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Ion beam bonding of thin films
881
Nuclear Instruments and Methods in Physics Research B7/8 (1985) 881-885 North-Holland, Amsterdam
ION BEAM BONDING J.E.E. BAGLIN
OF THIN FILMS
and G.J. CLARK
IBM Thomas J. Watson Research Cenrer, Yorktown Heights, New York 10598, USA
The phenomenon of irradiation enhanced adhesion of thin films has been studied for the case of 700 A Cu films deposited on alumina, fused quartz and glass-ceramic. Peel test measurements were made as a function of ion dose from lOI to 5 X 1016ions/cm2, for both 200 keV He+ ions and 280 keV Ne” ions. Subsequent heating at 450°C for 1 h further increased the adhesion of the irradiated interfaces by as much as a factor of 10. The thermal stability of the adhesion was also demonstrated by an interface wetting test. In view of the inability of Cu to reduce these substrates chemically, the strength and stability of the enhanced adhesion are surprising. Some possible explanations are proposed. The ability of both He+ and Ne+ to produce strong bonding indicates that enhanced adhesion can be produced by both nuclear and electronic components of ion energy loss.
1. Intmluction It is not surprising to find good adhesion at the interface between mutually reactive materials, or to find that good adhesion can be produced by ion beam mixing at such interfaces. It is surprising, however, to find strong adhesion produced between non-reactive, immiscible materials as a result of irradiation through the interface with a beam of ions. Greatly enhanced adhesion in many such systems has recently been reported [l-5], resulting from treatment with a variety of ion species and energies. The origin of this bonding process has not been established. Tombrello et al. [4] have suggested an empirical model in which the enhancement results from electronic interactions rather than nuclear interactions produced by the passage of ions through the interface. This concept has gained qualitative sup port from reports of adhesion enhancement by electrons and photons [5-g]. However, quantitative relations~ps have not been determined with certainty. As to the bond itself, it would appear that the thermodynamically stable interface configuration for layered immiscible and non-reacting materials should be abrupt and should involved relatively weak adhesion. In that case, it might be expected that the atomic configurations at an interface bonded by irradiation would be metastable. retuming to their original form after suitable heat treatments. If this were the case, it could greatly diminish the practical value of radiation enhanced adhesion phenomena. In this experiment, quantitative peel tests have been used to determine the dependence of adhesion upon ion beam dose for thin copper films deposited on substrates of polished alumina, fused quartz and a glass-ceramic [lo]. By comparison of results for He+ and Ne+ irradiation, the relative significance of electronic or nuclear 0168-583X/85/%03.30 6 Elsevier Science Publishers B.V. INorth-Holland Physics Publishing Division)
energy deposition at the interface can be assessed. Further tests of adhesion following thermal treatment indicate a large improvement in bonding, contrary to expectation. The thermal stability of the system has also been displayed by means of substrate wetting tests. The evidence of thermally stable adhesion in non-reacting systems augurs well for practical usage of this process. It raises intriguing questions, yet unanswered, about the atomic-scale bonding process involved.
2. Peel test for adhesion Substrates (5 cm x 5 cm) were scrubbed with detergent solution, rinsed repeatedly in deionized water and dried in a centrifuge. They were then coated with 700 A Cu, which was e-beam evaporated at 40 A/s in a vacuum system of base pressure 5 X lo-* Torr. Ion irradiation was carried out using a beam collimated to a V-shaped profile and a rotating sample carrier to produce a logarithmically graded dose ranging smoothly from 1014 ions/cm* at one edge of each substrate up to 5 x lOI ions/cm* at the opposite edge. After brief (but very necessary) plasma cleaning of the implanted surfaces. Cu stripes (1.6 mm wide, 10 pm thick) were added by e-beam deposition through a thin mask in the pattern shown in fig 1. In order to facilitate gripping of these stripes for the test, a band of Aquadag was painted on the substrate at the low-dose end prior to the stripe deposition, thus providing a release layer at the end of each stripe. For peel testing, the end of a stripe could be attached to a sensitive load-cell apparatus, providing a continuous record of tension required to slowly and steadily peel the Cu (including the initial layer) off the substrate. The record of peel force vs. position on the sample could then readily be converted XI. FINE LINE STR./DEPOSITION/ADHESION
J. E. E. Baglm, G.J. Clark / ion beam banding of thin films
882 PEEL
1
10fim STRIPES
Cu ADDED
Fig. 1. Peel test sample. A layer of 700 A Cu deposited on a 5 cm x 5 cm substrate is irradiated to produce a dose logarithmically graded in the direction of the peel stripes.
