THE PREPARATION OF TRANSMISSION ELECTRONMICROSCOPY SPECIMENS OF AS-DRAWN GOLD WIRE

K. Noguchi, M. Araki and Y. Ohno*Graduate School of Science and Technology, Kumamoto University, Kurokami, Kumamoto 860,

Japan *Department of Materials Science, Kumamoto University, Kurokami, Kumamoto 860, Japan

(Received November 30, 1999)(Accepted in revised form March 13, 2000)

Keywords:Cold rolling; Transmission electron microscopy; Gold wire; Young’s modulus; Texture

Introduction

Recently, microelectronic devices are becoming smaller and lighter. The Au wire, which connects theAl pads on the Si chip and external electrodes generally called lead frame, is now drawn to less than30 mm in diameter [1,2] and the pitch of Au wire bonded on the Al pads closes to less than 80mm [3].Au has been used widely as a wire material in the semiconductor industry because of its excellentoxidation resitance, electric conductivity and plasticity [4,5]. The chip is packaged with an epoxy moldresin to avoid contamination by moisture and dust after wire bonding [6]. However, Au wire isdeformed when the resin is flowed into the chip surface. This phenomenon is generally called “wiresweep” [7]. When adjacent wires contact each other by wire sweep, the device can fail. It is well knownthat no wire sweep occurs in Au wire with high Young’s modulus [8]. Therefore, it is important tomaintain high strength even in narrow wires [9,10]. It is important to understand the relation betweenthe microstructure and mechanical property of the wire. However, it is difficult to prepare as-drawn Auwire specimens for analyzing the microstructure since Au is a very soft metal. In this study, transmis-sion electron microscopy specimens of as-drawn Au wires have been prepared by two differenttechniques. In addition, the Young’s modulus of as-drawn Au wire has been measured by a tensile testand by a resonant method [11].

Experimental Procedure

The purity of the as-received Au wire used in this study is 99.99%. The wire has been already drawnto about 30mm in diameter. Scanning electron microscopy (SEM) and transmission electron micros-copy (TEM) were used for analyzing the microstructure of the wire. Aqua regia was applied as theetchant for SEM observation. Specimens for TEM observation parallel to the drawn direction (D.D.)were made by two methods described as follows. One preparation made use of the ultramicrotome. Afew wires were fixed with the epoxy resin so as not to bend. The resin was cut perpendicular to thedrawn direction (T.D.) at a cutting velosity of 0.5 mm/sec and at a thickness of 30 nm. The very thinfilm was picked up with a Cu mesh which was covered with a colloid film. The film was used for TEMobservation after evaporizing the carbon layer on the specimens for a few minutes. A secondpreparation method is shown in Fig. 1. About thirty Au wires were put into a Cu tube with an outerdiameter of 2 mmf and an inner diameter of 1 mmf. This tube was then put into a second Cu tube with

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an outer diameter of 4 mmf and an inner diameter of 3 mmf as shown in Fig. 1(a). There were lotsof huge pores between wires after Au wires were put into a Cu tube. Therefore, a cold-rolling wascarried out to decrease the pore area without the wires’ extreme deformation. In addition, two Cu tubeswere applied so as not to add an extreme stress to wires directly during cold-rolling. Two Cu tubes havean effect as a shock absorber. The tube was cold-rolled to approximately 3 mm in an external diameteras shown in Fig. 1(b). A disk about 0.5 mm in thickness was cut from the rolled tube and ground toabout 50mm in thickness mechanically. Both sides of the disk were reinforced with Mo grids with adiameter of 3 mmf and a hole of 0.5 mmf (Fig. 1(c)) and then thinned by ion milling. A specimen forTEM observation of the T.D. cross-section was made as shown in Fig. 1(d). About ten as-drawn wireswithout cold-rolling were arranged on the center of a Mo grid and adhered with epoxy resin. The gridwas reinforced with another Mo grid and thinned by ion milling. Therefore the specimens for TEMobservation of the T.D. cross-section possess the microstructure of the as-drawn wires. TEM observa-tions were carried out in a JEOL-2000FX microscope operated at 200 kV. The resonant method wasperformed to measure Young’s modulus of the as-drawn Au wires at room temprature [11].

