- Email: [email protected]
Development of a high mass-resolution TOF-ERDA system for a wide mass range
__ __ EB
m
Nuclear Instruments and Methods in Physics Research B 124 (I 997) 95-99
NNMIB
Beam Interactions with Materials 8 Atoms
ELSEVIER
Development of a high mass-resolution TOF-ERDA system for a wide mass range W. Hong a7bv*,S. Hayakawa a, K. Maeda b, S. Fukuda b, M. Yanokura b, M. Aratani ‘, K. Kimura b, Y. Gohshi a, I. Tanihata b a.Univerdy of b The Institute
of’ Physical
’ Instituteftir
Tokyo,
3-1.
Hongo
7 chome, Bunkyo-Ku,
and Chemical Reseurch (RIKEN). Environmentd
Received 24 September
Tokyo, Jupun
Wake, Saitamu, 351-01, Jopun
Sciences. Rokkashomura.
Aomori.
19%; revised form received 9 December
Jupan
1996
Abstract A high mass-resolution TOF-ERDA system installed at RILAC of RIKEN is presented. The target chamber was designed for TOF-ERDA. A charged-particle detector and a time detector for TOF measurements were installed in the chamber. The data-acquisition system, comprising CAMAC modules, is controlled by a personal computer. The analysis of the spectra is performed off-line. The mass resolution of the system for masses of around A = 28 was evaluated to be 1.9. H, C, N and 0 in a PIQ film could be measured simultaneously, and the peaks of C, N and 0 were separated. Keywords:
TOF-ERDA;
Light element analysis
1. Introduction Elastic Recoil Detection Analysis (ERDA) is one of the most promising methods for the determination of light elements in solid-state samples, especially when information along with depth is required. In particular, the technique can be applied to study of the materials with light and medium elements in a heavy matrix, such as trace-hydrogen analysis in solid samples [l-4]. To optimize the sensitivity for light elements in a heavy matrix, conventional ERDA employing a stopper foil in front of the detector has been performed at RIKEN [4,5]. The stopper foil prevents any incident particles scattered by matrix from reaching the detector, while the recoiled matrix elements are being counted. Using this method, not only hydrogen, but also multielement analyses have been carried out for several solid samples. However, since the energy spread of recoiled particles is broadened while passing through the stopper foil, the mass resolution was not sufficient to distinguish neighboring elements in the energy spectrum. A number of methods have been reported for extracting the extra recoil atomic number and mass information,
* Corresponding
author. Fax:
+ 81
48
4624689;
including A E-E recoil telescopes [6-81 and time-of-flight (TOF)-E recoil telescopes [3,9- 1 I]. Although A E-E recoil telescopes give recoil atomic number and energy information, they have limited applications due to the energy dispersion at the window of the AE gas detector. TOF-E recoil telescopes, which provide recoil mass-energy information, on the other hand, have good energy resolution. To realize both a higher mass resolution and a lower detection limit, a new TOF-E recoil telescope system using high-energy heavy ions has been set up. Although low-energy ion beams are used in several TOF-E recoil telescopes [ 1,2,1 I], better energy resolution can be obtained with heavier ions accelerated to high energy. This system also has a large advantage in the case that the beam time is limited, because of the time sharing between many users of an accelerator facility such as that in RIKEN, since reconstruction of the spectrum is available off-line.
2. Experimental 2.1.
email:
[email protected] 0168-583X/97/$17.00 0 PIf SO1 68-583X(97)0008
1997 Elsevier Science B.V. All rights reserved l-5
Measurement
system
The experimental arrangement is shown in Fig. 1. Ions accelerated by the RIKEN heavy-ion linear accelerator (RILAC) [ 12,131 are used as a probe. The incident energy can be selected up to 2.4 MeV/amu. The range of avail-
96
W. Hong rt d./Nucl.
Instr. und Meth. in Phys. Res. B I24 (1997) 95-99
Fig. 1. Setup of the TOF-E recoil spectrometer.
able ions is from hydrogen to xenon. In this experiment, “OAr ions accelerated to 41.5 MeV were employed. The target chamber is built of stainless steel and the inner diameter is I m. It contains a turntable with two arms for mounting detectors and a collimator. These can be rotated independently around a sample holder located at the center of the chamber. The sample holder, itself, can also be rotated. The incident angle of the beam can be changed from 0” to 90”, and was chosen to be 55” in this work. Ten samples can be mounted at the same time and moved along the vertical direction by remote control. A tantalum collimator having a knife edge is placed in front of the sample holder. The hole diameter can be changed from I mm 0 to 5 mm 0, and was selected to be 2 mm 0 in this work. The distance from the collimator to target is 30 cm. The collimator can be removed on line.