to show the dependence of peel strength on ion implantation dose. The peel test method was chosen because of its ability to produce quantitative and reproducible measurements of the adhesion strength. A mechanical analysis of this test [11] indicates that the peel strength should in fact be directly proportional to the interface energy of the sample. It will also depend slightly on the thickness and elastic moduli of the stripe material. Internal stress built up in the film and in the deposited stripe could in principle become large enough to reduce the measured peel strength. However, in our experience with copper films we have not found any significant change of results with thickness of deposited film, since the internal stress retained in deposited Cu is relatively low. It may therefore be noted in advance that the adhesion enhancement produced by the ion beam treatment was evidently not a result of simple stress relaxation due to radiation.
3. Peel test results A typical result is shown in fig. 2, where the peel strength for Cu on alumina is seen to be increased
01 10”
10’5 DOSE
10’6 (iont/cmz)
Fig. 3. Peel strength as a function of log (ion dose) for Cu on polished glass ceramic.
dramatically after irradiation with about 5 X 10” ions per cm2. Similar effects were found for the Cu/glassceramic system, as shown in fig. 3, and for Cu on fused quartz. These peel strength curves display a saturation value of adhesion after doses of 1 or 2 times lOi ions per cm2. The Ne+ treatment produced higher saturation values of peel strength than the He+ treatment, although He+ was relatively much more effective for the glass-ceramic substrate than for alumina.
4. Energy deposition processes The specific energy loss of the two species of ions chosen was evaluated for the plane of the Cu/substrate interface, using the TRIM Monte Carlo program of Biersack et al. [12,13]. Values for the energy ultimately deposited at the interface in the form of electronic processes or in the form of nuclear collisional processes are shown in table 1. For comparison, values for both the Cu and the substrate material are given.
Table 1 Specific energy deposition (dE/dx) m the region of the interface between 700 A Cu and a quartz substrate. The table shows energy deposited in the form of nuclear recoils and in electronic processes by 200 keV He and 280 keV Ne ions. Units are eV per (lOI atom/cm’). He+(200keV) __________-...
______---0
10 DOSE
Fig.
2.
Peel
20 (ions/cm2
30
_
40
Ion
Interface material
d E/dx electronic
d E/dx nuclear
He+
in Cu in SiOa
45 57
-2 -2
Ne+
in Cu in SiO,
50 65
x1015)
strength as a function of ion dose for Cu on polished ahnnina.
33 24
883
J. E. E. Baglin, G.J. Clark / Ion beam bonding of thin fdms
Electronic energy deposition is essentially the same for both beams. Therefore, if electronic energy loss were entirely responsible for adhesion enhancement, both beams should produce identical effects. The data of figs. 2 and 3 clearly deny this identity. However, the nuclear energy loss by the He+ beam is extremely small, so that if enhanced adhesion were a consequence of purely nuclear energy loss, He+ should offer only a few percent of the effect of Ne+ . In the case of glass-ceramic, this is clearly not supported. However, the overall superiority of the Ne + irradiation implies that the nuclear energy loss component is the more important in the present systems. Other work [4-91 in which electrons or high energy ions have been successful in improving adhesion shows that electronic processes alone can produce bonding enhancement. In view of the evidence, it appears that both forms of energy deposition contribute to adhesion enhancement. No simple scaling of their relative contributions is evident at present, however.
