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Temperature programmed desorption studies of the copper-sapphire (0001) system
Materials Science and Engineering, A162 (1993) 131-134
131
Temperature programmeddesorption studies of the copper-sapphire(0001) system Y. Ding, J. B l a k e l y a n d R. Raj Department of Materials Science and Engineering, Cornell University, Ithaca, N Y 14853-1501 (USA)
Abstract Temperature programmed desorption (TPD) studies of copper atoms were conducted on sapphire (0001) surfaces. Submonolayer to multimonolayer coverages of copper atoms were deposited onto the surfaces at room temperature. Subsequent copper desorption spectra at submonolayer coverages showed two peaks. The thermal activation energies associated with these peaks were determined to be 2.7 and 2.9 eV. The TPD spectra were consistent with copper island formation on the surface. The effects of annealing and oxidation on the samples were investigated.
1. Introduction
2. Experimental details
An important topic in the fundamental understanding of the mechanical properties of metal-ceramic interfaces is the correlation between the electronic structure and the macroscopic adhesion strength of the interface [1-4]. For the Cu-sapphire system, several experiments and theoretical calculations have been carried out to study metal-ceramic bonding. Of particular interest are the studies using surface science techniques [5-7], including Auger electron spectroscopy (AES), electron energy-loss spectroscopy (EELS), UV photoelectron spectroscopy (UPS) and low energy electron diffraction (LEED). These studies provide information on the bonding between deposited copper atoms and oxygen sites on the surface of sapphire, and are in agreement with theoretical calculations of self-consistent field X-alpha scattered-wave cluster molecular-orbital models [8]. Regarding the maximum temperature of thermal stability, there have been conflicting reports in the literature ranging from 430 °C to 800 °C [6, 7]. Since the other studies were conducted using methods such as AES without actually measuring the flow of copper atoms, it had been suggested that a thermal desorption study would provide more detailed information on the interaction between copper atoms and the sapphire surface. In this paper, we present results of temperature programmed desorption (TPD) experiments. The TPD spectra show a two-peak structure at low copper coverages, suggesting a complex bonding mechanism at the interface.
The experiments were performed in an ultrahigh vacuum (UHV) chamber, shown schematically in Fig. 1. The system includes a quadrupole mass spectrometer, an ion gun, an electron gun, a cylindrical mirror analyzer and a gas inlet for introducing high purity oxygen and argon. The main pump in the system is a Varian model 912-6003 with a speed of 801 s-1. A separate Perkin-Elmer Ultek ion pump (model 20-
0921-5093/93/$6.00
Cu souf'ce
• gun
I e- gun
"
Fig. 1. Schematic top view of the chamber. © 1993 - Elsevier Sequoia. All fights reserved
132
Y. Ding et al.
/
TPD of Cu/sapphire(O001)
044) with a speed of 50 1 s -1 was used to pump the quadrupole mass spectrometer section. The quadrupole mass spectrometer used was a UTI model 100C. The base pressure after a thorough bakeout was 9.0 x 10 - 10Torr. The polished sapphire (0001) sample (Saphikon Inc.), with a thickness of 1.7 mm, was mounted with a slot about 0.5 mm wide, cut in the middle of the sample using an Isomet low speed diamond saw. The sample was suspended on the electrical heater made of tantalum. A tungsten-rhenium thermal couple was attached to the sample on the back using a ceramic cement, Ceramabond 503 (Aremco Inc.). High temperature cure and U H V bakeout was performed in a separate chamber to insure a good thermal contact and minimal contamination. A tantalum heat shield was built around the sample with an opening pointing towards the probe of the quadrupole mass spectrometer. A 99.99% pure copper wire with 0.1 nun diameter, placed 5 cm away from the sample surface, was attached to a tantalum wire as an evaporation source; the rate of deposition of copper onto sapphire was approximately 0.5 A min- 1 Prior to the desorption experiments, AES spectra were taken to ensure that the surface was clean. When carbon traces were found, the sample was annealed at an oxygen partial pressure of 1 × 10 -8 Torr at 1000 °C to remove the impurities. All the TPD data were obtained with a linear temperature increase rate of 4 °C s-1 using electrical resistive heating. The initial copper coverages were calibrated using AES data. For TPD spectra, each of the curves plotted in the figures was smoothed by taking averages of adjacent points, and the universal background was subtracted. The TPD spectra were reproducible and the peak positions are consistently within a range of 10 °C for identical coverages.
