Recrystallization and Grain Growth of Cold-Drawn Gold Bonding Wire

Transcription

1 Recrystallization and Grain Growth of Cold-Drawn Gold Bonding Wire J.-H. CHO, J.-S. CHO, J.-T. MOON, J. LEE, Y.H. CHO, Y.W. KIM, A.D. ROLLETT, and K.H. OH Recrystallization and grain growth of gold bonding wire have been investigated with electron backscatter diffraction (EBSD). The bonding wires were wire-drawn to an equivalent strain greater than 11.4 with final diameter between 25 and 30 mm. Annealing treatments were carried out in a salt bath at 300 C, and 400 C for 1, 10, 60 minutes, and 1 day. The textures of the drawn gold wires contain major 111, minor 100, and small fractions of complex fiber components. The 100 oriented regions are located in the center and surface of the wire, and the complex fiber components are located near the surface. The 111 oriented regions occur throughout the wire. Maps of the local Taylor factor can be used to distinguish the 111 and 100 regions. The 111 oriented grains have large Taylor factors and might be expected to have higher stored energy as a result of plastic deformation compared to the 100 regions. Both 111 and 100 grains grow during annealing. In particular, 100 grains in the surface and the center part grow into the 111 regions at 300 C and 400 C. Large misorientations (angles 40 deg) are present between the 111 and 100 regions, which means that the boundaries between them are likely to have high mobility. Grain average misorientation (GAM) is greater in the 111 than in the 100 regions. It appears that the stored energy, as indicated by geometrically necessary dislocation content in the subgrain structure, is larger in the 111 than in the 100 regions. I. INTRODUCTION FINE wires of pure Au, Cu, or Al, are used for interconnection in semiconductor packaging. As packaging technology continues to advance, improved properties in bonding wire are needed. In particular, the ball shape, breaking load, elongation and the homogeneity of microtexture and microstructure are important. These characteristics are related to the purity of the original materials, the drawing process and the annealing process. The homogeneity of the microtexture and the microstructure can affect the swing or looping of bonded wires and theses are major factors for failure of packaging. Most bonding wires also undergo a final annealing before the packaging process. During final annealing, breaking load decreases and elongation increases. Therefore it is necessary to optimize annealing processes in order to obtain optimum bonding wire. High purity gold ( %wt Au) is too soft and unstable for obtaining good properties for bonding when it is drawn and annealed. Generally, the annealing and recrystallization temperature for pure gold is in the range of C and it has been reported that highly deformed pure gold will show recovery and recrystallization at room temperature [1]. Therefore bonding wire commonly has various dopants at J.-H. CHO, Researcher, Y.W. KIM, Research Associate Professor, and K.H. OH, Professor, are with the School of Materials Science and Engineering, Seoul National University, Seoul , Korea. Contact kyuhwan@shu.ac.kr. J.-S. CHO, Senior Researcher, J.-T. MOON, Principal Researcher, and J. LEE, R&D Head, are with MKE Electronics, Kyunggi-do , Korea. Y.W. CHO, Principal Researcher, is with the Nano-materials Research Center, Materials Science and Technology Division, KIST, Seoul , Korea. A.D. ROLLETT, Professor, is with the Materials Science and Engineering Department, Carnegie Mellon University, Pittsburgh, PA Manuscript submitted January 31, parts per million (ppm) level in order to control annealing response and to obtain better thermal and mechanical properties. Impurities, even at these low levels, are important for controlling the final mechanical properties and microstructures of gold wire by raising the recrystallization temperature and preventing grain growth. [2,3] Such small concentrations of impurities are known to strongly affect the migration rate of grain boundaries in many materials. [4,5] Recrystallization, recovery and grain growth all occur during annealing, and they affect the microstructures, microtextures and mechanical properties of gold wires. [6-9] The evolution of textures during wire drawing has been investigated by many researchers. The wire drawing textures of fcc metals typically have 111 and 100 fiber texture components. The textures of aluminum, copper, and brass wires have been investigated for cyclic symmetry. [10] The texture of drawn silver wires have radial symmetry, which is related to twinning. [11] Recrystallized grains are formed near the surface due to frictional heating in the die. Heizmann et al. have reported that the strength of the cyclic texture increases as the die angle increases. [12] Taylor s theoretical analysis shows that all grains rotate so that either a 111 or 100 crystal direction is aligned with the extension axis, depending on which direction the extension axis is closest to at zero strain. The crystal axes of grains near 101 will tend to rotate toward the 111 or the 100 axis. [13] English et al. has shown that the ratio of the 111 and 100 fiber texture components varies depending on the stacking fault energy. [14] Low stacking fault energy metals, i.e., silver, have stronger 100 components than 111. High and intermediate stacking fault energy metals, however, such as aluminum or copper, exhibit a stronger 111 than 100. Work on wire drawing textures in fcc metals, i.e., silver, gold, copper, aluminum, and brass, has also shown that the final texture components are 111 and 100. [15,16] METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 34A, MAY

