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Determination of nitrogen using the 14N(α, p)17O nuclear reaction
Nuclear Instruments and Methods in Physics Research B79 (1993) 498-500 North-Holland
Determination
Nl0R4l B
Beam Interactions with Materials 8 Atoms
of nitrogen using the 14N(CX,p)170 nuclear reaction
Z. Lin, L.C. McIntyre, Jr., J.A. Leavitt, M.D. Ashbaugh and R.P. Cox Department of Physics, University of Arizona, Tucson, AZ 85721, USA
A technique for quantifying nitrogen in thin films using the 14N(~, p)170 reaction is reported. An incident alpha particle beam with energy near 3.9 MeV is used and emitted protons are detected at a lab angle of 135”. A 15 pm Kapton foil is placed over the detector to stop elastically scattered alpha particles. A thin film containing a known area1 density of nitrogen is used for calibration. We present relative yield data on protons from the 14N(a, P,,)~~O reaction at 135” lab angle from 3.4 MeV to 4.0 MeV as well as an example of the application of this technique in determining nitrogen content in thin films. Possible interfering reactions, particularly %i((u, pj31P, are also discussed.
1. Introduction
In many cases, nuclear reaction methods for determination of nitrogen in thin films are superior to other ion beam analysis techniques. Although Rutherford backscattering spectrometry (RBS) using 4He ions provides a simple and fast method of thin film analysis, the determination of light elements such as nitrogen is often difficult because of overlapping signals from heavier elements in the film or backing material. Another problem in the application of backscattering to light elements is the non-Rutherford nature of the scattering cross section, even at incident 4He energies near 2 MeV [l]. A recent review article on ion beam analysis of nitrogen contains references to several nuclear reactions used for this purpose, including 14N(d, o1)“C, 14N(d, p)15N, 14N(a, y)18F, as well as 14N(a, p)170 [2]. Another recent study investigated both alpha elastic scattering and the (01, p) reaction on nitrogen in the energy range 5-7.5 MeV [3]. In this article we describe a technique of determining the area1 density of nitrogen in thin films using the 14N(ol, p)170 nuclear reaction. A bombarding energy of 3880 keV is used to take advantage of the essentially constant cross section for this reaction for about 100 keV below this energy. An example using this technique is reported.
2. Experimental Beams of 4He+ ions were obtained from the University of Arizona 5.5 MV CN (High Voltage Engineering Corp.) Van de Graaff accelerator. A large area (450 mm*) surface barrier detector was added to a 0168-583X/93/$06.00
standard backscattering setup [4] to detect protons from the (IX, p) reaction. The detector was located 45 mm from the target at a scattering angle of 135”. Two layers of 7.5 p,rn Kapton foil were placed in front of the detector to stop elastically scattered alpha particles. In addition, a 2 mm wide curved slit was placed in front of the detector to limit the spread in acceptance angle. This was found necessary when analyzing samples containing silicon since protons from the ‘%(ol, P)~~P reaction could not be resolved from those produced from nitrogen unless the spread in acceptance angle was reduced. With this slit in place, the angular acceptance was 135 f 1.3” and the solid angle was approximately 24 msr. Except for this slit, the arrangement is the same as used in our previous work using the ((u, p) reaction to determine phosphorus [5], fluorine [6], and boron [7]. Since the Q value of the 14N(ol, p)170 reaction is - 1193 keV, the emerging protons have low energy and the foil covering the detector must be chosen carefully. At an incident alpha particle energy of 3880 keV, the emerging protons have about 1320 keV energy and lose about 500 keV in 15 km of Kapton. This thickness of Kapton will stop alpha particles up to 3200 keV, therefore alpha particles with incident energy of 3880 keV which are elastically scattered (at 135”) from elements heavier than about zinc could penetrate the cover. If samples with heavy backings are analyzed with this method, increased cover thickness may be necessary. Protons from the interfering ‘%(a, pj31P reaction, which has a Q-value of - 1916 keV, have energy of about 1220 keV (before reaching the cover) and appear just below the proton peak due to nitrogen. Fig. la shows a proton spectrum resulting from bombardment of a target consisting of a 35 nm layer of AlN on top of a 10 nm layer of Ti on a carbon backing.