3. Results and discussion In this section, the TPD spectra of as-deposited, annealed and oxidized samples will be discussed.
3.1. As-deposited samples Figure 2 shows a series of desorption spectra taken from a sapphire (0001) surface with various initial copper coverages ranging from 0.03 monolayer (ML) to 3.8 M L deposited at room temperature. Figure 3 is an enlargement of desorption spectra for lower initial copper coverages. It can be seen that at the lowest coverages, there are two separable peaks. As the initial coverage increases, both peaks shift toward higher temperatures with the spacing between them gradually diminishing. At a
o
Rote of Desorption
of Cu from
Sapphire
((:;)001) Surfoces
J Cu Coveroge in Monolayers q
3.82 3.02 1.71 1.50 1.11 0.70 0.4-2 0.29 0,16 0.09 0.07 0.03
"~ ~'~o
~ r
z~ o oo
J
__~,
Q o
m
Q
?
0.0
.300.0
600.0 Temperoture(Celsius)
900.0
1200.0
Fig. 2. Copper TPD spectra of Cu/sapphire. The numbers on the left are coverages of copper in monolayers, in the same order as the curves. The dashed line is the calculated common leading edge for zero-order desorption curves with an activation energy
of 2.9 eV.
0.29
//~
0.16
j
i
S g k c5
0.0
300 0
600,0
900.0
1200.0
temperature(Celsius)
Fig. 3. Copper TPD spectra of Cu/sapphire for lower copper coverages (an enlargement of the TPD spectra from Fig. 1).
coverage of 0.7 ML, the two peaks merge into a broad peak, this peak becomes sharper as the coverage is further increased. At a coverage of 1.5 ML, the desorption spectrum is a single peak, with characteristics of zero-order desorption. TPD data show that the copper deposit is thermally stable up to a temperature of about 600 °C. Earlier reports [6, 7] gave a temperature of 430 °C at which AES peaks begin to change. As discussed later in this paper, it is possible that copper island formation might have caused this change. It is to be expected that for high initial coverages of copper, the desorption spectra should closely resemble a zero-order peak, corresponding to the desorption of
Y. Ding et al.
/
TPD of Cu/sapphire(O001)
copper from an infinitely thick film. It can be seen that in fact at highest coverages the spectra exhibit zeroorder desorption. A n estimate of the activation energy for this zero-order desorption yielded a value of 2.90 + 0.02 eV. In comparison, the heat of formation of copper is 3.4 eV. The difference between the calculated activation energy and the bulk heat of formation data is common in systems in which a metal forms islands on another material [9]. At lower coverages, the spectra exhibit two peaks that move closer and toward higher temperature with increasing coverages. Each peak has an order of desorption of less than unity, as a first-order desorption would have featured a peak that remained stationary at one temperature with increasing coverage. The application of a zero-order model to the data gives values of 2.66 and 2.87 eV respectively for the lower coverage spectra, while a first-order model gives values in the range 2.6-2.95 eV for the two peaks. These two methods give the upper and lower limits for the actual activation energies of desorption. That both peaks shift toward higher temperature with increasing coverage is an indication that the desorption processes of copper atoms are not independent, in other words since the order of the desorption is not unity, it is unlikely that copper atoms were desorbing independently of their neighbors from individual sites on the surface. Some form of Cu-Cu bonding could have occurred before the desorption. Therefore copper atoms would have started either to form copper islands at an early stage of deposition where the coverage is low, or to form islands during the heating processes of TPD experiments. EELS and UPS results published earlier [6, 7] have suggested the formation of a chemical bond between copper and oxygen for coverages of less than 2 ik of copper layers, attributed to an electronic transition from the antibonding molecular orbitals of energies consisting of Cu(3p) and O(2p) near the top of the A1203 valence band to energy levels below the conduction band minimum. Our TPD data indicate that in fact at low copper coverages there are at least two competing mechanisms for copper bonding to the surface. These findings, especially the closeness of the two activation energies, are surprising and cannot be explained by the models proposed in the literature. The merging of the two peaks at higher coverages suggests a role of oxygen in the bonding mechanisms at low coverages.
