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Microbeam analysis of microelectronic circuits
Nuclear Instruments and Methods North-Holland, Amsterdam
MICROBEAM
in Physics
ANALYSIS
Research
661
B15 (1986) 661-663
OF MICROELECTRONIC
CIRCUITS
W.G. MORRIS Ckwral Electric Corporate Research and Deaelopment, Schenectady, NY 12J01, USA II. BAKHRU
and A.W. HABERL
J tote University of New York at Alhan): 1400 Wushingon Awnue, Albany, NY 12222, USA
Rutherford backscattering spectrometry (RBS) and particle-induced X-ray emission (PIXE) analysis have been performed on microelectronic circuits with a spatial resolution of approximately 2 pm. Differences in X-ray and RBS yields from various thick analysis are targetsfor 2 MeV He+ and 2 MeV Ht ion excitation have been studied. Examples of chemical and microstructural s70wn.
2. Experimental setup
1. Introduction While fG,cused
the first
use of ion-induced
or collimated
ion
beams
X-rays dates
from
with
highly
the
early
of particle induced X-ray emission (PIXE) [1,2], r.lpid growth in facilities combined with Rutherford backscattering spectroscopy (RBS) has occurred only nithin the past several years [3]. The well-known advantages of a nuclear microprobe with PIXE and RBS systems are: the sensitivity of heavy particle induced X-ray emission relative to electron induced emission; the depth information available from RBS; light isotope detection with nuclear reactions; and the relatively unambiguous interpretation of the data. Nuclear microprobes have been used in solid state physics and material science [4] to obtain chemical and microstructural information from the near surface region of small-scale device structures. There have been considerable improvements during the past several years to obtain a small (l-5 pm) size beam spot using magnetic lenses [5-91. Small beam diameters are particularly important in experiments where the sample is very small as in the case of microelectronic circuits. The possibility of extending PIXE and RBS analysis down to the micron level has led to a joint project between General Electric and the State University of New York at Albany. In the present work, we report on the RBS and PIXE analyses of microelectronic circuits using 2 MeV He+ and Ht ions with a spatial resolution of approximately 2 pm. It is seen that PIXE and RBS can be done with either He+ or Ht ions with some interesting differences. In order to understand these differences, X-ray and RBS yields from various targets have been studied. days
0168-583X/86/$03.50 0 Elsevier Science Publishers (Worth-Holland Physics Publishing Division)
B.V
The details of our experimental setup are given by Morris et al. [lO,ll]. Beams of 2 MeV He’ or H + are obtained using a 4 MeV dynamitron accelerator. A quadrupole doublet lens is the final element used to focus the beam on the sample. X-rays are analyzed with a 6 mm diam. X 3 mm cooled Si(Li) detector which is placed inside the target chamber facing 45” to the target plane. The X-ray detector has a resolution of 160 eV at 5.9 keV. For RBS the backscattered ions are detected and energy-analyzed by a surface barrier detector placed at 150” back angle with a solid angle of 160 msr. The RBS detector had a resolution of 18 keV for 6 MeV alpha particles. A unique feature of this microprobe is that the beam has an autoalign system which greatly simplifies setup and control of the microbeam line. An electronic scanning system is used to form secondary electron images of the surface in order to locate features of interest. Hence. chemical and microstructural information based on the spot, line or frame mode can be obtained. The beam size was checked using a 1000 mesh electroformed copper grid and found to be approximately 2 pm with a beam current of 150 pA. In the present setup. both RBS and PIXE information can be collected simultaneously. An Apple II microcomputer is used to control the multichannel analyzers and to store the RBS and PIXE data. The same computer can be used to store the digitized image and to control the x-y stage for the sample holder. Alignment procedures are simplified by using an autoalign system (ref. [lo]) which automatically corrects XIII. ION BEAM MICROANALYSIS
W. G. Morm
662
et al. /
Micrnheam
ana!vsiJ of microelectrot~ic
circuits
2 hiev 2 MeV
H +
150i
.. OA
L x-ray Ko X-ray
.A.
Fig. 1. K, X-ray and L X-ray yields for targets MeV He+ and H’ ion excitation.
Ti-Zn
for 2
for small amounts of beam drift and keeps the incident beam centered on the slits which define the object size. The smallest beam size at the sample has been obtained under conditions where the main yuadrupote lens (a) gives a spot no larger than 1 mm and (b) gives minimum sweep as the lens current is varied slightly. The probe forming quadrupole lens is also aligned to give minimum sweep (displacement of the scanned image) as its current is varied.
