Phase diagram of Au–Al–Pd at 500 °C

Abstract

The isothermal phase diagram of Au–Al–Pd at 500 °C was constructed using diffusion couples and equilibrated bulk alloys. Electron microprobe analyses were used to determine the phase compositions and phase relationships. Thirteen three-phase equilibria and two ternary phases (T₁ and T₂) were identified in this study. Phase T₁ contained 37 at% Au, 26 at% Pd, and 37 at% Al. Phase T₂ contained 44 at% Au, 24 at% Pd, and 32 at% Al. The Au solubility in the Al₃Pd₂, AlPd, and AlPd₂ phases were 6, 15, and 34 at%, respectively. The Pd solubility in the AuAl₂, Au₂Al, Au₈Al₃, and Au₄Al phases were 5, 10, 13, and 3 at%, respectively.

Introduction

Gold wires have been used as bonding wires in integrated circuit (IC) packages because of the ductility and anticorrosion characteristics of gold. Recently, the increase in the input and output (I/O) number of ICs from 200 to 300 has necessitated a decrease in the diameter of gold wires from 25 μm to less than 20 μm to match the decrease in the area of the Al pad. However, the stiffness of gold wires must be enhanced to avoid short circuits caused by wire sweep. Au–Pd alloy wires (such as the Au–1 wt% Pd alloy wire) are one of the several alloy bonding wires that have been recently developed [1]. The bonding wire reacts with the Al pad and forms intermetallic phases to maintain the electrical connection. A low-temperature Au–Al–Pd phase diagram could provide the information required for understanding the reactions between the Au–Pd alloy wire and the Al pad. The stability of the intermetallic phases is closely related to the reliability of wire bonding [1, 2].

In 1991, Okamoto [3] updated the Al–Au phase diagram reported by Murray [4]. In a crystal structure study, the stoichiometry of a compound existing at approximately 72 at% Au was revealed to be in the Au₈Al₃ phase. In accordance with the aforementioned references, the Al–Au phase diagram was revised again by Okamoto in 2005 [5].

The Al–Pd phase diagram was constructed in 1986 by McAlister [6] and revised by Okamoto [7].

Based on the thermodynamic model, Okamoto [8] derived the Au–Pd phase diagram. Three ordered structure phases exist in this system. The Au₃Pd [9] and AuPd₃ [10] phases were confirmed by conducting experiments, and the AuPd phase was not confirmed because of its existence at a low temperature (below 100 °C).

In general, isothermal phase diagrams are constructed using bulk alloys, prepared at various compositions, and annealed at a certain high temperature to reach phase equilibrium. Because bulk alloys lead to large dendrites, they require additional time to reach phase equilibrium. According to the references, these bulk alloys require annealing for 14 days at 800 °C [11] and for 30 days at 600 °C [12]. Moreover, depending on the complexity of the isothermal phase diagram, 30 to 50 samples would be required for a study. Because the recovery of Au, a precious metal, is not possible, and a long period is required for the bulk alloys to reach phase equilibrium at low temperatures, this method is too expensive to be used in the present study.

This study focused on the investigation of the Au–Al–Pd phase diagram at 500 °C by combining the diffusion couple technique and the method of equilibrated bulk alloys. The diffusion couple technique follows the principle that phase interface would form local equilibrium, and then, we judge these phases existing in each other when at the same temperature. This study also referenced the theoretical details of the research conducted by Kodentsov et al. [13].

Experimental procedures

The Au _x Pd_{100 − x} (x = 10 to 80) and Au _x Pd _y Al_{100 − x − y} alloys were prepared by mini-arc melting pure Au, Pd, and Al in a pure Ar atmosphere. The Au–Pd alloys were arc-melt under Ar atmosphere and turned upside down for seven times and then sucked into 2-mm-diameter copper mold. The ingot length is between 15 and 20 mm. Au–Pd alloy forms a solid solution, and the homogeneity was checked under the electron probe microanalyzer (EPMA). Eight Au–Pd binary alloys (D1–D8) and seven Au–Al–Pd ternary alloys (A1–A7) were prepared for this phase diagram study. The alloy compositions are listed in Tables 1 and 2.

