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Pd surface finish through catalytic modification
Corrosion Science 146 (2019) 112–120
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Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Significant improvement of the thermal stability and electrochemical corrosion resistance of the Au/Pd surface finish through catalytic modification Y.H. Huanga, S.P. Yanga, P.T. Leea,b, T.T. Kuoa,c, C.E. Hoa,
T
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a
Department of Chemical Engineering & Materials Science, Yuan Ze University, Taiwan, ROC Department of Materials Science & Engineering, National Taiwan University, Taiwan, ROC c Taiwan Uyemura Limited Company, Taiwan, ROC b
A R T I C LE I N FO
A B S T R A C T
Keywords: Au/Pd/Cu Au/Pd/Au/Cu Au nanolayer Nanovoid free Electrochemical corrosion Catalytic modification
The thermal reliability and electrochemical corrosion of Au/Pd (electroless palladium/immersion gold, EPIG) and Au/Pd/Au (immersion gold/electroless palladium/immersion gold, IGEPIG) surface finishes over Cu traces were characterized in this study. The predeposition of a Au nanolayer over Cu (i.e., IGEPIG case) served as the catalysts for the electroless reduction of Pd2+ ions and modified the polycrystalline Pd structure (EP) into a dense Pd film with limited grain boundaries and nanochannels/nanovoids. This phenomenon greatly improved the thermal reliability and corrosion resistance of the Au/Pd/Cu structure (EPIG case), even though the predeposited Au film was only 10 nm thick.
1. Introduction Recently, the demands of high-speed signal transfer in the fifth generation (5G) mobile communication and the wide-range signal detection in autopilot systems have driven the electronic industry towards the development of high-frequency circuits with low signal degradation. In the microwave frequency region, the skin effect becomes an important factor for signal loss considerations [1,2], where the alternating electric current tends to distribute near the outer surface of an electrical conductor, leading to an increase in the surface resistivity and conductor loss. As Cu metal is easily oxidized in atmospheric environment, a Ni(P)-based metallization accompanied with noble metal finishes, such as electroless nickel/electroless palladium/immersion gold (i.e., Au/Pd/Ni(P), ENEPIG), are commonly codeposited over the Cu circuits to enhance the solderability and the oxidation/corrosion resistance. The topmost Au film serves as oxidation resistance. The intermediate Pd film prevents the galvanic corrosion [3] resulting from the Au electrolyte (i.e., black pads [4–6]), and inhibits the out-diffusion of Ni atoms through high-diffusivity paths [7]. The underlying Ni(P) finish can act as a good diffusion barrier against interdiffusion between Cu and Sn, preventing the formation of excessive Cu-Sn intermetallic (e.g., Cu6Sn5 and Cu3Sn) and Kirkendall voids [8,9]. Additionally, Ni(P) has an amorphous structure, which inhibits the grain boundary diffusion of Cu and O and promotes the Cu oxidation resistance. However,
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the permeability (μ = 1.25 × 10−4 H/m) and resistivity (ρ = 7.4 μΩ cm) of Ni are orders of magnitude higher than those of Cu (μ = 1.25 × 10-6 H/m; ρ = 1.7 μΩ cm) [1] and inevitably cause a noticeable signal loss that arises from the skin effect in high-frequently applications [1]. Modification of ENEPIG into a Ni-free surface finish through the direct deposition of electroless palladium/immersion gold (EPIG) over the Cu circuits has recently been attempted and adopted in communication devices. The solderability and thermal reliability of the EPIG surface finish have been investigated in the literature [9–13]. Ho et al. [9,12–13] revealed that the mechanical properties of microelectronic joints depend on the Pd thickness (δPd) or the Pd concentration in the solder (CPd). An appropriate Pd deposition (δPd = 0–0.4 μm) [12] or minor Pd alloying addition (CPd = 0–0.3 wt.%) to a solder alloy [9] can avoid the palladium-embrittlement phenomenon [13] and effectively enhance the joint strength due to the suppression of the Cu3Sn growth and Kirkendall void formation. Nevertheless, the solderability of the Sn/Cu joint system can be greatly improved with the EPIG deposition, a series of nanovoids might nucleate at the Pd/Cu interface in the as-deposited state, which might degrade its thermal reliability [14]. Elimination of such nanovoids from the Pd/Cu interface via Au catalytic modification (i.e., predeposition of an additional Au nanolayer over the Cu circuits prior to the Pd deposition; IGEPIG system) is now being attempted in the microelectronic industry. The objective of this study is to
Corresponding author. E-mail address: [email protected] (C.E. Ho).
https://doi.org/10.1016/j.corsci.2018.10.030 Received 5 July 2018; Received in revised form 11 September 2018; Accepted 19 October 2018 Available online 30 October 2018 0010-938X/ © 2018 Elsevier Ltd. All rights reserved.
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Fig. 1. BF-STEM cross-sectional images of (a) the as-deposited Au/Pd/Cu (EPIG case) and (b) Au/Pd/Au/Cu (IGEPIG case). (a’)–(b’) Selected-area diffraction patterns (SADPs) taken from the Pd layer (EP) of EPIG and IGEPIG, respectively.
in combination with selected-area diffraction (SAD) analysis was employed to investigate the interior microstructure of the Au/Pd/Cu and Au/Pd/Au/Cu multilayers. The beam size utilized in the SAD analysis was approximately 120 nm, which enables investigation of the crystallographic difference(s) of these two multilayers, specifically for the Pd films. The TEM specimens were prepared using a FIB apparatus and were mounted onto a Mo grid for the TEM analysis to avoid disturbances resulting from the grid. In the electrochemical corrosion test, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) studies were performed in a 1.5 mol/L sulfuric acid solution through a standard three-electrode cell using a VersaSTAT 4 potentiostat (Princeton Applied Research) at room temperature (25 °C). A fixed coating area (3.14 cm2; radius: 1 cm) was examined. A saturated mercury-mercurous sulfate electrode (SMSE) and platinum (Pt) were used as the reference electrode and auxiliary electrode, respectively. Before the electrochemical analysis, the samples were kept in the solution for 1 h to stabilize the open circuit potential (OCP). The voltages applied in the potentiodynamic polarization test was varied in the range of –1.7 V to 0 V at a scanning rate of 1 mV/s. EIS measurements were performed in the frequency range of 10–2 Hz to 105 Hz at the OCP with a sinusoidal voltage of a 10 mV amplitude. The EIS spectra were analyzed and modeled using the ZSimpWin program.
