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Reliable single-phase micro-joints with high melting point for 3D TSV chip stacking
Journal of Alloys and Compounds 828 (2020) 154468
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Reliable single-phase micro-joints with high melting point for 3D TSV chip stacking Ye Tian a, c, *, Heng Fang a, Ning Ren a, Yatao Zhao a, Boli Chen a, Fengshun Wu b, Kyung-Wook Paik c a b c
School of Mechanical and Electrical Engineering, Henan University of Technology, Zhengzhou, 450052, China School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, 430074, China Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Daejeon, 305-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 November 2019 Received in revised form 18 February 2020 Accepted 21 February 2020 Available online 22 February 2020
As micro solder-joints downsize to less than 10 mm in 3-dimensional (3D) TSV (Through Silicon Via) chip stacking, rapidly fabricating reliable micro-joints without collapse in chip stacking is regarded as a critical issue. A practicable method with a suitable temperature gradient (TG) and Ni/Sn(10 mm)/Ni interconnection structure was developed compared with conventional transient liquid phase (TLP) bonding. Using this method, the micro-joint fully composed of Ni3Sn4 intermetallic was fabricated within 17 min bonding time. It is nearly three times faster than that under conventional TLP bonding, which may overcome this technology limitation caused by longer bonding times. This fast TG-TLP method provided an asymmetry growth characteristic with thicker Ni3Sn4 at the cold end and thinner Ni3Sn4 at the hot end. The basic micro-joint formation mechanisms were suggested and experimentally verified. Furthermore, the properties of the fabricated Ni3Sn4 micro-joint were evaluated in this study. The results demonstrated its desirable features such as high melting temperature, stable single-phase constitution, stronger mechanical strength and low thermal resistance. Finally, the collapsed micro solder-joint issue was completely solved resulting in higher reliability. © 2020 Published by Elsevier B.V.
Keywords: Micro solder-joint Chip stacking Intermetallic compounds Reliability
1. Introduction Nowadays, as semiconductor packaging technology approaches the physical limitation, 3D ICs stacking technology becomes one promising solution in high density packaging to break through this limitation and continuously extend Moore’s law in microelectronic industry. Micro solder-joints have been commonly employed to serve as interconnections between vertically stacked 3D TSV chips [1,2]. The stand-off height of solder joint layer will be scaled down from current one hundred microns to several microns in near future [3]. In this dramatical height change, the bonding process and materials demand the fabricated micro-joints to not only be reliable but also stack chips without collapsed micro-joints [4,5]. The emergence of TLP bonding technology seems to overcome this technical challenge by generating fully transferred intermetallic compounds (IMCs) micro-joints with higher melting temperature
* Corresponding author. School of Mechanical and Electrical Engineering, Henan University of Technology, Zhengzhou, 450052, China. E-mail address: [email protected] (Y. Tian). https://doi.org/10.1016/j.jallcom.2020.154468 0925-8388/© 2020 Published by Elsevier B.V.
at low bonding temperature [6]. During past several years, many studies have reported to produce micro-joints with fully transferred IMCs using the TLP bonding technology. A typical process is implemented by using longer bonding time, until the Sn solder is completely converted into Cu6Sn5 IMCs in Cu/Sn/Cu joints. Nevertheless, this process has a common drawback that needs a long bonding time to consume all the Sn solder, ranging from tens of minutes to several hours [7e10]. The longer bonding time may damage the performance of silicon chips as well as produce excessive thermal stress/strain to lower the joints reliability. Moreover, the fabricated Cu6Sn5 micro-joint appears to be not reliable due to its formation unavoidably accompanied with the formation of Cu3Sn intermetallic and Kirkendall voids, as well as the allotropic transformation during solid cooling stage. These evolutions have been known to weaken the mechanical properties of micro-joints, thereby deteriorating their reliability [11e14]. Nickel is frequently used as a diffusion barrier for Cu substrate or other alternative substrates in solder joints [15,16]. The Ni3Sn4 intermetallic is only one reaction product during liquid-solid
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reaction of Ni and Sn solder under 300 C, and especially can remain thermally stable during aging treatments [17]. In addition, the Ni3Sn4 possesses higher melting point of 794.5 C, better mechanical performance as well as fracture toughness compared with typical copper IMCs (i.e., Cu6Sn5 and Cu3Sn). Therefore, Ni3Sn4 should be more favorable to work as micro-joints material of 3D TSV interconnection [18e20]. As a result, combining Ni3Sn4 desirable features with the excellent corrosion resistance of Ni, the Ni/ Sn/Ni interconnection is anticipated to become a promising candidate for the TLP bonding process. However, the long bonding time to produce fully Ni3Sn4 composed micro-joints is still a critical issue due to low Ni dissolution rate and slow Ni3Sn4 growth [21,22]. Recent investigations showed that the thermo-migration of interfacial atoms occurred in liquid-state reactions can significantly facilitate the IMCs rapid growth at the solder/Cu or Ni interface [23e25]. It may provide a potential solution to shorten bonding time for solving the previous issue. Consequently, this study is attempting to rapidly fabricate reliable micro-joints composed of only Ni3Sn4 intermetallic. The new bonding process in a combination of the TG-TLP bonding method and Ni/Sn(10 mm)/Ni interconnection structure was conducted based on conventional TLP bonding process. The basic micro-joint forming mechanism was investigated, and their properties were also evaluated in this study.