2 Montesin and Heizmann have reported an X-ray diffraction procedure for fine wires that included a diffraction volume correction. [17] Rajan and Petkie measured wire textures with electron backscatter diffraction (EBSD) and displayed the results with Rodrigues Frank maps in addition to inverse pole figures and standard pole figures. [18] The presence of an inhomogeneous distribution of twins and twinning reactions in copper wire was characterized, and it was suggested that the variations in the mesotexture could contribute to mechanical anisotropy. Recrystallization textures in drawn wires are also 111 and 100 fibers, which are similar to the deformation textures. The ratio of the 111 to the 100 varies with the annealing time and temperature. This suggests that the growth rates of 111 and 100 are different and that they are growing and competing against each other. It has been reported that copper wire develops 100 or 112 texture components in low-temperature annealing, but at high temperature, the major texture components are a mixture of 111 and 112 fiber textures. [19] During annealing, grain growth occurs as a result of grainboundary migration. Grain boundaries adopt curvatures based on local equilibrium at triple junctions, which are described by Herring s equations. [20,21,22] These curvatures lead to grainboundary migration. The grain-boundary migration rates depend on the grain-boundary energy and mobility, which in turn are a sensitive function of the grain-boundary structure. Second-phase particles result in resistance to migration with eventual pinning of the grain structure. Measurement of the misorientation distribution function (MDF) provides some information on grain-boundary types, grainboundary energy, and, thus, annealing characteristics. A knowledge of the thermodynamic and kinetic properties of grain boundaries according to crystallographic type will be useful for materials optimizations in the context of annealing. Up to now, the research on gold bonding wires has focused on their mechanical properties and recrystallization behavior in the heat-affected zone (HAZ) during bonding. For good bonding wire, it is necessary to understand the alloy design, optimized drawing process, and annealing process together. In this research, the microtextures and microstructures of gold bonding wires during drawing and annealing are investigated with high-resolution electron backscatter diffraction (HR-EBSD). In order to understand the grain-boundary characteristics, the MDF and the frequency of coincident site lattice (CSL) boundaries are also calculated from EBSD data. II. EXPERIMENTAL A. Materials and Sample Preparation The purity of gold wire used in this research is more than pct and it has some (intentional) dopants, such as Ca and Be that total less than 50 ppm by weight. Even at Parts Per Million levels, these dopants strongly affect (decrease) grain-boundary mobility and, hence, increase the recrystallization temperature. A typical recrystallization temperature of this gold is 320 C, based on isothermal annealing test after rod rolling a cast gold bar to an area reduction of 85 pct. The original cast gold bar was drawn through a series of diamond dies to a von Mises equivalent strain of Each die has less than 10 pct reduction in area in order to achieve homogeneous deformation. The EBSD measurements were performed on gold wires with diameters of 25 and 30 mm. For statistical reliability of the EBSD data, at least three wires were measured for the cold drawn and each of the annealed states. The number of grains measured and analyzed was approximately 5000 for the cold drawn wires and 500 for the annealed wires. Given the small diameter of the bonding wires, EBSD was more convenient and reliable than X-ray diffraction, as noted by Montesin and Heizmann. [17] Isothermal annealing for wires was carried out for 1 minute, 10 minutes, 60 minutes, and 24 hours at 300 C and 400 C. B. EBSD Measurement The bonding wire was mounted in epoxy and then sectioned and polished. The polished specimens were cleaned with ion milling. HR-EBSD (JEOL* 6500F with INCA/OXFORD *JEOL is a trademark of Japan Electron Optics Ltd., Tokyo. EBSD system) was used for measurement and the data analysis was made by Reprocessing of EBSD Data in SNU (REDS). [23] The operating voltage was 20 kv and the probe current was 4 na. A rectangular grid was used and the pixel spacing was mm. The EBSD maps were measured for transverse and longitudinal sections. The orientations maps were used for texture representations, and the misorientation distribution function (MDF) was used to characterize the grain-boundary characteristics. Taylor factor maps are also shown because they