Hungary) for the profilometer measurements. The technical assistance of the Van de Graaff accelerator operating staff at HAS ATOMKI is gratefully acknowledged. This work was supported by the EU co-funded Economic Competitiveness Operative Programme (GVOP-3.2.1.-2004-04-0402/3.0) and the Hungarian Research Fund OTKA No. NK73424. References [1] Alexey Kondyurin, Marcela Bilek, Ion Beam Treatment of Polymers, Elsevier, 2007, ISBN-13:978-0-08-044692-9. [2] J. Davenas, A. Dunlop, F. Rullier-Albenque, C. Jaouen, C. Templier, Materials under irradiation, Solid State Phenomena 30–31 (Tram Tech, 1993). [3] E. Szilágyi, F. Pászti, V. Quillet, F. Abel, Nucl. Instr. Meth. B 85 (1994) 63. [4] G. Boudreault, R.G. Elliman, R. Grotzschel, S.C. Gujrathi, C. Jeynes, W.N. Lennard, E. Rauhala, T. Sajavaara, H. Timmers, Y.Q. Wang, Nucl. Instr. Meth. B B222 (2004) 547. [5] I. Bogdanovic Radovic, Z. Medunic, M. Jaksic, Z. Siketic, N. Skukan, Nucl. Instr. Meth. B 231 (2005) 123. [6] B. Lovelace, H. Bakhru, A.W. Haberl, R.E. Benenson, Nucl. Instr. Meth. B 261 (2007) 452. [7] J. Tirira, Y. Serruys, P. Trocellier, H. Hofsäss, Forward Recoil Spectrometry: Applications to Hydrogen Determination in Solids, Plenum Press, New York, 1996, ISBN: 0-306-45249-9 (Chapter 5). [8] P. Reichart, G. Dollinger, A. Bergmaier, G. Datzmann, A. Hauptner, H.-J. Körner, Nucl. Instr. Meth. B 197 (2002) 134. [9] A. Macková, V. Havránek, V. Svorcík, N. Djourelov, T. Suzuki, Nucl. Instr. Meth. B 240 (2005) 245. [10] D. Spemann, R. Deltschew, M. Lorenz, T. Butz, Nucl. Instr. Meth. B 219–220 (2004) 693. [11] F. Abel, V. Quillet, M. Schott, Nucl. Instr. Meth. B 105 (1995) 86. [12] O. Horváth, R. Huszánk, Zs. Valicsek, Gy. Lendvay, Coord. Chem. Rev. 250 (2006) 1792. [13] C.H.M. Marée, A. Kleinpenning, A.M. Vredenberg, F.H.P.M. Habraken, Nucl. Instr. Meth. B 118 (1996) 301.
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[14] S.D. Minteer, Microfluidic Techniques: Reviews and Protocols; Humana Press, New Jersey, 2006. [15] M. Fleger, A. Neyer, Microelectron. Eng. 83 (2006) 1291. [16] I. Rajta, I. Borbély-Kiss, Gy. Mórik, L. Bartha, E. Koltay, Á.Z. Kiss, Nucl. Instr. Meth. B 109–110 (1996) 148. [17] A. Simon, T. Csákó, C. Jeynes, T. Szörényi, Nucl. Instr. Meth. B 249 (2006) 454. [18] A. Simon, A. Sulyok, M. Novák, G. Juhász, T. Lohner, M. Fried, A. Barna, R. Huszank, M. Menyhárd, Nucl. Instr. Meth. B 267 (2009) 2212. [19] Gy. G. Kiss, T. Rauscher, Gy. Gyürky, A. Simon, Zs. Fülöp, E. Somorjai, Phys. Rev. Lett. 101 (2008) 19:1101(4). [20] Zs. Kertész, Z. Szikszai, I. Uzonyi, A. Simon, Á.Z. Kiss, Nucl. Instr. Meth. B 231 (2005) 106. [21] I. Gomez-Morilla, A. Simon, R. Simon, Á.Z. Kiss, G.W. Grime, Nucl. Instr. Meth. B 249 (2006) 897. [22] Zs. Kertész, Z. Szikszai, Z. Szoboszlai, A. Simon, R. Huszank, I. Uzonyi, Nucl. Instr. Meth. B 267 (2009) 2236. [23] R. Huszank, A. Simon, E. Szilágyi, K. Keresztessy, I. Kovács, Nucl. Instr. Meth. B 267 (2009) 2132. [24] M.B.H. Breese, J.P. Landsberg, P.J.C. King, G.W. Grime, F. Watt, Nucl. Instr. Meth. B 64 (1992) 505. [25] P.J.C. King, M.B.H. Breese, G.R. Booker, P.R. Wilshaw, J. Whitehurst, G.W. Grime, F. Watt, M.J. Goringe, Nucl. Instr. Meth. B 77 (1993) 320. [26] A. Simon, C. Jeynes, R.P. Webb, R. Finnis, Z. Tabatabaian, P.J. Sellin, M.B.H. Breese, D.F. Fellows, R. Van den Broek, R.M. Gwilliam, Nucl. Instr. Meth. B 219– 220 (2004) 405. [27] D. Beasley, N.M. Spyrou, J. Radioanal. Nucl. Chem. 264 (2) (2005) 289. [28] Zs. Kertész, Z. Szikszai, E. Gontier, P. Moretto, J.E. Surleve-Bazeille, I.B. Kiss, I. Juhász, J. Hunyadi, Á.Z. Kiss, Nucl. Instr. Meth. B 231 (2005) 280. [29] Zs. Kertész, Z. Szikszai, P. Pelicon, J. Simcic, A. Telek, T. Bíró, Ion beam microanalysis of human hair follicles, Nucl. Instr. Meth. B 260 (2007) 218. [30] A. Simon, G. Kalinka, Nucl. Instr. Meth. B 231 (2005) 507. [31] A. Simon, L. Balkay, G. Kalinka, A. Kerek, D. Novák, A. Sipos, L. Végh, L. Trón,, J. Molnár, Nucl. Instr. Meth. A 546 (2005) 33. [32] A. Simon, G. Kalinka, M. Jakšic´, Zˇ. Pastuovic´, M. Novák, Á.Z. Kiss, Nucl. Instr. Meth. B 260 (2007) 304. [33] R. Huszank, S.Z. Szilasi, K. Vad, I. Rajta, Nucl. Instr. Meth. B 267 (2009) 2299. [34] G.W. Grime, M. Dawson, Nucl. Instr. Meth. B 104 (1995) 107. [35] E. Kótai, Nucl. Instr. Meth. B 85 (1994) 588. [36] E. Szilágyi, F. Pászti, G. Amsel, Nucl. Instr. Meth. B 100 (1995) 103. [37] R. Behrisch, W. Von der Linden, U. Von Toussaint, D. Grambole, Nucl. Instr. Meth. B 155 (1999) 440.