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Ion beam mixing of metal films on SiO2
Volume 2, number SA
June 1984
MATERIALS LETTERS
ION BEAM MIXING OF METAL FILMS ON SiO2 *
C.W. WHITE, G. FARLOW, J. NARAYAN Solid State Division,
Oak Ridge National
Laboratory,
Oak Ridge,
TN 37830,
USA
and G.J. CLARK and J.E.E. BAGLIN IBM Research
Laboratories,
Yorktown
Heights, NY 10598,
USA
Received 19 April 1984
Ion beam mixing of certain metals deposited on SiOa substrates causes reactions to occur which result in the formation of metal-rich silicides in the region of the interface and an Increase in the adhesion of the film to the substrate. For other metals, ion irradiation causes lateral transport and coalescence of metal atoms resulting in the formation of an island structure. The results obtained by ion irradiation are compared with previous studies of high-temperature thermal proces-
sing of metal films on SiOa.
Metallization schemes to improve the adhesion of deposited metal films to insulating substrates are important for the fabrication of electrically conductive paths on insulating substrates for semiconductor device fabrication. Interfacial reactions between the film and the insulating substrate can lead to improved adhesion in many cases. High-temperature thermal processing of metal films deposited on SiO, has been shown previously to result in the formation of metalrich silicides at the metal/Si02 interface [l-5]. In the work reported here, we have used ion beam mixing as a low-temperature processing technique to cause interface reactions between deposited metal films and an underlying Si02 substrate. Ion beam mixing has been used extensively to form silicides in the case of metal films deposited on silicon [6], and some work [7] has been reported on chemical effects of ion beam mixing of transition metals on SiO,. However, to our knowledge this is the first reported use of ion beam mixing to induce metal/SiO, contact reactions, and in a number of cases the result of the induced contact reaction is an increase in the * Research sponsored by the Division of Materials Sciences, U.S. Department of Energy under contract DE-ACOS840R21400 with Martin Marietta Energy Systems, Inc.
0 167-557x/84/$ 03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
adhesion of the film to the Si02 substrate. In the work reported here, metal films (200-300 A thick) of Nb, V, Cu, Pd, Ta, W, Ti, Hf, and Zr were deposited in an oil-free vacuum system onto oxidized silicon wafers. The Si02 thickness was =2000 A. Ion beam mixing was carried out in high vacuum (10v7 Torr) at room temperature using Xe+ ions at energies of 200-350 keV. For each irradiation, the ion energy was chosen such that the peak in the deposited energy distribution would be in the vicinity of metal/ SiO, interface. A dose of 1 X 1016 ions/cm2 was used for each irradiation. Following ion beam mixing, films were examined by Rutherford backscattering spectrometry (RBS), and secondary electron microscopy (SEM). Selected films were examined by glancing angle X-ray diffraction and transmission electron microscopy. Standard peel adhesion tests were used to determine relative changes in the adherence of the film to the substrate. The results of these experiments fall into two general categories: (a) metal/SiO, systems, which undergo interface reactions during ion beam mixing; (b) metal/SiO, systems in which the deposited metal undergoes lateral segregation and coalescence during ion irradiation with no indication of interface reactions. An example of case (a) is shown by the RBS 367
Nb 13008)
ON S102/S1 He+
B OXYGEN
June 1984
MATERIALS LETTERS
Volume 2, number 5A
STEP
20
MeV
Nb 1300
---
&ON
SO2 .fSi
IN S102
SURFACE
OF Nb 7200
A- AS DEPOSITED o -Xe
(lx10’6/cm21
MIXED
6300 800
600 3600
500 4oc
A-As 0
-Xe
Nb(SURFACE)
DEPOSITED (ix10’6/cm2)
MIXED
3oc 1800 2oc 900 IOC
0.65
0.75
085 ENERGY
0.95
I .05
I .I5
(M&‘)
1.4
1.5
1.6 ENERGY
17
(McV)
Fig. 1. Rutherford backscattering spectrum for Nb(300 A) on SiOz/Si before and after Xe+ ion irradiation. Room temperature.
