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Electronic sputtering from HOPG: A study of angular dependence
Nuclear Instruments and Methods in Physics Research B 212 (2003) 402–406 www.elsevier.com/locate/nimb
Electronic sputtering from HOPG: A study of angular dependence A. Tripathi a,g,*, S.A. Khan a, S.K. Srivastava a, M. Kumar b, S. Kumar c, S.V.S.N. Rao d, G.B.V.S. Lakshmi d, Azhar. M. Siddiqui a, N. Bajwa e, H.S. Nagaraja a, V.K. Mittal f, A. Szokefalvi g, M. Kurth g, A.C. Pandey b, D.K. Avasthi a, H.D. Carstanjen g a
Nuclear Science Centre, Aruna Asaf Ali Marg, P.O. Box 10502, New Delhi 110 067, India b University of Allahabad, Allahabad 211002, India c RBS college, Agra 282002, India d University of Hyderabad, Hyderabad 500 046, India e Panjab University, Chandigarh 160 014, India f Punjabi University, Patiala 147 002, India g Max Planck Institute, Heisenbergstrasse1, Stuttgart 70569, Germany
Abstract The angular distribution of the sputtering yield from highly oriented pyrolytic graphite sample irradiated with a 130 MeV Ag beam is studied. The beam was incident perpendicular to the sample and the sputtered carbon was collected on Si catcher foils which were studied using a high resolution ERDA set up. An anisotropic distribution of sputtering is observed with a distribution C ¼ A cos1:3 H þ B expððH 53Þ2 =r2 ) which shows that a peak lies at around 53 on a distribution which otherwise is a over-cosine function. The maximum sputtering yield is observed at 53, falling rapidly to almost zero at 90, with an average sputter yield of 5.5 · 105 atoms/ion. It is suggested that this anisotropy may be due to the crystal structure and formation of a pressure pulse. 2003 Elsevier B.V. All rights reserved. PACS: 79.20; 6180.Jh Keywords: Electronic sputtering; Angular distribution; Swift heavy ions; ERDA; HOPG
1. Introduction The movement of energetic heavy ion affects the lattice drastically due to a large energy deposition resulting in breaking of atomic bonds [1,2], for-
*
Corresponding author. E-mail address: [email protected] (A. Tripathi).
mation of latent tracks [3,4] and ejection of atoms/ molecules [5,6]. At higher energies with velocities comparable to electron Bohr velocity (2.2 · 106 m/s), swift heavy ions pass through a solid creating a highly dense energized nanometer cylindrical zone of intense electronic excitation along their trajectory on a picosecond time scale. The energy transfer at these energies is dominated by inelastic excitation of electrons, called electronic energy loss
0168-583X/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0168-583X(03)01742-7
A. Tripathi et al. / Nucl. Instr. and Meth. in Phys. Res. B 212 (2003) 402–406
as opposed to energy transfer by elastic collision with lattice atoms called nuclear energy loss. The mechanism of transfer of the electronic energy to the kinetic energy of lattice atoms is not very clearly understood, though models based on inelastic thermal spike [7] and a combination of Coulomb explosion and thermal spike [8] are used to explain the phenomena. At lower energies (<100 keV/u) the erosion of atomic and molecular species is referred as nuclear sputtering and this nuclear energy loss induced process is well understood in the framework of SigmundÕs theory [9]. The electronic energy loss induced erosion/ejection of material is referred as electronic sputtering of material. It has been shown that the sputtering yield increases at higher energies, specially in the case of insulators [10] where electronic energy loss process becomes dominant beyond a given threshold. Since the results were contrary to the prediction by Sigmund theory, the electronic sputtering process attracted the attention of the researchers. These studies are expected to provide a deep insight into the ion matter interaction besides having a large potential for applications. The sputtering from various carbon allotropes has earlier been studied by Behrisch et al. [11] and Pawlak et al. [12]. The structural dependence of the sputtering yield from hydrogenated amorphous carbon films [13,14] and the effect of film thickness on sputtering from fullerene film [15] and an Au thin film [16] has earlier been studied. It should be noted that most of the studies have studied the total yield, assuming the yield to be isotropic in all the directions. Toulemonde et al. recently studied the angular distribution of the sputtering yield from insulating samples such as LiF and SiO2 [17]. In the present work, we report the angular distribution of the sputtering yield from semi-mettalic highly oriented pyrolytic graphite (HOPG) sample.