investigate the thermal stability and electrochemical corrosion of the Cu traces deposited with EPIG and IGEPIG surface finishes. The effect of the Au catalytic modification on the two characteristics (thermal stability and electrochemical corrosion) and the underlying mechanism associated with this microstructural modification will be addressed in this study. 2. Experimental details The thermal reliability and electrochemical corrosion of EPIG and IGEPIG surface finishes over Cu traces were investigated in this study. The topmost Au film (IG) and the intermediate Pd film (EP) were approximately 0.05–0.1 μm and 0.1–0.2 μm thick, respectively. The bottom Au film (IG) has two different thickness, i.e., 0 μm (EPIG) and approximately 0.01 μm (10 nm) (IGEPIG). The thicknesses of the Au and Pd films were measured using an X-ray fluorescence spectrometer (XRF) and subsequently were verified via cross-sectional analysis using transmission electron microscopy (TEM) (Fig. 1a–b). To evaluate the thermal reliability of Au/Pd/Cu (EPIG case) and Au/Pd/Au/Cu (IGEPIG case), an isothermal annealing test was performed at 180 °C for 0 (as-deposited) to 50 h in an air convection oven. After isothermal annealing, the surfaces of EPIG and IGEPIG were analyzed using scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDS) and X-ray photoelectron spectroscopy (XPS). Additionally, focused ion beam (FIB) and field-emission TEM (FE-TEM) 113
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3. Results & discussion
3.2. Isothermal annealing test
3.1. As-deposited state
Fig. 2(a)–(b) presents the SEM images (backscattered electron mode) of the surfaces of the Au/Pd/Cu (EPIG case) and Au/Pd/Au/Cu (IGEPIG case) after annealing at 180 °C for 50 h, respectively. The inset (Fig. 2a’) shows a backscattered electron image (BEI) (gray) overlapped EDS elemental mappings of Cu (orange) and O (green) corresponding to Fig. 2(a). Numerous Cu oxides (CuxO) that appeared on the Au surface can be noted in the EPIG case (Fig. 2a, a’) after annealing of 180 °C for a few hours. In contrast, no meaningful Cu oxides were detected for the IGEPIG case (Fig. 2b). Fig. 2(c)–(d) displays FIB-processed cross sections of the EPIG and IGEPIG shown in Fig. 2(a)–(b), respectively, demonstrating the interior microstructures of Au/Pd/Cu and Au/Pd/Au/ Cu after annealing. The FIB images confirm the existence of an additional material (CuxO) with a thickness of a few nanometers over the EPIG surface but not the IGEPIG surface. These observations indicate that many Cu atoms had diffused towards the free surface of the Au/Pd/ Cu multilayer through the crystalline Au/Pd dual layer, where they nucleated as Cu oxides. The observations also reveal that the predeposition of an Au nanolayer (10 nm) above Cu can greatly reduce the out-diffusion of Cu, thereby preventing the formation of Cu oxides. Next, further analysis of the Cu oxides via XPS was conducted. Fig. 3 shows the XPS spectra (Cu 2p) that were acquired from the surfaces of the EPIG and IGEPIG shown in Fig. 2. The analysis area was
Fig. 1(a)–(b) shows the cross-sectional, bright-field scanning transmission electron microscopy (BF-STEM) images of the as-deposited EPIG and IGEPIG multilayers over the Cu traces. Several nanovoids formed at the Pd/Cu interface of the EPIG case (Fig. 1a), while no such material defects were observed in the alternative case (i.e., IGEPIG, Fig. 1b), suggesting that predeposition of a Au nanolayer between Pd and Cu can inhibit such nanovoid formation. Another microstructural difference between the EPIG and IGEPIG cases was that some nanochannels along the film deposition direction existed among the Pd grains of the EPIG case (Fig. 1a), which were not observed in the IGEPIG case (Fig. 1b). Moreover, the Pd film of the latter case (IGEPIG) possessed a denser microstructure with larger grain sizes (Fig. 1b) than that of the EPIG case (Fig. 1a). The Pd reciprocal lattices of the EPIG and IGEPIG taken from the TEM selected-area diffraction patterns (SADPs) display regular ring pattern (Fig. 1a’) and diffraction spots (Fig. 1b’), respectively, confirming the observations of the BF-STEM images (Fig. 1a–b). The formation mechanism of the nanovoids and nanochannels is discussed in Section 3.4.
Fig. 2. Backscattered electron images (BEIs) showing the surface appearances of (a) Au/Pd/Cu (EPIG) and (b) Au/Pd/Au/Cu (IGEPIG) after annealing at 180 °C for 50 h. (a’) BEI overlapped EDS elemental mappings of Cu and O corresponding to (a). (c)–(d) FIB-processed cross sections of EPIG and IGEPIG shown in (a)–(b), respectively. 114
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400 μm (diameter), providing a wide-range characterization of the surface chemistry after annealing at 180 °C for 50 h. Charging effects in the spectra were calibrated with reference to adventitious C 1s at 284.5 eV [10,15]. Four sharp peaks can be noted in the EPIG spectra. Based on the XPS database of National Institute of Standard and Technology (NIST), two peaks of Cu 2p3/2 and 2p1/2 with binding energies of 932.30 eV and 952.10 eV were identified as cupric oxide (CuO). Additionally, the satellite peaks near 940–950 eV and 960–965 eV were caused by electron shake-up processes and also belonged to CuO [16]. The alternatives were at 931.60 eV and 951.40 eV with bonding energies of Cu 2p3/2 and 2p1/2, corresponding to cuprous oxide (Cu2O). In contrast, the peak intensities of CuO and Cu2O for the IGEPIG case were significantly lower than those for the EPIG case, agreeing well with the observations of Fig. 2, where no meaningful Cu oxides were created in the IGEPIG case after the same annealing process. Fig. 4(a)–(b) shows the BF-STEM images of the cross sections for Au/Pd/Cu (EPIG case) and Au/Pd/Au/Cu (IGEPIG case) after annealing at 180 °C for 50 h. A significant increase in the nanovoid size between Pd and Cu occurred in the former case after annealing (Figs. 1a and 4 a), whereas no such voids were observed in the latter case (Fig. 4b). Fig. 4(c) shows a TEM-SADP acquired from the CuxO (+ Au) shown in Fig. 4(a). The reciprocal lattices of CuxO exhibited Debye rings,
Fig. 3. X-ray photoelectron spectroscopy (XPS) spectra of the Cu 2p region for the Au surface of Au/Pd/Cu (EPIG case) and Au/Pd/Au/Cu (IGEPIG case) after annealing at 180 °C for 50 h.
Fig. 4. (a)–(b) BF-STEM cross-sectional images of Au/Pd/Cu (EPIG case) and Au/Pd/Au/Cu (IGEPIG case) after annealing at 180 °C for 50 h, respectively. (c) Selected-area diffraction pattern (SADP) acquiring from CuxO (+ Au). 115
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Fig. 5. (a) Potentiodynamic polarization curves of Au/Pd/Cu (EPIG case) and Au/Pd/Au/Cu (IGEPIG case) in a sulfuric acid solution (1.5 mol/L). (b)–(c) Backscattered electron images (BEIs) overlapped EDS elemental mappings of Au, Pd, and Cu: (b) EPIG case; (c) IGEPIG case. The insets (b’ & c’) are BEIs of (b)–(c).
3.3. Electrochemical corrosion
Table 1 Corrosion current density (Icorr), corrosion potential (Ecorr), and corrosion inhibition efficiency (IE%) acquired from the potentiodynamic polarization analysis of the bare Cu, EPIG and IGEPIG cases. Specimen Bare Cu EPIG IGEPIG
Icorr (A/cm2) −5
4.8 × 10 2.8 × 10−5 5.3 × 10−6
Ecorr (V/SMSE)
IE (%)
–1.07 –1.40 –1.30
NA 41.66 88.96
Fig. 5(a) shows the potentiodynamic polarization curves of Au/Pd/ Cu (dark) and Au/Pd/Au/Cu (red) in a sulfuric acid solution (1.5 mol/ L), providing a comparison of the electrochemical corrosion between the two examined surface finishes (i.e., EPIG and IGEPIG). The potentiodynamic polarization curves can be subdivided into cathodic and anodic polarization curves, and the former (cathodic polarization curve) is related to the hydrogen reduction and the latter (anodic polarization curve) to the metal layer(s) (i.e., Au/Pd/Cu and Au/Pd/Au/ Cu) oxidation [23]. The intersection point of the two polarization curves determines the corrosion potential (Ecorr), as shown in Fig. 5(a). Subsequently, the corrosion current density (Icorr) can be acquired through the intersection point of the fitted regression lines of the cathodic and anodic polarization curves in the Tafel region [24], where a ± 100 mV variation of Ecorr was taken. Generally, a more positive Ecorr value indicates a decrease in Icorr [25]. A smaller Icorr value implies a slower corrosion rate of the corrosion system [23]. With the corrosion current density of bare Cu (Icorr(o)), the corrosion inhibition efficiency (IE%) can be calculated [24]:
indicating that this newly formed nanolayer possesses a polycrystalline structure. Moreover, the Debye rings with the d-spacings matched well with the face-centered cubic Au (space group: Fm 3¯ m; a = 0.4072 nm [17]), monoclinic CuO (space group: C2/c; a = 0.4653 nm, b = 0.3410 nm, c = 0.5108 nm, and β = 99.48° [18]), and cubic Cu2O (space group: Pn 3¯ m; a = 0.4250 nm [19]). The deviations between the measured and theoretical d-spacings are all smaller than 3% in the indexation of Fig. 4(c). The above analysis shows that the CuxO primarily consisted of CuO and Cu2O nanograins, confirming the XPS characterization. According to the literature [20,21], grain boundaries are the dominant diffusion path of Cu in Pd at 175–250 °C. Therefore, we propose that out-diffusion of Cu along the grain boundaries of the Au/ Pd dual layer (Fig. 2c) to the free surface in the 180 °C aging treatment was the root cause of the voiding enlargement between Pd and Cu and the formation of CuO and Cu2O species over the Au surface, as observed in Figs. 2(a), (c), and 4(a). The presence of a significant amount of Cu oxides on the Au surface of the Au/Pd/Cu multilayer (EPIG case) and the aggregation of numerous nanovoids in the vicinity of the Cu substrate (Figs. 2a, c, and 4 a) after isothermal annealing at 180 °C for 50 h inevitably deteriorate the solderability of microelectronic packages. According to the literature [22], disproportionation and complexation reactions occur between Cu oxides and organic acid flux during a reflow process. A water bubble will be produced by the above reactions during an early stage of the soldering reaction and subsequently might induce bump void formation, degrading the solder joint integrity. Next, the investigation of the corrosion resistance of the EPIG and IGEPIG surface finishes was conducted.