133
were designed to find out more about the mobility of copper atoms on the surface at elevated temperature. In the annealing experiments, after copper deposition, the samples were held in vacuum at about 450 °C for 1 h before they were subjected to the temperature ramp. As shown in Fig. 4, while the peak positions do not show any noticeable changes, the relative height of peaks does change substantially. The high temperature peaks have grown at the expense of the low temperature peaks. TPD spectra of samples with 5 min annealing did not show any significant changes compared with the as-deposited samples. The change in relative peak heights may not be attributed to slow desorption during annealing at 450 °C. A simple estimate can be made as follows: assume a medium activation energy of 2.8 eV, the ratio of desorbing rate of copper atoms at the peak temperature T 1= 650 °C to temperature T2 = 450 °C is then exp ( - E/kT1) = exp ( - E/kT2)
1.7 × 104
It can be shown that the number of copper atoms desorbed throughout the annealing process is less than 1% of the total. Considering the magnitude of the change, this possibility can be ruled out. We must also consider the possibility that the change in the spectra described above resulted from oxidation of copper. The following oxidation experiment was carried out to explore this possibility.
3.3. Oxidized samples In the oxidation experiments, a layer of copper film was deposited and then the sample was quickly raised to 450 °C and exposed to pure 02 at a partial pressure
0.15 0.10 007
1-h annealing ~ ~ i
ca o_
utn d
L
oL d i
3.2. Annealed samples As discussed in the preceding section, it was unclear whether copper atoms formed islands during desorption (at room temperature) or during the heating ramp in the desorption experiment. Annealing experiments
i 0.0
3000
6000 temperature(Celslus)
900 0
1200,0
Fig. 4. Copper TPD spectra of Cu/sapphire after annealing for 1 h at 450 °C.
134
Y. Ding et aL /
TPD of Cu/sapphire(O001)
Copper islands
0.0.09 04
post-ox~
Y~o J
Fig. 6. The desorption energy of copper atoms from a copper island may be site dependent. For example, sites A and B may have different coordination with adjacent Cu/O atoms.
0.0
300.0
600.0
Temperature(Celsius)
900.0
1200.0
Fig. 5. Copper TPD spectra of oxidized Cu/sapphire after exposure for 15 Langmuirs of oxygen at 450 °C. of 5 x 10 -7 Torr for 30 s. The TPD spectra for this specimen are shown in Fig. 5. The results do not differ significantly from those for the as-deposited samples. 3.4. S u m m a r y o f results
With the possibility of oxidation ruled out, the change in the TPD spectra in annealed samples can only be attributed to the realignment of copper atoms on the sapphire surface by a thermally activated process. The fact that the spectra change continuously with annealing time suggests that copper formed islands during the deposition process, and that island formation could have occurred even at coverages as small as several hundredths of a monolayer. The two peak desorption spectra show that there are at least two mechanisms of copper bonding to the sapphire surface. Annealing the specimens leads to a greater population of the lower energy sites at the expense of the higher energy sites. It should be noted that while the activation energy measured in a TPD experiment is a good phenomenological measurement of the binding energy, it is not necessarily equal to the bonding strength of an atom to the substrate. For example, it is possible for a metal to bond weakly with the substrate and yet have a high desorption energy owing to strong metal-metal bonding. The activation energy for desorption reflects the local environment of the desorbing atom at the time of desorption. For example, as shown in Fig. 6, atoms at sites A and B may have different coordinations with adjacent Cu/O atoms, leading to different desorption activation energies. Aluminum atoms in the second layer of sapphire (0001) from the free surface may also contribute to the binding energies of copper atoms, but the effect may be smaller.