A silicon surface barrier (SSB) detector is installed in the chamber. To realize TOF measurements a time detector comprising two MCPs and a 10 kg/cm* carbon foil were produced [ 141 and installed between the sample and the SSB detector. The SSB detector, time detector, sample and incident beam are in the same plane. The detector angle can be changed from 0” to 180” with respect to the incident beam direction, and was set at 55” in this work. The length of the flight path between the carbon foil and the SSB detector is variable, and was chosen to be 314 mm. The SSB detector, the time detector and the flight path are covered with an aluminum case in order to protect them from scattered ions. Secondary electrons from the carbon foil are accelerated to the MCPs by a bias voltage between the carbon foil and the MCPs, and provides a fast chargepulse. To minimize the width of the timing pulse, the flight distance between the carbon foil and the MCPs was minimized at 8 mm. The time resolution of the detector was under 100 ps. Two turbo-molecular pumps are used for evacuation; the typical pressure during measurements was 3 X IO-’ Torr. 2.2. Data-acquisition
system
An overview of the counting system is shown in Fig. 2. The signal of the SSB detector is used not only for energy measurements but also to provide a timing signal for the TOF spectrometer. To collect the signals efficiently, the timing output of the SSB detector is used as a start input, and the output of the time detector is delayed and used as a stop input for the time-to-digital converter (TX). A CA-
Exper.
Am I
Room
CFD
1
Control Room
CAMAC
Fig. 2. Electronic setup for the TOF-E recoil specrometer.
W. Hong er al./Nucl.
Instr. und Meth. in Phys. Res. B 124 (1997) 95-99
97
1000,
MAC system is used to control the counting system using the KODAQ code [ 151 on a personal computer. The KODAQ code is set so as to use the PC screen as a multiparameter MCA. The monitor display can be redesigned very easily by changing the source code of KODAQ depending on the purpose of the experiment. The PC screen is divided into three windows; also, a time spectrum, an energy spectrum and a two-dimensional (time-energy) scatter plot are shown simultaneously in real time. Data are saved event by event. After a measurement, the spectra are reconstructed by the KODAQ code with the region of interest determined by an input gate defined anew on the scatter plot. The gate can be defined by two gate lines, an upper line and a bottom line; the region which is enclosed by the gate lines and the lines connecting the end points of the gate lines is selected. The gate established on a scatter plot affects the other windows so that only data corresponding to the selected data of the scatter plot appear in the energy and time spectra. Using this method, a peak can be separated from the other peaks and from the background in the energy and time spectra. A Si wafer and a PIQ (polyimide quinone) film on a Si wafer were used as samples. The size of the samples was all IO X I9 mm. The beam current was such that the counting time was approximately 20 min for the inorganic sample. For the organic PIQ sample, the beam current was reduced in order to make the beam damage small. The counting time was approximately two hours.
E
c
600
..i._-,.
s
600 _..
E
400
.;:..:
Zoc~\ 0 ;f-_
de.
:
‘1 _;,,
0
(4 ‘-si
,I
,:-
?I:
I
0
100
200
300
400
500
energy (ch)
00
100
200
300
energy
(ch)
400
500
600 1 E ;
(c)
400
200
0 l-/IL 0
200
400
600
800
1000
time (ch) Fig. 3. Raw spectra of a Si wafer. (a) TOF-E scatter plot, (b) energy spectrum, (c) time spectrum. Counting time: 20 min.