results for Nb (300 A) on SiOZ in fig. 1. These RBS spectra were obtained using 2.0 MeV He+ ions in a grazing exit angle geometry for enhanced depth resolution. Following ion beam mixing (Xei, 300 keV, 1 X 1016/cm2), there is a significant decrease in the scattering yield from Nb in the region of the metal/ oxide interface. The change in scattering yield near the interface suggests an interfacial reaction between the metal and the insulating substrate. The interfacial reaction appears to have consumed ~40% of the original film thickness. In fig, 1, Xe+ irradiation results in the transport of silicon in Si02 toward the surface, again consistent with an interfacial reaction between Nb and SiOz during ion beam mixing. The movement of Si is also through -40% of the film. Analysis of the backscattering results (in fig. 1) shows that the amounts of reacted Nb and Si are consistent with the formation of Nb,Si at the metal/oxide interface during ion beam mixing. In fig, 1, ion beam mix368
ing also gives rise to an increase in the oxygen content at the surface and a decrease in the scattering yield from Nb near the front surface. These results are consistent with the formation of a metal oxide at the front surface during ion beam mixing, but the resulting oxide is thin and the stoichiometry cannot be determined. Finally, oxygen is distributed throughout the mixed film. From the RBS results, the stoichiometry in the mixed region near the interface is (approximately) 3 parts Nb, 1 part Si, and 1 part oxygen. For the case of Nb on Si02, SEM images of the surface obtained before and after ion irradiation are smooth and featureless. Glancing angle X-ray diffraction using a Debye Hall powder camera show that a polycrystalline phase with a preferred orientation was produced by ion beam mixing. Standard peel adhesion tests on the Nb fti showed a factor of.4 improvement in adhesion (from 0.5 to 2.0 g/mm2) following ion bombardment. Transmission electron microscopy
Volume 2, number 5A
MATERIALS LETTERS
Fig. 2. Cross-section micrographs and microdiffraction pattern of Nb(300 A) on SiOz. Micrographs are shown in the as deposited state (a) and following Xe+ (1 X 1016/cm2) irradiation (b). The diffraction pattern (c) was obtained from the mixed film near the interface.
was used to determine the phases produced in the case of Nb on SiO,. Fig. 2a is a cross-section micrograph showing a 320 A thick niobium layer on the Si02 substrate. After ion beam mixing (Xe+, 1 X 1016/cm2), the thickness of the mixed region increases to 480 A, as shown in fig. 2b. Microdiffraction analysis was carried out to determine the nature and structure of the phases present in the mixed film. Fig. 2c shows a microdiffraction pattern obtained from the mixed film near the interface. The amorphous ring and crystalline diffraction spots are consistent with the presence of amorphous and crystalline Nb3Si phases with a lattice constant of 5.2 A. This metastable phase of Nb3Si has been found to be superconducting in previous studies. The possibility of the presence of niobium oxide in the interface region (although not found) could not be ruled out from the present studies. Similar RBS and glancing angle X-ray diffraction analysis showed interfacial reactions in the V(300 A)/ Si02 system, Ti(300 A)/SiO, and Ta(300 A)/SiO,, suggesting the formation of a silicide during ion irradiation (Xe+, 1 X 1016/cm2). Improvements in adhesion (by up to a factor of 3) as a result of ion irradiation were found for V, Ti, Hf, and Zr films, again suggesting interfacial reactions in these systems. A number of metal/Si02 systems do not appear
June 1984
to undergo interfacial reactions during ion irradiation. Examples of this category of results are shown by the SEM images in fig. 3 of the surface topography of Cu (300 A) and Pd (300 A) on Si02 following ion irradiation. Prior to irradiation, the surfaces of the deposited films are smooth and featureless. Ion irradiation leads to lateral transport of metal atoms over thousands of ingstroms followed by coalescence into the island structure. This transport during ion irradiation takes place at temperatures near room temperature. In the case of Cu, a similar island structure develops even if the substrate is held at liquidnitrogen temperature during ion irradiation, as shown in fig. 4. This shows that the mobility of these atoms is extremely high during ion irradiation. We cannot rule out the possibility that interfacial reaction has occurred over a very limited depth because RBS analysis cannot be used to determine interface reactions when the surface topography has been changed so radically by the ion irradiation process. The formation of a silicide during ion beam mixing of Nb (and possibly for V, Ti, Ta, Hf, and Zr) on Si02 which we have observed is the first time that silicides have been formed on SiO, at temperatures near room temperature using ion beam mixing. The resulting increase in adhesion suggests the potential use of ion beam mixing to increase the adherence of metal circuit lines to insulators by low-temperature processing. Previous work by others has shown that thermal processing at elevated temperatures of similar deposited metal films on Si02 gives rise to interfacial reactions which result in the formation of silicides [l-5]. In that work it was found that metal-rich silicides could be formed when group IVa and Va metal films were subjected to thermal processing at elevated temperatures (>7OO’C). For other metals (such as Cu and Pd) interfacial reactions did not occur during thermal processing and the metal was observed to coalesce to form an island structure [l] . In our experiments, the interfacial reaction is driven by the energy deposited into the collision cascade and not by simple thermal activation because the temperature at the surface during irradiation was always less than 100°C. In the cases where interfacial reactions do occur (such as Nb on SiO,), we believe that ion beam mixing initiates atomic motion and the reaction is then driven by chemical forces until stoichiometric metal-rich silicides (Nb3Si) are formed. The previous 369
Volume 2, number 5A
June 1984
MATERIALS LETTERS
AS DEPOSITED
Xe (4 x4016/i/cm2) MIXED
Fig. 3. Surface topography of Cu and Pd films (300 A thick) deposited on SiOt before and after Xe* irradiation (1016/cmz).