2. Experimental The samples were irradiated with a 130 MeV Ag9þ beam from the 15 MV Pelletron [18] at the Nuclear Science Centre, New Delhi. The beam was incident perpendicular to the sample and the cur-
403
Fig. 1. The experimental arrangement.
rent was measured from the sample frame. The sample frame was surrounded by a secondary electron suppressor having a voltage of )120 V. Sputtered carbon atoms were collected on 10 mm · 5 mm Si catcher foils which were kept at a distance of 5 cm from the samples. The catcher foils were mounted on the secondary electron suppressor in such a way, so that all the catchers were equidistant in the horizontal plane. These were mounted at angles of 10–90 from the beam direction. The arrangement for mounting the catcher foils is shown in Fig. 1. The incident current was 0.6 particle nanoamperes (1 particle nA ¼ 6.2 · 109 ) ions/s) and the sample was irradiated for a fluence of 3.6 · 1013 ions/cm2 to collect sufficient carbon on small catchers for ERD analysis. Since the exposure time was long, the beam was scanned over an area of 5 mm · 10 mm to avoid any damage to the sample. It should be noted that both the catchers and scanned area was kept 5 mm in horizontal direction to reduce the error in angle measurement. Each catcher subtends an angle of 6, with an error of ±3 in the angular measurement, and an equal contribution is expected from the error due to scanning. After the irradiation was complete, a 20 nm Al layer was deposited on the sample to avoid the collected carbon getting sputtered during the subsequent ERDA study [19]. A dummy was also placed in the chamber so that we can measure only the sputtered carbon, as contributions from hydrocarbons and other contaminants present in the chamber are also expected. The dummy was
404
A. Tripathi et al. / Nucl. Instr. and Meth. in Phys. Res. B 212 (2003) 402–406
placed in the chamber on the outer wall of the suppressor so that it does not see the sputtered atoms. This catcher went through all the processes, including the Al layer deposition and the carbon collected on this dummy was measured to find the carbon contribution from various contaminants. This contribution was subtracted while calculating the actual quantity of sputtered carbon. The carbon collected on the catcher foils is analysed using the ERDA technique using the 1.3 MeV Arþ beam from the 6 MV Pelletron at the Max Planck Institute, Germany. Since the deposited carbon layer is of the order of a few nanometers, the analysis of the sputtered carbon was done using a high resolution spectrometer [20] set up. The sample was kept at an angle of 20 to the beam direction and the recoils were detected at an angle of 40. The recoils were detected using the time of flight (TOF) set up. The stop signal was obtained by the beam chopper providing a pulsed beam (width 60 ns, repetition rate 2 ls) and the start signal was obtained using the position sensitive micro-channel plate. The spectrometer has an energy acceptance window of 2.9% and hence the complete depth profile was obtained in two or more settings. The complete spectra was then obtained by combining the individual spectra and generating complete spectra after removing the overlapping energies. To obtain the absolute quantity of collected carbon a graphite sample was also used for normalization. The spectra were collected for the same incident charge and the recoil spectra recorded.
