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Low energy sputtering of neutral Cu2 molecules
Volume 79A, number 1
PHYSICS LETTERS
15 September 1980
LOW ENERGY SPUTTERING OF NEUTRAL Cu2 MOLECULES C. Burleigh COOPER Physics Department, University of Delaware, Newark, DE 19711, USA
and James R. WOODYARD Wayne State University, Detroit, MI 48202, USA Received 14 April 1980 Revised manuscript received 23 June 1980
The ratios of relative yields of neutral sputtered Cu2 molecules to neutral sputtered 4Cu ions atoms (energy were50—90 found eV), to beaslinearly determined by secondary proportional to the sputtering neutral mass yield spectrometry. of Cu, from a Cu target under bombardment by Ar
This letter reports on experiments done by mass spectrometry to measure the intensities of neutral Cu atoms and Cu 2 molecules sputtered from a polyçrystalline copper target by Ar~ions with energies in the range from 50 to 90 eV. Several models have been suggested to explain the emission of neutral secondary clusters from ion bornbarded metal surfaces. One of these is the recombination model of emission [1,2] in which independently sputtered atoms (resulting from the impact on the surface of a single bombarding ion) are pictured as agglomerating into molecules outside (but near) the surface. Another model [3—6]represents the cluster as being directly emitted from the metal surface in the form of a cluster. Winograd et at [71have published a series of papers on cluster emission using a computer simulation of the sputtering process. Their conclusions are that the recombination model dominates when the cluster does not have a special identity in the solid (e.g., for clean metals), but that this model does not apply to the emission of molecular adsorbates (e.g., CO). There is a limited amount of experimental data on the emission of neutral clusters appearing in the literature to test the models of cluster emission. Gerhard and Oechsner [8] using secondary neutral mass spec124
trometry have measured the intensity of emission of neutral clusters from a number of metallic targets, using Ar~ion bombardment with energies from 100— 1200 eV. They found that the quantity RI2 (the ratio of the intensity of neutral sputtered diatomic molecules, Me2, to neutral sputtered atoms, Me) for different metals was linearly proportional to the sputtering yields of the metals, and they consider this as evidence favoring the recombination model. Bernhardt et a!. [9] have studied the energy distribution of neutral sputtered Ag and Ag2 molecules from polycrystalline Ag under bombardment of Ar+ ions of energy 1 keY. The results again are interpreted by the recombination model. Konnen et al. [1] have used the recombination mod el of emission to predict the expected energy distribution of emitted dimers. They cite the experimental results of Baede et al. [10] on the emission of neutral K2 clusters due to bombardment of a polycrystalline
K target by keV Ar~ions in support of the recombination model. Considerable experimental data appear in the literature on the emission by sputtering of ion clusters. However the above models of cluster emission refer to the emission of neutral clusters. Thus, for testing the models of cluster emission, experiments on neutral clusters are preferable to those on ion clusters since
Volume 79A, number 1
PHYSICS LETTERS
15 September 1980
the variations in the probability of ionization can overshadow the kinematic emission process. For a review of experimental ion cluster work, see, e.g. ref. [11]. Wittmaack [6] has recently published data on the intensities of Si~ion clusters (3
the arc plasma potential was measured to be about ÷8 V, this correction was applied to the measured target voltages to obtain the energy of the bombarding ions. The results are plotted in fIg. 1. Quantities plotted along the ordinate are the ratios “S2”/”S1”, where
amorphous Si by inert gas ions of energy 3—30 keY, and draws the conclusion that his results support the direct emission process rather than the recombination model of cluster emission. Winograd et al. [7] cite experimental data which show that the ratios of neutral 0~2!~L(calculated from their computer simulation) agree with the measured values obtained by Barber et al. [12] of Ni~/Ni~ ions sputtered from the three low index crystal faces of a Ni single crystal. The apparatus used in the present experiments was a mass spectrometer in which the neutral sputtered particles were post ionized and then mass analyzed. The apparatus is described in detail elsewhere [13,141. Briefly, a magnetically confined, thermionically sustained inert gas arc plasma is set up in the mass spectrometer source region. The target is immersed in the plasma, and when biased negatively is bombarded normally by arc ions. The plasma is run with electron energies such that essentially no doubly charged ions are formed. The sputtered neutral particles from the target proceed through the arc and a fraction of them are ionized by the primary electron beam, and these ions then proceed through the sector magnet momentum filter. Intensities and energy distributions of the postionized species can be measured. Typical operating parameters are: primary electron beam energy, 40 eY; pressure in the arc region, about 10~ Torr; pressure in the analyzer region, about 10~ Torr; ion current density on the target, about 3 mA!