3. Results and discussion To evaluate the performance of the TOF-ERDA system, a thin polyimide quinone (PIQ) film deposited on a Si wafer was measured and the spectrum compared with that obtained with the Si wafer. Fig. 3a shows a TOF-E scatter plot of the Si wafer. The higher channels of the vertical axis represent short times, because the start and stop signals are reversed to each other. Fig. 3b and c shows the energy spectrum and the time spectrum, respectively. A line of the scatter plot represents particles having the same mass. The two short lines at the top of the scatter plot are signals from carbon and oxygen. The oxygen is from the natural oxide layer at the Si wafer surface. Carbon appears because of the low vacuum (3 X 1O-7 Torr) of the target chamber. In the time spectrum, the peak at channel 822 is from carbon and the peak at channel 804 is from oxygen. However the oxygen peak is hard to be recognized in the energy spectrum because of its low peak height and short distance to the carbon peak. In TOF-E spectrometry the mass is represented by
where M and E are the mass and energy of the observed particle, respectively; t is the time of flight, and I is the length of the flight path. Therefore, the mass resolution of the measurement system was calculated according to (;)z=(;)2+(~)2+(~)2.
(2)
The last term of Eq. (2) is negligible because the path difference between the particles is very small. The energy resolution can be calculated from the shoulder of the Si peak in the energy spectrum of Fig. 3. It was 530 keV at 13.2 MeV. The time resolution can be calculated from the shoulder of the Si peak in the time spectrum of Fig. 3, and was 0.9 ns at 33.0 ns. The time resolution of the SSB detector accounted for a large portion of the total time resolution, since the time resolution of the time detector was below 100 ps [ 141. The mass resolution was found to be about 1.9 at a mass number 28. The energy resolution of a SSB detector is related to the energy of a detected particle, i.e. SE&Z,
5) = A + 5E”3.
(3)
The constants A and 5 depend on the atomic number of the detected particle, The values of A and 5 for several
W. Hong et ul./ Nucl. Insn. and Metb. in Phys. Res. B 124 (1997)
98
elements have been given by O’Connor et al. [ 161, but not for Si. A and B for Si were calculated by a least-square fitting of the values of O’Connor et al.; also, SE, calculated by Eq. (3) was determined to be 164.8 keV for Si. This is smaller than the energy resolution found in this work. The energy and time resolution may be overestimated because of carbon and oxide layers on the surface. Therefore, the actual mass resolution may be better than the measured value. To separate the light-element peaks from those of the heavy matrix, an input gate was established on the scatter plot and the spectrum was reconstructed using the KODAQ code. Figs. 4 and 5 represent the spectra of a PIQ film on a Si wafer. The gated region of Fig. 4a is shown in Fig. 5a. The oxygen, nitrogen and carbon peaks in Fig. 5b and c were completely separated from the background peak from Si, which is the matrix element of the substrate. It is important for an improvement of the signal-to-noise ratio to separate any small peaks from the heavy matrix peak. Although the hydrogen peak at channel 31 of Fig. 4b is separated from the other light element peaks, the carbon (9.7 MeV, 239 ch), nitrogen (10.5 MeV, 253 ch) and
100
95-99
150
200
(ch)
energy
(ch)
E
a o
200
100
800 , (c)
600 E
g 0
400 200
L----L 0
200
400 time
600
s
600
E .=
4cm
350
300
300
0
z
250
energy
600
600
1000
400
500
(ch)
t.~.~k~.~J 0
100
200 energy
300 (ch)
Fig. 5. Gated spectra of a PIQ film. (a) TOF-E scatter plot, (b) energy spectrum, (c) time spectrum. (d) Separated peaks of each light element. Counting time: 2 hr.
100
600
E a 0
400 200 0
200
300
energy
(ch)
Irl
0
200
400
600
400
500
w
800
1000
time (ch)
Fig. 4. Raw spectra of a PIQ film. (a) TOF-E scatter plot. (b) energy spectrum, (c) time spectrum. Counting time: 20 min.
oxygen (I I. I MeV 267 ch) peaks are not separated from each other in the energy spectrum because of their small differences in energy. However, they are clearly separated in the scatter plot (Fig. 5a). This means that the mass resolution is better than 2 at a mass number of 16. The energy spectra of the different light elements are shown in Fig. 5d. Although these spectra are contained in one peak of the energy spectrum of Fig. 5b, they are now separated by the gate setting for each element in the scatter plot. The hydrogen peak is not presented in the figure, since it is sufficiently separated from the other light-element peaks.