3. Results and discussion The recoils with different m=q values are displayed using two dimensional spectra showing TOF versus energy (E) plots. The carbon recoils with 2þ charge state are identified by the energy of the recoils from the surface and by comparing it with the spectra of the graphite sample, which has carbon recoils with same surface energy. The two dimensional TOF–E spectra corresponding to carbon was then projected onto the E axis to generate one dimensional recoil energy spectra of carbon. The ERD spectra for the 10 catcher foils
Fig. 2. ERD spectra up to a depth of 20 nm for five catcher foils.
was recorded. For clarity, we have shown the spectra for only five catchers at alternate angles in Fig. 2. The peak in the spectra shows the sputtered carbon. The spectra do not show a sharp edge towards the surface, which is due to the fact that the high energy resolution possible with the spectrometer has deteriorated due to 20 nm thick Al layer deposited on top of the sputtered carbon layer. Similarly the lower energy of the spectra has a long tail. The possibility of a large carbon impurity in the Si catcher was ruled out by studying a virgin Si catcher wafer which had almost negligible carbon content. We assume the long tail on the lower energy side to be due to double/multiple scattering effects. This tail may also be due to the increased roughness of catcher surface due to deposited carbon. We were able to see this profile due to excellent depth resolution from the spectrometer and we have restricted the measurements of carbon near the surface with recoils with energies between 435 and 455 keV, representing a depth of approximately 20 nm. All the samples show a carbon content which is significantly higher than the carbon content of the dummy. The total carbon sputtered is calculated for each of the catcher foils from the counts between the energies 435 and 455 keV in the spectra and the same is plotted in Fig. 3. ERD of HOPG sample is also done for normalizing and calculating the actual carbon content. As is clear from the figure, the
A. Tripathi et al. / Nucl. Instr. and Meth. in Phys. Res. B 212 (2003) 402–406
Fig. 3. The areal density of carbon collected on all the ten catcher foils, which represents the angular distribution of the sputtering yield. The v2 and R2 values for the fit are 0.08 and 0.99 respectively.
measured quantity of the sputtered carbon does not vary much for the 10, 20 and 30 catchers with a total content of approximately 4.2 · 1016 atoms/cm2 for the 10 and 20 catchers. There is a rise in the carbon content in the next two wafers with the 50 catcher showing a maximum content of 5.6 · 1016 atoms/cm2 , after which it goes down slightly in the 60 catcher. The catchers at 70, 80 and 90 show a sudden decrease with the content going down to 0.2 · 1016 atoms/cm2 , a content even lower than the catchers at 10. The dummy is found to have carbon content of 4.3 · 1016 atoms/ cm2 , which is subtracted from all the catchers while calculating the concentration mentioned above. We have calculated the total sputtering yield from the measured quantity of collected carbon on the catchers. Since the sputtering yield is angle dependent, the value of total carbon collected by all the catcher foils is calculated by taking a weighted addition. The weight given to carbon for a given foil is proportional to the cosine of the angle of the measurement and the total quantity of sputtered carbon is extrapolated from the solid angle of 0.05 sr subtended by each catcher. Since our measurements are restricted to only one azimuthal plane we have assumed the distribution to be isotropic in vertical plane for calculating the total yield. For the total incident beam fluence of
405
