- Email: [email protected]
Slow, highly charged ions from a 7.25 GHz ECR ion source
m
Nuclear Instruments
and Methods in Physics Research B 98 (1995) 52X-531
NOMB
Beam Interactions with Materials8 Atoms
ELSEVIER
Slow, highly charged ions from a 7.25 GHz ECR ion source D. Henke a3*, H. Tyrroff a, R. Griitzschel a, H. Wirth ’ ’ Institutjiir lonenstrahlphysik und Materialforschung, Forschungszentrum Rossendorf e. V.. POB 510119, 01314 Dresden, Germany h Institut fir Kern- und Teilchenphysik der TlJ Dresden, Pratzschwilzer Str. 15, 01796 Pirna. Germany Abstract
The layout of a surface treating facility using a beam of slow, highly charged ions is presented. Optimization results for the ion output of different charge states for argon and xenon are given. The maximum outputs of Ar”+ and Xe*‘+ amount to 350 and 120 el. nA. The properties of an ion deceleration system are described which can reduce the kinetic energy of the ions by a factor of 200.
1. Introduction Our single stage, floating disk supported ECR ion source [l] can deliver an ion output of the order of 1 pA/cm2 for Ar”+ and Xe”+ using extraction voltages of U, = 5 kV. These charge states represent potential energies of 2 and 4 keV per ion. To distinguish between the influence of potential energy and kinetic energy on surface reactions involving these ions they should be slowed down to less than 100 eV X q (q = charge state). The mentioned current densities correspond to deposition rates of about 5 X 10” particles per cm’s, which is too low for direct deposition of chemically active species. However, such rare gas ion current densities could assist thin film production by transfering potential energy to the surface while the chemically active layer material is deposited from the gas phase. A corresponding facility for surface treatment is shown in Fig. 1. It combines film deposition and analysis equipment in an UHV target chamber with the highly charged ion source and a van de Graaff accelerator.
astigmatism of the beam. Fig. 2 illustrates the situation at the first waist. Lens 1 produces a further waist at the entrance of the toroidal condenser. The condenser inflects the beam into the van der Graaff beam line. The imaging parameters of all electrostatic elements including lens 2 are chosen so as to guide the beam through a neck represented by the bakeable valve, to deliver the right input for the decelerating lens and to realize the required final pressure of the order of 10-i” mbar inside the target chamber. This pressure is reached using beam waist apertures and corresponding pump stations. A four-electrode lens was calculated (code SAM-P [2]). Its behaviour is illustrated in Fig. 3 and the main parameters are compiled in Table 1. Target and target station are held at ground potential and the ion source at +AU (AU X q X e means the final kinetic energy of the ions) while the rest of the beam line is kept on ( - U* + AU). Until now all system components excluding the deceleration lens are mounted. Transmission experiments confirmed that 95% of the ion current measured at the first Faraday cup reach the UHV target chamber.
2. ECR ion beam line
3. Operational
The ions are emitted by the ECR plasma through an 8 mm hole, accelerated from 1 to 10 kV over a single gap, accepted with 40 mrad by an analyzing magnet and focused to a first beam waist. A quadrupole singlet lens between the ion source and bending magnet corrects the
In a first experiment we set the extraction voltage to 5 kV and optimized the output of several charge states at the first beam waist. This was done by varying the gas flow rates of main and cooling gas, the exciting current I,,, of the mirror field and the UHF power p. The xenon-136 optimization results are shown in Fig. 4. Comparable behaviour was also found for argon. Table 2 summarizes the maximum output for different argon and xenon charge states and the corresponding sets of operational parameters. The ion output was increased by a factor of two especially for the low pressure regimes if the disk was
* Corresponding author. Tel. +4Y 351 591 3296, fax +4Y 351 591 3285, e-mail [email protected]. 0168-583X/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved .SSDI 0168-583X(95)00181-6
parameters
D. Henke et al. / Nucl. Instr. and Meth. in Phys. Res. B 98 (I 995) 528-531
analyzi ing’ magnet
-0
ECR ion source
529
UHF
feed
-0Sm quadrupole singlet lens with divergence aperture - 1 .Om
Jlens
torodial
‘condenser
11
all-metal
le?S 2
valve
chamber
target
decelerating
lens
Fig. 1. Beam handling between ECR ion source and target chamber.
