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Adhesion improvement of diamond and other films after MeV ion irradiation
Vacuum/volume
IPIoaaes 999 to 1003/1997 0 1998Elsevier Science Ltd All rights reserved. Printed in Great Britain 0042-207X/98 $19.00+.00
Pergamon PII: 80042-207X(97)001
48/number
12-7
Adhesion improvement of diamond and other films after MeV ion irradiation U R Mhatre,” A N Kale,” D C Kothari,” P M Raole,b M K Totalani,” D KanjilaLd Atul Kulkarni,” S M Kanetkar” and S B Ogale,e aDepartment of Physics, University of Bombay, Vidyanagari, Bombay 400 098, India; bRSIC, Indian Institute of Technology, Powai, Bombay 400 076, India; “Metallurgy Division, Bhabha Atomic Research Centre, Trombay, Bombay 400 085, India, dNuclear Science Centre, POB 10502, New Delhi 110067, India, eDepartment of Physics, University of Poona, Ganeshkhind, Pune-4 11007, India received
6 August 1997
This paper reports the effects of MeV ion irradiation on the adhesion and quality of diamond films on WC-Co tool-bits and silver films on soda glass. Diamond films of about 3pm thickness were prepared, using the hot filament CVD technique, on WC-Co tool-bits. Silver films of IOOOA thickness were prepared, using the Joule evaporation technique, on soda glass. 100 MeV lz71beam was irradiated on the films at various doses up to 1 x IO” ions/cm’. Adhesion strength was measured using a pin-pull test. Scotch tape test was also performed on some of the films. SEM images show no morphological modifications of the diamond films after irradiation. Laser Raman spectra show that the irradiation causes almost complete elimination of non-diamond carbon present in the films. Adhesion strength of the diamond films as determined by Pin-pull test show improvement at least by 65kgf/cm2 after irradiation. Improvement in the adhesion of silver films is found to be dependent on the amount of electronic energy loss IS,) at the film-substrate interface. It is shown that for silver film on soda glass S,> 0.7keV/A is needed if the adhesion is required to be achieved at a dose of 1 x 1013ions/cm’ using ion irradiation of 100 MeV lz71beam. Main cause of the improvements in the adhesion and quality of the films is suggested to be the energy deposited to the electronic system of the material by the projectile ion. 0 1998 Elsevier Science Ltd. All rights reserved
Introduction Improvement in adhesion strength at the metal-insulator interface using ion irradiation has been reported by various groups.’ Ion beams of energies ranging from a few keV,’ MeV” to a few GeV4 have been used. Ion beams can improve adhesion even at an interface where no chemical affinity exists between the film and the substrate. The ion irradiation technique of adhesion improvement has found use in technologically important couples such as metal-polymer, metal-glass and metal-ceramics required in semiconductor industry. In the present work an application is chosen from metal cutting tool industry. Diamond coated WCCo tools are selected for the study. Adhesion of diamond film on WC-Co tool bit determines its usefulness as a tool. It is explored whether the adhesion of this technologically important couple can be improved using ion irradiation technique. Ion beam irradiation of diamond films and diamond like carbon (DLC) films have been studied by various groups.5,6 It has been generally observed that diamond quality deteriorates because of violent collisions encountered by diamond films during irradiation. It is argued that due to elastic collisions the sp’ bonds of diamond get transformed to sp2 bonds of non-diamond carbon phase and thus the quality of diamond film gets deterio-
rated. However, these reported studies, have been carried out at keV energies, which cause a lot of damage in the film due to collision cascades. In recent years, a number of authors have also studied the effect of high energy ion beams on DLC films.’ Increase in disorder after MeV ion irradiation of DLC tilm has been reported. MeV ions initially lose their energy mainly by electronic excitation and ionization. Only when the energy reduces to keV that the collision cascades play an important role. The energy loss in excitation and ionization is called as ‘electronic-loss’ and the corresponding dE/dx is called as electronic stopping power (S,). As the ion comes close to the end of its trajectory the energy loss is predominantly by elastic collision which is termed as ‘nuclear-loss’ and the corresponding dE/dx is called as nuclear stopping power (S,,). It was interpreted that the changes observed in the irradiated DLC films were mainly because of the electronic-loss. We carried out high energy irradiation on diamond films to know if the effects are similar to those observed on DLC films.’ The present experiments are performed to know the effects of the electronic-loss on the structure of diamond film and adhesion strength modifications at the interface. Although it has been recognized that the ion irradiation can 999
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improve adhesion strength, the mechanism of improvement is poorly understood. Adhesion improvement using keV beams could be caused due to nuclear-loss induced ion beam mixing of few atomic layers at the interface. At MeV and higher beam energies the electronic-loss could play an important role. A noted attempt to understand the mechanism of adhesion improvement using MeV ion beams was by Tombrello,’ who proposed that the electronic-loss is likely to induce desirable chemical changes at the interface. To verify the applicability of the model we decided to carry out some experiments on silver films, results of which are also presented in this paper. Experimental Diamond films of about 3 pm thickness were prepared, using the Hot Filament Chemical Vapor Deposition (HFCVD) technique, on WC-Co tool-bits. Details of the HFCVD apparatus and the deposition parameters are given elsewhere.