200 C at both ends of the sample. These two ends were respectively defined as the hot end and cold end. Two thermocouples fixed at the two ends were used to in-situ measure the temperatures for ensuring the temperatures stabilization in the duration of the liquid Sn. The bonding time ranged from 2 to 17 min to provide different samples for studying the interfacial IMCs growth. The cold-wind gun was used to cool down samples rapidly at the cooling stage to avoid the continuous solid-liquid reactions, which can provide a more precise relationship between IMCs growth and bonding time under this stable TG. For comparison, the micro solder-joints by conventional TLP bonding process were also prepared using the same samples at a uniform temperature of 265 C for designed durations. The fabricated micro-joint samples were mounted in an epoxy, manually ground and then polished to characterize the crosssectional microstructure by scanning electron microscope (SEM). Energy dispersive spectroscope (EDS) and Micro-region X-Ray diffraction (Micro-XRD) were employed to identify the IMCs phases. The existence of residual Sn in the micro-joint was examined using differential scanning calorimeter (DSC) at the maximum temperature of 300 C for heating rate of 5 C/min. Additionally, the tensile testing was also carried out to examine the joints tensile strength at a tensile speed of 0.05 mm/min. The thicknesses of the interfacial IMCs at the both ends were measured, and then analyzed by the Image-J software.
2. Experimental and simulation process 2.1. Simulation process A two-step method was carried out to lessen the solder squeezing phenomenon and maintain the micro-joints stand-off height during the TG-TLP bonding. Firstly, the Ni/Sn/Ni interconnections were prepared using pure Ni (99.99%) wire and pure Sn (99.99%) foil by a solder reflow process at the maximum temperature of 250 C for 20 s above the melting-point temperature. A specially designed fixture was utilized to control the interlayer Sn thickness as 10 mm between two Ni wires. The formed interconnection structure was shown in Fig. 1. The resultant IMC thickness was measured as approximately 0.4 mm which can be nearly negligible for the subsequent TG-TLP bonding. Fig. 2 shows schematic diagrams of experimental configuration under the TG-TLP bonding. The top and bottom heating blocks were fixed by Ni sheets at their both sides, and two ends of the fabricated interconnection sample were respectively fixed in the central slot-lines of these two blocks by high temperature adhesive, which avoided the melting Sn undergoing any force to remain the joint’s stand-off height, after the adhesive was cured, another fixed blocks with central slot-line were pressed on the sample. After bonding, the sample was removed from the blocks by dissolving the adhesive. A temperature gradient (TG) was introduced by holding 330 C and
Owing to the small joints size, it is difficult to measure the TG across the Sn interlayer during bonding process. Therefore, finite element simulation with computational fluid dynamics (CFD) was employed to obtain the temperature gradient. The temperatures loaded at both the ends keep the same as the stable values (330 C and 200 C) measured by the thermocouples during the bonding period. The thermal conductivities of Ni and liquid Sn solder were 90.7W/(m K) and 30.7W/(m K) respectively [26]. With respect to boundary conditions, the ambient temperature and gravity vector were set as 20 C and 9.81 m/s2 respectively. The boussinesq approximation and zero equation of turbulence were utilized to simulate the natural convection. The hot end represented the Ni/Sn interface adjacent to the top heating block while the cold end represented the Ni/Sn interface near the bottom heating block. Fig. 3 shows the temperature distribution in the Ni/Sn(10 mm)/Ni interconnection during the TG-TLP bonding. The temperature difference between the two ends is 4.6 C, and accordingly the temperature gradient was computed to be 4600 C/mm. For more precise comparison, the temperature of 265 C at the hot end was adopted as the bonding temperature of the conventional TLP process. 3. Results and discussion 3.1. The growth characteristic of Ni3Sn4 intermetallic under TG-TLP bonding
Fig. 1. Cross-sectional structure of a fabricated Ni/Sn/Ni interconnection.
Fig. 4 (a)-(d) show cross-sectional back-scattered electron (BSE) images of the Ni/Sn/Ni interconnections for TG-TLP bonding at 2, 6, 11 and 17 min. As observed in Fig. 4 (a), only one type of IMCs appeared at the both end interfaces, showing an asymmetry growth phenomenon with thicker IMCs at the cold end and thinner IMCs at the hot end. The IMC morphologies represented rod-like and block-like shape at the coldend, and rod-like shape at the hotend, respectively. This morphological discrepancy at the both ends should be associated with the interfacial reactions induced by the TG-TLP process. It was also observed that the block-shaped IMC
Y. Tian et al. / Journal of Alloys and Compounds 828 (2020) 154468
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Fig. 2. Schematic diagrams of the (a) TG-TLP bonding experimental configuration and (b) Ni/Sn/Ni interconnection structure.
Fig. 3. Temperature distribution in the Ni/Sn(10 mm)/Ni interconnection of (a) 1/4 global structure and (b) Local Sn solder joint during the TG-TLP bonding.
Fig. 4. Cross-sectional BSE images of the Ni/Sn(10 mm)/Ni interconnection structure after the TG-TLP bonding for (a) 2, (b) 6, (c) 11, and (d) 17 min.