“S2” is the relative yield of sputtered neutral Cu2 molecules, and “S1” the relative yield of sputtered neutral Cu atoms. Quantities plotted along the abscissa are the values of the total relative sputtering yield, “S”, of the Cu target (“~~“ + 2”S2”), for each bornbarding ion energy. The results of the experiments as plotted in fig. 1, show that the ratios “S2”f”S1” vary linearly with “S”. An empirical formula describing the results in~fig. 1 would be “S2”/”S1” k”S”, where k is a constant. Based on the recombination model it has been suggested [21that the intensity I~of an emitted neutral metallic cluster Mn would be given by: = k Sn 1
cm
2
n
fl
for n ~ 3, where k~is a constant, and S is the sputtering yield of the metal. Thus, the results presented in fig. 1 are in agreement with eq. (1), which here becomes ‘2/’l = kS. The range of bombarding ion energies used in this experiment is such that there is a large change in the sputtering yield. (The ratio of sputtering
I I
I
2
4
6
-~ ,~
002
E
.
In the present experiment, the target was a polycrystalline copper (99 .999% pure) sphere 3 mm in diameter. This target was bombarded normally by Ar~ ions whose energies ranged from 50 to 90 eV. Sputtered neutral Cu atoms and neutral Cu2 molecules were detected in the mass spectrometer. The relative yields, “5”, of the various sputtered species were determined by taking the ratio of the mass spectrometer collector ion current at maximum peak height to the ion current bombarding the target. Typical values of these relative yields, S varied from about 10 X 10b0 for primary ion energies of 50 eV to about 50 X 1010 for primary ion energies of 90 eV. Since ~
,,
.
,
00
-
-
I
0
8
0
s Fig. 1. (Ratios of neutral sputtered molecules S2 ) of to relative neutral yields sputtered Cu atoms (“Si”) Cu2 versus relative sputtering yields of Cu (“5”) (arbitrary units) due to Ar~ion bombardment. Energies of bombarding ~ ions are 50—90 eV.
125
Volume 79A, number 1
PHYSICS LETTERS
yields at bombarding ion energies of 90 and 50 eY is about 5 : 1.) A Cu target is used throughout, eliminating the problem of possible changes in kn for different elements [6]. The linear relationship of the present measurements is in agreement with the measurements of Gerhard and Oechsner [8] on the sputtering of Cu by Ar~ions of higher energy (100—1000 eY). In summary, the intensity ratios of neutral sputtered Cu 2 molecules to neutral sputtered Cu atoms was found to be linearly proportional to the sputtering yield of Cu, under normal bombardment by Ar~ions (energy 50—90 eY), as determined by secondary neutral mass spectrometry.
121 [3] 141 [5] [6]
[7] [8] [9] [10] [11] [12]
This work has been partially supported by the National Science Foundation. [13]
References [14] [1] G. Konnen, G. Tip and A.E. deVries, Radiat. Eff. 21 (1974) 269.
126
15 September 1980
W. Gerhard, Z. Phys. B22 (1975) 31. G. Staudenmaier, Radiat. Eff. 18 (1973) 181. P. Joyes, J. Phys. B4 (1971) L15. IS. Bitenskii and E.S. Parilis, Soy. Phys. Tech. Phys. 23 (1979) 1104. K. Wittmaack, Phys. Lett. 69A (1979) 322. N. Winograd, K.E. Foley, B.J. Garrison and D.E. Harrison Jr., Phys. Lett. 73A (1979) 253. W. Gerhard and H. Oechsner, Z. Phys. B22 (1975) 41. F. Bernhardt, H. Oechsner and E. Stumpe, Nuci. Instrum. Methods 132 (1976) 329. A.P.M. Baede, W.F. Jungmann and J. Los, Physica 54 (1971.) 459. K. Wittmaack, in: Inelastic ion—surface collisions, eds. Tolk, Tuily, Heiland and White (Academic Press, New York, 1977) p. 153. M. Barber, R. Bordoli, J. Wolstenholme and J.C. Vickerman, in: Proc. Ilird Intern. Conf. on Solid surfaces (Vienna, Austria, 1977). J.R. Woodyard and C.B. Cooper, J. Appl. Phys. 35 (1964) 1107. A.B. Campbell and C.B. Cooper, J. Appl. Phys. 43 (1972) 863.