0 1993 - Elsevier Science Publishers B.V. All rights reserved
Z. Lin et al. / N determination using ‘:N(a, p)170
499
B(Lab) = 135’ 250
Ea=3880keV Q=25& 14N(a,pf70
P
ml
2 0 .
150-
3;
J 28si(a,P)~‘P
J
.
5
0
1W
0
300
2a3
Channel Number
Channel Number
Fig. 1. (a) Energy spectrum of protons resulting from incident 3880 keV alpha particles on a target containing a 35 nm layer of AlN on a carbon backing. The detector was covered with 15 km of Kapton. Protons leaving I70 in the ground state and 3oSi in the ground and first three excited states are evident. (b) Same as (a) except the target contained a 75 nm layer of Si,N, on a carbon backing. Protons from the (cy, p) reaction on N and Si are evident.
The energy loss of the incident 3880 keV alpha beam in the AlN layer is about 10 keV. In addition to the large peak from the 14N(cy, p)170 reaction we see four peaks due to the 27Al(cz, p)30Si reaction. This reaction has a Q-value (for po> of +2372 keV and these high
energy protons do not interfere with those due to nitrogen. Fig. lb shows a proton spectrum from a target consisting of a 20 nm layer of Ta on top of a 75 nm layer of Si,N, on a carbon backing. The two proton peaks are due to nitrogen and silicon and illustrate the necessity of restricting the angular acceptance of the detector when silicon is present. Without the 2 mm wide slit, these peaks would be unresolved due to the large angular spread allowed by our unrestricted detector in the geometry described above. The large yield of protons from the “Si(a, pj31P reaction is due to a
18 7
“N(a, p)“O
16 7 $
.*
l.
14:
F 12 : 6 &
@(Lab)= 135’ . .
. 10. .
.
. f
a: 6:
.
.
’ .
. 4:
.
.
2:
. . . . .*** 0 ,,,,‘,,,,‘,,,,’ 34al
3500
l*........#‘*
. .
,,,‘, 36QO
3700
,,‘,,,.
3800
3900
Fig. 2. Relative yield of protons from the 14N(a, p)170 reaction at a lab angle of 135” as a function of incident alpha particle energy. The target was approximately 10 keV thick.
Cr,Fe,Ni,Ti
. J
Q=4opC
J
Q=4O@C
“N(a, p)170
Ea=3880keV
Eo=388OkeV i
4Mx1
Eo (kev)
B(Lab) = 135’ @(Lab)= 170’
.
;:
l
.. :i . !
8
.
0 0
200
400
Km
Channel Number
8w
loo0
0
50
100
150
m
Channel Number
Fig. 3. (a) Energy spectrum of backscattered alpha particles from a 100 nm TiN-INCONEL multilayer sample on a Si backing. (b) Energy spectrum of protons taken simultaneously with the alpha spectrum of (a). VII. PIXE, MICROPROBES,
...
500
Z. Lin et al. / N detetmination using 14N((u,p)170
resonance in this reaction at about 3870 keV. Unfortunately, this reaction is not suitable for quantifying the silicon content at this energy since the cross section is rapidly varying with energy due to this rather narrow (8.5 keV) resonance [8,9]. Fig. 2 shows the relative proton yield at 135” lab angle for the 14N(a, p0)170 reaction between 3.4 and 4.0 MeV incident energy. The AlN target described in connection with fig. la was used for these measurements. These results are consistent with previous measurements [lO,ll]. The broad resonance at about 3700 keV has been used to enhance the nitrogen detection sensitivity for this reaction [12]. A resonance at this energy is also observed in alpha elastic scattering from nitrogen [10,11,13].
a Si backing. The nitrogen signal is barely visible in the elastic scattering spectrum because of the Si backing, whereas the proton signal from nitrogen is completely resolved with low background. The two smaller proton peaks below the nitrogen peak in fig. 3b are due to two resonances (at 3870 and 3740 keV [8,9]) in the ?Si(a, P)~~P reaction taking place in the thick Si backing. The total nitrogen area1 density in this sample was determined to be 300 f 30 X 1015 atoms/cm* by using the equation given above. This analysis took about 7 minutes with a 90 nA incident alpha beam. In general, we estimate the uncertainty in results using this technique to be S-10%. In conclusion, we have presented a method for the determination of nitrogen area1 density in thin films which can be used in cases where RBS is ineffective.
