Investigation of Recrystallization and Grain Growth of Copper and Gold Bonding Wires

Investigation of Recrystallization and Grain Growth of Copper and Gold Bonding Wires JAE-HYUNG CHO, A.D. ROLLETT, J.-S. CHO, Y.-J. PARK, J.-T. MOON, and K.H. OH Copper bonding wires were characterized using electron backscatter diffraction (EBSD). During drawing, shear components are mainly located under the surface and,111. and,100. fiber texture components develop with similar volume fractions. Grain average misorientation (GAM) and scalar orientation spread (SOS) of the,100. component are lower than those of the,111. or other orientations. Also,,100. components grow into other texture orientations during recrystallization. Copper wires experience three stages of microstructure change during annealing. The first stage is subgrain growth to keep elongated grain shapes overall and to be varied in aspect ratio. The grain sizes of the,111. and,100. components increase. The volume fraction of the,100. component increases, whereas that of the,111. decreases. The second stage is recrystallization, during which equiaxed grains appear and coexist with elongated ones. The third stage is grain growth, which eliminates the elongated grains. The,111. and,100. grains compete with each other, and the,111. grains grow faster than the,100. grains during the third stage. Comparison of recrystallization and grain growth processes in copper and gold wires reveals many common microstructural features. I. INTRODUCTION RECENTLY, copper bonding wire has attracted attention as an alternative to gold bonding wire. Copper has better electrical conductivity than gold, and it is cheaper to produce. However, fabrication of copper wire still needs more research on drawing and annealing processes in order to optimize the quality of the bonding wire. Axisymmetrically drawn products of face-centered-cubic (fcc) metals with medium and high stacking fault energies have typical textures consisting of double fibers with a majority,111. component and a minority,100.. [1 8] It is known that the,100. fiber becomes dominant after recrystallization but changes back to the,111. component upon further annealing at higher temperatures. This texture transition has been reported in copper wire. [8] In the early stage of the annealing process, the,100. component is prevalent due to recrystallization. In the later stage, the,111. or,112. component becomes dominant because of abnormal grain growth. These investigations are based on both X-ray diffraction and electron backscatter diffraction (EBSD) measurement. Recrystallization and grain growth of gold bonding wires were previously investigated using EBSD. [9] The,100. component grows into the,111. fiber during recrystallization. In addition, the,100. and,111. fibers consume other texture components. As coarsening takes place, the average grain size of both,100. and,111. oriented grains increases. Grain boundaries with high misorientation angles between the,100. and,111. orientations tend to migrate into the,111., and the overall texture shows a higher volume fraction of the,100. component after JAE-HYUNG CHO and K.H. OH, MSEs, are with Seoul National University, Seoul, South Korea 151-742. Contact e-mail: kyuhwan@snu.ac.kr A.D. ROLLETT, MSE, is with Carnegie Mellon University, Pittsburgh, PA 15213. J.-S. CHO, Y.-J. PARK, and J.-T. MOON, MKE Research Lab, are with MK Electron, Pogok-Myeon, Yongin-Si, Kyunggi-Do, South Korea 151-742. Manuscript submitted February 12, 2006. recrystallization. This is explained by an energy advantage for the,100. fiber. During plastic deformation, gradients in the plastic strain give rise to geometrically necessary dislocations (GNDs) in order to maintain lattice continuity. [10,11,12] The energy stored through cold work must include a fraction stored as GNDs. Small lattice rotations due to GNDs can be measured and characterized using local orientation measurements with EBSD. [13] Thus, the internally stored energy in a material can be at least partially measured via intragrain misorientation angles, for example, with grain average misorientation (GAM) or scalar orientation spread (SOS). The SOS or GAM of the,100. component was measured to be lower than that of the,111. in gold wires. [9] This difference in stored energies provides driving force for the,100. component to grow into the,111. or other orientation components. In the beginning of annealing, subgrain growth (or the migration of low-angle grain boundaries (LAGBs)) occurred within each fiber texture component. [9] Low-angle twist boundaries between grains in the same fiber that share a common crystallographic