2.1 Sample preparation and observation

An Al wire was bonded to a metallization pad mounted on a silicon chip using an automatic ultrasonic silicon Al wire bonding machine FK64000. The wire was Al-1 wt.% Si with a diameter of 40 μm. The pad on the substrate consisted of three layers of Au/TiW/NiCr with thickness of 0.8 μm, 50 nm and 60 nm, respectively. The three layers were successively sputtered on the substrate by magnetron. The ultrasonic resonance frequency was about 60 kHz. The bonding was carried out at the bonding force in the range of 19-27 gf, the ultrasonic power in the range of 65-85 mW and the bonding time in the range of 17-35 ms.

In order to observe the atomic diffusion, the cross-sections of Al-Au ultrasonic bonding were obtained using Helios G4 CX focused ion beam (FIB). The specific processing scheme was shown in Figures S1(a) and (b). The Al-Au interface and atomic distribution were observed and measured by SEM with EDS equipment. The crystal structures were characterized by Talos F200X TEM. TEM samples were prepared by dual-beam FIB system. The destructive shear test was conducted on the CONDOR 70-3 tester at a speed of 300 μm/s, and shear strength was obtained by 10 samples for each bonding parameter.

2.2 Simulation procedures

3D FE model for wire bonding was shown in Figure S1(c). The wedge was applied to Al wire with an amount of bonding force, causing Al wire to deform and come into close contact with Au pad. At the same time, ultrasonic waves begin to act through the wedge. Due to the large size of Si chip and relatively small deformation, it was regarded as a rigid body, while the TiW and NiCr layers were ignored in order to simplify the calculation due to their very thin thickness. Compared with Al wire and Au pad, the stiffness of the wedge was so large that the wedge can be regarded as rigid materials. The material properties of Al and Au were given in Table 1 [23, 24].

3.1 Al-Au interface and atomic diffusion at interface

3.1.1 Effect of ultrasonic bonding parameters

The three positions, including the bond heel, intermediate and bond toe areas, are selected to observe the Al-Au interface shown in Figure S1(b). Figure 1 shows the effect of bonding force on the cross-section interface of the Al-Au ultrasonic bonding at an ultrasonic power of 75 mW and a bonding time of 21 ms. From Figure 1, it is seen that no obvious unbonded region is found at the suitable bonding forces of 21 gf and 23 gf shown in Figures 1(b) and (c). But, when the bonding force of 19 gf is too small (Figure 1(a)), there is a few holes in the intermediate area because relative sliding easily occurs at this position, which is resulted from the combined effect of ultrasonic vibration sliding and lower pressure stress. As can be confirmed from the results of FE simulation (Figure 2(a)), higher stresses are concentrated at the periphery of the Al-Au ultrasonic bonding compared to the intermediate area. In addition, when the bonding forces of 25 gf and 27 gf are too high, the holes are found in the bond heel (Figure 1(d)), the intermediate and the bond toe areas (Figures 1(e) and (f)). Especially, more unbonded regions are observed at a bonding force of 27 gf, and the overall Al-Au interface is poor. As the bonding force increases, the contact between the Al wire and the Au pad becomes closer, and the diffusion process is rapidly activated and a connection forms between Al and Au. At this time, to a certain extent, the bonding force plays a role in promoting the diffusion. ZHOU et al [25] pointed out the bonding force was conductive to removing the surface oxides during Au wire bonding process, which contributed to the metal contact and atomic diffusion. However, with the continuous increase of bonding force, on the one hand, excessive force will cause damage to the interface due to the combined effect of higher bonding force and ultrasonic vibration sliding. From Figure 2(b), it is also confirmed that the stress along section line of Al wire keeps increasing with increasing bonding force. According to the above-mentioned observation of the Al-Au interface and the calculated stress distribution, it is concluded that the bonded quality of the Al-Au interface is poor at lower and higher bonding forces. Especially, the bonded interface is easy to be destroyed and cannot effectively form the stable area at higher bonding forces [26].