3.8 · 1013 ions/cm2 , the average sputter rate of 5.5 · 103 atoms/ion is obtained. This value is higher than the value obtained by Ghosh [21] for a 200 MeV Ag beam with an Se value 21.7 keV/nm for graphite and an average sputter yield of 1 · 103 atoms/ion. With an electronic energy sputter yield dependence of Y / Se1:7 [21], we would expect a yield of 5 · 102 atoms/ion for 130 MeV Ag beam with an Se value of 14.6 keV/nm. The observed increase in sputtering yield is expected to be due to the large ion fluence as compared to the earlier study. In the present study, the fluence is kept at 3.8 · 1013 ions/cm2 , which is more than the track overlap fluence and we expect that amorphisation of surface may be responsible for the high sputtering yield, which is closer to the sputtering yield value for amorphised carbon [13,21]. The measured carbon content for all the catchers is shown in Fig. 3. The analysis of the distribution shows an anisotropic distribution with moderate sputtering at smaller angles. However a maximum is observed around 53 to the beam direction, after which the sputtering yield goes rapidly down to almost zero at 90. The spectra is fitted to a distribution C ¼ A cos1:3 H þ 2 B expððH 53Þ =r2 Þ, with a chi square value of 0.07, which shows that a Gaussian peak lies at around 53 on a distribution which otherwise has a over-cosine distribution. A similar jet like anisotropy is observed by Toulemonde et al. in swift heavy ion induced sputtering of insulating LiF sample. The inelastic thermal spike model [22] explains this anisotropy in LiF is as a result of hydrodynamic process driven by the pressure induced by the vapour phase formed in the bulk due to a thermal spike. It is envisaged that initially a gas phase induces a radial pressure in the track core, which is released by a jet of atoms from the impact area giving an angular anisotropy. At a later stage the evaporation from the heated track zone becomes dominant giving an isotropic distribution. Toulemonde et al. have suggested that the beam incidence strongly influences the geometry of the impact zone and track formation and non-normal incidence adds to the anisotropy as the radial pressure releases towards the surface. The main difference in our observation lies in the fact we have observed the anisotropic peak at 53 in case of
406
A. Tripathi et al. / Nucl. Instr. and Meth. in Phys. Res. B 212 (2003) 402–406
HOPG though it is observed at 0 in case of LiF [15]. It should be emphasised that the calculations based on sum of impulses by Johnson et al. predict a peak at 45 for the normal incidence [23] and observed by Ens et al. [24]. We feel that the structure of the crystal, in the initial stage of irradiation, might also play an important role in the jet like component. The LiF has a fcc structure and it is expected that the pressure pulse propagates along the axis, specially in case of non-normal incidence. Since we have perpendicular beam incidence, the ejected atoms are more likely to travel along the axis at 60 to the surface normal for the hcp structure of HOPG with [0 0 0 1] surface. 4. Conclusion We have studied the angular distribution of the sputtering yield from semiconducting HOPG sample irradiated with a 130 MeV Ag beam. The study is made for perpendicular incident beam with the sputtered carbon being collected on Si catcher foils. The catcher foils were studied using high resolution ERDA set up. The main result of the study is that the sputtering yield has an anisotropic distribution. There is only a moderate sputtering yield at smaller angles and a maxima at around 53 after which the sputtering yield goes rapidly down to almost zero at 90. The average sputtering yield is found to be 5.5 · 105 atoms/ion. The anisotropy is explained on the basis of pressure pulse model and the crystal structure of HOPG.
References [1] J. Provost, Ch. Simon, M. Hervieu, D. Groult, V. Hardy, F. Studer, M. Toulemonde, MRS Bull. 22 (1995) 22.