thermal processing results have been explained [l] on the basis of the heats of formation of the products compared with those of the reactants; if the heat of reaction of the products is less than that of the reactants for the reaction M, t SiO, + MYSit M,_,O,
(I)
(where M is the metal) then the free-energy change is negative and the reaction is thermodynamically possible and is often observed. This explanation favors reaction for those metal films which have a high affinity for oxygen, and it is those films which we observe to react readily. A similar explanation to that described above is believed to operate in the 370
ion beam mixing case also because the same metal/ SiOz systems are observed to undergo interfacial reactions during ion i~ad~tion and thermal processing. Others [S] have noted the similarity of results in ion mixing and high-temperature thermal processing of metal fihns on silicon. This appears to be the case also for metal films on SiO;z. In those cases such as Cu and Pd on SiOz where island coalescence is observed during ion irradiation (and during high-temperature thermal processing [l]), the freeenergy change is positive for reactions described by eq. (l), implying that interfacial reactions are not likely. in these cases the energy deposited during ion irradiation is transferred into atomic mo-
Volume 2, number SA
June 1984
MATERIALS LETTERS
ION IRRlOlATlON
OF Cu (38Ok
ON Si02
VIRGIN
Fig. 4. Surface topography produced by Xe+ irradiation of Cu on SiOz at room temperature and liquid-nitrogen temperature.
tion and surface diffusion takes place over thousands of &gstriims. Each incident ion must mobilize and transport at least ten metal atoms. Transport of metal atoms over these distances implies that in these systems the surface mobilities are extremely high during ion irradiation. Those systems which undergo lateral transport and coalescence during ion irradiation may have interesting catalytic properties due to the very large surface areas that can be produced by ion irradiation at room temperature or below. In summary, we have shown that ion irradiation
can give rise to interfacial reactions in certain metal/ SiO, systems at temperatures near room temperature. The systems which react are the same as those that react during thermal processing at elevated temperatures Interfacial reactions initiated factor of 4. For systems in the free-energy change would interfacial reactions are not observed metal film undergoes island structure during irradiation SiO2). 371
Volume 2, number 5A
MATERIALS LETTERS
In these systems a similar phenomenon has been observed during thermal processing at elevated temperatures [ 11.
References [l] R. PretoIius, J.M. Harris and M.A. Nicolet, Solid State Electron. 24 (1978) 667. [2] H. KIiiutle, W.K. Chu, M.A. Nicolet, J.W. Mayer and K.N. Tu, in: Applications of ion beams to metals, eds. S.T. Picraux, E.P. EerNisse and F.L. Vook (Plenum Press, New York, 1974) p. 193.
372
June 1984
[ 31 H. Krautle, M.A. Nicolet and J.W. Mayer, J. Appl. Phys. 45 (1974) 3304. [4] K.N. Tu, J.F. Ziegler and C.J. Kircher, Appl. Phys. Letters 23 (1973) 493. [S] H. KrLutle, M.A. Nicolet and J.W. Mayer, Phys. Stat. Sol. 20 (1973) K33. [6] B.Y. Tsaur, S.S. Lau and L.S. Hung, Nucl. Instr. Methods 182/183 (1981) 1, and references therein. [ 71 T. Banwell, B.X. Liu, I. Golecki and M.A. Nicolet; Nucl. Instr. Methods 209/210 (1983) 125. [8] F. d’Heurle, C.S. Petersson and M.Y. Tsai, J. Appl. Phys. 53 (1982) 8765.
