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Application of low energy charged particle beams for chlorine analysis in silicon samples
Nuclear
450
Instruments
and Methods
in Physics
Research
B22 (1987)
North-Holland,
APPLICATION ANALYSIS
F.M.
OF LOW
IN SILICON
EL-ASHRY*,
Institute
for
Nuclear
CHARGED
PARTICLE
BEAMS
FOR CHLORINE
SAMPLES
M. GOCLOWSKI,
Studies,
J. MARCZEWSKI
ENERGY
450-455
Amsterdam
L. GLOWACKA
and M. JASKhA
PL 00-681 Warsaw. Poland
and A. WOLKENBERG
Institute of Electron Technology CEMI. 02-668 Warsaw, Poland
Low-energy proton and helium beams have been used for chlorine determination in SiO, layers on (100) and (111) oriented 0 silicon wafers. The thickness of the SiOz layers was about 1200 A. The obtained results indicate that the chlorine content in near surface silicon samples is fairly proportional to the HCI content in a gaseous oxidant ambient.
1. Introduction For
impurity
analysis
in materials
research,
in gen-
energy of the analyzing incident particles should be chosen according to the combination of impurities and matrix elements in the sample. In particular for the determination of light elements (Z ~20) in solid state samples and the analysis of near-surface layers, proton energies well below 1 MeV could be comparable or superior to high energy proton (2-3 MeV) excitation methods [1,2]. In charged-particle excitation, the main background contribution in the X-ray spectra is the bremsstrahlung from the secondary electrons produced in the target by the impinging ions. This background radiation decreases rapidly when the energy becomes larger than the the maximum energy T, - 4mE,,/M, which represents energy that can be transferred from a projectile with mass M and energy E, to a free electron of mass m. The production of these electrons takes place through the same process which produces an inner shell vacancy leading to the emission of characteristic X-rays. In the case of low energy proton or helium beams the X-ray emission cross section is lower than at a few MeV; however bremsstrahlung in the X-ray energy range lower than about 5 keV from secondary electrons is much lower than that at a few MeV. The comparison between the sensitivity of trace element analysis of elements with Z < 18 in light matrices is higher for low energy protons than for few MeV protons [3,4]. In the literature there are now many applications and examples which indicate that a low energy PIXE method seems to be favourable for the eral,
the
* On leave Science,
optimum
of absence from Tanta University. Department of Physics, Tanta, Egypt.
Faculty
0168-583X/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
of
B.V.
detection of light impurities in solid state matrices [3-7). In this paper a low energy proton and ‘He+ ion beam method has been used to determine the chlorine content in the oxide layers built on (100) and (111) oriented silicon wafers obtained in a gaseous ambient which contains HCl and Oz.
2. Experiment
2.1.
Target preparation
The samples used for the experiment were n-type 3 0 cm (100) and ( 111) Si slices. They were oxidize at 1100°C in various mixtures of HCl and dry 0,. The flow rate of O2 was always maintained at 3000 ccimin. The thickness of the oxide was between 1100 and 1300 A, and was determined with ellipsometry. The electrical properties were checked on MOS structures with Al gates fabricated in the standard way. 2.2.
Irradiation
and measurement
The PIXE measurements were performed in an aluminium vacuum chamber which houses nine silicon samples mounted at 30” to the beam axis on a movable holder for sequential analysis. The chamber was insulated from the beam tube, so it behaved as a Faraday cup for thick targets. The details of the apparatus used have been published elsewhere [8,9]. The Si(Li) spectrometer, with an energy resolution of about 170 eV for the iron X-ray line, was positioned at 90” to the incident beam. The emitted X-rays emerged from the vacuum chamber through a 25 pm mylar window and traversed a 1 cm air gap and a 1.5 /*rn thick beryllium
F. M. El-Ashry
et al. I Cl analysis in silicon samples
detector window before detection. The intense silicon K X-ray lines were suppressed by an additional 1.4mgicm’ mylar foil absorber positioned in the air space in front of the detector window. To remove the charge build-up on the insulating silicon samples, a thin carbon foil was placed a few cm before the target [9]. The incident beam produces a number of 6electrons in the forward direction from this foil which were sufficient to neutralize the positive charge accumulating on the insulating thick targets. The diameter of the proton and helium beams was 2 mm and was defined by a graphite collimator system. Beam currents employed ranged from 20 to 70 nA, depending on the incident beam energy accelerated by a Van de Graaff accelerator. 2.3.
