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Correlation between elasticity modulus of metallic films and threshold dose for MeV ion-induced adhesion
Nuclear Instruments North-Holland
and Methods
in Physics
Research
B44 (1990) 445-448
CORRELATION BETWEEN ELASTICITY MODULUS DOSE FOR MeV ION-INDUCED ADHESION
OF METALLIC
445
FILMS AND THRESHOLD
Zhengmin LIU and Jianjun DONG Department Received
of Modern Physics, Lanzhou 14 August
University, PR China
1989 and in revised form 2 October
1989
Films of In, Sn, Al, Cu, Pd and Ni were deposited on Teflon substrates and bombarded with 6 MeV F ions. The Scotch tape test was employed to measure adhesion changes as a function of dose. It is found that the threshold dose seems to be proportional to the ratio of the film material elasticity modulus to the electronic stopping power. An empirical formula for calculating threshold dose is presented. The experimental results suggest that the enhanced adhesion effect may be correlated with changes of internal stresses in the film and substrate. Some of the phenomena involving MeV ion-induced adhesion are explained in terms of this stress change model.
1. Introduction Although a lot of experimental data on thin film enhanced adhesion induced by MeV ion beam inadiation has been accumulated during the last ten years, the basic mechanism which is responsible for the adhesion improvement is still not clear. Various hypotheses have been advanced to account for the mechanism, such as the formation of interfacial compounds [1,2], atomic mixing [3,4], wetting due to the induced change in surface tesion [5], etc. But none of these can completely interpret the full range of phenomena observed in this active area of research. It is often assumed that the difficulties in determining the exact mechanism are probably due to the lack of a quantitative adhesion test [5], the variety of experimental conditions [8] and the existence of several different processes involved in the enhanced adhesion effect [7]. The operative mechanism therefore might not be the same for different situations. In this paper, we present experimental results on metal-Teflon systems bombarded by 6 MeV F ions and propose a model which can be used to explain qualitatively more phenomena observed in previous experiments than other models do. According to thin film physics, the origin of the adhesion of thin films to substrates lies in the Van der Waals forces or the chemical binding. Mechanical properties of a thin film, including adhesion, are largely determined by the film structure and the extent of the internal stress in the film. The internal stresses in films depend strongly on the actual method of film preparation, and may reach large values of the order of 10 4-10 5 N/cm’ [9]. We believe that such large internal stresses in films could affect the adhesion of films to substrates. As a visible proof of this assumption, it may be noticed 0168-583X/90/$03.50 (North-Holland)
0 Elsevier Science Publishers
B.V.
that after a film is peeled from a substrate the film curls, or when using a Scotch tape to test a poor adhesion system the peeled tape, on which the film is stuck, curls dramatically. The film curling indicates that the internal stress tends to be against the Van der Waals force or chemical binding and therefore reduces the film adhesion. If the status of internal stress in a film is changed by a physical or chemical process, the film adhesion would probably be changed. During an irradiation of a film-substrate system, the bombardment of energetic ions generates new stresses in the film and the near surface area of the substrate [7,12]. Those new stresses must compete with initial internal stresses and they give rise to a new status for the internal stress. Subsequently, the adhesion of the film to the substrate may be changed. Based on this idea, we performed experiments on metal-Teflon systems using 6 MeV F ion bombardments for the purpose of looking for a correlation between the elasticity modulus of metallic films and the threshold dose for enhanced adhesion. The reason for choosing Teflon as the substrate material is that for low melting-point metals and soft plastics, the internal stresses in them disappear rather rapidly due to “selfannealing” or creeping at room temperature [9]. In such a case, internal stresses in soft substrates can be negligible, so that only stresses in films should be considered.
2. Experimental The Teflon substrates used were cut from commercial and polished sheets 3.5 mm in thickness. They were cleaned in hot detergent, rinsed.in deionized water and
446
Z. Liu, J. Dong / MeV ion-induced adhesion
high purity alcohol. In, Sn, Al, Cu, Pd and Ni films were deposited on substrates using a diffusion-pumped evaporator operating at < 9 X lop6 Torr. Thicknesses of the films were not controlled precisely during evaporation; the films were accepted as long as they were thin enough to ensure that 6 MeV F ion bombardment was in the electronic stopping regime (see section 3).