axis consist of dislocation networks. [9] Recovery resulted in the rapid elimination of such boundaries and a consequent increase in aspect ratio. Further annealing resulted in decrease in aspect ratio as regular grain growth took place and the grains became more compact in shape. [9] The,100. component in drawn fcc metals (Al, Cu, Au, Ag, etc.) grows at the expense of the,111. fiber during recrystallization. A variety of models have been used to explain this pattern of texture development. [14,15] The,100. component reduces after recrystallization and further annealing gives rise to the growth texture, which is dominated by the,111. fiber. In aluminum, the average mobility of,111. tilt boundaries is higher than that of,100. tilt boundaries at 400 C. [16,17] The,111. texture might be attributed to the,111. tilt boundaries having a higher average mobility than the,100. tilt boundaries in fcc metals. Shin et al. [15] discussed the deformation and METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 37A, OCTOBER 2006 3085

annealing texture of silver wire. Silver is a typical material in which the major texture component is,111. at low reductions in area (RAs) (less than about 90 pct RA), as observed for other fcc metals, but changes to,100. fiber at higher reductions in area (99 pct RA). A Cu-7.3 pct Al alloy, which has a very low stacking fault energy, was drawn to a 70 pct RA. [18] The orientation density ratio of,100./,111. during drawing decreases with increasing RA up to 50 pct and then increases with further RA. In gold and copper, as RA increases during drawing, and the,111. component increases at the expense of the,100. fiber and other components. The microstructural evolution of copper bonding wires has been characterized using EBSD in a similar way to previous work on gold bonding wires. [9] The effects of drawing deformation and intermediate annealing processes were investigated on the volume fractions and microstructure of,111. and,100. fiber components. The role of three microstructural stages during isothermal annealing for both gold and copper bonding wires is discussed, such as subgrain growth within each component, continuous/discontinuous recrystallization between,111. and,100. grains, and general grain growth. It seems that continuous recrystallization between,111. and,100. grains is a part of extended subgrain growth. [19] II. EXPERIMENTAL Copper casting bars with some intentional dopant of Ag, Mg, Ca, and Zr were cold drawn through several die sets and were made into fine bonding wires with 25-mm diameter. The copper was 99.99 pct pure. Intermediate anneals between drawing steps were added to facilitate the wire drawing process. These intermediate and final annealing steps give more flexibility for controlling the microstructure and texture of the copper bonding wires with optimal properties. Experimental details on drawing processes (RA) and annealing conditions (time and temperature) are summarized in Table I. According to the drawing and annealing steps, texture and microstructure evolution were investigated. In addition, isothermal annealing for the copper wires with 25-mm diameter was also carried out for 1 min, 10 min, 60 min, and 1 day. To prevent copper wires from oxidation during isothermal annealing, a furnace with protective nitrogen atmosphere was used. At least two or three distinguishable wires were selected and used for EBSD works for each temperature and time condition. These isothermal annealing experiments focused on recrystallization and grain growth in the bonding wire. The initial texture of the copper casting bar with 7-mm diameter was measured with X-ray diffraction. Three incomplete pole figures, (111), (200), and (220), were measured on a cross section of the casting bar using the back-reflection X-ray diffraction method with Cu K a radiation on a Seifert 3000PTS diffractometer (RICH, SEIFERT & Co., Germany). Texture and microstructure of wires smaller than 1000-mm diameter were measured and characterized using EBSD. Specimens of copper wires for EBSD measurements were prepared in the same way as previously described for gold bonding wires. [9] The copper wires were mounted in epoxy and then sectioned and polished. The polished specimens were cleaned with ion milling. High-resolution EBSD (using a JEOL* 6500F scanning *JEOL is a trademark of Japan Electron Optics Ltd., Tokyo. electron microscope with an INCA/OXFORD EBSD system) was used for measurement, and the data analysis was performed using the REDS system. [20] The finite-element scanning electron microscopy EBSD has a spatial resolution of 1.5 nm and angular resolution of 