4. Conclusions TPD studies provided a unique perspective on the copper-sapphire bonding mechanisms by providing a link between the macroscopic properties and the atomistic structure. It is shown that the C u sapphire(0001) system is thermally stable up to 600 °C. At low copper coverages, TPD spectra show two distinguishable peaks with activation energies estimated at 2.7 and 2.9 eV respectively. The TPD data suggest that copper forms islands on the surface even at coverages of several hundredths of a monolayer. Annealing leads to stronger bonds of copper to the surface.
Acknowledgments This research was supported by the Office of Naval Research under grant N0014-88-K-0331 under the direction of Dr. S. G. Fishman. Support was also received from the National Science Foundation through the use of the Facilities of the Materials Science Center at Cornell University.
References 1 2 3 4 5 6 7 8 9
M. Ruhle and A. G. Evans, Mater. Sci. Eng., A107(1989) 187. J.T. Klomp, Mater. Res. Soc. Bull., 59(1980) 794. M. Nicholas, J. Mater. Sci., 3 (1968) 571. F. Ernst, P. Pirouz and A. H. Heuer, Mater. Res. Soc. Syrup. Proc., 38(1989) 557. R.V. Kasowski, F. S. Ohuchi and R. H. French, Physica B, 150 (1988)44. Q. Guo and P. J. Moiler, Surf. Sci., 244 (1991) 228. Q. Guo and P. J. Moiler, Vacuum, 41 (4-6) (1990) 1114. K. H. Johnson and S. V. Pepper, J. Appl. Phys., 53 (10) (1982) 6634. M. Vollmer and F. Traeger, Lect. Notes Phys., 269 (1987) 25.
131
Temperature programmeddesorption studies of the copper-sapphire(0001) system Y. Ding, J. B l a k e l y a n d R. Raj Department of Materials Science and Engineering, Cornell University, Ithaca, N Y 14853-1501 (USA)
Abstract Temperature programmed desorption (TPD) studies of copper atoms were conducted on sapphire (0001) surfaces. Submonolayer to multimonolayer coverages of copper atoms were deposited onto the surfaces at room temperature. Subsequent copper desorption spectra at submonolayer coverages showed two peaks. The thermal activation energies associated with these peaks were determined to be 2.7 and 2.9 eV. The TPD spectra were consistent with copper island formation on the surface. The effects of annealing and oxidation on the samples were investigated.
1. Introduction
2. Experimental details
An important topic in the fundamental understanding of the mechanical properties of metal-ceramic interfaces is the correlation between the electronic structure and the macroscopic adhesion strength of the interface [1-4]. For the Cu-sapphire system, several experiments and theoretical calculations have been carried out to study metal-ceramic bonding. Of particular interest are the studies using surface science techniques [5-7], including Auger electron spectroscopy (AES), electron energy-loss spectroscopy (EELS), UV photoelectron spectroscopy (UPS) and low energy electron diffraction (LEED). These studies provide information on the bonding between deposited copper atoms and oxygen sites on the surface of sapphire, and are in agreement with theoretical calculations of self-consistent field X-alpha scattered-wave cluster molecular-orbital models [8]. Regarding the maximum temperature of thermal stability, there have been conflicting reports in the literature ranging from 430 °C to 800 °C [6, 7]. Since the other studies were conducted using methods such as AES without actually measuring the flow of copper atoms, it had been suggested that a thermal desorption study would provide more detailed information on the interaction between copper atoms and the sapphire surface. In this paper, we present results of temperature programmed desorption (TPD) experiments. The TPD spectra show a two-peak structure at low copper coverages, suggesting a complex bonding mechanism at the interface.