[2] H. Dammak, A. Dunlop, D. Lesuer, A. Brunnelle, S. Della-Negra, Y.L. Beyec, Phys. Rev. Lett. 74 (1995) 1135. [3] R.E. Johnson, B.U.R. Sundquist, Phys. Today (March) (1992) 28. [4] S. Ghosh, A. Tripathi, T. Som, S.K. Shrivastava, V. Ganeshan, D.K. Avasthi, Radiat Eff. Def. 154 (2001) 151. [5] C. Trautmann, M. Toulemonde, K. Schwartz, A. Muller, Nucl. Instr. and Meth. B 164–165 (2000) 365. [6] E.M. Bringa, R.E. Johnson, Phys. Rev. Lett. 88 (2002) 165501. [7] E.H. Lee, Nucl. Instr. and Meth. B 151 (1999) 29. [8] P. Sigmund, Phys. Rev. 184 (1969) 383. [9] M. Toulemonde, Ch. Dufour, E. Paumier, F. Pawlak, MRS Proc., Boston 504 (1997). [10] J. Chaumont, H. Bernas, A. Kusnetsov, C. Clerc, L. Damoulin, Nucl. Instr. and Meth. B 129 (1997) 436. [11] R. Behrisch, V.M. Prozesky, H. Huber, W. Assmann, Nucl. Instr. and Meth. B 118 (1996) 262. [12] F. Pawlak, Ch. Dufour, A. Laurent, E. Paumier, J. Perriere, J.P. Stoquert, M. Toulemonde, Nucl. Instr. and Meth. B 151 (1999) 140. [13] S. Ghosh, A. Ingale, T. Som, D. Kabiraj, A. Tripathi, S. Mioshra, S. Zhang, X. Hong, D.K. Avasthi, Solid State Commun. 120 (2001) 445. [14] S. Ghosh, D.K. Avasthi, T. Som, A. Tripathi, D. Kabiraj, A. Ingale, S. Mishra, V. Ganeshan, S. Zhang, X. Hong, Nucl. Instr. and Meth. B 190 (2002) 164. [15] S. Ghosh, D.K. Avasthi, A. Tripathi, S.K. Srivastava, S.V.S. Nageswara Rao, T. Som, V.K. Mittal, F. Gruner, W. Assman, Nucl. Instr. and Meth. B 190 (2002) 169. [16] A. Gupta, D.K. Avasthi, Phys. Rev. B 64 (2001) 155407. [17] M. Toulemonde, W. Assmann, C. Trautmann, F. Gruner, Phys. Rev. Lett. 88 (2002) 057602. [18] G.K. Mehta, A.P. Patro, Nucl. Instr. and Meth. B 129 (1997) 423. [19] J. LÕEcuyer, C. Brassard, C. Cardinal, J. Chabbal, L. Deschenes, J.P. Labrie, B. Terreault, J.G. Martel, R.S. Jacques, J. Appl. Phys. 47 (1976) 381. [20] H.D. Carstanjen, Nucl. Instr. and Meth. B 136–138 (1998) 1183. [21] Santanu Ghosh, Ph.D. Thesis, 2001. [22] M. Toulemonde, Ch. Dufour, A. Meftah, E. Paumier, Nucl. Instr. and Meth. B 166–167 (2000) 903. [23] R.E. Johnson, B.U.R. Sundquist, A. Hedin, D. Fenyo, Phys. Rev. B 40 (1989) 49. [24] W. Ens, B.U.R. Sundquist, P. Hakansson, A. Hedin, G. Jonsson, Phys. Rev. B 39 (1989) 763.
Electronic sputtering from HOPG: A study of angular dependence A. Tripathi a,g,*, S.A. Khan a, S.K. Srivastava a, M. Kumar b, S. Kumar c, S.V.S.N. Rao d, G.B.V.S. Lakshmi d, Azhar. M. Siddiqui a, N. Bajwa e, H.S. Nagaraja a, V.K. Mittal f, A. Szokefalvi g, M. Kurth g, A.C. Pandey b, D.K. Avasthi a, H.D. Carstanjen g a
Nuclear Science Centre, Aruna Asaf Ali Marg, P.O. Box 10502, New Delhi 110 067, India b University of Allahabad, Allahabad 211002, India c RBS college, Agra 282002, India d University of Hyderabad, Hyderabad 500 046, India e Panjab University, Chandigarh 160 014, India f Punjabi University, Patiala 147 002, India g Max Planck Institute, Heisenbergstrasse1, Stuttgart 70569, Germany
Abstract The angular distribution of the sputtering yield from highly oriented pyrolytic graphite sample irradiated with a 130 MeV Ag beam is studied. The beam was incident perpendicular to the sample and the sputtered carbon was collected on Si catcher foils which were studied using a high resolution ERDA set up. An anisotropic distribution of sputtering is observed with a distribution C ¼ A cos1:3 H þ B expððH 53Þ2 =r2 ) which shows that a peak lies at around 53 on a distribution which otherwise is a over-cosine function. The maximum sputtering yield is observed at 53, falling rapidly to almost zero at 90, with an average sputter yield of 5.5 · 105 atoms/ion. It is suggested that this anisotropy may be due to the crystal structure and formation of a pressure pulse. 2003 Elsevier B.V. All rights reserved. PACS: 79.20; 6180.Jh Keywords: Electronic sputtering; Angular distribution; Swift heavy ions; ERDA; HOPG
1. Introduction The movement of energetic heavy ion affects the lattice drastically due to a large energy deposition resulting in breaking of atomic bonds [1,2], for-
*
Corresponding author. E-mail address: [email protected] (A. Tripathi).
