Recrystallization and Grain Growth of. Cold drawn Gold bonding Wire. J.-H. Cho 1,2, J.-S. Cho 3, J.-T. Moon 3, J. Lee 3, Y.H. Cho 4, Y.W.

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1 Recrystallization and Grain Growth of Cold drawn Gold bonding Wire J.-H. Cho 1,2, J.-S. Cho 3, J.-T. Moon 3, J. Lee 3, Y.H. Cho 4, Y.W. Kim 2 A.D. Rollett 1 and K.H. Oh 2 1 Carnegie Mellon University, Pittsburgh, PA Seoul National University, Seoul Korea MKE Electronics, Pogok-Myeon, Yongin-Si, Kyunggi-Do, Korea 4 Korea Institute of Science and Technology, Seoul, Korea Corresponding Author : kyuhwan@snu.ac.kr

2 Abstract Recrystallization and grain growth of gold bonding wire have been investigated with Electron Back Scatter Diffraction (EBSD). The bonding wires were wire-drawn to an equivalent strain greater than 11.4 with final diameter between 25 and 30µm. Annealing treatments were carried out in a salt bath at 300 C, 400 C for 1 min, 10 min, 60 min, 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. <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 and 400 C. Large misorientations (angles > 40 ) 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 <100> regions. It appears that the stored energy as indicated by geometrically necessary dislocation content in the subgrain structure is larger in <111> regions than <100>. Keywords: Gold bonding wire, Recrystallization, Grain Growth, Grain Boundary, EBSD

3 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 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].

4 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 aluminium, copper and brass wires have been investigated for cyclic symmetry [10]. The textures 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 direciton the extension axis is closest to at zero strain. The crystal axes of grains near <101> will tend to rotate toward to the <111> or the <100> axes [13]. English 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, aluminium, and brass, has also shown that the final texture components are <111> and <100> [15, 16]. 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 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.

5 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 grain boundary migration. Grain boundaries adopt curvatures based on local equilibrium at triple junctions, which are described by Herring s equations [20-22]. These curvatures lead to grain boundary 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 MDF (Misorientation Distribution Function) provides some information on grain boundary types, grain boundary 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 HAZ (heat affected zone) during bonding. For the good bonding wire, it is necessary to understand the alloy design, optimized drawing process and annealing process together. In this research, the mircrotextures 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

6 MDF and the frequency of CSL (Coincident Site Lattice) boundaries are also calculated from EBSD data. Experimental Materials and sample preparation The purity of gold wire used in this research is more than 99.99% and it has some (intentional) dopants, such as Ca and Be that total less than 50 ppm by weight. Even at ppm 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%. 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% reduction in area in order to achieve homogeneous deformation. EBSD measurements were performed on gold wires with diameters of 25 and 30µm. 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 et al. [17]. Isothermal annealing for wires were carried out for 1min, 10min, 60min, 24hours at 300 o C and 400 o C. 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 EBSD

7 system) was used for measurement and the data analysis was made by REDS (Repressing of EBSD Data in SNU) [23]. The operating voltage was 20kV and the probe current was 4nA. A rectangular grid was used and the pixel spacing was 0.239µm. 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]. {111}<110> slip systems are assumed and the velocity gradient, ε ij, is as follows, ε ij = (1) 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.

8 Statistical analysis of microstructures The fluctuations or variations of material microstructures can be described by so-called secondorder 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 window W is given by ΕV ( Ξ W), then the variance, var, of the volume of Ξ W is 2 2 varv ( Ξ W) = ΕV ( Ξ W) -[ ΕV ( Ξ W)] (2) where ΕV 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)) = Ε[( f ( x) Εf ( x))( f ( y) Εf ( y))] (3) The normalized covariance function is called the correlation function, ρ f ( x ) f ( y ), and takes values between 1 and +1. ρ = cov ( f ( x), f ( y)) f ( x) f ( y) σ σ (4) f ( x) f ( y) where, σ f ( x) and σ f ( y) are the standard deviations of f (x) and f (y), respectively. A value of 1. 0 or 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 whether or not the <100> grains of the cold drawn wire exhibited a higher image quality than <111> grains.