The experiments were performed in an ultrahigh vacuum (UHV) chamber, shown schematically in Fig. 1. The system includes a quadrupole mass spectrometer, an ion gun, an electron gun, a cylindrical mirror analyzer and a gas inlet for introducing high purity oxygen and argon. The main pump in the system is a Varian model 912-6003 with a speed of 801 s-1. A separate Perkin-Elmer Ultek ion pump (model 20-
0921-5093/93/$6.00
Cu souf'ce
• gun
I e- gun
"
Fig. 1. Schematic top view of the chamber. © 1993 - Elsevier Sequoia. All fights reserved
132
Y. Ding et al.
/
TPD of Cu/sapphire(O001)
044) with a speed of 50 1 s -1 was used to pump the quadrupole mass spectrometer section. The quadrupole mass spectrometer used was a UTI model 100C. The base pressure after a thorough bakeout was 9.0 x 10 - 10Torr. The polished sapphire (0001) sample (Saphikon Inc.), with a thickness of 1.7 mm, was mounted with a slot about 0.5 mm wide, cut in the middle of the sample using an Isomet low speed diamond saw. The sample was suspended on the electrical heater made of tantalum. A tungsten-rhenium thermal couple was attached to the sample on the back using a ceramic cement, Ceramabond 503 (Aremco Inc.). High temperature cure and U H V bakeout was performed in a separate chamber to insure a good thermal contact and minimal contamination. A tantalum heat shield was built around the sample with an opening pointing towards the probe of the quadrupole mass spectrometer. A 99.99% pure copper wire with 0.1 nun diameter, placed 5 cm away from the sample surface, was attached to a tantalum wire as an evaporation source; the rate of deposition of copper onto sapphire was approximately 0.5 A min- 1 Prior to the desorption experiments, AES spectra were taken to ensure that the surface was clean. When carbon traces were found, the sample was annealed at an oxygen partial pressure of 1 × 10 -8 Torr at 1000 °C to remove the impurities. All the TPD data were obtained with a linear temperature increase rate of 4 °C s-1 using electrical resistive heating. The initial copper coverages were calibrated using AES data. For TPD spectra, each of the curves plotted in the figures was smoothed by taking averages of adjacent points, and the universal background was subtracted. The TPD spectra were reproducible and the peak positions are consistently within a range of 10 °C for identical coverages.
3. Results and discussion In this section, the TPD spectra of as-deposited, annealed and oxidized samples will be discussed.
3.1. As-deposited samples Figure 2 shows a series of desorption spectra taken from a sapphire (0001) surface with various initial copper coverages ranging from 0.03 monolayer (ML) to 3.8 M L deposited at room temperature. Figure 3 is an enlargement of desorption spectra for lower initial copper coverages. It can be seen that at the lowest coverages, there are two separable peaks. As the initial coverage increases, both peaks shift toward higher temperatures with the spacing between them gradually diminishing. At a
o
Rote of Desorption
of Cu from
Sapphire
((:;)001) Surfoces
J Cu Coveroge in Monolayers q
3.82 3.02 1.71 1.50 1.11 0.70 0.4-2 0.29 0,16 0.09 0.07 0.03
"~ ~'~o
~ r
z~ o oo
J
__~,
Q o
m
Q
?
0.0
.300.0
600.0 Temperoture(Celsius)
900.0
1200.0
Fig. 2. Copper TPD spectra of Cu/sapphire. The numbers on the left are coverages of copper in monolayers, in the same order as the curves. The dashed line is the calculated common leading edge for zero-order desorption curves with an activation energy
of 2.9 eV.
