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Ion-beam-enhanced adhesion of Au films on Si and SiO2
Nuclear Instruments and Methods in Physics Research B7/8 (1985) 877-880 North-Holland, Amsterdam
ION-BUM-EN~N~ED
ADHE!SION OF Au FILMS ON Si AND SO, *
A.E. BERKOWITZ ‘), R.E. BENENSON *), R.L. FLEISCHER ‘), L. WIELUNSKI W.A. LANFORD *) ‘) General Electric Research and Development Center, P. 0. Box 8, Schenectady, NY 12301, USA
*) and
‘) Department of Physics, State University of New York, Albany, NY 12222, USA
Samples consisting of 500 A Au evaporated onto Si and vitreous silica substrates were irradiated with beams of “N “P “F, and “C with energies between 3.0 and 6.8 MeV and fluences up to IO” ions/cm2. Adhesion was tested by the ‘Scotch tape bull kt” (ST) and by abrading with a dry Q-tip (QT). For all beams, fluences were found that produced (ST) and (QT) adhesion on Si. For SiO, substrates, adhesion was found only for the 31P irradiations, and this was (ST), but not (QT}. Blisters l-10 pm diameter were often found in the irradiated films on Si, probably due to desorption of adsorbed atmosphere gases. The irradiated Si02 substrates always showed evidence of compaction (cracks and depressed regions), and fine scale film porosity was caused by the “P irradiations. Althougfi the predominant energy loss mechanism for those beams in those substrates is electronic, it is possible that atomic collisions can produce the observed adhesion.
1. Introduction
Recent reports of enhanced adhesion of metal films to various substrates after irradiation with high energy (MeV/amu) beams have stimulated speculation concerning the mechanism involved [I]. A principal question is whether the bonding results from mixing produced by atomic collisions at the interface (as found with low-energy ion implantation) or from electronic effects at the interface between the film and substrate atoms. Since these high energy beams are in the electronic stopping region, the latter hypothesis seems plausible. Furthermore, in a dielectric, the electronic excitations can produce atomic displacements [2]. These considerations are combined in the case of a metal film on Si. Although the transfer of electronic excitation into atomic displacement is not expected in either film or substrate, there is always an SiO* film on the Si under ordinary handling conditions. In this thin ( - 20 A) layer atomic displa~ment could be produced by electronic energy loss. We have examined this situation by irradiating Au films on both Si and SiO, substrates. Werner et al. [3] have reported (ST) adhesion for these combinations using 20 MeV “Cl beams. We used beams of lighter atoms and lower energies, albeit still in the electronic stopping region. 2. Experimental The semiconductor grade Si and vitreous SiO, substrates were vapor-degreased in Freon and then * Work supported in part by the Army Research Office. 016g-583X/85/$03.30 @ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
plasma-ashed. 500 A Au films were deposited by evaporation from a filament at 10 A/s in 10m6 Torr. Irradiation of the samples was carried out in the SUNY/Albany Dynamitron accelerator in lo-’ Torr. Beams of 3.4 and 6.5 MeV “N; 3.4 and 6.8 MeV 3’P; 3.0 MeV “F; and 3.3 MeV “C were used. The collimated beam dimensions were 1 x 3 mm2 as measured on an irradiated plastic film. Currents ranged from 40-200 nA. The fluences we quote are based on these beam dimensions and currents. However, the sizes of the irradiated regions showing enhanced adhesion and the sizes of the irradiated regions darkened by polymerized organic contamination from the pumping system, were significantly smaller than 1 X 3 mm2. These facts suggest that the beam inhomogeneity was such that the quoted fluence values are certainly low, perhaps by a factor of 4, Adhesion was tested by pressing a piece of Scotch tape onto the sample, firmly rubbing the back of the tape with an eraser, and slowly pulling off the tape (ST). Another test was vigorous abrasion of the area with a dry Q-tip (Q’b
3. Resutts Many of the irradiated films were darkened by polymerization by the beams of the organic material deposited from the pumping system. Several tests indicated that these organic coatings did not influence adhesion. First, Au films on Si and SiO, irradiated with the same fluences exhibited radically different adhesions. Then, H profiling using the ‘H(lSN, ‘He)12 C XI. FINE LINE ~./DE~SITION/ADHESiON
A. E. Berkowitz et al. / Adhesion 01 Au firmc on ‘Si and SiO,
878
.