Icorr(i) ⎞ IE% = ⎜⎛1⎟ × 100% I ⎝ corr(o) ⎠
(1)
where Icorr(i) represents the corrosion current density for the Cu substrate coated with surface finish i (= EPIG or IGEPIG). A lager value of the corrosion inhibition efficiency (IE%) suggests a better corrosion resistance of the system. Table 1 lists Ecorr, Icorr(i), Icorr(o), and IE% of bare Cu, Au/Pd/Cu (EPIG case), and Au/Pd/Au/Cu (IGEPIG case), which are acquired from the Tafel extrapolation with Fig. 5(a) and Eq. (1). The IGEPIG case possesses a smaller Icorr (5.3 × 10–6 A/cm2), a more positive Ecorr (1.3 V), and a larger IE% (88.96%) than that of the EPIG case (where Icorr = 2.8 × 10–5 A/cm2, Ecorr = –1.4 V, and IE% = 41.66%), indicating that IGEPIG possesses a better corrosion resistance than EPIG does. Fig. 5(b)–(c) shows backscattered electron images (BEIs, Fig. 5b’–c’) overlapped Au, Pd, and Cu elemental mappings of the EPIG and IGEPIG cases after the potentiodynamic polarization test (Fig. 5a), respectively. The colors yellow, blue, and orange displayed in the BEIs (Fig. 5b–c) represent Au, Pd, and Cu elemental distributions, respectively. A 116
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Z” represent the impedance, the real component impedance, and the imaginary component impedance, respectively, and the dash lines represent the fitting values with the experimental measurements. The diameter of the semicircle in the Nyquist plot is a good indication of the corrosion characteristic of a specimen, and a larger diameter implies a better corrosion resistance [26]. The IGEPIG case displays a larger impedance arc diameter than that of EPIG (Fig. 6a) suggests that IGEPIG over Cu might possess a better anticorrosion performance than that of EPIG. The characteristic frequencies were unavailable in the Nyquist plot (Fig. 6a). This is because the EIS measurements were performed in the frequency range of 10–2 Hz to 105 Hz at constant OCP with a sinusoidal voltage of a 10 mV amplitude, such that the initiation of corrosion in the two examined cases (Au/Pd/Cu and Au/Pd/Au/Cu) can be well identified after the EIS analysis (see TEM analysis, Fig. 7). Phase angle (φ) and impedance module (log|Z|) as a function of frequency were shown in the Bode plot (Fig. 6b). The log|Z| values of Au/Pd/Cu (EPIG case) and Au/Pd/Cu (IGEPIG case) were approximately the same in the frequency range of 10–2 Hz to 105 Hz, suggesting that the impedance of the corrosion system was not significantly altered with the additional Au deposition. Phase angle in the high-, medium-, and low-frequency characterizes the surface defects, corrosion products within the coating(s) (i.e., EPIG or IGEPIG), and the charge transfer at the coating/metal interface, respectively [27]. A significant change in the medium-/high-frequency phase angles occurred (Fig. 6b), suggesting that the deposition of either EPIG or IGEPIG has a certain influence on the corrosion products within the EPIG and IGEPIG surface finishes and the charge transfer at the coating/metal interface. Three well-defined time constants can be noted in the φ spectra (Fig. 6b), indicating that three distinct electrochemical processes occurred in the EPIG and IGEPIG systems. Therefore, the EIS analysis results of the EPIG and IGEPIG cases can be analyzed through the equivalent circuit with three resistor-capacitor (RC) circuits, as shown in Fig. 6(c). The measured impedance and phase angle (Fig. 6a–b) were fitted using the ZSimpWin program with the equivalent circuit shown in Fig. 6(c). Table 2 lists the EIS parameters corresponding to the equivalent circuits of the EPIG and IGEPIG cases, where Rs, Rpore, Rcp, and Rct represent the solution resistance between the reference electrode (SMSE) and working electrode (Au/Pd/Cu or Au/Pd/Au/Cu), the resistance of the coating in pore areas, the resistance of corrosion product, and the resistance of charge transfer, respectively; n is the dispersion index and it is non-unity (Table 2). The constant phase element (CPE) was therefore employed to describe the frequency dependence of non-ideal capacitive behavior [28]. CPE1, CPE2, and CPE3 represent the multilayer coating including pores in the outer layer coating, the multilayer coating within the defect, and the double layer, respectively. The largest percentage error and the chi-squared value of the EPIG case were only 14.3% and 3.90 × 10–4, respectively, and those of IGEPIG were 27.1% and 3.43 × 10–4, respectively. The good agreement between the measured impedance/phase angle spectroscopies (Fig. 6a–b) and the simulation results confirms the validity of the equivalent circuit proposed in Fig. 6(c). The Rct values are much larger than the Rs, Rpore, and Rcp values, indicating that the charge transfer process at the coating/substrate interface (i.e., EPIG/Cu and IGEPIG/Cu interfaces) might dominate the corrosion mechanism of the two surface finishes. Furthermore, IGEPIG exhibits a larger Rct value than EPIG (Table 2), reflecting that IGEPIG possesses a better anticorrosion performance than EPIG does. The EIS analysis results (Fig. 6 and Table 2) are consistent with that obtained from the electrochemical polarization (Fig. 5a) and SEM-EDS analyses (Fig. 5c, b), which indicate an additional Au nanolayer acts as an effective barrier against the corrosion of a sulfuric acid solution. Theoretically, the existence of large and/or many pores in a corrosion system might cause small Rpore because of a large cross-section area. However, the opposite trend was observed in Table 2, where the Rpore value of the EPIG case (12.28 Ω· cm2) was slightly larger than that of the IGEPIG case (5.786 Ω cm2), although some nanovoids indeed did exist between
Fig. 6. (a) Nyquist plot and (b) Bode plot of EPIG case (dark) and IGEPIG case (red) at 2 h of immersion in a sulfuric acid solution (1.5 mol/L). (c) Equivalent electrical circuit of EPIG case and IGEPIG case used for the impedance plots fitting (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
significant amount of surface damages can be clearly observed in the EPIG case (Fig. 5b), where the Au/Pd dual layer was completely eliminated and the underlying Cu substrate was exposed accordingly after the potentiodynamic polarization test. In contrast to the EPIG case (Fig. 5b), only a few peeling areas were caused in the IGEPIG case after the corrosion test (Fig. 5c), indicating that less corrosion had occurred in the IGEPIG system. These observations agree well with the data listed in Table 1 and show that the predeposition of a Au nanolayer serves as a good protection against sulfuric acid corrosion. Fig. 6(a)–(b) presents electrochemical impedance spectroscopy (EIS) analysis at individual OCP of the EPIG (dark triangle) and IGEPIG (red dot) surface finishes after immersion in a sulfuric acid solution (1.5 mol/L) for 2 h: (a) Nyquist plot; and (b) Bode plot, where Z, Z’, and 117
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Fig. 7. (a)–(b) BF-STEM cross-sectional images of Au/Pd/Cu (EPIG case) and Au/Pd/Au/Cu (IGEPIG case) after EIS analysis, respectively. (c)–(d) Zoomed-in view of the Pd/Cu interface corresponding to (a)–(b). Table 2 EIS parameters corresponding to the equivalent circuits of the EPIG and IGEPIG cases. Specimen
Rs (Ω cm2)
CPE1 (S secn)
n1
Rpore (Ω cm2)
CPE2 (S secn)
n2
Rcp (Ω cm2)
CPE3 (S secn)
n3
Rct (Ω cm2)
chisquared
EPIG IGEPIG
3.838 3.998
1.95 × 10−4 2.78 × 10−4
0.868 0.858
12.28 5.786
7.46 × 10−4 1.00 × 10−3
0.850 0.827
215.9 255.5
1.57 × 10−3 8.79 × 10−4
0.737 0.824
1.80 × 104 2.99 × 104
3.90 × 10−4 3.43 × 10−4
3.4. Effect of the Au nanolayer (Au catalysts)
Pd and Cu in the former case (EPIG) (Fig. 1a). Here we argued that the examined multilayers (i.e., Au/Pd/Cu and Au/Pd/Au/Cu) basically possessed a dense structure and the existence of a limited amount of material defects (e.g., nanovoids) posed a minor influence on the global resistance of the corrosion system, such that the Rpore values of the two examined cases both were quite small and approximately indistinguishable (Table 2). This opposite trend might result from the deviation between experimental measurements and the simulation results (Fig. 6a–b). Further study is required to validate the above hypothesis. Fig. 7 shows the BF-STEM images of (a, c) Au/Pd/Cu and (b, d) Au/ Pd/Au/Cu after the EIS test, which provides further information associated with the microstructural differences between the EPIG and IGEPIG cases after the electrochemical corrosion test. A series of voids nucleated at the Pd/Cu interface of the EPIG case after the EIS analysis (Fig. 7a, c), indicating the occurrence of serious corrosion at the interface between the Cu substrate and the surface finish (EPIG) in the EIS analysis. In contrast, only a limited number of nanovoids can be observed between the surface finish (Au/Pd/Au) and Cu (Fig. 7b, d) under the same test condition. The TEM results confirm that the charge transfer process (Rct) at the coating/substrate (Cu) interface indeed plays a dominant role in the corrosion mechanism of the Au/Pd/Cu and Au/Pd/Au/Cu multilayers, and the predeposition of a Au nanolayer over Cu (i.e., IGEPIG case) greatly enhances the corrosion protection.
