Low-temperature direct copper-to-copper bonding enabled by creep on highly (111)-oriented Cu surfaces

Liu, Chien Min; Lin, Han Wen; Chu, Yi Cheng; Chen, Chih; Lyu, Dian Rong; Chen, Kuan‐Neng; Tu, King‐Ning

doi:10.1016/j.scriptamat.2014.01.040

Cited by 95 publications

(47 citation statements)

References 22 publications

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“…Further, surface diffusion coefficient at the bonding interface plays vital role in enhancement of interdiffusion. Surface diffusion coefficient of <111> oriented Cu plane is 3 to 4 orders of higher magnitude compared to other oriented Cu plane at the bonding interface [12] . In order to evaluate the preferable Cu plane orientation at the surface of an optimized layer of passivated Cu surface was subjected to same thermal stress as that of bonding conditions and subsequent Glancing angle X-Ray Diffraction (GI-XRD) was performed.…”

Section: A Optimization Of Passivation Layer Thickenss Of Pd Layermentioning

confidence: 94%

Achieving of Intensified Conductive Interconnections for Flex-on-Flex by Using Metal Passivated Copper – Copper Thermocompression Bonding

Cheemalamarri

Panigrahy

Bonam

et al. 2018

2018 IEEE 68th Electronic Components and Technology Conference (ECTC)

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Abstract-There is a gradual increase in demand for flexible electronics due to the way it is going to empower the end userto bent, roll/fold and arrange randomly in 3-D space, the devices without sacrificing the performance and reliability of devices, in a trend focused towards ever shrinking device footprint area. One of the prime mover towards realizing flexible electronics is interconnect scaling, which is motivating us to move towards three-dimensional interconnect integration. In this paper, we propose the interconnection of two flexible-flexible substrates by Thermocompression bonding between copper-to-copper, with their surface passivated with palladium, at low temperature and low pressure, to overcome the limitations imposed by conventional anisotropic conductive film (ACF) and nonconductive paste (NCP) approaches. The enhancement in interdiffusion of atomic species between the two surfaces, at low temperature and pressure, is possible only with unoxidized surface which has low RMS roughness or with surface which has varied film density. We have systematically optimized the thickness of palladium passivation layer for high quality Cu-Cu bonding, at low temperature of 140 o C and at low pressure of 5 bar. 2.06 nm RMS surface roughness was observed for Palladium passivation layer when its thickness was optimized at 5 nm, which made bonded Cu-Cu junction interface free from any copper oxide, as it was confirmed by X-ray Diffraction (XRD) and Energy Dispersive Spectroscopy (EDS) analysis. To confirm the quality of bonded interface, the samples were subjected to stress by folding just after Thermocompression bonding. Absence of any voids between the interfaces during Scanning Acoustic Microscopy (SAM) imaging confirms the superior quality of Cu-Cu bonding. The bonded interface image from cross-sectional Scanning Electron Microcopy (X-SEM) further reaffirms the quality of interface. Besides, very low contact resistance of ~2 E-7 Ω -cm 2 obtained for a fabricated daisy chain pattern with 70 um pitch and 100 um × 100 um bonding contact, in addition, confirms the good quality of bonding interface. The demonstrated bonding approach with metal passivated interconnect technique will be one of the prime contestant for future high bandwidth applications.

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Section: A Optimization Of Passivation Layer Thickenss Of Pd Layermentioning

confidence: 94%

Achieving of Intensified Conductive Interconnections for Flex-on-Flex by Using Metal Passivated Copper – Copper Thermocompression Bonding

Cheemalamarri

Panigrahy

Bonam

et al. 2018

2018 IEEE 68th Electronic Components and Technology Conference (ECTC)

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“…12 The thickness of the solder in the microbump is reduced from more than 10 microns to a range between a few microns and 10 microns. The volume reduction is approximately two to three orders of magnitude less than that of a traditional fl ip-chip solder joint.…”

Section: In This Issuementioning

confidence: 99%

Materials challenges in three-dimensional integrated circuits

Chen

2015

MRS Bull.