* Silver films of 1000 A thickness were prepared, using the Joule evaporation technique, on soda glass substrates. 1OOMeV ‘*‘I beam was irradiated on the films at various doses upto 1 x lOI ions/cm’ using a Pelletron accelerator at NSC, New Delhi. It is known that violent collisions present at the end of the trajectories of the ions cause deterioration in quality of diamond films. Thus if the thickness of diamond film chosen is less than the range of MeV ions, the energy loss via collision cascade would take place in the substrate rather than in the diamond film. In the case of diamond films the range of 100 MeV ‘171ions is 13 pm, whereas the film thickness is about 3 pm. Silver films are of thickness 1000 A which is much lower than the range of IOOMeV 12’1ions in silver. Thus in both the cases the electronic-losses are expected to play a major role in causing modifications in the film properties. Surface morphology of the diamond films was studied using a CAMECA-SV-SEM probe analytical scanning electron microscope. Two different samples were chosen to observe morphology in irradiated and un-irradiated films. Laser Raman spectra of the diamond films were recorded using JOBIN-YVON RAMANOR HG-2S spectrometer employing Art Laser. Laser Raman spectra were recorded in the same specimen but two different regions namely irradiated and un-irradiated. The spectra were also recorded in two different specimens viz. irradiated and un-irradiated. Adhesion strength was measured using Pin Pull test. A cylindrical pin of cross-sectional area A was attached to the film using a special epoxy. The pin was pulled from the clamped substrate and a maximum force F,,,,,, required to detach the film was recorded. The adhesion strength = (&,,)/A measured in units of kgf/cm2 is determined and reported here. The adhesion strength was also measured on irradiated and un-irradiated portion of the same specimen to estimate the variation in different specimens which was found to be less than 5%. A Scotch tape test was also performed on some of the silver films. A piece of scotch tape is firmly applied to the film and the tape is pulled away. If the film is detached then it is ‘failed’, otherwise it is ‘passed’.
Figlure 1. (a and b) SEM images of un-irradiated diamond film on co tool bit.
Results and discussions Figure I(a and b) shows SEM images of the un-irradiated diamond films at different magnifications. The SEM picture reveals clear faceting and mixed morphology. Irradiated diamond films show similar morphology as can be seen in Figure 2(a and b). Thus SEM results suggest that there are no morphological changes in the diamond film after irradiation. Different grain size in the SEMs of Figures 1 and 2 are attributed to the fact that these 1000
Fig;ure 2. (a and b) SEM images of diamond film on WC-Co tot )I bit: irri rdiated using 100 MeV 12’1beam at a dose of 5 x 10” ions/cm’.
U R Mhatre et al: Adhesion
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Figure 3. Laser Raman spectra of diamond films on WC-Co tool bit: un2: irradiated and irradiated using 100 MeV 12’1 beam. 1: Un-irradiated. Irradiated to a dose of 5 x IO” ions/cm’.
pictures are of two different specimens. Laser Raman spectra (Figure 3) reveal that the quality of the diamond films has improved. This result has been observed in irradiated and unirradiated regions of the same specimen and also in two different specimens namely irradiated and un-irradiated. Nucleation of diamond during HFCVD processes has been found to contain small quantity of non-diamond phase. Raman spectrum of the un-irradiated film shows a broad peak at about 1550 cm-‘. This broad peak is characteristics of non-diamond sp’ bonded phase of carbon.” Although the sensitivity of the Raman technique to the sp’ bonded phase of the carbon is about 50 times greater than that of the sp’ bonded (diamond) phase, the broad peak indicates presence of non-diamond carbon in minute quantity. However, after irradiation the broad peak disappears. This indicates that the non-diamond carbon has been eliminated from the films. We now attempt to understand the observed effect of elimination of non-diamond carbon from the film, in view of the reported electronic-loss induced modifications of the material. As pointed out earlier, thickness of diamond film chosen is such that the electronic-loss is predominant in the film. Recently many electronic-loss induced effects have been reported in literature such as in-elastic sputtering,‘” defect formation in metals,” phase transformation in metals,” etc. In-elastic sputtering of discontinuous gold film’” and latent track formation in similar film has been reported.13 In-elastic sputtering and latent track formation are thought to be due to the evaporation of isolated grain which could trap copious number of available electrons in isolated potential wells.” The non-diamond carbon in our films is in minute quantity as interpreted from the laser Raman spectrum. It is most likely that the non-diamond carbon exist in discontinuous form on the diamond film. As the diamond crys-