Fig. 1. 500 A Au-in Si. Irradiated with 2.5 X lbiw3.4MeV P. After Scotch tape test. - Half of adhering region. The maximum width of the adhered region is 0.3 mm. (4.43 MeV) reaction showed no H in the Au films, although H was observed at the film surface and at the substrate interface. Finally, SIMS analysis showed a
surface build-up of C that increased approximately linearly with dosage, and a fixed small amount at the interface, but none in the Au film. Fig. 1 shows about half of an adhering Au film on Si after the (ST) test. The film had been irradiated with 3.4 MeV 31P at a fluence of 2.5 X 1015/cm2. The areas of adhering Au films decreased with decreasing fluence, due to the higher ion density in the center of the beam. Most irradiated films on Si exhibited the thin domeshaped blisters seen in fig. 1. These are more clearly visible in fig. 2 after (ST) for a higher fluence of 3.4 .MeV “P. Blister diameters were generally l-10 pm in the central (highest fluence) portions of the irradiated regions, with a few much larger blisters on the outer edges. It is very likely that the blisters are produced by the radiation induced desorption of adsorbed atmospheric gases such as CO, O,, N,, H,O, although thermally induced stress relief can also produce film delamination. When non-irradiated films on Si or Si02 were heated for 10 min at temperature > 350°C, blisters developed with a density that increased with temperature in a manner consistent with an Arrhenius dependence. (Desorption of atmospheric gases is commonly encountered for temperatures > 200°C). Furthermore, some of the substrates showed groups of blisters before irradiation. It is interesting (although of uncertain reliability) to estimate sample temperatures during irradiation from blister density using the data from heated samples. Inferred Si substrate sample temperatures ranged from - 250°C for a fluence of 3.3 x 1016 3.4 MeV “N to - 650°C for fluence of 5 x 10” 3.0 MeV
%.
20pm Fig. 2. 500 A Au on Si. Irradiated with 1.1 X 10’6/cm2 3.4 MeV P. After Scotch tape test.
2r-lm Fig. 3. 500 A Au on Si. Irradiated with 1.1 X 1016/cm2 3.4 MeV P. After Scotch tape test.
879
A. E. Berkowrtz er al. / Adhesion of Au films on Si and SiO,
Table 1 Beam parameters and adhesion thresholds Energy (MeV)
J “,
@E/dxL,
(dE/dxL,
primary ionization
(eV/A)
(eV/A)
Range (am)
Adhesion threshold fluence (cm-*)
MN*+
6.5
3.4 3.3 3.0 3.4
139 133 110 154 188
0.31 0.53 0.35 1.1 4.2
5.8 3.8 4.2 3.2 2.8
;y&ym
15N+
18.1 21.3 17.0 29.1 47.5
6.5 3.4 3.3 3.0 3.4 6.8
18.1 21.3 17.0 29.1 47.5 46.7
131 126 104 145 177 234
0.31 0.54 0.35 1.1 4.3 2.5
5.1 3.5 4.1
IOIl
Displacements ‘) per atom
SlbCOfl
12c+ 19F+
3lP+
b’
Vitreous silica 15N2+ lsN+ 12c+ 19F+
31p+ xp2+
2x10’5 < 1.3 x 10’6 <5x10’s 2x10’5
0.27
(ST) b,
2.7
> 1.5 x 10’6 > 10” > 4x10’6 > 2.5 x 1016 2.5 x 1016 < 1.3 x 10’6
a) J values relative to ionization of a 0.5 MeV proton. b, Thresholds for Si for both S(T) and (QT); thresholds for SiO, for (ST) only. ‘) Calculated from modified Rinchin-Pease relation using 4 X threshold fluence in table, (d E/dx),,, an atom.