9 Average Lattice Orientation, Grain Orientation Spread (GOS) and Grain Average Misorientation (GAM) 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) [29-31]. In addition to average orientation, Grain Orientation Spread (GOS) and Grain Average Misorientation (GAM) can be calculated also [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 n i= 1 j= i+ 1 trace(( Pi min acos C 2 n m P 2 1 j ) S) 1 (5) 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. 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. GAM and GOS are size dependent and they increase as the grain sizes increase.

10 Results Transverse section of as-drawn wire (30µm) 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 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 1-f. Each grain in the wire can be partitioned into two types by their Taylor factors (TF), figure 1-b 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 o 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 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 of on the surface regions of the wire, while <111> fibers or high Taylor factor regions are located throughout the wire. <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. Figure 1-c shows this deviation by coloring only these grains that lie more than 15 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

11 multi-layer structures of wire are related to shear deformation and original microstructures. FCC metals like gold typically exhibits 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 (IQ). 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 equation 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 figure 2-a, b. 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-b.

12 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 1 3, 1 3, 1 3. The latter Rodrigues vector is equivalent to a 60 <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 around the <111> misorientation axis [32-34]. By contrast, the <100> regions are concentrated on <100> misorientation axes and their Rodrigues vectors are located between ( 0, 0, 0) and ( 2 1, 0, 0). The latter Rodrigues vector is equivalent to a 45 <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. 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-b shows a projection of the fundamental zone in Rodrigues space and the location of most of the low-sigma 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. CSLs along the <111> axis, figure 4-b, with Σ3, 7, 13b, 21a and 31a are found

13 frequently in the <111> fiber regions. The most frequent CSLs in the <100> component are Σ5, 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 so that the CSLs are mainly Σ3, 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 and the maximum angle is 62.8 ; this maximum misorientation for two cubic crystals is found for combinations such as the rotated cube, {100}<011>, and Goss, {110}<001> 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-b. Three different distributions are plotted separately based on each of the fiber regions, i.e. <111>, <100> and intermediate orientations. All three exhibit non-random distributions. Most misorientation angles in the <100> regions are less than 40, whereas the <111> regions exhibit angles up to 60. 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 between 45 and 60. Note that this peak increased to 55 from about 45 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.

14 Recrystallization and Grain Growth during Annealing (Transverse and Longitudinal sections; 25µm diameter wires) The gold wires were characterized by EBSD after isothermal annealing at 300 C and 400 C for 1min, 10min, 60min and 24 hours, and orientation maps are shown in figure 6 and 8. As in the as-drawn wire in figure 1, the orientation maps during annealing shows that most of the material is aligned with either <111> or <100>. During annealing at 300 C, figure 6-a, b, c, 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 non-uniform. 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, figure 6-e, f, g, 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 1.5~2 (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-b). The average grain size in the <111> and <100> regions bracket the average grain size. The equivalent grain size increases gradually from 0.7µm to 5µm. The <100>

15 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 min, 10min, 60min, and 24 hours at 300 C and 400 C. 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 is occurring 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, figure 8-d, h, there are islands of small grains within large <100> or <111> grains that are surrounded by Σ3 boundaries and appear to be stable. Annealing twins with immobile Σ3 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 o 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 o C annealing show more interesting results.

16 The aspect ratio for 1 min at 300 o 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 min and 60 min at 300 o 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 o 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 increase, 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 o C and 400 o 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 transverse sections, longitudinal sections show that <111> and <100> grains at 400 o C grow faster than at 300 o C and <100> grains seem to grow faster than <111>. The larger grains size in longitudinal sections than transverse comes from the elongated grain shapes. Volume fraction changes measured in longitudinal sections are similar to the transverse sections. <100> grains increase and <111> grains decrease as annealing time accumulates. At

17 300 o C, <100> grains grow continuously and it s the <100> volume fraction is larger than that of <111>. At 400 o 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. It means that <111> grains growing again through grain boundary movement. Discussion 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], see figure 1-b. 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

18 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 TEM and EBSD image for as-drawn gold wire. The grains are elongated along drawing directions. TEM image shows that grains have less than 1µm along transverse direction and the elongated grains have subgrain boundaries. Comparing the TEM image to the EBSD image maps, a tolerance angle of 5 o for grain identification resulted in more reasonable grain shapes than a choice of 15 o. 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 angle increases up to 5 o. After 5 o, 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 o between them. The aspect ratio changes show that most of subgrains with low misorientation angles are aligned along the longitudinal direction and they appear to be twist boundaries. Most of the tilt boundaries along transverse direction have a misorientation angle greater than 5 o. The increase of aspect ratio of the wires under 300 o C annealing during 10min, 60min 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>.