0.5 deg. The operating voltage for the EBSD measurements was 20 kv and the probe current was 4 na. A rectangular grid was used and the size of the pixel was varied with wire diameters. The 25-mm wire was measured with a 0.239-mm step size. The EBSD maps were measured along the longitudinal sections of wires. Inverse pole figure (IPF) maps from EBSD were used for texture and microstructure characterization. Misorientation angles between adjacent pixels were used for grain identification (ID). Any two adjacent pixels with a grain ID angle smaller than the cut-off value are considered as a part of the same grain. Most deformed or recrystallized grains have subgrain microstructures, and their overall structures can be described by misorientation measures calculated over a set of pixels contained within the grain. There are three types of misorientation measures commonly used in a grain: grain orientation spread (GOS), SOS, and grain average misorientation (GAM). [21 24] The first, GOS, characterizes the magnitude of misorientation between all pairs of pixels in a grain. The second, SOS, is calculated between each pixel and the average orientation. The third, GAM, is computed for adjacent pixels only, and gives information about the nearest neighbor correlations. The GAM value is generally smaller than either the GOS or SOS. Considering P i as an orientation at a point (x i ) and P j as another orientation at adjacent point (x j ) in a grain, the GAM can be calculated with a misorientation angle, which is given for two adjacent orientations at P i and P j, GAM ¼ n + u mis i i n [1] Table I. Fabrication of Copper Bonding Wires through Intermediate Annealing and Drawing Processes; Cast Bar with 7-mm Diameter was Drawn down to 25 mm Start 1 2 3 4 5 6 Diameter 7000 mm 1000 mm 105 mm 105 mm 48 mm 25 mm 25 mm Case 1 cast bar RA 97.9 pct no annealing RA 99.98 pct 700 C, 0.5 s RA 79.1 pct RA 94.3 pct 555 C, 0.5 s Case 2 cast bar RA 97.9 pct 600 C 1 h RA 98.9 pct 650 C, 0.5 s RA 79.1 pct RA 94.3 pct 555 C, 0.5 s Note: Reduction in area (RA) is overall reduction after annealing. 3086 VOLUME 37A, OCTOBER 2006 METALLURGICAL AND MATERIALS TRANSACTIONS A

"!# trace ððp where u mis i P 1 j ÞSÞ 1 ¼ min acos 2 Here, S is the symmetry operators belonging to the appropriate crystal class concerned and subscripts in rotation P refer only to position. [25,26] To calculate grain size, the number of data points or pixels in a grain are calculated, and, using the known pixel step size, the grain area is determined. The most convenient measure of grain size from grain area is the equivalent circle diameter. [27] III. RESULTS Figure 1 shows a (111) X-ray pole figure measured in cross section with the axial direction (AD), parallel to the casting axis. The terms RD and TD are the radial direction and the transverse direction, respectively. A well-developed,100. texture is evident and the main texture component is close to the rotated cube, {100},011.. This (111) pole figure is similar to that of gold cast bar in Reference 28, which was also provided by MKE Laboratory for research and development. We have pointed out that either equiaxed or columnar grains of the gold cast bar with alignment of,110.//rd and,100.//ad could result in such a texture, and this also seems to apply to the copper cast bar. A. Intermediate Annealing during Drawing Cold-drawn copper wires were characterized with EBSD after each drawing step. Intermediate annealing (IA) and final annealing (FA) steps were carried out in the annealing furnace for about 0.5 seconds. The microstructure and texture of the drawn wires during drawing and annealing processes are shown in Figure 2. There are two different cases in Figure 2. Case one is the process without an intermediate annealing step for the wire with a diameter of 1000 mm, whereas case 2 does include an intermediate annealing for the wire with a diameter of 1000 mm. The value of RA depends on both annealing and drawing processes. In Figure 2(a), the EBSD map for the 1000-mm-diameter wire (Figure 2(a)-1) shows that,100. grains are located under the surface and,111. grains occur mainly in the center. Intermediate annealing at 700 C for drawn 105-mm wire (Figure 2(a)-2) increases the,100. component, and such grains are located in the center and under the surface, as found in Figure 2(a)-3. Further drawing (Figures 2(a)-4 and 5) resulted in the elongation of grains and an intermixing of the,100.,,111., and other components. The,100. fiber decreases or remains the same during further drawing, while the,111. fiber increases steadily with increasing RA. Figure 2(b) shows another case in which intermediate