June 1984
MATERIALS LETTERS
ION BEAM MIXING OF METAL FILMS ON SiO2 *
C.W. WHITE, G. FARLOW, J. NARAYAN Solid State Division,
Oak Ridge National
Laboratory,
Oak Ridge,
TN 37830,
USA
and G.J. CLARK and J.E.E. BAGLIN IBM Research
Laboratories,
Yorktown
Heights, NY 10598,
USA
Received 19 April 1984
Ion beam mixing of certain metals deposited on SiOa substrates causes reactions to occur which result in the formation of metal-rich silicides in the region of the interface and an Increase in the adhesion of the film to the substrate. For other metals, ion irradiation causes lateral transport and coalescence of metal atoms resulting in the formation of an island structure. The results obtained by ion irradiation are compared with previous studies of high-temperature thermal proces-
sing of metal films on SiOa.
Metallization schemes to improve the adhesion of deposited metal films to insulating substrates are important for the fabrication of electrically conductive paths on insulating substrates for semiconductor device fabrication. Interfacial reactions between the film and the insulating substrate can lead to improved adhesion in many cases. High-temperature thermal processing of metal films deposited on SiO, has been shown previously to result in the formation of metalrich silicides at the metal/Si02 interface [l-5]. In the work reported here, we have used ion beam mixing as a low-temperature processing technique to cause interface reactions between deposited metal films and an underlying Si02 substrate. Ion beam mixing has been used extensively to form silicides in the case of metal films deposited on silicon [6], and some work [7] has been reported on chemical effects of ion beam mixing of transition metals on SiO,. However, to our knowledge this is the first reported use of ion beam mixing to induce metal/SiO, contact reactions, and in a number of cases the result of the induced contact reaction is an increase in the * Research sponsored by the Division of Materials Sciences, U.S. Department of Energy under contract DE-ACOS840R21400 with Martin Marietta Energy Systems, Inc.
0 167-557x/84/$ 03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
adhesion of the film to the Si02 substrate. In the work reported here, metal films (200-300 A thick) of Nb, V, Cu, Pd, Ta, W, Ti, Hf, and Zr were deposited in an oil-free vacuum system onto oxidized silicon wafers. The Si02 thickness was =2000 A. Ion beam mixing was carried out in high vacuum (10v7 Torr) at room temperature using Xe+ ions at energies of 200-350 keV. For each irradiation, the ion energy was chosen such that the peak in the deposited energy distribution would be in the vicinity of metal/ SiO, interface. A dose of 1 X 1016 ions/cm2 was used for each irradiation. Following ion beam mixing, films were examined by Rutherford backscattering spectrometry (RBS), and secondary electron microscopy (SEM). Selected films were examined by glancing angle X-ray diffraction and transmission electron microscopy. Standard peel adhesion tests were used to determine relative changes in the adherence of the film to the substrate. The results of these experiments fall into two general categories: (a) metal/SiO, systems, which undergo interface reactions during ion beam mixing; (b) metal/SiO, systems in which the deposited metal undergoes lateral segregation and coalescence during ion irradiation with no indication of interface reactions. An example of case (a) is shown by the RBS 367
Nb 13008)
ON S102/S1 He+
B OXYGEN
June 1984
MATERIALS LETTERS
Volume 2, number 5A
STEP
20
MeV
Nb 1300
---
&ON
SO2 .fSi
IN S102
SURFACE
OF Nb 7200
A- AS DEPOSITED o -Xe
(lx10’6/cm21
MIXED
6300 800
600 3600
500 4oc
A-As 0
-Xe
Nb(SURFACE)
DEPOSITED (ix10’6/cm2)
MIXED
3oc 1800 2oc 900 IOC
0.65
0.75
085 ENERGY
0.95
I .05
I .I5
(M&‘)
1.4
1.5
1.6 ENERGY
17
(McV)
Fig. 1. Rutherford backscattering spectrum for Nb(300 A) on SiOz/Si before and after Xe+ ion irradiation. Room temperature.
