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SIMS system for the analysis of sputtered ions during ion implantation
Volume 2, number SB
MATERIALS LETTERS
SIMS SYSTEM FOR THE ANALYSIS D. HABERLAND,
August 1984
OF SPUTTERED IONS DURING ION IMPLANTATION
P. HARDE, H. NELKOWSKI and W. SCHLAAK *
Institut fti’r FestkGrperphysik,
Technische
Universittit Berlin, D-1 000 Berlin 12, West Germany
Received 15 June 1984
A system is described which allows secondary ion mass spectroscopy (SIMS) measurements during implantation at primary energies up to 170 keV. The secondary ions are produced by the implantation beam itself. The system has been assembled to study the stoichiometric disturbances near the surface of compound semiconductors caused by the bombardment with ions. Furthermore it is possible to examine the influence of sputter effects during implantation on the doping profile. The arrangement also provides means for standard SIMS and Auger electron spectroscopy (AES).
1. Introduction
Ion implantation is a promising method for doping compound semiconductors especially for III-V compounds [ 11. Compared to implantation in silicon one has to solve at least two additional problems: (1) preferential sputtering and recoil effects of substrate components; (2) changes of electric properties by knockon effects of atoms adsorbed at the surface. The first has been reviewed by several authors [2] and may cause significant disturbances of the stoichiometry [3]. The second can lead to a change in transport properties. For example, carbon can form electroactive centers in InP [4]. In the following a system will be described to measure sputtering effects during implantation in UHV and to analyze the depth profile of the components of the substrate and the dopant in the implanted layers.
2. General design
An overview of the ion implantation system with the analysis chamber is shown in fig. 1. The main parts are bought of Danfysik or built by our own. The hot cathode ion source (I) (temperature range up to * Present address: Heinrich-Hertz-Institut DlOOO Berlin 10, West Germany.
458
Berlin GmbH,
175O’C) allows the use of both gaseous and solid elements. After an acceleration to 20 keV the ions are separated by a 90” magnetic mass separator (M) with a dispersion of 500/Mmm (M:atomic mass of the singly charged ion or cluster). Then the beam is post accelerated to its final energy by a multi-gap linear accelerator (A). The ion beam can be focused on the target to ~1 mm in diameter. An electrostatic x-y scanning unit (X) enables homogeneous doping of the targets up to 20 X 20 mm2. At this point (X) ions are separated from neutrals by a bias voltage. In this part of the system (HV) there is a pressure of ~2 X 10d4 Pa. The ion implanter is controlled by a personal computer. The necessary data are transmitted by an optical tibre transmission system. The computer adjusts the ion source to the desired parameters, selects the ions to be implanted by regulating the current of the magnetic mass separator, checks all parameters, realizes the error detection of the implanter and gives the values of the controlled parameters on a display or a line printer. Fig. 2 shows a schematic drawing of the ultra-highvacuum chamber. The beam has to pass a differential pumping stack (4 X lop6 Pa) to obtain a pressure in the lop8 Pa range in the UHV chamber. Deflecting plates are placed here also in the beam line to suppress the neutrals and to direct the ions on the target. The sample manipulator is rotatable and linearly movable in three dimensions. It can be loaded with eight samples through a sample transfer system which consists
Volume
2, number
MATERIALS
SB
LETTERS
August
1984
Fig. 1. 170 kV ion implantation system combined with a chamber for surface analysis. Ion source (I), mass separator (M), acceleration tube (A), xPy scanner (X), HV target chamber (HV), differential pumping stack (D), chamber for surface analysis (UHV), sample transfer system (S).
of a separate vacuum system, a caroussel for eleven samples and a linear transfer system. So it is possible to obtain a pressure of 2 X 10V7 Pa within 30 min after reloading the sample holder. A secondary ion optic with a quadrupole mass filter [S] is mounted under 90” to the direction of the high-energy ion beam. This arrangement is used to analyze the sputtered secondary ions produced by the implantation beam during the implantation. For measuring depth profiles after implantation Pumping stack Man!pulotor
Pumpmg
Fig. 2. Schematic
stack
drawing
of the chamber
for surface
analysis.