Figure 1全屏查看

The effect of bonding force on the interface morphology of cross-section at an ultrasonic power of 75 mW and bonding time of 21 ms in the bond toe area (b, c, e), in the intermediate area (a, f) and in the bond heel area (d): (a) 19 gf; (b) 21 gf; (c) 23 gf; (d) 25 gf; (e, f) 27 gf

Figure 2全屏查看

The effective stress distribution using FE simulation at an ultrasonic power of 75 mW and bonding time of 21 ms: (a) Effective stress field at a bonding force of 19 gf; (b) Effective stress along section line of Al wire at different bonding forces

Figure 3 shows the effect of ultrasonic power on the cross-section interface of the Al-Au ultrasonic bonding at a bonding force of 23 gf and bonding time of 21 ms. From Figure 3(a), it is seen that there are only sporadic unbonded regions near the bond heel area at an ultrasonic power of 65 mW. At the ultrasonic powers of 70 mW and 75 mW (Figures 3(b) and 1(c)), no obvious unbonded region is found at the entire Al-Au interface. When the ultrasonic power reaches 80 mW (Figure 3(c)), some small unbonded regions exist near the bond toe area. When the ultrasonic power is up to 85 mW (Figure 3(d)), the intermediate areas are also found some holes. According to the above-mentioned observation, it is concluded that the ultrasonic power has some effect on the Al-Au interface morphology, the bonded quality of the Al-Au interface is poor at lower and higher ultrasonic powers. The main reason is that the ultrasonic energy is applied alternately to Al wire in the form of sinusoidal waves, and the energy will generate relative motion between Al wire and Au pad, which exists in the form of microslip or gross sliding, and the mechanism depends on the ultrasonic power and bonding force [27]. In the process of ultrasonic bonding, on the one hand, the relative movement at the Al-Au interface can promote the atomic diffusion through intensifying the plastic deformation of Al wire and Au pad. On the other hand, the sliding is in competition with the interface strength, which affects the formation of the interface. When the ultrasonic power increases to 80 mW, the excessive ultrasonic power intensifies the relative movement between Al wire and Au pad, and the bond strength of the interface is not enough to inhibit the relative movement, resulting in a formation of some holes. Similarly, LUM et al [28] studied the footprint of Al-Cu wire bonding and found that the footprint morphology changed from microslip to gross sliding with increasing ultrasonic power, resulting in an increase in bonding area. However, TIAN et al [29] pointed out that excessive ultrasonic power damaged the forming bonding area due to over bonding. In addition, the increase of ultrasonic power can also lead to the accumulation of interface oxides, promoting the formation of holes and making the interface quality worse, which will be discussed in detail in the Section 3.2.

Figure 3全屏查看

The effect of ultrasonic power on the interface morphology of cross-section at a bonding force of 23 gf and bonding time of 21 ms at especial positions: (a) 65 mW in the bond heel area; (b) 70 mW in the intermediate area; (c) 80 mW in the bond toe area; (d) 85 mW in the intermediate area

3.1.2 Quantitative model of atomic diffusion

Figure 4 shows the concentration of Al atom at the Al-Au interface at different bonding forces and ultrasonic powers using EDS measurement. At different bonding forces or ultrasonic powers, there is no obvious pattern in the distribution of Al atom. From Al wire to Au pad, the concentration of Al atom decreases, indicating significant atomic diffusion during the bonding process. The external loads such as ultrasound and pressure cause the atomic diffusion at the Al-Au interface, thus forming a stable connection [30].