results for Nb (300 A) on SiOZ in fig. 1. These RBS spectra were obtained using 2.0 MeV He+ ions in a grazing exit angle geometry for enhanced depth resolution. Following ion beam mixing (Xei, 300 keV, 1 X 1016/cm2), there is a significant decrease in the scattering yield from Nb in the region of the metal/ oxide interface. The change in scattering yield near the interface suggests an interfacial reaction between the metal and the insulating substrate. The interfacial reaction appears to have consumed ~40% of the original film thickness. In fig, 1, Xe+ irradiation results in the transport of silicon in Si02 toward the surface, again consistent with an interfacial reaction between Nb and SiOz during ion beam mixing. The movement of Si is also through -40% of the film. Analysis of the backscattering results (in fig. 1) shows that the amounts of reacted Nb and Si are consistent with the formation of Nb,Si at the metal/oxide interface during ion beam mixing. In fig, 1, ion beam mix368
ing also gives rise to an increase in the oxygen content at the surface and a decrease in the scattering yield from Nb near the front surface. These results are consistent with the formation of a metal oxide at the front surface during ion beam mixing, but the resulting oxide is thin and the stoichiometry cannot be determined. Finally, oxygen is distributed throughout the mixed film. From the RBS results, the stoichiometry in the mixed region near the interface is (approximately) 3 parts Nb, 1 part Si, and 1 part oxygen. For the case of Nb on Si02, SEM images of the surface obtained before and after ion irradiation are smooth and featureless. Glancing angle X-ray diffraction using a Debye Hall powder camera show that a polycrystalline phase with a preferred orientation was produced by ion beam mixing. Standard peel adhesion tests on the Nb fti showed a factor of.4 improvement in adhesion (from 0.5 to 2.0 g/mm2) following ion bombardment. Transmission electron microscopy
Volume 2, number 5A
MATERIALS LETTERS
Fig. 2. Cross-section micrographs and microdiffraction pattern of Nb(300 A) on SiOz. Micrographs are shown in the as deposited state (a) and following Xe+ (1 X 1016/cm2) irradiation (b). The diffraction pattern (c) was obtained from the mixed film near the interface.
was used to determine the phases produced in the case of Nb on SiO,. Fig. 2a is a cross-section micrograph showing a 320 A thick niobium layer on the Si02 substrate. After ion beam mixing (Xe+, 1 X 1016/cm2), the thickness of the mixed region increases to 480 A, as shown in fig. 2b. Microdiffraction analysis was carried out to determine the nature and structure of the phases present in the mixed film. Fig. 2c shows a microdiffraction pattern obtained from the mixed film near the interface. The amorphous ring and crystalline diffraction spots are consistent with the presence of amorphous and crystalline Nb3Si phases with a lattice constant of 5.2 A. This metastable phase of Nb3Si has been found to be superconducting in previous studies. The possibility of the presence of niobium oxide in the interface region (although not found) could not be ruled out from the present studies. Similar RBS and glancing angle X-ray diffraction analysis showed interfacial reactions in the V(300 A)/ Si02 system, Ti(300 A)/SiO, and Ta(300 A)/SiO,, suggesting the formation of a silicide during ion irradiation (Xe+, 1 X 1016/cm2). Improvements in adhesion (by up to a factor of 3) as a result of ion irradiation were found for V, Ti, Hf, and Zr films, again suggesting interfacial reactions in these systems. A number of metal/Si02 systems do not appear
June 1984
to undergo interfacial reactions during ion irradiation. Examples of this category of results are shown by the SEM images in fig. 3 of the surface topography of Cu (300 A) and Pd (300 A) on Si02 following ion irradiation. Prior to irradiation, the surfaces of the deposited films are smooth and featureless. Ion irradiation leads to lateral transport of metal atoms over thousands of ingstroms followed by coalescence into the island structure. This transport during ion irradiation takes place at temperatures near room temperature. In the case of Cu, a similar island structure develops even if the substrate is held at liquidnitrogen temperature during ion irradiation, as shown in fig. 4. This shows that the mobility of these atoms is extremely high during ion irradiation. We cannot rule out the possibility that interfacial reaction has occurred over a very limited depth because RBS analysis cannot be used to determine interface reactions when the surface topography has been changed so radically by the ion irradiation process. The formation of a silicide during ion beam mixing of Nb (and possibly for V, Ti, Ta, Hf, and Zr) on Si02 which we have observed is the first time that silicides have been formed on SiO, at temperatures near room temperature using ion beam mixing. The resulting increase in adhesion suggests the potential use of ion beam mixing to increase the adherence of metal circuit lines to insulators by low-temperature processing. Previous work by others has shown that thermal processing at elevated temperatures of similar deposited metal films on Si02 gives rise to interfacial reactions which result in the formation of silicides [l-5]. In that work it was found that metal-rich silicides could be formed when group IVa and Va metal films were subjected to thermal processing at elevated temperatures (>7OO’C). For other metals (such as Cu and Pd) interfacial reactions did not occur during thermal processing and the metal was observed to coalesce to form an island structure [l] . In our experiments, the interfacial reaction is driven by the energy deposited into the collision cascade and not by simple thermal activation because the temperature at the surface during irradiation was always less than 100°C. In the cases where interfacial reactions do occur (such as Nb on SiO,), we believe that ion beam mixing initiates atomic motion and the reaction is then driven by chemical forces until stoichiometric metal-rich silicides (Nb3Si) are formed. The previous 369
Volume 2, number 5A
June 1984
MATERIALS LETTERS
AS DEPOSITED
Xe (4 x4016/i/cm2) MIXED
Fig. 3. Surface topography of Cu and Pd films (300 A thick) deposited on SiOt before and after Xe* irradiation (1016/cmz).