tallites nucleate and grow, the sp* bond is known to exist on the surface of the crystallites.‘4 Thus one expects that major part of the non-diamond carbon is present on the surface of diamond films, Also it is known that non-diamond carbon forms during the closing down operation of the HFCVD process. Non-diamond carbon in discontinuous form on the surface of the diamond film can be easily in-elastically sputtered using high energy ions. We believe that the in-elastic sputtering could be responsible for the observed elimination of non-diamond carbon from our diamond films after irradiation. Electronic-loss induced phase transformation is another likely effect which needs to be considered. The E to w phase transformation in titanium” has been explained using coulomb explosion model” which predicts high pressure shock-wave like situation during high energy irradiation. Thermal spike modellh predicts local melting and rapid cooling along the tracks of the ions, which could also predict phase transformations. These high pressure and high temperature could induce phase transformation from non-diamond to diamond phase. However. as deterioration of diamond quality after irradiation has been reported in the literature,6 we believe that the above mechanisms of phase transformation may not be playing major role in elimination of non-diamond carbon phase from our films. Figure 4 shows the adhesion strength of un-irradiated and irradiated diamond films on WC-Co substrates. Adhesion strength has been improved at least by 65 kgf/cm’. 100 MeV “7 beam deposits 1.2 keV/A to the electronic system and 0.012 keV/A in elastic collisions (nuclear loss) near the diamond film-substrate interface. As the nuclear loss is insignificant it is not likely to cause any changes at the interface. No beam mixing is expected at such a low dose. Thus the adhesion improvement can not be due to mixing caused by the nuclear stopping power S,,. As proposed by Tombrello’ the electronic stopping S, can cause changes in the interface chemistry and improve adhesion. Adhesion improvement of Cu on Teflon by GeV ion irradiation
2201
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Figure 4. Adhesion strengths of diamond films on WC-Co tool bit: unirradiated and irradiated using 100MeV ‘I’1 beam at different doses. Specimen 1: un-irradiated. Specimen 2: irradiated to a dose of 5 x lOi ions/cm’. Specimen 3: irradiated to a dose of 1 x lOI ions/cm’. 1001
U R Mhatre
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after MeV ion irradiation
1 40 “G
B B Z30 F! 3 8 ‘W 20
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Existence of the threshold dose D,,, for adhesion improvement has been proposed.’ This D,,, is different for different systems. Tombrello’ has developed a functional relationship between the threshold dose and electronic energy loss (S,) of the ion at the interface. According to this model, for adhesion one needs transfer of electrons across the interface where the required energy is provided by S,,. When one ion crosses the interface it transfers a number of electrons across the interface. The electrons transferred across the interface are utilized in the formation of new bonds. Thus we may write
-
5 lo-
0
-
3
1
Specimen Numbex Figure 5. Adhesion strengths of silver films on soda glass: w-irradiated and irradiated using 100MeV ‘?‘I beam at different doses. Specimen 1: m-irradiated. Specimen 2: irradiated to a dose of 5 x lO”ions/cm’. Specimen 3: irradiated to a dose of 1 x 10” ions/‘cm’.
using the melting in thermal spikes.‘h Similar explanation could be given even for the present results of adhesion improvement. To assess the role of the electronic stopping S, in the improvement of adhesion further experiments were carried out on the silver films. Figure 5 shows the adhesion strengths of un-irradiated and irradiated silver films on soda glass. As can be seen, even nominal dose of 5 x 10” ions/cm’ is sufficient to improve adhesion. To determine the role of electronic stopping power .S,. in adhesion improvement. Scotch tape test was used on un-irradiated and irradiated Ag films on soda glass. Three specimens were irradiated using 100 MeV “‘I beam at a dose of I x IO” ions/cm’. Two of the specimens were covered using thin Al foils of two different thicknesses during irradiation. The specimen without the Al foil cover gets exposed to 100 MeV ion beam and at the interface the beam deposits S, = 2.6 keV/A. One specimen was covered using .5pm thick Al foil. As the range of 1OOMeV “‘1 beam is 15 /Lrn, the beam is transmitted through the foil. The energy of the transmitted beam is 50 MeV which falls on the Ag film and at the interface it deposits S, = 1.88keV/A. The third specimen was covered using a 12 pm Al foil. The energy of transmitted beam through this foil is about 16MeV and thus the electronic energy loss (S,) deposited at the interface was about 0.7 keV/A. The results are shown in Table 1. was explained
The minimum number of bonds required for adhesion is constant for given film-substrate interface. The minimum number of ions passing across the interface is directly related to the threshold dose D,,,. The number of electrons transferred by one ion is directly related to the electronic energy loss S,.. This model has been tested for many couples and D,,, is found to be proportional to (S,))“, where n takes values between 1 and 3 for various filmsubstrate couples, For example, the exponent n determined using the scotch tape test is found to be 1.42 in the case of Au film on tantalum.” Another way of looking at the above model is that if the dose is kept constant then to form minimum number of bonds for adhesion one ion should transfer minimum number of electrons. Thus for a constant dose a threshold S,. value is required for adhesion. which was observed in our Scotch tape experiment (see Table 1). The Scotch tape ‘passed’ the first two specimens whereas the third specimen ‘failed’ in the test. Also un-irradiated specimens ‘failed’ in the test, Therefore it is established that S, > 0.7 keV/A is needed if the adhesion is required to be achieved at a dose of 1 x 1O’j ions/cm* using ion irradiation of 100 MeV ‘*‘I beam. As the nuclear stopping power is 0.027 keV/A for 100 MeV ‘*‘I beam in Ag, the dose of I x 10” ionsl’cm’ works out to be 0.003 dpa. Such a low value of dpa could barely induced any mixing at the interface. Hence the adhesion improvement due to ion beam mixing caused by the nuclear-loss is ruled out. Therefore, the present results indicate that electronic stopping power is responsible for the adhesion improvement. One should be able to achieve similar improvement in adhesion strength using either electron beam or even gamma ray irradiations in which large amount of energy is deposited to the electronic system of the material.