PHYSICS LETTERS
15 September 1980
LOW ENERGY SPUTTERING OF NEUTRAL Cu2 MOLECULES C. Burleigh COOPER Physics Department, University of Delaware, Newark, DE 19711, USA
and James R. WOODYARD Wayne State University, Detroit, MI 48202, USA Received 14 April 1980 Revised manuscript received 23 June 1980
The ratios of relative yields of neutral sputtered Cu2 molecules to neutral sputtered 4Cu ions atoms (energy were50—90 found eV), to beaslinearly determined by secondary proportional to the sputtering neutral mass yield spectrometry. of Cu, from a Cu target under bombardment by Ar
This letter reports on experiments done by mass spectrometry to measure the intensities of neutral Cu atoms and Cu 2 molecules sputtered from a polyçrystalline copper target by Ar~ions with energies in the range from 50 to 90 eV. Several models have been suggested to explain the emission of neutral secondary clusters from ion bornbarded metal surfaces. One of these is the recombination model of emission [1,2] in which independently sputtered atoms (resulting from the impact on the surface of a single bombarding ion) are pictured as agglomerating into molecules outside (but near) the surface. Another model [3—6]represents the cluster as being directly emitted from the metal surface in the form of a cluster. Winograd et at [71have published a series of papers on cluster emission using a computer simulation of the sputtering process. Their conclusions are that the recombination model dominates when the cluster does not have a special identity in the solid (e.g., for clean metals), but that this model does not apply to the emission of molecular adsorbates (e.g., CO). There is a limited amount of experimental data on the emission of neutral clusters appearing in the literature to test the models of cluster emission. Gerhard and Oechsner [8] using secondary neutral mass spec124
trometry have measured the intensity of emission of neutral clusters from a number of metallic targets, using Ar~ion bombardment with energies from 100— 1200 eV. They found that the quantity RI2 (the ratio of the intensity of neutral sputtered diatomic molecules, Me2, to neutral sputtered atoms, Me) for different metals was linearly proportional to the sputtering yields of the metals, and they consider this as evidence favoring the recombination model. Bernhardt et a!. [9] have studied the energy distribution of neutral sputtered Ag and Ag2 molecules from polycrystalline Ag under bombardment of Ar+ ions of energy 1 keY. The results again are interpreted by the recombination model. Konnen et al. [1] have used the recombination mod el of emission to predict the expected energy distribution of emitted dimers. They cite the experimental results of Baede et al. [10] on the emission of neutral K2 clusters due to bombardment of a polycrystalline
K target by keV Ar~ions in support of the recombination model. Considerable experimental data appear in the literature on the emission by sputtering of ion clusters. However the above models of cluster emission refer to the emission of neutral clusters. Thus, for testing the models of cluster emission, experiments on neutral clusters are preferable to those on ion clusters since
Volume 79A, number 1
PHYSICS LETTERS
15 September 1980
the variations in the probability of ionization can overshadow the kinematic emission process. For a review of experimental ion cluster work, see, e.g. ref. [11]. Wittmaack [6] has recently published data on the intensities of Si~ion clusters (3
the arc plasma potential was measured to be about ÷8 V, this correction was applied to the measured target voltages to obtain the energy of the bombarding ions. The results are plotted in fIg. 1. Quantities plotted along the ordinate are the ratios “S2”/”S1”, where
amorphous Si by inert gas ions of energy 3—30 keY, and draws the conclusion that his results support the direct emission process rather than the recombination model of cluster emission. Winograd et al. [7] cite experimental data which show that the ratios of neutral 0~2!~L(calculated from their computer simulation) agree with the measured values obtained by Barber et al. [12] of Ni~/Ni~ ions sputtered from the three low index crystal faces of a Ni single crystal. The apparatus used in the present experiments was a mass spectrometer in which the neutral sputtered particles were post ionized and then mass analyzed. The apparatus is described in detail elsewhere [13,141. Briefly, a magnetically confined, thermionically sustained inert gas arc plasma is set up in the mass spectrometer source region. The target is immersed in the plasma, and when biased negatively is bombarded normally by arc ions. The plasma is run with electron energies such that essentially no doubly charged ions are formed. The sputtered neutral particles from the target proceed through the arc and a fraction of them are ionized by the primary electron beam, and these ions then proceed through the sector magnet momentum filter. Intensities and energy distributions of the postionized species can be measured. Typical operating parameters are: primary electron beam energy, 40 eY; pressure in the arc region, about 10~ Torr; pressure in the analyzer region, about 10~ Torr; ion current density on the target, about 3 mA!