the films to temperatures to 550°C for 10 min did not enhance adhesion. Fig. 3 shows the remains of a blister at the edge of the adhering region after (ST). The secondary electron yield is different in the area under the blister as compared to the adjacent region. This implies a difference in composition. If the blister is assumed to represent a region of desorbed atmospheric gases, one possibility is that these gases interacted with the Si substrate. Outside the regions where Au adhered after (ST), circular regions on the Si similar to that of fig. 3 indicated the location of blisters that were produced by irradiation. Blisters were rare in films on SiO, substrates. Some were found on films after irradiation by 3.4 MeV 31P. The SiO, substrates consistently showed the compaction that is often reported as due to irradiation [4]. The 1 x 3 mm2 beam area was always visible as a region depressed 1000-2000 A below the sample surface as measured with a Dektak. Large cracks around the irradiated area resulted from the compaction stresses. Irradiation with 3.4 MeV P produced the structure shown in fig. 4. The dark regions are pores in the Au film; the light regions are Au. Table 1 lists some properties of the beams and adhesion threshold data. As noted above, the threshold fluences are lower limits due to beam inhomogeneity. The threshold data for Au on Si indicate adhesion for (ST) and (QT), whereas for Au on SiO,, no (QT) adhesion was observed, and only the “P beams produced (ST) adhesion. Electronic energy loss values, are interpolated from Northcliffe and (dE/dx),,, 19F. Heating
0.09 0.03
and assuming 25 eV to displace
Schilling [S]; atomic energy loss values, (also called nuclear) (dE/dx),,, from Smith [6]. Primary ionization values, J, are calculated by the formula given by Bartlett et al. [7]. For Si substrates, threshold fluences are 2 x 10” for both 3.4 MeV 15N and “P, even though the 31P beam has significantly higher primary ionization, electronic and atomic energy losses. These threshold values are four times the (ST) threshold fluences reported by the Caltech group for 20 MeV “Cl [3]. The higher threshold for 6.5 MeV 15N seems reasonable when compared with 3.4 MeV ‘lP since the J and dE/dx values are significantly lower than those for the ‘lP. However, the threshold for 3.4 MeV 15N is also significantly lower than that for the 6.5 MeV “N even though the J and dE/dx parameters are quite similar. The incomplete data for “C and 19F beams are not inconsistent with the same order of threshold values. The much higher (dE/dx),, for the 3.4 MeV P invites consideration of atomic mixing as the adhesion mechanism even though (d E/dx)=, is very much larger in all cases. Using (dE/dx),, and increasing the quoted thresholds by a factor of four due to beam inhomogeneity, the displacements per atom (DpA) were calculated using the modified Kin&in-Pease relation. The resulting values are shown in the last column in table 1. Even with the uncertainties in the magnitude of the threshold, these rather high dpa values make it difficult to rule out an atomic mixing mechanism due to atomic stopping even though the electronic energy loss is very much higher. XI. FINE LINE STR./DEWSITION/ADHESION
880
A. E. Berkowitz et ui. / Adhesion of Au /ilm
an Si and SiO,
materials on irradiation with ion beams. Using 20 MeV 35Cl, the Caltech group found an erosion yield for SiO, that was more than two orders of magnitude higher than that for Si [I]. We can offer no convincing explanation, other than the effects of different porosities, for a (ST) threshold for ‘*P but not for “C? r9F, or r5N. The implied values for these latter beams seem to be inordinately high, especially in view of the threshold of 2 x 10rs found by Werner et al. f3] for both Au and Ag on SiOz. One possibility is that film preparation conditions play a significant role.
4. conclusions
Fig, 4. 560 A on SiOr. Irradiated with 8 X After Scotch tape test.
10’6/ctn2
3.4 h&V P.