annealing was carried out at a wire diameter of 1000 mm; its microstructure has equiaxed,111. and,100. grains and many annealing twins (Figure 2(b)-1). After drawing down to a diameter of 105 mm (Figure 2(b)-2), the grains were elongated into long bands. Orientation distributions in the EBSD map display longer and thicker grain bands in Figure 2(b)-2 than in Figure 2(a)-2. The grain size of the 105 mm wire after intermediate annealing at 650 C was Fig. 1 (a) A schematic diagram for wire sections measured. RD denotes the radial direction, TD the transverse direction, and the casting axis is parallel to the AD, axial direction. (b) 111 pole figure of copper casting bar of 7-mm diameter measured from cross section. smaller (Figure 2(b)-3) than that at 700 C (Figure 2(a)-3). The microstructure after intermediate annealing (Figure 2(a)-3 or 2(b)-3) is different from that after final annealing (Figures 2(a)-6 or 2(b)-6). Figure 2(a)-3 shows a mixed microstructure with evidence of subgrain growth and discontinuous recrystallization, while Figure 2(a)-6 displays a microstructure resulting from grain growth with equiaxed grains. We will discuss microstructure evolution further based on isothermal annealing experiments at lower temperatures of 300 C and 400 C. During isothermal annealing, METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 37A, OCTOBER 2006 3087

Fig. 2 Inverse pole figure maps (grain ID angle, 15 deg) of copper wires during drawing and annealing. Each map is notified by the reduction area and its process step number. Steps from 2 to 6 show the full sections of wires but 1000 mm shows half sections. IA: intermediate annealing, and FA: final annealing. (a) 7f/ 1000 mm (1) / 105 mm (2) / IA (3) / 48 mm (4) / 25 mm (5) / FA (6). (b) 7f/ 1000 mm / IA (1) / 105 mm (2) / IA (3) / 48 mm (4) / 25 mm (5) / FA (6). Fig. 5 Inverse pole figure maps (grain ID angle, 15 deg) of copper wire during isothermal annealing at (a) 300 C and (b) 400 C. Examples of HAGBs for continuous recrystallization are specified with white rectangles in (a), 10 min and 60 min. Wires are 25-mm diameter. 3088 VOLUME 37A, OCTOBER 2006 METALLURGICAL AND MATERIALS TRANSACTIONS A

it became clear that some grains in the,100. and,111. fibers grow at the beginning of recrystallization. In particular, the,100. component grows faster than the,111.. During grain growth at higher temperatures, however, the,111. grains grow faster than the,100.. The volume fractions of,111.,,100., and other components were calculated from EBSD data in Figure 3. Over the entire cross section, similar volume fractions of the,111. and,100. are observed with increasing RA; other components decrease. Other components include all grains except those within 15 deg of having,111. or,100. fibers parallel to the wire axis. These orientations are usually concentrated in the near-surface region rather than in the center region. This is a consequence of the inhomogeneous deformation that arises from the friction between the die and copper wire. When we define the relative radial position, or ratio of s/s o, where s is the distance from the wire center and s o is the wire radius, the center is equivalent to s 5 0 and the surface is s 5 1, respectively. Overall, the surface region in the study corresponds to the ratio between 0.7 # s # 1.0, and the center region the ratio between 0 # s # 0.3. In the center region, the deformation is Fig. 3 Variations in volume fraction of copper wire as a function of process step. IA: intermediate annealing, and FA: final annealing. The numbers in the figures are the link to the processes of wire fabrication shown in Fig. 2: (a) whole section, (b) surface region, (c) center region for Fig. 2(a), (d) whole section, (e) surface region, and (f) center region for Fig. 2(b). Fig. 4 Variations in grain size and aspect ratio of copper wires as a function of process step. The process step number is the link to the processes in Fig. 2. (a) Grain size and aspect ratio for Fig. 2(a). (b) Grain size and aspect ratio for Fig. 2(b). METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 37A, OCTOBER 2006 3089