distinguish the 111 and 100 fiber texture components. The Taylor factor is calculated based on the standard slip systems for fcc metals and velocity gradient appropriate to uniaxial extension. [24,25] The {111} 110 slip systems are assumed and the velocity gradient, ij, is as follows: ij Image quality or pattern quality is the term given to describe the quality of an electron backscatter diffraction pattern (EBSP). Many factors control the quality of the EBSP, which can be assigned a numerical value. This pattern quality value is derived from the Hough transform of each diffraction pattern. [26] In order to calculate the grain size, the number of data points or pixels in the grain is calculated. Using the known pixel step size and numbers, the grain area is calculated. The most convenient measure of grain size from grain area is the equivalent circle diameter (ECD) or equivalent grain size, which is the diameter of a circle having the same area. [27] Other measures of grain size are available but not used in this work. C. Statistical Analysis of Microstructures The fluctuations or variations of material microstructures can be described by so-called second-order characteristics such as the variance of the volume of a microstructural component or phase. When the mean or the first moment, E, of the volume, V, of a component Ξ restricted to a spatial win- [1] 1114 VOLUME 34A, MAY 2003 METALLURGICAL AND MATERIALS TRANSACTIONS A

3 dow W is given by EV (Ξ W), then the variance, var, of the volume of Ξ W is var V (Ξ W) EV 2 (Ξ W) [EV (Ξ W)] 2 where EV 2 (Ξ W) is the second moment of volume of the Ξ phase. If f(x) and f(y) are the probabilities of random variables, x and y, the covariance of the pair ( f(x), f(y)) is given: cov ( f(x), f(y)) E[( f(x) Ef(x))( f(y) Ef(y)) The normalized covariance function is called the correlation function, r f(x)f(y), and takes values between 1 and 1. cov( f(x), f(y)) r f(x)f(y) [4] s f (x) s f(y) where s f (x) and s f (y) are the standard deviations of f(x) and f(y), respectively. A value of 1.0 or 1.0 indicates perfect linear correlation between f(x) and f(y), whereas a value of zero indicates absence of correlation. [28] Here, f(x) and f(y) are the Taylor factor and pattern quality at positions x and y, respectively. This correlation function was used to check if the 100 grains of the cold drawn wire exhibited a higher image quality than 111 grains. D. Average Lattice Orientation, Grain Orientation Spread, and Grain Average Misorientation The average orientation can be calculated for each grain, and it is useful for characterization of grain substructure. Recently, Barton and Dawson defined an average orientation based on misorientation angles, which leads to a nonlinear least-squares problem that can be solved numerically (Appendix). [29,30,31] In addition to average orientation, grain orientation spread (GOS) and grain average misorientation (GAM) can be calculated. [24] Considering P i as an orientation at a point (x i ), the GOS in a grain can be calculated with misorientation angles, which are given for two arbitrarily chosen orientations. n 1 a n a i 1 j i 1 min c acos e trace ((P i P 1 j ) S) 1 fd 2 where S is the symmetry operator belonging to the appropriate crystal class concerned [32,33] and the subscripts in rotation P refer only to position. The term 2 C n m is the combination that selects a subset containing two orientations among a given set with n m orientations. The GAM is determined in the same way as the GOS, but it includes only the first nearest neighbor orientations within a grain in the average in contrast to GOS. Therefore, it is a more local measure of orientation spread. Both GAM and GOS are size dependent and they increase as the grain sizes increase. III. 2C n m RESULTS A. Transverse Section of as-drawn Wire (30 mm) In order to analyze the fiber texture of the drawn wire, both transverse and longitudinal transverse sections of wires were investigated with EBSD. The main fiber components observed during drawing are 111 //ND and 100 //ND. These [2] [3] [5] two texture components are known as the typical fibers of fcc wires. A set of EBSD maps of the as-drawn wire is shown in Figure 1. The color index for the orientation maps is shown in Figure 1f. Each grain in the wire can be partitioned into two types by their Taylor factors (Figure 1) or by their crystallographic orientation, i.e., 111 or 100 //ND (Figure 1(a)). The partitioning of orientations is based on the misorientation angle, 15 deg, between grains. The average Taylor factor of cold drawn wire was calculated using the standard 12 slip systems for fcc metals, and its value was found to be about Regions with a Taylor factor lower than 2.87 are predominantly 100 oriented, whereas regions with a Taylor factor greater than 2.87 are predominantly 111. As expected, the images partitioned by orientation or by Taylor factor are very similar to each other. Most of the 100 or low Taylor factor regions are located in the center and some on the surface regions of the wire, while 111 fibers or high Taylor factor regions are located throughout the wire. The 100 oriented grains in the surface regions are not axisymmetric. The region between