Investigations of TEM/FIB (Figs. 1, 2c–d, 4, and 7), SEM/EDS (Figs. 2a–b and 5 b–c), XPS (Fig. 3), potentiodynamic polarization (Figs. 5a), and EIS (Fig. 6) showed that Au/Pd/Au/Cu (IGEPIG case) possesses a better thermal stability and corrosion resistance than that of Au/Pd/Cu (EPIG case), preventing the formation of Cu oxides in isothermal annealing (180 °C) and the corrosion of Cu substrate in a sulfuric acid solution (1.5 mol/L). The improvements in the thermal/electrochemical characteristics of the Au/Pd/Cu structure (EPIG case) can be ascribed to the microstructural modification of the Pd film via predeposition of the additional Au nanolayer (IGEPIG case), as presented in Fig. 1(b). To gain a better understanding of the Pd microstructural modification, the electroless Pd (EP) deposition and its pretreatment over the Cu substrate should be examined. Fig. 8(a)–(b) shows BEIs overlapped EDS elemental mappings of Au (yellow), Pd (blue), and Cu (orange) for the Cu substrates deposited with Pd catalysts and a Au nanolayer (∼10 nm), respectively, demonstrating the catalytic distributions prior to the electroless Pd reduction in the EPIG and IGEPIG cases. Unlike that for uniform distribution of a Au film (catalysts) on Cu (Fig. 8b), there was only a limited amount of the Pd catalysts scattered over the Cu surface in the EPIG case (Fig. 8a). This SEM-EDS examination reveals that the catalytic deposition prior to the EP process was distinctly 118
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Fig. 8. Backscattered electron images (BEIs) overlapped EDS elemental mappings of Au, Pd, and Cu after catalytic deposition: (a) Pd catalyst (EPIG case); (b) Au catalyst (IGEPIG case). (c)–(f) Mechanism of Pd (EP) deposition through the use of different catalysts: (c, e) Pd catalyst (EPIG case); (d, f) Au catalyst (IGEPIG case).
deposited over the Cu traces were evaluated. In the high-temperature (180 °C) storage test, only a limited amount of Cu oxides formed on the Au surface of the Au/Pd/Au/Cu multilayer after annealing for 50 h, suggesting that predeposition of an additional Au nanolayer can prevent the out-diffusion of Cu and that IGEPIG possesses better oxidation resistance than that of EPIG. The potentiodynamic polarization and EIS analyses showed that IGEPIG has a smaller Icorr (5.3 × 10–6 A/cm2), a more positive Ecorr (–1.3 V), a larger IE% (88.96%) and Rct (2.99 × 104 Ω cm2) than those of EPIG (where Icorr = 2.8 × 10–5 A/cm2, Ecorr = –1.4 V, IE% = 41.66%, and Rct = 1.80 × 104 Ω cm2), indicating that IGEPIG possesses a better anticorrosion performance than EPIG does. Further EIS results in combination with TEM examination confirmed that the charge transfer process at the coating/substrate (Cu) interface plays a dominant role in the corrosion mechanism and that the corrosion resistance can be greatly enhanced by the predeposition of the Au nanolayer (i.e., IGEPIG case). We propose that modification of a polycrystalline Pd film into a nanochannel-/nanovoid-free structure by the Au catalytic modification (Au nanolayer) can promote the thermal/ electrochemical characteristics of an EPIG surface finish.
different in the two examined cases, and the replacement of the Pd catalysts by Au (i.e., Au nanolayer) greatly enhances the uniformity of the catalytic distribution for the subsequent Pd reduction reaction. Based on the catalytic modification (Fig. 8a–b), we proposed a possible mechanism associated with the Pd microstructures obtained in Fig. 1(a)–(b). As only a limited amount of Pd catalysts was deposited over the Cu substrate in the EPIG case (Fig. 8a and c), the reduction reaction of electroless Pd was only limited to the specific Cu sites with Pd catalysts at the early stage of electroless Pd deposition. This phenomenon resulted in a loose Pd microstructure with a polycrystalline structure and numerous nanochannels within the Pd film (Figs. 1a and 8 e) as the film thickness was on the submicron scale. The presence of nanochannels within the Pd film might induce the galvanic corrosion of Cu in the subsequent Au plating process, leading to nanovoid formation in the as-deposited state (Fig. 1a). Additionally, the existence of numerous grain boundaries in the polycrystalline Pd facilitated the oudiffusion of Cu [14], yielding the nucleation of Cu oxides on the Au surface (Figs. 2a, 2c, 3, 4a, and 4c). In the alternative case (IGEPIG), however, a continuous catalytic layer (Au) was predeposited over the Cu substrate for the subsequent electroless Pd reaction (Fig. 8b and d), such that a reduction of Pd2+ into Pd occurred everywhere with the Au catalysts and a dense Pd film was yielded after the EP process (Figs. 1b and 8 f). Therefore, the predeposition of a 10 nm-thick Au layer over a Cu substrate (i.e., IGEPIG case) modified the loose Pd microstructure (Fig. 1a) into a dense film with limited grain boundaries (Fig. 1b), which avoided the formation of nanovoids/nanochannels at the Pd/Cu interface (or within the Pd film) (Figs. 1a, 2 c, and 4a), thereby enhancing the thermal/electrochemical characteristics of the Au/Pd/Cu structure.
Acknowledgments This study was supported by the Ministry of Science and Technology (R.O.C.) through Grant Nos. MOST105-2628-E-155-001-MY3, MOST105-2221-E-155-006-MY2, and MOST106-2622-E-155-006-CC3. The authors would like to thank Yu-Ling Chien (National Central University), Yin-Mei Chang (National Tsing Hua University), Yi-Chen Yu (National Tsing Hua University), Wei-Ping Dow (National Chung Hsing University), and Wun-Jheng Zeng (Taiwan Uyemura) for their assistance with the experiments.