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Investigation of hydrogen depletion of organic materials upon ion beam irradiation by simultaneous micro-RBS and micro-ERDA techniques A. Simon *, R. Huszank, M. Novák, Z. Pintye Institute of Nuclear Research of the Hungarian Academy of Sciences, H-4001 Debrecen, P.O. Box 51, Hungary
a r t i c l e
i n f o
Article history: Available online 25 February 2010 Keywords: Polymer degradation Radiation stability Ion beam modification Focussed ion beam Hydrogen depletion Micro-RBS Micro-ERDA
a b s t r a c t Radiation stability investigations in a form of hydrogen depletion of organic materials induced by a focussed 1.6 MeV He+ scanning ion beam within a beam intensity range of 10–50 pA/lm2 is reported. The hydrogen depletion was measured by applying Elastic Recoil Detection Analysis (ERDA) and Rutherford Backscattering Spectrometry (RBS) techniques, simultaneously. H-count rates were measured and further processed as a function of the deposited ion beam fluence. Our results show, that within the deposited fluence range of 1014–1015 ions/cm2 both the Kapton-foil and porphyrin complex (Fe(III)TPP) do not suffer from hydrogen depletion process, indicating only minor chemical structural changes. There is a slight decrease of the hydrogen content of the Mylar-foil while a remarkable loss of hydrogen occurs in case of poly(dimethyl-siloxane) (PDMS) suggesting the degradation of the polymer chain. A full description of our versatile and complex particle detection system for nuclear microprobe applications is also given in the paper. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction The fundamental processes in ion beam interaction with polymers are of high interest and are being actively studied [1,2]. Ion irradiation of polymers is accompanied by radiation-induced effects changing the structure and properties of the irradiated material. The ion irradiation leads to the degradation of polymeric chains, chemical bond cleavage, forming of free radicals and release of gaseous degradation products. The type and degree of the degradation strongly depend on the chemical groups attached to the main chain of the polymer. These processes are also relevant to the development of several applications, such as ion beam lithography, improvement of polymer adhesion, polymer doping by ion implantation, etc. The industrial use of polymers ranges across a broad field of structural, mechanical, electrical and optical applications in microelectronics, opto-electronics and in medicine, as well. Moreover, polymer films, especially Mylar-foil and Kapton-foil, are widely used as H-standards in Elastic Recoil Detection Analysis (ERDA) [3–6]. However, the decomposition of polymer materials when irradiated with a milli- and microbeam also reported in several papers [7–11]. As we also experienced the ion beam damaging effect at least in the form of darkening of the polymers, we have
* Corresponding author. Address: Institute of Nuclear Research of the Hungarian Academy of Sciences (ATOMKI), H-4001, P.O. Box 51, Debrecen, Hungary. Tel.: +36 52 509 211; fax: +36 52 416 181. E-mail address: [email protected] (A. Simon). 0168-583X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2010.02.087