Conclusions Table 1. Results of Scotch tape test
Energy 100 MeV 50 MeV 16MeV Un-irradiated
Electronic (keV/A) 2.6 1.8 0.7
loss .S,. Scotch tape test results Passed Passed Failed Failed
1OOMeV ‘? ion beam irradiation improves the quality of diamond film and improves adhesion at the interface of diamond film and WC-Co substrate. SEM images show no morphological modifications of the films after irradiation. Laser Raman spectra show that the irradiation causes almost complete removal of nondiamond carbon present in the films. The ion beam irradiation also gives rise to significant improvement in adhesion strength of silver films on soda glass. Low dose of 5 x lO’*ions/cm’ is sufficient to cause the improvement. Minimum electronic energy
U R Mhatre et a/:Adhesion
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loss S, is required to get improvement in adhesion strength. The present work has shown that for silver film on soda glass S, > 0.7 keV/A is needed if the adhesion is required to be achieved at a dose of I x 10”ions/cm2 using ion irradiation of 1OOMeV 12’1beam. The main cause of the improvements in the adhesion and quality of the films is suggested to be the energy deposited to the electronic system of the material by the projectile ion. References Baglin, J.E.E.E., in Ion Beam modification ofInsulators, chapter 15, P. Mazzoldi and R.W. Arnold (eds.), Elsevier, Amsterdam, 1987. Baglin, J.E.E.E. and Clark, G.J., Nucl. Znstr. and Methods. 1985, B7/8, 88 1. Tombrello, T.A., Materials Science and Engineering, 1985. 69. 443. Wang, L., Anquert, N., Ruck, D., Trautmann, C., Vetter, J., Quan, Z. and Hantsche, H., Nwl. Instr. and Methods, 1993, B83, 503. Wu, R.L.C., Miyoshi, K., Korenyi-Both, A.L., Garscadden, A. and Barnes, P.N., Surface Coating Technology, 1993, 62, 589.
6. Barshilia, H.C., Sha, S., Mehta, B.R., Vankar, V.D., Avasthi, D.K., Kabiraj, D. and Jaipal, Mehta. G.K., Thin Solid Films, 1995, 258, 123. 7. Tombrello, T.A., Proc. Mar. Res. Sot., 1984, 25, 173. 8. Kanetkar, SM., Kulkarni, A.A., Vaidya, A., Vispute, R.D., Ogale, S.B., Kshirsagar, S.T. and Purandare, SC.. Appl. Phys. Left., 1993, 63, 740. 9. Wada, N. and Solin, S.A., Physica B and C, 1981, 105B, 353. IO. Baranov, I.A., Obnorskii, V.V. and Tsepelevich, S.O., Nucl. Instr. and Methods in Physics Research, 1990, B52, 9. 11, Dunlop, A. and Lesueur, D., Radiation Ejfbcts and Defkrs in Solids, 1993, 126, 123. 12. Dammak, A., Barbu, A., Dunlop, A., Lesueur, D. and Lorenzelli. N., Phil. Msg. Lett., 1993, 67A, 253. 13. Merkle, K.L., Phys. Ren. Lett., 1962, 9, 150. 14. Pate, B.B., Surl: Sci., 1986, 165. 83. 15. Seitz, F., Disc. Faraday Sot., 1949, 5, 27 1. 16. Varley, J.H.D., Namrr, 1954, 174, 886. 17. Sugdan, S., Sofield, C.J. and Murrell, M.P.. Nucl. Instr. and Methods, 1989, B44, 137.