4. Conclusions References The thermal reliability and electrochemical corrosion characteristics of EPIG (Au/Pd) and IGEPIG (Au/Pd/Au) surface finishes
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Significant improvement of the thermal stability and electrochemical corrosion resistance of the Au/Pd surface finish through catalytic modification Y.H. Huanga, S.P. Yanga, P.T. Leea,b, T.T. Kuoa,c, C.E. Hoa,
T
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a
Department of Chemical Engineering & Materials Science, Yuan Ze University, Taiwan, ROC Department of Materials Science & Engineering, National Taiwan University, Taiwan, ROC c Taiwan Uyemura Limited Company, Taiwan, ROC b
A R T I C LE I N FO
A B S T R A C T
Keywords: Au/Pd/Cu Au/Pd/Au/Cu Au nanolayer Nanovoid free Electrochemical corrosion Catalytic modification
The thermal reliability and electrochemical corrosion of Au/Pd (electroless palladium/immersion gold, EPIG) and Au/Pd/Au (immersion gold/electroless palladium/immersion gold, IGEPIG) surface finishes over Cu traces were characterized in this study. The predeposition of a Au nanolayer over Cu (i.e., IGEPIG case) served as the catalysts for the electroless reduction of Pd2+ ions and modified the polycrystalline Pd structure (EP) into a dense Pd film with limited grain boundaries and nanochannels/nanovoids. This phenomenon greatly improved the thermal reliability and corrosion resistance of the Au/Pd/Cu structure (EPIG case), even though the predeposited Au film was only 10 nm thick.
1. Introduction Recently, the demands of high-speed signal transfer in the fifth generation (5G) mobile communication and the wide-range signal detection in autopilot systems have driven the electronic industry towards the development of high-frequency circuits with low signal degradation. In the microwave frequency region, the skin effect becomes an important factor for signal loss considerations [1,2], where the alternating electric current tends to distribute near the outer surface of an electrical conductor, leading to an increase in the surface resistivity and conductor loss. As Cu metal is easily oxidized in atmospheric environment, a Ni(P)-based metallization accompanied with noble metal finishes, such as electroless nickel/electroless palladium/immersion gold (i.e., Au/Pd/Ni(P), ENEPIG), are commonly codeposited over the Cu circuits to enhance the solderability and the oxidation/corrosion resistance. The topmost Au film serves as oxidation resistance. The intermediate Pd film prevents the galvanic corrosion [3] resulting from the Au electrolyte (i.e., black pads [4–6]), and inhibits the out-diffusion of Ni atoms through high-diffusivity paths [7]. The underlying Ni(P) finish can act as a good diffusion barrier against interdiffusion between Cu and Sn, preventing the formation of excessive Cu-Sn intermetallic (e.g., Cu6Sn5 and Cu3Sn) and Kirkendall voids [8,9]. Additionally, Ni(P) has an amorphous structure, which inhibits the grain boundary diffusion of Cu and O and promotes the Cu oxidation resistance. However,
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the permeability (μ = 1.25 × 10−4 H/m) and resistivity (ρ = 7.4 μΩ cm) of Ni are orders of magnitude higher than those of Cu (μ = 1.25 × 10-6 H/m; ρ = 1.7 μΩ cm) [1] and inevitably cause a noticeable signal loss that arises from the skin effect in high-frequently applications [1]. Modification of ENEPIG into a Ni-free surface finish through the direct deposition of electroless palladium/immersion gold (EPIG) over the Cu circuits has recently been attempted and adopted in communication devices. The solderability and thermal reliability of the EPIG surface finish have been investigated in the literature [9–13]. Ho et al. [9,12–13] revealed that the mechanical properties of microelectronic joints depend on the Pd thickness (δPd) or the Pd concentration in the solder (CPd). An appropriate Pd deposition (δPd = 0–0.4 μm) [12] or minor Pd alloying addition (CPd = 0–0.3 wt.%) to a solder alloy [9] can avoid the palladium-embrittlement phenomenon [13] and effectively enhance the joint strength due to the suppression of the Cu3Sn growth and Kirkendall void formation. Nevertheless, the solderability of the Sn/Cu joint system can be greatly improved with the EPIG deposition, a series of nanovoids might nucleate at the Pd/Cu interface in the as-deposited state, which might degrade its thermal reliability [14]. Elimination of such nanovoids from the Pd/Cu interface via Au catalytic modification (i.e., predeposition of an additional Au nanolayer over the Cu circuits prior to the Pd deposition; IGEPIG system) is now being attempted in the microelectronic industry. The objective of this study is to
Corresponding author. E-mail address: [email protected] (C.E. Ho).
https://doi.org/10.1016/j.corsci.2018.10.030 Received 5 July 2018; Received in revised form 11 September 2018; Accepted 19 October 2018 Available online 30 October 2018 0010-938X/ © 2018 Elsevier Ltd. All rights reserved.
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Fig. 1. BF-STEM cross-sectional images of (a) the as-deposited Au/Pd/Cu (EPIG case) and (b) Au/Pd/Au/Cu (IGEPIG case). (a’)–(b’) Selected-area diffraction patterns (SADPs) taken from the Pd layer (EP) of EPIG and IGEPIG, respectively.
in combination with selected-area diffraction (SAD) analysis was employed to investigate the interior microstructure of the Au/Pd/Cu and Au/Pd/Au/Cu multilayers. The beam size utilized in the SAD analysis was approximately 120 nm, which enables investigation of the crystallographic difference(s) of these two multilayers, specifically for the Pd films. The TEM specimens were prepared using a FIB apparatus and were mounted onto a Mo grid for the TEM analysis to avoid disturbances resulting from the grid. In the electrochemical corrosion test, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) studies were performed in a 1.5 mol/L sulfuric acid solution through a standard three-electrode cell using a VersaSTAT 4 potentiostat (Princeton Applied Research) at room temperature (25 °C). A fixed coating area (3.14 cm2; radius: 1 cm) was examined. A saturated mercury-mercurous sulfate electrode (SMSE) and platinum (Pt) were used as the reference electrode and auxiliary electrode, respectively. Before the electrochemical analysis, the samples were kept in the solution for 1 h to stabilize the open circuit potential (OCP). The voltages applied in the potentiodynamic polarization test was varied in the range of –1.7 V to 0 V at a scanning rate of 1 mV/s. EIS measurements were performed in the frequency range of 10–2 Hz to 105 Hz at the OCP with a sinusoidal voltage of a 10 mV amplitude. The EIS spectra were analyzed and modeled using the ZSimpWin program.