the center and surface regions contains complex orientations, which deviate from 111 and 100 by more than 15 deg. Figure 1(c) shows this deviation by coloring only those grains that lie more than 15 deg from either 100 or 111. Figure 1(d) shows the overall structure of the gold wire after the drawing process. Most of the grains show orientations parallel to 111, whereas the center and some parts of the surface are parallel to 100. The complex regions are located between the center and the surface. These multilayer structures of wire are related to shear deformation and original microstructures. The fcc metals such as gold typically exhibit a majority of 111 and a minority of 100 fiber. The 100 fiber in the center is related to the microstructure and texture of the initial gold bar, which has mainly large 100 grains. The image quality map of the transverse section of the cold drawn wire is shown in Figure 1(e) and suggests that 100 regions are associated with high image quality. The 100 region on the surface in Figure 1(e) shows a higher pattern quality than other regions. In order to quantify this observation, a correlation function was calculated with the two variables, orientation (or Taylor factor) and image quality. This approach is based on the fact that high Taylor factor regions have high stored energy and exhibit a low pattern quality, whereas the low Taylor factor regions have low stored energy and higher pattern quality. [24] Using the orientations, the Taylor factor was calculated, and then the image quality of the orientations was combined, as shown in Eq. [4]. The resulting value was 0.22, which is a mild negative correlation. This suggests that high Taylor factor regions have low pattern quality and low Taylor factor regions have high pattern quality in keeping with the qualitative observation made previously. The 100 oriented material on the surface is likely to be a consequence of shear deformation or dynamic recrystallization. [11] The 100 grains in the center part are related to original cast bar and they are also highly deformed regions as are the 111 grains. Discrete inverse pole figure maps for the pixels partitioned by either their orientation or Taylor factor in Figure 1 are shown in Figures 2(a) and. The complex regions of cold-drawn wire are shown in Figure 2(c). The drawn wire apparently has major 111 and minor 100 texture components. The Taylor factor can separate the 111 and 100 regions successfully, as shown in Figure 2. METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 34A, MAY

4 Fig. 6 Orientation maps of transverse sections of 25-mm gold wires. Misorientation angle, 5 deg is used for identifying the grains: (a) 1min, 10 min, (c) 60 min and (d ) 1 day at 300 C; and (e) 1 min, ( f) 10 min, (g) 60 min, and (h) 1 day at 400 C. Fig. 1 Orientation maps for a cold-drawn gold bonding wire (30 mm). The EBSD data are separated into 具111典 具100典 and 具high典 具low典 Taylor factor regions. Their differences are complex regions mainly: (a) orientation image maps for 具111典 具100典 regions, orientation image maps for 具high典 具low典 Taylor factor regions, (c) complex regions, (d ) schematic plot for structure of gold wire, (e) image quality, and ( f ) orientation color key. Fig. 8 Orientation maps of 25-mm gold wires after isothermal annealing. Misorientation angle, 5 deg is used for identifying the grains. Circles show islands with 3 boundaries: (a) 1 min, 10 min, (c) 60 min and (d ) 1 day at 300 C; and (e) 1 min, ( f ) 10 min, (g) 60 min and (h) 1 day at 400 C VOLUME 34A, MAY 2003 METALLURGICAL AND MATERIALS TRANSACTIONS A

5 (a) (c) (a) (c) Fig. 3 Misorientation angle/axis distribution of the drawn wire. These data come from crystallographic regions in Fig. 1(a): (a) 111 regions, 100 regions, and (c) all part. B. Misorientations in the as-drawn Wire Misorientation distributions in Rodrigues Frank space based on the crystallographic partitioning of the as-drawn wire are given in Figure 3. These maps show that the misorientation distributions of 111 fibers, as expected, are concentrated on 111 misorientation axes and their Rodrigues vector components (R1, R2, R3) take values from (0, 0, 0) to a 1. The latter Rodrigues vector is equivalent to a 3, 1 3, 1 3 b 60 deg 111 misorientation angle/axis pair. The length of the Rodrigues vector is equal to the tangent of half the misorientation angle; therefore, 111 regions have a large range of misorientation angles, i.e., from 0 to 60 deg, around the 111 misorientation axis. [32,33,34] By contrast, the 100 regions are concentrated on 100 misorientation axes and their Rodrigues vectors are located between (0, 0, 0) and ( 22 1, 0, 0). The latter Rodrigues vector is equivalent to a 45 deg 100 misorientation. Grain boundaries in the 100 regions have smaller misorientation angles than in the 111 regions. Some boundaries in the 111 and complex regions have 110 misorientation axes (Figure 3(c)). METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 34A, MAY