The results for the SiO, substrates are spay different from the Si data. No threshold was found for (QT), and only the 31P beams produced (ST) adhesion. Furthermore, structure produced by the “P beams in fig 4 introduces questions about the (ST) adhesion. The extreme porosity of the Au film may so limit film contact with the tape that an apparent (ST) adhesion might result without significant interfacial bonding. Since the J and d E/dx values are essentially the same for the SiO* as for Si, it is plausible that the lack of adhesion on the SiO, is a consequence of the beam interaction with the SiO, that produces the compaction observed. Compaction in vitreous silica can be produced by X-rays, y-rays, electrons, ions and neutrons [4f. Ionization is most often invoked a a mechanism, although atomic displacements can also be: involved in some cases. Since o&y a [ST) adhesion was found with just one of the beams used, it appears that compaction absorbs energy that might otherwise improve adhesion either by intermixing or interfacial bonding. Another Factor that may account For the much weaker adhesion of films on SiO, as compared to those on Si is tbe difference in erosion yield (back sputtering) of these
It is estimated that atomic collisions can produce significant DPA in Si after irradiation with “P and *‘N beams. Therefore, it is possible that the very strong adhesion of 500 A Au films on Si substrates arises from atomic mixing for “N, 3’P, 19F, and 12C beams even though (dE/dx)& B (dE/dx),,. The lack of strong adhesion on the SiO, substrates for any of the beams might be a consequence either of the atomic processes involved in the SiO, dilation on irradiation, or of the substantial backsputtering from SiO,, The (ST) adhesion on SiO, after the 31P irradiation may result from the fine scale porosity produced in the Au film, rather than from interfacial bonding. We are grateful to C. Robertson for the SEM work and to GA. Smith for the SIMS analyses. We also appreciate useful discussions with A. Mogro-Campero,
References [l] T.A. Tombrello, Nucl. Instr. and Meth. 218 (1983) 679 (Proc. 6th Int. Conf. on Ion Beam Analysis). 121R.L. Fleischer, P.B. Price and R.M. Walker, Nuclear Tracks in Solids (Univ. of California Press, Berkeley, 1975). [3] B.T. Werner, T. Vreeland, Jr., M.H. Mendenhall, Y. Qiu and T.A. Tombrello, Thin Solid Films 104 (1983) 163. [4] W. Primak, Compaction States of Vitreous Silica (Gordon and Breach, New York, 1975). 153L.C. Northciiffe and R.F. Schihmg. Nucl. Data A7 (19701 233. [6] B. Smith, Ion Implantation Range Data for Si and Ge Device Technatogies (Harwell, 1977). ]7] D.F. Bartlett, D. Soo, R.L. Fleischer, H.R. Hart and Mogro-Campero, Phys. Rev. D24 (1981) 612.
ION-BUM-EN~N~ED
ADHE!SION OF Au FILMS ON Si AND SO, *
A.E. BERKOWITZ ‘), R.E. BENENSON *), R.L. FLEISCHER ‘), L. WIELUNSKI W.A. LANFORD *) ‘) General Electric Research and Development Center, P. 0. Box 8, Schenectady, NY 12301, USA
*) and
‘) Department of Physics, State University of New York, Albany, NY 12222, USA
Samples consisting of 500 A Au evaporated onto Si and vitreous silica substrates were irradiated with beams of “N “P “F, and “C with energies between 3.0 and 6.8 MeV and fluences up to IO” ions/cm2. Adhesion was tested by the ‘Scotch tape bull kt” (ST) and by abrading with a dry Q-tip (QT). For all beams, fluences were found that produced (ST) and (QT) adhesion on Si. For SiO, substrates, adhesion was found only for the 31P irradiations, and this was (ST), but not (QT}. Blisters l-10 pm diameter were often found in the irradiated films on Si, probably due to desorption of adsorbed atmosphere gases. The irradiated Si02 substrates always showed evidence of compaction (cracks and depressed regions), and fine scale film porosity was caused by the “P irradiations. Althougfi the predominant energy loss mechanism for those beams in those substrates is electronic, it is possible that atomic collisions can produce the observed adhesion.