Calibration
of the PIXE
system
Chemically pure thin samples of known areal density ranging from about 5 pgicm” to about 40 pgicm” were made by vacuum evaporation of elements or compounds of the elements onto a thick ultrapure graphite backing and used as standards. These standards can be used over a long period of time. The standards used by us earlier, which were evaporated
r.9
9 U,lOC n-
E
2 2 5(
2(
10
E
2
l_
0 Fig.
I
2
L
6
8
10
!\
!c
l2 E,[keVl
1. Calibration curve of the X-ray yield as a function atomic number for a 0.5 MeV proton beam.
of
451
onto thin plastic materials, gave worse results. Such standard samples become fragile when irradiated for a period of time and the standards were then easily damaged during handling. Also, the quality of the sample surface flatness of thin standards is not so good as that of the standards evaporated onto a thick graphite backing. Ten suitable calibrators in the form of pure elements or compounds of elements (Al, Ti, Fe, Zn, Cu,S, BaCl, and KBr) were selected to cover the range of elements of interest, and the X-ray yield per PC was determined for each calibrator. The thicknesses of these calibrators were determined by separate backscattering measurements using a 2 MeV 4He’ beam. A calibration curve of the X-ray yield in photons PC’ pg-’ cm* as a function of atomic number obtained for a 0.5 MeV proton beam is presented in fig. 1.
3. Results Fig. 2 shows a comparison of X-ray spectra excited by 2.0, 1.5, 1.0 and 0.5 MeV proton beams for a silicon wafer with the symbol “4/O”. It is clear from the figure that spectra obtained by 0.5 MeV proton bombardment have superior sensitivities in analyzing the chlorine content in the studied sample (relative to 1.0, 1.5 and 2.0MeV). The Cl,, X-ray signal-to-background ratio (NJN,) for measurement of Cl in sample “410” is about 0.6 with 0.5 MeV, about 0.4 with 1 MeV, about 0.26 with 1.5 MeV and 0.12 with 2.0MeV protons. This indicates the advantage of the low energy proton beam. The measurements for other silicon samples were performed using the 0.5 MeV proton beam. Fig. 3 shows the X-ray spectra for some of the studied silicon targets. For comparison a 1 MeV helium beam was also applied for chlorine analysis in silicon samples. The essential advantage of using a low energy helium beam instead of 0.5 MeV protons is that the 1 MeV ‘He’ beam is believed to be much more sensitive, as a larger signal-to-noise ratio in detection of chlorine can be obtained. For 0.5 MeV protons the with 0.5 keV for 1 MeV ‘H’ Trill- 1 keV in comparison ions. The signal-to-noise ratio for 1 MeV 4He ’ ions for sample “410” is about 0.9. The obtained results for other silicon samples indicate that the 1 .O MeV 4He+ beam gives better precision than the 0.5 MeV proton beam. Examples of the X-ray spectra excited by the 1 MeV 4Het beam are shown in fig. 4. Tables 1 and 2 show the compilation of the chlorine content in the studied samples by means of low energy proton and helium beams. There is no significant discrepancy between proton and helium excitation besides the precision. For proton beams the mean accuracy of the chlorine determination is about 35%, and for “He beams about 24%. IV. GEOLOGICAL
AND
OTHER
APPLICATIONS
452
Fig. 2. Comparison
Table 1 Content of Cl in near-surface
of X-ray
spectra
SiO, layers
for silicon
built on (111) content
sample
and
“4/O”
(100)
oriented
j
Symbol of (100) sample
( pg/cm’)
Symbol of (111) sample
HCI content in oxidant ambient (%)
Chlorine
E, = 1 MeV
E, =O.SMeV
l/l 211 311 t/1 i/l )I1
0 3 5 7 8 10
0 0.17 0.26 0.40 0.37 0.48
0 0.10 0.22 0.25 0.30
excited
Ii0 210 3/o 410 510 610
by 2.0. 1.5, 1.0 and 0.5 MeV protons.