Figure 4全屏查看

Al atom fraction of the Al-Au interface at different ultrasonic bonding parameters: (a) Bonding force at an ultrasonic power of 75 mW and bonding time of 21 ms; (b) Ultrasonic power at a bonding force of 23 gf and bonding time of 21 ms

For the Al-Au bonding system, the driving force of Al and Au atom movement comes from the concentration gradient of the interfacial atoms, which is subject to Fick’s second law. The diameter of Al wire is 40 μm and the thickness of Au layer is about 0.8 μm, which is large enough to provide enough atoms in the diffusion process. Therefore, Fick’s second law in the one-dimensional case can be used to describe the diffusion process of Al atoms near the Al-Au interface. The model is expressed as [31]:

(1)

where D is the diffusion coefficient (m²/s); C represents the volume concentration of the diffused element (g/m³ or mol/m³); t is the time (s) and x is diffusion distance (m). And there are two boundary conditions. When t=0, the initial condition is described as:

(2)

When t0, the boundary condition is described as:

(3)

Therefore, combined with Eqs. (1)-(3), the concentration distribution equation of Al atom in the diffusion process is obtained as:

(4)

where C₁ is the concentration of Al atom at the beginning of diffusion; C₂ is the concentration of Al atom at the end of diffusion; and erf is the Gaussian error function. The diffusion coefficient D is obtained by:

(5)

where D₀ is the diffusion constant; Q is the diffusion activation energy (kJ/mol); R is the gas constant (8.3145 J/(mol·K)), and T is the temperature at diffusion process (K).

The concentration of atoms at different positions of the Al-Au interface extracted from Figure 4 is shown in Figure S2. Bonding force has an approximate quadratic function relationship with the concentration of Al atoms at different positions, while the relationship between ultrasonic power and Al atomic concentration is approximately cubic polynomial (Figures S2(a) and (b)). The modified diffusion coefficient is obtained by substituting the bonding force and ultrasonic power into Eq. (5), and is expressed as:

(6)

where F is the bonding force (gf); P is the ultrasonic power (mW); A₁, A₂, A₃, A₄ and A₅ are material constants.

Figures S2(c) and (d) show the relationship between the bonding parameters and the Al atomic diffusion starting concentration and Al atomic diffusion ending concentration. These results indicate that the relationships between bonding parameters and C₁ and C₂ are both approximately cubic function. The corrected C₁ and C₂ can be obtained as:

(7) (8)

where A₆, A₇, A₈, A₉, A₁₀, A₁₁, A₁₂, A₁₃, A₁₄, A₁₅, A₁₆, A₁₇, A₁₈ and A₁₉ are material constants.

In summary, an atomic diffusion model associated with ultrasonic bonding parameters, time and distance at the Al-Au interface is established as:

(9)

According to previous EDS results, the fraction of Al atomic at different ultrasonic bonding parameters are used as the sample data for the optimization of model parameters. The genetic algorithm toolbox in MATLAB is applied for optimizing the model parameters. Then, the material parameters are shown in Table 2.

Table 2全屏查看

Material parameters of Al-Au interfacial diffusion model

Parameter	Value	Parameter	Value
A0	-101.6	A1	-18.4
A2	5.7	A3	52.1
A4	-1.2	A5	-0.2
A6	0.4	A7	-398.8
A8	17.5	A9	6.8
A10	1200.2	A11	152.8
A12	-4.3	A13	0.5
A14	126.6	A15	1.2
A16	-3.5	A17	-48.4
A18	-48.5	A19	-1.5

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The comparison between the calculated and experimental fraction of Al atomic is shown in Table 3. The maximum relative error is 7.35%, which indicates that the model has good predictability. The accurate model lays the foundation for understanding the evolution of the Al-Au interface in the further study.

Table 3全屏查看

Comparison of the calculated and experimental fraction of Al atom

Bonding force/gf	Ultrasonic power/mW	Bonding time/ms	Position/μm	Calculated atom fraction/%	Experimental atom fraction/%	Relative error/%
23	80	21	-0.15	79.32	74.99	5.77
23	80	21	-0.10	73.44	68.78	6.79
23	80	21	-0.05	65.48	63.81	2.61
23	80	21	0	52.80	54.49	-3.10
23	80	21	0.05	39.91	37.94	5.18
23	80	21	0.10	32.15	30.29	6.12
23	80	21	0.15	26.28	24.70	6.38
25	75	21	-0.15	79.87	82.40	-3.07
25	75	21	-0.10	77.59	79.56	-2.47
25	75	21	-0.05	66.40	68.60	-3.22
25	75	21	0	52.53	43.84	4.20
25	75	21	0.05	38.67	41.74	-7.35
25	75	21	0.10	27.48	28.98	-5.19
25	75	21	0.15	23.78	23.07	3.06

