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Shear strength of off-eutectic AuGe joints at high-temperature
Microelectronics Reliability 99 (2019) 31–43
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Shear strength of off-eutectic AueGe joints at high-temperature a,b,⁎
Andreas Larsson
c
, Torleif A. Tollefsen , Knut E. Aasmundtveit
d
T
a
TECHNI AS, Dept. of Applied Physics, Borre, Norway University of South-Eastern Norway (USN), Dept. of Microsystems, Borre, Norway TEGma AS, Oslo, Norway d University of South-Eastern Norway (USN), Dept. of Microsystems, Borre, Norway b c
ARTICLE INFO
ABSTRACT
Keywords: Joining Bonding High-temperature Off-eutectic AueGe TLP SLID
High-temperature compatible devices of SiC have been demonstrated in recent years. Together with a hightemperature compatible die-attach, they can form a basis for future reliable high-temperature electronics. This article presents shear strength capacity of Au-rich off-eutectic AueGe die-attach at temperatures above the eutectic melting point. Metallized (Ni/Au) SiC dice were joined to metallized (Cu/Ni/Au) Si3N4 substrates with eutectic AueGe preforms. The joints demonstrated a significant shear strength capacity up to 410 °C (50 °C above the eutectic melting point). High-quality Au-rich off-eutectic AueGe joints, comprising a AueGe mixture with 8 ± 4 at.% Ge, were formed with a low bond line pressure (240 kPa). At room temperature the shear strength was measured to be approximately 140 MPa, falling to approximately 40 MPa at 410 °C. This demonstrates that off-eutectic joints might be considered for use at temperatures above the eutectic melting point. The fracture mode was mainly cohesive in the off-eutectic AueGe layer when tested at a temperature below the eutectic melting point. Above the eutectic melting point, the fracture mode was complex with signs of a partially melted off-eutectic AueGe compound. Within this compound, columnar structures of the primary α phase (Au) was surrounded by an apparently melted and solidified phase. In other regions, an adhesive fracture at the interface between a slightly off-eutectic compound and a GeNi layer was discovered. The microstructure was found to be inhomogeneous with large regional variations within joints. From almost pure Au, primary α phase with up to 2–3 at.% Ge, up to near eutectic AueGe regions with an overall composition of 20–30 at.% Ge. In general, Ge was segregated into explicit domains within a primary α phase.
1. Introduction Since the beginning of modern electronics, joining components with metals have been a critical system component. Already in the 1950s, gold was used on germanium to form a contact that enabled the demonstration of a point contact transistor leading to the Noble prize in physics in 1956 awarded to William Bradford Shockley, John Bardeen and Walter Houser Brattain [1]. Since then, many different types of joining technologies have been developed and thoroughly explored for a wide variety of electronic applications. Such joining technologies include; soldering, brazing, welding, contact melting [2,3], thermocompression, ultrasonic, diffusion, solid-liquid interdiffusion (SLID) [4–8]/transient liquid phase (TLP) [9–11] bonding and sintering [12]. In many applications, high-temperature compatibility is a desired property of the final joint. It can be when application requirements request high operation temperatures or when the joint needs to comply
with subsequent high-temperature processes during fabrication. In recent years, high-temperature compatible components have been developed and demonstrated to operate at high temperatures, > 200 °C, such as silicon on insulator (SOI), gallium arsenide (GaAs), gallium nitride (GaN), and silicon carbide (SiC) semiconductor devices. Together with ceramic substrates, such as alumina (Al2O3), aluminum nitride (AlN) or silicon nitride (Si3N4), and a suitable die attach they form a basis for high-temperature compatible electronics. Joints formed by TLP/SLID type and sintering technologies may be used at temperatures significantly higher than the process temperature required to create the joints. This is one of the main reasons why a significant research effort has been made on such technologies over the years. The downside is that they typically require long process times [9] or high bond line pressures [13]. Now that high-temperature compatible components and substrates have been developed, traditional brazes, or hard solders, have regained interest for high-temperature applications,
Corresponding author. E-mail addresses: [email protected], [email protected] (A. Larsson), [email protected] (T.A. Tollefsen), [email protected] (K.E. Aasmundtveit). ⁎
https://doi.org/10.1016/j.microrel.2019.05.002 Received 8 February 2019; Received in revised form 8 April 2019; Accepted 7 May 2019 0026-2714/ © 2019 Published by Elsevier Ltd.
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Fig. 1. Illustration of the binary phase diagram of the AueGe system. The target off-eutectic composition, c0, of the fabricated joints is marked with a dashed region in. The phase diagram was adapted from Okamoto and Massalski [25]. Note that the eutectic melting point varies slightly in the pertinent literature; 356–361 °C [26,27].
Fig. 2. Schematic illustration of the material stack of the investigated system, prior to bonding. A eutectic AueGe preform is sandwiched between a SiC die and a Si3N4 substrate metallized with Au seed layers. The layer thicknesses are presented in parenthesis.
due to their relatively easy process requirements with low bond line pressure and quick process scheme. Off-eutectic compounds melt incongruently into both a liquid and a solid component. Within this binary material state, such compounds
could have enough volume fraction of the solid component to form a continuous and coherent solid phase extending through the liquid phase, like a sponge, or a metal foam. If there is sufficient solid phase, it could be used to adjoin components at a temperature above the eutectic
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Fig. 3. A characteristic temperature profile during bonding. The eutectic melting point is marked with a dashed line.
Preform
Self-aligning tool head
Fig. 4. A sample placed on the shear tester hot plate after calibration tests. A small piece of eutectic preform is seen on the top of the die used as visual reference of temperature above the eutectic melting point.
temperature. Thus, such a compound could combine the same attractive property of SLID/TLP and sintering; i.e., a lower process temperature than the final operation temperature, with the short process times and low bond line pressures of brazing. Eutectic gold based joints made with AueGe, AueSi and AueIn have demonstrated significant strength capacity at high homologous temperatures [14–16]. The AueGe system was chosen due to its relatively low process temperature compared to common brazes [15], and its stability and performance at high-temperature (~300 °C) [14].