another ion gun, part of a standard SIMS system (type telefocus, Atomica [6]), is mounted rectangular to the quadrupole mass filter tilted by 30” to the implantation beam. This ion gun is designed to operate with noble and with reactive gases. The ion beam is mass separated by a Wien filter and the selected ions are focused on the sample with an additional lens to 200 m beam diameter. This lens has a trap to suppress the neutrals and a built-in raster scanning unit with an electronic window. This is necessary to minimize crater edge effects for depth profiling, and to reach a high dynamic range of typical five orders of magnitude [7,8]. To analyze the samples with AES the manipulator has to be turned by 13.5”. SIMS as well as AES are controlled by another computer system. Especially for the SIMS measurements it is important to optimize the adjustment of the mass separator and the target potential [9]. For depth profiling the secondary ion intensities of eight different masses can be measured in sequence with separate adjustment of the quadrupole mass analyzer and of the target potential for each mass. Fig. 3 shows the block diagram of the electronic arrangement. During sputtering of high-resistance samples charge effects are observed. These effects are compensated automatically in our system. The target potential will be changed in steps controlled by the computer until the maxi459
August 1984
MATERIALS LETTERS
Volume 2, number SB
-
Counter Timer
Ion Source
Secondary
y
I----
Ion Optic
--1
I
-t
1 --&I!!I JI L-_--_-1
Fig. 3. Block diagram of the automatic control and data acquisition system for secondary ion mass spectroscopy.
In
kl
Zn
1 60
66
76
84
92
100
_I
106
MASS [ul Fig. 4. Typical mass spectrum of ZnSe produced by 120 keV lo7Ag+ beam.
460
mum of the secondary ion intensity is reached again. Fig. 4 shows a mass spectrum which was produced by the 170 keV implantation plant as an ion source and the quadrupole mass filter of the SIMS configuration as the analyzer. The sample - a ZnSe crystal was irradiated by lo7Ag+ ions with an energy of 120 keV. The spectrum shows the different isotopes of Zn and of Se but only the isotope loTAg. Because the ion beam is not scanned the intensities are not comparable with the ones in fig. 5. Fig. 5 shows the change of the secondary ion intensities of b4Zn’, 8oSe+ and lo7Ag+ during the implantation with lo7Ag+. Just as in fig. 4 the secondary ions were produced by the 120 keV Ag+ beam itself. The scanned area was ~50 mm2. The change of intensities of each isotope indicates a change in the composition of the sample. The rise of Zn+ signal and the simultaneous decrease of the Se+ signal suggest an enrichment of Zn at the surface because of the Ag implantation. AES measurements before and after implantation show the same characteristics [lo]. Without further investigations it cannot be distinguished between preferen-
MATERIALS LETTERS
Volume 2, number SB
August 1984
References
2 1E3-
s 8
2
*
*
*
* *
*
1E2 * 0
5
10
15
20
TIME OF IMPLANTATIONC min 1 Fig. 5. Change of the secondary ion intensity during implantation into ZnSe produced by a 120 keV 1°‘Ag+ beam.
tial sputtering or recoil implantation as the cause of the change in the composition of the surface.
3. Summary It has been shown that by combining an ion implanter with SIMS and AES the ions sputtered as a result of ion implantation can be measured. The change of composition at the surface of the compound semiconductor target is observed too and shows the theoretically predicted disturbances of stoichiometry [3]. The investigation of depth profiles in compound semiconductors with this equipment will be described elsewhere [11,12].