1003
IPIoaaes 999 to 1003/1997 0 1998Elsevier Science Ltd All rights reserved. Printed in Great Britain 0042-207X/98 $19.00+.00
Pergamon PII: 80042-207X(97)001
48/number
12-7
Adhesion improvement of diamond and other films after MeV ion irradiation U R Mhatre,” A N Kale,” D C Kothari,” P M Raole,b M K Totalani,” D KanjilaLd Atul Kulkarni,” S M Kanetkar” and S B Ogale,e aDepartment of Physics, University of Bombay, Vidyanagari, Bombay 400 098, India; bRSIC, Indian Institute of Technology, Powai, Bombay 400 076, India; “Metallurgy Division, Bhabha Atomic Research Centre, Trombay, Bombay 400 085, India, dNuclear Science Centre, POB 10502, New Delhi 110067, India, eDepartment of Physics, University of Poona, Ganeshkhind, Pune-4 11007, India received
6 August 1997
This paper reports the effects of MeV ion irradiation on the adhesion and quality of diamond films on WC-Co tool-bits and silver films on soda glass. Diamond films of about 3pm thickness were prepared, using the hot filament CVD technique, on WC-Co tool-bits. Silver films of IOOOA thickness were prepared, using the Joule evaporation technique, on soda glass. 100 MeV lz71beam was irradiated on the films at various doses up to 1 x IO” ions/cm’. Adhesion strength was measured using a pin-pull test. Scotch tape test was also performed on some of the films. SEM images show no morphological modifications of the diamond films after irradiation. Laser Raman spectra show that the irradiation causes almost complete elimination of non-diamond carbon present in the films. Adhesion strength of the diamond films as determined by Pin-pull test show improvement at least by 65kgf/cm2 after irradiation. Improvement in the adhesion of silver films is found to be dependent on the amount of electronic energy loss IS,) at the film-substrate interface. It is shown that for silver film on soda glass S,> 0.7keV/A is needed if the adhesion is required to be achieved at a dose of 1 x 1013ions/cm’ using ion irradiation of 100 MeV lz71beam. Main cause of the improvements in the adhesion and quality of the films is suggested to be the energy deposited to the electronic system of the material by the projectile ion. 0 1998 Elsevier Science Ltd. All rights reserved
Introduction Improvement in adhesion strength at the metal-insulator interface using ion irradiation has been reported by various groups.’ Ion beams of energies ranging from a few keV,’ MeV” to a few GeV4 have been used. Ion beams can improve adhesion even at an interface where no chemical affinity exists between the film and the substrate. The ion irradiation technique of adhesion improvement has found use in technologically important couples such as metal-polymer, metal-glass and metal-ceramics required in semiconductor industry. In the present work an application is chosen from metal cutting tool industry. Diamond coated WCCo tools are selected for the study. Adhesion of diamond film on WC-Co tool bit determines its usefulness as a tool. It is explored whether the adhesion of this technologically important couple can be improved using ion irradiation technique. Ion beam irradiation of diamond films and diamond like carbon (DLC) films have been studied by various groups.5,6 It has been generally observed that diamond quality deteriorates because of violent collisions encountered by diamond films during irradiation. It is argued that due to elastic collisions the sp’ bonds of diamond get transformed to sp2 bonds of non-diamond carbon phase and thus the quality of diamond film gets deterio-
rated. However, these reported studies, have been carried out at keV energies, which cause a lot of damage in the film due to collision cascades. In recent years, a number of authors have also studied the effect of high energy ion beams on DLC films.’ Increase in disorder after MeV ion irradiation of DLC tilm has been reported. MeV ions initially lose their energy mainly by electronic excitation and ionization. Only when the energy reduces to keV that the collision cascades play an important role. The energy loss in excitation and ionization is called as ‘electronic-loss’ and the corresponding dE/dx is called as electronic stopping power (S,). As the ion comes close to the end of its trajectory the energy loss is predominantly by elastic collision which is termed as ‘nuclear-loss’ and the corresponding dE/dx is called as nuclear stopping power (S,,). It was interpreted that the changes observed in the irradiated DLC films were mainly because of the electronic-loss. We carried out high energy irradiation on diamond films to know if the effects are similar to those observed on DLC films.’ The present experiments are performed to know the effects of the electronic-loss on the structure of diamond film and adhesion strength modifications at the interface. Although it has been recognized that the ion irradiation can 999
U R Mhatre
et a/: Adhesion
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after MeV ion irradiation
improve adhesion strength, the mechanism of improvement is poorly understood. Adhesion improvement using keV beams could be caused due to nuclear-loss induced ion beam mixing of few atomic layers at the interface. At MeV and higher beam energies the electronic-loss could play an important role. A noted attempt to understand the mechanism of adhesion improvement using MeV ion beams was by Tombrello,’ who proposed that the electronic-loss is likely to induce desirable chemical changes at the interface. To verify the applicability of the model we decided to carry out some experiments on silver films, results of which are also presented in this paper. Experimental Diamond films of about 3 pm thickness were prepared, using the Hot Filament Chemical Vapor Deposition (HFCVD) technique, on WC-Co tool-bits. Details of the HFCVD apparatus and the deposition parameters are given elsewhere.* Silver films of 1000 A thickness were prepared, using the Joule evaporation technique, on soda glass substrates. 