Fig. 6 Variations in grain properties during annealing at 300 C. The transitions from the stage of subgrain growth via recrystallization to grain growth are shown by vertical dashed lines. GAM and SOS are given in degrees: (a) grain size, (b) aspect ratio, (c) volume fraction, (d) GAM, and (e) SOS. more homogeneous than under the surface and, therefore, the,111. component is stronger. As RA increases, the,111. component strengthens, whereas the,100. component decreases or remains the same. Intermediate annealing changes the ratio of the,100. and,111. components. Usually, the,100. component increases and the,111. decreases. After final annealing of the 25-mm wires, the ratio of volume fractions of the,100. and,111. goes to unity. Variations in grain size and aspect ratio with process steps are plotted in Figure 4. Increasing drawing deformation results in increasing aspect ratio and decreasing grain size, whereas annealing decreases the aspect ratio and increases the grain size for both cases. B. Isothermal Annealing after Drawing Copper bonding wires fabricated from the process of Figure 2(a), or case 1 without final annealing for the 1-mmdiameter wire, were used to carry out the isothermal annealing experiment for the investigation on recrystallization and grain growth. After the drawing processes in Figure 2, the total RA of copper wire from a cast bar to a 25-mm wire is 99.999 pct. The net RA is 94.3 pct if one accounts for the effect of intermediate annealing. Figure 5 shows EBSD maps of copper wires during isothermal annealing at 300 C and 400 C for 1 minute, 10 minutes, 60 minutes, and 1 day. The EBSD map after annealing at 300 C for 10 minutes shows that most grains still have elongated shapes. After annealing at 300 C for 60 minutes, some elongated grains have both widened and lengthened and some equiaxed grains are moving into the elongated grains. After annealing at 300 C for 1 day, all elongated grains have been eliminated. A microstructure with both elongated and equiaxed grains (as is found after annealing at 300 C for 60 minutes) is also found after annealing at 400 C for 1 minute. This is as expected 3090 VOLUME 37A, OCTOBER 2006 METALLURGICAL AND MATERIALS TRANSACTIONS A

Fig. 7 Variations in grain properties during annealing at 400 C. The transitions from the stage of subgrain growth with recrystallization to grain growth are shown by vertical dashed lines. GAM and SOS are given in degrees: (a) grain size, (b) aspect ratio, (c) volume fraction, (d) GAM, and (e) SOS. because higher temperature accelerates the annealing process. The EBSD maps in Figure 5 imply that there are three stages of microstructural evolution in the copper bonding wires during annealing. In the first stage, apparent for annealing at 300 C, subgrain growth and continuous recrystallization are dominant for 10 minutes and the grain shape remains elongated. In the second stage, equiaxed grains from discontinuous recrystallization grow into the elongated deformed structure (after annealing for 60 minutes). The EBSD map for 1 day at 300 C shows only equiaxed grains and grain growth occurs thereafter as the third stage. This will be discussed further in Section IV. It is also notable that the second stage of discontinuous recrystallization clearly nucleates in the subsurface region and spreads to the rest of the cross section, as observed by Waryoba et al. [29] The as-deformed copper wire of 25-mm diameter has both,111. and,100. fiber components. During annealing, texture and microstructure change. Figures 6 and 7 show variations in grain size, aspect ratio, GAM, SOS, and volume fractions of fiber components during annealing at 300 C and 400 C, respectively. Based on the microstructural evolution displayed in Figure 5, three stages of subgrain growth, recrystallization, and grain growth are specified in Figures 6 and 7. Grain size increases with annealing time and,100. grains are usually larger than the others in Figures 6(a) and 7(a). The only exception is for the longest annealing (1 day) at 400 C, Figure 7(a), which is a consequence of extended grain growth. It is clear that,100. grains grow faster than others during recrystallization at both 300 C and 400 C. Aspect ratios during annealing at 300 C (Figure 6(b)) increase slightly after 1 minute and then decrease. This behavior was also observed in gold bonding wires and is related to subgrain growth along the drawing axis. [9] Grains along the drawing axis have predominantly pure twist grain METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 37A, OCTOBER 2006 3091

