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A reaction sputtering type rf ion source for solid elements
136
Nuclear
A REACTION Cheng Shichang, Ion Beam
SPU’ITERING
TYPE
Instruments
and Methods
RF ION SOURCE
in Physics Research B37/38 (1989) 136-139 North-Holland, Amsterdam
FOR SOLID ELEMENTS
Fu Dejun, Zhang Hui and Pan Xianzheng
Physics Luboratory
Physics Dept.,
Wuhan
University,
Wuhan,
China
A new type of ion source for refractory elements was developed for low energy ion beam deposition. The source was made by employing a pair of plate shape sputtering electrodes in an inductance coupled rf ion source. The electrode was composed of the solid element needed or its compounds and its gaseous oxide or halide was used as a working gas. Owing to the fact that the gaseous compounds, composed, by means of reactive sputtering, of the reactive gas ions and the solid element in the electrode, entered the ion source, the proportion of solid particles was increased and a larger ion beam current of the solid was obtained. The source possesses the advantages of the normal rf ion source. About 80 PA of carbon ion beam was obtained under optimal conditions.
1. Introduction Low energy ion beam deposition has been developed as a new technique for thin film growth taking advantage of the singularity of deposition particles, the deposition energy and dose measurable and controllable and the target which can be placed in high or ultrahigh vacuum. Direct deposition of a low energy ion beam requires that the ion source produces ions of the deposition element, which generally has a high melting point. Also, in order to deposit thick enough films, the ions should have doses larger than 1017 ions/cm2. Hence the source should have the ability of producing large currents of ion beams and have a long lifetime. High temperature ion sources such as Freeman [l], Nielsen [2] and hollow cathode [3] ion sources were developed and widely used which melt the required substance in a crucible and then let the vapor immediately into the ion source for ionization. These sources are effective for substances with melting points between 600 and 1500°C but fail for refractory elements such as boron and carbon. One resolution to this problem is to use the gaseous or low melting point compounds of the refractory element [4]. Due to the complication of the compounds and the small proportion of required ions in the plasma, however, it is often difficult to get a high ion beam current. In another method [5,6], sputtering was employed in the ion source, and ions were produced by sputtering the electrode consisting of the required element. The electrode was negatively biased and inert gas was used for sputtering. However, as a high voltage was applied to the electrode, some of the substances sputtered off easily contaminated the insulating parts of the ion source and relieved the source from working long and stably. This paper describes the reaction sputtering type rf ion source for production of low energy ion beams of
0168-583X/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
refractory elements. Carbon and boron ion beams were extracted.
2. Structure and processes The structure of the ion source is shown in fig. 1. The discharge chamber was made from quartz glass with a T-shape. On each terminal of the tube was fixed a sputtering electrode composed of the solid substance needed or its compounds. The two electrodes were biased with the same value of voltage in the range of O-2500 V negative to the plasma. The working gas was released into the chamber through the gas inlet with its flux controlled by a pin valve. The rf power with a frequency of 13.56 MHz, produced from a rf oscillator, was coupled into the chamber through the inductance coil. The ion beams were extracted by a plate shape extracting system, which is composed of a BN piece with an aperture of 2.5 mm diameter and an extractor with a hole of 1.5 mm diameter and 3 mm length which are separated by 3 mm. The extracting voltage, which is the potential difference between plasma and extractor, can be controlled in the range of O-8000 V. In order to produce carbon ion beams, CO, was used as a working gas and the electrode was made from high purity graphite. The CO, gas was ionized and decomposed by the rf power. In the chamber there existed great quantities of atoms and molecules of the oxide and their ions, atoms and ions of carbon and atoms, molecules and their ions of carbon oxide and carbon dioxide. Under the influence of the electric field of the bias voltage between the electrode and the plasma, positive ions were accelerated to the electrode where the graphite reacted with the oxide ions to compose CO which moved back to the plasma region. Thus the proportion of carbons and the density of carbon ions in
Cheng Shichang et al. / A reaction sputtering type
rf ion murce
137
Sputtering Electrode
t
*
‘1 I
II
\
-\
Anode
‘On 6eam \
Extractor
Fig. 1. The structure of the reaction sputtering type rf
the plasma were increased. The secondary electrons produced from the electrode sputtered were also accelerated to the plasma by the above electric field. These energetic electrons enhanced the ionization and decomposition of the gases in the chamber and further increased the quantity of carbon ions. The atoms and molecules of the oxide and their ions diffusing onto the wall of the discharge tube reacted with the absorbed carbons to form carbonous gases, cleared out the carbon contamination to the chamber and made the ion source work longer.