1OOMeV ‘*‘I beam was irradiated on the films at various doses upto 1 x lOI ions/cm’ using a Pelletron accelerator at NSC, New Delhi. It is known that violent collisions present at the end of the trajectories of the ions cause deterioration in quality of diamond films. Thus if the thickness of diamond film chosen is less than the range of MeV ions, the energy loss via collision cascade would take place in the substrate rather than in the diamond film. In the case of diamond films the range of 100 MeV ‘171ions is 13 pm, whereas the film thickness is about 3 pm. Silver films are of thickness 1000 A which is much lower than the range of IOOMeV 12’1ions in silver. Thus in both the cases the electronic-losses are expected to play a major role in causing modifications in the film properties. Surface morphology of the diamond films was studied using a CAMECA-SV-SEM probe analytical scanning electron microscope. Two different samples were chosen to observe morphology in irradiated and un-irradiated films. Laser Raman spectra of the diamond films were recorded using JOBIN-YVON RAMANOR HG-2S spectrometer employing Art Laser. Laser Raman spectra were recorded in the same specimen but two different regions namely irradiated and un-irradiated. The spectra were also recorded in two different specimens viz. irradiated and un-irradiated. Adhesion strength was measured using Pin Pull test. A cylindrical pin of cross-sectional area A was attached to the film using a special epoxy. The pin was pulled from the clamped substrate and a maximum force F,,,,,, required to detach the film was recorded. The adhesion strength = (&,,)/A measured in units of kgf/cm2 is determined and reported here. The adhesion strength was also measured on irradiated and un-irradiated portion of the same specimen to estimate the variation in different specimens which was found to be less than 5%. A Scotch tape test was also performed on some of the silver films. A piece of scotch tape is firmly applied to the film and the tape is pulled away. If the film is detached then it is ‘failed’, otherwise it is ‘passed’.
Figlure 1. (a and b) SEM images of un-irradiated diamond film on co tool bit.
Results and discussions Figure I(a and b) shows SEM images of the un-irradiated diamond films at different magnifications. The SEM picture reveals clear faceting and mixed morphology. Irradiated diamond films show similar morphology as can be seen in Figure 2(a and b). Thus SEM results suggest that there are no morphological changes in the diamond film after irradiation. Different grain size in the SEMs of Figures 1 and 2 are attributed to the fact that these 1000
Fig;ure 2. (a and b) SEM images of diamond film on WC-Co tot )I bit: irri rdiated using 100 MeV 12’1beam at a dose of 5 x 10” ions/cm’.
U R Mhatre et al: Adhesion
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1700
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I
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(c nil)
Figure 3. Laser Raman spectra of diamond films on WC-Co tool bit: un2: irradiated and irradiated using 100 MeV 12’1 beam. 1: Un-irradiated. Irradiated to a dose of 5 x IO” ions/cm’.
pictures are of two different specimens. Laser Raman spectra (Figure 3) reveal that the quality of the diamond films has improved. This result has been observed in irradiated and unirradiated regions of the same specimen and also in two different specimens namely irradiated and un-irradiated. Nucleation of diamond during HFCVD processes has been found to contain small quantity of non-diamond phase. Raman spectrum of the un-irradiated film shows a broad peak at about 1550 cm-‘. This broad peak is characteristics of non-diamond sp’ bonded phase of carbon.” Although the sensitivity of the Raman technique to the sp’ bonded phase of the carbon is about 50 times greater than that of the sp’ bonded (diamond) phase, the broad peak indicates presence of non-diamond carbon in minute quantity. However, after irradiation the broad peak disappears. This indicates that the non-diamond carbon has been eliminated from the films. We now attempt to understand the observed effect of elimination of non-diamond carbon from the film, in view of the reported electronic-loss induced modifications of the material. As pointed out earlier, thickness of diamond film chosen is such that the electronic-loss is predominant in the film. Recently many electronic-loss induced effects have been reported in literature such as in-elastic sputtering,‘” defect formation in metals,” phase transformation in metals,” etc. In-elastic sputtering of discontinuous gold film’” and latent track formation in similar film has been reported.13 In-elastic sputtering and latent track formation are thought to be due to the evaporation of isolated grain which could trap copious number of available electrons in isolated potential wells.” The non-diamond carbon in our films is in minute quantity as interpreted from the laser Raman spectrum. It is most likely that the non-diamond carbon exist in discontinuous form on the diamond film. As the diamond crys-