W. Hong et ul./Nucl.
Instr. und Meth. in Phys. Rex B 124 (1997) 95-99
4. Summary By using the TOF system the mass resolution of ERDA is improved remarkably. The main results are: (1) hydrogen, carbon, nitrogen and oxygen were detected simultaneously; (2) the mass resolution of the system was 1.9 for masses of around 28; (3) carbon, nitrogen and oxygen peaks were separated in the TOF-E scatter plot, and the energy spectra for each element were obtained using the KODAQ code. The resulting low-background spectra are promising for detecting peaks due to trace elements which are difficult to detect with conventional ERDA. Determining the contents and depth profiles of light elements will be the main subject of the next step in this work.
References
111_I. Tirira, P. Trocellier, J.P. Frontier and P. Trouslard,
Nucl. Instr. and Meth. B 45 (1990) 203. Dl Y. Wang, B. Huang, D. Cao, J. Cao, D. Zhu and K. Shen, Nucl. Instr. and Meth. B 84 (1994) 111. 131 E. Arai, A. Zounek, M. Sekino, K. Takemoto and 0. Nittono, Nucl. Instr. and Meth. B 85 (1994) 226. [41 H. Nagai, S. Hayashi, M. Aratani, T. Nozaki, M. Yanokura,
99
1. Kohno, 0. Kuboi and Y. Yatsurugi, Nucl. Instr. and Meth. B 28 (1987) 59. [5] K. Kanazawa, M. Yanokura, M. Aratani and H. Akiyama, Vacuum 44 ( 1993) 7. [6] J.P. Stoquert, G. Guillaume, M. Hage-Ah, J.J. Grob. C. Gamer and P. Siffert, Nucl. Instr. and Meth. B 44 (1989) 184. [71 D.K. Avasthi, D. Kabiraj, A. Bhagwat. G.K. Mehta. V.D. Vankar and S.B. Ogale, Nucl. Instr. and Meth. B 93 (1994) 480. 181W. Assmann, Th. Reichelt, T. Eisenhammer, H. Huber, A. Mahr, H. Schellinger and R. Wohlgemuth, Nucl. Instr. and Meth. B 113 (1996) 303. [91 H.J. Whitlow, G. Possnert and C.S. Petersson, Nucl. Ins&. and Meth. B 27 (1987) 448. [lOI M.E. Bouanani, M. Hult et al., Nucl. Instr. and Meth. B 94 (1994) 530. [Ill J.H. Arps and R.A. Weller, Nucl. Instr. and Meth. B 100 (1995) 331. [121 M. Odera, RIKEN Accel. Prog. Rep. 15 (1981) 14. [131 T. Kubo, T. Kambara et al., RIKEN Accel. Prog. Rep. 15 (1981) 128. [141 K. Kimura and J. Wada, Phys. Rev. B 48 (1993) 15535. [151 K. Omata, Y. Fuji& N. Yoshikawa, M. Sekiguchi and Y. Yoshida, IEEE Trans. Nucl. 39 (1992) 143. D.J. O’Connor and T. Chunyu, Nucl. Instr. and Meth. B 36 [I61 (1989) 178.
m
Nuclear Instruments and Methods in Physics Research B 124 (I 997) 95-99
NNMIB
Beam Interactions with Materials 8 Atoms
ELSEVIER
Development of a high mass-resolution TOF-ERDA system for a wide mass range W. Hong a7bv*,S. Hayakawa a, K. Maeda b, S. Fukuda b, M. Yanokura b, M. Aratani ‘, K. Kimura b, Y. Gohshi a, I. Tanihata b a.Univerdy of b The Institute
of’ Physical
’ Instituteftir
Tokyo,
3-1.
Hongo
7 chome, Bunkyo-Ku,
and Chemical Reseurch (RIKEN). Environmentd
Received 24 September
Tokyo, Jupun
Wake, Saitamu, 351-01, Jopun
Sciences. Rokkashomura.
Aomori.