3. Results and discussion The performance of the ion source was tested on the low energy ion beam deposition machine. The static pressure of the vacuum was 0.5 mPa and while the system worked it was 1.0-2.0 mPa. The accelerating voltage was 30 kV and the mass resolving power of the magnetic analyzer was 200. Argon was first used to test the ion source. Then two of the refractory elements, carbon and boron, were extracted, respectively. Mass spectra were taken in order to analyse the composition of the plasma at different stages. The extraction characteristic of the source tested using argon is shown in fig. 2. Like that of any rf ion source, the extracting ion beam current changes with the extracting voltage and at the given voltage appears a peak which increases in magnitude and its position shifts to higher extracting voltage as the rf power raises. Nevertheless the negative bias voltage applied to the sputtering electrode did not evidently influence the extraction characteristic. In the case of extracting carbon ion beams, however, the composition of the plasma was greatly influenced by the bias voltage. Fig. 3 shows the spectra of the ion beams at both zero and 400 V bias voltage. One can see by comparing them that at the higher bias voltage, there
ion source
appear decreases of the contents of Ot and 0: and remarkable increases of the CO+ and C+ beam currents. This is evidently due to the effect of the bias voltage under which the positive ions in the plasma were accelerated towards an impact on the graphite electrode. The carbons reacted with either O+ and 0: to compose CO gas. The dissociation and dissociative ionization of CO resulted in a corresponding increase of the content of carbon ions. To confirm this result we did measure the beam currents of CO+ and C+ on the target which was placed in the deposition chamber, as well as the current of the ions incident on the sputtering electrode. The tendencies of their change with bias voltage were observed. The experiment indicated that the CO+ beam current on the target increased rapidly with increasing
1.5
RF Power
IWI
\ ‘.
l
IC
*
\\
l
x l
‘\*r
\
*A0
0.5
\.y
\ J r 0 1 0
Ov
I
I
I
2
4
6
EXTRACTING
VOLTAGE
(kV)
Fig. 2. The extraction characteristics of the reaction sputtering type rf ion source. II. ION SOURCES
Cheng Shichang et al. / A reaction sputiering type
138
rf ion source
50
40
30 MASS (AMU)
Fig. 3. Mass spectra of the carbon ion source at different bias voltages.
bias voltage when the latter was below 300 V. However, when the bias voltage exceeded this value, the CO+ current no longer changed. This agrees well with the change of the sputtering ion current, as shown in fig. 4. Fig. 5 shows the carbon ion beam current as a function of the bias voltage. The carbon ion beam current behaves similarly to the CO+ beam current, consistent with the results shown in figs. 3 and 4. All this shows that what happened in the ion source was a chemically reactive sputtering instead of the physical sputtering process in which the sputtering yield increases with the ion energy. In the former case, the sputtering yield is independent of the ion energy but proportional to the quantity of reactive ions incident on the sputtering target. Carbon ions were produced mainly from CO. It can be seen from figs. 4 and 5 that the change of the carbon ion beam current with bias voltage slightly
f
I
I
200 BIAS VOLTAGE
Fig. 4. The CO+
1
I
I
1
a beam
differs from that of the CO+ current in that the carbon ion beam current decreases when the bias voltage exceeds 400 V. In fact, as the energetic ions bombarded the sputtering electrode, a large quantity of secondary electrons were produced while the sputtering and reaction processes took place. The quantity of secondary electrons increases with the ion energy. Under the influence of the bias voltage, the secondary electrons were accelerated, moved into the plasma and were involved in the ionization process. The rise of the bias voltage increased the kinetic energy of the secondary electrons. Generally the ionization cross sections of atoms and molecules are closely related to the electron energy. When the electron energy surpasses 400 eV, the ionization cross sections of both carbon oxide and carbon decrease [7]. This explains the results shown in fig. 5. Boron ion beams were also extracted from the ion source using La$ and BF, as electrode material and
0 300
I
400
I 600
400 BIAS VOLTAGE
(V)
current and sputtering the bias voltage.