1. Introduction
Recent reports of enhanced adhesion of metal films to various substrates after irradiation with high energy (MeV/amu) beams have stimulated speculation concerning the mechanism involved [I]. A principal question is whether the bonding results from mixing produced by atomic collisions at the interface (as found with low-energy ion implantation) or from electronic effects at the interface between the film and substrate atoms. Since these high energy beams are in the electronic stopping region, the latter hypothesis seems plausible. Furthermore, in a dielectric, the electronic excitations can produce atomic displacements [2]. These considerations are combined in the case of a metal film on Si. Although the transfer of electronic excitation into atomic displacement is not expected in either film or substrate, there is always an SiO* film on the Si under ordinary handling conditions. In this thin ( - 20 A) layer atomic displa~ment could be produced by electronic energy loss. We have examined this situation by irradiating Au films on both Si and SiO, substrates. Werner et al. [3] have reported (ST) adhesion for these combinations using 20 MeV “Cl beams. We used beams of lighter atoms and lower energies, albeit still in the electronic stopping region. 2. Experimental The semiconductor grade Si and vitreous SiO, substrates were vapor-degreased in Freon and then * Work supported in part by the Army Research Office. 016g-583X/85/$03.30 @ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
plasma-ashed. 500 A Au films were deposited by evaporation from a filament at 10 A/s in 10m6 Torr. Irradiation of the samples was carried out in the SUNY/Albany Dynamitron accelerator in lo-’ Torr. Beams of 3.4 and 6.5 MeV “N; 3.4 and 6.8 MeV 3’P; 3.0 MeV “F; and 3.3 MeV “C were used. The collimated beam dimensions were 1 x 3 mm2 as measured on an irradiated plastic film. Currents ranged from 40-200 nA. The fluences we quote are based on these beam dimensions and currents. However, the sizes of the irradiated regions showing enhanced adhesion and the sizes of the irradiated regions darkened by polymerized organic contamination from the pumping system, were significantly smaller than 1 X 3 mm2. These facts suggest that the beam inhomogeneity was such that the quoted fluence values are certainly low, perhaps by a factor of 4, Adhesion was tested by pressing a piece of Scotch tape onto the sample, firmly rubbing the back of the tape with an eraser, and slowly pulling off the tape (ST). Another test was vigorous abrasion of the area with a dry Q-tip (Q’b
3. Resutts Many of the irradiated films were darkened by polymerization by the beams of the organic material deposited from the pumping system. Several tests indicated that these organic coatings did not influence adhesion. First, Au films on Si and SiO, irradiated with the same fluences exhibited radically different adhesions. Then, H profiling using the ‘H(lSN, ‘He)12 C XI. FINE LINE ~./DE~SITION/ADHESiON
A. E. Berkowitz et al. / Adhesion 01 Au firmc on ‘Si and SiO,
878
.
Fig. 1. 500 A Au-in Si. Irradiated with 2.5 X lbiw3.4MeV P. After Scotch tape test. - Half of adhering region. The maximum width of the adhered region is 0.3 mm. (4.43 MeV) reaction showed no H in the Au films, although H was observed at the film surface and at the substrate interface. Finally, SIMS analysis showed a
surface build-up of C that increased approximately linearly with dosage, and a fixed small amount at the interface, but none in the Au film. Fig. 1 shows about half of an adhering Au film on Si after the (ST) test. The film had been irradiated with 3.4 MeV 31P at a fluence of 2.5 X 1015/cm2. The areas of adhering Au films decreased with decreasing fluence, due to the higher ion density in the center of the beam. Most irradiated films on Si exhibited the thin domeshaped blisters seen in fig. 1. These are more clearly visible in fig. 2 after (ST) for a higher fluence of 3.4 .MeV “P. Blister diameters were generally l-10 pm in the central (highest fluence) portions of the irradiated regions, with a few much larger blisters on the outer edges. It is very likely that the blisters are produced by the radiation induced desorption of adsorbed atmospheric gases such as CO, O,, N,, H,O, although thermally induced stress relief can also produce film delamination. When non-irradiated films on Si or Si02 were heated for 10 min at temperature > 350°C, blisters developed with a density that increased with temperature in a manner consistent with an Arrhenius dependence. (Desorption of atmospheric gases is commonly encountered for temperatures > 200°C). Furthermore, some of the substrates showed groups of blisters before irradiation. It is interesting (although of uncertain reliability) to estimate sample temperatures during irradiation from blister density using the data from heated samples. Inferred Si substrate sample temperatures ranged from - 250°C for a fluence of 3.3 x 1016 3.4 MeV “N to - 650°C for fluence of 5 x 10” 3.0 MeV
%.
20pm Fig. 2. 500 A Au on Si. Irradiated with 1.1 X 10’6/cm2 3.4 MeV P. After Scotch tape test.
