JAEA-Review-2010-065.pdf:15.99MB - 日本原子力研究開発機構

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Positive Secondary Ion Emission from PMMA upon
Energetic C8 Cluster and Mo Ion Impacts
K. Hirata a) , Y. Saitoh b) , A. Chiba b) , K. Yamada b) , Y. Takahashi b) , K. Narumi c) and T. Kojima b)
a) National Institute of Advanced Industrial Science and Technology (AIST),
b) Department of Advanced Radiation Technology, TARRI, JAEA,
c) Advanced Science Research Center, JAEA
Secondary ions (SIs) are ejected from the surface when the
primary ions bombard the target. Cluster ion impact on a target
produces different SI emission yields compared with those for
monoatomic ions because of their peculiar irradiation effects
caused by simultaneous energy transfer from the constituent
atoms of the cluster to a small area of the target surface. In this
paper, we report comparison of SI emission yields from a
Poly(methyl methacrylate) (PMMA) target for incident ion
+ +
impacts of energetic cluster ions (0.8 MeV C8 , 2.4 MeV C8 , and
+
4.0 MeV C8 ) and heavy monoatomic molybdenum ions
(4.0 MeV Mo + , and 14 MeV Mo 4+ ) with similar mass to C8 ion by
time-of-flight (TOF) SI mass analysis combined with SI electric
current measurements.
SI emission yield measurements for PMMA were performed
using a TOF mass analyzer combined with pulsed ion beams
produced by a 3 MV tandem accelerator at the Japan Atomic
Energy Agency (JAEA)/Takasaki 1) , which has been described
elsewhere 2, 3) . For quantitative comparison of SI emission yields
per incident ion impact, peak intensities of the TOF spectra were
plotted by scaling the total count of each spectrum based on qIs/Io (q: incident ion charge number, Io: incident beam electric current,
IS: positive SI current for each irradiation condition), respectively.
Io and Is were measured by highly sensitive electrometers,
respectively, connected to a Faraday cup and a movable metal
2)
plate with a grid, as described elsewhere .
Figure 1 shows the positive SI TOF spectra from m/z (mass to
+
charge ratio) = 58 to 72 of PMMA for (a) 0.8 MeV C8 , (b)
+ + +
2.4 MeV C8 , (c) 4.0 MeV C8 , (d) 4.0 MeV Mo , and (e)
14 MeV Mo 4+ , respectively. The major peaks observed in the
+
figure can be attributed to C2H3O2 (m/z=59) and C4H5O +
(m/z=69), respectively. The total count of each spectrum was
scaled based on qIs/Io for each irradiation condition in order to
quantitatively compare the SI emission yields per incident ion
impact among the spectra.
The relative SI emission yield per C8 impact increases in the
+ + +
order of 4.0 MeV C8 > 2.4 MeV C8 > 0.8 MeV C8 . SI
emission yields are influenced by energy transfer processes and
their deposited energy densities around the impact-points. In an
incident energy range of 0.1-0.5 MeV for C (corresponding to
that of 0.8-4.0 MeV for C8), the energies transferred by electronic
and nuclear processes increases and decreases with increasing the
incident energy, respectively, provided the transferred energies
for a cluster ion with n identical atoms is n times that of a
monoatomic ion with the same velocity. Considering that the
+
relative SI emission yields are higher in the order of 4.0 MeV C8 ,
+ +
2.4 MeV C8 , and 0.8 MeV C8 , the electronic energy transfer
process dominantly contributes to the SI emission.
Comparison of the SI emission yields for the C8 impacts with
+
those for the Mo impacts reveals that the 4.0 MeV C8 impact
provides higher SI emission yields than the Mo ion impacts of not
only for 4.0 MeV but also for 14 MeV. This shows that the
JAEA-Review 2010-065
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higher SI emission yields are obtained with lower incident ion
energy using cluster ions rather than swift heavy monoatomic ion.
Energetic cluster ion impacts provide emission yields of the
positive SIs of m/z=59 and 69 for PMMA more effectively than
impacts of heavy monoatomic ion with similar mass to that of the
cluster ion, which is advantageous for highly sensitive surface
analysis using TOF SI mass analysis.
Fig. 1 Positive secondary ion TOF spectra of PMMA
for (a) 0.8 MeV C 8 + , (b) 2.4 MeV C8 + , (c) 4.0 MeV
C 8 + , (d) 4.0 MeV Mo + , and (e) 14 MeV Mo 4+ .
Peaks at m/z=59 and 69 are attributed to C 2H 3O 2 + and
C4H 5O + , respectively.
References
1) Y. Saitoh et al., Nucl. Instrum. Meth. Phys. Res. A 452
(2000) 61.
2) K. Hirata et al., Appl. Phys. Lett., 86, (2005) 044105.
3) K. Hirata et al., Nucl. Instrum. Meth. Phys. Res. B in press.