Applications of Nuclear Reaction Analysis for
Semiconductor Industry
Luncun Wei
Charles Evans & Associates, 810 Kifer Road, Sunnyvale, CA 94086
Abstract. Many thin film samples used in the semiconductor industry contain C, N and O. The detection
limits and accuracy obtained by Rutherford Backscattering Spectroscopy (RBS) measurement are limited due
to the small cross section values. High energy non-Rutherford backscattering is often used to enhance the
sensitivities. But non-Rutherford cross section values are irregular and can not be calculated as normal
Rutherford backscattering values. It is also difficult to find an appropriate energy window that for all these
elements, and high-energy ions are needed. In this paper, the Nuclear Reaction Analysis (NRA) method is
used to simultaneously measure C, N and O. several applications in the semiconductor research,
development, and manufacturing areas are presented .
Introduction
Experimental
RBS is widely used for bulk compositional
measurements. Due to the low scattering cross
section of light elements, RBS detection limits for
light elements are poor, generally in the range of 5 to
7%(atomic) for C, N and O. Carbides, nitrides and
oxides are used in semiconductor manufacturing,
hence C, N or O are either the major components or
important impurities in thin films.
Their
characterization is sometimes crucial for the
performance and characteristics of devices. NRA can
be used to improve the detection limits for these
elements and by combining RBS and NRA, it is
possible to measure down to 1% of these elements in
thin films.
To simultaneously measure C, N and O, 1.0MeV
deuterons were used with a solid state detector
located at 160 . In the front of this detector, a 13µ
polyethylene foil was placed to stop all scattered
incident deuterons and to allow nuclear reaction
emitted high energy particles to pass through. ,The
energies of the protons emitted by ,12C(d, p)13C,
14
N(d, p4,5)15N, 16O(d, p1)17O were 3.0MeV, 1.9MeV
and 1.6MeV. They were well separated and easy to
measure.
RBS analysis uses 2.275MeV He with two detectors,
one located at 160 and other located at a smaller
detection angle depending on the sample thickness to
enhance the depth resolution.
For thin films
deposited on a crystal substrate (for example, Si
wafer), channeling is often used to reduce
backscattering from the substrate in order to improve
detection of C, N and O.
NRA uses energetic, low mass ions, such as protons,
deuterons or helium-3 to induce nuclear reactions on
light elements. The particles emitted by these nuclear
reactions are used to measure the light element
contents and profiles in thin films. The following
reactions are most useful for light elements
characterization: 7Li(p, α)α, 11B(p, α)α, 12C(d, p)13C,
14
N(d, p)15N, 16O(d, p)17O and 19F(p, α)16O.
Applications
(1) TaNx Film Analysis
Although NRA is very sensitive to measure light
elements, its depth resolution is not as good as
normal RBS.
By combing RBS and NRA, it is
possible to give not only accurate composition, but
also the depth profile information.
TaN is widely used as a dielectric layer in
semiconductor manufacturing. Due to the very high
scattering cross section of Ta, it is hard to get a good
channeling effect to enhance the N signal to
background ratio. The detection limit by RBS alone
is about 5-7% for N, C and O when the TaN is
deposited on Si. With NRA, N, C and O areal density
Three typical applications of RBS/NRA are presented
in this paper: TaNx, Hi-κ HfOx and DLC films.
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
464
areal densities of these elements. The Ta areal
density was measured accurately by RBS. The N/Ta
ratio can thus be calculated. This ratio can be used to
fit the RBS spectra to get depth profiles if the TaNx
layer is not uniform. The C, N and O detection limits
for this type of samples are about 1%. Thick samples
will have even lower detection limits.
can be measured easily. In Figure 1, a RBS spectrum
of 14nm Ta0.525N0.357O0.07C0.044 on Si substrate is
shown. The RBS fitting for N is very difficult due to
the low signal to background ratio. The NRA
spectrum of the same sample is shown in Figure 2.
N, C and O peaks are clearly marked. The proton
yields from N, C and O were used to calculate the
2x104
RBS 2.275MeV He at 160o
Ta
4
2x10
Yield
14nm Ta0.525N0.357O0.07C0.044
1x104
5x103
C N O
Si
0
0
100
200
300
400
500
Channel Number
Figure 1: RBS spectrum of 14nm Ta0.525N0.357O0.07C0.044/Si with 2.275 MeV He
C
500
NRA 1.0MeVDeuteron at 160o
14nm Ta0.525N0.357O0.07C0.044/Si
Yield
400
300
200
N
100
0
O
0
100
200
300
400
500
Channel Number
Figure 2: NRA spectrum of 14nm Ta0.525N0.357O0.07C0.044/Si with 1.00 MeV D
465
(2) HfOx Film Analysis
Figure 3 shows an example of a RBS spectrum of
26nm HfOx. From this RBS spectrum, the exact ratio
of O/Hf can be determined. But it is difficult to see
any C and N present in this film. Figure 2 shows the
NRA spectrum obtained with 1.0MeV deuterons. In
this spectrum it is clear that this film has some carbon
impurity. The exact content of this film was
determined to be Hf0.32O0.64C0.036Zr0.004.