5. Abrupt interface After the removal of peel strips from an irradiated sample, a Rutherford backscattering (RBS) analysis was made for both the newly peeled copper surface and the residual exposed substrate. Even for heavily irradiated and bonded samples, no trace of substrate metal (e.g. Al of Al,O,) was found transferred to the Cu stripe. The sensitivity of RBS would enable the detection of a few monolayers of Al transferred to Cu. Its sensitivity to residual Cu on the substrate would be less than a monolayer; yet no transferred Cu was found. It is therefore apparent that the peel has not occurred at an intermediate or “ion beam mixed” layer; rather the bonded interface probably remains “abrupt” on the scale of a few monolayers at most. The enhanced adhesion must therefore be attributed to bonds set up between Cu atoms and substrate atoms at or very near to the interface.
6. Thermal stability As a test of thermal stability, samples which had been irradiated and peel-tested at room temperature (figs. 2 and 3) were heated in flowing purified helium at 450°C for 1 h. These tests were made six weeks after the ion irradiation. The results of subsequent peel tests for Cu on alumina are shown in fig. 4. Heating produced a large increase in adhesion, the improvement depending strongly on the prior irradiation dose. For comparison, the peel strength obtained by heating a sample which was not irradiated is also shown. The heating cycle increased the best adhesion in the Ne+-irradiated sample by a factor of 2. In the He+-irradiated sample,
-30
---- He+(200k&) .-.-.-He++450’C
t
E
.-.-
1 hr
Ne+(250keV) Ne++450’C
.’
1 hr
./
./-.
k.
\. ‘.\
/’ i
P
.i’
2
,,.i’
l/l
= lo-
/
./--
/
ki! 45O’C. o-_.
1hr.
no lmo+ . .
10”
I
10’5 DOSE
10”
10’
(ions/cm*)
Fig. 4. Effect of heat treatment (450°C 1 h) on peel strength for Cu on polished ahunina
adhesion improved by a factor of 20, producing a strong bond. Further heat cycles up to 8 h at 450°C made no appreciable changes. The microscopic process whereby interfaces preactivated by irradiation will develop good adhesion under thermal treatment is not clear. It is particularly surmising to note the large difference in thermal performance following He+ or Ne+ bombardment. From this work it is evident, however, that room temperature ion irradiation sets up a condition in the region of the interface which assists adhesion at room temperature and which remains “active” for long periods. Thermal treatment can then relax this interface condition in a way that produces a thermally stable, more strongly adhering interface.
7. Wetting tests A further demonstration of thermal stability of the irradiated interface was carried out in the following way. Cu films of thickness 100 A were e-beam deposited on room temperature substrates of alumina, sapphire and fused quartz, after following the substrate cleaning procedure described above. After deposition, scanning electron microscopy (SEM) showed the films to be smooth, continuous and featureless. Some of the samples were then heated in flowing helium at 450” for 1 h. In each case. The Cu formed beads or islands on the substrate surface, indicating the existence of a large Cu-substrate interface energy (poor wetting), as shown on the left hand side of fig. 5. A second set of Cu/substrate samples was irradiated with 2 x 10’6/cm2 of Ne+ ions at 280 keV (as used for adhesion enhancement), before the heating cycle at 45O“C. These films showed no tendency to form beads or islands, as shown in the right hand column of fig. 5. XI. FINE LINE STR./DEPOSITION/ADHESION
884
J.E. E. Baglm, G.J. Clurk / Ion beam bonding of thin films
Fig. 5. Interface wetting. The scanning electron micrographs show: (left-hand side) bead formation for 100 A Cu on substrates of (a) sapphire, (c) fused quartz, (e) polished alumina, following heat treatment at 45O*C for 1 h in pure helium; (right-hand side) Cu film integrity retained when irradiation with 1016 Ne+/cm’ preceded heat treatment (b), (d), and (f). [The lines evident in (e) and (f) are
polishing marks on the substrate.]
The only features evident were a few steps or ledges in the Cu surface. The absence of beading indicates that
ion irradiation produced a system with low interface energy (consistent with good adhesion), and that the low energy interface was stable at 450°C. The fact that islands did not begin to form implies that the adhesive interface forms very quickly at 450°C - probably as interface atoms relax their positions by only a few lattice spacings. Qualitatively similar effects in the Pt/ zirconia system have also been reported recently [14].