Table 1 Summary of phase equilibria in eight diffusion couples

Table 2 Summary of phase equilibria in seven bulk alloy samples

Figure 1 is the schematic diagram of the diffusion couple setup. The Au–Pd binary alloy ingots (D1–D8) were sliced into 1-mm-thick discs, and a pure aluminum bar with a 2-mm diameter was sliced into 2-mm-thick discs. The discs were joined to form a diffusion couple. The screw of the holder exerted pressure on the diffusion couple. The holder was sealed in a quartz tube in which a vacuum was created and maintained at 500 °C for 24 h; the quartz tube was then quenched in ice water. The ternary alloys (A1–A7) were sealed in the quartz tube vacuum. These alloy samples were maintained at 500 °C for 34 days.

Fig. 1

The sketch diagram of diffusion couple setup

An electron probe microanalyzer (EPMA) was used to determine the equilibrium phase compositions of the diffusion couple samples and ternary alloy samples. The isothermal phase diagram of the Au–Al–Pd alloy annealed at 500 °C was constructed based on these results.

Results

Isothermal section at 500 °C

The phase equilibria of the diffusion couples are summarized in Table 1. As shown in Fig. 2a–c, the D1 diffusion couple formed the equilibrium phase layers Al/AuAl₂/AuAl/Au₂Al/Au₂Al+AlPd/T₁+Au₂Al/T₂/Au₈Al₃+AlPd₂/AlPd₂+α(Au,Pd)/α(Au,Pd). The phase layer thickness of AuAl₂, AuAl, and Au₂Al was approximately 10, 5, and 130 μm, respectively, and the Pd solubility was approximately 10 at% in the Au₂Al phase. The T₂–Au₈Al₃–AlPd₂ three-phase equilibrium was generated on the interface between the T₂ one-phase layer and Au₈Al₃+AlPd₂ two-phase layer. These three-phase equilibria T₂-Au₈Al₃-AlPd₂ were confirmed through the bulk alloy (A1) as shown in Fig. 3. The interface between the Au₈Al₃+AlPd₂ and AlPd₂+α two-phase layers was also examined. The existence of Au₈Al₃–AlPd₂–α three-phase equilibrium was confirmed by analyzing the bulk alloy (A2), as shown in Fig. 4. Four three-phase equilibria were observed in the D1 diffusion layers: Au₂Al–AlPd–T₁, T₁–Au₂Al–T₂, AlPd₂–T₂–Au₈Al₃, and α–Au₈Al₃–AlPd₂. The phase equilibria of the alloy equilibrium samples are summarized in Table 2. The phase diagram constructed according to the results in Tables 1 and 2 is shown in Fig. 5. In the diffusion-couple experiment, 11 three-phase equilibria, resembling the T₂–Au₈Al₃–AlPd₂ or Au₈Al₃–AlPd₂–α three-phase equilibria, were determined from the phase relationships. The remaining three-phase equilibria, Au₄A1–α–Au₈Al₃ and Au₂Al–AuAl–AlPd, were determined by applying alloy equilibrium methods to the alloy samples A3 and A4 (Table 2). These 13 sets of three-phase equilibria are shown as solid lines in Fig. 5. In addition, the AlPd₂–T₂–AlPd three-phase equilibria were estimated to meet the phase relationships shown as dashed lines in Fig. 5. This experiment revealed that most binary phases conform to the Al–Au [5] and Al–Pd [7] binary phase diagrams. The Au–Pd [8] system exists in the α phase only at 500 °C. In this study, Al solubility in the α phase was also determined. In the experiment, 13 three-phase equilibria were observed in the Au–Al–Pd ternary system: T₂–Au₂Al–Au₈Al₃, T₁–Au₂Al–T₂, T₁–Au₂Al–AlPd, AuAl–AlPd–Au₂Al, AuAl₂–AlPd–AuAl, Al₂₁Pd₈–AuAl₂–Al₃Pd₂, AlPd–AuAl₂–Al₃Pd₂, Al₄Pd–AuAl₂–Al₂₁Pd₈, Al–AuAl₂–Al₄Pd, T₁–AlPd–T₂, T₂–AlPd₂–Au₈Al₃, AlPd₂–Au₈Al₃–α, and Au₄Al–Au₈Al₃–α. The three-phase equilibrium AlPd–T₂–AlPd₂ was estimated to demonstrate the phase relationship. T₁ and T₂ are newly discovered ternary phases.