19%; revised form received 9 December
Jupan
1996
Abstract A high mass-resolution TOF-ERDA system installed at RILAC of RIKEN is presented. The target chamber was designed for TOF-ERDA. A charged-particle detector and a time detector for TOF measurements were installed in the chamber. The data-acquisition system, comprising CAMAC modules, is controlled by a personal computer. The analysis of the spectra is performed off-line. The mass resolution of the system for masses of around A = 28 was evaluated to be 1.9. H, C, N and 0 in a PIQ film could be measured simultaneously, and the peaks of C, N and 0 were separated. Keywords:
TOF-ERDA;
Light element analysis
1. Introduction Elastic Recoil Detection Analysis (ERDA) is one of the most promising methods for the determination of light elements in solid-state samples, especially when information along with depth is required. In particular, the technique can be applied to study of the materials with light and medium elements in a heavy matrix, such as trace-hydrogen analysis in solid samples [l-4]. To optimize the sensitivity for light elements in a heavy matrix, conventional ERDA employing a stopper foil in front of the detector has been performed at RIKEN [4,5]. The stopper foil prevents any incident particles scattered by matrix from reaching the detector, while the recoiled matrix elements are being counted. Using this method, not only hydrogen, but also multielement analyses have been carried out for several solid samples. However, since the energy spread of recoiled particles is broadened while passing through the stopper foil, the mass resolution was not sufficient to distinguish neighboring elements in the energy spectrum. A number of methods have been reported for extracting the extra recoil atomic number and mass information,
* Corresponding
author. Fax:
+ 81
48
4624689;
including A E-E recoil telescopes [6-81 and time-of-flight (TOF)-E recoil telescopes [3,9- 1 I]. Although A E-E recoil telescopes give recoil atomic number and energy information, they have limited applications due to the energy dispersion at the window of the AE gas detector. TOF-E recoil telescopes, which provide recoil mass-energy information, on the other hand, have good energy resolution. To realize both a higher mass resolution and a lower detection limit, a new TOF-E recoil telescope system using high-energy heavy ions has been set up. Although low-energy ion beams are used in several TOF-E recoil telescopes [ 1,2,1 I], better energy resolution can be obtained with heavier ions accelerated to high energy. This system also has a large advantage in the case that the beam time is limited, because of the time sharing between many users of an accelerator facility such as that in RIKEN, since reconstruction of the spectrum is available off-line.
2. Experimental 2.1.
email:
[email protected] 0168-583X/97/$17.00 0 PIf SO1 68-583X(97)0008
1997 Elsevier Science B.V. All rights reserved l-5
Measurement
system
The experimental arrangement is shown in Fig. 1. Ions accelerated by the RIKEN heavy-ion linear accelerator (RILAC) [ 12,131 are used as a probe. The incident energy can be selected up to 2.4 MeV/amu. The range of avail-
96
W. Hong rt d./Nucl.
Instr. und Meth. in Phys. Res. B I24 (1997) 95-99
Fig. 1. Setup of the TOF-E recoil spectrometer.
able ions is from hydrogen to xenon. In this experiment, “OAr ions accelerated to 41.5 MeV were employed. The target chamber is built of stainless steel and the inner diameter is I m. It contains a turntable with two arms for mounting detectors and a collimator. These can be rotated independently around a sample holder located at the center of the chamber. The sample holder, itself, can also be rotated. The incident angle of the beam can be changed from 0” to 90”, and was chosen to be 55” in this work. Ten samples can be mounted at the same time and moved along the vertical direction by remote control. A tantalum collimator having a knife edge is placed in front of the sample holder. The hole diameter can be changed from I mm 0 to 5 mm 0, and was selected to be 2 mm 0 in this work. The distance from the collimator to target is 30 cm. The collimator can be removed on line.