investigate the thermal stability and electrochemical corrosion of the Cu traces deposited with EPIG and IGEPIG surface finishes. The effect of the Au catalytic modification on the two characteristics (thermal stability and electrochemical corrosion) and the underlying mechanism associated with this microstructural modification will be addressed in this study. 2. Experimental details The thermal reliability and electrochemical corrosion of EPIG and IGEPIG surface finishes over Cu traces were investigated in this study. The topmost Au film (IG) and the intermediate Pd film (EP) were approximately 0.05–0.1 μm and 0.1–0.2 μm thick, respectively. The bottom Au film (IG) has two different thickness, i.e., 0 μm (EPIG) and approximately 0.01 μm (10 nm) (IGEPIG). The thicknesses of the Au and Pd films were measured using an X-ray fluorescence spectrometer (XRF) and subsequently were verified via cross-sectional analysis using transmission electron microscopy (TEM) (Fig. 1a–b). To evaluate the thermal reliability of Au/Pd/Cu (EPIG case) and Au/Pd/Au/Cu (IGEPIG case), an isothermal annealing test was performed at 180 °C for 0 (as-deposited) to 50 h in an air convection oven. After isothermal annealing, the surfaces of EPIG and IGEPIG were analyzed using scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDS) and X-ray photoelectron spectroscopy (XPS). Additionally, focused ion beam (FIB) and field-emission TEM (FE-TEM) 113
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3. Results & discussion
3.2. Isothermal annealing test
3.1. As-deposited state
Fig. 2(a)–(b) presents the SEM images (backscattered electron mode) of the surfaces of the Au/Pd/Cu (EPIG case) and Au/Pd/Au/Cu (IGEPIG case) after annealing at 180 °C for 50 h, respectively. The inset (Fig. 2a’) shows a backscattered electron image (BEI) (gray) overlapped EDS elemental mappings of Cu (orange) and O (green) corresponding to Fig. 2(a). Numerous Cu oxides (CuxO) that appeared on the Au surface can be noted in the EPIG case (Fig. 2a, a’) after annealing of 180 °C for a few hours. In contrast, no meaningful Cu oxides were detected for the IGEPIG case (Fig. 2b). Fig. 2(c)–(d) displays FIB-processed cross sections of the EPIG and IGEPIG shown in Fig. 2(a)–(b), respectively, demonstrating the interior microstructures of Au/Pd/Cu and Au/Pd/Au/ Cu after annealing. The FIB images confirm the existence of an additional material (CuxO) with a thickness of a few nanometers over the EPIG surface but not the IGEPIG surface. These observations indicate that many Cu atoms had diffused towards the free surface of the Au/Pd/ Cu multilayer through the crystalline Au/Pd dual layer, where they nucleated as Cu oxides. The observations also reveal that the predeposition of an Au nanolayer (10 nm) above Cu can greatly reduce the out-diffusion of Cu, thereby preventing the formation of Cu oxides. Next, further analysis of the Cu oxides via XPS was conducted. Fig. 3 shows the XPS spectra (Cu 2p) that were acquired from the surfaces of the EPIG and IGEPIG shown in Fig. 2. The analysis area was
Fig. 1(a)–(b) shows the cross-sectional, bright-field scanning transmission electron microscopy (BF-STEM) images of the as-deposited EPIG and IGEPIG multilayers over the Cu traces. Several nanovoids formed at the Pd/Cu interface of the EPIG case (Fig. 1a), while no such material defects were observed in the alternative case (i.e., IGEPIG, Fig. 1b), suggesting that predeposition of a Au nanolayer between Pd and Cu can inhibit such nanovoid formation. Another microstructural difference between the EPIG and IGEPIG cases was that some nanochannels along the film deposition direction existed among the Pd grains of the EPIG case (Fig. 1a), which were not observed in the IGEPIG case (Fig. 1b). Moreover, the Pd film of the latter case (IGEPIG) possessed a denser microstructure with larger grain sizes (Fig. 1b) than that of the EPIG case (Fig. 1a). The Pd reciprocal lattices of the EPIG and IGEPIG taken from the TEM selected-area diffraction patterns (SADPs) display regular ring pattern (Fig. 1a’) and diffraction spots (Fig. 1b’), respectively, confirming the observations of the BF-STEM images (Fig. 1a–b). The formation mechanism of the nanovoids and nanochannels is discussed in Section 3.4.
Fig. 2. Backscattered electron images (BEIs) showing the surface appearances of (a) Au/Pd/Cu (EPIG) and (b) Au/Pd/Au/Cu (IGEPIG) after annealing at 180 °C for 50 h. (a’) BEI overlapped EDS elemental mappings of Cu and O corresponding to (a). (c)–(d) FIB-processed cross sections of EPIG and IGEPIG shown in (a)–(b), respectively. 114
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400 μm (diameter), providing a wide-range characterization of the surface chemistry after annealing at 180 °C for 50 h. Charging effects in the spectra were calibrated with reference to adventitious C 1s at 284.5 eV [10,15]. Four sharp peaks can be noted in the EPIG spectra. Based on the XPS database of National Institute of Standard and Technology (NIST), two peaks of Cu 2p3/2 and 2p1/2 with binding energies of 932.30 eV and 952.10 eV were identified as cupric oxide (CuO). Additionally, the satellite peaks near 940–950 eV and 960–965 eV were caused by electron shake-up processes and also belonged to CuO [16]. The alternatives were at 931.60 eV and 951.40 eV with bonding energies of Cu 2p3/2 and 2p1/2, corresponding to cuprous oxide (Cu2O). In contrast, the peak intensities of CuO and Cu2O for the IGEPIG case were significantly lower than those for the EPIG case, agreeing well with the observations of Fig. 2, where no meaningful Cu oxides were created in the IGEPIG case after the same annealing process. Fig. 4(a)–(b) shows the BF-STEM images of the cross sections for Au/Pd/Cu (EPIG case) and Au/Pd/Au/Cu (IGEPIG case) after annealing at 180 °C for 50 h. A significant increase in the nanovoid size between Pd and Cu occurred in the former case after annealing (Figs. 1a and 4 a), whereas no such voids were observed in the latter case (Fig. 4b). Fig. 4(c) shows a TEM-SADP acquired from the CuxO (+ Au) shown in Fig. 4(a). The reciprocal lattices of CuxO exhibited Debye rings,
Fig. 3. X-ray photoelectron spectroscopy (XPS) spectra of the Cu 2p region for the Au surface of Au/Pd/Cu (EPIG case) and Au/Pd/Au/Cu (IGEPIG case) after annealing at 180 °C for 50 h.
Fig. 4. (a)–(b) BF-STEM cross-sectional images of Au/Pd/Cu (EPIG case) and Au/Pd/Au/Cu (IGEPIG case) after annealing at 180 °C for 50 h, respectively. (c) Selected-area diffraction pattern (SADP) acquiring from CuxO (+ Au). 115
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Fig. 5. (a) Potentiodynamic polarization curves of Au/Pd/Cu (EPIG case) and Au/Pd/Au/Cu (IGEPIG case) in a sulfuric acid solution (1.5 mol/L). (b)–(c) Backscattered electron images (BEIs) overlapped EDS elemental mappings of Au, Pd, and Cu: (b) EPIG case; (c) IGEPIG case. The insets (b’ & c’) are BEIs of (b)–(c).
3.3. Electrochemical corrosion
Table 1 Corrosion current density (Icorr), corrosion potential (Ecorr), and corrosion inhibition efficiency (IE%) acquired from the potentiodynamic polarization analysis of the bare Cu, EPIG and IGEPIG cases. Specimen Bare Cu EPIG IGEPIG
Icorr (A/cm2) −5
4.8 × 10 2.8 × 10−5 5.3 × 10−6
Ecorr (V/SMSE)
IE (%)
–1.07 –1.40 –1.30
NA 41.66 88.96
Fig. 5(a) shows the potentiodynamic polarization curves of Au/Pd/ Cu (dark) and Au/Pd/Au/Cu (red) in a sulfuric acid solution (1.5 mol/ L), providing a comparison of the electrochemical corrosion between the two examined surface finishes (i.e., EPIG and IGEPIG). The potentiodynamic polarization curves can be subdivided into cathodic and anodic polarization curves, and the former (cathodic polarization curve) is related to the hydrogen reduction and the latter (anodic polarization curve) to the metal layer(s) (i.e., Au/Pd/Cu and Au/Pd/Au/ Cu) oxidation [23]. The intersection point of the two polarization curves determines the corrosion potential (Ecorr), as shown in Fig. 5(a). Subsequently, the corrosion current density (Icorr) can be acquired through the intersection point of the fitted regression lines of the cathodic and anodic polarization curves in the Tafel region [24], where a ± 100 mV variation of Ecorr was taken. Generally, a more positive Ecorr value indicates a decrease in Icorr [25]. A smaller Icorr value implies a slower corrosion rate of the corrosion system [23]. With the corrosion current density of bare Cu (Icorr(o)), the corrosion inhibition efficiency (IE%) can be calculated [24]:
indicating that this newly formed nanolayer possesses a polycrystalline structure. Moreover, the Debye rings with the d-spacings matched well with the face-centered cubic Au (space group: Fm 3¯ m; a = 0.4072 nm [17]), monoclinic CuO (space group: C2/c; a = 0.4653 nm, b = 0.3410 nm, c = 0.5108 nm, and β = 99.48° [18]), and cubic Cu2O (space group: Pn 3¯ m; a = 0.4250 nm [19]). The deviations between the measured and theoretical d-spacings are all smaller than 3% in the indexation of Fig. 4(c). The above analysis shows that the CuxO primarily consisted of CuO and Cu2O nanograins, confirming the XPS characterization. According to the literature [20,21], grain boundaries are the dominant diffusion path of Cu in Pd at 175–250 °C. Therefore, we propose that out-diffusion of Cu along the grain boundaries of the Au/ Pd dual layer (Fig. 2c) to the free surface in the 180 °C aging treatment was the root cause of the voiding enlargement between Pd and Cu and the formation of CuO and Cu2O species over the Au surface, as observed in Figs. 2(a), (c), and 4(a). The presence of a significant amount of Cu oxides on the Au surface of the Au/Pd/Cu multilayer (EPIG case) and the aggregation of numerous nanovoids in the vicinity of the Cu substrate (Figs. 2a, c, and 4 a) after isothermal annealing at 180 °C for 50 h inevitably deteriorate the solderability of microelectronic packages. According to the literature [22], disproportionation and complexation reactions occur between Cu oxides and organic acid flux during a reflow process. A water bubble will be produced by the above reactions during an early stage of the soldering reaction and subsequently might induce bump void formation, degrading the solder joint integrity. Next, the investigation of the corrosion resistance of the EPIG and IGEPIG surface finishes was conducted.