_ L
positioned properly along the axis of the ion source. The presented values can be reproduced fairly well. However, after exposure of the source to air it shbuld run for at least two weeks for proper operation. In another experiment the ion source parameters were chosen to obtain the high charge state output regime for Ar. While keeping constant the source regime (i.e. the plasma density) we measured the current density of all beam components q/h4 passing through a divergence aperture (see Fig. 1) as a function of the extraction gap
01.09.94 t36Xe-, 02., H2 mitiure Extraction: s W/t rn~ i-l power: 92 w
0 excitation of the analyzing magnet
Fig. 2. Typical ion beam composition. indicated
Xe charge
states
are
lens axis [mm]
213
Fig. 3. The four electrode lens: deceleration of a beam with an entrance emittance ellipse of r0 = 2.5 mm and (Y, = 64 mrad to 0, / G0 = 0.005.
5. PRODUCTION/METHODS/APPLICATIONS
530
D. Henke et al. /Nucl.
Instr. and Meth. in Phys. Res. B 98 (1995) 528-531
Table 1 Data of the deceleration lens: c, / 11, - image object energy ratio, U?-4 - electrode voltages, U, - acceleration voltage, M magnification, C - spherical aberration, B, - linear axial chromatic acceleration, B, - linear radial chromatic aberration, ri beam radius at the target, E, - field strength on the target surface for U,=lOkV ui/L.o
1
u2/u,
0.858 0.858 0 1.2 1.526 0.722 0.370 2.5 9.5
u3/uA
Q/f-!, -M C [ml - B, [ml - B, [ml ri [mm1 Ei [V/cm]
0.1
0.01
-0.3 -1.4 -0.1 1.0 2.257 0.479 0.104 2.4 -36
0.005
-1.3 -0.25 -0.15 1.4 9.915 7.082 0.709 3.3 -34
- 0.45 - 0.09 - 0.08 1.9 5.054 5.315 0.451 3.7.. (4.8) -18
J 15
30
45
60
extraction voltage FVl Fig. 5. Density of total Ar ion current and Ar ion beam components versus acceleration voltage. The ll+ output is figured tenfold magnified.
7
22
20
18
16
14
12
10
8
6
I
voltage. Fig. 5 shows that the Arq+ current density components and their sum I,,, increase with the extraction voltage. In our case of a constant plasma density the change of the ion current densities beyond the mentioned aperture is essentially a result of the change of the beam divergence. The beam divergence for our low current densities depends mainly on the plasma curvature, on the focussing strength of the extraction gap and on the magnetic fringing field of the ion source. The observed deviations in the voltage dependence of the beam components Ar@ possibly indicate a charge state dependent focusing effect of the extended magnetic fringing field of the ion source. Although we do not exclude the influence of charge state dependent emittances [3].
2
charge state q Fig. 4. Charge state distributions obtained for different ion source regimes. Charge state with maximized output is indicated.
Table 2 Maximized Ar and Xe output I and corresponding operational parameters excitation. Gas flow in rel. units. D,...- total oressure in 10m6 mbar
for several charge states 4 +
. p - UHF power, I,,, - mirror field
4”,4&+
1
2
3
4
6
7
8
9
11
12
P WI 1, [Al Ar 0 Pt0t I [PAI
2 587 60 off 10 57
64 610 40 off 1.3 20.5
92 756 36 off 0.8 18.5
65 795 36
65 820 32 20 0.5 13
65 820 32 20 0.7 11.5
59 867 30 26 0.9 15.5
75 865 28 32 0.7 8
85 909 26 32 0.8 1.5
85 909 26 32 0.8 0.35
r36Xeq+ P [WI I,,, [Al Xe 0 Pt0t I [PAI
3 3 890 28 off 1.4 7
5 78 720 22 Off
1.1 6.2
Off
0.7 15
7
10
12
14
16
18
20
21
35 860 18 14 0.85 6.8
40 910 16 15 0.94 6.5
46 920 15 15 1.0 4.5
75 956 12.5 16.5 1.2 2.8
70 941 10.5 19.5 1.0 2.1
98 940 10 21 1.0 1.5
98 940 10 21 1.0 0.75
98 940 10 21 1.0 0.5
22 98 940 10 21 1.0 0.12
D. Henke et al. / Nucl. Instr. and Meth. in Phys. Res. B 98 (1995) 528-531
4. Comparison Our nitrogen output of 5 p,A for Nh+ is comparable to the value of Antaya’s single stage 6.4 GHz source [4]. However, in comparison to his double stage 6.4 GHz source [5] our Ar output is 5 times lower and the Xe output by one order of magnitude. That means the floating disk operation cannot fully replace a high performance two-stage operation.