2. Materials and methods 2.1. Materials and fabrication Eutectic AueGe preforms were sandwiched between silicon carbide (SiC) dice and silicon nitride (Si3N4) substrates. The SiC dice (Fairchild Semiconductor) had sputtered nickel silicide (Ni2Si)/nickel (Ni)/gold (Au) layers and a 5 μm thick electroplated Au top layer. The Si3N4 substrates (Denka Chemicals) had active metal bonded (AMB) copper (Cu) conductors plated with a nickel–phosphorus (7 wt% P) top layer on the back and front sides. The substrates were electroplated with a uniform Au layer (5 μm), using a gold cyanide solution at 60–65 °C and
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(a)
Fig. 5. Image captures of a die during a high-temperature shear test. A piece of eutectic AueGe preform is visible on top of the die. Squeeze out of eutectic AueGe is visible on the substrate surface around the die. (a) The hot plate temperature is set to 320 °C, (b) The hot plate temperature is increased to 400 °C and the image was captured before the tool head contacts the die. The preform has melted and both preform and the substrate surface around the die have changed color to a silvery color. (c) The hot plate temperature is still set to 400 °C, and image was capture after die contacts the tool head. The preform has solidified and changed color back to yellow. Sections of the substrate surface near the tool head have changed color back to gold (marked with dashed blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Tool head
Au72Ge28 Preform
(b)
(c)
a current density of 2.7 mA/cm2. The Au layer thickness was chosen such that the final joint would have a Au-rich off-eutectic composition. The Ni layer was used as a diffusion barrier. The plated substrates and dice were diced into 6 mm × 6 mm and 3.4 mm × 1.9 mm sized samples. The eutectic AueGe preform, i.e., 72 at.% Au and 28 at.% Ge, (Goodfellow) was (35 ± 2) μm thick and manually cut into pieces with
the same footprint as the dice. This results in an overall nominal Au:Ge ratio of (12.2 ± 0.4) at.% Ge in the entire system. The target off-eutectic composition is marked with a dashed region in the binary AueGe phase diagram depicted in Fig. 1. The materials stack prior to bonding is shown in Fig. 2. Dice and preforms were manually placed and aligned on top of
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Fig. 6. Graph of the FEA results showing the joint temperature as function of time in the joints during high-temperature shear testing for four different hot plate set points: 300 °C, 400 °C, 415°, and 460 °C. Model calibration was done by correlating melting, m, and solidification, s, of the 400 °C curve to match video recordings from shear testing. Two different durations of contact between tool head and die are shown for the calibration curve 2 s (dashed) and 5 s (solid).
substrates forming a SiC/Ni/Au/AueGe/Au/NieP/Cu/Si3N4/Cu/NieP configuration. The stack was clamped together with spring-loaded ball plungers in a custom-made fixture and placed into a vacuum bonder (Budatec VS160UG). A characteristic temperature profile is shown in Fig. 3. The joints spent roughly 10 min in a molten state before solidification. A small force was used to secure a thermal contact through the stack. Two different bond line pressures were used, 30 kPa and 240 kPa. The bond process was carried out in a dry nitrogen atmosphere, ~1 atm, or in a vacuum, ~1 mTorr.
(a)
(b)
(c)
2.2. Characterization Cross-sections of fabricated samples were evaluated with the use of optical microscopy (Neophot 32, NA 0.9, up to 1000× magnification), and scanning electron microscopy (SEM) (Hitachi SU8230). Samples were prepared for cross-section analysis by two different methods. In the first method, the samples were ground and polished and finished by flat ion milling. The grinding stopped at 2000 grit grade before preparation was continued with polishing using a cloth and a 1 μm
SiC Au–Ge Cu
200 µm
SiC Au–Ge Cu
200 µm
SiC Au–Ge Cu
200 µm
Fig. 7. Cross-sections of virgin joints formed by three different processes (bond line pressure and atmosphere); (a) 30 kPa in N2, (b) 240 kPa in N2, and (c) 240 kPa in vacuum. Voids can be seen in the center of the joint at the original bond line, for (a) and (b).
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(a)
Au
20 µm
(b)
Au
Ge
(c)
40 µm
Lc
Void
30 µm Fig. 8. Cross-sections of virgin joints formed by three different processes (bond line pressure and atmosphere); (a) 30 kPa in N2, (b) 240 kPa in N2, and (c) 240 kPa in vacuum. Ge domains are located primarily at the joint center and at the interface to the Ni layer on both sides. A void is clearly seen in the center of figure (c). The line Lc marks the line scan results shown in Fig. 11.
diamond paste for the final step. Then the samples were finished with flat ion milling (Hitachi IM4000, Ar) to reveal potential voids or other defects possibly concealed by smearing. The second method was cutting the sample into two halves and then perform a cross-section cut with ion-milling. The composition and microstructure were evaluated by SEM, energy-dispersive X-ray spectroscopy (EDX) (Oxford X-MAX 150), and electron backscattered diffraction (EBSD) (Oxford Nordlys Nano).
A shear force was applied on five dice while they were heated to ~600 °C to reveal significant remelting of the joint (liquification) above the eutectic melting point at 361 °C. The shear load was (1 ± 0.5) MPa and was applied manually to the die. The shear strength was then quantified at four different joint temperatures; 25 °C, 270 °C, 370 °C, and 410 °C. The shear tester (XYZTEC, Condor Sigma) was equipped with a 100 kgf load cell, a hot plate, and a self-aligning tool head. A 36
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SiC
Au
Lb
were taken at different positions through the samples. In some regions, there was a complete absence of Ge in the Au phase (Fig. 8(a)). The composition variation between regions was; from almost pure Au, primary α-phase, with up to 2–3 at.% Ge dissolved in the α-phase, up to near eutectic AueGe regions with a Ge composition of 20–30 at.%. The overall composition of virgin (as bonded) joints was roughly a Au-rich off-eutectic composition with (8 ± 4) at.% Ge, measured by EDX. Regions with a higher Ge content typically had regions with a lamella type AueGe structure and explicit Ge domains in grain boundaries in the Au phase, as can be seen in Figs. 8(c) and 9. EDX maps of Au, Ge, and Ni of the section shown in Fig. 9 is shown in Fig. 10, illustrating that Ge has segregated into explicit domains. One can also see that Ge and Ni have reacted at both the top and bottom interfaces of the joint. Line scans across virgin joints (Fig. 11) confirmed that the main body of the joint comprised mainly Au and Ge and that a GeNi phase was formed at the Ni interface. The microstructure of these off-eutectic joints differ from the microstructure in joints formed with eutectic AueGe. Eutectic AueGe joints have a lamella or speckled microstructure throughout the joints [14,15,17–19]. The final joint thickness varied between 20 and 30 μm which is smaller than the initial bond layer thicknesses combined, i.e., Au (5 μm) + preform (35 μm) + Au (5 μm). This thickness difference was due to a squeeze-out of melted eutectic material during joining. To quantify the actual composition of our resulting joints, EDX was used throughout our work.
GeNi
Void Ge
Ni–P Cu
GeNi 30 µm
Fig. 9. A section with a few Ge domains. A GeNi layer is visible on both sides of the joint as well as a small spherical void. The line Lb marks the line scan results shown in Fig. 11.
sample placed on the hot plate is shown in Fig. 4. The shear height was 60 μm, and the shear speed was 100 μm/s. The hot plate was preheated to 25 °C, 300 °C or 320 °C before the samples were placed onto it. For the 370 °C and 410 °C groups, the temperature was then increased at a rate of 20 °C/min to the correct setpoint. The local temperature in the sample drops when the tool head of the shear tester contacts the sample. A finite element model was developed to determine the actual temperature in the joints. The model was calibrated to agree with the dynamic behavior of the real system. This was done by trimming the model until melting and solidification corresponded accurately with the real system. Pieces of eutectic AueGe preforms were used as reference points during model calibration. Visual inspection of the phase transformation (melting and solidification) of the preform and a color shift on the substrate surface was used to verify the onset of melting and solidification during tests, see Fig. 5. The duration dice were in contact with the shear tool was correlated with the force-time graphs generated during shear testing. The resulting transient temperature profiles of the joint from the model are shown in Fig. 6. Also, visual inspection of recorded videos from shear testing was done post-test to verify that no solidification could be seen in the 370 °C and 410 °C group joints. Fractography was performed by visual inspection of the fracture surfaces with an optical microscope, SEM and EDX.