Icorr(i) ⎞ IE% = ⎜⎛1⎟ × 100% I ⎝ corr(o) ⎠
(1)
where Icorr(i) represents the corrosion current density for the Cu substrate coated with surface finish i (= EPIG or IGEPIG). A lager value of the corrosion inhibition efficiency (IE%) suggests a better corrosion resistance of the system. Table 1 lists Ecorr, Icorr(i), Icorr(o), and IE% of bare Cu, Au/Pd/Cu (EPIG case), and Au/Pd/Au/Cu (IGEPIG case), which are acquired from the Tafel extrapolation with Fig. 5(a) and Eq. (1). The IGEPIG case possesses a smaller Icorr (5.3 × 10–6 A/cm2), a more positive Ecorr (1.3 V), and a larger IE% (88.96%) than that of the EPIG case (where Icorr = 2.8 × 10–5 A/cm2, Ecorr = –1.4 V, and IE% = 41.66%), indicating that IGEPIG possesses a better corrosion resistance than EPIG does. Fig. 5(b)–(c) shows backscattered electron images (BEIs, Fig. 5b’–c’) overlapped Au, Pd, and Cu elemental mappings of the EPIG and IGEPIG cases after the potentiodynamic polarization test (Fig. 5a), respectively. The colors yellow, blue, and orange displayed in the BEIs (Fig. 5b–c) represent Au, Pd, and Cu elemental distributions, respectively. A 116
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Z” represent the impedance, the real component impedance, and the imaginary component impedance, respectively, and the dash lines represent the fitting values with the experimental measurements. The diameter of the semicircle in the Nyquist plot is a good indication of the corrosion characteristic of a specimen, and a larger diameter implies a better corrosion resistance [26]. The IGEPIG case displays a larger impedance arc diameter than that of EPIG (Fig. 6a) suggests that IGEPIG over Cu might possess a better anticorrosion performance than that of EPIG. The characteristic frequencies were unavailable in the Nyquist plot (Fig. 6a). This is because the EIS measurements were performed in the frequency range of 10–2 Hz to 105 Hz at constant OCP with a sinusoidal voltage of a 10 mV amplitude, such that the initiation of corrosion in the two examined cases (Au/Pd/Cu and Au/Pd/Au/Cu) can be well identified after the EIS analysis (see TEM analysis, Fig. 7). Phase angle (φ) and impedance module (log|Z|) as a function of frequency were shown in the Bode plot (Fig. 6b). The log|Z| values of Au/Pd/Cu (EPIG case) and Au/Pd/Cu (IGEPIG case) were approximately the same in the frequency range of 10–2 Hz to 105 Hz, suggesting that the impedance of the corrosion system was not significantly altered with the additional Au deposition. Phase angle in the high-, medium-, and low-frequency characterizes the surface defects, corrosion products within the coating(s) (i.e., EPIG or IGEPIG), and the charge transfer at the coating/metal interface, respectively [27]. A significant change in the medium-/high-frequency phase angles occurred (Fig. 6b), suggesting that the deposition of either EPIG or IGEPIG has a certain influence on the corrosion products within the EPIG and IGEPIG surface finishes and the charge transfer at the coating/metal interface. Three well-defined time constants can be noted in the φ spectra (Fig. 6b), indicating that three distinct electrochemical processes occurred in the EPIG and IGEPIG systems. Therefore, the EIS analysis results of the EPIG and IGEPIG cases can be analyzed through the equivalent circuit with three resistor-capacitor (RC) circuits, as shown in Fig. 6(c). The measured impedance and phase angle (Fig. 6a–b) were fitted using the ZSimpWin program with the equivalent circuit shown in Fig. 6(c). Table 2 lists the EIS parameters corresponding to the equivalent circuits of the EPIG and IGEPIG cases, where Rs, Rpore, Rcp, and Rct represent the solution resistance between the reference electrode (SMSE) and working electrode (Au/Pd/Cu or Au/Pd/Au/Cu), the resistance of the coating in pore areas, the resistance of corrosion product, and the resistance of charge transfer, respectively; n is the dispersion index and it is non-unity (Table 2). The constant phase element (CPE) was therefore employed to describe the frequency dependence of non-ideal capacitive behavior [28]. CPE1, CPE2, and CPE3 represent the multilayer coating including pores in the outer layer coating, the multilayer coating within the defect, and the double layer, respectively. The largest percentage error and the chi-squared value of the EPIG case were only 14.3% and 3.90 × 10–4, respectively, and those of IGEPIG were 27.1% and 3.43 × 10–4, respectively. The good agreement between the measured impedance/phase angle spectroscopies (Fig. 6a–b) and the simulation results confirms the validity of the equivalent circuit proposed in Fig. 6(c). The Rct values are much larger than the Rs, Rpore, and Rcp values, indicating that the charge transfer process at the coating/substrate interface (i.e., EPIG/Cu and IGEPIG/Cu interfaces) might dominate the corrosion mechanism of the two surface finishes. Furthermore, IGEPIG exhibits a larger Rct value than EPIG (Table 2), reflecting that IGEPIG possesses a better anticorrosion performance than EPIG does. The EIS analysis results (Fig. 6 and Table 2) are consistent with that obtained from the electrochemical polarization (Fig. 5a) and SEM-EDS analyses (Fig. 5c, b), which indicate an additional Au nanolayer acts as an effective barrier against the corrosion of a sulfuric acid solution. Theoretically, the existence of large and/or many pores in a corrosion system might cause small Rpore because of a large cross-section area. However, the opposite trend was observed in Table 2, where the Rpore value of the EPIG case (12.28 Ω· cm2) was slightly larger than that of the IGEPIG case (5.786 Ω cm2), although some nanovoids indeed did exist between
Fig. 6. (a) Nyquist plot and (b) Bode plot of EPIG case (dark) and IGEPIG case (red) at 2 h of immersion in a sulfuric acid solution (1.5 mol/L). (c) Equivalent electrical circuit of EPIG case and IGEPIG case used for the impedance plots fitting (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
significant amount of surface damages can be clearly observed in the EPIG case (Fig. 5b), where the Au/Pd dual layer was completely eliminated and the underlying Cu substrate was exposed accordingly after the potentiodynamic polarization test. In contrast to the EPIG case (Fig. 5b), only a few peeling areas were caused in the IGEPIG case after the corrosion test (Fig. 5c), indicating that less corrosion had occurred in the IGEPIG system. These observations agree well with the data listed in Table 1 and show that the predeposition of a Au nanolayer serves as a good protection against sulfuric acid corrosion. Fig. 6(a)–(b) presents electrochemical impedance spectroscopy (EIS) analysis at individual OCP of the EPIG (dark triangle) and IGEPIG (red dot) surface finishes after immersion in a sulfuric acid solution (1.5 mol/L) for 2 h: (a) Nyquist plot; and (b) Bode plot, where Z, Z’, and 117
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Fig. 7. (a)–(b) BF-STEM cross-sectional images of Au/Pd/Cu (EPIG case) and Au/Pd/Au/Cu (IGEPIG case) after EIS analysis, respectively. (c)–(d) Zoomed-in view of the Pd/Cu interface corresponding to (a)–(b). Table 2 EIS parameters corresponding to the equivalent circuits of the EPIG and IGEPIG cases. Specimen
Rs (Ω cm2)
CPE1 (S secn)
n1
Rpore (Ω cm2)
CPE2 (S secn)
n2
Rcp (Ω cm2)
CPE3 (S secn)
n3
Rct (Ω cm2)
chisquared
EPIG IGEPIG
3.838 3.998
1.95 × 10−4 2.78 × 10−4
0.868 0.858
12.28 5.786
7.46 × 10−4 1.00 × 10−3
0.850 0.827
215.9 255.5
1.57 × 10−3 8.79 × 10−4
0.737 0.824
1.80 × 104 2.99 × 104
3.90 × 10−4 3.43 × 10−4
3.4. Effect of the Au nanolayer (Au catalysts)
Pd and Cu in the former case (EPIG) (Fig. 1a). Here we argued that the examined multilayers (i.e., Au/Pd/Cu and Au/Pd/Au/Cu) basically possessed a dense structure and the existence of a limited amount of material defects (e.g., nanovoids) posed a minor influence on the global resistance of the corrosion system, such that the Rpore values of the two examined cases both were quite small and approximately indistinguishable (Table 2). This opposite trend might result from the deviation between experimental measurements and the simulation results (Fig. 6a–b). Further study is required to validate the above hypothesis. Fig. 7 shows the BF-STEM images of (a, c) Au/Pd/Cu and (b, d) Au/ Pd/Au/Cu after the EIS test, which provides further information associated with the microstructural differences between the EPIG and IGEPIG cases after the electrochemical corrosion test. A series of voids nucleated at the Pd/Cu interface of the EPIG case after the EIS analysis (Fig. 7a, c), indicating the occurrence of serious corrosion at the interface between the Cu substrate and the surface finish (EPIG) in the EIS analysis. In contrast, only a limited number of nanovoids can be observed between the surface finish (Au/Pd/Au) and Cu (Fig. 7b, d) under the same test condition. The TEM results confirm that the charge transfer process (Rct) at the coating/substrate (Cu) interface indeed plays a dominant role in the corrosion mechanism of the Au/Pd/Cu and Au/Pd/Au/Cu multilayers, and the predeposition of a Au nanolayer over Cu (i.e., IGEPIG case) greatly enhances the corrosion protection.