Fig. 2

a The whole view of D1 sample microstructure. b The detail phase layers of the A region in a. c The detail phase layers of the B region in a

Fig. 3

The microstructure of A1 alloy sample including Au₈Al₃, T₂ and AlPd₂ phases

Fig. 4

The microstructure of A2 alloy sample including Au₈Al₃, α(Au,Pd) and AlPd₂ phases

Fig. 5

The isotherm of Au-Al-Pd at 500 °C determined by this study

Solubility range of binary intermetallic phases

The Au solubility in the Al₃Pd₂, AlPd, and AlPd₂ phases was 6, 15, and 34, respectively. The Pd solubility in AuAl₂, Au₂Al, and Au₈Al₃ phases was 5, 10, and 13 at%, respectively. In the Au₄Al phase, Al atoms were exchanged with Pd atoms, and the Pd solubility in the Au₄Al phase was 3 at%. The maximal solubility of the AlPd₂ and Au₈Al₃ phases was determined based on the bulk alloys (A2) shown in Fig. 4. The Al₄Pd, Al₂₁Pd₈, and AuAl phases exhibited extremely low solubility.

T₁ and T₂ ternary phases

The T₁ and T₂ phases were observed in the D1 and D2 diffusion couple. The compositions of the T₁ and T₂ ternary phases were determined by conducting EPMA analysis on the D1 and D2 diffusion samples, as shown in Fig. 6. The T₁ phase contained 37 at% Au, 26 at% Pd, and 37 at% Al. The T₂ phase contained 44 at% Au, 24 at% Pd, and 32 at% Al.

Fig. 6

The compositions of T₁ and T₂ ternary phases determined from D2 diffusion couple sample

Discussion

In this study, a classical semiinfinite diffusion couple was applied, meaning that the end couples maintained their original compositions after the diffusion couple was annealed. If volume diffusion in a semiinfinite couple is observed to be a rate-limiting step, a local equilibrium is assumed to exist; therefore, the rules described previously can be applied to relate the reaction zone morphology developed during isothermal diffusion to the phase diagram. The phase composition of the reaction zone is independent of time, whereas that of the diffusion path is fixed. The effectiveness of this technique in constructing isothermal cross-sections of ternary systems has been documented extensively [13].

The bulk alloy compositions and D2 and D6 diffusion paths are marked in Fig. 7. The microstructure of the D2 diffusion couple is shown in Fig. 8a, b. The D2 diffusion couple formed the equilibrium phase layers Al/AuAl₂/AuAl/Au₂Al/Au₂Al+AlPd/T₁/T₂/AlPd₂/AlPd₂+α(Au,Pd)/α(Au,Pd). A two-phase layer (Au₂Al+AlPd) was in contact with the T₁ phase layer; therefore, the Au₂Al–AlPd–T₁ three-phase equilibrium was generated. The D2 diffusion path is plotted in Fig. 8c, and its corresponding microstructure is shown in Fig. 8a, b. The solid a–b line crosses the α(Au, Pd) single-phase field, b–c is in the AlPd₂-α(Au, Pd) two-phase region, c–d is in the AlPd₂ phase region, e–f is in the T₂ phase region, g–h is in the T₁ phase region, and the dashed h–i line crosses the Au₂Al–AlPd–T₁ three-phase region, indicating the presence of an interface in the diffusion couple, with equilibrium between the single-phase T₁ and Au₂Al–AlPd two-phase region. The j–k–l–m–n–o–p line in Fig. 8c shows that the diffusion path crosses four single-phase layers, the Au₂Al, AuAl, and AuAl₂ phases and the Al end phase. The phase layer thickness of AuAl₂, AuAl, and Au₂Al was approximately 24, 35, and 50 μm, respectively. There are voids formed within the phase layer such as AuAl phase in Fig. 8a or Kirkendall voids build up at interface. If the diffusion couples were held for longer time, crack may form between certain phase layers.