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“…52 Figure 7 a presents a cross-sectional transmission electron microscopy (TEM) image of a bonded Cu fi lm after bonding at 150°C for 1 h. 52 The dark-fi eld TEM image shown in Figure 7b indicates that the bonding surfaces are all (111) oriented. An excellent interface can be obtained because the surface diffusion coeffi cient on the (111) surface of Cu is three to four orders of magnitude faster than on other surfaces.…”

Section: Vertical Interconnects By Cu-to-cu Direct Bondingmentioning

confidence: 99%

Vertical interconnects of microbumps in 3D integration

Chen

2015

MRS Bull.

Self Cite

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With two-dimensional large-scale integrated circuits (ICs) approaching the limit of Moore's Law, the most promising solution is three-dimensional (3D) ICs based on chip stacking. 1 -3 The critical architectural element in 3D ICs is the vertical interconnect developed using through-Si via (TSV) and Cu microbump techniques. Thousands of microbumps are typically present on a given TSV chip. Pb-free solder has been employed for the microbumps to join two chips vertically. Overall, the solder joint is a very mature form of technology widely used in fl ip-chip technology, 4 in which solder bumps are fabricated on Si dies, and then the Si dies are fl ipped over to join with polymer substrates to form vertical interconnects.Figure 1 a presents a cross-sectional scanning electron microscope image of a typical fl ip-chip joint with Sn2.5Ag solder. 5 The joint is 100 μ m in diameter, and the solder is approximately 70 μ m in height. As labeled in the fi gure, the under-bump metallization (UBM) on the chip side is 5 μ m Cu/3 μ m Ni. On the substrate side, the metallization is 5 μ m electroless Ni on Cu traces. The solder has reacted with the metallization on the chip and substrate sides to form Ni 3 Sn 4 intermetallic compounds of 1.0 μ m thickness, such that the joint can provide electrical and thermal conduction, as well as mechanical strength to hold the chip and the substrate. The solder volume is estimated to be 6 × 10 5 μ m 3 , which is much larger than that of the UBM.As the microelectronic industry shifts to 3D ICs, more inputs/outputs are needed. Therefore, microbumps of 20-μ m diameter are currently being adopted to be the vertical interconnects between chips. The microbumps have been successfully fabricated by refl ow or thermo-compression. 6 -14 Typically, refl ow is carried out in an oven at a temperature above the melting point of the solder for approximately one minute, whereas thermo-compression is achieved by a bonder with a compressive force above the melting point for a few seconds. Figure 1b shows the structure of a typical Sn2.5Ag microbump with 5 μ m Cu/3 μ m Ni UBM on both top and bottom chips. The solder height decreases to only 6.2 μ m. Nevertheless, the thickness of the UBM cannot be scaled down accordingly due to consideration of metallurgical reactions. If the UBM is too thin, intermetallics (IMCs) would detach from the UBM when the UBM is consumed. 15 Therefore, the UBM thickness of the microbump remains approximately the same as that of the fl ip-chip joint. It is noteworthy that the transistor has gradually scaled down in the past three decades. For example, the minimum feature length shrank from 90 nm to 65 nm nodes With the electronics packaging industry shifting increasingly to three-dimensional packaging, microbumps have been adopted as the vertical interconnects between chips. Consequently, solder volumes have decreased dramatically, and the solder thickness has reduced to a range between a few and 10 microns. The solder volume of a microbump is approximately two orders of magnitude smalle...

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Low-temperature direct copper-to-copper bonding enabled by creep on highly (111)-oriented Cu surfaces

Cited by 95 publications

References 22 publications

Achieving of Intensified Conductive Interconnections for Flex-on-Flex by Using Metal Passivated Copper – Copper Thermocompression Bonding

Achieving of Intensified Conductive Interconnections for Flex-on-Flex by Using Metal Passivated Copper – Copper Thermocompression Bonding

Materials challenges in three-dimensional integrated circuits

Vertical interconnects of microbumps in 3D integration

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