[l] KG. Stephens, Nut. Instr. Methods 209/210 (1983) 589. [2] H.H. Andersen, Appl. Phys. 18 (1979) 131; G. Betz and G.K. Wehner, in: Topics in applied physics, Vol. 52. Sputtering by particle bombardment II, ed. R. Behrisch (Springer, Berlin, 1983) pp. 11-90; H. Wiedersich, in: Surface modification and alloying by laser ion and electron beams, NATO Conference Series VI, Vol. 8, eds. J.M. Poate, G. Foti and D.C. Jacobson (Plenum Press, New York, 1983) p. 261. 131 L.A. Christel and J.F. Gibbons, J. Appl. Phys. 52 (1981) 5050. 141 E. KuphaI, J. Crystal Growth 54 (1981) 117. 151 K. Wittmaack, in: Proceedings of the 7th International Vacuum Congress and the 3rd International Conference on Solid Surfaces, eds. R. Dobrozemsky et al. (Vienna, 1977) p. 2573. 161 J.L. Maul, in: Chemical physics, Vol. 9. Secondary ion mass spectrometry, SIMS II, eds. A. Benninghoven, C.A. Evans Jr., R.A. Powell, R. Shimizu and H.A. Storms (Springer, Berlin, 1979) p. 206. [7] J.B. Clegg,in: Chemical physics, Vol. 19. Secondary ion mass spectrometry, SIMS III, eds. A. Benninghoven, .I. Giter, J. Lrizzlo, M. Riedel and H.W. Werner (Springer, Berlin, 1982) p. 308. [8] C.W. Maggee, R.E. Honig and C.A. Evans, in: Chemical physics, Vol. 19. Secondary ion mass spectrometry, SIMS III, eds. A. Benninghoven, J. Giter, J. La’zzlo, M. Riedel and H.W. Werner (Springer, Berlin, 1982) p. 172. [9] K. Wittmaack, Appl. Phys. Letters 29 (1976) 552. [lo] D. Haberland, H. Nelkowski and W. Schlaak, Verhandl. Deut. Physik. Ges. 18 (1983) 664. [ 111 D. Haberland, P. Harde, H. Nelkowski and W. Schlaak, in : Ion implantation and ion beam processing of materials, MRS Symposia Proceedings, Vol. 27, eds. B.K. Hubler, C.W. White, O.W. Holland and C.R. Clayton (Elsevier, New York, 1984), to be published. [ 121 D. Haberland, B. Krabel, H. Nelkowski and W. Schlaak, Surface Interface Anal. (1984), to be published.
461
MATERIALS LETTERS
SIMS SYSTEM FOR THE ANALYSIS D. HABERLAND,
August 1984
OF SPUTTERED IONS DURING ION IMPLANTATION
P. HARDE, H. NELKOWSKI and W. SCHLAAK *
Institut fti’r FestkGrperphysik,
Technische
Universittit Berlin, D-1 000 Berlin 12, West Germany
Received 15 June 1984
A system is described which allows secondary ion mass spectroscopy (SIMS) measurements during implantation at primary energies up to 170 keV. The secondary ions are produced by the implantation beam itself. The system has been assembled to study the stoichiometric disturbances near the surface of compound semiconductors caused by the bombardment with ions. Furthermore it is possible to examine the influence of sputter effects during implantation on the doping profile. The arrangement also provides means for standard SIMS and Auger electron spectroscopy (AES).
1. Introduction
Ion implantation is a promising method for doping compound semiconductors especially for III-V compounds [ 11. Compared to implantation in silicon one has to solve at least two additional problems: (1) preferential sputtering and recoil effects of substrate components; (2) changes of electric properties by knockon effects of atoms adsorbed at the surface. The first has been reviewed by several authors [2] and may cause significant disturbances of the stoichiometry [3]. The second can lead to a change in transport properties. For example, carbon can form electroactive centers in InP [4]. In the following a system will be described to measure sputtering effects during implantation in UHV and to analyze the depth profile of the components of the substrate and the dopant in the implanted layers.
2. General design
An overview of the ion implantation system with the analysis chamber is shown in fig. 1. The main parts are bought of Danfysik or built by our own. The hot cathode ion source (I) (temperature range up to * Present address: Heinrich-Hertz-Institut DlOOO Berlin 10, West Germany.
