Quantification of the H Content in Diamondlike Carbon and Polymeric Thin Films by Reflection Electron Energy Loss Spectroscopy

Quantification of the H content in diamondlike carbon and polymeric thin films by reflection electron energy loss spectroscopy F. Yubero,a兲V. J. Rico, J. P. Espinós, J. Cotrino, and A. R. González-Elipe ICMSE (CSIC-USE) Américo Vespucio s/n, E-41092 Sevilla, Spain 共Received 24 March 2005; accepted 28 June 2005; published online 15 August 2005兲 A nondestructive method to determine the hydrogen content at the surface of diamondlike carbon and polymeric thin films is proposed. The method relies on the analysis of the elastic peak produced by backscattering of electrons from the hydrogen atoms present at the sample surface. Quantitative analysis of the H content at the surface is achieved through use of a phenomenological sensitivity factor for elastic electron backscattering by H atoms with respect to other atoms present at the surface of reference polymeric samples. The validity of the method is checked with elastic recoil detection measurements and infrared spectroscopy analysis of the same samples. The accuracy of the method in the determination of H content at the sample surface is estimated to be ±10%. © 2005 American Institute of Physics.关DOI: 10.1063/1.2011786兴 Quantification of the hydrogen content in diamondlike carbon 共DLC兲and polymeric thin films is of the most importance because their properties are tightly linked to this content.1Typically the hydrogen content in this type of materials has been determined with techniques such as elastic recoil detection analysis 共ERDA兲, nuclear reaction analysis 共NRA兲using the 1H共15N, ␣ 兲12C nuclear reaction, nuclear magnetic resonance 共NMR兲, thermal hydrogen evolution, or infrared spectroscopy 共IR兲. Conscious of the great demand of alternative and simple methods,1–3 we have developed a procedure to quantify the hydrogen content at the surface of DLC and polymeric thin films. As it will be shown in this paper, the H content within the surface region of these type of materials can be determined by looking at the elastically backscattered electrons with kinetic energies in the order of 1 to 2 keV as in standard reflection electron energy-loss spectroscopy 共REELS兲analysis. The basis of the quantification is the same as that of Rutherford backscattering spectroscopy 共RBS兲, with the difference that in RBS the probe particles are usually alpha particles and in our method electrons 共note the difference of mass between these two projectiles兲. In this respect, it must be mentioned that very recently Orosz et al.4 have reported that H atoms in polyethylene can be detected by this technique. It is also worth mentioning that REELS was used in the past to detect H induced chemical shifts in the plasmon excitations.5 The presence of H can be easily detected by standard REELS measurements with 1 to 2 keV electron beams. The physical basis for the detection of H atoms at the surface of materials containing light elements is the expected difference in the recoil energy ⌬Er 6in the interaction of a fast electron with a static nucleus, either hydrogen or another type of atom, which can be estimated by the expression ⌬Er=4me MH E0sin2 ␪ 2 冉 1− 1 A 冊 ,共1兲 where me,MH, and Aare the electron mass, the atomic weight of the H atom and the other nucleus considered, respectively, ␪ is the scattering angle, and E0is the primary energy of the incoming electrons. This expression predicts that, for a typical scattering angle of ␪ =120° and a primary energy of 1500 eV, the difference in recoil energies between H and other atoms heavier than C is ⬃2.1 eV, so that we might easily distinguish between H and these other atoms present in the sample with the standard energy resolution of REELS measurements 共i.e., few tenths of eV兲. Figure 1 shows the REELS spectra, taken with a primary electron beam of 1500 eV, for several reference samples 关polycarbonate 共PC, -C16H14O3-兲, polystyrene 共PS, -C8H8-兲, polymethyl-methacrylate 共PMMA, -C5H8O2-兲, and polytetrafluorethylene 共PTFE, -CF2-兲and highly oriented pyrolitic graphite 共HOPG, C兲兴. The REELS measurements were performed with special care to minimize electron damage at the surface of the samples. Thus, typically the measuring time was of ⬃1 min. with electron beam currents below 1 ␮ A/cm2.7The angle of incidence of the electron beam with respect to the surface normal was 60° 共i.e., ␪ =120°兲and the electron