Growth of Intermetallic Compounds in Thermosonic Copper Wire ...

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Growth of Intermetallic Compounds in Thermosonic Copper Wire Bonding on Aluminum Metallization 129 Fig. 9. Cu-Al IMC growth in Cu-Al bonds under different thermal aging times and temperatures. Table II. Growth rate constants for IMCs in Cu-Al bonds at different temperatures T, Temperature (°C) K, Growth Rate Constants for Cu-Al IMCs (m 2 /s) 200 2.27 9 10 18 250 2.46 9 10 17 300 1.67 9 10 16 where K 0 is a prefactor, Q is the activation energy for intermetallic growth, R is the molar gas constant, and T is absolute temperature. An Arrhenius plot, as shown in Fig. 10, is obtained from the data in Table II. The activation energy, Q, can be calculated from the slope of a curve of ln K versus 1/T. The obtained activation energy was 1.01 eV, which was different from the reported activation energy, 1.14 eV to 1.43 eV, obtained for the bulk Cu/Al diffusion couples. 25,26 A lower activation energy is generally considered an indication of the formation of IMCs by shortcircuit diffusion via structural defects such as grain boundaries and dislocations. Actually, the atomic diffusion of Cu and Al through the IMCs is the main defining process for the IMCs growth during thermal aging. During diffusion, one atom moves into an empty lattice position (vacancy), and another atom moves into the empty position of the first. Therefore, the diffusion rate of one metal into another is dependent on the number of defects (e.g., vacancies, dislocations, and grain boundaries) in the crystal lattice. The more defects it has, the faster the diffusion processes. The thermosonic bonding process results in numerous lattice defects for Cu-Al bonds due to large plastic deformations and high stresses, thus, interdiffusion in thermosonic bonds is more rapid than in bulk Cu/Al couples; this explains the Fig. 10. ln K versus 1/T dependence. Fig. 11. TEM image presenting two layers of IMCs in Cu-Al bond after thermal aging at 250°C for 1 h. lower activation energy of Cu-Al IMCs in thermosonic Cu-Al bonds. As a consequence, poorly bonded bonds would fail more rapidly as IMCs grow more quickly. Identification of Cu-Al IMCs Figure 11 presents a BF TEM image of the interface region of a Cu-Al bond after thermal aging for 1 h at 250°C. Two layers of IMCs are clearly observed between copper and aluminum. STEM–EDX was conducted to identify these IMC layers. The results of EDX given in Table III from
130 Table III. STEM–EDX results of regions 1–6 in Fig. 11 Regions O K (at.%) Al K (at.%) Cu K (at.%) 1 10.8 46.2 43.0 2 9.9 46.7 43.4 3 11.1 26.5 62.4 4 12.1 27.2 60.6 5 48.9 31.0 20.2 6 40.5 37.2 22.3 Fig. 12. TEM image of a Cu-Al bond after thermal aging at 250°C for 100 h. regions 1 and 2 of the IMC-1 layer indicate that the ratio of Cu to Al was close to 1:1; however, an approximate 9:4 Cu-to-Al ratio was found in regions 3 and 4 of the IMC-2 layer. Therefore, the two types of IMCs (CuAl and Cu 9Al 4) existed in the layers of IMC-1 and IMC-2, respectively. It was also found with EDX that oxygen is relatively rich at the interface between the two IMCs (approximately 40 at.% to 50 at.%) in comparison with inside the IMCs (approximately 10 at.%). A TEM image of a sample after thermal aging for 100 h at 250°C is shown in Fig. 12. Two thicker layers of IMCs existed between the copper and aluminum. Line-scanning TEM–EDX analysis was conducted across those two layers of Cu-Al IMCs, and the results are shown in Fig. 13. As there are no changes in the composition between the two layers of IMCs, it indicates that there could be only one type of IMC between the Cu and Al. The decrease of intensity of all elements at the interface between Xu, Liu, Silberschmidt, and Chen Fig. 13. Results of line-scanning TEM–EDX across two layers of IMCs in Fig. 12. Fig. 14. SAD patterns of (a) IMC-1 and (b) IMC-2 in Fig. 12. the two layers of IMCs indicates the presence of voids. Due to the inability to identify the crystallographic structure by means of TEM–EDX analysis, further investigation was carried out by TEM–SAD, with two diffraction patterns of Cu-Al IMCs for the sample aged at 250°C for 100 h presented in Fig. 14a and b. Cu9Al4 (p-43m, a = 0.870 nm) was identified as the phase for both IMC layers. For the sample aged at 250°C for 1 h, SAD results showed that both IMCs are polycrystalline, although further identification is still ongoing. However, it should be noted that STEM–EDX suggested that two layers of IMCs might be CuAl and Cu 9Al 4, respectively. Previous work investigating interdiffusion in bulk copper–aluminum couples detected all five equilibrium phases by microprobe analysis, 15 and observed CuAl2, CuAl and Cu9Al4 in thinfilm copper–aluminum couples. 16,17 The presence of specific intermetallic phases can depend on the relative amounts of metals present in the initial composition. Bulk copper–aluminum couples and thin-film couples with varying relative amounts of metals could produce different compounds. Consequently, for copper bonds on aluminum metallization, the amount of copper is much higher than that of aluminum, producing copperrich IMCs, i.e., Cu9Al4. However, it is possible that other phases are initially formed at the early stage of diffusion but are dissolved or transferred to another phase when one of the materials is consumed in the diffusion process or reaction. We believe that,
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