Irradiation of the samples was performed by F4+ ions with E = 6.0 MeV and doses $I = 1.0 x 101l-l.O X 10’3/cm2 at beam current density of 9 nA/cm2. Dimensions of the beam spot on the target were 3.5 X 4.5 mm2. In order to investigate the dependence of threshold dose for enhanced adhesion on the beam current density, three currents: 9, 85 and 180 nA/cm2 were used to bombard the %-Teflon sample in the dose range as mentioned above. The Scotch tape test was used to measure threshold doses for all samples.
3. Results and discussion Prior to ion beam bombardment, all films on Teflon substrates were so poorly adherent that they did not pass the Scotch tape test. After the irradiations the higher dose bombarded areas passed the test because of adhesion improvement. Typical results on the adhesion enhancement of the metallic films on Teflon are shown in fig. 1. Threshold doses measured by the Scotch tape test with other parameters are summarized in table 1. It is found that the threshold dose seems to be proportional to the ratio of the film material elasticity modulus to the electronic stopping power. Plotted in fig. 2 is that linear relationship. Fitting the straight line in fig. 2, we obtain an empirical formula as follows
-r
‘g
6.0
P
N
0
a”
3.0
0 10
20
M,/ (dE / dx),
30
40
(kg / MeV.cm)
Fig. 2. Threshold dose versus ratio of elasticity modulus to electronic stopping power for metallic films on Teflon.
where Dth is the threshold dose for a given film on Teflon; M, is the elasticity modulus (Young’s modulus) of the film material; (dE/dx), is the electronic stopping power for a bombarding ion in the film; k = 1.5 x 101’ MeV/kg . cm and Do = 4.5 X 10n/cm2. The source of error in k and Do was mainly from the measurements of threshold doses in our experiments. The dose steps used for this study were 4 or 8 X 101’/cm2. The actual threshold doses for adhesion could thus be about 15%25% lower than the measured doses. It should be noted that the elasticity moduli of metal films are in agreement with the bulk values [9], which can therefore be used in the formula. Using the formula, we could calculate threshold doses for other metal-Teflon systems or particles. Griffith and co-workers irradiated Au films on Teflon with 1 MeV He, 2 MeV H and 5 MeV F, and found the threshold doses for enhanced adhesion were approximately 2 X 1013, < 5 X 1014 and 3 X 1012/cm2, respec-
Fig. 1. Photographs of typical samples after irradiation and application of the Scotch tape test. Black squares are strong bonded films remaining on substrates. Areas marked by arrows are carbonized beam spots caused by high dose ion bombardments.
441
2. IA, J. Dong / MeV ion-induced adhesion
Table 1 Film parameters and adhesion thresholds Film
Thickness
M, a)
(dE/dx),
(A)
Elasticity
(MeV/cm)
h,
modulus 1780 900 700 1900 1400 1300
1.10x 10’ 4.24x 10’ 1.18 x lo6 1.10x106 7.0 x105 2.0 x 106
Adhesion
(dE/dx),
dose (cmm2)
(kg/MeV
(kg/cm’) In Sn Pd CU Al Ni
Mi
2.84~ IO4 2.73 x lo4 4.88 x lo4 4.48~10” 2.18x IO“ 4.61 x IO4
3.90 15.8 24.2 24.6 32.1 43.4
threshold cm)
osx10’2 1.7 x 10’2 2.5 x lOI 2.9~ 10” 4.5 x 10’2 6.1 x lOI
‘) Values from ref. [15]. b, For 6 MeV F ion stopping in films. From ref. [14].