detection was set normal to the sample surface. In the elastic region of these spectra 共between ⬃1497 and 1502 eV兲the H signal can be clearly identified as a weak peak at 2.1 eV lower kinetic energy than the more intense feature in the spectra at 1500 eV. This later peak is due to the electrons backscattered by the atoms other than H present at the surface of the samples. Note that the small peak at 2.1 eV is only present in the H containing polymers 共i.e., PS, PMMA, and PC兲and is not detected in the case of PTFE or HOPG, as expected. This peak shifts to 3.0 eV lower kinetic energy than the main peak when the excitation energy is changed to 2000 eV 共not shown兲. This result confirms its assignment as electrons backscattered by the H atoms in the samples. At this point it is worth mentioning that using primary beam energies larger than ⬃1800 eV has the effect of increasing the background between the two contributions to the elastic peak probably due to multiple elastic scattering and damage. On the other hand, the two contributions to the elastic peak do not separate enough to facilitate quantification when primary electron energies smaller than ⬃1200 eV are considered. Besides the small feature at ⬃2.1 eV from the main elastic peak for E0=1500 eV, these spectra are characterized by the typical ␲ plasmon features at ⬃6eV energy loss, corresponding to the benzoic rings in PC and a兲Author to whom correspondence should be addressed; electronic mail: [email protected] APPLIED PHYSICS LETTERS 87, 084101 共2005兲 0003-6951/2005/87共8兲/084101/3/$22.50 © 2005 American Institute of Physics87, 084101-1 12 June 2025 14:52:16 PS, and to the sp2character of graphite. Besides, the corresponding ␴ + ␲ bulk plasmons are indicated for each material. All these reference samples were also analyzed by XPS to confirm the level of surface contamination. It was found that a 5%–10% of O contamination was present at the surface of polymers that do not contain oxygen in their structures. A similar deviation from the theoretical oxygen content was found in those materials such as PC or PMMA that contain this element in their structures. From the relative intensity of the H peak with respect to the main feature of the spectra, it is possible to estimate a phenomenological sensitivity factor ␴ exp that, in the form of a calibration curve, serves to correct for the different cross sections for elastic electron backscattering by H or other atoms 共C and, to a minor extent, O atoms兲present at the surface of the polymers. The inset in Fig. 1 shows the correlation between the intensity ratio IH/Ino_H, and the nominal ratio between H and other type of atoms 共C and or O兲present in the polymers considered in this study. Here IHis the area of the H signal in the backreflected electrons and Ino_His the area of the elastic peak coming from atoms other than H. It can be observed that there exist a linear correlation within the experimental uncertainties between the nominal atomic ratio and the experimental intensity ratio. In this analysis, we did not attempt quantification of high-H content polymers such as polypropylene 共-C3H6-兲or polyethylene 共-CH2-兲because they are known to degrade easily with release of hydrogen when they are bombarded with low-energy electrons.7By performing a linear regression passing through the origin of coordinates with the other points in the graph, the slope of the fitted line is 37. This value can be considered as a phenomenological relative cross section ␴ exp between the two contributions to the elastic peak signal. Note that the value obtained for ␴ exp is in reasonable agreement with the predictions of the NIST database for elastic scattering cross sections8共neglecting the presence of oxygen atoms at the surface and multiple elastic scattering, that may be significant in this case9兲. A rough theoretical estimation of ␴ exp is 42, obtained as the ratio of the differential elastic scattering cross section with respect to the solid angle between C and H for 120° scattering angle and 1500 eV electron energy.8In the following we will used our phenomenological ␴ exp=37 to quantify the H content in DLC samples. Thus, the H quantification at the surface of unknown samples by REELS can be performed according to the expression %H= 100 IH ␴ exp IH ␴ exp +Ino_H ,共2兲 where the meaning of the different magnitudes has been introduced previously. To check the validity of this procedure, a first comparative study