8. Discussion The experimental results seem to present a contradiction. On the one hand, it is known that Cu will
form neither solid solutions nor ternary compounds with the stoichiometric substrate materials in the bulk. Also, Cu can not chemically reduce the oxides of Al or Si in order to produce reaction or to facilitate ion beam mixing. On the other hand, ion beam treatment produces adhesion of the order of strength of chemical or metallic bonds at an abrupt interface between Cu and substrate. Furthermore, this bonding is thermally stable (at least up to 45O”Q as shown by its persistence or improvement after extended heat treatments capable of mobilizing interface Cu. One expl~ation would be the production by the beam of more intimate Cu-substrate contact or a better-packed arrangement of Cu atoms on the substrate. The beam no doubt produces defects at the interface in the process, whose movement during heat treatment
J. E. E. Baglin, G.J. Clark / Ion beam bonding of thin jilms
might assist the interface relaxation to a minimum-energy state. Since in Al r03 and SiO,, 4% terminal outer surface consists of oxygen atoms, the strength of such adhesion would depend on Cu attachment to surface oxygen atoms. It is difficult to see how such adhesion could reach the observed strength, however. A different model for the Cu-Al,Os system would involve a disordering by the ion beam of a few monoiayers near the interface, thereby allowing metallic bonding of CU-Al to occur at the interface pfane. IIeat treatment, however would be expected to assist such a system in reverting to segregated Cu and Al,O,, unless the interface configuration of Cu-Al-O indeed forms a phase stabilized by the planar nature of the bonding in contrast to the bulk situation where a mixture is not thermalty stable. At present, this proposal can only be speculative. It seems clear, at least, that the enhanced bonding phenomena occur at or very close to the original metal-substrate interface. Further studies of the detailed bond structure will be required in order to account satisfactorily for the observed formation and thermal stability of radiation-enhanced bonding at metal-ceramic and metal-glass interfaces. The authors wish to acknowledge the vital collaboration and interest of G. Coleman (sample preparation and peel testing), R. Fiorio (ion implantation), C. Aliotta (SEMI, and the staff of the Central Scientific Services department (J. Cuomo)). They also wish to thank R. Kelly, F.M. D’IIeurle and V. Brusic for many helpful discussions.
885
References [l] J.E. Griffith, Y. Qiu and T.A. Tomb&lo, Nuci. Instr and Meth. 198 (1982) 607. [2] S. Jacobson, B. Jonsson and B. Sundqvist, Thin Solid Films 107 (1983) 89. (31 J.E.E. Baglin, G.J. Clark and J. Battiger, in: Thin Films and interfaces II, eds., J.E.E. Baglin, D.S. Campbell and W.K. Chu (Elsevier, New York, 1984) p. 179. [4] T.A. Tombrello, ibid., p. 173. [5] I.V. Mitchell, J.S. Williams, D.K. Sood, K.T. Short, S. Johnson, and R.G. Elliman, ibid., p. 189. [6] J. Bottiger, J.E.E. Baglin, V. Brusic, G.J. Clark and D. Anfiteatro, ibid., p, 203. (71 C.J. Sofield, C.J. Woods, C. Wild, J.C. Riviere and L.S. Welch, ibid., p. 197. [S] I.V. Mitchell, J.S. Wilhams, P. Smith and R.G. E&man, Appl. Phys. Lett. 44 (1984) 193. 19) I.V. Mitchell, G. Nyberg and R.G. Elliman, Appl. Phys. L&t. 45 (1984) 137. (lo] Corning “MACOR”; approx. composition by weight: SiOa-46%; Al,O,-16%; MgO-17%; KaO-10%; F-4%; 8203 -7%. [Ill S. Wu, Polymer Interface and Adhesion (Marcel Dekker, New York, 1982) p. 536. [12] J.P. Biersack and L.G. Haggmark, Nucl. Instr. and Meth. 174 (1980) 257, [13) J.F. Ziegler, J.P. Biersack and U. Littmark, The Stopping and Range of Ions in Solids, vol. 1 (Pergamon, New York, 1984). [I4] D.K. Sood, P.D. Bond and S.P.S. BadwaI, Proc. Mat. Res. Sot. 27 (1984) 565.