458
Berlin GmbH,
175O’C) allows the use of both gaseous and solid elements. After an acceleration to 20 keV the ions are separated by a 90” magnetic mass separator (M) with a dispersion of 500/Mmm (M:atomic mass of the singly charged ion or cluster). Then the beam is post accelerated to its final energy by a multi-gap linear accelerator (A). The ion beam can be focused on the target to ~1 mm in diameter. An electrostatic x-y scanning unit (X) enables homogeneous doping of the targets up to 20 X 20 mm2. At this point (X) ions are separated from neutrals by a bias voltage. In this part of the system (HV) there is a pressure of ~2 X 10d4 Pa. The ion implanter is controlled by a personal computer. The necessary data are transmitted by an optical tibre transmission system. The computer adjusts the ion source to the desired parameters, selects the ions to be implanted by regulating the current of the magnetic mass separator, checks all parameters, realizes the error detection of the implanter and gives the values of the controlled parameters on a display or a line printer. Fig. 2 shows a schematic drawing of the ultra-highvacuum chamber. The beam has to pass a differential pumping stack (4 X lop6 Pa) to obtain a pressure in the lop8 Pa range in the UHV chamber. Deflecting plates are placed here also in the beam line to suppress the neutrals and to direct the ions on the target. The sample manipulator is rotatable and linearly movable in three dimensions. It can be loaded with eight samples through a sample transfer system which consists
Volume
2, number
MATERIALS
SB
LETTERS
August
1984
Fig. 1. 170 kV ion implantation system combined with a chamber for surface analysis. Ion source (I), mass separator (M), acceleration tube (A), xPy scanner (X), HV target chamber (HV), differential pumping stack (D), chamber for surface analysis (UHV), sample transfer system (S).
of a separate vacuum system, a caroussel for eleven samples and a linear transfer system. So it is possible to obtain a pressure of 2 X 10V7 Pa within 30 min after reloading the sample holder. A secondary ion optic with a quadrupole mass filter [S] is mounted under 90” to the direction of the high-energy ion beam. This arrangement is used to analyze the sputtered secondary ions produced by the implantation beam during the implantation. For measuring depth profiles after implantation Pumping stack Man!pulotor
Pumpmg
Fig. 2. Schematic
stack
drawing
of the chamber
for surface
analysis.
another ion gun, part of a standard SIMS system (type telefocus, Atomica [6]), is mounted rectangular to the quadrupole mass filter tilted by 30” to the implantation beam. This ion gun is designed to operate with noble and with reactive gases. The ion beam is mass separated by a Wien filter and the selected ions are focused on the sample with an additional lens to 200 m beam diameter. This lens has a trap to suppress the neutrals and a built-in raster scanning unit with an electronic window. This is necessary to minimize crater edge effects for depth profiling, and to reach a high dynamic range of typical five orders of magnitude [7,8]. To analyze the samples with AES the manipulator has to be turned by 13.5”. SIMS as well as AES are controlled by another computer system. Especially for the SIMS measurements it is important to optimize the adjustment of the mass separator and the target potential [9]. For depth profiling the secondary ion intensities of eight different masses can be measured in sequence with separate adjustment of the quadrupole mass analyzer and of the target potential for each mass. Fig. 3 shows the block diagram of the electronic arrangement. During sputtering of high-resistance samples charge effects are observed. These effects are compensated automatically in our system. The target potential will be changed in steps controlled by the computer until the maxi459
August 1984
MATERIALS LETTERS
Volume 2, number SB
-
Counter Timer
Ion Source
Secondary
y
I----
Ion Optic
--1
I
-t
1 --&I!!I JI L-_--_-1
Fig. 3. Block diagram of the automatic control and data acquisition system for secondary ion mass spectroscopy.
In
kl
Zn
1 60
66
76
84
92
100
_I
106
MASS [ul Fig. 4. Typical mass spectrum of ZnSe produced by 120 keV lo7Ag+ beam.