mation of latent tracks [3,4] and ejection of atoms/ molecules [5,6]. At higher energies with velocities comparable to electron Bohr velocity (2.2 · 106 m/s), swift heavy ions pass through a solid creating a highly dense energized nanometer cylindrical zone of intense electronic excitation along their trajectory on a picosecond time scale. The energy transfer at these energies is dominated by inelastic excitation of electrons, called electronic energy loss
0168-583X/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0168-583X(03)01742-7
A. Tripathi et al. / Nucl. Instr. and Meth. in Phys. Res. B 212 (2003) 402–406
as opposed to energy transfer by elastic collision with lattice atoms called nuclear energy loss. The mechanism of transfer of the electronic energy to the kinetic energy of lattice atoms is not very clearly understood, though models based on inelastic thermal spike [7] and a combination of Coulomb explosion and thermal spike [8] are used to explain the phenomena. At lower energies (<100 keV/u) the erosion of atomic and molecular species is referred as nuclear sputtering and this nuclear energy loss induced process is well understood in the framework of SigmundÕs theory [9]. The electronic energy loss induced erosion/ejection of material is referred as electronic sputtering of material. It has been shown that the sputtering yield increases at higher energies, specially in the case of insulators [10] where electronic energy loss process becomes dominant beyond a given threshold. Since the results were contrary to the prediction by Sigmund theory, the electronic sputtering process attracted the attention of the researchers. These studies are expected to provide a deep insight into the ion matter interaction besides having a large potential for applications. The sputtering from various carbon allotropes has earlier been studied by Behrisch et al. [11] and Pawlak et al. [12]. The structural dependence of the sputtering yield from hydrogenated amorphous carbon films [13,14] and the effect of film thickness on sputtering from fullerene film [15] and an Au thin film [16] has earlier been studied. It should be noted that most of the studies have studied the total yield, assuming the yield to be isotropic in all the directions. Toulemonde et al. recently studied the angular distribution of the sputtering yield from insulating samples such as LiF and SiO2 [17]. In the present work, we report the angular distribution of the sputtering yield from semi-mettalic highly oriented pyrolytic graphite (HOPG) sample.
2. Experimental The samples were irradiated with a 130 MeV Ag9þ beam from the 15 MV Pelletron [18] at the Nuclear Science Centre, New Delhi. The beam was incident perpendicular to the sample and the cur-
403
Fig. 1. The experimental arrangement.
rent was measured from the sample frame. The sample frame was surrounded by a secondary electron suppressor having a voltage of )120 V. Sputtered carbon atoms were collected on 10 mm · 5 mm Si catcher foils which were kept at a distance of 5 cm from the samples. The catcher foils were mounted on the secondary electron suppressor in such a way, so that all the catchers were equidistant in the horizontal plane. These were mounted at angles of 10–90 from the beam direction. The arrangement for mounting the catcher foils is shown in Fig. 1. The incident current was 0.6 particle nanoamperes (1 particle nA ¼ 6.2 · 109 ) ions/s) and the sample was irradiated for a fluence of 3.6 · 1013 ions/cm2 to collect sufficient carbon on small catchers for ERD analysis. Since the exposure time was long, the beam was scanned over an area of 5 mm · 10 mm to avoid any damage to the sample. It should be noted that both the catchers and scanned area was kept 5 mm in horizontal direction to reduce the error in angle measurement. Each catcher subtends an angle of 6, with an error of ±3 in the angular measurement, and an equal contribution is expected from the error due to scanning. After the irradiation was complete, a 20 nm Al layer was deposited on the sample to avoid the collected carbon getting sputtered during the subsequent ERDA study [19]. A dummy was also placed in the chamber so that we can measure only the sputtered carbon, as contributions from hydrocarbons and other contaminants present in the chamber are also expected. The dummy was