3. Results and discussion Various metals are used for the contacts and conductor runs in microelectronic circuits. It is necessary to
Fig. 2. Secondary
electron
image of a region of a microelectronic
0:
14
18
1.6 Energy
20
( MeV I
Fig. 3. RBS spectrum taken in a spot mode. conductor, (b) between the metal conductors.
(a) on a bright
understand the differences in X-ray and RBS spectra using He+ and ti’ ion excitation. We have undertaken a program to obtain the X-ray yields for K and L lines for various thick and thin targets. In this report the thick target yields are given for Z = 22 (Ti) to Z = 30 (Zn). It was observed that 2 MeV H+ ions excite the K lines and L lines of Cu with comparable strengths, and yield a spectrutn sitnilar to that obtained when doing microanalysis with a 30 keV electron beam. A 2 MeV
test circuit.
Frame
scan 5 s, 2 MeV He’.
0.1 nA
He+ beam, however, has a lower velocity and is significantly less effective in exciting the Cu K lines. The results are summarized in fig. 1 which shows the K, X-ray yield for elements between Ti and Zn for both Ile+ and H+ ions. The K, X-ray yield with He’ ions is much
smaller
than
the
K,
X-ray
yield
with
H’
ions.
the L X-ray yield for elements between Fe and Zn using both 2 MeV He+ and 2 MeV II+ excitation. The L yields are similar for Cu and Zn l?hen comparing H+ and He+ excitation, with He+ ljeing a slightly more efficient exciter for lighter elelnents (Fe. Z = 26). The secondary electron image of a portion of a lnicroelectronic circuit is shown in fig. 2. The metallized rectangles in the center to upper right region are about 1.0 to 50 pm. The conductors in the lower left region are 1, pm lines with 3 pm separations. Fig. 3 shows RBS spectra taken with H+ ions for a portion of this microcircuit. The upper curve of fig. 3(a) ij taken using 2 MeV H + ions in a spot mode on a bright conductor which is analyzed to be 1 pm Al on 120 nm of SiO, on a Si substrate. The lower curve of fig. 3(b) is taken under similar conditions with the beam positioned between the metal conductors, showing SiO, on Si. Spectra were also obtained from these areas using a 2 LleV He+ beam. Comparison with the spectra obtained lmder H+ impact shows that, at the same energy, H+ i:ms allow compositional analysis to much larger depths rhan He+ ions whereas the latter provide much better depth resolution than the former. I:ig.
1 also shows
4. Conclusions
spatial resolution of approximately 2 pm can provide complimentary chemical and microstructural information. In order to understand differences in the spectra, X-ray and RBS yields from various targets must be known. The advantage of using He+ ions for RBS analysis are better depth resolution, better mass resolution and a more quantitative analysis with simulations. The advantages of using H+ ions for RBS analysis are greater depth penetration. and better resolution of the oxide layers. PIXE analysis using 2 MeV H’ ion excitation is nearly 10-100 times more sensitive for K, X-rays and approximately equal for L lines as compared with that using 2 MeV He+ excitation for thick targets of atomic number Z between 25 and 30.
References PI J.A. Cookson and M. Poole, New Scientist 1 (1970) 404. PI T.B. Johanason, R. Akselsson and S.A.E. Johansson, Nucl. Instr. and Meth. 84 (1970) 141. [31 R.G. Musket, Nucl. Instr. and Meth. 218 (1983) 420. [41 B.L. Doyle and N.D. Wing, IEEE Trans. Nucl. Sci. NS-30 (1983) 1214. [51 B.L. Doyle. N.D. Wing and P.S. Peercy, Microbeam Analysis 1981, ed., R.H. Gleiss, (San Francisco Press. 1981). [61 F.W. Martin and R. Goloskie, Appl. Phys. Lett. 40 (1982) 191. 171 J.A. Cookson. Nucl. Instr. and Meth. 165 (1979) 477. PI G.J.F. Legge, Nucl. Instr. and Meth. 197 (1982) 243. 191 R. Nobiling. Nucl. Instr. and Meth. 218 (1983) 197. PO1 W.G. Morris, W. Katz, H. Bakhru and A.W. Haberl. J. Vat. Sci. Technol. B3 (1985) 391. 1111 W.G. Morris. H. Bakhru and A.W. Haberl. Microbeam Analysis 1984. eds., A.D. Romig Jr and J.I. Goldstein (San Francisco Press. 1984).