Fig. 8 Misorientation distributions of copper wires: (a) 300 C and (b) 400 C. boundaries. The grains with twist boundaries share a common axis, i.e.,,111. or,100.. This suggests that the twist boundaries have higher mobility than the boundaries between the elongated grains, at least during the subgrain coarsening stage. The peak in aspect ratio at 400 C was probably missed because of faster recrystallization than at 300 C (Figure 7(b)). Variations in the volume fraction at 300 C show that the,111. fiber decreases whereas the,100. increases up to 60 minutes (Figure 6(c)). However the,111. component increases again after 60 minutes at the expense of the,100. component. Typically,,100. is known as the recrystallization texture component, and it grows during recrystallization in order to lower the stored energy of deformation. [14] It is necessary to distinguish continuous recrystallization from discontinuous recrystallization. The,100. component increases mainly during continuous recrystallization. During discontinuous recrystallization, however, the elongated,100. grains decrease in frequency. The,111. component is favored by grain growth (at higher temperatures than those required for recrystallization). During grain growth, the grain boundary mobility is the major factor that determines the texture and microstructure evolution. Similarly, during annealing at 400 C for 1 minute (Figure 7(c)), the microstructure is already in the grain growth stage so,111. increases while,100. decreases. Misorientation distributions show that,100. oriented grains have a lower value of GAM or SOS than,111. grains in the cold-drawn data. These trends seem to continue in Figures 6(d) and (e) for 300 C and Figures 7(d) and (e) for 400 C during recrystallization. Figure 8 shows the misorientation angle distributions for various cases, based on a 5-deg cut-off angle. The as-drawn wire has a high frequency of both lower misorientation angles than 15 deg and higher misorientation angles than 50 deg. During annealing at 300 C and400 C, the frequency of LAGBs decreases and a peak appears around 60 deg, which is mainly from S3 boundaries. Annealing at 300 C results in a stronger S3 peakthan that at 400 C. IV. DISCUSSION A similar study of recrystallization and grain growth in gold bonding wires was reported in Reference 9. Both gold and copper are fcc metals with low-medium stacking fault energy and the materials in both the former and the current studies have parts per million (ppm) level dopants. Therefore, it is reasonable to expect similar behavior in the two different materials. Comparison of isothermal annealing experiments for gold and copper bonding wires with 25-mm diameter reveals similar microstructural evolution, as anticipated. The volume fraction ratio of,100. to,111. fiber for cold-drawn copper wires is 0.34:0.32 (Figures 6 and 7) and 0.1:0.8 in gold wires in Reference 9. The copper wire has more,100. component than the gold wire during drawing. The,100. fraction increases in the beginning of the annealing process in both copper and gold. Coalescence along the wire axis between,100. oriented grains occurs by the elimination of twist boundaries. The same process occurs in the,111. component. Grain boundary migration takes place between,111. and,100. oriented grains, which is biased toward movement into (i.e., consumption of) the,111. component. During the first stage of annealing, the twist grain boundaries move faster than others, which results in a maximum of the aspect ratio of grains (Figures 6(b) and 7(b)). These processes are found in the beginning of annealing, which is a subgrain coarsening phase. Typical microstructure and texture of the copper and gold wires are shown in Figure 9. The initially elongated grains in a copper wire are replaced by a mixture of elongated and equiaxed grains after annealing at 400 C for 1 minute (Figure 9(b)). The elongated grains are wider than those of the as-drawn wires. Equiaxed,111. grains grow into the elongated,100. grains. As pointed out previously, the mixture of elongated and newly grown grains shows that both subgrain growth (or continuous recrystallization) and discontinuous recrystallization have taken place. Unlike copper wires, Figure 9(e), however, shows 3092 VOLUME 37A, OCTOBER 2006 METALLURGICAL AND MATERIALS TRANSACTIONS A

Fig. 9 Comparison of typical microstructure and texture of 25-mm-diameter copper and gold wires. CSLs are shown in (c) and (f) with yellow lines for S3, green for S5, and blue for S7. (a) EBSD map for as drawn, (b) and (c) EBSD map and CSL for annealed copper wire, (d) EBSD map for as drawn, and (e) and (f) EBSD map and CSL for annealed gold wire in Fig. 8 of Ref. 9. Fig. 13 Schematic diagrams for HAGBs and microstructure during stage 2. The IPF map is the microstructure image from annealing at 400 C for 1 min. (a) Diagram of HAGBs for discontinuous recrystallization. (b) IPF map for discontinuous recrystallization. that coalescence continues to occur along the wire axis in both the,111. and,100. components in gold. Therefore, most,111. and,100. grains retain elongated shapes. The slower kinetics observed in the gold suggests that the overall mobility of grain boundaries is lower in gold than in copper at the same temperature. This may be a consequence of different impurity (dopant) levels and types in the two materials. Most boundaries are either,111. or,100. tilt boundaries after coalescence has eliminated most twist METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 37A, OCTOBER 2006 3093