19 Figure 12 shows the variations of aspect ratio during annealing as a fucntion of tolerance angle. Grain aspect ratios for <111> and <100> grains decrease after 1min at 300 o C. <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 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 orientation image and pattern quality map along 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 above, 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, Grain Average Misorientation (GAM) are shown in figure 14. The presence of dislocations is associated with variations in orientation

20 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. The GAM has slightly lower value in <100> than other regions during annealing in 300 o C and 400 o C up to 60 minutes. It means that a lower density of geometrically necessary dislocations exists in <100> and it has a lower stored energy. <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 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. Conclusion In this study, recrystallization and grain growth of gold bonding wire have been investigated during isothermal annealing at 300 C, 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.

21 2. There is a weak correlation between image quality and orientation in the cold drawn wire, and it suggests a lower stored dislocation density in the <100> component than in the <111>. 3. During annealing, both the <100> and <111> oriented regions coarsen. <100> grains grow into <111> grains at 300 C and 400 C and increase the <100> volume fraction. At 400 o C for 24 hours <111> fraction approaches <100>. 4. 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 <111>. 6. 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 large misorientation angles greater than Most of the low angle boundaries under 5 o in as-drawn 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 Acknowledgement This research is supported by the BK21 project of the Ministry of Education & Human Resources Development in Korea and MKE Electronics. Partial support of the Mesoscale

22 Interface Mapping Project at Carnegie Mellon University under NSF grant No is acknowledged.

23 References 1.T.H. Ramsey: Solid State Technology, 1973, vol. 16, pp S.Omiyama, Y.Fukui: Gold Bulletin, 1980, vol. 15, p B.L. Gehman: Solid State Technology, 1980, vol. 23, pp K. T. Aust and J. W. Rutter: Trans, TMS-AIME, 1959, vol. 215, pp E.M. Fridman, C.V. Kopezky and L.S. Shvindlerman: Z. Metallkunde, 1975, vol. 66, pp K.Busch, H.U.Künzi, B.Ilschner: Scripta Metallurgica, 1988, vol. 22, pp K. Hausmann, B. Ilschner, H.U. Künzi: DVS Berichte, 1986, vol. 102, pp R. Hofbeck, K. Hausmann, B. Ilschner, H.U. Künzi: Scripta Metallurgica, 1986, vol. 20, pp G. Qi and S. Zhang: Journal of Materials Processing Technology, 1997, vol. 68, pp G. Linßen, H.D. Mengelberg, H.P. Stüwe: Z. Metallkunde, 1964, vol. 55, pp E. Aernouldt, I. Kokubo, H.P. Stüwe: Z. Metallkunde, 1966, vol. 57, pp J.J. Heizmann, C. Laruelle, A. Vadon, and A.Abdellaoui: ICOTOM11 Proceedings, 1996, pp G. I. Taylor: J. Inst. Metals, 1938, vol. 62, pp A. T. English and G. Y. Chin: Acta Metall, 1965, vol. 13, H. Ahlborn: Z. Metallkunde, 1965, vol. 56, pp H. Ahlborn: Z. Metallkunde, 1965, vol. 56, pp T. Montesin, J.J. Heizmann: J. Appl. Cryst. 1992, vol. 25, pp K. Rajan, R. Petkie: Materials Science and Engineering, 1998, vol. A257 pp