3.2. Shear strength All samples were able to carry the applied 1 MPa shear load up to 600 °C. The shear strength quantification results at four different temperatures are shown in Fig. 12. Off-eutectic AueGe joints show tremendous strength at room temperature. The measured strength, (142 ± 7) MPa, is 2–3 times higher than typically found in the pertinent literature for eutectic AueGe joints [17,18,20–22]. It is at the very top of the highest numbers reported for eutectic AueGe joints at room temperature [15,23]. At a higher temperature, 250–300 °C the off-eutectic system tested here is about 30–80% stronger than eutectic AueGe joints [15,16,23]. The shear strength at 270 °C was (81 ± 5) MPa. Table 1 show a comparison of the high-temperature shear strength with eutectic AueGe joints, SLID type joints from the CueSn, AueSn and AueIn systems and Ag sintered joints. At 370 °C, about 10 °C above the eutectic melting point, the shear strength was measured to (31 ± 8) MPa. This shear strength is roughly 10 MPa higher than the room temperature shear strength of regular PbeSn solders. PbeSn solders, such as PbSn 60/40, PbSn 90/10, and PbSn 95/5, melts (liquidus) between 183 °C (eutectic melting point) and roughly 300–310 °C depending on the Sn content, which makes them unfeasible at temperatures near 300 °C. At 410 °C, the shear strength was measured to be (39 ± 9) MPa, i.e., a small increase from the 370 °C group. The requirement in US military standard (method 2019.8) is 3.9 MPa for a 6.3 mm2 large die-attach [24]. The results show that there is potential for high-temperature use of off-eutectic at temperatures above the eutectic melting point. The fracture mode changed as a function of test temperature. A mixed adhesive fracture at the GeNi/Ni interface as well as a cohesive fracture in several sections in the Au-rich off-eutectic layer were found at room temperature (Fig. 13(a)). At 270 °C, a cohesive fracture through the Au-rich off-eutectic AueGe joint was the main fracture mode (Fig. 13(b)). When the test temperature was above the eutectic melting point, the fracture mode was complex. It was mainly found to be a mixed fracture with sections showing a cohesive fracture through a Au-
3. Results and discussion 3.1. Microstructure Investigation of the effect of bond line pressure and atmosphere showed that there were fewer voids when the bond line pressure was 240 kPa and performed in a vacuum as can be seen in Fig. 7. Thus, this process was chosen for fabrication of the shear tested samples. The fabricated joints were uniform, and no interface at the original bond line could be seen. This indicates proper wetting and a complete dissolution and solidification of the preform with the adjoined Au seed layers. The virgin samples had an overall off-eutectic AueGe structure. Cross-sections further showed an inhomogeneous microstructure were some regions were of mainly Au (Fig. 8(b) and Fig. 9). Other regions had a significantly higher Ge content (Fig. 8(c)). This apparent inhomogeneity varied through individual samples when cross-sections
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Au
30 µm
Ge
30 µm
Ni
30 µm Fig. 10. EDX maps of the section shown in Fig. 9. From top to bottom; Au, Ge, and Ni.
rich off-eutectic AueGe compound and as an adhesive fracture between a slightly off-eutectic AueGe layer and a mainly GeNi layer as can be seen in Fig. 13(c) and (d). The samples tested at 370 °C showed a fracture extending through a Au-rich off-eutectic AueGe compound (partially liquid sections during testing) and at the interface between a slightly off-eutectic AueGe compound (mainly liquid during testing) and an adjoining solid layer typically comprising the primary α phase (Au) and GeNi phase as can be seen in Fig. 14. A closer inspection of the cohesive fracture surfaces in the Au-rich off-eutectic AueGe compound (with a Ge content of
4–10 at.%) showed a ductile fracture behavior. Within these regions, columnar structures of the primary α phase (Au) were found. They were surrounded by a AueGe compound that had clearly been in a liquid state with a free surface before solidification (Fig. 15). These Au structures showed clear signs of a ductile fracture with elongation of the material, a well-defined fracture surface, and clear slip bands at the necking region as can be seen in Fig. 16. The compound surrounding the columnar features was off-eutectic AueGe with a Ge concertation varying between a few at.% to a composition near the eutectic composition (28 at.%).
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Fig. 11. EDX line scans marked with Lb and Lc in Fig. 8 and Fig. 9.
Fig. 12. The measured shear strength of off-eutectic AueGe joints, shear tested at four different temperatures.
Table 1 A comparison of high-temperature shear strength for six different high-temperature joining materials. Type
Off-eutectic AueGe
Eutectic AueGe
SLID CueSn
SLID AueSn
SLID AueIn
Sintering Ag
Temperature (°C) Shear strength (MPa) Source
270 81 ± 5 This work
300 26–54.5 [15,16,23]
300 42–61 [28]
300 20 [29]
300 40–60 [15,30]
300 10–20 [15,23]
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(b)
(a)
Cohesive
Adhesive
1 mm
Adhesive
Cohesive
Adhesive
Cohesive
1 mm
(d)
(c)
Cohesive
Adhesive
1 mm
1 mm
Fig. 13. SEM micrographs of typical fracture surfaces from each shear test temperature group. (a) 25 °C: Mainly an adhesive fracture at the GeNi/Ni interface with four sections showing a cohesive fracture in the Au-rich off-eutectic layer. (b) 270 °C: Mainly a cohesive fracture through an off-eutectic AueGe section. (c) 370 °C and (d) 410 °C: The fracture extends mainly as a cohesive fracture through a Au-rich off-eutectic region, but also as an adhesive fracture between a primary α (Au) layer or the adhesion layer on the die and a mainly GeNi layer.
Si3N4 substrates with eutectic AueGe preforms. High quality off-eutectic AueGe (8 ± 4 at.% Ge) joints were formed with a low bond line pressure (240 kPa). The joints demonstrated a significant shear strength capacity up to 410 °C. At room temperature, the shear strength was measured to be (142 ± 7) MPa falling to (39 ± 9) MPa at 410 °C (50 °C above the melting point of eutectic AueGe (28 at.% Ge)). 39 MPa is ten times higher than the requirement in the MIL-STD-883H standard for such joints. The shear strength at 410 °C is roughly up to twice as strong as regular SnePb and SAC solders at room temperature. This demonstrates that off-eutectic AueGe joints have the potential to be used at temperatures higher than their eutectic melting point. The fracture mode changed with temperature. At room temperature, a mixed mode with an adhesive fracture at a GeNi/Ni interface combined with a cohesive fracture in an off-eutectic Au layer was found. At 270 °C, the fracture mode was mainly cohesive in an off-eutectic AueGe layer. Above the eutectic melting point, the fracture mode was complex with signs of a partially melted off-eutectic AueGe compound. Within this compound, columnar structures of primary α (Au) surrounded by an apparently melted and solidified phase were found. In other regions an adhesive fracture at the interface between a slightly off-eutectic compound and a layer comprising the GeNi phase was present. The microstructure was found to be inhomogeneous with large regional variations within joints. From almost pure Au, primary α phase with up to 2–3 at.% Ge, up to near eutectic AueGe regions with 20–30 at.% Ge. In general, Ge was segregated into explicit domains within a primary α phase. Two concerns of particular interest need to be investigated before final conclusions can be made. First; how will the kinetics of off-eutectic compound behave when it is exposed to prolonged durations in a partially liquid state? Will Ge diffuse along the grain boundaries between Au grains creating a liquid interlayer compromising the structural integrity? Second: How will thermal cycling above the eutectic melting point affect the microstructure?