existed in the solder matrix adjacent to the cold end. By the EDX result from Fig. 5 and NieSn binary phase diagram [27], the IMC was identified as Ni3Sn4 intermetallic. After 5 min TG-TLP bonding,
the asymmetric growth appeared greatly more pronounced as presented in Fig. 4 (b), and even the large block-shaped Ni3Sn4 growing from the cold end touched the hot end after 11 min as
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Fig. 5. EDX analysis of the fabricated micro-joint by the TG-TLP bonding process at (a) the marked red frame in Fig. 4 (a), and (b) the marked red line in Fig. 4 (d). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
observed in Fig. 4 (c). This remarkable asymmetry growth phenomenon with thicker IMCs at the cold end and thinner IMCs at the hot end is strongly associated with the directional diffusion of the interfacial atoms driven by the TG,which would be discussed in detail in the next section. It was worth noting that the block-shaped Ni3Sn4 intermetallic grows more significantly, and become predominant at the cold end interface as bonding time increased. This can be explained by the Ostwald Ripening theory defined as the growth of larger crystals at the expense of smaller ones by reducing total interfacial energy. As the bonding time increased to 17 min, as depicted in Fig. 4 (d), there is no Sn solder observed in the microjoint, meaning the Sn solder was completely transformed into the IMCs. The DSC result was further verified no Sn remained in the micro-joint as shown in Fig. 6. Furthermore, the resulting microjoint was also determined to contain only Ni3Sn4 by EDX results in Fig. 5 (b) and XRD results in Fig. 7. It can be anticipated that Ni3Sn4 micro-joints with high melting temperature of 794.5 C may be robust to withstand the subsequent chip-stacking without collapsed joint during the vertical interconnection process of 3D TSV chips. Fig. 8 (a) and (b) show cross-sectional BSE images of the Ni/Sn/ Ni interconnections after a conventional TLP bonding for a homogeneous temperature of 265 C at 17 and 48 min. As is well known, the IMCs growth in the solder joint is considerably sensitive to the temperature. Hence, for more precise comparison with the TG-TLP bonding, the conventional TLP bonding temperature maintained the same temperature as the hot end temperature of the TG-TLP bonding evaluated from the simulation. As shown in Fig. 8 (a), the Ni3Sn4 intermetallic displayed identical thickness and morphologies of long-needle and large-block at both ends after 17 min TLP bonding. Furthermore, numerous Sn solder was found to still
Fig. 6. DSC analysis of the fabricated micro-joint at 17 min by the TG-TLP and TLP bonding methods.
exist in the Ni/Sn/Ni joint. The DSC result can further verify substantial Sn solder left in the micro-joint as shown in Fig. 6. As the bonding time extended to 48 min, the Sn solder disappeared as presented in Fig. 8 (b), indicating that all the solder was converted into the Ni3Sn4 intermetallic. Clearly, the formation rate of the fully Ni3Sn4 composed micro-joint is approximately 3 times faster in the TG-TLP bonding in relative to the conventional TLP bonding. Consequently, the introduced TG dominates the fast and asymmetry growth of the Ni3Sn4 intermetallic in the Ni/Sn/Ni interconnection. 3.2. Growth kinetics analysis of interfacial Ni3Sn4 under TG-TLP bonding The thermo-migration can be described that both the heat and mass transfer occur simultaneously in a mixture due to the imposed temperature gradient across the mixture. The Ni flux JTM subjected to a temperature gradient vT=vx can be expressed as:
JTM ¼ C
D Q* vT kT T vx
Where D is the diffusion coefficient in a liquid solder, k is the Boltzmann’s constant, T is the absolute temperature, Q * is the transport heat, C is the concentration. Clearly, the fast formation of IMCs joints is essentially attributed to continuous Ni thermomigration from the hot end to the cold end driven by the TG. The Ni flux JTM is directly proportional to the temperature gradient vT=vx, suggesting that the formation rate is dependent on the TG magnitude. Additionally, it was well documented that the dissolution and growth of interfacial IMCs occur simultaneously during
Fig. 7. XRD analysis of the fabricated micro-joint at 17 min for the TG-TLP bonding.
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Fig. 8. Cross-sectional BSE images of the Ni/Sn/Ni interconnects using a conventional TLP bonding at (a) 17 and (b) 48 min.
soldering [28].The dissolution rate can be written by:
dC S ¼ Kd ðCs CÞ dt V Where Kd is a constant, S is the surface area of interfacial IMCs, V is the solder volume, and Cs is the Cu solubility in the solder. Obviously, the higher CseC tends to cause the faster dissolution of interfacial IMCs. Fig. 9 presents schematic diagrams of Ni atomic fluxes in the Ni/Sn/Ni interconnection during the TG-TLP bonding. At the cold end, JTM presented an opposite direction with Ni chemical potential flux (JChem ), i.e., grain boundary diffusion flux JGB , as explained in Fig. 9. As a result, Ni atoms in the liquid solder around the cold end always remained saturated or near-saturated, giving rise to a low dissolution rate because of lower CseC. Therefore, the growth dominates the interfacial Ni3Sn4, and makes them thicken fast at the cold end. Actually, both the thickened Ni3Sn4 layer and lower dissolution rate induced by the smaller CseC could effectively inhibit the Ni atoms diffusion from the cold end resulting in negligible JChem. Therefore, one concluded that JTM dominates the interfacial Ni3Sn4 growth at the cold end. Fig. 10 displays the Ni3Sn4 thickness as a function of bonding times. The Ni3Sn4 layer was increased from 3 mm up to 10.5 mm for 2 and 11 min respectively at the cold end. The growth rate of interfacial Ni3Sn4 was 0.92 mm/min and 0.33 mm/min using the TG-TLP bonding and traditional TLP bonding respectively. In contrast, the same direction of Jchem and JTM at the hot end can keep Cu atoms always unsaturated in the liquid Sn adjacent to the hot end, which produces a larger CseC value. At this end, Ni atoms may rapidly diffuse across the thin Ni3Sn4 layer by grain boundary diffusion at the initial reaction stage, resulting in a large Jchem. It is reasonable to suppose that most of Jchem need to provide JTM, and thereby leaving a minor of Ni atoms to enable the Ni3Sn4 growth. As bonding time prolonged, the thickened Ni3Sn4 layer would gradually hinder the Ni diffusion until Jchem ¼ JTM, inducing a dynamic
Fig. 10. Interfacial Ni3Sn4 thickness as a function of bonding time using both TG-TLP and conventional TLP bonding methods.