3. Method and examples Acknowledgements
In this section we will describe the method of calibration and give an example of the use of the t4N(a p)170 reaction to quantify nitrogen area1 density in’s thin film. An incident energy of 3880 keV was chosen, which results in an approximately constant cross section for this reaction throughout thin films less than 100 keV thick. A calibration was done using samples with known nitrogen area1 density (Nt) measured by Rutherford backscattering. The samples used for the spectra shown in fig. 1 were used. The number of protons detected per p,C beam charge per atom/cm* of nitrogen was determined for our particular geometry. This determined the constant K in the equation Nt = K x (proton counts/&
beam charge)
used to calculate nitrogen area1 density (Nt) in unknown samples. For the geometry described above, K was determined to be 3.0 f 0.2 if the area1 density is given in units of 101’ atoms/cm*. The minimum detectable signal within a reasonable counting time corresponds to about 1 count per uC. This corresponds to 3 X 1015 atoms/cm*; however, in our case an increase in solid angle of an order of magnitude is possible if no silicon is present, since the narrow slit may be removed from the 450 mm* detector. An attractive feature of this method of nitrogen analysis is that it requires only a simple addition of a covered detector to a standard RBS experimental arrangement. Proton spectra can be taken simultaneously with alpha elastic scattering spectra at 3880 keV. Figs. 3a and 3b show an alpha elastic scattering spectrum (at 170.5” lab angle) and a proton spectrum (at 135” lab angle) taken simultaneously. The target was a TiN-INCONEL (0, Fe, Ni) multilayer about 100 nm thick on
The authors would like to thank Patrick Diehl of Arizona State University for providing the TiN-INCONEL multilayers. This work was partially supported by the Optical Data Storage Center which is administered by the Optical Sciences Center, University of Arizona.
References [l] J.A. Leavitt and L.C. McIntyre Jr., Nucl. Instr. and Meth. B56/57 (1991) 734. [2] K. Bethge, Nucl. Instr. and Meth. B66 (1992) 146. [3] H. Artigalas, A. Chevarier, N. Chevarier, M. El Bouanani, E. Gerlic, N. Moncoffre, B. Roux, M. Stem and J. Tousset, Nucl. Instr. and Meth. B66 (1992) 237. [4] J.A. Leavitt, Nucl. Instr. and Meth. B24/25 (1987) 717. [5] L.C. McIntyre Jr., J.A. Leavitt, B. Dezfouly-Arjomandy and J. Oder, Nucl. Instr. and Meth. B35 (1988) 446. [6] L.C. McIntyre Jr., J.A. Leavitt, M.D. Ashbaugh, B. Dezfouly-Arjomandy, Z. Lin, J. Oder, R.F.C. Farrow and S.S.P. Parkin, Nucl. Sci. Techn. (China) 1 (1990) 56. [7] L.C. McIntyre Jr., J.A. Leavitt, M.D. Ashbaugh, Z. Lin and J.O. Stoner Jr., Nucl. Instr. and Meth. B66 (1992) 221. [8] P.J. Riley, G.A. Lock, J.A. Rawlins and Y.M. Shin, Nucl. Phys. A96 (1967) 641. [9] Z. Lin, private communication. [lo] D.F. Herring, R. Chiba, B.R. Gaston and H.T. Richards, Phys. Rev. 112 (1958) 1210. [ll] E. Kashy, P.D. Miller and J.R. Risser, Phys. Rev. 112 (1958) 547. [12] B.L. Doyle, D.M. Follstaedt, S.T. Picraux, F.G. Yost, L.E. Pope and J.A. Knapp, Nucl. Instr. and Meth. B7/8 (1985) 166. [13] D.F. Herring, Phys. Rev. 112 (1958) 1217.