“S2” is the relative yield of sputtered neutral Cu2 molecules, and “S1” the relative yield of sputtered neutral Cu atoms. Quantities plotted along the abscissa are the values of the total relative sputtering yield, “S”, of the Cu target (“~~“ + 2”S2”), for each bornbarding ion energy. The results of the experiments as plotted in fig. 1, show that the ratios “S2”f”S1” vary linearly with “S”. An empirical formula describing the results in~fig. 1 would be “S2”/”S1” k”S”, where k is a constant. Based on the recombination model it has been suggested [21that the intensity I~of an emitted neutral metallic cluster Mn would be given by: = k Sn 1
cm
2
n
fl
for n ~ 3, where k~is a constant, and S is the sputtering yield of the metal. Thus, the results presented in fig. 1 are in agreement with eq. (1), which here becomes ‘2/’l = kS. The range of bombarding ion energies used in this experiment is such that there is a large change in the sputtering yield. (The ratio of sputtering
I I
I
2
4
6
-~ ,~
002
E
.
In the present experiment, the target was a polycrystalline copper (99 .999% pure) sphere 3 mm in diameter. This target was bombarded normally by Ar~ ions whose energies ranged from 50 to 90 eV. Sputtered neutral Cu atoms and neutral Cu2 molecules were detected in the mass spectrometer. The relative yields, “5”, of the various sputtered species were determined by taking the ratio of the mass spectrometer collector ion current at maximum peak height to the ion current bombarding the target. Typical values of these relative yields, S varied from about 10 X 10b0 for primary ion energies of 50 eV to about 50 X 1010 for primary ion energies of 90 eV. Since ~
,,
.
,
00
-
-
I
0
8
0
s Fig. 1. (Ratios of neutral sputtered molecules S2 ) of to relative neutral yields sputtered Cu atoms (“Si”) Cu2 versus relative sputtering yields of Cu (“5”) (arbitrary units) due to Ar~ion bombardment. Energies of bombarding ~ ions are 50—90 eV.
125
Volume 79A, number 1
PHYSICS LETTERS
yields at bombarding ion energies of 90 and 50 eY is about 5 : 1.) A Cu target is used throughout, eliminating the problem of possible changes in kn for different elements [6]. The linear relationship of the present measurements is in agreement with the measurements of Gerhard and Oechsner [8] on the sputtering of Cu by Ar~ions of higher energy (100—1000 eY). In summary, the intensity ratios of neutral sputtered Cu 2 molecules to neutral sputtered Cu atoms was found to be linearly proportional to the sputtering yield of Cu, under normal bombardment by Ar~ions (energy 50—90 eY), as determined by secondary neutral mass spectrometry.
121 [3] 141 [5] [6]
[7] [8] [9] [10] [11] [12]
This work has been partially supported by the National Science Foundation. [13]
References [14] [1] G. Konnen, G. Tip and A.E. deVries, Radiat. Eff. 21 (1974) 269.
126
15 September 1980
W. Gerhard, Z. Phys. B22 (1975) 31. G. Staudenmaier, Radiat. Eff. 18 (1973) 181. P. Joyes, J. Phys. B4 (1971) L15. IS. Bitenskii and E.S. Parilis, Soy. Phys. Tech. Phys. 23 (1979) 1104. K. Wittmaack, Phys. Lett. 69A (1979) 322. N. Winograd, K.E. Foley, B.J. Garrison and D.E. Harrison Jr., Phys. Lett. 73A (1979) 253. W. Gerhard and H. Oechsner, Z. Phys. B22 (1975) 41. F. Bernhardt, H. Oechsner and E. Stumpe, Nuci. Instrum. Methods 132 (1976) 329. A.P.M. Baede, W.F. Jungmann and J. Los, Physica 54 (1971.) 459. K. Wittmaack, in: Inelastic ion—surface collisions, eds. Tolk, Tuily, Heiland and White (Academic Press, New York, 1977) p. 153. M. Barber, R. Bordoli, J. Wolstenholme and J.C. Vickerman, in: Proc. Ilird Intern. Conf. on Solid surfaces (Vienna, Austria, 1977). J.R. Woodyard and C.B. Cooper, J. Appl. Phys. 35 (1964) 1107. A.B. Campbell and C.B. Cooper, J. Appl. Phys. 43 (1972) 863.