A similarly complex fracture behavior as for the 370 °C group was found for the 410 °C group with two distinct differences. First, a larger fraction of the fracture surface comprised a cohesive fracture through the Au-rich off-eutectic AueGe compound. Second, sections of the primary α phase (Au) was found, mainly in the center region of the joints. In general, the overall Ge content of the fracture surfaces was significantly lower than what was found in the fracture surfaces in the 370 °C group. This explains the recorded increase in shear strength from the 370 °C group to the 410 °C group. That is, a larger fraction of the joint was in a liquid state for the 370 °C group than for the 410 °C group. I.e., more solid was present to carry the load at 410 °C than at 370 °C. Inspection of the substrate surface around the periphery of the joint revealed that Ge had diffused along the Au surface and started to react with the underlying Ni layer when the temperature was increased above the eutectic melting point (Fig. 17). A slight microstructural change, coarsening of the surface, was observed between samples tested at 25 °C (Fig. 17(a)) and 270 °C (Fig. 17(b)). The overall composition on the surface changed by 5–10 at.% Ge to 55–60 at.% Ge for the 270 °C group. Au precipitates formed during fabrication were dispersed across the compound surface for both groups. A fine lamella structure of Au and Ge had developed between the Au precipitates on the surface of the samples from the 370 °C group (Fig. 17(c)). The overall Ge content had decreased on the surface to a near eutectic composition, and the surface was smooth with clear signs of solidification of a free surface as can be seen in Fig. 17. At 370 °C, the Au crystals had grown outward from the surface forming rounded particles on the surface increasing the surface roughness (Fig. 18). After exposure to 410 °C, the Ge had reacted with the underlying Ni and Cu layers forming a Cu–Ge–Ni compound between large Au precipitates (Fig. 17(d)). 4. Summary and conclusions Metallized (Ni/Au) SiC dice were joined to metallized (Cu/Ni/Au) 40
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Au
(a) SiC Au + GeNi(60) + Au + GeNi(62) + Cu(3) Cu(3) + Ni(4) + + Ni(11) + Si(2) Si(3) Au–Ge(12) Au–Ge(20) Au + GeNi(11) + Ni(2) Au–Ge(5) + GeNi(3)
Ge
Au–Ge(30) Ni
Au
(b) SiC
Au–Ge(28) + Si(2)
Au–Ge(25) + Si(1) Au–Ge(12) + GeNi(54) + Si(1)
Ge
Au–Ge(16) + GeNi(42) + Si(1) Au–Ge(4) Au–Ge(16)
Ni
Au–Ge(4) + Si(1)
Fig. 14. SEM micrographs of the fracture surfaces for one substrate (a) and corresponding die (b) of one sample tested at 370 °C. EDX maps of Au, Ge, and Si from the same section are shown to the right. Fig. 15. SEM micrograph of the cohesive fracture surface in the off-eutectic AueGe compound of a sampled shear tested at 370 °C. Columnar structures (solid) are surrounded by a phase that have apparently been in a liquid state (melted). A magnified image of the marked (dashed lines) feature is shown in Fig. 16(a).
Melted
Solid
50 µm 41
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Fig. 16. SEM micrograph of columnar structures discovered in the fracture surface from a sample shear tested at 370 °C. The features appeared in a Au-rich off-eutectic (approximately 4–9 at.% Ge) section. The features show clear signs of mechanical stress. There is a sheared fracture surface at the top of the columns. Slip bands are visible along the side's indicating plastic deformation of the feature before fracture.
Sheared surface
Slip bands 10 µm
Fig. 17. SEM micrographs and EDX maps of the substrate surface around the periphery of the joints after shear testing at (a) 25 °C, (b) 270 °C, (c) 370 °C, and (d) 410 °C. Substrate Joint Substrate Joint Substrate Joint Substrate Joint The top row shows both the substrate surface and fracture surface of the joint at the 200 µm 200 µm 200 µm 200 µm joint periphery. The bottom middle row shows a magnified image of the substrate surface. Au precipitates are surrounded by a Ge rich off-eutectic AueGe compound. At 370 °C, a finer lamella structure developed within the Ge rich off-eutectic AueGe compound. After exposure to 410 °C, Ge 50 µm 50 µm 50 µm 50 µm had reacted with Ni and Cu in the underlaying layers forming a GeNi phase with Cu dissolve within it. Color key: Au is yellow, Ge is blue, Ni is cyan, and Cu is orange. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) (a)
(c)
(b)
Fig. 18. SEM image showing precipitates of primary α (Au) crystals growing outward from the substrate surface around the periphery of a joint shear tested at 370 °C. A magnified image of the dashed region in (a) is shown in (b) were a smooth freely solidified surface is clear and a fine lamella structure is visible between the precipitates.
(b) Fracture surface
Substrate surface
(a)
(d)
Au–Ge(37) Au(α)
50 µm
20 µm
Acknowledgment
[2] [3] [4] [5] [6]
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The authors would like to acknowledge Ole Martin Løvvik (SINTEF Industry) for his support and fruitful discussions. This work was supported by the Research Council of Norway (244915). References [1] The Nobel Prize in Physics, http://www.nobelprize.org/nobel_prizes/physics/ laureates/1956/, (1956) , Accessed date: 13 November 2018.
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A. Larsson, et al. [10] W.D. MacDonald, T.W. Eagar, Annu. Rev. Mater. Sci. 22 (1992) 23–46. [11] W.F. Gale, D.A. Butts, Sci. Technol. Weld. Join. 9 (2004) 283–300. [12] V.R. Manikam, K.Y. Cheong, Components, Packag. Manuf. Technol. IEEE Trans. 1 (2011) 457–478. [13] S.A. Paknejad, S.H. Mannan, Microelectron. Reliab. 70 (2017) 1–11. [14] V. Chidambaram, H.B. Yeung, G. Shan, J. Electron. Mater. 41 (2012) 2107–2117. [15] A. Drevin-Bazin, F. Lacroix, J.F. Barbot, J. Electron. Mater. 43 (2014) 695–701. [16] S. Tanimoto, K. Matsui, Y. Murakami, H. Yamaguchi, H. Okumura, IMAPS Int. Conf. High Temp. Election. (HiTEC), IMAPS, Albuquerque, NM, USA, 2010. [17] S. Egelkraut, L. Frey, M. Knoerr, A. Schletz, In IEEE 12th Proc. Electron. Packag. Technol. Conf. (EPTC), Singapore, (2010). [18] F. Lang, H. Yamaguchi, H. Ohashi, H. Sato, J. Electron. Mater. 40 (2011) 1563–1571. [19] V. Chidambaram, J. Hald, J. Hattel, J. Alloys Compd. 490 (2010) 170–179. [20] M.J. Palmer, R.W. Johnson, B.H. Ecedept, Int. High Temp. Electron., Santa Fe, NM, USA, (2006).