Determination
Nl0R4l B
Beam Interactions with Materials 8 Atoms
of nitrogen using the 14N(CX,p)170 nuclear reaction
Z. Lin, L.C. McIntyre, Jr., J.A. Leavitt, M.D. Ashbaugh and R.P. Cox Department of Physics, University of Arizona, Tucson, AZ 85721, USA
A technique for quantifying nitrogen in thin films using the 14N(~, p)170 reaction is reported. An incident alpha particle beam with energy near 3.9 MeV is used and emitted protons are detected at a lab angle of 135”. A 15 pm Kapton foil is placed over the detector to stop elastically scattered alpha particles. A thin film containing a known area1 density of nitrogen is used for calibration. We present relative yield data on protons from the 14N(a, P,,)~~O reaction at 135” lab angle from 3.4 MeV to 4.0 MeV as well as an example of the application of this technique in determining nitrogen content in thin films. Possible interfering reactions, particularly %i((u, pj31P, are also discussed.
1. Introduction
In many cases, nuclear reaction methods for determination of nitrogen in thin films are superior to other ion beam analysis techniques. Although Rutherford backscattering spectrometry (RBS) using 4He ions provides a simple and fast method of thin film analysis, the determination of light elements such as nitrogen is often difficult because of overlapping signals from heavier elements in the film or backing material. Another problem in the application of backscattering to light elements is the non-Rutherford nature of the scattering cross section, even at incident 4He energies near 2 MeV [l]. A recent review article on ion beam analysis of nitrogen contains references to several nuclear reactions used for this purpose, including 14N(d, o1)“C, 14N(d, p)15N, 14N(a, y)18F, as well as 14N(a, p)170 [2]. Another recent study investigated both alpha elastic scattering and the (01, p) reaction on nitrogen in the energy range 5-7.5 MeV [3]. In this article we describe a technique of determining the area1 density of nitrogen in thin films using the 14N(ol, p)170 nuclear reaction. A bombarding energy of 3880 keV is used to take advantage of the essentially constant cross section for this reaction for about 100 keV below this energy. An example using this technique is reported.
2. Experimental Beams of 4He+ ions were obtained from the University of Arizona 5.5 MV CN (High Voltage Engineering Corp.) Van de Graaff accelerator. A large area (450 mm*) surface barrier detector was added to a 0168-583X/93/$06.00
standard backscattering setup [4] to detect protons from the (IX, p) reaction. The detector was located 45 mm from the target at a scattering angle of 135”. Two layers of 7.5 p,rn Kapton foil were placed in front of the detector to stop elastically scattered alpha particles. In addition, a 2 mm wide curved slit was placed in front of the detector to limit the spread in acceptance angle. This was found necessary when analyzing samples containing silicon since protons from the ‘%(ol, P)~~P reaction could not be resolved from those produced from nitrogen unless the spread in acceptance angle was reduced. With this slit in place, the angular acceptance was 135 f 1.3” and the solid angle was approximately 24 msr. Except for this slit, the arrangement is the same as used in our previous work using the ((u, p) reaction to determine phosphorus [5], fluorine [6], and boron [7]. Since the Q value of the 14N(ol, p)170 reaction is - 1193 keV, the emerging protons have low energy and the foil covering the detector must be chosen carefully. At an incident alpha particle energy of 3880 keV, the emerging protons have about 1320 keV energy and lose about 500 keV in 15 km of Kapton. This thickness of Kapton will stop alpha particles up to 3200 keV, therefore alpha particles with incident energy of 3880 keV which are elastically scattered (at 135”) from elements heavier than about zinc could penetrate the cover. If samples with heavy backings are analyzed with this method, increased cover thickness may be necessary. Protons from the interfering ‘%(a, pj31P reaction, which has a Q-value of - 1916 keV, have energy of about 1220 keV (before reaching the cover) and appear just below the proton peak due to nitrogen. Fig. la shows a proton spectrum resulting from bombardment of a target consisting of a 35 nm layer of AlN on top of a 10 nm layer of Ti on a carbon backing.