5. Plasma diagnostics Measurements of characteristic X-rays emitted by ions of this ECR source have been performed. This experiment established an obvious difference between the charge state distributions of the plasma and the ion beam. Furthermore, the energy distribution of the hot ECR plasma electrons was determined using the bremsstrahlung spectrum emitted along the magnetic mirror field of the source [6].
6. Conclusion Shortly the deceleration lens will be mounted and tested. In a first application we intend to investigate the influence of our ion beam on the epitaxial growth and the morphology of layers on semiconductor surfaces. Furthermore, to improve the energy resolution of the X-ray spectra a crystal spectrometer will be connected to the ion source. This could help to obtain more information on the
531
ECR discharge mechanism. More time should be spent to explain the complicated nature of beam extraction from an ECR ion source.
Acknowledgements Support by Bundesminister ftir Forschung und Technologie, S?ichsisches Staatsministerium ftir Wissenschaft und Kunst and Volkswagen Stiftung is greatly appreciated. We thank G. Franz, P. Hartmann and F. Niitzold for technical assistance.
References [l] L. Friedrich, E. Huttel, R. Hentschel and H. Tyrroff, Proc. 11th Int. Workshop on ECR Ion Sources, May 1993, Groningen (1993) p. 19. [2] .I. Teichert, D. Janssen and M.A. Tiunov, Report ZFK-667 (1989) p. 75. [3] D.J. Clark, Proc. Int. Conf. on ECR Ion Sources, East Lansing (1987) p. 433. [4] D.K. Bose and T.A. Antaya, Proc. Int. Conf. on ECR Ion Sources, Dec. 1987, East Lansing, (1987) p. 371. [5] T.A. Antaya, S. Alfredson, D. Cole, K. Harrison and P. Osborne, Proc. 11th Int. Workshop on ECR Ion Sources, Groningen (1993) p. 37. [6] R. Friedlein, S. Herpich, U. Lehnert, H. Tyrroff, C. Zippe and G. Zschornack, these Proceedings (7th Int. Conf. on the Physics of Highly Charged Ions, Vienna, Austria, 1994) Nucl. Instr. and Meth. B 98 (1995) 585.
5. PRODUCTION/METHODS/APPLICATIONS
Nuclear Instruments
and Methods in Physics Research B 98 (1995) 52X-531
NOMB
Beam Interactions with Materials8 Atoms
ELSEVIER
Slow, highly charged ions from a 7.25 GHz ECR ion source D. Henke a3*, H. Tyrroff a, R. Griitzschel a, H. Wirth ’ ’ Institutjiir lonenstrahlphysik und Materialforschung, Forschungszentrum Rossendorf e. V.. POB 510119, 01314 Dresden, Germany h Institut fir Kern- und Teilchenphysik der TlJ Dresden, Pratzschwilzer Str. 15, 01796 Pirna. Germany Abstract
The layout of a surface treating facility using a beam of slow, highly charged ions is presented. Optimization results for the ion output of different charge states for argon and xenon are given. The maximum outputs of Ar”+ and Xe*‘+ amount to 350 and 120 el. nA. The properties of an ion deceleration system are described which can reduce the kinetic energy of the ions by a factor of 200.
1. Introduction Our single stage, floating disk supported ECR ion source [l] can deliver an ion output of the order of 1 pA/cm2 for Ar”+ and Xe”+ using extraction voltages of U, = 5 kV. These charge states represent potential energies of 2 and 4 keV per ion. To distinguish between the influence of potential energy and kinetic energy on surface reactions involving these ions they should be slowed down to less than 100 eV X q (q = charge state). The mentioned current densities correspond to deposition rates of about 5 X 10” particles per cm’s, which is too low for direct deposition of chemically active species. However, such rare gas ion current densities could assist thin film production by transfering potential energy to the surface while the chemically active layer material is deposited from the gas phase. A corresponding facility for surface treatment is shown in Fig. 1. It combines film deposition and analysis equipment in an UHV target chamber with the highly charged ion source and a van de Graaff accelerator.
astigmatism of the beam. Fig. 2 illustrates the situation at the first waist. Lens 1 produces a further waist at the entrance of the toroidal condenser. The condenser inflects the beam into the van der Graaff beam line. The imaging parameters of all electrostatic elements including lens 2 are chosen so as to guide the beam through a neck represented by the bakeable valve, to deliver the right input for the decelerating lens and to realize the required final pressure of the order of 10-i” mbar inside the target chamber. This pressure is reached using beam waist apertures and corresponding pump stations. A four-electrode lens was calculated (code SAM-P [2]). Its behaviour is illustrated in Fig. 3 and the main parameters are compiled in Table 1. Target and target station are held at ground potential and the ion source at +AU (AU X q X e means the final kinetic energy of the ions) while the rest of the beam line is kept on ( - U* + AU). Until now all system components excluding the deceleration lens are mounted. Transmission experiments confirmed that 95% of the ion current measured at the first Faraday cup reach the UHV target chamber.