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3.2 Mechanism of Al-Au ultrasonic bonding

Figure 5 exhibits the TEM observations of the Al-Au interface at a bonding force of 23 gf, ultrasonic power of 75 mW and bonding time of 21 ms. In Figure 5(b), an IMC layer between the Al and Au layers is formed, with a thickness of approximately 200 nm. And there is a transition layer of about 55 nm between the Al layer and the IMC layer, as shown in Figure 5(c). Fast-Fourier transform (FFT) pattern in Figure 5(f) reveal that the crystal zone axis of IMC layer is [772]. Meanwhile, from the IFFT diagram (Figure 5(e)), the interplanar crystal spacing is 0.2183 nm, which is consistent with the crystal structure of Au₈Al₃. Combined with EDS results in Figure 5(b), it is further explained that IMC is Au₈Al₃. During diffusion process, when the content of solute exceeds the solubility of the solid solution, based on forming the solid solution, IMC with crystal structure different from Al and Au is also formed.

Figure 5全屏查看

TEM images of the Al-Au interface with a bonding force of 23 gf, ultrasonic power of 75 mW and bonding time of 21 ms: (a) Bright field (BF) image; (b) High-angle annular dark-field (HAADF) image of area A in (a); (c) BF image of area B in (b); (d) High resolution transmission electron microscopy (HRTEM) image of area C in (c); (e) Inverse fast-Fourier transform (IFFT) image of area D in (d); (f) FFT image of area D in (d)

Figure 6 shows the TEM observations of the Al-Au interface at a bonding force of 27 gf, ultrasonic power of 75 mW and bonding time of 21 ms. From Figures 6(c)-(e), there is a region lacking Al, Au and O elements at the Al-Au interface, indicating that the region is a nanoscale hole. In the process of ultrasonic bonding, the Al-Au interface is subjected to external loads in two directions. One of loads is the bonding force, which is from the static load with the wedge in the vertical direction, and the other is the periodic dynamic loads from ultrasound along the horizontal direction. The stress at the Al-Au interface is equivalent to the vector synthesis of the two loads. As the bonding force increases, the stress along the vertical direction increases, inhibiting the relative movement of the Al-Au interface. Therefore, when the bonding force increases, on the one hand, it promotes the Al wire and the Au pad to form a better metallurgical connection. But on the other hand, it causes that the regions completing the connection are subjected to excessive stress, thus breaking the weak region and forming holes, as discussed in Section 3.1.1 (Figure 1). EDS results in Figure 6(f) indicates that alumina particles present at the Al-Au interface. The crystal structure of IMC is determined by FFT, as shown in Figure 6(i). The results show that the phase transformation of IMC is not caused by the increase of bonding force, and Au₈Al₃ is still formed first in the Al-Au interface as the preferred phase.

Figure 6全屏查看

TEM images of the Al-Au interface with a bonding force of 27 gf, ultrasonic power of 75 mW and bonding time of 21 ms: (a) BF image; (b) HAADF image of area A in (a) and relative corresponding elemental maps for (c) Al, (d) Au and (e) O; (f) EDS line scanning result along the line in (b); (g) BF image of area A in (a); (h) HRTEM image of area B in (g); (i) FFT image of area C in (h)