silicon
wafers
WC1 content in oxidant ambient (7~) 0 3 5 7 8 10
Chlorine
content{
pglcm’)
E, = 1 MeV
E, = 0.S MeV
0 0.23 _ 0.29 0.41
0 0.11 0.14 0.43 0.20 0.37
F.M.
El-Ashry
et al.
I Cl anaivsis
CHANNEL Fig. 3. X-ray
spectra
Table 2 As table
excited
by a 0.5 MeV proton
1, but for another
series
NUMBER beam
of silicon
in SiOz layers
“O”, “2”, “S’, “7”. lA, 2A, 18, 2B,
HCl content in oxidant ambient
(loo)
0
(100) (IOU) (111) (111) (111) (111) (111)
3 5 7 0 10 0 10
(%h)
on (111)
oriented
silicon
wafers
samples Chlorine
Symbol of silicon samole
453
in silicon samples
content
E_ = 1 MeV
(pgicm’) E_ = 0.5 MeV
0
0
0.05 0.18 0.22 0 0.29 0 0.28
0.08 0.20 0.23
IV. GEOLOGICAL
AND
OTHER
APPLICATIONS
F. M. El-Ashry et al. I Cl analysis in .silicon samples
454
CHANNEL Fig. 4. X-ray
spectra
excited
NUMBER
by 1 MeV ‘He ’ ions in SiO, layers
on (111)
oriented
silicon wafers
4. Conclusion
i TGl 2 ?2 .5_ .L_ .3 _
Oz.’ a/l
.2 _
‘,/O
.I 0
12
I 3‘
I
I 5
11 6
7
11 8
9
' IO
L96HCI1 Fig. 5. Chlorine content in SiOz layers versus the HCI (%) content in gaseous oxidant. 0 ~ Points for (111); 0 -points for (100).
The presented results show that in thin oxide layers obtained in a gaseous ambient which contains HCI and 0, many chlorine atoms are present. The chlorine content in SiO, layers on (100) and (111) oriented silicon wafers is fairly proportional to the HCI content in gaseous oxidant, which is shown in fig. 5. The chlorine content in investigated silicon slices can be approximated by the formulae: [Cls,02] z 0.04 X [HCI]; where [Cls,,,2] is in pg/cm’, and [HCl] in percentage content in gaseous ambient. The obtained results indicate that the low energy helium beam seems to be more favorable for the detection of
F.M.
chlorine in near proton beam.
surface
layers
than
El-Ashry
et al. I Cl analysis in silicon samples
the low energy [4]
References [I] F. Folkmann, G. Gaarde, T. Huus and K. Kemp, Nucl. Instr. and Meth. 116 (1974) 487. [2] F. Folkmann, J. Borggreen and A. Kjelogaard, Nucl. Instr. and Meth. 119 (1974) 117. [3] T. Shiokawa. T.C. Chu, V.R. Navarrete, H. Kaji, G.
[5] [6] [7] [8] [9]
455
Izawa, K. Ishii, S. Morita and H. Tawara, Nucl. Instr. and Meth. 142 (1977) 199. Y. Mariya, Y. Ato and S. Miyagawa, Nucl. Instr. and Meth. 150 (1978) 523. V. RGssiger, Nucl. Instr. and Meth. 196 (1982) 483. G. Lindner, Nucl. Instr. and Meth. B3 (1984) 130. G. Brunner and V. Rtissiger, Nucl. Instr. and Meth. B3 (1984) 275. B. Babinski, M. Jaskbia, M. Kucharski and L. Zemlo, Nukleonika 26 (1981) 1. M. Gociowski, M. Jask6Ia and L. Zemio, Nucl. Instr. and Meth. 204 (1983) 553.