Fig. 10 CSL distributions as a function of annealing time and temperature along the vertical and horizontal directions in the bonding wire axis. (a) Asdrawn, (b) 1 min, (c) 60 min, and (d) 1 day at 300 C for copper wire; and (e) as-drawn, (f) 1 min, (g) 60 min, and (h) 1 day at 300 C for gold wire. boundaries. S3, 7, 13b, 21a, 31a types and near-coincident site lattice boundaries (CSLs) with,111. misorientation axes are found frequently in the,111. fiber regions. The most frequent CSLs in the,100. regions are S5, 13a, 17a, 25a, and 29a with,100. axes, as expected. Boundaries between grains with the same fiber show smaller misorientation angles on average than boundaries between grains with different fibers, again as expected. Almost all of the boundaries between the,111. and,100. components have misorientation angles above 40 deg so those CSL types are mainly S3, 9, 11, 17b, 25b, 31b, and 33c. Figure 10 shows the distributions of CSL boundaries for both copper and gold. Separate plots of boundaries with normals parallel (horizontal) to and perpendicular (vertical) to the 3094 VOLUME 37A, OCTOBER 2006 METALLURGICAL AND MATERIALS TRANSACTIONS A

Fig. 11 Variations in aspect ratio with annealing time and temperature: (a) gold wire and (b) copper wire. Fig. 12 Schematic diagrams for patterns of grain boundary migration during stage 1. LAGB: low-angle grain boundary, and HABG: high-angle grain boundary. (a) Subgrain growth between the same fibers along the wire axis, (b) subgrain growth between,100. fibers along the transverse direction, (c) subgrain growth between,111. fibers along the transverse direction, and (d) continuous recrystallization with growth of a,100. grain into a neighboring,111. grain. wire axis are shown. The copper wires exhibit almost negligible frequencies of CSL boundaries other than S3, whereas the gold wires show strong peaks for boundaries with,111. misorientation axes, i.e., S3, S7, S13b, and S21a. These peaks decrease as annealing time passes (Figures 10(a) through (d)). This trend is more obvious in the gold wire (Figures 10(e) through (h)). The change in aspect ratio is obviously related to grain shape change during subgrain growth, recrystallization, and grain growth. Both copper and gold show a slight increase in aspect ratio at short times at 300 C (Figure 11). Although the aspect ratio depends to some extent on the choice of the cut-off angle used for grain identification, the trend of increase followed by decrease during annealing at 300 C is consistently present. As the cutoff angle used for grain identification decreases from 5 to 1 deg, the aspect ratio also decreases. As pointed out in Reference 9, LAGBs are located along the longitudinal direction, so employing a smaller grain cut-off angle results in a smaller aspect ratio. Most of the low-angle boundaries under 5 deg in the as-drawn wires consist of pure twist boundaries. These are a source of subgrain growth from dislocation tangles at the beginning of recovery during annealing. The presence of dislocation tangles was also confirmed by transmission electron microscopy in the gold wires. [9] The increase of the aspect ratio is a consequence of subgrain growth or coalescence along the longitudinal direction. Figures 6 and 7 show the variations in grain size, aspect ratio, and the volume fractions with annealing time in the copper wire, which reveal results of interest. Both,111. and,100. grain sizes of copper in 300 C and 400 C increase during annealing (recrystallization and grain growth processes). The volume fraction of,100. and,111., however, moves in the opposite directions. The same trend was found in the gold wire. [9] At 300 C,,111. and,100. grains both grow up to 24 hours (Figure 6) and the volume fraction of the,111. decreases monotonically. After 24 hours, the,111. fraction increases again. The volume fraction of the,100. component shows exactly the opposite behavior. At 400 C, the grain size of,111. and,100. grains increases during annealing (Figure 7). By contrast to the lower temperature, however, the volume fraction of the,111. increases and that of the,100. decreases after 1 minute of annealing. This happens because recrystallization is complete after a METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 37A, OCTOBER 2006 3095