0.29
//~
0.16
j
i
S g k c5
0.0
300 0
600,0
900.0
1200.0
temperature(Celsius)
Fig. 3. Copper TPD spectra of Cu/sapphire for lower copper coverages (an enlargement of the TPD spectra from Fig. 1).
coverage of 0.7 ML, the two peaks merge into a broad peak, this peak becomes sharper as the coverage is further increased. At a coverage of 1.5 ML, the desorption spectrum is a single peak, with characteristics of zero-order desorption. TPD data show that the copper deposit is thermally stable up to a temperature of about 600 °C. Earlier reports [6, 7] gave a temperature of 430 °C at which AES peaks begin to change. As discussed later in this paper, it is possible that copper island formation might have caused this change. It is to be expected that for high initial coverages of copper, the desorption spectra should closely resemble a zero-order peak, corresponding to the desorption of
Y. Ding et al.
/
TPD of Cu/sapphire(O001)
copper from an infinitely thick film. It can be seen that in fact at highest coverages the spectra exhibit zeroorder desorption. A n estimate of the activation energy for this zero-order desorption yielded a value of 2.90 + 0.02 eV. In comparison, the heat of formation of copper is 3.4 eV. The difference between the calculated activation energy and the bulk heat of formation data is common in systems in which a metal forms islands on another material [9]. At lower coverages, the spectra exhibit two peaks that move closer and toward higher temperature with increasing coverages. Each peak has an order of desorption of less than unity, as a first-order desorption would have featured a peak that remained stationary at one temperature with increasing coverage. The application of a zero-order model to the data gives values of 2.66 and 2.87 eV respectively for the lower coverage spectra, while a first-order model gives values in the range 2.6-2.95 eV for the two peaks. These two methods give the upper and lower limits for the actual activation energies of desorption. That both peaks shift toward higher temperature with increasing coverage is an indication that the desorption processes of copper atoms are not independent, in other words since the order of the desorption is not unity, it is unlikely that copper atoms were desorbing independently of their neighbors from individual sites on the surface. Some form of Cu-Cu bonding could have occurred before the desorption. Therefore copper atoms would have started either to form copper islands at an early stage of deposition where the coverage is low, or to form islands during the heating processes of TPD experiments. EELS and UPS results published earlier [6, 7] have suggested the formation of a chemical bond between copper and oxygen for coverages of less than 2 ik of copper layers, attributed to an electronic transition from the antibonding molecular orbitals of energies consisting of Cu(3p) and O(2p) near the top of the A1203 valence band to energy levels below the conduction band minimum. Our TPD data indicate that in fact at low copper coverages there are at least two competing mechanisms for copper bonding to the surface. These findings, especially the closeness of the two activation energies, are surprising and cannot be explained by the models proposed in the literature. The merging of the two peaks at higher coverages suggests a role of oxygen in the bonding mechanisms at low coverages.
133
were designed to find out more about the mobility of copper atoms on the surface at elevated temperature. In the annealing experiments, after copper deposition, the samples were held in vacuum at about 450 °C for 1 h before they were subjected to the temperature ramp. As shown in Fig. 4, while the peak positions do not show any noticeable changes, the relative height of peaks does change substantially. The high temperature peaks have grown at the expense of the low temperature peaks. TPD spectra of samples with 5 min annealing did not show any significant changes compared with the as-deposited samples. The change in relative peak heights may not be attributed to slow desorption during annealing at 450 °C. A simple estimate can be made as follows: assume a medium activation energy of 2.8 eV, the ratio of desorbing rate of copper atoms at the peak temperature T 1= 650 °C to temperature T2 = 450 °C is then exp ( - E/kT1) = exp ( - E/kT2)
1.7 × 104
It can be shown that the number of copper atoms desorbed throughout the annealing process is less than 1% of the total. Considering the magnitude of the change, this possibility can be ruled out. We must also consider the possibility that the change in the spectra described above resulted from oxidation of copper. The following oxidation experiment was carried out to explore this possibility.