Investigations of TEM/FIB (Figs. 1, 2c–d, 4, and 7), SEM/EDS (Figs. 2a–b and 5 b–c), XPS (Fig. 3), potentiodynamic polarization (Figs. 5a), and EIS (Fig. 6) showed that Au/Pd/Au/Cu (IGEPIG case) possesses a better thermal stability and corrosion resistance than that of Au/Pd/Cu (EPIG case), preventing the formation of Cu oxides in isothermal annealing (180 °C) and the corrosion of Cu substrate in a sulfuric acid solution (1.5 mol/L). The improvements in the thermal/electrochemical characteristics of the Au/Pd/Cu structure (EPIG case) can be ascribed to the microstructural modification of the Pd film via predeposition of the additional Au nanolayer (IGEPIG case), as presented in Fig. 1(b). To gain a better understanding of the Pd microstructural modification, the electroless Pd (EP) deposition and its pretreatment over the Cu substrate should be examined. Fig. 8(a)–(b) shows BEIs overlapped EDS elemental mappings of Au (yellow), Pd (blue), and Cu (orange) for the Cu substrates deposited with Pd catalysts and a Au nanolayer (∼10 nm), respectively, demonstrating the catalytic distributions prior to the electroless Pd reduction in the EPIG and IGEPIG cases. Unlike that for uniform distribution of a Au film (catalysts) on Cu (Fig. 8b), there was only a limited amount of the Pd catalysts scattered over the Cu surface in the EPIG case (Fig. 8a). This SEM-EDS examination reveals that the catalytic deposition prior to the EP process was distinctly 118
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Fig. 8. Backscattered electron images (BEIs) overlapped EDS elemental mappings of Au, Pd, and Cu after catalytic deposition: (a) Pd catalyst (EPIG case); (b) Au catalyst (IGEPIG case). (c)–(f) Mechanism of Pd (EP) deposition through the use of different catalysts: (c, e) Pd catalyst (EPIG case); (d, f) Au catalyst (IGEPIG case).
deposited over the Cu traces were evaluated. In the high-temperature (180 °C) storage test, only a limited amount of Cu oxides formed on the Au surface of the Au/Pd/Au/Cu multilayer after annealing for 50 h, suggesting that predeposition of an additional Au nanolayer can prevent the out-diffusion of Cu and that IGEPIG possesses better oxidation resistance than that of EPIG. The potentiodynamic polarization and EIS analyses showed that IGEPIG has a smaller Icorr (5.3 × 10–6 A/cm2), a more positive Ecorr (–1.3 V), a larger IE% (88.96%) and Rct (2.99 × 104 Ω cm2) than those of EPIG (where Icorr = 2.8 × 10–5 A/cm2, Ecorr = –1.4 V, IE% = 41.66%, and Rct = 1.80 × 104 Ω cm2), indicating that IGEPIG possesses a better anticorrosion performance than EPIG does. Further EIS results in combination with TEM examination confirmed that the charge transfer process at the coating/substrate (Cu) interface plays a dominant role in the corrosion mechanism and that the corrosion resistance can be greatly enhanced by the predeposition of the Au nanolayer (i.e., IGEPIG case). We propose that modification of a polycrystalline Pd film into a nanochannel-/nanovoid-free structure by the Au catalytic modification (Au nanolayer) can promote the thermal/ electrochemical characteristics of an EPIG surface finish.
different in the two examined cases, and the replacement of the Pd catalysts by Au (i.e., Au nanolayer) greatly enhances the uniformity of the catalytic distribution for the subsequent Pd reduction reaction. Based on the catalytic modification (Fig. 8a–b), we proposed a possible mechanism associated with the Pd microstructures obtained in Fig. 1(a)–(b). As only a limited amount of Pd catalysts was deposited over the Cu substrate in the EPIG case (Fig. 8a and c), the reduction reaction of electroless Pd was only limited to the specific Cu sites with Pd catalysts at the early stage of electroless Pd deposition. This phenomenon resulted in a loose Pd microstructure with a polycrystalline structure and numerous nanochannels within the Pd film (Figs. 1a and 8 e) as the film thickness was on the submicron scale. The presence of nanochannels within the Pd film might induce the galvanic corrosion of Cu in the subsequent Au plating process, leading to nanovoid formation in the as-deposited state (Fig. 1a). Additionally, the existence of numerous grain boundaries in the polycrystalline Pd facilitated the oudiffusion of Cu [14], yielding the nucleation of Cu oxides on the Au surface (Figs. 2a, 2c, 3, 4a, and 4c). In the alternative case (IGEPIG), however, a continuous catalytic layer (Au) was predeposited over the Cu substrate for the subsequent electroless Pd reaction (Fig. 8b and d), such that a reduction of Pd2+ into Pd occurred everywhere with the Au catalysts and a dense Pd film was yielded after the EP process (Figs. 1b and 8 f). Therefore, the predeposition of a 10 nm-thick Au layer over a Cu substrate (i.e., IGEPIG case) modified the loose Pd microstructure (Fig. 1a) into a dense film with limited grain boundaries (Fig. 1b), which avoided the formation of nanovoids/nanochannels at the Pd/Cu interface (or within the Pd film) (Figs. 1a, 2 c, and 4a), thereby enhancing the thermal/electrochemical characteristics of the Au/Pd/Cu structure.
Acknowledgments This study was supported by the Ministry of Science and Technology (R.O.C.) through Grant Nos. MOST105-2628-E-155-001-MY3, MOST105-2221-E-155-006-MY2, and MOST106-2622-E-155-006-CC3. The authors would like to thank Yu-Ling Chien (National Central University), Yin-Mei Chang (National Tsing Hua University), Yi-Chen Yu (National Tsing Hua University), Wei-Ping Dow (National Chung Hsing University), and Wun-Jheng Zeng (Taiwan Uyemura) for their assistance with the experiments.
4. Conclusions References The thermal reliability and electrochemical corrosion characteristics of EPIG (Au/Pd) and IGEPIG (Au/Pd/Au) surface finishes
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