Fig. 7

The bulk alloy samples and the diffusion paths of D2 and D6 samples

Fig. 8

a The whole view of D2 sample microstructure. b The detail phase layers of the A region in a. c The diffusion path of D2 diffusion couple and sketch figure

The D6 diffusion path crossed three three-phase regions (AuAl₂–AlPd–Al₃Pd₂, AuAl₂–Al₃Pd₂–Al₂₁Pd₈, and AuAl₂–Al₂₁Pd₈–Al₄Pd) with AuAl₂ as the common matrix phase. Figure 9a shows the diffusion path near the Al–Pd binary region, and Fig. 9b shows the plot displaying the equilibrium of these three phases and their related two-phase regions. According to Fig. 9b, f–g is the AuAl₂–AlPd–Al₃Pd₂ three-phase equilibrium, g–h is the AuAl₂–Al₃Pd₂ two-phase region, h–i is the AuAl₂–Al₃Pd₂–Al₂₁Pd₈ three-phase equilibrium, i–j is the AuAl₂–Al₂₁Pd₈ two-phase region, j–k is the AuAl₂–Al₂₁Pd₈–Al₄Pd three-phase equilibrium, k–l is the AuAl₂–Al₄Pd two-phase region, and l–m is the Al₄Pd single-phase region, and then in equilibrium with the Al edge phase. The microstructure and corresponding sketch are shown in Fig. 9c and are labeled from f to m, as described previously.

Fig. 9

a The diffusion path of D6 diffusion couple near the Al–Pd binary region. b The sketch figure of D6 diffusion path. c The microstructure of D6 diffusion couple with the label (f to m) and sketch figure

For the wire bonding process, Fig. 10 shows the microstructure of the Au–0.9Pd wire sample under 165 °C and 3,000-h aging [1]. The (Au, Pd)₄Al and Au₅Al₂ (as Au₈Al₃) phases tangled with the Pd-rich layer as α(Au, Pd). Because these three phases will maintain three-phase equilibrium as this study and this stable interface can hold for long time means better wire bond reliability.

Fig. 10

The microstructure of the Au–0.9Pd wire sample under 165 °C and 3,000-h aging [1]

For the intermetallic phases, AuAl₂ is purple and used for jewellery applications. The solubility of Pd in the purple AuAl₂ phase is limited in Fig. 8a and has 5 at% in D5 diffusion couple. And both AuAl₂ phase still maintain purple color.

Conclusion

This study is the first to investigate a Au–Al–Pd system. The isothermal section of a Au–Al–Pd system annealed at 500 °C was presented in this study. The entire composition range included two ternary phases (T₁ and T₂) and 14 three-phase equilibria: T₂–Au₂Al–Au₈Al₃, T₁–Au₂Al–T₂, T₁–Au₂Al–AlPd, AuAl–AlPd–Au₂Al, AuAl₂–AlPd–AuAl, Al₂₁Pd₈–AuAl₂–Al₃Pd₂, AlPd–AuAl₂–Al₃Pd₂, Al₄Pd–AuAl₂–Al₂₁Pd₈, Al–AuAl₂–Al₄Pd, T₁–AlPd–T₂, T₂–AlPd₂–Au₈Al₃, AlPd₂–Au₈Al₃–α, and Au₄Al–Au₈Al₃–α. The three-phase equilibrium AlPd–T₂–AlPd₂ was estimated to exhibit the phase relationship. T₁ and T₂ are newly discovered ternary phases. The T₁ phase contained 37 at% Au, 26 at% Pd, and 37 at% Al. The T₂ phase contained 44 at% Au, 24 at% Pd, and 32 at% Al.

The Au solubility in the Al₃Pd₂, AlPd, and AlPd₂ phases was 6, 15, and 34 at%, respectively, by exchanging Pd atoms. The Pd solubility in the AuAl₂, Au₂Al, and Au₈Al₃ phases was 5, 10, and 13 at%, respectively, by exchanging Au atoms. The Pd solubility in Au₄Al phase was 3 at% by exchanging Al atoms.