short time and the,111. fraction increases during subsequent grain growth. Figure 12 shows diagrams for characteristic patterns of grain boundary migration between the various combinations of,111. and,100. oriented grains. At short annealing times, subgrain growth occurs that eliminates LAGBs between grains belonging to the same fiber. Continuous recrystallization involving migration of high-angle grain boundaries (HAGBs) between different fibers also occurs. This is defined as stage 1 during annealing. Topologically, most grains still have elongated grain shapes. Subgrain growth occurs between the same grains in the same fiber along both the longitudinal and transverse directions. Migration of LAGBs causes the grain size to increase during annealing but does not change the volume fraction of either the,111. or the,100. component. The increase in aspect ratio during annealing is a consequence of LAGB migration along the longitudinal direction. Migration of HAGBs is mainly responsible for changes in the volume fraction of,111. and,100. grains. Continuous recrystallization shown in Figure 12(d) contributes to increases in the,100. fiber and decreases in the,111.. During continuous recrystallization, the overall elongated grain shapes are remained. Examples of HAGBs for continuous recrystallization are specified with white rectangles in Figure 5(a), 10 and 60 min. Another type of HAGBs is measured after 60 minutes at 300 C or 1 minute at 400 C in copper wires. Newly grown and equiaxed,111. grains come into contact with elongated,100. grains. Figure 13 shows the schematic diagram for this type of HAGBs. The motion of these boundaries results in discontinuous recrystallization and the fraction of HAGB decreases except for S3. This is defined as stage 2 during annealing, during which most of the elongated grains disappear and equiaxed grains grow (Figures 5(a) from 60 minutes to 1 day; and Figures 5(b) from1minuteto10minutes).afterstage 2, normalgrain growth of equiaxed grains occurs, which is defined as stage 3. The volume fraction of,100. increases during stages 1 and 2, whereas,111. increases during stage 3. In fact, it is difficult to separate these steps clearly and the microstructural evolution in copper and gold wires shows all the processes of subgrain growth/continuous recrystallization, discontinuous recrystallization, and grain growth at various points during annealing. V. CONCLUSIONS Microstructure and texture in copper bonding wires were characterized by EBSD and compared with previous work in gold bonding wires during the drawing and annealing processes. 1. During drawing of copper and gold, the shear components are located mainly in the near-surface region. The,111. component develops more than the,100. in gold wires, whereas the,111. component has a similar volume fraction to,100. in copper wires. 2. Isothermal annealing at 300 C and 400 C shows that,100. grains grow faster than,111. at the beginning of annealing for both gold and copper. The,111. component grows faster than,100. in the final grain growth stage, however. 3. The aspect ratio of grain shape at 300 C increases slightly at short annealing times for both gold and copper. This is the result of subgrain growth along the longitudinal direction. 4. Orientation spread, measured by GAM and SOS, in the,100. component is lower than in the,111. and other components in the as-drawn state. This provides a driving force (based on differences in stored energy) for the,100. component to grow into the other texture components in both gold and copper during recrystallization. 5. During annealing, three stages can be defined for both copper and gold wires. Stage 1 is defined by the persistence of elongated grain shapes and increasing grain size in both,111. and,100.. The volume fraction of,100., increases whereas,111. decreases. Stage 2 is defined by the appearance of a mixture of newly recrystallized grains and elongated ones. Stage 3 is grain growth. Only equiaxed grains are found. The,111. component grows at the expense of the,100. at this stage. ACKNOWLEDGMENTS This research is supported by the BK21 project of the Ministry of Education and Human Resources Development in South Korea and MKE Electron. Partial support of the Mesoscale Interface Mapping Project at Carnegie Mellon University under NSF Grant No. DMR-0520425 is acknowledged. REFERENCES 1. A.T. English and G.Y. Chin: Acta Metall., 1965, vol. 13, pp. 1013-16. 2. G. Linßen, H.D. Mengelberg, and H.P. Stüwe: Z. Metallkd., 1964, vol. 55, pp. 600-04. 3. E. Aernouldt, I. Kokubo, and H.P. Stüwe: Z. Metallkd., 1966, vol. 57, pp. 216-20. 4. J.J. Heizmann, C. Laruelle, A. 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