equilibrium presence of the solution and growth. This can produce a critical layer with a stable thickness. When the Ni3Sn4 layer exceeds this critical thickness, the dissolution becomes dominant and the Ni3Sn4 layer would become thinner because of Jchemþ Jdis ¼ JTM. Obviously, such a critical thickness is strongly dependent on the dynamic equilibrium between the chemical potential gradient and the TG. As shown in Fig. 10, the Ni3Sn4 layer thickened slowly before 2 min, and maintained nearly unchanged from 2 to 6 min at the hot end. These experimental results were well agreed with the theory explained above. Nevertheless, the thickness showed a slow rise after 6 min. This may be a consequence of the lowered TG caused by significantly thinned Sn interlayer because of the consumption of substantial Sn in interfacial reactions. 3.3. The properties evaluation of the fabricated Ni3Sn4 micro-joint In this section, the properties of the fabricated Ni3Sn4 micro-
Fig. 9. Schematic diagrams of Ni atomic fluxes in the Ni/Sn/Ni micro-joint during a TG-TLP bonding.
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Fig. 11. Fracture surface images of the fabricated micro-joints of (a) Sn micro solder -joint and (b) full Ni3Sn4 IMCs micro solder-joint after a tensile test.
joint were characterized. The tensile testing was carried out to estimate the joint mechanical property. The tensile strengths of the Ni3Sn4 and Sn micro-joints were measured as 180.8 MPa and 97.3 MPa, respectively. Apparently, the former possesses higher tensile strength. Fig. 11 shows fracture surfaces of these two joints after the tensile loading. As observed in Fig. 11(a), the fracture surface of Sn micro-joint is composed of deeply equiaxed-dimples, representing a typical ductile fracture occurred into the Sn solder matrix. This is because the Sn solder has higher volume fraction in the micro solder-joint after solder reflowing. However, only fractured Ni3Sn4 intermetallic were exposed on the fracture surface, indicating the fracture completely occurred inside them as shown in Fig. 11(b). Although the Ni3Sn4 intermetallic is more brittle than the Sn solder, the Ni3Sn4 micro-joint may display more reliable mechanical properties than the solder micro-joint owing to higher mechanical strength of the Ni3Sn4 intermetallic. A four-point probe was performed to examine the electrical resistivity of the Ni3Sn4 micro-joints. The average electrical resistivity was obtained as 25.9 mU/cm that is well acceptable for the 3D TSV interconnections. Apart from the mechanical and electronic properties, the thermal property of the Ni3Sn4 micro-joint was evaluated by calculating thermal resistance r. The calculation formula can be expressed as r ¼ d=k, where d is the measured thickness and k is the thermal conductivity. The thermal conductivities of solid-state Sn and Ni3Sn4 are known as 6.9 W/(m K) and 19.6 W/(m K) respectively [29]. The thermal resistance of the Ni3Sn4 micro-joint was calculated as 0.612 mm2KW1. In the case of the micro solder-joint, since the Sn layer is greatly thicker in relative to the Ni3Sn4 layer formed after solder reflow, the micro solder-joint can be assumed to only contain Sn solder. The corresponding thermal resistance was calculated as 1.74 mm2KW1, which is much higher than that of the Ni3Sn4 micro-joint. One primary limitation facing 3D ICs is known to be effective heat dissipation, particularly from the upper chips for greatly dense chips integration. It is anticipated that the fabricated Ni3Sn4 micro-joint can effectively reduce the vertical thermal resistance due to its lower thermal resistance. This can be is beneficial to enhance the heat dissipation of 3D ICs.
process. This faster IMCs asymmetric growth phenomenon was primarily due to the Ni directional thermal-migration driven by the TG across the Sn interlayer. The resultant Ni3Sn4 micro-joint exhibited desirable features for the near future interconnections of the 3D TSV packaging, i.e., high melting temperature, stable single-phase constitution, strong mechanical strength, and low thermal resistance. The fabricated Ni3Sn4 micro-joint can solve the solder joint collapse issue and thus obtaining higher reliability. Consequently, this TG-TLP process can provide a promising solution to stack 3D TSV chips.
4. Conclusion
References
A practical approach to fabricate intermetallic micro-joints, only consisting of Ni3Sn4 intermetallic, was suggested by soldering Ni/ Sn(10 mm)/Ni interconnection structure under a suitable temperature gradient. During this process, the interfacial IMCs grew asymmetrically with thicker Ni3Sn4 layer at the cold end and thinner Ni3Sn4 layer at the hot end in the Ni/Sn/Ni interconnection. Such a growth continuously intensified until fully composed Ni3Sn4 micro-joint rapidly formed. The corresponding formation rate is approximately 3 times faster than that using a conventional TLP
Funding This work was supported by the National Natural Science Foundation of China [grant numbers U1504507, U1704147], and Science and Technology Program of Henan Province [grant number 182102410048]. The authors also acknowledge the valuable supports from The Youth Backbone Teacher Training Program of Henan University of Technology. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Ye Tian: Conceptualization, Investigation, Resources, Writing original draft, Writing - review & editing, Project administration, Funding acquisition. Heng Fang: Methodology, Software, Formal analysis, Writing - original draft, Visualization. Ning Ren: Methodology, Software, Validation, Formal analysis. Yatao Zhao: Methodology, Validation, Data curation. Boli Chen: Methodology, Validation, Data curation. Fengshun Wu: Supervision, Writing review & editing. Kyung-Wook Paik: Supervision, Writing - review & editing.