tively [lo]. Substituting the parameters for the Au film case into the formula, the threshold doses obtained by the calculations are 1.8 X lo”, 1.6 X lOI and 2.4 X 10’*/cm2, in reasonable agreement with the experimental values. The formula can be easily understood according to our model. The internal stress in a film is proportional to the film elasticity modulus [9]. For a film which has a larger value of Young’s modulus, changing the status of the initial internal stress in the film needs more energy, which is equivalent to needing more ions to deposit the energy in the film during an ion bombardment. Also, the threshold dose for enhanced adhesion could be larger for a given case, depending upon the adhesion test method. The energy deposited in the film by an ion is approximately proportional to the electronic stopping power provided that the film is thin enough. Thus, the threshold dose is related to the reciprocal of the stopping power. It might be surprising that the threshold dose in our experiments is independent of the film thickness. The result seems to be at variance with other work [6]. As a possible explanation for this discrepancy, we assume that for a film-nonplastic substrate system, the initial iternal stress in the near surface area of the substrate, which could influence the behavior of the film adhesion, cannot be negligible as it is for a Teflon substrate. Ion beam bombardment then changes the state of stress in the substrate as well as that in the film. The changes of both film and substrate stresses would simultaneously contribute to the film adhesion enhancement. But the energy deposited in the substrate by a bombarding ion depends strongly on the film thickness. As a result, a dependence of threshold dose to film thickness would appear for such a system. Under circumstances where neglecting the stress in the near surface area of a substrate is appropriate, as in a Teflon or a low melting-point substrate, the average initial stress in a film is almost independent of its thickness [9]. While the
thickness of the film increases: the total stress in the film increases proportionally for a given irradiated area on the film, which means the energy needed to change the total stress increases proportionally, too. But the energy deposited in the film by an ion, as mentioned above, is almost proportional to the thickness of the film. Therefore, the film thickness becomes unimportant in the process. The result of changing the beam current used to bombard the Sn-Teflon sample showed that the beam heating effect did not influence the threshold dose, because a constant value of threshold dose for different beam currents was found. For some of the higher dose areas on the samples, the Teflon could not withstand the long beam bombardment and was slightly carbonized, which led to the films peeling from the substrates (fig. 1). Using the model presented above, some of phenomena of MeV ion-enhanced adhesion can qualitatively be explained: (1) The threshold dose for film enhanced adhesion depends on the method and condition of film preparation [7]. Because the status of initial internal stresses in a film and near surface area of a substrate depends on the method and condition of film preparation [9], the dose needed to change significantly the adhesion should be different. (2) The threshold doses for film-Teflon systems arc quite low, of the order of 10’2-10’3/cm2; for film-metal systems the threshold doses are higher, of the order of 10”-10’4/cm2; whereas for film-semiconductor or inorganic insulator systems the thresholds are the highest. of the order of 10’4-10’6/cm2 [lo]. According to our model, it is assumed that the changes of both film and substrate stresses would simultaneously affect the behavior of the film adhesion. These changes must be related to the elasticity moduli of the materials. For Teflon, metal, and semiconductor or inorganic insulator materials used in previous experiments, the values of
448
Z. Liu, J. Dong / MeV ion-induced adhesion
the elasticity moduli just fall into low, medium, and high regimes in the same way as their threshold doses do. The larger the modulus of a substrate material is, the higher the expected threshold dose. (3) It has been found that keV electron or UV irradiation is sufficient to enhance adhesion [13]. Any process which deposits enough energy in a film or substrate to change the internal stresses could change the film adhesion, including enhancing the adhesion and de-adhesion, depending on how the internal stresses change (see (5)). (4) Aging effect: Experiments performed before showed that ion bombardment sometimes does not produce enhanced bonding when tested immediately after the irradiation for several film-substrate combinations, but when tested a couple of days later, it produces bonding [lO,ll]. The cause of the aging effect may be creep in the film and/or substrate materials, which is induced by new internal stresses after an irradiation. The process of creep relieves the stresses gradually and thereby gives rise to an aging effect in the film adhesion enhancement. (5) De-adhesion effect: For some film-substrate combinations an ion irradiation can cause film de-adhesion and blistering [7,11]. This phenomenon can also be interpreted by our model. If the initial stress in a film is opposite in direction to the beam induced stress, the stress in the film, at first, would be relieved by the irradiation, and enhanced adhesion could occur. After equilibrium, the beam induced stresses in the film and substrate become larger and larger with increasing irradiation dose. As a result, the film adhesion would be reduced so that in the limit de-adhesion and blisters may occur. The experimental results of Cu films on MO substrates bombarded by 3 MeV silicon and nickel may be examples of such an effect [3]. On the other hand, the internal stress in a film might be in the same direction as the beam-induced stress, in this case, the stress in the film would not be relieved and the film enhanced adhesion could never occur, only de-adhesion and blisters would happen. The case of Au films deposited on sapphire irradiated by N ions and Pt films on glass bombarded by He ions may be in this situation [7,Ill.
4. Conclusions a) The enhanced adhesion effect induced by MeV ion bombardment may be correlated with changes of internal stresses in a film and a substrate. The elasticity modulus of film and substrate materials affects the threshold dose for the enhanced adhesion. b) For metallic films on Teflon, the threshold dose is proportional to the ratio of the film material elasticity modulus to the electronic stopping power of the bombarding ion in the film. c) The empirical formula obtained by these experiments may be used to calculate or estimate the threshold dose for other film-Teflon systems and bombarding particles.