Figure 7 shows the TEM observations of the Al-Au interface at a bonding force of 23 gf, ultrasonic power of 85 mW and bonding time of 21 ms. Compared to Figure 5, the Al-Au interface still presents a multi-layer structure with increasing ultrasonic power, forming a solid solution and IMC between Al and Au, and the IMC is determined to be Au₈Al₃ by FFT, as shown in Figures 7(b)-(d). In addition, the EDS results in Figures 7(g)-(i) show that Al and O elements are enriched in region D but Au element is less abundant, indicating that there are residual alumina particles in the Al-Au interface. Meanwhile, the thickness of the IMC layer is about 200 nm in the region where alumina particles are present, while the thickness of the IMC is about 330 nm in the region without alumina particles. Excessive ultrasonic power is not conducive to the removal of oxides, resulting in residual alumina particles at the bonding interface, which significantly inhibits the mutual diffusion of Al and Au atoms and prevents the effective connection of the Al-Au interface. During the process of ultrasonic bonding, the surface of the Al wire contains a dense layer of alumina film, which hinders the inter-diffusion between the Al-Au atoms, so the removal of the alumina film is the premise of the interface connection. Cracks occur on the alumina under the effect of bonding force, and then the oxide is separated from the surface of Al wire under the action of ultrasound. The separated oxide is ground into small particles under the reciprocating sliding effect of bonding force and ultrasound. Small particles of oxide are then transported to the peripheral contact area. However, the higher ultrasonic power can’t completely remove the oxide particles from the Al-Au interface, as mentioned in Section 3.1.1 (Figure 3). XU et al [32] found in Au wire bonding that the residual alumina in Al pad would provide oxygen atoms, leading to the oxidation of IMC at the interface between IMC and alumina, and the migration of oxygen led to the growth of hole. Therefore, the accumulated alumina in the periphery caused by excessive ultrasonic power is not only unfavourable to the atomic diffusion, but also accelerates the formation of holes and further damage the Al-Au interface.

Figure 7全屏查看

TEM images of the Al-Au interface with a bonding force of 23 gf, ultrasonic power of 85 mW and bonding time of 21 ms: (a) BF image; (b) BF image of area A in (a); (c) HRTEM image of area B in (b); (d) FFT image of area C in (c); (e) HAADF image of the Al-Au interface; (f) HAADF image of area D in (e) and corresponding elemental maps for (g) Al, (h) Au and (i) O

Figure 8 shows the TEM observations of the Al-Au interface at a bonding force of 23 gf, ultrasonic power of 75 mW and bonding time of 35 ms. Combined with Figures 5(b) and 8(b), when the bonding time is 21 ms, the concentration of Al atom in the IMC layer is 45.65 at.% and the concentration of Au atom is 54.35 at.%. With increasing bonding time to 35 ms, the concentration of Al atom in IMC layer is 30.82 at.%, and the concentration of Au atom rises to 69.18 at.%, which indicates that Au is the main diffusion element in diffusion process. Therefore, with the extension of bonding time, the concentration of Au atoms in IMC layer increases significantly. As bonding time increases from 21 ms to 35 ms, the Al grains near the Al-Au interface are refined to 250 nm, indicating that the recrystallization of Al grains is intensified with the extension of bonding time. At the same time, the further diffusion of Al and Au atoms leads to the growth of IMC layer thickness from 195 nm to 330 nm. It can be seen from Figure 8(c) that there is a transition layer with a thickness of about 60 nm between the Al layer and the IMC layer, and the atomic arrangement in the Al-IMC transition layer is distorted in Figure 8(d), indicating that there are a large number of defects in the transition layer. In addition, there are also local highly ordered structures in the transition layer. FFT result in Figure 8(e) shows that Au₈Al₃ exists in the Al-IMC transition layer, which is also the composition of IMC layer (Figures 8(f)-(h)). This phenomenon indicates that when the solute exceeds the solid solubility, Au₈Al₃ is first formed. Meanwhile, there are residual alumina particles in the transition layer, as shown in Figure 8(i). Moreover, the thickness of IMC-Au transition layer near the alumina particles is obviously reduced, which directly illustrates that the residual alumina particles inhibit the inter-diffusion between Al and Au atoms. As shown in Figure 8(j), there is a poor periodic arrangement of atoms in the IMC-Au transition layer, and Au atoms are relatively easy to be activated, thus providing a fast diffusion channel for the inter-diffusion of Al and Au atoms.