1
200
ion current
vs
Fig. 5. Dependence of the carbon voltage at different
(VI
ion beam rf powers.
current
on bias
Cheng Shichang et al. / A reaction sputtering type rf ion source
working gas, respectively. As in the case discussed above, the boron ion beam was significantly increased by the reaction sputtering process.
139
tion of the refractory element in the ion source. The source has a lifetime of tens of hours.
References 4. Conclusion
It is deduced from the above results that by using the reaction sputtering type rf ion source, larger beam currents of ions of refractory elements can be obtained. In addition to the rf power, the bias voltage applied to the sputtering electrode was the main parameter influencing the composition of the ion source. Under optimal conditions, about 80 PA of carbon ion beams were obtained on the target. There was no contamina-
Ul J.H. Freeman, Nucl. Instr. and Meth. 22 (1963) 306. u-1K.O. Nielsen, Nucl. Instr. and Meth. 1 (1957) 289. [31 G. Sidenius, Nucl. Instr. and Meth. 36 (1965) 19. S. Misawa, S. Yoshida and S. Gonda, J. [41T. Miyazawa, Appl. Phys. 55 (1964) 186. PI S. Aisenberg and R. Chabot, J. Appl. Phys. 42 (1971) 2953. WI K.J. Hill and R.S. Nilson, Nucl. Instr. and Meth. 38 (1965) 15. Press, [71A. Von Engel, Ionized Gases (Oxford University 1965).
II. ION SOURCES
Nuclear
A REACTION Cheng Shichang, Ion Beam
SPU’ITERING
TYPE
Instruments
and Methods
RF ION SOURCE
in Physics Research B37/38 (1989) 136-139 North-Holland, Amsterdam
FOR SOLID ELEMENTS
Fu Dejun, Zhang Hui and Pan Xianzheng
Physics Luboratory
Physics Dept.,
Wuhan
University,
Wuhan,
China
A new type of ion source for refractory elements was developed for low energy ion beam deposition. The source was made by employing a pair of plate shape sputtering electrodes in an inductance coupled rf ion source. The electrode was composed of the solid element needed or its compounds and its gaseous oxide or halide was used as a working gas. Owing to the fact that the gaseous compounds, composed, by means of reactive sputtering, of the reactive gas ions and the solid element in the electrode, entered the ion source, the proportion of solid particles was increased and a larger ion beam current of the solid was obtained. The source possesses the advantages of the normal rf ion source. About 80 PA of carbon ion beam was obtained under optimal conditions.
1. Introduction Low energy ion beam deposition has been developed as a new technique for thin film growth taking advantage of the singularity of deposition particles, the deposition energy and dose measurable and controllable and the target which can be placed in high or ultrahigh vacuum. Direct deposition of a low energy ion beam requires that the ion source produces ions of the deposition element, which generally has a high melting point. Also, in order to deposit thick enough films, the ions should have doses larger than 1017 ions/cm2. Hence the source should have the ability of producing large currents of ion beams and have a long lifetime. High temperature ion sources such as Freeman [l], Nielsen [2] and hollow cathode [3] ion sources were developed and widely used which melt the required substance in a crucible and then let the vapor immediately into the ion source for ionization. These sources are effective for substances with melting points between 600 and 1500°C but fail for refractory elements such as boron and carbon. One resolution to this problem is to use the gaseous or low melting point compounds of the refractory element [4]. Due to the complication of the compounds and the small proportion of required ions in the plasma, however, it is often difficult to get a high ion beam current. In another method [5,6], sputtering was employed in the ion source, and ions were produced by sputtering the electrode consisting of the required element. The electrode was negatively biased and inert gas was used for sputtering. However, as a high voltage was applied to the electrode, some of the substances sputtered off easily contaminated the insulating parts of the ion source and relieved the source from working long and stably. This paper describes the reaction sputtering type rf ion source for production of low energy ion beams of
0168-583X/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
refractory elements. Carbon and boron ion beams were extracted.