References 111 J.E.E. Baglin, A.G. Schrott, R.D. Thompson,
K.N. Tu and A. Segmuller, Nucl. Instr. and Meth. B19/20 (1987) 782. PI Chin-An Chang, J.E.E. Baglin, A.G. Schrott and K.C. Lin, Appl. Phys. Lett. 51 (1987) 103. [31 D.C. Ingram and P.P. Pronko, Nucl. Instr. and Meth. B13 (1986) 462. A.I. Kamardin, Z.A. Iskanderova and 141 T.D. Radjabov, M.P. Parpiev, Nucl. Instr. and Meth. B28 (1987) 344. 151 D.K. Sood and J.E.E. Baghn, Nucl. Instr. and Meth. B19/20 (1987) 954. 161 R.G. Stokstad, P.M. Jacobs, I. Tserruya, L. Sapir and G. Mamane, Nucl. Instr. and Meth. B16 (1986) 465. [71 B. Daudin and P. Martin, Nucl. Instr. and Meth. B34 (1988) 181. @I P. Benjamin and C. Weaver, Proc. R. Sot. (London) A281 (1961) 516. PI L. Eckertova, Physics of Thin Films (Plenum. New York, 1977) chap. 6, p. 151. WI J.E. Griffith, Yuanxun Qiu and T.A. Tombrello, Nucl. Instr. and Meth. 198 (1982) 607. VI D.K. Sood, W.M. Skinner and J.S. Williams, Nucl. Instr. and Meth. B7/8 (1985) 893. R.P. Livi, T. WI C.R. Wie, C.R. Shi, M.H. Mendenhah, Vreeland, Jr, and T.A. Tombrello, Nucl. Instr. and Meth. B9 (1985) 20. t131 I.V. Mitchell, G. Nyberg and R.G. Elliman, Appl. Phys. Lett. 45 (1984) 137. and R.F. SchilIing, Nucl. Data Tables, u41 L.C. Northcliffe vol. 7 (Academic Press, New York, 1970). 1151 Handbook of Industrial Materials, 1st ed. (Trade & Technical Press).
and Methods
in Physics
Research
B44 (1990) 445-448
CORRELATION BETWEEN ELASTICITY MODULUS DOSE FOR MeV ION-INDUCED ADHESION
OF METALLIC
445
FILMS AND THRESHOLD
Zhengmin LIU and Jianjun DONG Department Received
of Modern Physics, Lanzhou 14 August
University, PR China
1989 and in revised form 2 October
1989
Films of In, Sn, Al, Cu, Pd and Ni were deposited on Teflon substrates and bombarded with 6 MeV F ions. The Scotch tape test was employed to measure adhesion changes as a function of dose. It is found that the threshold dose seems to be proportional to the ratio of the film material elasticity modulus to the electronic stopping power. An empirical formula for calculating threshold dose is presented. The experimental results suggest that the enhanced adhesion effect may be correlated with changes of internal stresses in the film and substrate. Some of the phenomena involving MeV ion-induced adhesion are explained in terms of this stress change model.
1. Introduction Although a lot of experimental data on thin film enhanced adhesion induced by MeV ion beam inadiation has been accumulated during the last ten years, the basic mechanism which is responsible for the adhesion improvement is still not clear. Various hypotheses have been advanced to account for the mechanism, such as the formation of interfacial compounds [1,2], atomic mixing [3,4], wetting due to the induced change in surface tesion [5], etc. But none of these can completely interpret the full range of phenomena observed in this active area of research. It is often assumed that the difficulties in determining the exact mechanism are probably due to the lack of a quantitative adhesion test [5], the variety of experimental conditions [8] and the existence of several different processes involved in the enhanced adhesion effect [7]. The operative mechanism therefore might not be the same for different situations. In this paper, we present experimental results on metal-Teflon systems bombarded by 6 MeV F ions and propose a model which can be used to explain qualitatively more phenomena observed in previous experiments than other models do. According to thin film physics, the origin of the adhesion of thin films to substrates lies in the Van der Waals forces or the chemical binding. Mechanical properties of a thin film, including adhesion, are largely determined by the film structure and the extent of the internal stress in the film. The internal stresses in films depend strongly on the actual method of film preparation, and may reach large values of the order of 10 4-10 5 N/cm’ [9]. We believe that such large internal stresses in films could affect the adhesion of films to substrates. As a visible proof of this assumption, it may be noticed 0168-583X/90/$03.50 (North-Holland)
0 Elsevier Science Publishers
B.V.