IV. GEOLOGICAL
AND
OTHER
APPLICATIONS
450
Instruments
and Methods
in Physics
Research
B22 (1987)
North-Holland,
APPLICATION ANALYSIS
F.M.
OF LOW
IN SILICON
EL-ASHRY*,
Institute
for
Nuclear
CHARGED
PARTICLE
BEAMS
FOR CHLORINE
SAMPLES
M. GOCLOWSKI,
Studies,
J. MARCZEWSKI
ENERGY
450-455
Amsterdam
L. GLOWACKA
and M. JASKhA
PL 00-681 Warsaw. Poland
and A. WOLKENBERG
Institute of Electron Technology CEMI. 02-668 Warsaw, Poland
Low-energy proton and helium beams have been used for chlorine determination in SiO, layers on (100) and (111) oriented 0 silicon wafers. The thickness of the SiOz layers was about 1200 A. The obtained results indicate that the chlorine content in near surface silicon samples is fairly proportional to the HCI content in a gaseous oxidant ambient.
1. Introduction For
impurity
analysis
in materials
research,
in gen-
energy of the analyzing incident particles should be chosen according to the combination of impurities and matrix elements in the sample. In particular for the determination of light elements (Z ~20) in solid state samples and the analysis of near-surface layers, proton energies well below 1 MeV could be comparable or superior to high energy proton (2-3 MeV) excitation methods [1,2]. In charged-particle excitation, the main background contribution in the X-ray spectra is the bremsstrahlung from the secondary electrons produced in the target by the impinging ions. This background radiation decreases rapidly when the energy becomes larger than the the maximum energy T, - 4mE,,/M, which represents energy that can be transferred from a projectile with mass M and energy E, to a free electron of mass m. The production of these electrons takes place through the same process which produces an inner shell vacancy leading to the emission of characteristic X-rays. In the case of low energy proton or helium beams the X-ray emission cross section is lower than at a few MeV; however bremsstrahlung in the X-ray energy range lower than about 5 keV from secondary electrons is much lower than that at a few MeV. The comparison between the sensitivity of trace element analysis of elements with Z < 18 in light matrices is higher for low energy protons than for few MeV protons [3,4]. In the literature there are now many applications and examples which indicate that a low energy PIXE method seems to be favourable for the eral,
the
* On leave Science,
optimum
of absence from Tanta University. Department of Physics, Tanta, Egypt.
Faculty
0168-583X/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
of
B.V.
detection of light impurities in solid state matrices [3-7). In this paper a low energy proton and ‘He+ ion beam method has been used to determine the chlorine content in the oxide layers built on (100) and (111) oriented silicon wafers obtained in a gaseous ambient which contains HCl and Oz.
2. Experiment
2.1.
Target preparation
The samples used for the experiment were n-type 3 0 cm (100) and ( 111) Si slices. They were oxidize at 1100°C in various mixtures of HCl and dry 0,. The flow rate of O2 was always maintained at 3000 ccimin. The thickness of the oxide was between 1100 and 1300 A, and was determined with ellipsometry. The electrical properties were checked on MOS structures with Al gates fabricated in the standard way. 2.2.
Irradiation
and measurement
The PIXE measurements were performed in an aluminium vacuum chamber which houses nine silicon samples mounted at 30” to the beam axis on a movable holder for sequential analysis. The chamber was insulated from the beam tube, so it behaved as a Faraday cup for thick targets. The details of the apparatus used have been published elsewhere [8,9]. The Si(Li) spectrometer, with an energy resolution of about 170 eV for the iron X-ray line, was positioned at 90” to the incident beam. The emitted X-rays emerged from the vacuum chamber through a 25 pm mylar window and traversed a 1 cm air gap and a 1.5 /*rn thick beryllium
F. M. El-Ashry
et al. I Cl analysis in silicon samples
detector window before detection. The intense silicon K X-ray lines were suppressed by an additional 1.4mgicm’ mylar foil absorber positioned in the air space in front of the detector window. To remove the charge build-up on the insulating silicon samples, a thin carbon foil was placed a few cm before the target [9]. The incident beam produces a number of 6electrons in the forward direction from this foil which were sufficient to neutralize the positive charge accumulating on the insulating thick targets. The diameter of the proton and helium beams was 2 mm and was defined by a graphite collimator system. Beam currents employed ranged from 20 to 70 nA, depending on the incident beam energy accelerated by a Van de Graaff accelerator. 2.3.