2. Structure and processes The structure of the ion source is shown in fig. 1. The discharge chamber was made from quartz glass with a T-shape. On each terminal of the tube was fixed a sputtering electrode composed of the solid substance needed or its compounds. The two electrodes were biased with the same value of voltage in the range of O-2500 V negative to the plasma. The working gas was released into the chamber through the gas inlet with its flux controlled by a pin valve. The rf power with a frequency of 13.56 MHz, produced from a rf oscillator, was coupled into the chamber through the inductance coil. The ion beams were extracted by a plate shape extracting system, which is composed of a BN piece with an aperture of 2.5 mm diameter and an extractor with a hole of 1.5 mm diameter and 3 mm length which are separated by 3 mm. The extracting voltage, which is the potential difference between plasma and extractor, can be controlled in the range of O-8000 V. In order to produce carbon ion beams, CO, was used as a working gas and the electrode was made from high purity graphite. The CO, gas was ionized and decomposed by the rf power. In the chamber there existed great quantities of atoms and molecules of the oxide and their ions, atoms and ions of carbon and atoms, molecules and their ions of carbon oxide and carbon dioxide. Under the influence of the electric field of the bias voltage between the electrode and the plasma, positive ions were accelerated to the electrode where the graphite reacted with the oxide ions to compose CO which moved back to the plasma region. Thus the proportion of carbons and the density of carbon ions in
Cheng Shichang et al. / A reaction sputtering type
rf ion murce
137
Sputtering Electrode
t
*
‘1 I
II
\
-\
Anode
‘On 6eam \
Extractor
Fig. 1. The structure of the reaction sputtering type rf
the plasma were increased. The secondary electrons produced from the electrode sputtered were also accelerated to the plasma by the above electric field. These energetic electrons enhanced the ionization and decomposition of the gases in the chamber and further increased the quantity of carbon ions. The atoms and molecules of the oxide and their ions diffusing onto the wall of the discharge tube reacted with the absorbed carbons to form carbonous gases, cleared out the carbon contamination to the chamber and made the ion source work longer.
3. Results and discussion The performance of the ion source was tested on the low energy ion beam deposition machine. The static pressure of the vacuum was 0.5 mPa and while the system worked it was 1.0-2.0 mPa. The accelerating voltage was 30 kV and the mass resolving power of the magnetic analyzer was 200. Argon was first used to test the ion source. Then two of the refractory elements, carbon and boron, were extracted, respectively. Mass spectra were taken in order to analyse the composition of the plasma at different stages. The extraction characteristic of the source tested using argon is shown in fig. 2. Like that of any rf ion source, the extracting ion beam current changes with the extracting voltage and at the given voltage appears a peak which increases in magnitude and its position shifts to higher extracting voltage as the rf power raises. Nevertheless the negative bias voltage applied to the sputtering electrode did not evidently influence the extraction characteristic. In the case of extracting carbon ion beams, however, the composition of the plasma was greatly influenced by the bias voltage. Fig. 3 shows the spectra of the ion beams at both zero and 400 V bias voltage. One can see by comparing them that at the higher bias voltage, there
ion source
appear decreases of the contents of Ot and 0: and remarkable increases of the CO+ and C+ beam currents. This is evidently due to the effect of the bias voltage under which the positive ions in the plasma were accelerated towards an impact on the graphite electrode. The carbons reacted with either O+ and 0: to compose CO gas. The dissociation and dissociative ionization of CO resulted in a corresponding increase of the content of carbon ions. To confirm this result we did measure the beam currents of CO+ and C+ on the target which was placed in the deposition chamber, as well as the current of the ions incident on the sputtering electrode. The tendencies of their change with bias voltage were observed. The experiment indicated that the CO+ beam current on the target increased rapidly with increasing
1.5
RF Power
IWI
\ ‘.