[21] P. Hagler, P. Henson, R.W. Johnson, IEEE Trans. Ind. Electron. 58 (2011) 2673–2682. [22] F. Lang, S. Tanimoto, H. Ohashi, H. Yamaguchi, In Eur. Microelectron. Packag. Conf., Rimini, Italy, (2009). [23] W. Sabbah, S. Azzopardi, C. Buttay, R. Meuret, E. Woirgard, Microelectron. Reliab. 53 (2013) 1617–1621. [24] R. Instructions and F. Productivity: Change, (1994), pp. 1–33. [25] H. Okamoto, T.B. Massalski, Bull. Alloy Phase Diagr. 5 (1984) 601–610. [26] R.P. Elliott, F.A. Shunk, Bull. Alloy Phase Diagr. 1 (1980) 51–54. [27] J. Wang, C. Leinenbach, M. Roth, J. Alloys Compd. 481 (2009) 830–836. [28] T.T. Luu, N. Hoivik, K. Wang, K.E. Aasmundtveit, A.S.B. Vardøy, Metall. Mater. Trans. A Phys. Metall. Mater. Sci. 46 (2015) 5266–5274. [29] T.A. Tollefsen, O.M. Løvvik, K. Aasmundtveit, A. Larsson, Metall. Mater. Trans. A 44 (2013) 2914–2916. [30] T. Luu, N. Hoivik, K. Wang, K.E. Aasmundtveit, and A.B. Vardøy: 2015, vol. 46, pp. 2637–45.
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Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Shear strength of off-eutectic AueGe joints at high-temperature a,b,⁎
Andreas Larsson
c
, Torleif A. Tollefsen , Knut E. Aasmundtveit
d
T
a
TECHNI AS, Dept. of Applied Physics, Borre, Norway University of South-Eastern Norway (USN), Dept. of Microsystems, Borre, Norway TEGma AS, Oslo, Norway d University of South-Eastern Norway (USN), Dept. of Microsystems, Borre, Norway b c
ARTICLE INFO
ABSTRACT
Keywords: Joining Bonding High-temperature Off-eutectic AueGe TLP SLID
High-temperature compatible devices of SiC have been demonstrated in recent years. Together with a hightemperature compatible die-attach, they can form a basis for future reliable high-temperature electronics. This article presents shear strength capacity of Au-rich off-eutectic AueGe die-attach at temperatures above the eutectic melting point. Metallized (Ni/Au) SiC dice were joined to metallized (Cu/Ni/Au) Si3N4 substrates with eutectic AueGe preforms. The joints demonstrated a significant shear strength capacity up to 410 °C (50 °C above the eutectic melting point). High-quality Au-rich off-eutectic AueGe joints, comprising a AueGe mixture with 8 ± 4 at.% Ge, were formed with a low bond line pressure (240 kPa). At room temperature the shear strength was measured to be approximately 140 MPa, falling to approximately 40 MPa at 410 °C. This demonstrates that off-eutectic joints might be considered for use at temperatures above the eutectic melting point. The fracture mode was mainly cohesive in the off-eutectic AueGe layer when tested at a temperature below the eutectic melting point. Above the eutectic melting point, the fracture mode was complex with signs of a partially melted off-eutectic AueGe compound. Within this compound, columnar structures of the primary α phase (Au) was surrounded by an apparently melted and solidified phase. In other regions, an adhesive fracture at the interface between a slightly off-eutectic compound and a GeNi layer was discovered. The microstructure was found to be inhomogeneous with large regional variations within joints. From almost pure Au, primary α phase with up to 2–3 at.% Ge, up to near eutectic AueGe regions with an overall composition of 20–30 at.% Ge. In general, Ge was segregated into explicit domains within a primary α phase.
1. Introduction Since the beginning of modern electronics, joining components with metals have been a critical system component. Already in the 1950s, gold was used on germanium to form a contact that enabled the demonstration of a point contact transistor leading to the Noble prize in physics in 1956 awarded to William Bradford Shockley, John Bardeen and Walter Houser Brattain [1]. Since then, many different types of joining technologies have been developed and thoroughly explored for a wide variety of electronic applications. Such joining technologies include; soldering, brazing, welding, contact melting [2,3], thermocompression, ultrasonic, diffusion, solid-liquid interdiffusion (SLID) [4–8]/transient liquid phase (TLP) [9–11] bonding and sintering [12]. In many applications, high-temperature compatibility is a desired property of the final joint. It can be when application requirements request high operation temperatures or when the joint needs to comply
with subsequent high-temperature processes during fabrication. In recent years, high-temperature compatible components have been developed and demonstrated to operate at high temperatures, > 200 °C, such as silicon on insulator (SOI), gallium arsenide (GaAs), gallium nitride (GaN), and silicon carbide (SiC) semiconductor devices. Together with ceramic substrates, such as alumina (Al2O3), aluminum nitride (AlN) or silicon nitride (Si3N4), and a suitable die attach they form a basis for high-temperature compatible electronics. Joints formed by TLP/SLID type and sintering technologies may be used at temperatures significantly higher than the process temperature required to create the joints. This is one of the main reasons why a significant research effort has been made on such technologies over the years. The downside is that they typically require long process times [9] or high bond line pressures [13]. Now that high-temperature compatible components and substrates have been developed, traditional brazes, or hard solders, have regained interest for high-temperature applications,
Corresponding author. E-mail addresses: [email protected], [email protected] (A. Larsson), [email protected] (T.A. Tollefsen), [email protected] (K.E. Aasmundtveit). ⁎
https://doi.org/10.1016/j.microrel.2019.05.002 Received 8 February 2019; Received in revised form 8 April 2019; Accepted 7 May 2019 0026-2714/ © 2019 Published by Elsevier Ltd.
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Fig. 1. Illustration of the binary phase diagram of the AueGe system. The target off-eutectic composition, c0, of the fabricated joints is marked with a dashed region in. The phase diagram was adapted from Okamoto and Massalski [25]. Note that the eutectic melting point varies slightly in the pertinent literature; 356–361 °C [26,27].
Fig. 2. Schematic illustration of the material stack of the investigated system, prior to bonding. A eutectic AueGe preform is sandwiched between a SiC die and a Si3N4 substrate metallized with Au seed layers. The layer thicknesses are presented in parenthesis.
due to their relatively easy process requirements with low bond line pressure and quick process scheme. Off-eutectic compounds melt incongruently into both a liquid and a solid component. Within this binary material state, such compounds
could have enough volume fraction of the solid component to form a continuous and coherent solid phase extending through the liquid phase, like a sponge, or a metal foam. If there is sufficient solid phase, it could be used to adjoin components at a temperature above the eutectic
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Fig. 3. A characteristic temperature profile during bonding. The eutectic melting point is marked with a dashed line.