that after a film is peeled from a substrate the film curls, or when using a Scotch tape to test a poor adhesion system the peeled tape, on which the film is stuck, curls dramatically. The film curling indicates that the internal stress tends to be against the Van der Waals force or chemical binding and therefore reduces the film adhesion. If the status of internal stress in a film is changed by a physical or chemical process, the film adhesion would probably be changed. During an irradiation of a film-substrate system, the bombardment of energetic ions generates new stresses in the film and the near surface area of the substrate [7,12]. Those new stresses must compete with initial internal stresses and they give rise to a new status for the internal stress. Subsequently, the adhesion of the film to the substrate may be changed. Based on this idea, we performed experiments on metal-Teflon systems using 6 MeV F ion bombardments for the purpose of looking for a correlation between the elasticity modulus of metallic films and the threshold dose for enhanced adhesion. The reason for choosing Teflon as the substrate material is that for low melting-point metals and soft plastics, the internal stresses in them disappear rather rapidly due to “selfannealing” or creeping at room temperature [9]. In such a case, internal stresses in soft substrates can be negligible, so that only stresses in films should be considered.
2. Experimental The Teflon substrates used were cut from commercial and polished sheets 3.5 mm in thickness. They were cleaned in hot detergent, rinsed.in deionized water and
446
Z. Liu, J. Dong / MeV ion-induced adhesion
high purity alcohol. In, Sn, Al, Cu, Pd and Ni films were deposited on substrates using a diffusion-pumped evaporator operating at < 9 X lop6 Torr. Thicknesses of the films were not controlled precisely during evaporation; the films were accepted as long as they were thin enough to ensure that 6 MeV F ion bombardment was in the electronic stopping regime (see section 3).
Irradiation of the samples was performed by F4+ ions with E = 6.0 MeV and doses $I = 1.0 x 101l-l.O X 10’3/cm2 at beam current density of 9 nA/cm2. Dimensions of the beam spot on the target were 3.5 X 4.5 mm2. In order to investigate the dependence of threshold dose for enhanced adhesion on the beam current density, three currents: 9, 85 and 180 nA/cm2 were used to bombard the %-Teflon sample in the dose range as mentioned above. The Scotch tape test was used to measure threshold doses for all samples.
3. Results and discussion Prior to ion beam bombardment, all films on Teflon substrates were so poorly adherent that they did not pass the Scotch tape test. After the irradiations the higher dose bombarded areas passed the test because of adhesion improvement. Typical results on the adhesion enhancement of the metallic films on Teflon are shown in fig. 1. Threshold doses measured by the Scotch tape test with other parameters are summarized in table 1. It is found that the threshold dose seems to be proportional to the ratio of the film material elasticity modulus to the electronic stopping power. Plotted in fig. 2 is that linear relationship. Fitting the straight line in fig. 2, we obtain an empirical formula as follows
-r
‘g
6.0
P
N
0
a”
3.0
0 10
20
M,/ (dE / dx),
30
40
(kg / MeV.cm)
Fig. 2. Threshold dose versus ratio of elasticity modulus to electronic stopping power for metallic films on Teflon.
where Dth is the threshold dose for a given film on Teflon; M, is the elasticity modulus (Young’s modulus) of the film material; (dE/dx), is the electronic stopping power for a bombarding ion in the film; k = 1.5 x 101’ MeV/kg . cm and Do = 4.5 X 10n/cm2. The source of error in k and Do was mainly from the measurements of threshold doses in our experiments. The dose steps used for this study were 4 or 8 X 101’/cm2. The actual threshold doses for adhesion could thus be about 15%25% lower than the measured doses. It should be noted that the elasticity moduli of metal films are in agreement with the bulk values [9], which can therefore be used in the formula. Using the formula, we could calculate threshold doses for other metal-Teflon systems or particles. Griffith and co-workers irradiated Au films on Teflon with 1 MeV He, 2 MeV H and 5 MeV F, and found the threshold doses for enhanced adhesion were approximately 2 X 1013, < 5 X 1014 and 3 X 1012/cm2, respec-
Fig. 1. Photographs of typical samples after irradiation and application of the Scotch tape test. Black squares are strong bonded films remaining on substrates. Areas marked by arrows are carbonized beam spots caused by high dose ion bombardments.