A silicon surface barrier (SSB) detector is installed in the chamber. To realize TOF measurements a time detector comprising two MCPs and a 10 kg/cm* carbon foil were produced [ 141 and installed between the sample and the SSB detector. The SSB detector, time detector, sample and incident beam are in the same plane. The detector angle can be changed from 0” to 180” with respect to the incident beam direction, and was set at 55” in this work. The length of the flight path between the carbon foil and the SSB detector is variable, and was chosen to be 314 mm. The SSB detector, the time detector and the flight path are covered with an aluminum case in order to protect them from scattered ions. Secondary electrons from the carbon foil are accelerated to the MCPs by a bias voltage between the carbon foil and the MCPs, and provides a fast chargepulse. To minimize the width of the timing pulse, the flight distance between the carbon foil and the MCPs was minimized at 8 mm. The time resolution of the detector was under 100 ps. Two turbo-molecular pumps are used for evacuation; the typical pressure during measurements was 3 X IO-’ Torr. 2.2. Data-acquisition
system
An overview of the counting system is shown in Fig. 2. The signal of the SSB detector is used not only for energy measurements but also to provide a timing signal for the TOF spectrometer. To collect the signals efficiently, the timing output of the SSB detector is used as a start input, and the output of the time detector is delayed and used as a stop input for the time-to-digital converter (TX). A CA-
Exper.
Am I
Room
CFD
1
Control Room
CAMAC
Fig. 2. Electronic setup for the TOF-E recoil specrometer.
W. Hong er al./Nucl.
Instr. und Meth. in Phys. Res. B 124 (1997) 95-99
97
1000,
MAC system is used to control the counting system using the KODAQ code [ 151 on a personal computer. The KODAQ code is set so as to use the PC screen as a multiparameter MCA. The monitor display can be redesigned very easily by changing the source code of KODAQ depending on the purpose of the experiment. The PC screen is divided into three windows; also, a time spectrum, an energy spectrum and a two-dimensional (time-energy) scatter plot are shown simultaneously in real time. Data are saved event by event. After a measurement, the spectra are reconstructed by the KODAQ code with the region of interest determined by an input gate defined anew on the scatter plot. The gate can be defined by two gate lines, an upper line and a bottom line; the region which is enclosed by the gate lines and the lines connecting the end points of the gate lines is selected. The gate established on a scatter plot affects the other windows so that only data corresponding to the selected data of the scatter plot appear in the energy and time spectra. Using this method, a peak can be separated from the other peaks and from the background in the energy and time spectra. A Si wafer and a PIQ (polyimide quinone) film on a Si wafer were used as samples. The size of the samples was all IO X I9 mm. The beam current was such that the counting time was approximately 20 min for the inorganic sample. For the organic PIQ sample, the beam current was reduced in order to make the beam damage small. The counting time was approximately two hours.
E
c
600
..i._-,.
s
600 _..
E
400
.;:..:
Zoc~\ 0 ;f-_
de.
:
‘1 _;,,
0
(4 ‘-si
,I
,:-
?I:
I
0
100
200
300
400
500
energy (ch)
00
100
200
300
energy
(ch)
400
500
600 1 E ;
(c)
400
200
0 l-/IL 0
200
400
600
800
1000
time (ch) Fig. 3. Raw spectra of a Si wafer. (a) TOF-E scatter plot, (b) energy spectrum, (c) time spectrum. Counting time: 20 min.
3. Results and discussion To evaluate the performance of the TOF-ERDA system, a thin polyimide quinone (PIQ) film deposited on a Si wafer was measured and the spectrum compared with that obtained with the Si wafer. Fig. 3a shows a TOF-E scatter plot of the Si wafer. The higher channels of the vertical axis represent short times, because the start and stop signals are reversed to each other. Fig. 3b and c shows the energy spectrum and the time spectrum, respectively. A line of the scatter plot represents particles having the same mass. The two short lines at the top of the scatter plot are signals from carbon and oxygen. The oxygen is from the natural oxide layer at the Si wafer surface. Carbon appears because of the low vacuum (3 X 1O-7 Torr) of the target chamber. In the time spectrum, the peak at channel 822 is from carbon and the peak at channel 804 is from oxygen. However the oxygen peak is hard to be recognized in the energy spectrum because of its low peak height and short distance to the carbon peak. In TOF-E spectrometry the mass is represented by
where M and E are the mass and energy of the observed particle, respectively; t is the time of flight, and I is the length of the flight path. Therefore, the mass resolution of the measurement system was calculated according to (;)z=(;)2+(~)2+(~)2.