6 (a) Fig. 4 Tilted grains in a drawing wire and projected CSL grain boundaries in the Rodrigues space in the 111 and 100 regions and between them: (a) 100 type tilted grains and Rodrigues vectors of CSLs. (a) Fig. 5 Misorientation angle distribution of initial wire: (a) random case from Mackenzie Handscomb and as-drawn gold wire. Figure 4 shows the drawing deformation and misorientation fundamental zone in Rodrigues space. The wires undergo an axisymmetric uniaxial deformation during drawing such that they have a C symmetry axis aligned with the wire axis (Figure 4(a)). As the grains are elongated during deformation, most of the boundary area has a normal perpendicular to the axis. Thus, most boundaries are either 111 or 100 tilt boundaries. Figure 4 shows a projection of the fundamental zone in Rodrigues space and the location of most of the lowsigma coincident site lattice (CSL) boundary types. In this figure, the 111 axis falls on top of the 110 axis along the hypotenuse of the triangle. The 100 axis projects along the lower, horizontal edge. The CSLs along the 111 axis (Figure 4), with 3, 7, 13b, 21a, and 31a, are found frequently in the 111 fiber regions. The most frequent CSLs in the 100 component are 5, 13a, 17a, 25a, and 29a on the 100 axis. Almost all of the boundaries between the 111 and 100 components have misorientation angles above 40 deg so that the CSLs are mainly 3, 9, 11, 17b, 25b, 31b, and 33c. Before showing the misorientation angle distribution of the drawn wire, the well-known Mackenzie plot for randomly distributed cubic crystals is shown in Figure 5(a). [35,36] The peak in frequency (fraction number) occurs at a misorientation angle near 45 deg and the maximum angle is 62.8 deg; this maximum misorientation for two cubic crystals is found for combinations such as the rotated cube, {100} 011, and Goss, {110} 100 orientations. The misorientation angle distributions of 800 combinations of randomly distributed single crystals are shown with symbols on the same plot. The two distributions are very similar. The misorientation angle distribution for cold-drawn gold wire is shown in Figure 5. Three different distributions are plotted separately based on each of the fiber regions, i.e., 111, 100, and intermediate orientations. All three exhibit nonrandom distributions. Most misorientation angles in the 100 regions are less than 40 deg, whereas the 111 regions exhibit angles up to 60 deg. High misorientations predominate for boundaries between the 111 and 100 regions. The peak in the misorientation distribution for boundaries between the 111 and 100 regions is located 1118 VOLUME 34A, MAY 2003 METALLURGICAL AND MATERIALS TRANSACTIONS A

7 (a) (c) Fig. 7 Aspect ratio, grain size, and volume fraction of gold wire along cross section during isothermal annealing at 300 C and 400 C in Fig. 6: (a) aspect ratio, equivalent grain size, and (c) volume fraction. between 45 and 60 deg. Note that this peak increased to 55 deg from about 45 deg in the random distribution shown in Figure 5(a). The large misorientation angles of boundaries between the 111 and 100 regions mean that these boundaries will tend to have higher energy and, possibly, higher mobility than the average boundary in this system. C. Recrystallization and Grain Growth during Annealing (Transverse and Longitudinal Sections; 25-mm Diameter Wires) The gold wires were characterized by EBSD after isothermal annealing at 300 C and 400 C for 1 minute, 10 minutes, 60 minutes, and 24 hours, and orientation maps are shown in Figures 6 and 8. As in the as-drawn wire in Figure 1, the orientation maps during annealing show that most of the material is aligned with either 111 or 100. During annealing at 300 C (Figures 6(a) through (d)), grain growth occurs in both the 111 and 100 regions. During this grain growth, some 111 grains consume other 111 grains and some 100 grains of the center and surface regions grow into 111 regions. The wire surface is not uniformly covered by 100 grains and so growth of the 100 fiber in the surface is correspondingly nonuniform. After 24 hours, the 100 regions have obviously coarsened. Consequently, the 111 volume fraction decreases and the 100 volume fraction increases during annealing. Isothermal annealing at 400 C (Figures 6(e) through (h)) causes faster growth of 111 and 100 grains than at 300 C, as expected for a thermally activated process. Coarsening occurs in all regions in the wire during annealing. At both annealing temperatures, coarsening of the 100 regions is clear also and the 100 volume fraction increases with annealing time. Figure 7 shows the aspect ratio, equivalent grain size, and volume fraction of 111 and 100 grains in transverse section. The aspect ratio of grain shape in the transverse section (Figure 7(a)), is in the range 1.5 to 2 (grains are elongated along the drawing direction) and annealing time and temperatures have little effect on the aspect ratio. Grain growth occurs in all areas of the wire and is more rapid at 400 C than at 300 C, as expected for thermally activated motion of grain boundaries (Figure 7). The average grain size in the 111 and 100 regions bracket the average grain size. The equivalent grain size increases gradually from 0.7 to 5 mm. The 100 region grows at the expense of the 111 region such that the volume fraction of 100 increases (Figure 7(c)), whereas the 111 volume fraction decreases. The volume fraction of complex orientations, i.e., the balance of the material from partitioning the orientations into 111, 100, and complex fibers, increases at first and then decreases at longer times. Figure 8 shows EBSD orientation maps for longitudinal sections of isothermally annealed gold wire after 1 minute, 10 minutes, 60 minutes, and 24 hours at 300 C and 400 C. METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 34A, MAY