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Reliable single-phase micro-joints with high melting point for 3D TSV chip stacking Ye Tian a, c, *, Heng Fang a, Ning Ren a, Yatao Zhao a, Boli Chen a, Fengshun Wu b, Kyung-Wook Paik c a b c
School of Mechanical and Electrical Engineering, Henan University of Technology, Zhengzhou, 450052, China School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, 430074, China Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Daejeon, 305-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 November 2019 Received in revised form 18 February 2020 Accepted 21 February 2020 Available online 22 February 2020
As micro solder-joints downsize to less than 10 mm in 3-dimensional (3D) TSV (Through Silicon Via) chip stacking, rapidly fabricating reliable micro-joints without collapse in chip stacking is regarded as a critical issue. A practicable method with a suitable temperature gradient (TG) and Ni/Sn(10 mm)/Ni interconnection structure was developed compared with conventional transient liquid phase (TLP) bonding. Using this method, the micro-joint fully composed of Ni3Sn4 intermetallic was fabricated within 17 min bonding time. It is nearly three times faster than that under conventional TLP bonding, which may overcome this technology limitation caused by longer bonding times. This fast TG-TLP method provided an asymmetry growth characteristic with thicker Ni3Sn4 at the cold end and thinner Ni3Sn4 at the hot end. The basic micro-joint formation mechanisms were suggested and experimentally verified. Furthermore, the properties of the fabricated Ni3Sn4 micro-joint were evaluated in this study. The results demonstrated its desirable features such as high melting temperature, stable single-phase constitution, stronger mechanical strength and low thermal resistance. Finally, the collapsed micro solder-joint issue was completely solved resulting in higher reliability. © 2020 Published by Elsevier B.V.
Keywords: Micro solder-joint Chip stacking Intermetallic compounds Reliability
1. Introduction Nowadays, as semiconductor packaging technology approaches the physical limitation, 3D ICs stacking technology becomes one promising solution in high density packaging to break through this limitation and continuously extend Moore’s law in microelectronic industry. Micro solder-joints have been commonly employed to serve as interconnections between vertically stacked 3D TSV chips [1,2]. The stand-off height of solder joint layer will be scaled down from current one hundred microns to several microns in near future [3]. In this dramatical height change, the bonding process and materials demand the fabricated micro-joints to not only be reliable but also stack chips without collapsed micro-joints [4,5]. The emergence of TLP bonding technology seems to overcome this technical challenge by generating fully transferred intermetallic compounds (IMCs) micro-joints with higher melting temperature
* Corresponding author. School of Mechanical and Electrical Engineering, Henan University of Technology, Zhengzhou, 450052, China. E-mail address: [email protected] (Y. Tian). https://doi.org/10.1016/j.jallcom.2020.154468 0925-8388/© 2020 Published by Elsevier B.V.
at low bonding temperature [6]. During past several years, many studies have reported to produce micro-joints with fully transferred IMCs using the TLP bonding technology. A typical process is implemented by using longer bonding time, until the Sn solder is completely converted into Cu6Sn5 IMCs in Cu/Sn/Cu joints. Nevertheless, this process has a common drawback that needs a long bonding time to consume all the Sn solder, ranging from tens of minutes to several hours [7e10]. The longer bonding time may damage the performance of silicon chips as well as produce excessive thermal stress/strain to lower the joints reliability. Moreover, the fabricated Cu6Sn5 micro-joint appears to be not reliable due to its formation unavoidably accompanied with the formation of Cu3Sn intermetallic and Kirkendall voids, as well as the allotropic transformation during solid cooling stage. These evolutions have been known to weaken the mechanical properties of micro-joints, thereby deteriorating their reliability [11e14]. Nickel is frequently used as a diffusion barrier for Cu substrate or other alternative substrates in solder joints [15,16]. The Ni3Sn4 intermetallic is only one reaction product during liquid-solid
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reaction of Ni and Sn solder under 300 C, and especially can remain thermally stable during aging treatments [17]. In addition, the Ni3Sn4 possesses higher melting point of 794.5 C, better mechanical performance as well as fracture toughness compared with typical copper IMCs (i.e., Cu6Sn5 and Cu3Sn). Therefore, Ni3Sn4 should be more favorable to work as micro-joints material of 3D TSV interconnection [18e20]. As a result, combining Ni3Sn4 desirable features with the excellent corrosion resistance of Ni, the Ni/ Sn/Ni interconnection is anticipated to become a promising candidate for the TLP bonding process. However, the long bonding time to produce fully Ni3Sn4 composed micro-joints is still a critical issue due to low Ni dissolution rate and slow Ni3Sn4 growth [21,22]. Recent investigations showed that the thermo-migration of interfacial atoms occurred in liquid-state reactions can significantly facilitate the IMCs rapid growth at the solder/Cu or Ni interface [23e25]. It may provide a potential solution to shorten bonding time for solving the previous issue. Consequently, this study is attempting to rapidly fabricate reliable micro-joints composed of only Ni3Sn4 intermetallic. The new bonding process in a combination of the TG-TLP bonding method and Ni/Sn(10 mm)/Ni interconnection structure was conducted based on conventional TLP bonding process. The basic micro-joint forming mechanism was investigated, and their properties were also evaluated in this study.