XI. FINE LINE STR./DEPOSITION/ADHESION
Nuclear Instruments and Methods in Physics Research B7/8 (1985) 881-885 North-Holland, Amsterdam
ION BEAM BONDING J.E.E. BAGLIN
OF THIN FILMS
and G.J. CLARK
IBM Thomas J. Watson Research Cenrer, Yorktown Heights, New York 10598, USA
The phenomenon of irradiation enhanced adhesion of thin films has been studied for the case of 700 A Cu films deposited on alumina, fused quartz and glass-ceramic. Peel test measurements were made as a function of ion dose from lOI to 5 X 1016ions/cm2, for both 200 keV He+ ions and 280 keV Ne” ions. Subsequent heating at 450°C for 1 h further increased the adhesion of the irradiated interfaces by as much as a factor of 10. The thermal stability of the adhesion was also demonstrated by an interface wetting test. In view of the inability of Cu to reduce these substrates chemically, the strength and stability of the enhanced adhesion are surprising. Some possible explanations are proposed. The ability of both He+ and Ne+ to produce strong bonding indicates that enhanced adhesion can be produced by both nuclear and electronic components of ion energy loss.
1. Intmluction It is not surprising to find good adhesion at the interface between mutually reactive materials, or to find that good adhesion can be produced by ion beam mixing at such interfaces. It is surprising, however, to find strong adhesion produced between non-reactive, immiscible materials as a result of irradiation through the interface with a beam of ions. Greatly enhanced adhesion in many such systems has recently been reported [l-5], resulting from treatment with a variety of ion species and energies. The origin of this bonding process has not been established. Tombrello et al. [4] have suggested an empirical model in which the enhancement results from electronic interactions rather than nuclear interactions produced by the passage of ions through the interface. This concept has gained qualitative sup port from reports of adhesion enhancement by electrons and photons [5-g]. However, quantitative relations~ps have not been determined with certainty. As to the bond itself, it would appear that the thermodynamically stable interface configuration for layered immiscible and non-reacting materials should be abrupt and should involved relatively weak adhesion. In that case, it might be expected that the atomic configurations at an interface bonded by irradiation would be metastable. retuming to their original form after suitable heat treatments. If this were the case, it could greatly diminish the practical value of radiation enhanced adhesion phenomena. In this experiment, quantitative peel tests have been used to determine the dependence of adhesion upon ion beam dose for thin copper films deposited on substrates of polished alumina, fused quartz and a glass-ceramic [lo]. By comparison of results for He+ and Ne+ irradiation, the relative significance of electronic or nuclear 0168-583X/85/%03.30 6 Elsevier Science Publishers B.V. INorth-Holland Physics Publishing Division)
energy deposition at the interface can be assessed. Further tests of adhesion following thermal treatment indicate a large improvement in bonding, contrary to expectation. The thermal stability of the system has also been displayed by means of substrate wetting tests. The evidence of thermally stable adhesion in non-reacting systems augurs well for practical usage of this process. It raises intriguing questions, yet unanswered, about the atomic-scale bonding process involved.
2. Peel test for adhesion Substrates (5 cm x 5 cm) were scrubbed with detergent solution, rinsed repeatedly in deionized water and dried in a centrifuge. They were then coated with 700 A Cu, which was e-beam evaporated at 40 A/s in a vacuum system of base pressure 5 X lo-* Torr. Ion irradiation was carried out using a beam collimated to a V-shaped profile and a rotating sample carrier to produce a logarithmically graded dose ranging smoothly from 1014 ions/cm* at one edge of each substrate up to 5 x lOI ions/cm* at the opposite edge. After brief (but very necessary) plasma cleaning of the implanted surfaces. Cu stripes (1.6 mm wide, 10 pm thick) were added by e-beam deposition through a thin mask in the pattern shown in fig 1. In order to facilitate gripping of these stripes for the test, a band of Aquadag was painted on the substrate at the low-dose end prior to the stripe deposition, thus providing a release layer at the end of each stripe. For peel testing, the end of a stripe could be attached to a sensitive load-cell apparatus, providing a continuous record of tension required to slowly and steadily peel the Cu (including the initial layer) off the substrate. The record of peel force vs. position on the sample could then readily be converted XI. FINE LINE STR./DEPOSITION/ADHESION
J. E. E. Baglm, G.J. Clark / ion beam banding of thin films
882 PEEL
1
10fim STRIPES
Cu ADDED
Fig. 1. Peel test sample. A layer of 700 A Cu deposited on a 5 cm x 5 cm substrate is irradiated to produce a dose logarithmically graded in the direction of the peel stripes.