8 Statistically, a longitudinal section contains many more grains than a transverse section. In contrast to the equiaxed shapes observed in the transverse sections, most grains have elongated shapes in the longitudinal sections during annealing. These grain shapes suggest that growth occurs in both the transverse and longitudinal directions. The as-drawn structure in Figure 8 shows 100 oriented regions both in the center and at the periphery, with 111 oriented material occupying most of the volume of the wire. As annealing time accumulates, both 111 and 100 grains grow and 100 grains grow into 111 grains. After 24 hours annealing (Figures 8(d) and (h)), there are islands of small grains within large 100 or 111 grains that are surrounded by 3 boundaries and appear to be stable. Annealing twins with immobile 3 boundaries are often observed in metals with low to moderate stacking fault energies such as gold. The island regions may be stable to further coarsening because their perimeter is a low mobility boundary. Figure 9 shows the aspect ratio, equivalent grain size, and volume fraction of 111 and 100 grains in longitudinal sections. The initial aspect ratio of grains is about 4.5 and it decreases gradually during annealing at 400 C. Considering that grain growth occurs in both the transverse and the longitudinal directions, the decrease in the aspect ratio during grain growth shows that coarsening is fastest in the transverse direction. The wires of 300 C annealing show more interesting results. The aspect ratio for 1 minute at 300 C decreases slightly and it reflects the effects of newly recrystallized grains or subgrain growth from dislocation tangles during recovery or first stage of recrystallization. There is also a slight decrease in the equivalent grain size. As annealing time increases, the aspect ratio for 10 and 60 minutes at 300 C increases again and it shows that most of the 111 grains merge with other 111 grains along the longitudinal direction, i.e., coalescence occurs. After 24 hours, grain growth occurs between grains with large misorientation angles and the aspect ratio drops to about 2. Boundaries between grains of the same fiber component tend to be tilt boundaries when the grain centers are connected by a radius (Figure 4(a)) and, by contrast, twist boundaries when they are lying along the wire axis. Pure tilt boundaries are known to exhibit higher mobility than twist or mixed boundaries. [4,37] Gold wire during annealing at 300 C shows that the mobilities of twist boundaries seem to be higher than tilt boundaries and they move rapidly at the beginning of recrystallization. Initial grains have an elongated shape with aspect ratio 1.5 along transverse and 5.5 along longitudinal. As annealing time increases, it converges to about 2 in both directions. The aspect ratio after 24 hours along the longitudinal direction is similar to that of the transverse section at both 300 C and 400 C. Equivalent grain size increases gradually according with annealing time, and the 100 grains are slightly larger than the 111 grains at 300 C and 400 C. As observed in the (a) (c) Fig. 9 Aspect ratio, grain size, and volume fraction of gold wire along longitudinal section during isothermal annealing at 300 C and 400 C in Fig. 8: (a) aspect ratio, equivalent grain size, and (c), volume fraction VOLUME 34A, MAY 2003 METALLURGICAL AND MATERIALS TRANSACTIONS A

9 transverse sections, longitudinal sections show that 111 and 100 grains at 400 C grow faster than at 300 C and 100 grains seem to grow faster than 111. The larger grain sizes in longitudinal sections than transverse comes from the elongated grain shapes. Volume fraction changes measured in longitudinal sections are similar to the transverse sections. The 100 grains increase and 111 grains decrease as annealing time accumulates. At 300 C, 100 grains grow continuously and the 100 volume fraction is larger than that of 111. At 400 C, individual grains grow to sizes comparable to the wire diameter after 24 hours annealing and the ratio of 111 and 100 approaches 1:1, which means that 111 grains grow again through grain boundary movement. IV. DISCUSSION The fcc metals typically exhibit a mixture of 111 and 100 fiber components, as expected from Taylor s original analysis of the effects of crystallographic dislocation glide on the reorientation of crystals during plastic deformation. The gold wire studied here exhibits this classical combination of texture components and, to first order, annealing only changes the relative volume fractions. In displaying the microstructures, the Taylor factor provides a convenient means of partitioning the material because the 100 oriented grains have the minimum Taylor factor, whereas the 111 oriented grains have the maximum Taylor factor under