200 C at both ends of the sample. These two ends were respectively defined as the hot end and cold end. Two thermocouples fixed at the two ends were used to in-situ measure the temperatures for ensuring the temperatures stabilization in the duration of the liquid Sn. The bonding time ranged from 2 to 17 min to provide different samples for studying the interfacial IMCs growth. The cold-wind gun was used to cool down samples rapidly at the cooling stage to avoid the continuous solid-liquid reactions, which can provide a more precise relationship between IMCs growth and bonding time under this stable TG. For comparison, the micro solder-joints by conventional TLP bonding process were also prepared using the same samples at a uniform temperature of 265 C for designed durations. The fabricated micro-joint samples were mounted in an epoxy, manually ground and then polished to characterize the crosssectional microstructure by scanning electron microscope (SEM). Energy dispersive spectroscope (EDS) and Micro-region X-Ray diffraction (Micro-XRD) were employed to identify the IMCs phases. The existence of residual Sn in the micro-joint was examined using differential scanning calorimeter (DSC) at the maximum temperature of 300 C for heating rate of 5 C/min. Additionally, the tensile testing was also carried out to examine the joints tensile strength at a tensile speed of 0.05 mm/min. The thicknesses of the interfacial IMCs at the both ends were measured, and then analyzed by the Image-J software.
2. Experimental and simulation process 2.1. Simulation process A two-step method was carried out to lessen the solder squeezing phenomenon and maintain the micro-joints stand-off height during the TG-TLP bonding. Firstly, the Ni/Sn/Ni interconnections were prepared using pure Ni (99.99%) wire and pure Sn (99.99%) foil by a solder reflow process at the maximum temperature of 250 C for 20 s above the melting-point temperature. A specially designed fixture was utilized to control the interlayer Sn thickness as 10 mm between two Ni wires. The formed interconnection structure was shown in Fig. 1. The resultant IMC thickness was measured as approximately 0.4 mm which can be nearly negligible for the subsequent TG-TLP bonding. Fig. 2 shows schematic diagrams of experimental configuration under the TG-TLP bonding. The top and bottom heating blocks were fixed by Ni sheets at their both sides, and two ends of the fabricated interconnection sample were respectively fixed in the central slot-lines of these two blocks by high temperature adhesive, which avoided the melting Sn undergoing any force to remain the joint’s stand-off height, after the adhesive was cured, another fixed blocks with central slot-line were pressed on the sample. After bonding, the sample was removed from the blocks by dissolving the adhesive. A temperature gradient (TG) was introduced by holding 330 C and
Owing to the small joints size, it is difficult to measure the TG across the Sn interlayer during bonding process. Therefore, finite element simulation with computational fluid dynamics (CFD) was employed to obtain the temperature gradient. The temperatures loaded at both the ends keep the same as the stable values (330 C and 200 C) measured by the thermocouples during the bonding period. The thermal conductivities of Ni and liquid Sn solder were 90.7W/(m K) and 30.7W/(m K) respectively [26]. With respect to boundary conditions, the ambient temperature and gravity vector were set as 20 C and 9.81 m/s2 respectively. The boussinesq approximation and zero equation of turbulence were utilized to simulate the natural convection. The hot end represented the Ni/Sn interface adjacent to the top heating block while the cold end represented the Ni/Sn interface near the bottom heating block. Fig. 3 shows the temperature distribution in the Ni/Sn(10 mm)/Ni interconnection during the TG-TLP bonding. The temperature difference between the two ends is 4.6 C, and accordingly the temperature gradient was computed to be 4600 C/mm. For more precise comparison, the temperature of 265 C at the hot end was adopted as the bonding temperature of the conventional TLP process. 3. Results and discussion 3.1. The growth characteristic of Ni3Sn4 intermetallic under TG-TLP bonding
Fig. 1. Cross-sectional structure of a fabricated Ni/Sn/Ni interconnection.
Fig. 4 (a)-(d) show cross-sectional back-scattered electron (BSE) images of the Ni/Sn/Ni interconnections for TG-TLP bonding at 2, 6, 11 and 17 min. As observed in Fig. 4 (a), only one type of IMCs appeared at the both end interfaces, showing an asymmetry growth phenomenon with thicker IMCs at the cold end and thinner IMCs at the hot end. The IMC morphologies represented rod-like and block-like shape at the coldend, and rod-like shape at the hotend, respectively. This morphological discrepancy at the both ends should be associated with the interfacial reactions induced by the TG-TLP process. It was also observed that the block-shaped IMC
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Fig. 2. Schematic diagrams of the (a) TG-TLP bonding experimental configuration and (b) Ni/Sn/Ni interconnection structure.
Fig. 3. Temperature distribution in the Ni/Sn(10 mm)/Ni interconnection of (a) 1/4 global structure and (b) Local Sn solder joint during the TG-TLP bonding.
Fig. 4. Cross-sectional BSE images of the Ni/Sn(10 mm)/Ni interconnection structure after the TG-TLP bonding for (a) 2, (b) 6, (c) 11, and (d) 17 min.