3.3. Oxidized samples In the oxidation experiments, a layer of copper film was deposited and then the sample was quickly raised to 450 °C and exposed to pure 02 at a partial pressure
0.15 0.10 007
1-h annealing ~ ~ i
ca o_
utn d
L
oL d i
3.2. Annealed samples As discussed in the preceding section, it was unclear whether copper atoms formed islands during desorption (at room temperature) or during the heating ramp in the desorption experiment. Annealing experiments
i 0.0
3000
6000 temperature(Celslus)
900 0
1200,0
Fig. 4. Copper TPD spectra of Cu/sapphire after annealing for 1 h at 450 °C.
134
Y. Ding et aL /
TPD of Cu/sapphire(O001)
Copper islands
0.0.09 04
post-ox~
Y~o J
Fig. 6. The desorption energy of copper atoms from a copper island may be site dependent. For example, sites A and B may have different coordination with adjacent Cu/O atoms.
0.0
300.0
600.0
Temperature(Celsius)
900.0
1200.0
Fig. 5. Copper TPD spectra of oxidized Cu/sapphire after exposure for 15 Langmuirs of oxygen at 450 °C. of 5 x 10 -7 Torr for 30 s. The TPD spectra for this specimen are shown in Fig. 5. The results do not differ significantly from those for the as-deposited samples. 3.4. S u m m a r y o f results
With the possibility of oxidation ruled out, the change in the TPD spectra in annealed samples can only be attributed to the realignment of copper atoms on the sapphire surface by a thermally activated process. The fact that the spectra change continuously with annealing time suggests that copper formed islands during the deposition process, and that island formation could have occurred even at coverages as small as several hundredths of a monolayer. The two peak desorption spectra show that there are at least two mechanisms of copper bonding to the sapphire surface. Annealing the specimens leads to a greater population of the lower energy sites at the expense of the higher energy sites. It should be noted that while the activation energy measured in a TPD experiment is a good phenomenological measurement of the binding energy, it is not necessarily equal to the bonding strength of an atom to the substrate. For example, it is possible for a metal to bond weakly with the substrate and yet have a high desorption energy owing to strong metal-metal bonding. The activation energy for desorption reflects the local environment of the desorbing atom at the time of desorption. For example, as shown in Fig. 6, atoms at sites A and B may have different coordinations with adjacent Cu/O atoms, leading to different desorption activation energies. Aluminum atoms in the second layer of sapphire (0001) from the free surface may also contribute to the binding energies of copper atoms, but the effect may be smaller.
4. Conclusions TPD studies provided a unique perspective on the copper-sapphire bonding mechanisms by providing a link between the macroscopic properties and the atomistic structure. It is shown that the C u sapphire(0001) system is thermally stable up to 600 °C. At low copper coverages, TPD spectra show two distinguishable peaks with activation energies estimated at 2.7 and 2.9 eV respectively. The TPD data suggest that copper forms islands on the surface even at coverages of several hundredths of a monolayer. Annealing leads to stronger bonds of copper to the surface.
Acknowledgments This research was supported by the Office of Naval Research under grant N0014-88-K-0331 under the direction of Dr. S. G. Fishman. Support was also received from the National Science Foundation through the use of the Facilities of the Materials Science Center at Cornell University.
References 1 2 3 4 5 6 7 8 9
M. Ruhle and A. G. Evans, Mater. Sci. Eng., A107(1989) 187. J.T. Klomp, Mater. Res. Soc. Bull., 59(1980) 794. M. Nicholas, J. Mater. Sci., 3 (1968) 571. F. Ernst, P. Pirouz and A. H. Heuer, Mater. Res. Soc. Syrup. Proc., 38(1989) 557. R.V. Kasowski, F. S. Ohuchi and R. H. French, Physica B, 150 (1988)44. Q. Guo and P. J. Moiler, Surf. Sci., 244 (1991) 228. Q. Guo and P. J. Moiler, Vacuum, 41 (4-6) (1990) 1114. K. H. Johnson and S. V. Pepper, J. Appl. Phys., 53 (10) (1982) 6634. M. Vollmer and F. Traeger, Lect. Notes Phys., 269 (1987) 25.