0 1993 - Elsevier Science Publishers B.V. All rights reserved
Z. Lin et al. / N determination using ‘:N(a, p)170
499
B(Lab) = 135’ 250
Ea=3880keV Q=25& 14N(a,pf70
P
ml
2 0 .
150-
3;
J 28si(a,P)~‘P
J
.
5
0
1W
0
300
2a3
Channel Number
Channel Number
Fig. 1. (a) Energy spectrum of protons resulting from incident 3880 keV alpha particles on a target containing a 35 nm layer of AlN on a carbon backing. The detector was covered with 15 km of Kapton. Protons leaving I70 in the ground state and 3oSi in the ground and first three excited states are evident. (b) Same as (a) except the target contained a 75 nm layer of Si,N, on a carbon backing. Protons from the (cy, p) reaction on N and Si are evident.
The energy loss of the incident 3880 keV alpha beam in the AlN layer is about 10 keV. In addition to the large peak from the 14N(cy, p)170 reaction we see four peaks due to the 27Al(cz, p)30Si reaction. This reaction has a Q-value (for po> of +2372 keV and these high
energy protons do not interfere with those due to nitrogen. Fig. lb shows a proton spectrum from a target consisting of a 20 nm layer of Ta on top of a 75 nm layer of Si,N, on a carbon backing. The two proton peaks are due to nitrogen and silicon and illustrate the necessity of restricting the angular acceptance of the detector when silicon is present. Without the 2 mm wide slit, these peaks would be unresolved due to the large angular spread allowed by our unrestricted detector in the geometry described above. The large yield of protons from the “Si(a, pj31P reaction is due to a
18 7
“N(a, p)“O
16 7 $
.*
l.
14:
F 12 : 6 &
@(Lab)= 135’ . .
. 10. .
.
. f
a: 6:
.
.
’ .
. 4:
.
.
2:
. . . . .*** 0 ,,,,‘,,,,‘,,,,’ 34al
3500
l*........#‘*
. .
,,,‘, 36QO
3700
,,‘,,,.
3800
3900
Fig. 2. Relative yield of protons from the 14N(a, p)170 reaction at a lab angle of 135” as a function of incident alpha particle energy. The target was approximately 10 keV thick.
Cr,Fe,Ni,Ti
. J
Q=4opC
J
Q=4O@C
“N(a, p)170
Ea=3880keV
Eo=388OkeV i
4Mx1
Eo (kev)
B(Lab) = 135’ @(Lab)= 170’
.
;:
l
.. :i . !
8
.
0 0
200
400
Km
Channel Number
8w
loo0
0
50
100
150
m
Channel Number
Fig. 3. (a) Energy spectrum of backscattered alpha particles from a 100 nm TiN-INCONEL multilayer sample on a Si backing. (b) Energy spectrum of protons taken simultaneously with the alpha spectrum of (a). VII. PIXE, MICROPROBES,
...
500
Z. Lin et al. / N detetmination using 14N((u,p)170
resonance in this reaction at about 3870 keV. Unfortunately, this reaction is not suitable for quantifying the silicon content at this energy since the cross section is rapidly varying with energy due to this rather narrow (8.5 keV) resonance [8,9]. Fig. 2 shows the relative proton yield at 135” lab angle for the 14N(a, p0)170 reaction between 3.4 and 4.0 MeV incident energy. The AlN target described in connection with fig. la was used for these measurements. These results are consistent with previous measurements [lO,ll]. The broad resonance at about 3700 keV has been used to enhance the nitrogen detection sensitivity for this reaction [12]. A resonance at this energy is also observed in alpha elastic scattering from nitrogen [10,11,13].
a Si backing. The nitrogen signal is barely visible in the elastic scattering spectrum because of the Si backing, whereas the proton signal from nitrogen is completely resolved with low background. The two smaller proton peaks below the nitrogen peak in fig. 3b are due to two resonances (at 3870 and 3740 keV [8,9]) in the ?Si(a, P)~~P reaction taking place in the thick Si backing. The total nitrogen area1 density in this sample was determined to be 300 f 30 X 1015 atoms/cm* by using the equation given above. This analysis took about 7 minutes with a 90 nA incident alpha beam. In general, we estimate the uncertainty in results using this technique to be S-10%. In conclusion, we have presented a method for the determination of nitrogen area1 density in thin films which can be used in cases where RBS is ineffective.