RBS and PIXE information on microelectronic circuits using 2 MeV He+ and 2 MeV H+ ions with a
XIII. ION BEAM MICROANALYSIS
MICROBEAM
in Physics
ANALYSIS
Research
661
B15 (1986) 661-663
OF MICROELECTRONIC
CIRCUITS
W.G. MORRIS Ckwral Electric Corporate Research and Deaelopment, Schenectady, NY 12J01, USA II. BAKHRU
and A.W. HABERL
J tote University of New York at Alhan): 1400 Wushingon Awnue, Albany, NY 12222, USA
Rutherford backscattering spectrometry (RBS) and particle-induced X-ray emission (PIXE) analysis have been performed on microelectronic circuits with a spatial resolution of approximately 2 pm. Differences in X-ray and RBS yields from various thick analysis are targetsfor 2 MeV He+ and 2 MeV Ht ion excitation have been studied. Examples of chemical and microstructural s70wn.
2. Experimental setup
1. Introduction While fG,cused
the first
use of ion-induced
or collimated
ion
beams
X-rays dates
from
with
highly
the
early
of particle induced X-ray emission (PIXE) [1,2], r.lpid growth in facilities combined with Rutherford backscattering spectroscopy (RBS) has occurred only nithin the past several years [3]. The well-known advantages of a nuclear microprobe with PIXE and RBS systems are: the sensitivity of heavy particle induced X-ray emission relative to electron induced emission; the depth information available from RBS; light isotope detection with nuclear reactions; and the relatively unambiguous interpretation of the data. Nuclear microprobes have been used in solid state physics and material science [4] to obtain chemical and microstructural information from the near surface region of small-scale device structures. There have been considerable improvements during the past several years to obtain a small (l-5 pm) size beam spot using magnetic lenses [5-91. Small beam diameters are particularly important in experiments where the sample is very small as in the case of microelectronic circuits. The possibility of extending PIXE and RBS analysis down to the micron level has led to a joint project between General Electric and the State University of New York at Albany. In the present work, we report on the RBS and PIXE analyses of microelectronic circuits using 2 MeV He+ and Ht ions with a spatial resolution of approximately 2 pm. It is seen that PIXE and RBS can be done with either He+ or Ht ions with some interesting differences. In order to understand these differences, X-ray and RBS yields from various targets have been studied. days
0168-583X/86/$03.50 0 Elsevier Science Publishers (Worth-Holland Physics Publishing Division)
B.V
The details of our experimental setup are given by Morris et al. [lO,ll]. Beams of 2 MeV He’ or H + are obtained using a 4 MeV dynamitron accelerator. A quadrupole doublet lens is the final element used to focus the beam on the sample. X-rays are analyzed with a 6 mm diam. X 3 mm cooled Si(Li) detector which is placed inside the target chamber facing 45” to the target plane. The X-ray detector has a resolution of 160 eV at 5.9 keV. For RBS the backscattered ions are detected and energy-analyzed by a surface barrier detector placed at 150” back angle with a solid angle of 160 msr. The RBS detector had a resolution of 18 keV for 6 MeV alpha particles. A unique feature of this microprobe is that the beam has an autoalign system which greatly simplifies setup and control of the microbeam line. An electronic scanning system is used to form secondary electron images of the surface in order to locate features of interest. Hence. chemical and microstructural information based on the spot, line or frame mode can be obtained. The beam size was checked using a 1000 mesh electroformed copper grid and found to be approximately 2 pm with a beam current of 150 pA. In the present setup. both RBS and PIXE information can be collected simultaneously. An Apple II microcomputer is used to control the multichannel analyzers and to store the RBS and PIXE data. The same computer can be used to store the digitized image and to control the x-y stage for the sample holder. Alignment procedures are simplified by using an autoalign system (ref. [lo]) which automatically corrects XIII. ION BEAM MICROANALYSIS
W. G. Morm
662
et al. /
Micrnheam
ana!vsiJ of microelectrot~ic
circuits
2 hiev 2 MeV
H +
150i
.. OA
L x-ray Ko X-ray
.A.
Fig. 1. K, X-ray and L X-ray yields for targets MeV He+ and H’ ion excitation.
Ti-Zn
for 2
for small amounts of beam drift and keeps the incident beam centered on the slits which define the object size. The smallest beam size at the sample has been obtained under conditions where the main yuadrupote lens (a) gives a spot no larger than 1 mm and (b) gives minimum sweep as the lens current is varied slightly. The probe forming quadrupole lens is also aligned to give minimum sweep (displacement of the scanned image) as its current is varied.