441
2. IA, J. Dong / MeV ion-induced adhesion
Table 1 Film parameters and adhesion thresholds Film
Thickness
M, a)
(dE/dx),
(A)
Elasticity
(MeV/cm)
h,
modulus 1780 900 700 1900 1400 1300
1.10x 10’ 4.24x 10’ 1.18 x lo6 1.10x106 7.0 x105 2.0 x 106
Adhesion
(dE/dx),
dose (cmm2)
(kg/MeV
(kg/cm’) In Sn Pd CU Al Ni
Mi
2.84~ IO4 2.73 x lo4 4.88 x lo4 4.48~10” 2.18x IO“ 4.61 x IO4
3.90 15.8 24.2 24.6 32.1 43.4
threshold cm)
osx10’2 1.7 x 10’2 2.5 x lOI 2.9~ 10” 4.5 x 10’2 6.1 x lOI
‘) Values from ref. [15]. b, For 6 MeV F ion stopping in films. From ref. [14].
tively [lo]. Substituting the parameters for the Au film case into the formula, the threshold doses obtained by the calculations are 1.8 X lo”, 1.6 X lOI and 2.4 X 10’*/cm2, in reasonable agreement with the experimental values. The formula can be easily understood according to our model. The internal stress in a film is proportional to the film elasticity modulus [9]. For a film which has a larger value of Young’s modulus, changing the status of the initial internal stress in the film needs more energy, which is equivalent to needing more ions to deposit the energy in the film during an ion bombardment. Also, the threshold dose for enhanced adhesion could be larger for a given case, depending upon the adhesion test method. The energy deposited in the film by an ion is approximately proportional to the electronic stopping power provided that the film is thin enough. Thus, the threshold dose is related to the reciprocal of the stopping power. It might be surprising that the threshold dose in our experiments is independent of the film thickness. The result seems to be at variance with other work [6]. As a possible explanation for this discrepancy, we assume that for a film-nonplastic substrate system, the initial iternal stress in the near surface area of the substrate, which could influence the behavior of the film adhesion, cannot be negligible as it is for a Teflon substrate. Ion beam bombardment then changes the state of stress in the substrate as well as that in the film. The changes of both film and substrate stresses would simultaneously contribute to the film adhesion enhancement. But the energy deposited in the substrate by a bombarding ion depends strongly on the film thickness. As a result, a dependence of threshold dose to film thickness would appear for such a system. Under circumstances where neglecting the stress in the near surface area of a substrate is appropriate, as in a Teflon or a low melting-point substrate, the average initial stress in a film is almost independent of its thickness [9]. While the
thickness of the film increases: the total stress in the film increases proportionally for a given irradiated area on the film, which means the energy needed to change the total stress increases proportionally, too. But the energy deposited in the film by an ion, as mentioned above, is almost proportional to the thickness of the film. Therefore, the film thickness becomes unimportant in the process. The result of changing the beam current used to bombard the Sn-Teflon sample showed that the beam heating effect did not influence the threshold dose, because a constant value of threshold dose for different beam currents was found. For some of the higher dose areas on the samples, the Teflon could not withstand the long beam bombardment and was slightly carbonized, which led to the films peeling from the substrates (fig. 1). Using the model presented above, some of phenomena of MeV ion-enhanced adhesion can qualitatively be explained: (1) The threshold dose for film enhanced adhesion depends on the method and condition of film preparation [7]. Because the status of initial internal stresses in a film and near surface area of a substrate depends on the method and condition of film preparation [9], the dose needed to change significantly the adhesion should be different. (2) The threshold doses for film-Teflon systems arc quite low, of the order of 10’2-10’3/cm2; for film-metal systems the threshold doses are higher, of the order of 10”-10’4/cm2; whereas for film-semiconductor or inorganic insulator systems the thresholds are the highest. of the order of 10’4-10’6/cm2 [lo]. According to our model, it is assumed that the changes of both film and substrate stresses would simultaneously affect the behavior of the film adhesion. These changes must be related to the elasticity moduli of the materials. For Teflon, metal, and semiconductor or inorganic insulator materials used in previous experiments, the values of