Calibration
of the PIXE
system
Chemically pure thin samples of known areal density ranging from about 5 pgicm” to about 40 pgicm” were made by vacuum evaporation of elements or compounds of the elements onto a thick ultrapure graphite backing and used as standards. These standards can be used over a long period of time. The standards used by us earlier, which were evaporated
r.9
9 U,lOC n-
E
2 2 5(
2(
10
E
2
l_
0 Fig.
I
2
L
6
8
10
!\
!c
l2 E,[keVl
1. Calibration curve of the X-ray yield as a function atomic number for a 0.5 MeV proton beam.
of
451
onto thin plastic materials, gave worse results. Such standard samples become fragile when irradiated for a period of time and the standards were then easily damaged during handling. Also, the quality of the sample surface flatness of thin standards is not so good as that of the standards evaporated onto a thick graphite backing. Ten suitable calibrators in the form of pure elements or compounds of elements (Al, Ti, Fe, Zn, Cu,S, BaCl, and KBr) were selected to cover the range of elements of interest, and the X-ray yield per PC was determined for each calibrator. The thicknesses of these calibrators were determined by separate backscattering measurements using a 2 MeV 4He’ beam. A calibration curve of the X-ray yield in photons PC’ pg-’ cm* as a function of atomic number obtained for a 0.5 MeV proton beam is presented in fig. 1.
3. Results Fig. 2 shows a comparison of X-ray spectra excited by 2.0, 1.5, 1.0 and 0.5 MeV proton beams for a silicon wafer with the symbol “4/O”. It is clear from the figure that spectra obtained by 0.5 MeV proton bombardment have superior sensitivities in analyzing the chlorine content in the studied sample (relative to 1.0, 1.5 and 2.0MeV). The Cl,, X-ray signal-to-background ratio (NJN,) for measurement of Cl in sample “410” is about 0.6 with 0.5 MeV, about 0.4 with 1 MeV, about 0.26 with 1.5 MeV and 0.12 with 2.0MeV protons. This indicates the advantage of the low energy proton beam. The measurements for other silicon samples were performed using the 0.5 MeV proton beam. Fig. 3 shows the X-ray spectra for some of the studied silicon targets. For comparison a 1 MeV helium beam was also applied for chlorine analysis in silicon samples. The essential advantage of using a low energy helium beam instead of 0.5 MeV protons is that the 1 MeV ‘He’ beam is believed to be much more sensitive, as a larger signal-to-noise ratio in detection of chlorine can be obtained. For 0.5 MeV protons the with 0.5 keV for 1 MeV ‘H’ Trill- 1 keV in comparison ions. The signal-to-noise ratio for 1 MeV 4He ’ ions for sample “410” is about 0.9. The obtained results for other silicon samples indicate that the 1 .O MeV 4He+ beam gives better precision than the 0.5 MeV proton beam. Examples of the X-ray spectra excited by the 1 MeV 4Het beam are shown in fig. 4. Tables 1 and 2 show the compilation of the chlorine content in the studied samples by means of low energy proton and helium beams. There is no significant discrepancy between proton and helium excitation besides the precision. For proton beams the mean accuracy of the chlorine determination is about 35%, and for “He beams about 24%. IV. GEOLOGICAL
AND
OTHER
APPLICATIONS
452
Fig. 2. Comparison
Table 1 Content of Cl in near-surface
of X-ray
spectra
SiO, layers
for silicon
built on (111) content
sample
and
“4/O”
(100)
oriented
j
Symbol of (100) sample
( pg/cm’)
Symbol of (111) sample
HCI content in oxidant ambient (%)
Chlorine
E, = 1 MeV
E, =O.SMeV
l/l 211 311 t/1 i/l )I1
0 3 5 7 8 10
0 0.17 0.26 0.40 0.37 0.48
0 0.10 0.22 0.25 0.30
excited
Ii0 210 3/o 410 510 610
by 2.0. 1.5, 1.0 and 0.5 MeV protons.
silicon
wafers
WC1 content in oxidant ambient (7~) 0 3 5 7 8 10
Chlorine
content{
pglcm’)
E, = 1 MeV
E, = 0.S MeV
0 0.23 _ 0.29 0.41
0 0.11 0.14 0.43 0.20 0.37
F.M.