404
A. Tripathi et al. / Nucl. Instr. and Meth. in Phys. Res. B 212 (2003) 402–406
placed in the chamber on the outer wall of the suppressor so that it does not see the sputtered atoms. This catcher went through all the processes, including the Al layer deposition and the carbon collected on this dummy was measured to find the carbon contribution from various contaminants. This contribution was subtracted while calculating the actual quantity of sputtered carbon. The carbon collected on the catcher foils is analysed using the ERDA technique using the 1.3 MeV Arþ beam from the 6 MV Pelletron at the Max Planck Institute, Germany. Since the deposited carbon layer is of the order of a few nanometers, the analysis of the sputtered carbon was done using a high resolution spectrometer [20] set up. The sample was kept at an angle of 20 to the beam direction and the recoils were detected at an angle of 40. The recoils were detected using the time of flight (TOF) set up. The stop signal was obtained by the beam chopper providing a pulsed beam (width 60 ns, repetition rate 2 ls) and the start signal was obtained using the position sensitive micro-channel plate. The spectrometer has an energy acceptance window of 2.9% and hence the complete depth profile was obtained in two or more settings. The complete spectra was then obtained by combining the individual spectra and generating complete spectra after removing the overlapping energies. To obtain the absolute quantity of collected carbon a graphite sample was also used for normalization. The spectra were collected for the same incident charge and the recoil spectra recorded.
3. Results and discussion The recoils with different m=q values are displayed using two dimensional spectra showing TOF versus energy (E) plots. The carbon recoils with 2þ charge state are identified by the energy of the recoils from the surface and by comparing it with the spectra of the graphite sample, which has carbon recoils with same surface energy. The two dimensional TOF–E spectra corresponding to carbon was then projected onto the E axis to generate one dimensional recoil energy spectra of carbon. The ERD spectra for the 10 catcher foils
Fig. 2. ERD spectra up to a depth of 20 nm for five catcher foils.
was recorded. For clarity, we have shown the spectra for only five catchers at alternate angles in Fig. 2. The peak in the spectra shows the sputtered carbon. The spectra do not show a sharp edge towards the surface, which is due to the fact that the high energy resolution possible with the spectrometer has deteriorated due to 20 nm thick Al layer deposited on top of the sputtered carbon layer. Similarly the lower energy of the spectra has a long tail. The possibility of a large carbon impurity in the Si catcher was ruled out by studying a virgin Si catcher wafer which had almost negligible carbon content. We assume the long tail on the lower energy side to be due to double/multiple scattering effects. This tail may also be due to the increased roughness of catcher surface due to deposited carbon. We were able to see this profile due to excellent depth resolution from the spectrometer and we have restricted the measurements of carbon near the surface with recoils with energies between 435 and 455 keV, representing a depth of approximately 20 nm. All the samples show a carbon content which is significantly higher than the carbon content of the dummy. The total carbon sputtered is calculated for each of the catcher foils from the counts between the energies 435 and 455 keV in the spectra and the same is plotted in Fig. 3. ERD of HOPG sample is also done for normalizing and calculating the actual carbon content. As is clear from the figure, the
A. Tripathi et al. / Nucl. Instr. and Meth. in Phys. Res. B 212 (2003) 402–406
Fig. 3. The areal density of carbon collected on all the ten catcher foils, which represents the angular distribution of the sputtering yield. The v2 and R2 values for the fit are 0.08 and 0.99 respectively.