Figure 8全屏查看

TEM images of the Al-Au interface with a bonding force of 23 gf, ultrasonic power of 75 mW and bonding time of 35 ms: (a) BF image; (b) BF image of area A in (a); (c) BF image of area B in (b); (d) HRTEM image of area E in (c); (e) FFT image of area F in (d); (f) BF image of area C in (b); (g) HRTEM image of area G in (f); (h) FFT image of area H in (g); (i) BF image of area D in (b); (j) HRTEM image of area I in (i)

According to TEM observations, the process of Al-Au ultrasonic bonding is shown in Figure 9. First, with the effect of rapid and periodic ultrasound, the dense alumina film on the surface of the Al wire is removed, so that Al wire and Au pad are in close contact, which is the premise of the Al-Au interface connection. Subsequently, Al wire happens severe plastic deformation and the number of dislocations increases. At the same time, along with the recrystallization of Al grains near the interface, the number of grain boundaries increases. The diffusion between the elements is promoted, because there are a large number of grain boundaries and dislocations. Therefore, the activation energy required for diffusion is reduced, which contributes to promoting the formation of IMC layer. Among Al-Au IMCs, Au₈Al₃ has the lowest effective heat of formation [33], so it is preferred to be formed.

Figure 9全屏查看

Schematic diagram of Al-Au ultrasonic bonding process: (a) Before ultrasonic bonding; (b) The period of ultrasonic bonding; (c) After ultrasonic bonding

3.3 Shear strength of Al-Au ultrasonic bonding

Figure 10 exhibits the effect of ultrasonic bonding parameters on the shear strength of Al-Au ultrasonic bonding. As the bonding force increases, the shear strength becomes larger (Figure 10(a)). Under the condition of low bonding force, the shear strength of Al-Au ultrasonic bonding is small because of the thin diffusion layer thickness. When the bonding force increases to 23 gf, the shear strength of Al-Au ultrasonic bonding increases, because the diffusion layer became thicker. When the bonding force is up to 27 gf, the thickness of IMC layer increases, and the Al grain size near the Al-Au interface is further refined, which is beneficial to the improvement of the interface strength. From Figure 10(b), it is seen that the shear strength of Al-Au ultrasonic bonding presents a small upward trend with increasing ultrasonic power. When the ultrasonic power is small, the shear strength is lower due to the thin diffusion layer thickness at the Al-Au interface. With increasing ultrasonic power, the thickness of the Al-Au interface diffusion layer increases, and the Al grain size near the interface is further refined, which is beneficial to the improvement of interface strength. However, the residual alumina particles at the Al-Au interface only lead to a small growth in bond strength at an ultrasonic power of 85 mW. As shown in Figure 10(c), with the extension of bonding time, the shear strength increases slightly. When the bonding time is small, the thickness of diffusion layer at the Al-Au interface is thin, so the shear strength is lower. While the bonding time reaches 35 ms, the thickness of the diffusion layer at the Al-Au interface is higher, the grain size of Al near the interface is further reduced, and the shear strength is further increased.

Figure 10全屏查看

The effect of ultrasonic bonding parameters on shear strength: (a) Bonding force at an ultrasonic power of 75 mW, bonding time of 21 ms; (b) Ultrasonic power at a bonding force of 23 gf, bonding time of 21 ms; (c) Bonding time at a bonding force of 23 gf, ultrasonic power of 75 mW

According to above-mentioned analysis, it is concluded that the shear strength of Al-Au ultrasonic bonding increases with increasing bonding force, ultrasonic power and bonding time. However, excessive bonding force and ultrasonic power promote the formation of holes at the Al-Au interface, and holes will induce the initiation and development of cracks in practical applications. For instance, NOOLU et al [34, 35] investigated that in the process of high temperature, holes at the bonding interface continue to form and aggregate into void lines, resulting in continuous crack propagation and degradation of ultrasonic bonding performance. Therefore, the optimal ultrasonic bonding parameter is confirmed to be a bonding force of 23 gf, ultrasonic power of 75 mW and bonding time of 21 ms.