existed in the solder matrix adjacent to the cold end. By the EDX result from Fig. 5 and NieSn binary phase diagram [27], the IMC was identified as Ni3Sn4 intermetallic. After 5 min TG-TLP bonding,
the asymmetric growth appeared greatly more pronounced as presented in Fig. 4 (b), and even the large block-shaped Ni3Sn4 growing from the cold end touched the hot end after 11 min as
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Fig. 5. EDX analysis of the fabricated micro-joint by the TG-TLP bonding process at (a) the marked red frame in Fig. 4 (a), and (b) the marked red line in Fig. 4 (d). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
observed in Fig. 4 (c). This remarkable asymmetry growth phenomenon with thicker IMCs at the cold end and thinner IMCs at the hot end is strongly associated with the directional diffusion of the interfacial atoms driven by the TG,which would be discussed in detail in the next section. It was worth noting that the block-shaped Ni3Sn4 intermetallic grows more significantly, and become predominant at the cold end interface as bonding time increased. This can be explained by the Ostwald Ripening theory defined as the growth of larger crystals at the expense of smaller ones by reducing total interfacial energy. As the bonding time increased to 17 min, as depicted in Fig. 4 (d), there is no Sn solder observed in the microjoint, meaning the Sn solder was completely transformed into the IMCs. The DSC result was further verified no Sn remained in the micro-joint as shown in Fig. 6. Furthermore, the resulting microjoint was also determined to contain only Ni3Sn4 by EDX results in Fig. 5 (b) and XRD results in Fig. 7. It can be anticipated that Ni3Sn4 micro-joints with high melting temperature of 794.5 C may be robust to withstand the subsequent chip-stacking without collapsed joint during the vertical interconnection process of 3D TSV chips. Fig. 8 (a) and (b) show cross-sectional BSE images of the Ni/Sn/ Ni interconnections after a conventional TLP bonding for a homogeneous temperature of 265 C at 17 and 48 min. As is well known, the IMCs growth in the solder joint is considerably sensitive to the temperature. Hence, for more precise comparison with the TG-TLP bonding, the conventional TLP bonding temperature maintained the same temperature as the hot end temperature of the TG-TLP bonding evaluated from the simulation. As shown in Fig. 8 (a), the Ni3Sn4 intermetallic displayed identical thickness and morphologies of long-needle and large-block at both ends after 17 min TLP bonding. Furthermore, numerous Sn solder was found to still
Fig. 6. DSC analysis of the fabricated micro-joint at 17 min by the TG-TLP and TLP bonding methods.
exist in the Ni/Sn/Ni joint. The DSC result can further verify substantial Sn solder left in the micro-joint as shown in Fig. 6. As the bonding time extended to 48 min, the Sn solder disappeared as presented in Fig. 8 (b), indicating that all the solder was converted into the Ni3Sn4 intermetallic. Clearly, the formation rate of the fully Ni3Sn4 composed micro-joint is approximately 3 times faster in the TG-TLP bonding in relative to the conventional TLP bonding. Consequently, the introduced TG dominates the fast and asymmetry growth of the Ni3Sn4 intermetallic in the Ni/Sn/Ni interconnection. 3.2. Growth kinetics analysis of interfacial Ni3Sn4 under TG-TLP bonding The thermo-migration can be described that both the heat and mass transfer occur simultaneously in a mixture due to the imposed temperature gradient across the mixture. The Ni flux JTM subjected to a temperature gradient vT=vx can be expressed as:
JTM ¼ C
D Q* vT kT T vx
Where D is the diffusion coefficient in a liquid solder, k is the Boltzmann’s constant, T is the absolute temperature, Q * is the transport heat, C is the concentration. Clearly, the fast formation of IMCs joints is essentially attributed to continuous Ni thermomigration from the hot end to the cold end driven by the TG. The Ni flux JTM is directly proportional to the temperature gradient vT=vx, suggesting that the formation rate is dependent on the TG magnitude. Additionally, it was well documented that the dissolution and growth of interfacial IMCs occur simultaneously during
Fig. 7. XRD analysis of the fabricated micro-joint at 17 min for the TG-TLP bonding.
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Fig. 8. Cross-sectional BSE images of the Ni/Sn/Ni interconnects using a conventional TLP bonding at (a) 17 and (b) 48 min.
soldering [28].The dissolution rate can be written by:
dC S ¼ Kd ðCs CÞ dt V Where Kd is a constant, S is the surface area of interfacial IMCs, V is the solder volume, and Cs is the Cu solubility in the solder. Obviously, the higher CseC tends to cause the faster dissolution of interfacial IMCs. Fig. 9 presents schematic diagrams of Ni atomic fluxes in the Ni/Sn/Ni interconnection during the TG-TLP bonding. At the cold end, JTM presented an opposite direction with Ni chemical potential flux (JChem ), i.e., grain boundary diffusion flux JGB , as explained in Fig. 9. As a result, Ni atoms in the liquid solder around the cold end always remained saturated or near-saturated, giving rise to a low dissolution rate because of lower CseC. Therefore, the growth dominates the interfacial Ni3Sn4, and makes them thicken fast at the cold end. Actually, both the thickened Ni3Sn4 layer and lower dissolution rate induced by the smaller CseC could effectively inhibit the Ni atoms diffusion from the cold end resulting in negligible JChem. Therefore, one concluded that JTM dominates the interfacial Ni3Sn4 growth at the cold end. Fig. 10 displays the Ni3Sn4 thickness as a function of bonding times. The Ni3Sn4 layer was increased from 3 mm up to 10.5 mm for 2 and 11 min respectively at the cold end. The growth rate of interfacial Ni3Sn4 was 0.92 mm/min and 0.33 mm/min using the TG-TLP bonding and traditional TLP bonding respectively. In contrast, the same direction of Jchem and JTM at the hot end can keep Cu atoms always unsaturated in the liquid Sn adjacent to the hot end, which produces a larger CseC value. At this end, Ni atoms may rapidly diffuse across the thin Ni3Sn4 layer by grain boundary diffusion at the initial reaction stage, resulting in a large Jchem. It is reasonable to suppose that most of Jchem need to provide JTM, and thereby leaving a minor of Ni atoms to enable the Ni3Sn4 growth. As bonding time prolonged, the thickened Ni3Sn4 layer would gradually hinder the Ni diffusion until Jchem ¼ JTM, inducing a dynamic
Fig. 10. Interfacial Ni3Sn4 thickness as a function of bonding time using both TG-TLP and conventional TLP bonding methods.