to show the dependence of peel strength on ion implantation dose. The peel test method was chosen because of its ability to produce quantitative and reproducible measurements of the adhesion strength. A mechanical analysis of this test [11] indicates that the peel strength should in fact be directly proportional to the interface energy of the sample. It will also depend slightly on the thickness and elastic moduli of the stripe material. Internal stress built up in the film and in the deposited stripe could in principle become large enough to reduce the measured peel strength. However, in our experience with copper films we have not found any significant change of results with thickness of deposited film, since the internal stress retained in deposited Cu is relatively low. It may therefore be noted in advance that the adhesion enhancement produced by the ion beam treatment was evidently not a result of simple stress relaxation due to radiation.
3. Peel test results A typical result is shown in fig. 2, where the peel strength for Cu on alumina is seen to be increased
01 10”
10’5 DOSE
10’6 (iont/cmz)
Fig. 3. Peel strength as a function of log (ion dose) for Cu on polished glass ceramic.
dramatically after irradiation with about 5 X 10” ions per cm2. Similar effects were found for the Cu/glassceramic system, as shown in fig. 3, and for Cu on fused quartz. These peel strength curves display a saturation value of adhesion after doses of 1 or 2 times lOi ions per cm2. The Ne+ treatment produced higher saturation values of peel strength than the He+ treatment, although He+ was relatively much more effective for the glass-ceramic substrate than for alumina.
4. Energy deposition processes The specific energy loss of the two species of ions chosen was evaluated for the plane of the Cu/substrate interface, using the TRIM Monte Carlo program of Biersack et al. [12,13]. Values for the energy ultimately deposited at the interface in the form of electronic processes or in the form of nuclear collisional processes are shown in table 1. For comparison, values for both the Cu and the substrate material are given.
Table 1 Specific energy deposition (dE/dx) m the region of the interface between 700 A Cu and a quartz substrate. The table shows energy deposited in the form of nuclear recoils and in electronic processes by 200 keV He and 280 keV Ne ions. Units are eV per (lOI atom/cm’). He+(200keV) __________-...
______---0
10 DOSE
Fig.
2.
Peel
20 (ions/cm2
30
_
40
Ion
Interface material
d E/dx electronic
d E/dx nuclear
He+
in Cu in SiOa
45 57
-2 -2
Ne+
in Cu in SiO,
50 65
x1015)
strength as a function of ion dose for Cu on polished ahnnina.
33 24
883
J. E. E. Baglin, G.J. Clark / Ion beam bonding of thin fdms
Electronic energy deposition is essentially the same for both beams. Therefore, if electronic energy loss were entirely responsible for adhesion enhancement, both beams should produce identical effects. The data of figs. 2 and 3 clearly deny this identity. However, the nuclear energy loss by the He+ beam is extremely small, so that if enhanced adhesion were a consequence of purely nuclear energy loss, He+ should offer only a few percent of the effect of Ne+ . In the case of glass-ceramic, this is clearly not supported. However, the overall superiority of the Ne + irradiation implies that the nuclear energy loss component is the more important in the present systems. Other work [4-91 in which electrons or high energy ions have been successful in improving adhesion shows that electronic processes alone can produce bonding enhancement. In view of the evidence, it appears that both forms of energy deposition contribute to adhesion enhancement. No simple scaling of their relative contributions is evident at present, however.