Thin high-κ films, generally refractory metal oxides,
such as HfOx, and ZrOx, play an increasingly
important role in semiconductor manufacturing.
Light elements, such as C, N and O are normally
present at concentrations of several percent. Their
presence has a big impact on the performance of
high-κ films. It is crucial to accurately measure these
light elements. By normal RBS techniques, it is
possible to get the oxygen to metal ratio, but RBS has
relatively high detection limits for light elements
(about atomic 6%). The combination of RBS and
NRA can accurately measure the oxygen to metal
ratios and low content C and N in thin high-k films.
The combination of RBS/NRA can be used to
measure MCxNyOz films (M=Hf, Zr, Ta, Ti, Ru, W,
Y, etc) composition with detection limits less than
atomic 1% for C, N and O.
6000
Hf
26nm Hf0.32O0.64C0.036Zr0.004
5000
2.275KeV He RBS at 95o
Experimental
Theoretical
Yield
4000
O
3000
C
X10
2000
1000
0
100
200
300
400
500
Channel Number
Figure 3: RBS spectrum of 26nm HfOx with 2.275MeV He
(3) DLC Film Analysis
Diamond-like carbon (DLC) films are often
deposited on disk drives and device surfaces as a
protective layer. The film thickness is nominally
3nm to 20nm for most applications. RBS can not
measure such a thin carbon layer on a high Z
substrate. However, NRA can be used to measure C,
N and O areal densities. HFS (Hydrogen Forward
Scattering) is used to quantify the H content in DLC
film. By the combination of NRA and HFS, it is
possible to get DLC composition from films that are
several nanometers thick.
Figure 5 shows a HFS spectrum with 2.275MeV He.
A detector located at 30 with an Al foil in front of it
was used to collect the HFS spectrum. The aluminum
foil is used to stop forward scattered He and let H (or
proton) to pass through. The NRA spectrum with
1.0MeV deuterons from the same is shown in Figure
6. It is clear from the NRA spectrum that this DLC
film contains C, N and O. This DLC film thickness
was 18nm and H0.185C0.629N0.091O0.095.
NRA is especially useful to measure very thin DLC
466
Film thicknesses. For example, the thinnest film that
can be measured by NRA in our system is
approximately 0.2nm. Further improvement can be
made by using an ultra-high vacuum system to
reduce hydrocarbon deposition during NRA data
acquisition.
300
O
26nm Hf0.32O0.64C0.036Zr0.004
250
1.0MeV Deuteron NRA
Yield
200
150
100
C
50
0
0
100
200
300
400
500
Channel Number
Figure 4: NRA spectrum of 26nm HfOx with 1.0 MeV D
200
Yield
150
18nm H0.185C0.629N0.091O0.095 on NiFe HFS
Experimental
Theoretical
100
50
0
100
200
300
400
500
Channel Number
Figure 5: HFS spectrum of 18nm H0.185C0.629N0.091O0.095 on NiFe with 2.275MeV He
467
5000
18nm H0.185C0.629N0.091O0.095 on NiFe
1.0MeVDeuteron NRA
C
Yield
4000
3000
O
N
2000
1000
0
0
100
200
300
400
500
Channel Number
Figure 6: NRA spectrum of 18nm H0.185C0.629N0.091O0.095 on NiFe with 1.0MeV D
Summary
Reference
By using the combination of RBS and NRA
analytical techniques, the detection limits for C, N
and O are improved from about 6% to less than 1%.
This combination is important for light element
characterization in thin films used in semiconductor
manufacturing and other industries. NRA can also be
used to measure other light elements, such as Li, B
and F.
1.
Wei-Kan Chu. James W. Mayer and Marc-A
Nicolet, Backscattering Spectrometry, Academic
Press (1978).
2.
Joseph R. Tesmer and Michael Nastasi, ed.,
Handbook of Modern Ion Beam Materials
Analysis, Materials Research Society (1995).
3.
J. W. Mayer and E. Rimini, ed., Ion Beam
Handbook for Materials Analysis, Academic
Press (1977).
Acknowledgments
The author would like to thank S. Baumann of Evans
Texas for reviewing and discussion of this paper.
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