measured quantity of the sputtered carbon does not vary much for the 10, 20 and 30 catchers with a total content of approximately 4.2 · 1016 atoms/cm2 for the 10 and 20 catchers. There is a rise in the carbon content in the next two wafers with the 50 catcher showing a maximum content of 5.6 · 1016 atoms/cm2 , after which it goes down slightly in the 60 catcher. The catchers at 70, 80 and 90 show a sudden decrease with the content going down to 0.2 · 1016 atoms/cm2 , a content even lower than the catchers at 10. The dummy is found to have carbon content of 4.3 · 1016 atoms/ cm2 , which is subtracted from all the catchers while calculating the concentration mentioned above. We have calculated the total sputtering yield from the measured quantity of collected carbon on the catchers. Since the sputtering yield is angle dependent, the value of total carbon collected by all the catcher foils is calculated by taking a weighted addition. The weight given to carbon for a given foil is proportional to the cosine of the angle of the measurement and the total quantity of sputtered carbon is extrapolated from the solid angle of 0.05 sr subtended by each catcher. Since our measurements are restricted to only one azimuthal plane we have assumed the distribution to be isotropic in vertical plane for calculating the total yield. For the total incident beam fluence of
405
3.8 · 1013 ions/cm2 , the average sputter rate of 5.5 · 103 atoms/ion is obtained. This value is higher than the value obtained by Ghosh [21] for a 200 MeV Ag beam with an Se value 21.7 keV/nm for graphite and an average sputter yield of 1 · 103 atoms/ion. With an electronic energy sputter yield dependence of Y / Se1:7 [21], we would expect a yield of 5 · 102 atoms/ion for 130 MeV Ag beam with an Se value of 14.6 keV/nm. The observed increase in sputtering yield is expected to be due to the large ion fluence as compared to the earlier study. In the present study, the fluence is kept at 3.8 · 1013 ions/cm2 , which is more than the track overlap fluence and we expect that amorphisation of surface may be responsible for the high sputtering yield, which is closer to the sputtering yield value for amorphised carbon [13,21]. The measured carbon content for all the catchers is shown in Fig. 3. The analysis of the distribution shows an anisotropic distribution with moderate sputtering at smaller angles. However a maximum is observed around 53 to the beam direction, after which the sputtering yield goes rapidly down to almost zero at 90. The spectra is fitted to a distribution C ¼ A cos1:3 H þ 2 B expððH 53Þ =r2 Þ, with a chi square value of 0.07, which shows that a Gaussian peak lies at around 53 on a distribution which otherwise has a over-cosine distribution. A similar jet like anisotropy is observed by Toulemonde et al. in swift heavy ion induced sputtering of insulating LiF sample. The inelastic thermal spike model [22] explains this anisotropy in LiF is as a result of hydrodynamic process driven by the pressure induced by the vapour phase formed in the bulk due to a thermal spike. It is envisaged that initially a gas phase induces a radial pressure in the track core, which is released by a jet of atoms from the impact area giving an angular anisotropy. At a later stage the evaporation from the heated track zone becomes dominant giving an isotropic distribution. Toulemonde et al. have suggested that the beam incidence strongly influences the geometry of the impact zone and track formation and non-normal incidence adds to the anisotropy as the radial pressure releases towards the surface. The main difference in our observation lies in the fact we have observed the anisotropic peak at 53 in case of
406
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HOPG though it is observed at 0 in case of LiF [15]. It should be emphasised that the calculations based on sum of impulses by Johnson et al. predict a peak at 45 for the normal incidence [23] and observed by Ens et al. [24]. We feel that the structure of the crystal, in the initial stage of irradiation, might also play an important role in the jet like component. The LiF has a fcc structure and it is expected that the pressure pulse propagates along the axis, specially in case of non-normal incidence. Since we have perpendicular beam incidence, the ejected atoms are more likely to travel along the axis at 60 to the surface normal for the hcp structure of HOPG with [0 0 0 1] surface. 4. Conclusion We have studied the angular distribution of the sputtering yield from semiconducting HOPG sample irradiated with a 130 MeV Ag beam. The study is made for perpendicular incident beam with the sputtered carbon being collected on Si catcher foils. The catcher foils were studied using high resolution ERDA set up. The main result of the study is that the sputtering yield has an anisotropic distribution. There is only a moderate sputtering yield at smaller angles and a maxima at around 53 after which the sputtering yield goes rapidly down to almost zero at 90. The average sputtering yield is found to be 5.5 · 105 atoms/ion. The anisotropy is explained on the basis of pressure pulse model and the crystal structure of HOPG.
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