FABRICATION of NANOPOROUS COPPER by SELECTIVE DEALLOYING of Cu-Fe ALLOY

2009 INTERNATIONAL SYMPOSIUM ON NANO SCIENCE AND TECHNOLOGY Tainan, TAIWAN, 20-21 November 2009

FABRICATION OF NANOPOROUS COPPER FILMS BY SELECTIVE DEALLOYING OF Cu-Fe ALLOY

Bo-Jiun Yeh, Hsyi-En Cheng, Yu-Hung Hsieh

Department of Electro-Optical Engineering, Southern Taiwan University Tainan, Taiwan [email protected]

Abstract Nanoporous copper films were fabricated by dealloying on glass substrates. Cu-Fe alloy films and Cu conducting layer were deposited by dual e-beam evaporator. Iron was dissolved selectively from Cu-Fe alloy through electrochemical reactions. Selective dissolution of iron from Cu-Fe alloy occured at open-circuit potential of -0.3 V vs. Ag/AgCl, which leads to precipitation of porous layer of copper. The pore size was about 35 nm for Cu-20%Fe alloy. The pores size and pore density was further enhanced by vacuum annealing. After annealing, the pore size was increased to about 80 nm, and the pore density was improved by nearly 50%.

Introduction Nanoporous films have attracted increasing attention in recent years. It provided a wealth of scientific interest as well as of potential applications for magnetic recording, optical devices and sensors. Dealloying is a process during which an element is selectively dissolved from an alloy. It is a method to fabricate nanoporous films. In this work, nanoporous Cu films were fabricated by dealloying the Cu-Fe alloy. It has been reported that the steady state open-circuit potentials for the copper electrode and iron electrode in the chloride solution are -0.256 and -0.578 V, respectively [1]. Iron therefore can be dissolved selectively from Cu-Fe alloy using potential between -0.256 and -0.578 V.

Experimental An adhesion layer of titanium (200 nm) and a conduction layer of copper (200 nm) were deposited on glass substrates by e-beam evaporation. The Cu-Fe alloy with thickness of 1 μm was then deposited on the conduction layer by dual e-beam evaporation. Samples were annealed with a pressure of 3×10-5 torr and a temperature of

450℃ for 2 hours. Subsequently, the samples were etched in 1.0 M sodium chloride electrolyte with pH of 2 by potentiostat system at -0.3V vs. Ag/AgCl. The electrochemical process was finished as the current fall and approach zero. The composition was analyzed using auger electron spectrometer. The structures of the films were determined by X-ray diffractomter. The morphologies of films were observed by a scanning electron microscopy.

Results and Discussion The deposited Cu-Fe alloy was found to consist of 80% Cu and 20% Fe. Fig. 1 shows the SEM images of

127 2009 INTERNATIONAL SYMPOSIUM ON NANO SCIENCE AND TECHNOLOGY Tainan, TAIWAN, 20-21 November 2009

as-deposited and annealed Cu-Fe alloy films, respectively. Fig. 2 shows the XRD patterns of these films. A clear splitting of (110) Fe park from (111) Cu peak after annealing indicating a total decomposition of Cu and Fe in the sample.

(a) (b)

Fig.1 SEM images of (a) the as-deposited and (b) the annealed Cu-20%Fe films.

(a) (b)

Fig.2 XRD patterns of (a) the as-deposited and (b) the annealed Cu-20% Fe films.

The currents of dealloying of Cu-20%Fe films as a function of time are shown in Fig. 3. The current increases initially with time to reach a maximum, and then fall to nearly zero. The iron was dissolved completely as the current fall to zero.

(a) (b)

Fig. 3 Current vs. time curves of dealloying for (a) the as-deposited and(b) the annealed Cu-20% Fe films.

Lots of pores were formed on the surface of films after dealloying as shown in Fig. 4. The pore size was about 35 nm. However, the pore size was about increased to 80 nm, and the pore density was improved by has nearly 50% after

128 2009 INTERNATIONAL SYMPOSIUM ON NANO SCIENCE AND TECHNOLOGY Tainan, TAIWAN, 20-21 November 2009 annealing for the as-deposited film. The improvement of pore size and pore density is considered to be due to the decomposition and grain growth of Fe in the film.

(a) (b)

Fig. 4 SEM images of (a) the as-deposited and (b) the annealed Cu-20%Fe films after dealloying.

Conclusion Nanoporous copper films can be fabricated by dealloying. Vacuum annealing before dealloying improves the pores size and pore density.

REFERENCES

[1] S.S. E1-Egamy, Corrosion Science 50 (2008) 928-937.