Preform
Self-aligning tool head
Fig. 4. A sample placed on the shear tester hot plate after calibration tests. A small piece of eutectic preform is seen on the top of the die used as visual reference of temperature above the eutectic melting point.
temperature. Thus, such a compound could combine the same attractive property of SLID/TLP and sintering; i.e., a lower process temperature than the final operation temperature, with the short process times and low bond line pressures of brazing. Eutectic gold based joints made with AueGe, AueSi and AueIn have demonstrated significant strength capacity at high homologous temperatures [14–16]. The AueGe system was chosen due to its relatively low process temperature compared to common brazes [15], and its stability and performance at high-temperature (~300 °C) [14].
2. Materials and methods 2.1. Materials and fabrication Eutectic AueGe preforms were sandwiched between silicon carbide (SiC) dice and silicon nitride (Si3N4) substrates. The SiC dice (Fairchild Semiconductor) had sputtered nickel silicide (Ni2Si)/nickel (Ni)/gold (Au) layers and a 5 μm thick electroplated Au top layer. The Si3N4 substrates (Denka Chemicals) had active metal bonded (AMB) copper (Cu) conductors plated with a nickel–phosphorus (7 wt% P) top layer on the back and front sides. The substrates were electroplated with a uniform Au layer (5 μm), using a gold cyanide solution at 60–65 °C and
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(a)
Fig. 5. Image captures of a die during a high-temperature shear test. A piece of eutectic AueGe preform is visible on top of the die. Squeeze out of eutectic AueGe is visible on the substrate surface around the die. (a) The hot plate temperature is set to 320 °C, (b) The hot plate temperature is increased to 400 °C and the image was captured before the tool head contacts the die. The preform has melted and both preform and the substrate surface around the die have changed color to a silvery color. (c) The hot plate temperature is still set to 400 °C, and image was capture after die contacts the tool head. The preform has solidified and changed color back to yellow. Sections of the substrate surface near the tool head have changed color back to gold (marked with dashed blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Tool head
Au72Ge28 Preform
(b)
(c)
a current density of 2.7 mA/cm2. The Au layer thickness was chosen such that the final joint would have a Au-rich off-eutectic composition. The Ni layer was used as a diffusion barrier. The plated substrates and dice were diced into 6 mm × 6 mm and 3.4 mm × 1.9 mm sized samples. The eutectic AueGe preform, i.e., 72 at.% Au and 28 at.% Ge, (Goodfellow) was (35 ± 2) μm thick and manually cut into pieces with
the same footprint as the dice. This results in an overall nominal Au:Ge ratio of (12.2 ± 0.4) at.% Ge in the entire system. The target off-eutectic composition is marked with a dashed region in the binary AueGe phase diagram depicted in Fig. 1. The materials stack prior to bonding is shown in Fig. 2. Dice and preforms were manually placed and aligned on top of
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Fig. 6. Graph of the FEA results showing the joint temperature as function of time in the joints during high-temperature shear testing for four different hot plate set points: 300 °C, 400 °C, 415°, and 460 °C. Model calibration was done by correlating melting, m, and solidification, s, of the 400 °C curve to match video recordings from shear testing. Two different durations of contact between tool head and die are shown for the calibration curve 2 s (dashed) and 5 s (solid).
substrates forming a SiC/Ni/Au/AueGe/Au/NieP/Cu/Si3N4/Cu/NieP configuration. The stack was clamped together with spring-loaded ball plungers in a custom-made fixture and placed into a vacuum bonder (Budatec VS160UG). A characteristic temperature profile is shown in Fig. 3. The joints spent roughly 10 min in a molten state before solidification. A small force was used to secure a thermal contact through the stack. Two different bond line pressures were used, 30 kPa and 240 kPa. The bond process was carried out in a dry nitrogen atmosphere, ~1 atm, or in a vacuum, ~1 mTorr.
(a)
(b)
(c)
2.2. Characterization Cross-sections of fabricated samples were evaluated with the use of optical microscopy (Neophot 32, NA 0.9, up to 1000× magnification), and scanning electron microscopy (SEM) (Hitachi SU8230). Samples were prepared for cross-section analysis by two different methods. In the first method, the samples were ground and polished and finished by flat ion milling. The grinding stopped at 2000 grit grade before preparation was continued with polishing using a cloth and a 1 μm
SiC Au–Ge Cu
200 µm
SiC Au–Ge Cu
200 µm
SiC Au–Ge Cu
200 µm
Fig. 7. Cross-sections of virgin joints formed by three different processes (bond line pressure and atmosphere); (a) 30 kPa in N2, (b) 240 kPa in N2, and (c) 240 kPa in vacuum. Voids can be seen in the center of the joint at the original bond line, for (a) and (b).
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(a)
Au
20 µm
(b)
Au
Ge
(c)
40 µm
Lc
Void
30 µm Fig. 8. Cross-sections of virgin joints formed by three different processes (bond line pressure and atmosphere); (a) 30 kPa in N2, (b) 240 kPa in N2, and (c) 240 kPa in vacuum. Ge domains are located primarily at the joint center and at the interface to the Ni layer on both sides. A void is clearly seen in the center of figure (c). The line Lc marks the line scan results shown in Fig. 11.
diamond paste for the final step. Then the samples were finished with flat ion milling (Hitachi IM4000, Ar) to reveal potential voids or other defects possibly concealed by smearing. The second method was cutting the sample into two halves and then perform a cross-section cut with ion-milling. The composition and microstructure were evaluated by SEM, energy-dispersive X-ray spectroscopy (EDX) (Oxford X-MAX 150), and electron backscattered diffraction (EBSD) (Oxford Nordlys Nano).
A shear force was applied on five dice while they were heated to ~600 °C to reveal significant remelting of the joint (liquification) above the eutectic melting point at 361 °C. The shear load was (1 ± 0.5) MPa and was applied manually to the die. The shear strength was then quantified at four different joint temperatures; 25 °C, 270 °C, 370 °C, and 410 °C. The shear tester (XYZTEC, Condor Sigma) was equipped with a 100 kgf load cell, a hot plate, and a self-aligning tool head. A 36
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SiC
Au
Lb
were taken at different positions through the samples. In some regions, there was a complete absence of Ge in the Au phase (Fig. 8(a)). The composition variation between regions was; from almost pure Au, primary α-phase, with up to 2–3 at.% Ge dissolved in the α-phase, up to near eutectic AueGe regions with a Ge composition of 20–30 at.%. The overall composition of virgin (as bonded) joints was roughly a Au-rich off-eutectic composition with (8 ± 4) at.% Ge, measured by EDX. Regions with a higher Ge content typically had regions with a lamella type AueGe structure and explicit Ge domains in grain boundaries in the Au phase, as can be seen in Figs. 8(c) and 9. EDX maps of Au, Ge, and Ni of the section shown in Fig. 9 is shown in Fig. 10, illustrating that Ge has segregated into explicit domains. One can also see that Ge and Ni have reacted at both the top and bottom interfaces of the joint. Line scans across virgin joints (Fig. 11) confirmed that the main body of the joint comprised mainly Au and Ge and that a GeNi phase was formed at the Ni interface. The microstructure of these off-eutectic joints differ from the microstructure in joints formed with eutectic AueGe. Eutectic AueGe joints have a lamella or speckled microstructure throughout the joints [14,15,17–19]. The final joint thickness varied between 20 and 30 μm which is smaller than the initial bond layer thicknesses combined, i.e., Au (5 μm) + preform (35 μm) + Au (5 μm). This thickness difference was due to a squeeze-out of melted eutectic material during joining. To quantify the actual composition of our resulting joints, EDX was used throughout our work.