tensile deformation [38] (Figure 1). The Taylor factor is a measure of the ratio of microscopic shear or glide to macroscopic strain. Large values mean that more slip must take place in order to accommodate the imposed strain and, equivalently, that those grains will bear a higher (macroscopic) flow stress for the same critical resolved shear stress. This suggests that the 111 grains would be expected to have higher stored energies than the 100 grains. This should provide a driving force for 100 grains to grow into the 111 regions. In the cold-drawn wire, the 111 grains exist throughout the wire and they arise from the drawing deformation. The 100 grains are located in the center and the surface of the wire. The presence of the 100 component at the center is most likely inherited from the texture of casting bar, which probably had a columnar grain structure. The presence of 100 at or near the surface is related to friction between the wire and the dies during drawing. Figures 1, 6 and 8 show that 100 grains are distributed over the surface and that they coarsen independently of the 100 grains in the center region during annealing. Figure 10 shows the transmission electron microscopy (TEM) and EBSD image for as-drawn gold wire. The grains are elongated along drawing directions. The TEM image shows that grains have less than 1 mm along the transverse direction and the elongated grains have subgrain boundaries. Comparing the TEM image to the EBSD image maps, a tolerance angle of 5 deg for grain identification resulted in more reasonable grain shapes than a choice of 15 deg. The aspect ratio and equivalent grain size of the as-drawn gold wires were investigated as a function of tolerance angle used for grain identification (Figure 11). Both aspect ratio and equivalent grain size increase sharply as the tolerance Fig. 10 Grain shapes for cold-drawn gold wire. The elongated grains are shown in (a) TEM image, EBSD image (tol 15 deg), and (c) EBSD image (tol 5 deg). Tol: Tolerance angle for grain identification. angle increases to 5 deg. After 5 deg, the equivalent grain size increases slowly and continuously, but the aspect ratio decreases slightly. The variation in equivalent grain size shows that there are many subgrains with less than 5 deg between them. The aspect ratio changes show that most of the subgrains with low misorientation angles are aligned along the longitudinal direction and they appear to be twist boundaries. Most of the tilt boundaries along the transverse direction have a misorientation angle greater than 5 deg. The increase of aspect ratio of the wires under 300 C annealing during 10 and 60 minutes seems to be related to the motion of twist boundaries leading to coalescence of subgrains, as shown in Figure 9(a). After 24 hours, the grains grow in the transverse direction also. It seems that twist boundaries with low misorientation angles have higher mobility than others. After subgrain growth based on motion of twist boundaries, grain growth occurs by motion of tilt boundaries and high-angle grain boundaries between 111 and 100. Figure 12 shows the variations of aspect ratio during annealing as a function of tolerance angle. Grain aspect ratios for 111 and 100 grains decrease after 1 minute at 300 C. The 111 grains exhibit a maximum aspect ratio at 60 minutes and 100 grains have a maximum at 10 minutes. This suggests that coalescence in the longitudinal direction proceeds at different rates in the two fiber components. These trends are independent of tolerance angles used for grain identification. During primary recrystallization, boundaries of nucleated new grains sweep through a deformed structure and remove the dislocations that were stored during the prior plastic METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 34A, MAY

10 (a) Fig. 11 Aspect ratio and equivalent grain size variations of as-drawn gold wires according to grain identification angle or tolerance angle: (a) aspect ratio and equivalent grain size. (a) Fig. 12 Aspect ratio variations during annealing at 300 C according to grain identification angle or tolerance angle: (a) 100 grains and 111 grains. deformation. Higher dislocation densities therefore represent a higher driving force for recrystallization. It is reasonable to suppose that higher Taylor factor orientations such as 111 should contain higher stored energy. There is indeed evidence that the 100 grains grow into the 111 regions at 300 C and 400 C. To set against this view, however, there is little evidence for new grains growing in a deformed structure with the accompanying contrasts in image quality, for example. Instead, it appears that a general coarsening occurs throughout the material, i.e., subgrain growth. Figure 13 shows the orientation image and pattern quality map along the longitudinal direction. High pattern quality regions are assumed to be newly recrystallized or growing grains. The circles marked in Figure 13 show that most of them have elongated shapes and are growing by subgrain coarsening or grain growth. As noted previously, there is also some competition between the two main texture components, 111 and 100. All this suggests that more attention should be paid to the properties of the boundaries involved. In order to make a more accurate estimate of the stored energy of the grains as a function of orientation, GAMs are shown in Figure 14. The presence of dislocations is associated with variations in orientation within each grain. The GAM is the average misorientation (angle) between all neighboring pairs of points in a grain. The slight increase in GAM observed during grain growth is the result of the accumulation of low-angle boundaries within grains. In general, the frequency of high energy and high mobility boundaries decreases during annealing, whereas the frequency of low energy and low mobility boundaries such as low-angle boundaries and twin boundaries increases VOLUME 34A, MAY 2003 METALLURGICAL AND MATERIALS TRANSACTIONS A