decided to investigate in more details the possible hydrogen depletion of Kapton and Mylar foils for focussed microbeam analyses. These polymers are mainly applied as H-standards but they are used e.g. as backing film for atmospheric aerosols, as well. The investigation also included another two organic materials which are applied widely both in research and industrial fields. Recently, the application of organic dyes, for example different porphyrin derivatives as light absorbing materials in organic solar cells has gained considerable interest as they exhibit very high absorption coefficients [12]. Furthermore, they can be functionalized to obtain specific optical or electrical properties utilised e.g. in solar cells [13]. Poly(dimethyl-siloxane) (PDMS) is a silicon based flexible polymer, base material for many applications nowadays. It has advantageous properties, such as low surface energy, high electrical resistance, constant and high ductility over a wide range of temperatures, etc. It has been widely used to fabricate Micro-ElectroMechanical Systems (MEMS), microfluidic devices [14], biosensors, biochips or creating micro-stamps [15]. 2. Experimental details 2.1. Materials used for the analyses In this paper we present the analyses of four kinds of organic materials (a) poly(ethylene terephthalate) (PET, Mylar, C10H8O4); (b) poly(4,40 -oxydiphenylene-pyromellitimide) (PI, Kapton, C22H10N2O5) both from DuPont; (c) poly(dimethyl-siloxane) (PDMS,
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A. Simon et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 2197–2201
Ion beam
Optical microscope
Sample De t. C
BNC adaptors
et .B D
BNC adaptors
Det. A
Faraday-cup
Det.E Det. D
±180°
Fig. 1. Layout of the Atomki nuclear microprobe chamber for simultaneous charged particle detection. Dets. A-D: Ortec ULTRA™ Ion-Implanted Si detectors, Det. E: Hamamatsu S5821 Si PIN diode. The rotatable detector holder at transmission geometry also includes a Faraday-cup used for low beam current measurements and settings.
SiOC2H6) from Dow Corning as Sylgard 184 elastomer kit and (d) Fe(III) meso-tetraphenyl porphyrin (Fe(III)TPP, C44H28ClFeN4) from Aldrich. PDMS polymer samples were prepared by mixing the liquid silicone pre-polymer and the curing agent in the recommended ratio of 10:1. Then the pre-polymer was cured in a glass Petri-dish for 30 min. at a temperature of 125 °C and was left to finish curing overnight at a room temperature. The porphyrin thin film was spin-coated from a solution of 49.4 mg of Fe(III)TPP in 5 ml of chlorophorm (CH3Cl) onto a silicon substrate at 1000 rpm for 30 s. After the spin-coating the film was allowed to dry for overnight at room temperature in a desiccator. The uniformity of the samples was checked with an AMBIOS XP-I type profilometer before the IBA analyses and it was found to be uniform enough within the range of the scan size of the measurement. 2.2. Experimental setup Rutherford Backscattering Spectrometry (RBS) and Elastic Recoil Detection Analysis (ERDA) were implemented simultaneously with a focussed 1600 keV He+ ion beam on the Oxfordtype nuclear microprobe facility at HAS-ATOMKI, Debrecen, Hungary [16]. The beam was focussed to a 2 2 lm2 spot while the scan size was set to 500 500 lm2 – except for the Mylar-foil where 625 625 lm2 area was scanned. Data were collected in 256 256 pixel and the scanning speed was set to 10 ls/tick. The sample chamber is equipped with a High Precision Vacuum Generators
5-axes goniometer, which makes available both the high depthresolution micro-RBS and micro-ERDA measurements. The applied tilt angle was 80°, thus, the effective scan size was 2879 500 lm2 (3599 625 lm2 for the Mylar-foil). For the calculation of the total deposited fluence the effective scan size was taken. The layout of the microbeam chamber for particle detection measurements is shown in Fig. 1. Dets. A-D: Ortec ULTRA™ IonImplanted Si detectors (50 mm2 sensitive area and 18 keV system energy resolution). Dets. A&B are mainly used for micro-RBS measurements [17–19]. Using Det. A simultaneous Particle Induced X-ray Emission (micro-PIXE) and micro-RBS analysis can be implemented [20,21] even combined with micro-ERDA [22,23], too. Detectors setup: Det. A: H = 165° at Cornell geometry, O = 27.1 msrad; Det. B: H = 135° at IBM-geometry, O = 55.8 msrad. Det. C is used for ERDA: H = 30° at IBM-geometry, O = 15.1 msrad There is a 1.1 mm wide aperture on its front surface covered with a 9 lm thick Mylar-foil. There is a rotatable detector holder at transmission geometry equipped with Dets. D&E. A similar rotatable detector holder was used before at the University of Oxford for Scanning Transmission Ion Microscopy (STIM) [24], a Transmission Channelling version of STIM [25] is still in use at University of Surrey Ion Beam Centre [26,27]. In our case Det. E is a Hamamatsu S5821 Si PIN diode. A carbon Faraday-cup is also installed onto this rotatable flange and especially used for low beam current measurements and beam settings. Beside STIM [28,29] we widely use this rotatable detector holder for Ion Beam Induced Current (IBIC) investigations [30–32] and for Proton Beam Writing [33].
A. Simon et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 2197–2201
2.3. Spectrum and data evaluation Data were collected by the OM_DAQ 2004 system [34] in listmode. Micro-RBS and micro-ERDA spectra were evaluated with the RBX computer code [35] version 5.17. The RBX code was developed to evaluate precisely RBS and channelling measurements of simple 1D structured samples, including all the main energy spread contributions calculated by DEPTH [36]. To get the hydrogen-loss data the listmode files were further processed. At first, the RBS and ERDA count numbers were extracted from each data blocks together with the measurement parameters like charge and elapsed time. Then, the count rates of the ERDA and RBS signals were calculated using the time-stamps of each data blocks. The charge was collected on the sample but absolute total deposited charge values were determined by fitting the RBS spectra. In order to be independent of any possible beam current fluctuation the accumulated charge rate distribution was also calculated using the time-stamps in each data blocks. Finally, the calculated ERDA and RBS count rate distributions were normalised using the calculated charge rate distributions.
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or activated by the ion beam-, can be dislocated or leave the sample completely. As stated already in Section 1, the damage increases if micrometer sized beams with high areal beam densities are used for ion beam analysis. A sensitive analysis of concentration profiles becomes impossible if the profile itself is altered by the irradiation before sufficient amount of data is collected. Both displacement collisions induced by nuclear stopping and electronic stopping effects, like excitation and ionization play role in the damage effects and have to be considered. We report the micro-RBS and micro-ERDA spectra of Fe(III)TPP and PDMS here. The surface peaks of the constituting elements and spectra fits are shown in Figs. 2 and 3. We think that it is very useful that the investigated porphyrin molecule includes an iron metal centre which can be a marker element together with the low energy constituents for an accurate RBS energy calibration. In addition, this standard can be prepared as a thin layer on a substrate, thus, the accuracy of the ERDA spectrum evaluation can be increased, as well. All in all, this material can be ideal as an ERDA calibration standard.
3.2. Hydrogen count rate distributions 3. Results and discussion 3.1. Micro-RBS and micro-ERDA spectra Each ion beam analytical techniques inhere more or less the modification of the original sample. As a result, atoms -scattered
Under ion beam radiation, beside the different physical effects, complex chemical processes can take place in organic materials. For initial steps, loss of the kinetic energy of the penetrating ions occurs by inelastic collisions, resulting in ionization and excitation of the target material. The excited-state molecules may return to
Fig. 2. Micro-RBS spectrum (left) and micro-ERDA spectrum (right) of a PDMS sample. Measurement parameters:1.6 MeV He+ ion beam, 2 2 lm2 beam spot, 2879 500 lm2 scan area, RBS: H = 135°, ERDA: H = 30° tilt angle: 80°, total deposited charge: 1.1 lC.