2r-lm Fig. 3. 500 A Au on Si. Irradiated with 1.1 X 1016/cm2 3.4 MeV P. After Scotch tape test.
879
A. E. Berkowrtz er al. / Adhesion of Au films on Si and SiO,
Table 1 Beam parameters and adhesion thresholds Energy (MeV)
J “,
@E/dxL,
(dE/dxL,
primary ionization
(eV/A)
(eV/A)
Range (am)
Adhesion threshold fluence (cm-*)
MN*+
6.5
3.4 3.3 3.0 3.4
139 133 110 154 188
0.31 0.53 0.35 1.1 4.2
5.8 3.8 4.2 3.2 2.8
;y&ym
15N+
18.1 21.3 17.0 29.1 47.5
6.5 3.4 3.3 3.0 3.4 6.8
18.1 21.3 17.0 29.1 47.5 46.7
131 126 104 145 177 234
0.31 0.54 0.35 1.1 4.3 2.5
5.1 3.5 4.1
IOIl
Displacements ‘) per atom
SlbCOfl
12c+ 19F+
3lP+
b’
Vitreous silica 15N2+ lsN+ 12c+ 19F+
31p+ xp2+
2x10’5 < 1.3 x 10’6 <5x10’s 2x10’5
0.27
(ST) b,
2.7
> 1.5 x 10’6 > 10” > 4x10’6 > 2.5 x 1016 2.5 x 1016 < 1.3 x 10’6
a) J values relative to ionization of a 0.5 MeV proton. b, Thresholds for Si for both S(T) and (QT); thresholds for SiO, for (ST) only. ‘) Calculated from modified Rinchin-Pease relation using 4 X threshold fluence in table, (d E/dx),,, an atom.
the films to temperatures to 550°C for 10 min did not enhance adhesion. Fig. 3 shows the remains of a blister at the edge of the adhering region after (ST). The secondary electron yield is different in the area under the blister as compared to the adjacent region. This implies a difference in composition. If the blister is assumed to represent a region of desorbed atmospheric gases, one possibility is that these gases interacted with the Si substrate. Outside the regions where Au adhered after (ST), circular regions on the Si similar to that of fig. 3 indicated the location of blisters that were produced by irradiation. Blisters were rare in films on SiO, substrates. Some were found on films after irradiation by 3.4 MeV 31P. The SiO, substrates consistently showed the compaction that is often reported as due to irradiation [4]. The 1 x 3 mm2 beam area was always visible as a region depressed 1000-2000 A below the sample surface as measured with a Dektak. Large cracks around the irradiated area resulted from the compaction stresses. Irradiation with 3.4 MeV P produced the structure shown in fig. 4. The dark regions are pores in the Au film; the light regions are Au. Table 1 lists some properties of the beams and adhesion threshold data. As noted above, the threshold fluences are lower limits due to beam inhomogeneity. The threshold data for Au on Si indicate adhesion for (ST) and (QT), whereas for Au on SiO,, no (QT) adhesion was observed, and only the “P beams produced (ST) adhesion. Electronic energy loss values, are interpolated from Northcliffe and (dE/dx),,, 19F. Heating
0.09 0.03
and assuming 25 eV to displace
Schilling [S]; atomic energy loss values, (also called nuclear) (dE/dx),,, from Smith [6]. Primary ionization values, J, are calculated by the formula given by Bartlett et al. [7]. For Si substrates, threshold fluences are 2 x 10” for both 3.4 MeV 15N and “P, even though the 31P beam has significantly higher primary ionization, electronic and atomic energy losses. These threshold values are four times the (ST) threshold fluences reported by the Caltech group for 20 MeV “Cl [3]. The higher threshold for 6.5 MeV 15N seems reasonable when compared with 3.4 MeV ‘lP since the J and dE/dx values are significantly lower than those for the ‘lP. However, the threshold for 3.4 MeV 15N is also significantly lower than that for the 6.5 MeV “N even though the J and dE/dx parameters are quite similar. The incomplete data for “C and 19F beams are not inconsistent with the same order of threshold values. The much higher (dE/dx),, for the 3.4 MeV P invites consideration of atomic mixing as the adhesion mechanism even though (d E/dx)=, is very much larger in all cases. Using (dE/dx),, and increasing the quoted thresholds by a factor of four due to beam inhomogeneity, the displacements per atom (DpA) were calculated using the modified Kin&in-Pease relation. The resulting values are shown in the last column in table 1. Even with the uncertainties in the magnitude of the threshold, these rather high dpa values make it difficult to rule out an atomic mixing mechanism due to atomic stopping even though the electronic energy loss is very much higher. XI. FINE LINE STR./DEWSITION/ADHESION
880
A. E. Berkowitz et ui. / Adhesion of Au /ilm
an Si and SiO,
materials on irradiation with ion beams. Using 20 MeV 35Cl, the Caltech group found an erosion yield for SiO, that was more than two orders of magnitude higher than that for Si [I]. We can offer no convincing explanation, other than the effects of different porosities, for a (ST) threshold for ‘*P but not for “C? r9F, or r5N. The implied values for these latter beams seem to be inordinately high, especially in view of the threshold of 2 x 10rs found by Werner et al. f3] for both Au and Ag on SiOz. One possibility is that film preparation conditions play a significant role.