Results and Discussion

Figure 2(a) shows an SEM micrograph of the wire cross-section observed parallel to the drawingdirection (D.D. cross-sectional view). The grains in the wire are so fine that they can not be seen clearly

Figure 1. Illustration of the preparation of TEM specimen: (a) Au wires are inserted into Cu tubes, (b) the tubes are cold-rolled,(c) a disk which includes Au wires is reinforced with Mo grids for TEM observation of D.D. cross-section. (d) wires are arrangedon the Mo grids and adhered with epoxy resin for TEM observation of T.D. cross-section.

Figure 2. SEM micrographs of the as-received wire D.D. cross-sectional view (a) and surface (b), respectively.

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by the SEM. Figure 2 (b) shows an SEM micrograph of the wire surface. An arrow in Fig. 2 (b)indicates the drawing direction. Although the fiber structure can be observed, detailed information aboutthe grain size and morphology could not be obtained from Fig. 2. Figure 3 shows an SEM micrographof the Au wire and Cu tube cross section after cold-rolling. Wires have partially welded areas (arrowsin Fig. 3) where they contacted the Cu tube or each other. However, since most wires keep a circlarshape with approximately 30mm in diameter and this size is the same compared with that of theas-received wire shown in Fig. 2, they have no deformation and maintain the as-drawn microstructure.This aspect will be proved by TEM observations of the D.D. cross-section (Fig. 4) and the T.D.cross-section (Fig. 5).

Figures 4 (a) and (b) show bright field images (BFIs) of the D.D. cross-sectional view of a wirethinned by ion milling. Grains in both the center (Fig. 4 (a)) and the edge (Fig. 4 (b)) of the wire arevery fine. The average size of the grains is approximately 0.2mm. The structural morphology in thecenter and the edge of the wire is the same. Figures 4 (c) to (h) show micro-diffraction patterns fromgrains C to H in Fig. 4 (a), respectively. Only the diffraction pattern in Fig. 4 (c) is that of the,111.incidence. The incident beam directions of other patterns from Fig. 4 (d) to Fig. 4 (h) may be inclinedto the,111. direction. These patterns were observed in regardless of the center and the edge of thewires. Figure 5 (a) shows BFI of the center of the wire cross-section observed perpendicular to thedrawn direction (T.D. cross-sectional view). The black arrow in Fig. 5 (a) shows the drawn direction.The grains are extremely long and thin parallel to the drawn direction and the average grain sizeperpendicular to the drawn direction is about 0.2mm. This size corresponds to that in Fig. 4 (a). Therewas no change on the grain size in the whole area by T.D. cross-sectional view. It is also noted that longand thin grains can be seen. The correct length was not measured in the present study but grains arelonger than at least 5mm parallel to the drawing direction. Figures 5 (b) to (o) show micro diffractionpatterns from grains B to O in Fig. 5 (a), respectively. Since the long axis of the fine grains in Fig. 5(a) corresponds to the,111. direction in diffraction patterns, it is apparent that a,111. texture isformed parallel to the drawn direction by the drawing. The deviation from the,111. direction of someadjacent grains is a few degrees as shown in Fig. 5 (f) and Fig. 5 (i). The long axis of a few grainscorresponded to,110. or ,112. directions. Their detailed distribution, misorientaion to,111.texture and so on are under further study. These results support the ones obtained by the TEMobservation of the D.D. cross-section. It is considered that many of the incident beam directions usedfor D.D. cross-sectional view were the,111. direction or close to the,111.. Furthermore, the grainsize in the D.D. cross-section was the same compared with that in the T.D. cross-section, the specimensfor which were not cold-rolled discribed above. Therefore, the D.D. cross-section specimens preparedby cold-rolling remained the as-drawn microstructure.