24 19. F.J. Humphreys and M. Hatherly: Recrystallization and Related Annealing Phenomena, Pergamon press, 1995 p C. Herring: The Physics of Powder Metallurgy, McGraw-Hill Book Company, New York, 1951, p D. Kinderlehrer, I. Livshits, S. Ta asan, and E. E. Mason: ICOTOM12 Proceedings, Montreal, CA, 1999, pp C.C. Yang, A.D. Rollett and W.W. Mullins: Scripta Mater. 2001, vol. 44, pp REDS, Repressing of EBSD Data in Seoul National University, User Manual, Texture Control Lab, OIM 2.6, Software for analysis of electron backscatter diffraction patterns, User manual, TSL, S.I. Wright, B.L. Adams and K. Kunze: Mat. Sci. Eng., 1993, A160, pp OXFORD Instruments INCA Suite version F.J.Humphreys: J. Materials Science, 2001, vol.36, pp J.Ohser and F. Müchklich: Statistical Analysis of Microstructures in Materials Science, John Wiley & Sons, Ltd., 2000, Chap N.R. Barton and R.R. Dawson: Metall. Trans. A, 2001, vol. 32A, pp Morton E. Gurtin: An Introduction to Continuum Mechanics, vol, 158, Mathematice in Science and Engineegirng, Academic Press, New York, NY, 1981, sect Morris W. Hirsch and Stephen Smale: Differential Equations, Dynamical Systems, and Linear Algebra, Pure and Applied Mathematics, Academic Press, New York, NY, 1974, sect V. Randle: The Measurement of Grain Boundary Geometry, Institute of Physics, 1993, Chap. 2-3.

25 33. A. Heinz and P. Neumann: Acta Cryst., 1991, vol. A47, pp P. Neumann: Textures and Microstructures, 1991, vol , pp J. K. Mackenzie: Biometrika, 1958, vol. 45, pp D. C. Handscomb: Can. J. Math. 1958, vol. 10, pp S. Kohara, M.N. Parthasarathi, and P.A. Beck: Trans. TMS-AIME, 1958, vol. 212, p G.Y. Chin, W.L. Mammel and M.T. Dolan: Trans TMS-AIME, 1967, vol. 239, p.1854.

26 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-square approach can be used [29~31]. The average orientation (C a ) corresponding to a given set of lattice orientations, C (i), with i in the range from 1 to n, where n is the number of orientations, produces a misorientation C m between C a and C (i), below, ( i) m ( i) 1 C = ( Ca ) C (A1) (i) trace(( Cm ) S j) 1 ( i) θ m = min[acos{ }], i = 1,..., n; S1 = E, (A2) 2 (i) where θ m 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 [32, 33]. Since 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, (i) θ. f = 1 2 n i= 1 ( i) ( i) 2 φ ( θ ) (A3) where (i) φ is the weight of each of the i th misorientation and the sum of the weights is unity.

27 <111> <100> <111>+<100> a) <High> 5.67µm <Low> All b) c) d) e) f) Figure 1. Orientation maps for a cold drawn gold bonding wire (30µm). The EBSD data are separated into <111>+<100> and <high>+<low> Taylor factor (TF) regions. Their difference is complex regions mainly. a) Orientation Image maps for <111>+<100> regions b) Orientation Image maps for <high>+<low> Taylor factor regions c) Complex regions d) Schematic plot for Structure of gold wire e) Image quality f) Orientation color key

28 <111> <100> a) <high> <low> b) c) Figure 2. Inverse Pole figure (ND) of drawn wire in figure 1. a) Crystallographic <111> and <100> regions b) Taylor factor <high> and <low> regions c) Complex regions.

29 a) b) c) Figure 3. Misorientation angle/axis distribution of the drawn wire These data come from crystallographic <111>+<100> regions (in Fig. 1-a). a) <111> regions b) <100> regions c) All part

30 C a) R3 value 17b ( 2 1, 2 1,0) b) 1/4~1/3 1/5 1/8~1/6 1/11~1/9 0 <111> regions 19b 7 37c c 25b 11 GBs between <111> and <100> 39a 13b 31b 21a 31a (0,0,0) 41a 25a 37a 13a 17a <100> regions 5 29a ( 2 1,0,0) Figure 4. Tilted grains in a drawing wire and projected CSL grain boundaries in the Rodrigues space in the <111>, <100> regions and between them. a) <100> type tilted grains b) Rodrigues vectors of CSLs

31 From 800 random crystals from Mackenzie/Handscomb a) Density Density Angle of disorientation <100> fibers <111> fibers Bet. <111> and<100> b) Fraction number Misorientation angle ( o ) Figure 5. Misorientation angle distribution of initial wire. a) Random case from Mackenzie/Handscomb b) As drawn gold wire