2. ECR ion beam line
3. Operational
The ions are emitted by the ECR plasma through an 8 mm hole, accelerated from 1 to 10 kV over a single gap, accepted with 40 mrad by an analyzing magnet and focused to a first beam waist. A quadrupole singlet lens between the ion source and bending magnet corrects the
In a first experiment we set the extraction voltage to 5 kV and optimized the output of several charge states at the first beam waist. This was done by varying the gas flow rates of main and cooling gas, the exciting current I,,, of the mirror field and the UHF power p. The xenon-136 optimization results are shown in Fig. 4. Comparable behaviour was also found for argon. Table 2 summarizes the maximum output for different argon and xenon charge states and the corresponding sets of operational parameters. The ion output was increased by a factor of two especially for the low pressure regimes if the disk was
* Corresponding author. Tel. +4Y 351 591 3296, fax +4Y 351 591 3285, e-mail [email protected]. 0168-583X/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved .SSDI 0168-583X(95)00181-6
parameters
D. Henke et al. / Nucl. Instr. and Meth. in Phys. Res. B 98 (I 995) 528-531
analyzi ing’ magnet
-0
ECR ion source
529
UHF
feed
-0Sm quadrupole singlet lens with divergence aperture - 1 .Om
Jlens
torodial
‘condenser
11
all-metal
le?S 2
valve
chamber
target
decelerating
lens
Fig. 1. Beam handling between ECR ion source and target chamber.
_ L
positioned properly along the axis of the ion source. The presented values can be reproduced fairly well. However, after exposure of the source to air it shbuld run for at least two weeks for proper operation. In another experiment the ion source parameters were chosen to obtain the high charge state output regime for Ar. While keeping constant the source regime (i.e. the plasma density) we measured the current density of all beam components q/h4 passing through a divergence aperture (see Fig. 1) as a function of the extraction gap
01.09.94 t36Xe-, 02., H2 mitiure Extraction: s W/t rn~ i-l power: 92 w
0 excitation of the analyzing magnet
Fig. 2. Typical ion beam composition. indicated
Xe charge
states
are
lens axis [mm]
213
Fig. 3. The four electrode lens: deceleration of a beam with an entrance emittance ellipse of r0 = 2.5 mm and (Y, = 64 mrad to 0, / G0 = 0.005.
5. PRODUCTION/METHODS/APPLICATIONS
530
D. Henke et al. /Nucl.
Instr. and Meth. in Phys. Res. B 98 (1995) 528-531
Table 1 Data of the deceleration lens: c, / 11, - image object energy ratio, U?-4 - electrode voltages, U, - acceleration voltage, M magnification, C - spherical aberration, B, - linear axial chromatic acceleration, B, - linear radial chromatic aberration, ri beam radius at the target, E, - field strength on the target surface for U,=lOkV ui/L.o
1
u2/u,
0.858 0.858 0 1.2 1.526 0.722 0.370 2.5 9.5
u3/uA
Q/f-!, -M C [ml - B, [ml - B, [ml ri [mm1 Ei [V/cm]
0.1
0.01
-0.3 -1.4 -0.1 1.0 2.257 0.479 0.104 2.4 -36
0.005
-1.3 -0.25 -0.15 1.4 9.915 7.082 0.709 3.3 -34
- 0.45 - 0.09 - 0.08 1.9 5.054 5.315 0.451 3.7.. (4.8) -18
J 15
30
45
60
extraction voltage FVl Fig. 5. Density of total Ar ion current and Ar ion beam components versus acceleration voltage. The ll+ output is figured tenfold magnified.
