AMTC Letters
Vol. 2 (2010)© 2010 Japan Fine Ceramics Center

TEM Characterization of Sintered Copper Nanoparticles Covered by
Biopolymer Nanoskin
Kiyonobu Ida1,2, Katsuhiro Sasaki1, Yasuyuki Sugiyama1, Yuki Chujyo1, Tomoharu Tokunaga1,
Masanori Tomonari2, Tetsu Yonezawa3, Kotaro Kuroda1
1

2

Department of Quantum Engineering, Nagoya University, Nagoya, 464-8603, Japan
Functional Materials R＆D Division, Ishihara Sangyo Kaisya, LTD, Yokkaichi, 510-0842, Japan3
Department of Materials Science, Graduate School of Engineering, Hokkaido
University, Sapporo, 060-8628, Japan

Silver and copper nanoparticles have been studied greatly to apply toelectronic materials such as wiring materials and bonding materials in the field ofprinted electronics. In particular, copper which is a sufficiently cheap and widely usedindustrial metal has advantageous properties such as low melting point, high electricalconductivity and less electromigration property. However, because the copper is easily
oxidized that is a disadvantage, only few papers on practical synthetic methods andapplications of copper nanoparticles [1, 2].
Recently, oxidation-resistant copper nanoparticles with biopolymer nanoskin havebeen prepared by the authors [3] using the simple wet-process which is suitable formass production. In this method, average radii of the copper nanoparticles obtained canbe controlled by the concentration of a complex agent. The oxidation-resistance due tothe prevention of the oxidation by gelatin which covers the copper nanoparticles allows
us easier treatment of the copper powders in ambient atmosphere. We expect applicationof our nanoparticles to wiring, bonding and electrode materials.
In this study, we report the characterization of the above mentioned copper
nanoparticles sintered using in-situ heating TEM.
Copper nanoparticles used as specimens were synthesized using the
wet-chemical method as mentioned in reference [4]. Average radii of used copper
nanoparticles were 46nm, 125nm, and 175nm, and the surface of it was covered bygelatin layer with thickness of 5nm. In-situ heating TEM observations were performedin H-9000NAR at an accelerating voltage of 300 kV. The pressure atmosphere duringin-situ TEM observation was controlled between 2.0×10-5Pa, which is the raw vacuum
pressure of the TEM column, and 8.0×10-4 Pa with oxygen flow through the Kamino
holder. The characterization of the sintered copper nanoparticles was performed using
the electron energy loss spectroscope (EELS) and selected area electron diffraction(SAED).
As the results of in-situ TEM observation, all size of copper nanoparticles
sublimated without sintering at about 1273 K under vacuum pressure of 2.0×10-5Pa
without oxygen gas, and carbonized gelatin remained the shape of initial layer on theheating wire.
Under more than 2.0×10-4Pa of vacuum pressure, sintering of copper nanoparticles
were observed between 523K and 673K. Sintering temperature of copper nanoparticles
was similarly regardless of particle size. As a result of TG-DTA analysis, the gelatin
which was used as the protective agent in the synthesis of copper nanoparticles was
decomposed at higher than about 523K by oxidation in 1 atm of air. Therefore, it issuggested that the decomposition temperature of gelatin are more predominant than the88AMTC Letters
Vol. 2 (2010)© 2010 Japan Fine Ceramics Center

particle size in the sintering temperature of copper nanoparticles covered by gelatin.
In general, it is well known that impurities such as carbon element and metal oxide in
the circuit deteriorate the electro-conductivity. Therefore, we examined carbon element
and copper oxide in the sintering copper. Figure 1(a) and Figure 1(b) show sintering
behavior of copper nanoparticles. As a result of EEL spectrum shown in Figure 1(c), thecarbon element was not confirmed in the grain boundary soon after sintering between
copper nanoparticles. Analysis of Copper oxide was performed after the completion ofsintering. Figure 2(a) shows high resolution TEM image of completely sintered copper
nanoparticles. From this result, it is found that the gelatin layer is exactly covered on thesurface of the sintered copper. Moreover, as results of SAED pattern (Figure2(b)) andEEL spectrum (Figure2(c)) of sintered copper, it was confirmed that copper
nanoparticles have not been oxidized. On the other hand, when copper nanoparticles
were oxidized during the sintering, the gelatin layer was sparsely formed on the surface.
From these results, it was suggested that the gelatin layer could work as a prevention
layer against oxygen. Based on obtained findings in this study, further work is underway
to apply for real fabrication of wiring.
References[1] B.K.Park, et al., Thin Solid Films. 515, (2007)7706.
[2] H.S.Kim et al., Composites Sci. Tech. 69, (2009)1256.
[3] M. Tomonari et al., J. Sur. Fin. Soc. of Jap. 59, (2008)18(in Japanese).
[4] M. Tomonari et al., Nanosci. nanotech. 8, (2008)2468.

FIG. 1. Behavior of copper nanoparticles at 623K in 2.0×10-4 Pa of oxygen gas flow. Bright field
TEM images (a) before heating, and (b) in the initial stage of sintering. (c) EEL spectrum of grainboundary corresponding to the area indicated by the circle in (b).

FIG. 2. (a)High resolution TEM image after the completion of sintering. (b) corresponding SAED
pattern, and (c)EEL spectrum.

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