5. Abrupt interface After the removal of peel strips from an irradiated sample, a Rutherford backscattering (RBS) analysis was made for both the newly peeled copper surface and the residual exposed substrate. Even for heavily irradiated and bonded samples, no trace of substrate metal (e.g. Al of Al,O,) was found transferred to the Cu stripe. The sensitivity of RBS would enable the detection of a few monolayers of Al transferred to Cu. Its sensitivity to residual Cu on the substrate would be less than a monolayer; yet no transferred Cu was found. It is therefore apparent that the peel has not occurred at an intermediate or “ion beam mixed” layer; rather the bonded interface probably remains “abrupt” on the scale of a few monolayers at most. The enhanced adhesion must therefore be attributed to bonds set up between Cu atoms and substrate atoms at or very near to the interface.
6. Thermal stability As a test of thermal stability, samples which had been irradiated and peel-tested at room temperature (figs. 2 and 3) were heated in flowing purified helium at 450°C for 1 h. These tests were made six weeks after the ion irradiation. The results of subsequent peel tests for Cu on alumina are shown in fig. 4. Heating produced a large increase in adhesion, the improvement depending strongly on the prior irradiation dose. For comparison, the peel strength obtained by heating a sample which was not irradiated is also shown. The heating cycle increased the best adhesion in the Ne+-irradiated sample by a factor of 2. In the He+-irradiated sample,
-30
---- He+(200k&) .-.-.-He++450’C
t
E
.-.-
1 hr
Ne+(250keV) Ne++450’C
.’
1 hr
./
./-.
k.
\. ‘.\
/’ i
P
.i’
2
,,.i’
l/l
= lo-
/
./--
/
ki! 45O’C. o-_.
1hr.
no lmo+ . .
10”
I
10’5 DOSE
10”
10’
(ions/cm*)
Fig. 4. Effect of heat treatment (450°C 1 h) on peel strength for Cu on polished ahunina
adhesion improved by a factor of 20, producing a strong bond. Further heat cycles up to 8 h at 450°C made no appreciable changes. The microscopic process whereby interfaces preactivated by irradiation will develop good adhesion under thermal treatment is not clear. It is particularly surmising to note the large difference in thermal performance following He+ or Ne+ bombardment. From this work it is evident, however, that room temperature ion irradiation sets up a condition in the region of the interface which assists adhesion at room temperature and which remains “active” for long periods. Thermal treatment can then relax this interface condition in a way that produces a thermally stable, more strongly adhering interface.
7. Wetting tests A further demonstration of thermal stability of the irradiated interface was carried out in the following way. Cu films of thickness 100 A were e-beam deposited on room temperature substrates of alumina, sapphire and fused quartz, after following the substrate cleaning procedure described above. After deposition, scanning electron microscopy (SEM) showed the films to be smooth, continuous and featureless. Some of the samples were then heated in flowing helium at 450” for 1 h. In each case. The Cu formed beads or islands on the substrate surface, indicating the existence of a large Cu-substrate interface energy (poor wetting), as shown on the left hand side of fig. 5. A second set of Cu/substrate samples was irradiated with 2 x 10’6/cm2 of Ne+ ions at 280 keV (as used for adhesion enhancement), before the heating cycle at 45O“C. These films showed no tendency to form beads or islands, as shown in the right hand column of fig. 5. XI. FINE LINE STR./DEPOSITION/ADHESION
884
J.E. E. Baglm, G.J. Clurk / Ion beam bonding of thin films
Fig. 5. Interface wetting. The scanning electron micrographs show: (left-hand side) bead formation for 100 A Cu on substrates of (a) sapphire, (c) fused quartz, (e) polished alumina, following heat treatment at 45O*C for 1 h in pure helium; (right-hand side) Cu film integrity retained when irradiation with 1016 Ne+/cm’ preceded heat treatment (b), (d), and (f). [The lines evident in (e) and (f) are
polishing marks on the substrate.]
The only features evident were a few steps or ledges in the Cu surface. The absence of beading indicates that
ion irradiation produced a system with low interface energy (consistent with good adhesion), and that the low energy interface was stable at 450°C. The fact that islands did not begin to form implies that the adhesive interface forms very quickly at 450°C - probably as interface atoms relax their positions by only a few lattice spacings. Qualitatively similar effects in the Pt/ zirconia system have also been reported recently [14].