Figure 3. SEM micrograph of Au wire and Cu tube cross-sectional view after cold-rolling.

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Figures 6(a) and (b) show BFIs of the edge and the center of the wire D.D. cross-sectional viewprepared by ultramicrotomy. The arrows in Figs. 6 (a) and (b) indicate the cutting direction. It isapparent that very fine grains can be observed in both areas. The whole area in the wire had the samestructure. However, grains elongate parallel to the cutting direction and show a complicated shape. Soan accurate grain size was not obtained. This structure is quite different from that in Fig. 4. We suggestthat the wire was damaged and deformed by cutting. Therefore, the microstructure of the specimenobtained with the ultramicrotome is probably different from the as-drawn wire.

Figure 4. TEM micrographs of the wire D.D. cross-section thinned by the ion milling; (a) bright field image (BFI) of the centerof the Au wire, (b) BFI of the edge of the Au wire and (c);(h) micro diffraction patterns from grains C to H, respectively.

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Figure 5. TEM micrographs of the wire T.D. cross-sectional view; (a) BFI and (b);(o) micro diffraction patterns from grainsB to O, respectively.

Figure 6. TEM micrographs of the wire D.D. cross-section prepared with the microtome; (a) BFI of the edge and (b) BFI of thecenter of the wire, respectively.

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Table 1 shows the Young’s modulus of,111. and ,100. Au single crystal [9] and the wireobtained by the resonant method [11] in this study. It is well known that Au has an anisotropy ofmechanical properties according to the crystal direction [9]. The Young’s modulus of the Au wire isobviously almost the same as that of the,111. single crystal. So the higher Young’s modulus of theas-drawn wire compared to the,100. single crystal is due to the formation of,111. texture duringdrawing.

Concluding Remarks

TEM specimens of as-drawn Au wires were prepared by two different techniques. Both microstructureswere observed and compared in this study. A,111. texture were observed in the wire thinned by ionmilling. The grains were elongated and were approximately 0.2mm wide. The grains of the specimenprepared by ultramicrotomy were deformed parallel to the cutting direction. We suggest that the wirewas damaged by cutting. Young’s modulus of the Au wire was almost the same as that of,111. Ausingle crystal. Therefore, the,111. texture contributed to the high modulus of the wire.

References

1. H. Drack and L. Ainouz, Microelectron. J. 27(8), 54–56.2. G. Qi and S. Zhang, J. Mater. Process. Technol. 68, 288 (1997).3. K. Tatsmi, T. Uno, O. Kitamura, Y. Ohno, T. Katsumata, and M. Furusawa, Proc. Int. Electron. Manuf. Tech. Symp., pp.

295–298 (1995).4. M. Grimwade, Interdisciplinary Sci. Rev. 17(4), 371 (1992).5. S. Tomiyama and Y. Fukui, Gold Bull. 15(2), 43 (1982).6. J. E. Krzanowski, IEEE Trans. Components Hybrids Manufact. Technol. 13(1), 176–181 (1990).7. L. T. Nguyen, Poly. Eng. Sci. 14, 926 (1988).8. T. Yoshiwara, M. Yuki, T. Hayashida, and Y. Ohno, Trans. Inst. Electron. Inform. Commun. Eng. C-II. J. 81(4), 413 (1998).9. S.-P. Hannula, J. Wanagel, and C.-Y. Li, IEEE Trans. Components Hybrids Manufact. Technol. 6(4), 494 (1983).

10. K. Bush, H. U. Kunzi, and B. Ilschner, Scripta Metall. 1(22), 501 (1988).11. M. Araki, K. Noguchi, and Y. Ohno, Trans. Inst. Electron. Inform. Commun. Eng. C-II. J. 82(4), 165 (1999).

TABLE 1Young’s Modulus of,111., ,100. Au Single Crystal [9] and the

Au Wire in the Present Study

,111. ,100. Au wire

Young’s modulus (GPa) 112 41 100

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