448
Z. Liu, J. Dong / MeV ion-induced adhesion
the elasticity moduli just fall into low, medium, and high regimes in the same way as their threshold doses do. The larger the modulus of a substrate material is, the higher the expected threshold dose. (3) It has been found that keV electron or UV irradiation is sufficient to enhance adhesion [13]. Any process which deposits enough energy in a film or substrate to change the internal stresses could change the film adhesion, including enhancing the adhesion and de-adhesion, depending on how the internal stresses change (see (5)). (4) Aging effect: Experiments performed before showed that ion bombardment sometimes does not produce enhanced bonding when tested immediately after the irradiation for several film-substrate combinations, but when tested a couple of days later, it produces bonding [lO,ll]. The cause of the aging effect may be creep in the film and/or substrate materials, which is induced by new internal stresses after an irradiation. The process of creep relieves the stresses gradually and thereby gives rise to an aging effect in the film adhesion enhancement. (5) De-adhesion effect: For some film-substrate combinations an ion irradiation can cause film de-adhesion and blistering [7,11]. This phenomenon can also be interpreted by our model. If the initial stress in a film is opposite in direction to the beam induced stress, the stress in the film, at first, would be relieved by the irradiation, and enhanced adhesion could occur. After equilibrium, the beam induced stresses in the film and substrate become larger and larger with increasing irradiation dose. As a result, the film adhesion would be reduced so that in the limit de-adhesion and blisters may occur. The experimental results of Cu films on MO substrates bombarded by 3 MeV silicon and nickel may be examples of such an effect [3]. On the other hand, the internal stress in a film might be in the same direction as the beam-induced stress, in this case, the stress in the film would not be relieved and the film enhanced adhesion could never occur, only de-adhesion and blisters would happen. The case of Au films deposited on sapphire irradiated by N ions and Pt films on glass bombarded by He ions may be in this situation [7,Ill.
4. Conclusions a) The enhanced adhesion effect induced by MeV ion bombardment may be correlated with changes of internal stresses in a film and a substrate. The elasticity modulus of film and substrate materials affects the threshold dose for the enhanced adhesion. b) For metallic films on Teflon, the threshold dose is proportional to the ratio of the film material elasticity modulus to the electronic stopping power of the bombarding ion in the film. c) The empirical formula obtained by these experiments may be used to calculate or estimate the threshold dose for other film-Teflon systems and bombarding particles.
References 111 J.E.E. Baglin, A.G. Schrott, R.D. Thompson,
K.N. Tu and A. Segmuller, Nucl. Instr. and Meth. B19/20 (1987) 782. PI Chin-An Chang, J.E.E. Baglin, A.G. Schrott and K.C. Lin, Appl. Phys. Lett. 51 (1987) 103. [31 D.C. Ingram and P.P. Pronko, Nucl. Instr. and Meth. B13 (1986) 462. A.I. Kamardin, Z.A. Iskanderova and 141 T.D. Radjabov, M.P. Parpiev, Nucl. Instr. and Meth. B28 (1987) 344. 151 D.K. Sood and J.E.E. Baghn, Nucl. Instr. and Meth. B19/20 (1987) 954. 161 R.G. Stokstad, P.M. Jacobs, I. Tserruya, L. Sapir and G. Mamane, Nucl. Instr. and Meth. B16 (1986) 465. [71 B. Daudin and P. Martin, Nucl. Instr. and Meth. B34 (1988) 181. @I P. Benjamin and C. Weaver, Proc. R. Sot. (London) A281 (1961) 516. PI L. Eckertova, Physics of Thin Films (Plenum. New York, 1977) chap. 6, p. 151. WI J.E. Griffith, Yuanxun Qiu and T.A. Tombrello, Nucl. Instr. and Meth. 198 (1982) 607. VI D.K. Sood, W.M. Skinner and J.S. Williams, Nucl. Instr. and Meth. B7/8 (1985) 893. R.P. Livi, T. WI C.R. Wie, C.R. Shi, M.H. Mendenhah, Vreeland, Jr, and T.A. Tombrello, Nucl. Instr. and Meth. B9 (1985) 20. t131 I.V. Mitchell, G. Nyberg and R.G. Elliman, Appl. Phys. Lett. 45 (1984) 137. and R.F. SchilIing, Nucl. Data Tables, u41 L.C. Northcliffe vol. 7 (Academic Press, New York, 1970). 1151 Handbook of Industrial Materials, 1st ed. (Trade & Technical Press).