460
mum of the secondary ion intensity is reached again. Fig. 4 shows a mass spectrum which was produced by the 170 keV implantation plant as an ion source and the quadrupole mass filter of the SIMS configuration as the analyzer. The sample - a ZnSe crystal was irradiated by lo7Ag+ ions with an energy of 120 keV. The spectrum shows the different isotopes of Zn and of Se but only the isotope loTAg. Because the ion beam is not scanned the intensities are not comparable with the ones in fig. 5. Fig. 5 shows the change of the secondary ion intensities of b4Zn’, 8oSe+ and lo7Ag+ during the implantation with lo7Ag+. Just as in fig. 4 the secondary ions were produced by the 120 keV Ag+ beam itself. The scanned area was ~50 mm2. The change of intensities of each isotope indicates a change in the composition of the sample. The rise of Zn+ signal and the simultaneous decrease of the Se+ signal suggest an enrichment of Zn at the surface because of the Ag implantation. AES measurements before and after implantation show the same characteristics [lo]. Without further investigations it cannot be distinguished between preferen-
MATERIALS LETTERS
Volume 2, number SB
August 1984
References
2 1E3-
s 8
2
*
*
*
* *
*
1E2 * 0
5
10
15
20
TIME OF IMPLANTATIONC min 1 Fig. 5. Change of the secondary ion intensity during implantation into ZnSe produced by a 120 keV 1°‘Ag+ beam.
tial sputtering or recoil implantation as the cause of the change in the composition of the surface.
3. Summary It has been shown that by combining an ion implanter with SIMS and AES the ions sputtered as a result of ion implantation can be measured. The change of composition at the surface of the compound semiconductor target is observed too and shows the theoretically predicted disturbances of stoichiometry [3]. The investigation of depth profiles in compound semiconductors with this equipment will be described elsewhere [11,12].
[l] KG. Stephens, Nut. Instr. Methods 209/210 (1983) 589. [2] H.H. Andersen, Appl. Phys. 18 (1979) 131; G. Betz and G.K. Wehner, in: Topics in applied physics, Vol. 52. Sputtering by particle bombardment II, ed. R. Behrisch (Springer, Berlin, 1983) pp. 11-90; H. Wiedersich, in: Surface modification and alloying by laser ion and electron beams, NATO Conference Series VI, Vol. 8, eds. J.M. Poate, G. Foti and D.C. Jacobson (Plenum Press, New York, 1983) p. 261. 131 L.A. Christel and J.F. Gibbons, J. Appl. Phys. 52 (1981) 5050. 141 E. KuphaI, J. Crystal Growth 54 (1981) 117. 151 K. Wittmaack, in: Proceedings of the 7th International Vacuum Congress and the 3rd International Conference on Solid Surfaces, eds. R. Dobrozemsky et al. (Vienna, 1977) p. 2573. 161 J.L. Maul, in: Chemical physics, Vol. 9. Secondary ion mass spectrometry, SIMS II, eds. A. Benninghoven, C.A. Evans Jr., R.A. Powell, R. Shimizu and H.A. Storms (Springer, Berlin, 1979) p. 206. [7] J.B. Clegg,in: Chemical physics, Vol. 19. Secondary ion mass spectrometry, SIMS III, eds. A. Benninghoven, .I. Giter, J. Lrizzlo, M. Riedel and H.W. Werner (Springer, Berlin, 1982) p. 308. [8] C.W. Maggee, R.E. Honig and C.A. Evans, in: Chemical physics, Vol. 19. Secondary ion mass spectrometry, SIMS III, eds. A. Benninghoven, J. Giter, J. La’zzlo, M. Riedel and H.W. Werner (Springer, Berlin, 1982) p. 172. [9] K. Wittmaack, Appl. Phys. Letters 29 (1976) 552. [lo] D. Haberland, H. Nelkowski and W. Schlaak, Verhandl. Deut. Physik. Ges. 18 (1983) 664. [ 111 D. Haberland, P. Harde, H. Nelkowski and W. Schlaak, in : Ion implantation and ion beam processing of materials, MRS Symposia Proceedings, Vol. 27, eds. B.K. Hubler, C.W. White, O.W. Holland and C.R. Clayton (Elsevier, New York, 1984), to be published. [ 121 D. Haberland, B. Krabel, H. Nelkowski and W. Schlaak, Surface Interface Anal. (1984), to be published.
461