(2)
The last term of Eq. (2) is negligible because the path difference between the particles is very small. The energy resolution can be calculated from the shoulder of the Si peak in the energy spectrum of Fig. 3. It was 530 keV at 13.2 MeV. The time resolution can be calculated from the shoulder of the Si peak in the time spectrum of Fig. 3, and was 0.9 ns at 33.0 ns. The time resolution of the SSB detector accounted for a large portion of the total time resolution, since the time resolution of the time detector was below 100 ps [ 141. The mass resolution was found to be about 1.9 at a mass number 28. The energy resolution of a SSB detector is related to the energy of a detected particle, i.e. SE&Z,
5) = A + 5E”3.
(3)
The constants A and 5 depend on the atomic number of the detected particle, The values of A and 5 for several
W. Hong et ul./ Nucl. Insn. and Metb. in Phys. Res. B 124 (1997)
98
elements have been given by O’Connor et al. [ 161, but not for Si. A and B for Si were calculated by a least-square fitting of the values of O’Connor et al.; also, SE, calculated by Eq. (3) was determined to be 164.8 keV for Si. This is smaller than the energy resolution found in this work. The energy and time resolution may be overestimated because of carbon and oxide layers on the surface. Therefore, the actual mass resolution may be better than the measured value. To separate the light-element peaks from those of the heavy matrix, an input gate was established on the scatter plot and the spectrum was reconstructed using the KODAQ code. Figs. 4 and 5 represent the spectra of a PIQ film on a Si wafer. The gated region of Fig. 4a is shown in Fig. 5a. The oxygen, nitrogen and carbon peaks in Fig. 5b and c were completely separated from the background peak from Si, which is the matrix element of the substrate. It is important for an improvement of the signal-to-noise ratio to separate any small peaks from the heavy matrix peak. Although the hydrogen peak at channel 31 of Fig. 4b is separated from the other light element peaks, the carbon (9.7 MeV, 239 ch), nitrogen (10.5 MeV, 253 ch) and
100
95-99
150
200
(ch)
energy
(ch)
E
a o
200
100
800 , (c)
600 E
g 0
400 200
L----L 0
200
400 time
600
s
600
E .=
4cm
350
300
300
0
z
250
energy
600
600
1000
400
500
(ch)
t.~.~k~.~J 0
100
200 energy
300 (ch)
Fig. 5. Gated spectra of a PIQ film. (a) TOF-E scatter plot, (b) energy spectrum, (c) time spectrum. (d) Separated peaks of each light element. Counting time: 2 hr.
100
600
E a 0
400 200 0
200
300
energy
(ch)
Irl
0
200
400
600
400
500
w
800
1000
time (ch)
Fig. 4. Raw spectra of a PIQ film. (a) TOF-E scatter plot. (b) energy spectrum, (c) time spectrum. Counting time: 20 min.
oxygen (I I. I MeV 267 ch) peaks are not separated from each other in the energy spectrum because of their small differences in energy. However, they are clearly separated in the scatter plot (Fig. 5a). This means that the mass resolution is better than 2 at a mass number of 16. The energy spectra of the different light elements are shown in Fig. 5d. Although these spectra are contained in one peak of the energy spectrum of Fig. 5b, they are now separated by the gate setting for each element in the scatter plot. The hydrogen peak is not presented in the figure, since it is sufficiently separated from the other light-element peaks.
W. Hong et ul./Nucl.
Instr. und Meth. in Phys. Rex B 124 (1997) 95-99
4. Summary By using the TOF system the mass resolution of ERDA is improved remarkably. The main results are: (1) hydrogen, carbon, nitrogen and oxygen were detected simultaneously; (2) the mass resolution of the system was 1.9 for masses of around 28; (3) carbon, nitrogen and oxygen peaks were separated in the TOF-E scatter plot, and the energy spectra for each element were obtained using the KODAQ code. The resulting low-background spectra are promising for detecting peaks due to trace elements which are difficult to detect with conventional ERDA. Determining the contents and depth profiles of light elements will be the main subject of the next step in this work.
References
111_I. Tirira, P. Trocellier, J.P. Frontier and P. Trouslard,
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