El-Ashry
et al.
I Cl anaivsis
CHANNEL Fig. 3. X-ray
spectra
Table 2 As table
excited
by a 0.5 MeV proton
1, but for another
series
NUMBER beam
of silicon
in SiOz layers
“O”, “2”, “S’, “7”. lA, 2A, 18, 2B,
HCl content in oxidant ambient
(loo)
0
(100) (IOU) (111) (111) (111) (111) (111)
3 5 7 0 10 0 10
(%h)
on (111)
oriented
silicon
wafers
samples Chlorine
Symbol of silicon samole
453
in silicon samples
content
E_ = 1 MeV
(pgicm’) E_ = 0.5 MeV
0
0
0.05 0.18 0.22 0 0.29 0 0.28
0.08 0.20 0.23
IV. GEOLOGICAL
AND
OTHER
APPLICATIONS
F. M. El-Ashry et al. I Cl analysis in .silicon samples
454
CHANNEL Fig. 4. X-ray
spectra
excited
NUMBER
by 1 MeV ‘He ’ ions in SiO, layers
on (111)
oriented
silicon wafers
4. Conclusion
i TGl 2 ?2 .5_ .L_ .3 _
Oz.’ a/l
.2 _
‘,/O
.I 0
12
I 3‘
I
I 5
11 6
7
11 8
9
' IO
L96HCI1 Fig. 5. Chlorine content in SiOz layers versus the HCI (%) content in gaseous oxidant. 0 ~ Points for (111); 0 -points for (100).
The presented results show that in thin oxide layers obtained in a gaseous ambient which contains HCI and 0, many chlorine atoms are present. The chlorine content in SiO, layers on (100) and (111) oriented silicon wafers is fairly proportional to the HCI content in gaseous oxidant, which is shown in fig. 5. The chlorine content in investigated silicon slices can be approximated by the formulae: [Cls,02] z 0.04 X [HCI]; where [Cls,,,2] is in pg/cm’, and [HCl] in percentage content in gaseous ambient. The obtained results indicate that the low energy helium beam seems to be more favorable for the detection of
F.M.
chlorine in near proton beam.
surface
layers
than
El-Ashry
et al. I Cl analysis in silicon samples
the low energy [4]
References [I] F. Folkmann, G. Gaarde, T. Huus and K. Kemp, Nucl. Instr. and Meth. 116 (1974) 487. [2] F. Folkmann, J. Borggreen and A. Kjelogaard, Nucl. Instr. and Meth. 119 (1974) 117. [3] T. Shiokawa. T.C. Chu, V.R. Navarrete, H. Kaji, G.
[5] [6] [7] [8] [9]
455
Izawa, K. Ishii, S. Morita and H. Tawara, Nucl. Instr. and Meth. 142 (1977) 199. Y. Mariya, Y. Ato and S. Miyagawa, Nucl. Instr. and Meth. 150 (1978) 523. V. RGssiger, Nucl. Instr. and Meth. 196 (1982) 483. G. Lindner, Nucl. Instr. and Meth. B3 (1984) 130. G. Brunner and V. Rtissiger, Nucl. Instr. and Meth. B3 (1984) 275. B. Babinski, M. Jaskbia, M. Kucharski and L. Zemlo, Nukleonika 26 (1981) 1. M. Gociowski, M. Jask6Ia and L. Zemio, Nucl. Instr. and Meth. 204 (1983) 553.
IV. GEOLOGICAL
AND
OTHER
APPLICATIONS