GeNi
Void Ge
Ni–P Cu
GeNi 30 µm
Fig. 9. A section with a few Ge domains. A GeNi layer is visible on both sides of the joint as well as a small spherical void. The line Lb marks the line scan results shown in Fig. 11.
sample placed on the hot plate is shown in Fig. 4. The shear height was 60 μm, and the shear speed was 100 μm/s. The hot plate was preheated to 25 °C, 300 °C or 320 °C before the samples were placed onto it. For the 370 °C and 410 °C groups, the temperature was then increased at a rate of 20 °C/min to the correct setpoint. The local temperature in the sample drops when the tool head of the shear tester contacts the sample. A finite element model was developed to determine the actual temperature in the joints. The model was calibrated to agree with the dynamic behavior of the real system. This was done by trimming the model until melting and solidification corresponded accurately with the real system. Pieces of eutectic AueGe preforms were used as reference points during model calibration. Visual inspection of the phase transformation (melting and solidification) of the preform and a color shift on the substrate surface was used to verify the onset of melting and solidification during tests, see Fig. 5. The duration dice were in contact with the shear tool was correlated with the force-time graphs generated during shear testing. The resulting transient temperature profiles of the joint from the model are shown in Fig. 6. Also, visual inspection of recorded videos from shear testing was done post-test to verify that no solidification could be seen in the 370 °C and 410 °C group joints. Fractography was performed by visual inspection of the fracture surfaces with an optical microscope, SEM and EDX.
3.2. Shear strength All samples were able to carry the applied 1 MPa shear load up to 600 °C. The shear strength quantification results at four different temperatures are shown in Fig. 12. Off-eutectic AueGe joints show tremendous strength at room temperature. The measured strength, (142 ± 7) MPa, is 2–3 times higher than typically found in the pertinent literature for eutectic AueGe joints [17,18,20–22]. It is at the very top of the highest numbers reported for eutectic AueGe joints at room temperature [15,23]. At a higher temperature, 250–300 °C the off-eutectic system tested here is about 30–80% stronger than eutectic AueGe joints [15,16,23]. The shear strength at 270 °C was (81 ± 5) MPa. Table 1 show a comparison of the high-temperature shear strength with eutectic AueGe joints, SLID type joints from the CueSn, AueSn and AueIn systems and Ag sintered joints. At 370 °C, about 10 °C above the eutectic melting point, the shear strength was measured to (31 ± 8) MPa. This shear strength is roughly 10 MPa higher than the room temperature shear strength of regular PbeSn solders. PbeSn solders, such as PbSn 60/40, PbSn 90/10, and PbSn 95/5, melts (liquidus) between 183 °C (eutectic melting point) and roughly 300–310 °C depending on the Sn content, which makes them unfeasible at temperatures near 300 °C. At 410 °C, the shear strength was measured to be (39 ± 9) MPa, i.e., a small increase from the 370 °C group. The requirement in US military standard (method 2019.8) is 3.9 MPa for a 6.3 mm2 large die-attach [24]. The results show that there is potential for high-temperature use of off-eutectic at temperatures above the eutectic melting point. The fracture mode changed as a function of test temperature. A mixed adhesive fracture at the GeNi/Ni interface as well as a cohesive fracture in several sections in the Au-rich off-eutectic layer were found at room temperature (Fig. 13(a)). At 270 °C, a cohesive fracture through the Au-rich off-eutectic AueGe joint was the main fracture mode (Fig. 13(b)). When the test temperature was above the eutectic melting point, the fracture mode was complex. It was mainly found to be a mixed fracture with sections showing a cohesive fracture through a Au-
3. Results and discussion 3.1. Microstructure Investigation of the effect of bond line pressure and atmosphere showed that there were fewer voids when the bond line pressure was 240 kPa and performed in a vacuum as can be seen in Fig. 7. Thus, this process was chosen for fabrication of the shear tested samples. The fabricated joints were uniform, and no interface at the original bond line could be seen. This indicates proper wetting and a complete dissolution and solidification of the preform with the adjoined Au seed layers. The virgin samples had an overall off-eutectic AueGe structure. Cross-sections further showed an inhomogeneous microstructure were some regions were of mainly Au (Fig. 8(b) and Fig. 9). Other regions had a significantly higher Ge content (Fig. 8(c)). This apparent inhomogeneity varied through individual samples when cross-sections
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Au
30 µm
Ge
30 µm
Ni
30 µm Fig. 10. EDX maps of the section shown in Fig. 9. From top to bottom; Au, Ge, and Ni.
rich off-eutectic AueGe compound and as an adhesive fracture between a slightly off-eutectic AueGe layer and a mainly GeNi layer as can be seen in Fig. 13(c) and (d). The samples tested at 370 °C showed a fracture extending through a Au-rich off-eutectic AueGe compound (partially liquid sections during testing) and at the interface between a slightly off-eutectic AueGe compound (mainly liquid during testing) and an adjoining solid layer typically comprising the primary α phase (Au) and GeNi phase as can be seen in Fig. 14. A closer inspection of the cohesive fracture surfaces in the Au-rich off-eutectic AueGe compound (with a Ge content of
4–10 at.%) showed a ductile fracture behavior. Within these regions, columnar structures of the primary α phase (Au) were found. They were surrounded by a AueGe compound that had clearly been in a liquid state with a free surface before solidification (Fig. 15). These Au structures showed clear signs of a ductile fracture with elongation of the material, a well-defined fracture surface, and clear slip bands at the necking region as can be seen in Fig. 16. The compound surrounding the columnar features was off-eutectic AueGe with a Ge concertation varying between a few at.% to a composition near the eutectic composition (28 at.%).
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Fig. 11. EDX line scans marked with Lb and Lc in Fig. 8 and Fig. 9.
Fig. 12. The measured shear strength of off-eutectic AueGe joints, shear tested at four different temperatures.
Table 1 A comparison of high-temperature shear strength for six different high-temperature joining materials. Type
Off-eutectic AueGe
Eutectic AueGe
SLID CueSn
SLID AueSn
SLID AueIn
Sintering Ag
Temperature (°C) Shear strength (MPa) Source
270 81 ± 5 This work
300 26–54.5 [15,16,23]
300 42–61 [28]
300 20 [29]
300 40–60 [15,30]
300 10–20 [15,23]
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(b)
(a)
Cohesive
Adhesive
1 mm
Adhesive
Cohesive
Adhesive
Cohesive
1 mm
(d)
(c)
Cohesive
Adhesive
1 mm
1 mm
Fig. 13. SEM micrographs of typical fracture surfaces from each shear test temperature group. (a) 25 °C: Mainly an adhesive fracture at the GeNi/Ni interface with four sections showing a cohesive fracture in the Au-rich off-eutectic layer. (b) 270 °C: Mainly a cohesive fracture through an off-eutectic AueGe section. (c) 370 °C and (d) 410 °C: The fracture extends mainly as a cohesive fracture through a Au-rich off-eutectic region, but also as an adhesive fracture between a primary α (Au) layer or the adhesion layer on the die and a mainly GeNi layer.