equilibrium presence of the solution and growth. This can produce a critical layer with a stable thickness. When the Ni3Sn4 layer exceeds this critical thickness, the dissolution becomes dominant and the Ni3Sn4 layer would become thinner because of Jchemþ Jdis ¼ JTM. Obviously, such a critical thickness is strongly dependent on the dynamic equilibrium between the chemical potential gradient and the TG. As shown in Fig. 10, the Ni3Sn4 layer thickened slowly before 2 min, and maintained nearly unchanged from 2 to 6 min at the hot end. These experimental results were well agreed with the theory explained above. Nevertheless, the thickness showed a slow rise after 6 min. This may be a consequence of the lowered TG caused by significantly thinned Sn interlayer because of the consumption of substantial Sn in interfacial reactions. 3.3. The properties evaluation of the fabricated Ni3Sn4 micro-joint In this section, the properties of the fabricated Ni3Sn4 micro-
Fig. 9. Schematic diagrams of Ni atomic fluxes in the Ni/Sn/Ni micro-joint during a TG-TLP bonding.
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Fig. 11. Fracture surface images of the fabricated micro-joints of (a) Sn micro solder -joint and (b) full Ni3Sn4 IMCs micro solder-joint after a tensile test.
joint were characterized. The tensile testing was carried out to estimate the joint mechanical property. The tensile strengths of the Ni3Sn4 and Sn micro-joints were measured as 180.8 MPa and 97.3 MPa, respectively. Apparently, the former possesses higher tensile strength. Fig. 11 shows fracture surfaces of these two joints after the tensile loading. As observed in Fig. 11(a), the fracture surface of Sn micro-joint is composed of deeply equiaxed-dimples, representing a typical ductile fracture occurred into the Sn solder matrix. This is because the Sn solder has higher volume fraction in the micro solder-joint after solder reflowing. However, only fractured Ni3Sn4 intermetallic were exposed on the fracture surface, indicating the fracture completely occurred inside them as shown in Fig. 11(b). Although the Ni3Sn4 intermetallic is more brittle than the Sn solder, the Ni3Sn4 micro-joint may display more reliable mechanical properties than the solder micro-joint owing to higher mechanical strength of the Ni3Sn4 intermetallic. A four-point probe was performed to examine the electrical resistivity of the Ni3Sn4 micro-joints. The average electrical resistivity was obtained as 25.9 mU/cm that is well acceptable for the 3D TSV interconnections. Apart from the mechanical and electronic properties, the thermal property of the Ni3Sn4 micro-joint was evaluated by calculating thermal resistance r. The calculation formula can be expressed as r ¼ d=k, where d is the measured thickness and k is the thermal conductivity. The thermal conductivities of solid-state Sn and Ni3Sn4 are known as 6.9 W/(m K) and 19.6 W/(m K) respectively [29]. The thermal resistance of the Ni3Sn4 micro-joint was calculated as 0.612 mm2KW1. In the case of the micro solder-joint, since the Sn layer is greatly thicker in relative to the Ni3Sn4 layer formed after solder reflow, the micro solder-joint can be assumed to only contain Sn solder. The corresponding thermal resistance was calculated as 1.74 mm2KW1, which is much higher than that of the Ni3Sn4 micro-joint. One primary limitation facing 3D ICs is known to be effective heat dissipation, particularly from the upper chips for greatly dense chips integration. It is anticipated that the fabricated Ni3Sn4 micro-joint can effectively reduce the vertical thermal resistance due to its lower thermal resistance. This can be is beneficial to enhance the heat dissipation of 3D ICs.
process. This faster IMCs asymmetric growth phenomenon was primarily due to the Ni directional thermal-migration driven by the TG across the Sn interlayer. The resultant Ni3Sn4 micro-joint exhibited desirable features for the near future interconnections of the 3D TSV packaging, i.e., high melting temperature, stable single-phase constitution, strong mechanical strength, and low thermal resistance. The fabricated Ni3Sn4 micro-joint can solve the solder joint collapse issue and thus obtaining higher reliability. Consequently, this TG-TLP process can provide a promising solution to stack 3D TSV chips.
4. Conclusion
References
A practical approach to fabricate intermetallic micro-joints, only consisting of Ni3Sn4 intermetallic, was suggested by soldering Ni/ Sn(10 mm)/Ni interconnection structure under a suitable temperature gradient. During this process, the interfacial IMCs grew asymmetrically with thicker Ni3Sn4 layer at the cold end and thinner Ni3Sn4 layer at the hot end in the Ni/Sn/Ni interconnection. Such a growth continuously intensified until fully composed Ni3Sn4 micro-joint rapidly formed. The corresponding formation rate is approximately 3 times faster than that using a conventional TLP
Funding This work was supported by the National Natural Science Foundation of China [grant numbers U1504507, U1704147], and Science and Technology Program of Henan Province [grant number 182102410048]. The authors also acknowledge the valuable supports from The Youth Backbone Teacher Training Program of Henan University of Technology. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Ye Tian: Conceptualization, Investigation, Resources, Writing original draft, Writing - review & editing, Project administration, Funding acquisition. Heng Fang: Methodology, Software, Formal analysis, Writing - original draft, Visualization. Ning Ren: Methodology, Software, Validation, Formal analysis. Yatao Zhao: Methodology, Validation, Data curation. Boli Chen: Methodology, Validation, Data curation. Fengshun Wu: Supervision, Writing review & editing. Kyung-Wook Paik: Supervision, Writing - review & editing.
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