3. Results and discussion Various metals are used for the contacts and conductor runs in microelectronic circuits. It is necessary to
Fig. 2. Secondary
electron
image of a region of a microelectronic
0:
14
18
1.6 Energy
20
( MeV I
Fig. 3. RBS spectrum taken in a spot mode. conductor, (b) between the metal conductors.
(a) on a bright
understand the differences in X-ray and RBS spectra using He+ and ti’ ion excitation. We have undertaken a program to obtain the X-ray yields for K and L lines for various thick and thin targets. In this report the thick target yields are given for Z = 22 (Ti) to Z = 30 (Zn). It was observed that 2 MeV H+ ions excite the K lines and L lines of Cu with comparable strengths, and yield a spectrutn sitnilar to that obtained when doing microanalysis with a 30 keV electron beam. A 2 MeV
test circuit.
Frame
scan 5 s, 2 MeV He’.
0.1 nA
He+ beam, however, has a lower velocity and is significantly less effective in exciting the Cu K lines. The results are summarized in fig. 1 which shows the K, X-ray yield for elements between Ti and Zn for both Ile+ and H+ ions. The K, X-ray yield with He’ ions is much
smaller
than
the
K,
X-ray
yield
with
H’
ions.
the L X-ray yield for elements between Fe and Zn using both 2 MeV He+ and 2 MeV II+ excitation. The L yields are similar for Cu and Zn l?hen comparing H+ and He+ excitation, with He+ ljeing a slightly more efficient exciter for lighter elelnents (Fe. Z = 26). The secondary electron image of a portion of a lnicroelectronic circuit is shown in fig. 2. The metallized rectangles in the center to upper right region are about 1.0 to 50 pm. The conductors in the lower left region are 1, pm lines with 3 pm separations. Fig. 3 shows RBS spectra taken with H+ ions for a portion of this microcircuit. The upper curve of fig. 3(a) ij taken using 2 MeV H + ions in a spot mode on a bright conductor which is analyzed to be 1 pm Al on 120 nm of SiO, on a Si substrate. The lower curve of fig. 3(b) is taken under similar conditions with the beam positioned between the metal conductors, showing SiO, on Si. Spectra were also obtained from these areas using a 2 LleV He+ beam. Comparison with the spectra obtained lmder H+ impact shows that, at the same energy, H+ i:ms allow compositional analysis to much larger depths rhan He+ ions whereas the latter provide much better depth resolution than the former. I:ig.
1 also shows
4. Conclusions
spatial resolution of approximately 2 pm can provide complimentary chemical and microstructural information. In order to understand differences in the spectra, X-ray and RBS yields from various targets must be known. The advantage of using He+ ions for RBS analysis are better depth resolution, better mass resolution and a more quantitative analysis with simulations. The advantages of using H+ ions for RBS analysis are greater depth penetration. and better resolution of the oxide layers. PIXE analysis using 2 MeV H’ ion excitation is nearly 10-100 times more sensitive for K, X-rays and approximately equal for L lines as compared with that using 2 MeV He+ excitation for thick targets of atomic number Z between 25 and 30.
References PI J.A. Cookson and M. Poole, New Scientist 1 (1970) 404. PI T.B. Johanason, R. Akselsson and S.A.E. Johansson, Nucl. Instr. and Meth. 84 (1970) 141. [31 R.G. Musket, Nucl. Instr. and Meth. 218 (1983) 420. [41 B.L. Doyle and N.D. Wing, IEEE Trans. Nucl. Sci. NS-30 (1983) 1214. [51 B.L. Doyle. N.D. Wing and P.S. Peercy, Microbeam Analysis 1981, ed., R.H. Gleiss, (San Francisco Press. 1981). [61 F.W. Martin and R. Goloskie, Appl. Phys. Lett. 40 (1982) 191. 171 J.A. Cookson. Nucl. Instr. and Meth. 165 (1979) 477. PI G.J.F. Legge, Nucl. Instr. and Meth. 197 (1982) 243. 191 R. Nobiling. Nucl. Instr. and Meth. 218 (1983) 197. PO1 W.G. Morris, W. Katz, H. Bakhru and A.W. Haberl. J. Vat. Sci. Technol. B3 (1985) 391. 1111 W.G. Morris. H. Bakhru and A.W. Haberl. Microbeam Analysis 1984. eds., A.D. Romig Jr and J.I. Goldstein (San Francisco Press. 1984).
RBS and PIXE information on microelectronic circuits using 2 MeV He+ and 2 MeV H+ ions with a
XIII. ION BEAM MICROANALYSIS