Fig. 3. Micro-RBS spectrum (left) and micro-ERDA spectrum (right) of a 81 nm thick Fe(III)TPP layer on a Si substrate. Measurement parameters:1.6 MeV He+ ion beam, 2 2 lm2 beam spot, 2879 500 lm2 scan area, RBS: H = 135°, ERDA: H = 30° tilt angle: 80°, total deposited charge: 0.78 lC.
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Fig. 4. Hydrogen count rate distributions as a function of the Deposited Ion Beam Fluence. For the detailed calculation method please see text in Section 2.3. Both the Kaptonfoil and the Fe(III)TPP complex do not suffer from hydrogen depletion process indicating only minor chemical structural changes. There is a slight decrease of the hydrogen content of the Mylar-foil while a remarkable exponential loss of hydrogen occurs in case of PDMS suggesting the degradation of the polymer chain. The residual of the fit is also shown in the graphs.
the ground state through radiationless decay or undergo homolytic dissociation reactions to form free radicals. The free radicals are very reactive and cause a number of chemical reactions in polymers. In order to determine the radiation stability of the H content of the investigated organic materials, H-count rates were extracted from the listmode files. The graphs in Fig. 4 show such distributions. The H-count rate is in a linear correlation with the H concentrations. Please note, that a constant H-rate distribution means same detected count numbers in each time interval. i.e. a constant hydrogen content. The main advantage of our method is that with this unique data processing, the ion beam induced chemical degradations causing irreversible hydrogen depletion can be determined even after a routine ERDA measurement, as well. The hydrogen loss takes place by an irreversible process of homolytic scission of C–H bonds. The degree of the irreversibility mainly depends on the structure of the molecule. It is well known, that molecules containing aromatic functional groups are generally more stable under irradiation because of their ‘‘protection” effect. However, the Mylar-foil contains aromatic groups which provide relatively high radiation stability but it contains ethylene groups too, which can undergo bond scission can be responsible for some H-loss (see Fig. 4a). The short initial exponential part shows that the majority of the reactions causing H-loss take place during the first part of the irradiation period. The Kapton polymer contains almost only aromatic bonds, thus, it has one of the highest radiation stability among the polymers. Our data in Fig. 4b show a constant H-count rate distribution which means there is no irreversible process of homolytic scission of C–H bonds. The PDMS just the opposite type of polymer, it includes only single covalent bonds causing low radiation stability. It is clearly seen in Fig. 4c that the H loss is already significant at relatively low ion beam fluence. The distribution is fitted well with an exponential function which cor-
responds to a first order reaction kinetics. The chemical bonds in the Fe(III)TPP molecule are similar to the Kapton polymer’s one. The porphyrin molecule includes almost only aromatic bonds, so its radiation stability is also very high as it is shown in Fig. 4d. In overall, we can conclude that both data sets can be fitted by an exponential function – at least at the beginning of the damaging process. The results show a very similar trend than that of in [37] where an exponential model of the surface layer destruction was developed by Behrisch et al., This suggests, that in our case especially in case of PDMS and Mylar-foil a similar damaging mechanism occurs; but because of the different experimental conditions and chemical structure of the targets the damaging effect is not that significant than in [37].
4. Conclusions H-count rate as a function of the deposited ion beam fluence shows that within the deposited fluence range of 1014–1015 ions/ cm2 both the Kapton-foil and the Fe(III)TPP complex do not suffer from hydrogen depletion process indicating only minor chemical structural changes. There is a slight decrease of the hydrogen content of the Mylar-foil while a remarkable exponential loss of hydrogen occurs in case of PDMS suggesting the degradation of the polymer chain. The unique data processing used for these evaluations makes possible the determination of ion beam induced chemical degradations causing irreversible hydrogen depletion even following a routine ERDA measurement. The high radiation stability makes the porphyrin complex ideal as a calibration standard. On the other hand, the complex molecule includes an iron metal centre which can be an useful marker for an accurate RBS energy calibration. In addition, this standard can be prepared as thin layer on a substrate, thus, the accuracy of the ERDA spectrum evaluation can be increased, as well.
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