11 Fig. 13 Orientation image and pattern quality along longitudinal sections. High pattern quality regions are marked with circles at (d), (e), and (f ): (a) 111 regions, 100 regions, (c) pattern quality for as-drawn wire, (d) 111 regions, (e) 100 regions, (f ) pattern quality for 1 min at 300 C, (g) 111 regions, (h) 100 regions, and (i) pattern quality for 1 min at 400 C. (a) Fig. 14 The GAM of the gold wire along longitudinal section during annealing: (a) 300 C and 400 C. METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 34A, MAY

12 The GAM has a slightly lower value in 100 than other regions during annealing in 300 C and 400 C up to 60 minutes, which means that a lower density of geometrically necessary dislocations exists in 100 and it has a lower stored energy. The 100 can grow over other regions during recrystallization. In addition, the competition in coarsening depends on the characteristics of the various grain-boundary types such as energy and mobility. These important characteristics have yet to be measured. It should also be noted that the fact that grains in each major component are aggregated together means that the interfacial area between the two components is minimized. This in turn limits the rate at which one component can growth into the other. V. CONCLUSIONS In this study, recrystallization and grain growth of gold bonding wire have been investigated during isothermal annealing at 300 C and 400 C. 1. The cold-drawn bonding wire has a major 111 fiber component and a minor 100 component. The 100 oriented grains are located in the center and the surface regions. 2. There is a weak correlation between image quality and orientation in the cold-drawn wire, which suggests a lower stored dislocation density in the 100 component than in the During annealing, both the 100 and 111 oriented regions coarsen. The 100 grains grow into 111 grains at 300 C and 400 C and increase the 100 volume fraction. At 400 C for 24 hours, 111 fraction approaches The misorientation angle distributions show that grain boundaries within the 111 fiber have larger misorientation angles than in the 100 component. 5. The GAM for individual grains shows that 100 grains have lower orientation spreads than 111 -oriented grains during annealing. This suggests that strain energy based on geometrically necessary dislocation content in 100 is smaller than Other than low-angle boundaries, CSL boundaries in the 111 regions are predominantly of 111 axis type. Similarly the CSL boundaries in the 100 regions are of the 100 misorientation axis type. The CSLs between 111 and 100 have larger misorientation angles, greater than 40 deg. 7. Most of the low-angle boundaries under 5 deg in asdrawn wires consist of twist boundaries, and they are the source of subgrain growth from dislocation tangles at the beginning of recovery or recrystallization during annealing. ACKNOWLEDGMENTS This research is supported by the BK21 project of the Ministry of Education & Human Resources Development (Seoul, Korea) and MKE Electronics. Partial support of the Mesoscale Interface Mapping Project, Carnegie Mellon University, under NSF Grant No is acknowledged. APPENDIX Average lattice orientation by nonlinear approach Lattice orientation data are determined for a large number of pixels within each grain in an EBSD map, and the average of these pixel orientations is useful in data analysis. In order to define the average orientation based on misorientation angles, the nonlinear least-squares approach can be used. [29,30,31] The average orientation (C a ) corresponding to a given set of lattice orientations, C (i), with i in the range from l to n, where n is the number of orientations, produces a misorientation C m between C a and C (i) : C (i) m (C a ) C (i) 1 f 1 2 a n i 1 f (i) (v (i) ) 2 [A1] trace u (i) ((C(i) m ) S j ) 1 m min c acos e fd, [A2] 2 i 1, p, n; S 1 E where u m (i) is the misorientation angle between C a and C (i), and S j describes one of the p symmetry operations belonging to the appropriate crystal class concerned. [28,29] Because the misorientation angle is the metric by which distance is measured in orientation space, an average of a set of orientations can be found by minimizing the following function of the misorientation angle, u (i) : [A3] where f (i) is the weight of each of the ith misorientation and the sum of the weights is unity. 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