Si3N4 substrates with eutectic AueGe preforms. High quality off-eutectic AueGe (8 ± 4 at.% Ge) joints were formed with a low bond line pressure (240 kPa). The joints demonstrated a significant shear strength capacity up to 410 °C. At room temperature, the shear strength was measured to be (142 ± 7) MPa falling to (39 ± 9) MPa at 410 °C (50 °C above the melting point of eutectic AueGe (28 at.% Ge)). 39 MPa is ten times higher than the requirement in the MIL-STD-883H standard for such joints. The shear strength at 410 °C is roughly up to twice as strong as regular SnePb and SAC solders at room temperature. This demonstrates that off-eutectic AueGe joints have the potential to be used at temperatures higher than their eutectic melting point. The fracture mode changed with temperature. At room temperature, a mixed mode with an adhesive fracture at a GeNi/Ni interface combined with a cohesive fracture in an off-eutectic Au layer was found. At 270 °C, the fracture mode was mainly cohesive in an off-eutectic AueGe layer. Above the eutectic melting point, the fracture mode was complex with signs of a partially melted off-eutectic AueGe compound. Within this compound, columnar structures of primary α (Au) surrounded by an apparently melted and solidified phase were found. In other regions an adhesive fracture at the interface between a slightly off-eutectic compound and a layer comprising the GeNi phase was present. The microstructure was found to be inhomogeneous with large regional variations within joints. From almost pure Au, primary α phase with up to 2–3 at.% Ge, up to near eutectic AueGe regions with 20–30 at.% Ge. In general, Ge was segregated into explicit domains within a primary α phase. Two concerns of particular interest need to be investigated before final conclusions can be made. First; how will the kinetics of off-eutectic compound behave when it is exposed to prolonged durations in a partially liquid state? Will Ge diffuse along the grain boundaries between Au grains creating a liquid interlayer compromising the structural integrity? Second: How will thermal cycling above the eutectic melting point affect the microstructure?
A similarly complex fracture behavior as for the 370 °C group was found for the 410 °C group with two distinct differences. First, a larger fraction of the fracture surface comprised a cohesive fracture through the Au-rich off-eutectic AueGe compound. Second, sections of the primary α phase (Au) was found, mainly in the center region of the joints. In general, the overall Ge content of the fracture surfaces was significantly lower than what was found in the fracture surfaces in the 370 °C group. This explains the recorded increase in shear strength from the 370 °C group to the 410 °C group. That is, a larger fraction of the joint was in a liquid state for the 370 °C group than for the 410 °C group. I.e., more solid was present to carry the load at 410 °C than at 370 °C. Inspection of the substrate surface around the periphery of the joint revealed that Ge had diffused along the Au surface and started to react with the underlying Ni layer when the temperature was increased above the eutectic melting point (Fig. 17). A slight microstructural change, coarsening of the surface, was observed between samples tested at 25 °C (Fig. 17(a)) and 270 °C (Fig. 17(b)). The overall composition on the surface changed by 5–10 at.% Ge to 55–60 at.% Ge for the 270 °C group. Au precipitates formed during fabrication were dispersed across the compound surface for both groups. A fine lamella structure of Au and Ge had developed between the Au precipitates on the surface of the samples from the 370 °C group (Fig. 17(c)). The overall Ge content had decreased on the surface to a near eutectic composition, and the surface was smooth with clear signs of solidification of a free surface as can be seen in Fig. 17. At 370 °C, the Au crystals had grown outward from the surface forming rounded particles on the surface increasing the surface roughness (Fig. 18). After exposure to 410 °C, the Ge had reacted with the underlying Ni and Cu layers forming a Cu–Ge–Ni compound between large Au precipitates (Fig. 17(d)). 4. Summary and conclusions Metallized (Ni/Au) SiC dice were joined to metallized (Cu/Ni/Au) 40
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Au
(a) SiC Au + GeNi(60) + Au + GeNi(62) + Cu(3) Cu(3) + Ni(4) + + Ni(11) + Si(2) Si(3) Au–Ge(12) Au–Ge(20) Au + GeNi(11) + Ni(2) Au–Ge(5) + GeNi(3)
Ge
Au–Ge(30) Ni
Au
(b) SiC
Au–Ge(28) + Si(2)
Au–Ge(25) + Si(1) Au–Ge(12) + GeNi(54) + Si(1)
Ge
Au–Ge(16) + GeNi(42) + Si(1) Au–Ge(4) Au–Ge(16)
Ni
Au–Ge(4) + Si(1)
Fig. 14. SEM micrographs of the fracture surfaces for one substrate (a) and corresponding die (b) of one sample tested at 370 °C. EDX maps of Au, Ge, and Si from the same section are shown to the right. Fig. 15. SEM micrograph of the cohesive fracture surface in the off-eutectic AueGe compound of a sampled shear tested at 370 °C. Columnar structures (solid) are surrounded by a phase that have apparently been in a liquid state (melted). A magnified image of the marked (dashed lines) feature is shown in Fig. 16(a).
Melted
Solid
50 µm 41
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Fig. 16. SEM micrograph of columnar structures discovered in the fracture surface from a sample shear tested at 370 °C. The features appeared in a Au-rich off-eutectic (approximately 4–9 at.% Ge) section. The features show clear signs of mechanical stress. There is a sheared fracture surface at the top of the columns. Slip bands are visible along the side's indicating plastic deformation of the feature before fracture.
Sheared surface
Slip bands 10 µm
Fig. 17. SEM micrographs and EDX maps of the substrate surface around the periphery of the joints after shear testing at (a) 25 °C, (b) 270 °C, (c) 370 °C, and (d) 410 °C. Substrate Joint Substrate Joint Substrate Joint Substrate Joint The top row shows both the substrate surface and fracture surface of the joint at the 200 µm 200 µm 200 µm 200 µm joint periphery. The bottom middle row shows a magnified image of the substrate surface. Au precipitates are surrounded by a Ge rich off-eutectic AueGe compound. At 370 °C, a finer lamella structure developed within the Ge rich off-eutectic AueGe compound. After exposure to 410 °C, Ge 50 µm 50 µm 50 µm 50 µm had reacted with Ni and Cu in the underlaying layers forming a GeNi phase with Cu dissolve within it. Color key: Au is yellow, Ge is blue, Ni is cyan, and Cu is orange. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) (a)
(c)
(b)
Fig. 18. SEM image showing precipitates of primary α (Au) crystals growing outward from the substrate surface around the periphery of a joint shear tested at 370 °C. A magnified image of the dashed region in (a) is shown in (b) were a smooth freely solidified surface is clear and a fine lamella structure is visible between the precipitates.
(b) Fracture surface
Substrate surface
(a)
(d)
Au–Ge(37) Au(α)
50 µm
20 µm
Acknowledgment
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