was carried out by ERDA/RBS and REELS for a series of DLC samples prepared with fixed bias voltage and different content of H in the plasma gas. The DLC thin films were prepared by plasma Enhanced Chemical Vapor Deposition in a rf plasma reactor with a plate configuration. Mixtures of C2H2, Ar, and H2were used as the plasma gas, while a self-induced bias voltage was applied to the plate electrode containing the substrate of the samples. It was found that the percentage of H atoms in the DLC films measured by both techniques 共not shown兲was the same within 10%, so that, with the uncertainty limits of these techniques, they yield similar values for the H content of the film. This proves that REELS can be an alternative technique for the determination of H content at the surface in DLC films. To check further the validity of the method, the hydrogen content has been also determined for a series of DLC thin films prepared with a standard plasma composition by varying the bias voltage applied to the substrate plate. Bias voltage is recognized as a critical parameter for the control of the hardness and other properties of DLC thin films.10 For this series of samples, the percentage of hydrogen in the films has been plotted in Fig. 2 against the bias voltage. For comparison, this figure also includes the evolution of the intensity of the 2750–3050 cm−1 IR band, corresponding to the stretching C-H vibrations, normalized to the film thickness as determined by x-ray fluorescence spectroscopy in a scanning electron microscope. It is interesting that, within the error bars in both preparation and quantification protoFIG. 1. REELS spectra measured with 1500 eV kinetic energy of PTFE, graphite 共G兲, PC, PS, and PMMA. Inset: Correlation between the theoretical H content in the reference polymer samples and the ratio IH/Ino_Has determined by REELS measurements. FIG. 2. Correlation between the normalized intensity of the infrared C-H stretching modes, acceleration voltage during preparation and the atomic percentage of H in the film as determined by quantification of the elastic signal in REELS data. Full circles: Infrared quantification; open squares: REELS quantification. 084101-2 Yubero et al. Appl. Phys. Lett. 87, 084101 共2005兲 12 June 2025 14:52:16 cols, these two magnitudes follow similar tendencies. This evolution is characterized by a progressive decrease in the hydrogen content as the bias voltage increases. This correlation shows that, despite the surface character of REELS, its quantification data for hydrogen can be taken as roughly representative of bulk composition. To conclude, the present results have shown that REELS may provide a way of quantifying the amount of hydrogen present at the surface of DLC or polymeric materials where C and H are the majority constituent elements. The surface character of REELS is controlled by the mean free path of electrons that travel through the analyzed materials that, for the usual electron energies utilized in this technique, is ⬃3 nm. We want to emphasize that this analysis procedure measures directly the amount of hydrogen by a surface sensitive technique. It could thus be useful to combine REELS with XPS for surface analysis since XPS does not directly detect H. We thank the Spanish Ministry of Science and Education 共MAT2004-01558兲for financial support. 1J. Robertson, Mater. Sci. Eng., R. 37, 129 共2002兲. 2W. Jacob and M. Unger, Appl. Phys. Lett. 68, 475 共1996兲. 3J. Ristein, R. T. Stief, L. Ley, W. Beyer, J. Appl. Phys. 84, 3836 共1998兲. 4G. T. Orosz, G. Gergely, M. Menyhard, J. Toth, D. Varga, B. Lesiak, A. Jablonski, Surf. Sci. 566-568, 544 共2004兲. 5G. Gergely, Prog. Surf. Sci. 71,31共2002兲. 6A. J. Bevolo, J. Vac. Sci. Technol. A 3, 1312 共1985兲,and references there in. 7D. R. Baer, D. J. Gaspar, M. H. Engelhard, A. S. Lea, Beam Effects During AES and XPS Analysis in Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, edited by D. Briggs and J. T. Grant 共IM Publications and Surface Spectra Limited, UK, 2003兲,Chap.9. 8A. Jablonski, F. Salvat, C. J. Powell, NIST Electron Scattering Crosssection databaseVer. 3.0. National Institute of Standards and Technology, Gaithersburg, MD 共2002兲. 9A. Jablonski and C. J. Powell, Surf. Sci. 551, 106 共2004兲. 10A. R. González-Elipe, F. Yubero, and J. M. Sanz, Low Energy Ion Assisted Film Growth 共Imperial College Press, UK, 2003兲,Chap.5. 084101-3 Yubero et al. Appl. Phys. Lett. 87, 084101 共2005兲 12 June 2025 14:52:16