l
IC
*
\\
l
x l
‘\*r
\
*A0
0.5
\.y
\ J r 0 1 0
Ov
I
I
I
2
4
6
EXTRACTING
VOLTAGE
(kV)
Fig. 2. The extraction characteristics of the reaction sputtering type rf ion source. II. ION SOURCES
Cheng Shichang et al. / A reaction sputiering type
138
rf ion source
50
40
30 MASS (AMU)
Fig. 3. Mass spectra of the carbon ion source at different bias voltages.
bias voltage when the latter was below 300 V. However, when the bias voltage exceeded this value, the CO+ current no longer changed. This agrees well with the change of the sputtering ion current, as shown in fig. 4. Fig. 5 shows the carbon ion beam current as a function of the bias voltage. The carbon ion beam current behaves similarly to the CO+ beam current, consistent with the results shown in figs. 3 and 4. All this shows that what happened in the ion source was a chemically reactive sputtering instead of the physical sputtering process in which the sputtering yield increases with the ion energy. In the former case, the sputtering yield is independent of the ion energy but proportional to the quantity of reactive ions incident on the sputtering target. Carbon ions were produced mainly from CO. It can be seen from figs. 4 and 5 that the change of the carbon ion beam current with bias voltage slightly
f
I
I
200 BIAS VOLTAGE
Fig. 4. The CO+
1
I
I
1
a beam
differs from that of the CO+ current in that the carbon ion beam current decreases when the bias voltage exceeds 400 V. In fact, as the energetic ions bombarded the sputtering electrode, a large quantity of secondary electrons were produced while the sputtering and reaction processes took place. The quantity of secondary electrons increases with the ion energy. Under the influence of the bias voltage, the secondary electrons were accelerated, moved into the plasma and were involved in the ionization process. The rise of the bias voltage increased the kinetic energy of the secondary electrons. Generally the ionization cross sections of atoms and molecules are closely related to the electron energy. When the electron energy surpasses 400 eV, the ionization cross sections of both carbon oxide and carbon decrease [7]. This explains the results shown in fig. 5. Boron ion beams were also extracted from the ion source using La$ and BF, as electrode material and
0 300
I
400
I 600
400 BIAS VOLTAGE
(V)
current and sputtering the bias voltage.
1
200
ion current
vs
Fig. 5. Dependence of the carbon voltage at different
(VI
ion beam rf powers.
current
on bias
Cheng Shichang et al. / A reaction sputtering type rf ion source
working gas, respectively. As in the case discussed above, the boron ion beam was significantly increased by the reaction sputtering process.
139
tion of the refractory element in the ion source. The source has a lifetime of tens of hours.
References 4. Conclusion
It is deduced from the above results that by using the reaction sputtering type rf ion source, larger beam currents of ions of refractory elements can be obtained. In addition to the rf power, the bias voltage applied to the sputtering electrode was the main parameter influencing the composition of the ion source. Under optimal conditions, about 80 PA of carbon ion beams were obtained on the target. There was no contamina-
Ul J.H. Freeman, Nucl. Instr. and Meth. 22 (1963) 306. u-1K.O. Nielsen, Nucl. Instr. and Meth. 1 (1957) 289. [31 G. Sidenius, Nucl. Instr. and Meth. 36 (1965) 19. S. Misawa, S. Yoshida and S. Gonda, J. [41T. Miyazawa, Appl. Phys. 55 (1964) 186. PI S. Aisenberg and R. Chabot, J. Appl. Phys. 42 (1971) 2953. WI K.J. Hill and R.S. Nilson, Nucl. Instr. and Meth. 38 (1965) 15. Press, [71A. Von Engel, Ionized Gases (Oxford University 1965).
II. ION SOURCES