8. Discussion The experimental results seem to present a contradiction. On the one hand, it is known that Cu will
form neither solid solutions nor ternary compounds with the stoichiometric substrate materials in the bulk. Also, Cu can not chemically reduce the oxides of Al or Si in order to produce reaction or to facilitate ion beam mixing. On the other hand, ion beam treatment produces adhesion of the order of strength of chemical or metallic bonds at an abrupt interface between Cu and substrate. Furthermore, this bonding is thermally stable (at least up to 45O”Q as shown by its persistence or improvement after extended heat treatments capable of mobilizing interface Cu. One expl~ation would be the production by the beam of more intimate Cu-substrate contact or a better-packed arrangement of Cu atoms on the substrate. The beam no doubt produces defects at the interface in the process, whose movement during heat treatment
J. E. E. Baglin, G.J. Clark / Ion beam bonding of thin jilms
might assist the interface relaxation to a minimum-energy state. Since in Al r03 and SiO,, 4% terminal outer surface consists of oxygen atoms, the strength of such adhesion would depend on Cu attachment to surface oxygen atoms. It is difficult to see how such adhesion could reach the observed strength, however. A different model for the Cu-Al,Os system would involve a disordering by the ion beam of a few monoiayers near the interface, thereby allowing metallic bonding of CU-Al to occur at the interface pfane. IIeat treatment, however would be expected to assist such a system in reverting to segregated Cu and Al,O,, unless the interface configuration of Cu-Al-O indeed forms a phase stabilized by the planar nature of the bonding in contrast to the bulk situation where a mixture is not thermalty stable. At present, this proposal can only be speculative. It seems clear, at least, that the enhanced bonding phenomena occur at or very close to the original metal-substrate interface. Further studies of the detailed bond structure will be required in order to account satisfactorily for the observed formation and thermal stability of radiation-enhanced bonding at metal-ceramic and metal-glass interfaces. The authors wish to acknowledge the vital collaboration and interest of G. Coleman (sample preparation and peel testing), R. Fiorio (ion implantation), C. Aliotta (SEMI, and the staff of the Central Scientific Services department (J. Cuomo)). They also wish to thank R. Kelly, F.M. D’IIeurle and V. Brusic for many helpful discussions.
885
References [l] J.E. Griffith, Y. Qiu and T.A. Tomb&lo, Nuci. Instr and Meth. 198 (1982) 607. [2] S. Jacobson, B. Jonsson and B. Sundqvist, Thin Solid Films 107 (1983) 89. (31 J.E.E. Baglin, G.J. Clark and J. Battiger, in: Thin Films and interfaces II, eds., J.E.E. Baglin, D.S. Campbell and W.K. Chu (Elsevier, New York, 1984) p. 179. [4] T.A. Tombrello, ibid., p. 173. [5] I.V. Mitchell, J.S. Williams, D.K. Sood, K.T. Short, S. Johnson, and R.G. Elliman, ibid., p. 189. [6] J. Bottiger, J.E.E. Baglin, V. Brusic, G.J. Clark and D. Anfiteatro, ibid., p, 203. (71 C.J. Sofield, C.J. Woods, C. Wild, J.C. Riviere and L.S. Welch, ibid., p. 197. [S] I.V. Mitchell, J.S. Wilhams, P. Smith and R.G. E&man, Appl. Phys. Lett. 44 (1984) 193. 19) I.V. Mitchell, G. Nyberg and R.G. Elliman, Appl. Phys. L&t. 45 (1984) 137. (lo] Corning “MACOR”; approx. composition by weight: SiOa-46%; Al,O,-16%; MgO-17%; KaO-10%; F-4%; 8203 -7%. [Ill S. Wu, Polymer Interface and Adhesion (Marcel Dekker, New York, 1982) p. 536. [12] J.P. Biersack and L.G. Haggmark, Nucl. Instr. and Meth. 174 (1980) 257, [13) J.F. Ziegler, J.P. Biersack and U. Littmark, The Stopping and Range of Ions in Solids, vol. 1 (Pergamon, New York, 1984). [I4] D.K. Sood, P.D. Bond and S.P.S. BadwaI, Proc. Mat. Res. Sot. 27 (1984) 565.
XI. FINE LINE STR./DEPOSITION/ADHESION