32 1min 10min 60min 1 day 25µm a) b) c) d) e) f) g) h) Figure 6. Orientation maps of transverse sections of 25µm Gold wires. Misorientation angle, 5 o is used for identifying the grains a) 1min b) 10min c) 60min d) 1day at 300 o C e) 1min f) 10min g) 60min h) 1day at 400 o C

33 a) Aspect Ratio 3 2 All grains, 300 o C <111> grains, 300 o C <100> grains, 300 o C All grains, 400 o C <111> grains, 400 o C <100> grains, 400 o C b) Equivalent Grain Size, [µm] 5 All grains, 3 00 o C <111> grains, 300 o C <100> grains, 300 o C 4 All grains, 4 00 o C <111> grains, 400 o C <100> grains, 400 o C c) Volume Fraction other grains, 300 o C <111> grains, 300 o C <100> grains, 300 o C other grains, 400 o C <111> grains, 400 o C <100> grains, 400 o C Annealing time, [sec] Figure 7. Aspect Ratio, Grain size and Volume fraction of gold wire along cross section during isothermal annealing at 300 o C, 400 o C in figure 6. a) Aspect Ratio b) Equivalent Grain size c) Volume fraction

34 25µm As-drawn a) b) c) d) e) f) g) h) Figure 8. Orientation maps of 25µm Gold wires after isothermal annealing. Misorientation angle, 5 o is used for identifying the grains. Circles show islands with Σ3 boundaries. a) 1min b) 10min c) 60min d) 1day at300 o C e) 1min f) 10min g) 60min h) 1day at 400 o C

35 6 5 a) Aspect Ratio All grains, 300 o C <111> grains, 300 o C <100> grains, 300 o C All grains, 400 o C <111> grains, 400 o C <100> grains, 400 o C b) Equivalent Grain size, [µm] 5 All grains, 300 o C <111> grains, 300 o C <100> grains, 300 o C All grains, 400 o C <111> grains, 400 o C <100> grains, 400 o C c) Volume Fraction other grains, 300 o C <111> grains, 300 o C <100> grains, 300 o C other grains, 400 o C <111> grains, 400 o C <100> grains, 400 o C Annealing time, [sec] Figure 9. Aspect Ratio, Grain size and Volume fraction of gold wire along longitudinal section during isothermal annealing at 300 o C, 400 o C in figure 8. a) Aspect Ratio b) Equivalent Grain size c) Volume fraction

36 Drawing direction a) Drawing direction b) c) Figure 10. Grain shapes for cold drawn gold wire. The elongated grains are shown. a) TEM image b) EBSD image (tol =15 o ) c) EBSD image (tol =5 o )

37 6 5 a) Aspect Ratio 4 3 Aspect Ratio of <111> grains Aspect Ratio of <100> grains b) Equivalent Grain Size ECD of <111> grains ECD of <100> grains Misorientation Angle Tolerance Angle Figure 11. Aspect ratio and Equivalent Grain Size variations of as-drawn gold wires according to Grain identification angle or tolerance angle. a) aspect ratio b) equivalent grain size

38 a) Aspect Ratio <100> grains As-drawn wire 1min, 300 o C 10min, 300 o C 60min, 300 o C 1day, 300 o C b) Aspect Ratio <111> grains As-drawn wire 1min, 300 o C 10min, 300 o C 60min, 300 o C 1day, 300 o C Misorientation Angle Tolerance Angle Figure 12. Aspect ratio variations during annealing at 300 o C according to Grain identification angle or tolerance angle. a) <100> grains b) <111> grains

39 a) b) c) d) e) f) g) h) i) Figure 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 b) <100> regions c) pattern quality for as-drawn wire d) <111> regions e) <100> regions f) pattern quality for 1 min at 300 o C g) <111> regions h) <100> regions i) pattern quality for 1 min at 400 o C

40 1.5 a) GAM 1.0 <all> <111> <100> others Annealing Time, (s) b) GAM 1.0 <all> <111> <100> Others Annealing Time, (s) Figure 14. The Grain Average Misorientation (GAM) of the gold wire along longitudinal section during annealing a) 300 o C b) 400 o C