3. Method and examples Acknowledgements
In this section we will describe the method of calibration and give an example of the use of the t4N(a p)170 reaction to quantify nitrogen area1 density in’s thin film. An incident energy of 3880 keV was chosen, which results in an approximately constant cross section for this reaction throughout thin films less than 100 keV thick. A calibration was done using samples with known nitrogen area1 density (Nt) measured by Rutherford backscattering. The samples used for the spectra shown in fig. 1 were used. The number of protons detected per p,C beam charge per atom/cm* of nitrogen was determined for our particular geometry. This determined the constant K in the equation Nt = K x (proton counts/&
beam charge)
used to calculate nitrogen area1 density (Nt) in unknown samples. For the geometry described above, K was determined to be 3.0 f 0.2 if the area1 density is given in units of 101’ atoms/cm*. The minimum detectable signal within a reasonable counting time corresponds to about 1 count per uC. This corresponds to 3 X 1015 atoms/cm*; however, in our case an increase in solid angle of an order of magnitude is possible if no silicon is present, since the narrow slit may be removed from the 450 mm* detector. An attractive feature of this method of nitrogen analysis is that it requires only a simple addition of a covered detector to a standard RBS experimental arrangement. Proton spectra can be taken simultaneously with alpha elastic scattering spectra at 3880 keV. Figs. 3a and 3b show an alpha elastic scattering spectrum (at 170.5” lab angle) and a proton spectrum (at 135” lab angle) taken simultaneously. The target was a TiN-INCONEL (0, Fe, Ni) multilayer about 100 nm thick on
The authors would like to thank Patrick Diehl of Arizona State University for providing the TiN-INCONEL multilayers. This work was partially supported by the Optical Data Storage Center which is administered by the Optical Sciences Center, University of Arizona.
References [l] J.A. Leavitt and L.C. McIntyre Jr., Nucl. Instr. and Meth. B56/57 (1991) 734. [2] K. Bethge, Nucl. Instr. and Meth. B66 (1992) 146. [3] H. Artigalas, A. Chevarier, N. Chevarier, M. El Bouanani, E. Gerlic, N. Moncoffre, B. Roux, M. Stem and J. Tousset, Nucl. Instr. and Meth. B66 (1992) 237. [4] J.A. Leavitt, Nucl. Instr. and Meth. B24/25 (1987) 717. [5] L.C. McIntyre Jr., J.A. Leavitt, B. Dezfouly-Arjomandy and J. Oder, Nucl. Instr. and Meth. B35 (1988) 446. [6] L.C. McIntyre Jr., J.A. Leavitt, M.D. Ashbaugh, B. Dezfouly-Arjomandy, Z. Lin, J. Oder, R.F.C. Farrow and S.S.P. Parkin, Nucl. Sci. Techn. (China) 1 (1990) 56. [7] L.C. McIntyre Jr., J.A. Leavitt, M.D. Ashbaugh, Z. Lin and J.O. Stoner Jr., Nucl. Instr. and Meth. B66 (1992) 221. [8] P.J. Riley, G.A. Lock, J.A. Rawlins and Y.M. Shin, Nucl. Phys. A96 (1967) 641. [9] Z. Lin, private communication. [lo] D.F. Herring, R. Chiba, B.R. Gaston and H.T. Richards, Phys. Rev. 112 (1958) 1210. [ll] E. Kashy, P.D. Miller and J.R. Risser, Phys. Rev. 112 (1958) 547. [12] B.L. Doyle, D.M. Follstaedt, S.T. Picraux, F.G. Yost, L.E. Pope and J.A. Knapp, Nucl. Instr. and Meth. B7/8 (1985) 166. [13] D.F. Herring, Phys. Rev. 112 (1958) 1217.