thermal processing results have been explained [l] on the basis of the heats of formation of the products compared with those of the reactants; if the heat of reaction of the products is less than that of the reactants for the reaction M, t SiO, + MYSit M,_,O,
(I)
(where M is the metal) then the free-energy change is negative and the reaction is thermodynamically possible and is often observed. This explanation favors reaction for those metal films which have a high affinity for oxygen, and it is those films which we observe to react readily. A similar explanation to that described above is believed to operate in the 370
ion beam mixing case also because the same metal/ SiOz systems are observed to undergo interfacial reactions during ion i~ad~tion and thermal processing. Others [S] have noted the similarity of results in ion mixing and high-temperature thermal processing of metal fihns on silicon. This appears to be the case also for metal films on SiO;z. In those cases such as Cu and Pd on SiOz where island coalescence is observed during ion irradiation (and during high-temperature thermal processing [l]), the freeenergy change is positive for reactions described by eq. (l), implying that interfacial reactions are not likely. in these cases the energy deposited during ion irradiation is transferred into atomic mo-
Volume 2, number SA
June 1984
MATERIALS LETTERS
ION IRRlOlATlON
OF Cu (38Ok
ON Si02
VIRGIN
Fig. 4. Surface topography produced by Xe+ irradiation of Cu on SiOz at room temperature and liquid-nitrogen temperature.
tion and surface diffusion takes place over thousands of &gstriims. Each incident ion must mobilize and transport at least ten metal atoms. Transport of metal atoms over these distances implies that in these systems the surface mobilities are extremely high during ion irradiation. Those systems which undergo lateral transport and coalescence during ion irradiation may have interesting catalytic properties due to the very large surface areas that can be produced by ion irradiation at room temperature or below. In summary, we have shown that ion irradiation
can give rise to interfacial reactions in certain metal/ SiO, systems at temperatures near room temperature. The systems which react are the same as those that react during thermal processing at elevated temperatures Interfacial reactions initiated factor of 4. For systems in the free-energy change would interfacial reactions are not observed metal film undergoes island structure during irradiation SiO2). 371
Volume 2, number 5A
MATERIALS LETTERS
In these systems a similar phenomenon has been observed during thermal processing at elevated temperatures [ 11.
References [l] R. PretoIius, J.M. Harris and M.A. Nicolet, Solid State Electron. 24 (1978) 667. [2] H. KIiiutle, W.K. Chu, M.A. Nicolet, J.W. Mayer and K.N. Tu, in: Applications of ion beams to metals, eds. S.T. Picraux, E.P. EerNisse and F.L. Vook (Plenum Press, New York, 1974) p. 193.
372
June 1984
[ 31 H. Krautle, M.A. Nicolet and J.W. Mayer, J. Appl. Phys. 45 (1974) 3304. [4] K.N. Tu, J.F. Ziegler and C.J. Kircher, Appl. Phys. Letters 23 (1973) 493. [S] H. KrLutle, M.A. Nicolet and J.W. Mayer, Phys. Stat. Sol. 20 (1973) K33. [6] B.Y. Tsaur, S.S. Lau and L.S. Hung, Nucl. Instr. Methods 182/183 (1981) 1, and references therein. [ 71 T. Banwell, B.X. Liu, I. Golecki and M.A. Nicolet; Nucl. Instr. Methods 209/210 (1983) 125. [8] F. d’Heurle, C.S. Petersson and M.Y. Tsai, J. Appl. Phys. 53 (1982) 8765.