7
22
20
18
16
14
12
10
8
6
I
voltage. Fig. 5 shows that the Arq+ current density components and their sum I,,, increase with the extraction voltage. In our case of a constant plasma density the change of the ion current densities beyond the mentioned aperture is essentially a result of the change of the beam divergence. The beam divergence for our low current densities depends mainly on the plasma curvature, on the focussing strength of the extraction gap and on the magnetic fringing field of the ion source. The observed deviations in the voltage dependence of the beam components Ar@ possibly indicate a charge state dependent focusing effect of the extended magnetic fringing field of the ion source. Although we do not exclude the influence of charge state dependent emittances [3].
2
charge state q Fig. 4. Charge state distributions obtained for different ion source regimes. Charge state with maximized output is indicated.
Table 2 Maximized Ar and Xe output I and corresponding operational parameters excitation. Gas flow in rel. units. D,...- total oressure in 10m6 mbar
for several charge states 4 +
. p - UHF power, I,,, - mirror field
4”,4&+
1
2
3
4
6
7
8
9
11
12
P WI 1, [Al Ar 0 Pt0t I [PAI
2 587 60 off 10 57
64 610 40 off 1.3 20.5
92 756 36 off 0.8 18.5
65 795 36
65 820 32 20 0.5 13
65 820 32 20 0.7 11.5
59 867 30 26 0.9 15.5
75 865 28 32 0.7 8
85 909 26 32 0.8 1.5
85 909 26 32 0.8 0.35
r36Xeq+ P [WI I,,, [Al Xe 0 Pt0t I [PAI
3 3 890 28 off 1.4 7
5 78 720 22 Off
1.1 6.2
Off
0.7 15
7
10
12
14
16
18
20
21
35 860 18 14 0.85 6.8
40 910 16 15 0.94 6.5
46 920 15 15 1.0 4.5
75 956 12.5 16.5 1.2 2.8
70 941 10.5 19.5 1.0 2.1
98 940 10 21 1.0 1.5
98 940 10 21 1.0 0.75
98 940 10 21 1.0 0.5
22 98 940 10 21 1.0 0.12
D. Henke et al. / Nucl. Instr. and Meth. in Phys. Res. B 98 (1995) 528-531
4. Comparison Our nitrogen output of 5 p,A for Nh+ is comparable to the value of Antaya’s single stage 6.4 GHz source [4]. However, in comparison to his double stage 6.4 GHz source [5] our Ar output is 5 times lower and the Xe output by one order of magnitude. That means the floating disk operation cannot fully replace a high performance two-stage operation.
5. Plasma diagnostics Measurements of characteristic X-rays emitted by ions of this ECR source have been performed. This experiment established an obvious difference between the charge state distributions of the plasma and the ion beam. Furthermore, the energy distribution of the hot ECR plasma electrons was determined using the bremsstrahlung spectrum emitted along the magnetic mirror field of the source [6].
6. Conclusion Shortly the deceleration lens will be mounted and tested. In a first application we intend to investigate the influence of our ion beam on the epitaxial growth and the morphology of layers on semiconductor surfaces. Furthermore, to improve the energy resolution of the X-ray spectra a crystal spectrometer will be connected to the ion source. This could help to obtain more information on the
531
ECR discharge mechanism. More time should be spent to explain the complicated nature of beam extraction from an ECR ion source.
Acknowledgements Support by Bundesminister ftir Forschung und Technologie, S?ichsisches Staatsministerium ftir Wissenschaft und Kunst and Volkswagen Stiftung is greatly appreciated. We thank G. Franz, P. Hartmann and F. Niitzold for technical assistance.
References [l] L. Friedrich, E. Huttel, R. Hentschel and H. Tyrroff, Proc. 11th Int. Workshop on ECR Ion Sources, May 1993, Groningen (1993) p. 19. [2] .I. Teichert, D. Janssen and M.A. Tiunov, Report ZFK-667 (1989) p. 75. [3] D.J. Clark, Proc. Int. Conf. on ECR Ion Sources, East Lansing (1987) p. 433. [4] D.K. Bose and T.A. Antaya, Proc. Int. Conf. on ECR Ion Sources, Dec. 1987, East Lansing, (1987) p. 371. [5] T.A. Antaya, S. Alfredson, D. Cole, K. Harrison and P. Osborne, Proc. 11th Int. Workshop on ECR Ion Sources, Groningen (1993) p. 37. [6] R. Friedlein, S. Herpich, U. Lehnert, H. Tyrroff, C. Zippe and G. Zschornack, these Proceedings (7th Int. Conf. on the Physics of Highly Charged Ions, Vienna, Austria, 1994) Nucl. Instr. and Meth. B 98 (1995) 585.
5. PRODUCTION/METHODS/APPLICATIONS