tallites nucleate and grow, the sp* bond is known to exist on the surface of the crystallites.‘4 Thus one expects that major part of the non-diamond carbon is present on the surface of diamond films, Also it is known that non-diamond carbon forms during the closing down operation of the HFCVD process. Non-diamond carbon in discontinuous form on the surface of the diamond film can be easily in-elastically sputtered using high energy ions. We believe that the in-elastic sputtering could be responsible for the observed elimination of non-diamond carbon from our diamond films after irradiation. Electronic-loss induced phase transformation is another likely effect which needs to be considered. The E to w phase transformation in titanium” has been explained using coulomb explosion model” which predicts high pressure shock-wave like situation during high energy irradiation. Thermal spike modellh predicts local melting and rapid cooling along the tracks of the ions, which could also predict phase transformations. These high pressure and high temperature could induce phase transformation from non-diamond to diamond phase. However. as deterioration of diamond quality after irradiation has been reported in the literature,6 we believe that the above mechanisms of phase transformation may not be playing major role in elimination of non-diamond carbon phase from our films. Figure 4 shows the adhesion strength of un-irradiated and irradiated diamond films on WC-Co substrates. Adhesion strength has been improved at least by 65 kgf/cm’. 100 MeV “7 beam deposits 1.2 keV/A to the electronic system and 0.012 keV/A in elastic collisions (nuclear loss) near the diamond film-substrate interface. As the nuclear loss is insignificant it is not likely to cause any changes at the interface. No beam mixing is expected at such a low dose. Thus the adhesion improvement can not be due to mixing caused by the nuclear stopping power S,,. As proposed by Tombrello’ the electronic stopping S, can cause changes in the interface chemistry and improve adhesion. Adhesion improvement of Cu on Teflon by GeV ion irradiation
2201
7
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200 -
120 -
100
1
3
Figure 4. Adhesion strengths of diamond films on WC-Co tool bit: unirradiated and irradiated using 100MeV ‘I’1 beam at different doses. Specimen 1: un-irradiated. Specimen 2: irradiated to a dose of 5 x lOi ions/cm’. Specimen 3: irradiated to a dose of 1 x lOI ions/cm’. 1001
U R Mhatre
et a/: Adhesion
improvement
after MeV ion irradiation
1 40 “G
B B Z30 F! 3 8 ‘W 20
-
Existence of the threshold dose D,,, for adhesion improvement has been proposed.’ This D,,, is different for different systems. Tombrello’ has developed a functional relationship between the threshold dose and electronic energy loss (S,) of the ion at the interface. According to this model, for adhesion one needs transfer of electrons across the interface where the required energy is provided by S,,. When one ion crosses the interface it transfers a number of electrons across the interface. The electrons transferred across the interface are utilized in the formation of new bonds. Thus we may write
-
5 lo-
0
-
3
1
Specimen Numbex Figure 5. Adhesion strengths of silver films on soda glass: w-irradiated and irradiated using 100MeV ‘?‘I beam at different doses. Specimen 1: m-irradiated. Specimen 2: irradiated to a dose of 5 x lO”ions/cm’. Specimen 3: irradiated to a dose of 1 x 10” ions/‘cm’.
using the melting in thermal spikes.‘h Similar explanation could be given even for the present results of adhesion improvement. To assess the role of the electronic stopping S, in the improvement of adhesion further experiments were carried out on the silver films. Figure 5 shows the adhesion strengths of un-irradiated and irradiated silver films on soda glass. As can be seen, even nominal dose of 5 x 10” ions/cm’ is sufficient to improve adhesion. To determine the role of electronic stopping power .S,. in adhesion improvement. Scotch tape test was used on un-irradiated and irradiated Ag films on soda glass. Three specimens were irradiated using 100 MeV “‘I beam at a dose of I x IO” ions/cm’. Two of the specimens were covered using thin Al foils of two different thicknesses during irradiation. The specimen without the Al foil cover gets exposed to 100 MeV ion beam and at the interface the beam deposits S, = 2.6 keV/A. One specimen was covered using .5pm thick Al foil. As the range of 1OOMeV “‘1 beam is 15 /Lrn, the beam is transmitted through the foil. The energy of the transmitted beam is 50 MeV which falls on the Ag film and at the interface it deposits S, = 1.88keV/A. The third specimen was covered using a 12 pm Al foil. The energy of transmitted beam through this foil is about 16MeV and thus the electronic energy loss (S,) deposited at the interface was about 0.7 keV/A. The results are shown in Table 1. was explained
The minimum number of bonds required for adhesion is constant for given film-substrate interface. The minimum number of ions passing across the interface is directly related to the threshold dose D,,,. The number of electrons transferred by one ion is directly related to the electronic energy loss S,.. This model has been tested for many couples and D,,, is found to be proportional to (S,))“, where n takes values between 1 and 3 for various filmsubstrate couples, For example, the exponent n determined using the scotch tape test is found to be 1.42 in the case of Au film on tantalum.” Another way of looking at the above model is that if the dose is kept constant then to form minimum number of bonds for adhesion one ion should transfer minimum number of electrons. Thus for a constant dose a threshold S,. value is required for adhesion. which was observed in our Scotch tape experiment (see Table 1). The Scotch tape ‘passed’ the first two specimens whereas the third specimen ‘failed’ in the test. Also un-irradiated specimens ‘failed’ in the test, Therefore it is established that S, > 0.7 keV/A is needed if the adhesion is required to be achieved at a dose of 1 x 1O’j ions/cm* using ion irradiation of 100 MeV ‘*‘I beam. As the nuclear stopping power is 0.027 keV/A for 100 MeV ‘*‘I beam in Ag, the dose of I x 10” ionsl’cm’ works out to be 0.003 dpa. Such a low value of dpa could barely induced any mixing at the interface. Hence the adhesion improvement due to ion beam mixing caused by the nuclear-loss is ruled out. Therefore, the present results indicate that electronic stopping power is responsible for the adhesion improvement. One should be able to achieve similar improvement in adhesion strength using either electron beam or even gamma ray irradiations in which large amount of energy is deposited to the electronic system of the material.