4. conclusions
Fig, 4. 560 A on SiOr. Irradiated with 8 X After Scotch tape test.
10’6/ctn2
3.4 h&V P.
The results for the SiO, substrates are spay different from the Si data. No threshold was found for (QT), and only the 31P beams produced (ST) adhesion. Furthermore, structure produced by the “P beams in fig 4 introduces questions about the (ST) adhesion. The extreme porosity of the Au film may so limit film contact with the tape that an apparent (ST) adhesion might result without significant interfacial bonding. Since the J and d E/dx values are essentially the same for the SiO* as for Si, it is plausible that the lack of adhesion on the SiO, is a consequence of the beam interaction with the SiO, that produces the compaction observed. Compaction in vitreous silica can be produced by X-rays, y-rays, electrons, ions and neutrons [4f. Ionization is most often invoked a a mechanism, although atomic displacements can also be: involved in some cases. Since o&y a [ST) adhesion was found with just one of the beams used, it appears that compaction absorbs energy that might otherwise improve adhesion either by intermixing or interfacial bonding. Another Factor that may account For the much weaker adhesion of films on SiO, as compared to those on Si is tbe difference in erosion yield (back sputtering) of these
It is estimated that atomic collisions can produce significant DPA in Si after irradiation with “P and *‘N beams. Therefore, it is possible that the very strong adhesion of 500 A Au films on Si substrates arises from atomic mixing for “N, 3’P, 19F, and 12C beams even though (dE/dx)& B (dE/dx),,. The lack of strong adhesion on the SiO, substrates for any of the beams might be a consequence either of the atomic processes involved in the SiO, dilation on irradiation, or of the substantial backsputtering from SiO,, The (ST) adhesion on SiO, after the 31P irradiation may result from the fine scale porosity produced in the Au film, rather than from interfacial bonding. We are grateful to C. Robertson for the SEM work and to GA. Smith for the SIMS analyses. We also appreciate useful discussions with A. Mogro-Campero,
References [l] T.A. Tombrello, Nucl. Instr. and Meth. 218 (1983) 679 (Proc. 6th Int. Conf. on Ion Beam Analysis). 121R.L. Fleischer, P.B. Price and R.M. Walker, Nuclear Tracks in Solids (Univ. of California Press, Berkeley, 1975). [3] B.T. Werner, T. Vreeland, Jr., M.H. Mendenhall, Y. Qiu and T.A. Tombrello, Thin Solid Films 104 (1983) 163. [4] W. Primak, Compaction States of Vitreous Silica (Gordon and Breach, New York, 1975). 153L.C. Northciiffe and R.F. Schihmg. Nucl. Data A7 (19701 233. [6] B. Smith, Ion Implantation Range Data for Si and Ge Device Technatogies (Harwell, 1977). ]7] D.F. Bartlett, D. Soo, R.L. Fleischer, H.R. Hart and Mogro-Campero, Phys. Rev. D24 (1981) 612.