Conclusions Table 1. Results of Scotch tape test
Energy 100 MeV 50 MeV 16MeV Un-irradiated
Electronic (keV/A) 2.6 1.8 0.7
loss .S,. Scotch tape test results Passed Passed Failed Failed
1OOMeV ‘? ion beam irradiation improves the quality of diamond film and improves adhesion at the interface of diamond film and WC-Co substrate. SEM images show no morphological modifications of the films after irradiation. Laser Raman spectra show that the irradiation causes almost complete removal of nondiamond carbon present in the films. The ion beam irradiation also gives rise to significant improvement in adhesion strength of silver films on soda glass. Low dose of 5 x lO’*ions/cm’ is sufficient to cause the improvement. Minimum electronic energy
U R Mhatre et a/:Adhesion
improvement
after MeV ion irradiation
loss S, is required to get improvement in adhesion strength. The present work has shown that for silver film on soda glass S, > 0.7 keV/A is needed if the adhesion is required to be achieved at a dose of I x 10”ions/cm2 using ion irradiation of 1OOMeV 12’1beam. The main cause of the improvements in the adhesion and quality of the films is suggested to be the energy deposited to the electronic system of the material by the projectile ion. References Baglin, J.E.E.E., in Ion Beam modification ofInsulators, chapter 15, P. Mazzoldi and R.W. Arnold (eds.), Elsevier, Amsterdam, 1987. Baglin, J.E.E.E. and Clark, G.J., Nucl. Znstr. and Methods. 1985, B7/8, 88 1. Tombrello, T.A., Materials Science and Engineering, 1985. 69. 443. Wang, L., Anquert, N., Ruck, D., Trautmann, C., Vetter, J., Quan, Z. and Hantsche, H., Nwl. Instr. and Methods, 1993, B83, 503. Wu, R.L.C., Miyoshi, K., Korenyi-Both, A.L., Garscadden, A. and Barnes, P.N., Surface Coating Technology, 1993, 62, 589.
6. Barshilia, H.C., Sha, S., Mehta, B.R., Vankar, V.D., Avasthi, D.K., Kabiraj, D. and Jaipal, Mehta. G.K., Thin Solid Films, 1995, 258, 123. 7. Tombrello, T.A., Proc. Mar. Res. Sot., 1984, 25, 173. 8. Kanetkar, SM., Kulkarni, A.A., Vaidya, A., Vispute, R.D., Ogale, S.B., Kshirsagar, S.T. and Purandare, SC.. Appl. Phys. Left., 1993, 63, 740. 9. Wada, N. and Solin, S.A., Physica B and C, 1981, 105B, 353. IO. Baranov, I.A., Obnorskii, V.V. and Tsepelevich, S.O., Nucl. Instr. and Methods in Physics Research, 1990, B52, 9. 11, Dunlop, A. and Lesueur, D., Radiation Ejfbcts and Defkrs in Solids, 1993, 126, 123. 12. Dammak, A., Barbu, A., Dunlop, A., Lesueur, D. and Lorenzelli. N., Phil. Msg. Lett., 1993, 67A, 253. 13. Merkle, K.L., Phys. Ren. Lett., 1962, 9, 150. 14. Pate, B.B., Surl: Sci., 1986, 165. 83. 15. Seitz, F., Disc. Faraday Sot., 1949, 5, 27 1. 16. Varley, J.H.D., Namrr, 1954, 174, 886. 17. Sugdan, S., Sofield, C.J. and Murrell, M.P.. Nucl. Instr. and Methods, 1989, B44, 137.
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