BME-Assisted Fabrication of Porous Copper Films with Adjustable Porosity and Ligament Width for Microelectronic Interconnects

Monolithic porous Cu films are synthesized through the use of bicontinuous microemulsion (BME) to serve as a flexible template. The BME system is made up of a series of interconnected linked water and hydrocarbon phases. This one-of-a-kind structure enables precision Cu electrodeposition only in the aqueous phase, avoiding undesirable electrodeposition in the oil phase. We can fine-tune the porosity and ligament width of the resultant copper film by altering the oil phase proportion in the soft template. This Sn-plated porous Cu is capable of both low-temperature reflow and high-temperature service. The flexible template approach shows great potential for creating porous Cu with changeable porosity, allowing for more control over compositions. This approach has potential applications in advanced packaging technologies.


Introduction
Moore's Law has long been the driving force behind advancements in semiconductor technology [1].Nevertheless, with Si-based electronics nearing their physical limitations, the emergence of thirdgeneration semiconductor materials like GaN and SiC presents new avenues for technological progress.These materials offer superior performance characteristics, including enhanced efficiency, reduced size, quicker switching rates, and greater breakdown voltage [2].To harness the endless possibilities of such third-generation electronics, there is a pressing need for advanced packaging materials capable of withstanding demanding operational conditions, including elevated temperatures resulting from increased power density and component miniaturization.Transient Liquid Phase (TLP) bonding is one potential approach for creating reliable interconnections [3].The introduction of a lower melting point metal interlayer allows bonding via diffusion processes and the creation of intermetallic compounds (IMCs) with higher melting points.Sn and Cu are the preferred materials for microelectronics bonding due to their exceptional electrical and thermal conductivity as well as cost-effectiveness [4].Moreover, the integration of porous structures to boost the reaction surface accelerates the bonding process, which makes it a desirable bonding approach.
The fabrication of metallic porous structures may be accomplished in two ways: de-alloying [5] and templating [6].Chemical, electrochemical, and vapor phase de-alloying processes preferentially remove higher-reactivity metals in alloys, which creates a porous structure.Nevertheless, when subjected to mechanical stress, de-alloyed porous metals frequently display weakness and catastrophic failure, limiting their practical value.In contrast, there are two types of template methods: hard and soft.While hard templates like AAO, PS, and SiO2 provide structured frameworks for porous materials, their complete removal poses significant challenges [7].The soft template approach appears to be a potential answer to these problems.Soft templates generated by surfactant component accumulation give birth to a variety of structured polymers made of amphiphilic particles.However those methods often produce mesoporous materials, and macropores are required for bonding in semiconductor technology [8].As a result, the creation of a unique soft template technique with bigger dimensions offers significant potential for broadening the use of porous structures.
In this study, we introduce an innovative method for porous copper films using bicontinuous microemulsion (BME) to be the soft template for electroplating deposition.Additionally, we investigate how variations in the composition of BME influence the structure and appearance of the porous copper films.By modifying the ratio of oil phases in the BME, customizable porosity and ligament width in the porous copper films can be achieved.It opens up exciting possibilities for precise control of microstructure.Particularly, this unique technique aligns well with the wafer production process, allowing for low-temperature bonding and high-temperature service of the solder joints, which addresses the multifaceted challenges associated with electrical and thermal properties in die-attached packaging.

Preparation of BME
An aqueous phase was prepared by combining CuSO4, Sodium Dodecyl Sulfate (SDS), deionized water, H2SO4, and HCl.Simultaneously, an organic phase was created by mixing 2m2b with cyclohexane.The BME was generated by combining 0.10 mol of CuSO4, 0.06-0.10mol of surfactant, 200 mL of distilled water, 0.06 mol of H2SO4, 0.02 mol of HCl, and 200 mL of organic phase.The oil phase was progressively combined with the aqueous phase to create an emulsion.This method enhanced the emulsion's stability and homogeneity, resulting in a Winsor III emulsion with three distinct phases: an upper oil phase, the center BME phase, and an inferior aqueous solution.The resulting BME served to be a flexible template for the electroposition of porous copper films.

Electrodeposition of Monolithic Cu Film
The intermediate phase BME template was carefully separated from the glass beaker.Electrodeposition of the porous copper films was carried out on Cu plates without agitation.It occurred during 4 hours of ambient stability, with the BME solution functioning as the electrolyte.The cathode substrates consisted of Cu plates measuring 10 mm x 10 mm x 1 mm, while a Pt electrode of the same size was employed as the anode.

Sn Plating and Soldering Process
A confirmed Sn plating process was used to fill porosity in the porous copper films.The MK-504L flux was applied using a high-precision horizontal touchscreen printing, which aided the soldering process by eliminating oxides and improving wetting between the Sn and Cu layers of material.In a reflow oven, Sn plating-porous copper films were heated to 170 °C for 3 min and kept at 250 °C for 10 min under 1 MPa before cooling to room temperature in ambient air.

Characterization
The features of the porous copper films were examined using a field emission-scanning electron microscope (FE-SEM, JSM-7001FA, JEOL, Japan) equipped with an energy-dispersive X-ray spectrometer (XM4).The cross-sectional morphology of the solder junctions was evaluated through mechanical grinding using SiC sheets with grit sizes ranging from 80 to 4000, followed by polishing using a 0.25 μm diamond solution.SEM was employed for the cross-sectional examination of the polished samples.

Effect of Current Density on Porous Cu Morphology
The investigation into the impact of electrical density on the structure of porous Cu revealed intriguing results.When we reduced the electrical density to 1 A/dm 2 , even with extended electrodeposition periods, it yielded a smooth Cu surface without any projecting Cu arrays.This phenomenon can be ascribed to the dynamic flow characteristics of the flexible template, which allow for continual moving and rearranging of the BME phases on the substrate's surface Consequently, the aqueous and oil phase zones remained in perpetual motion during the electrodeposition process.At lower electrical densities, this dynamic flow hindered the deposition of the desired bumps before the flexible template could flow, resulting in a flat Cu layer with no protrusions.However, upon increasing the electrical density to 2 A/dm 2 (Fig. 2b), a noteworthy transformation occurred.The Cu arrays obtained exhibited uniform sizes, primarily falling within the 20 μm range.This current density appeared to strike the right balance for achieving the desired morphology.Conversely, further elevating the electrical density to 3 A/dm 2 led to the formation of numerous Cu dendrites.Unfortunately, these randomly formed Cu dendrites lacked the requisite solidity and failed to align with the objectives of our experiment.Therefore, 2 A/dm 2 was finally selected as the ideal current density for our electrodeposition process

Influence of BME Phase Composition on Porous Cu Morphology
We extended our investigation to assess the impact of varying the composition of the BME phase (BME-1, BME-2, and BME-3) on the morphology of the deposited porous Cu.In the BME-1 phase (oil-rich), the Cu arrays exhibited homogeneous sizes.The average distance among separate Cu arrays was around 1 µm.Enhancing the fraction of the aqueous phase in the BME phase in the BME-2 phase (Fig. 3b) resulted in significant increases in the dimensions and densities of the Cu arrays.Consequently, the spacing between Cu arrays was greatly decreased.Notably, in addition to the submicron-sized Cu arrays, we observed the formation of pagoda-shaped Cu arrays.These Cu arrays featured radial diameters ranging from 1 to 3 µm.Further increasing the water phase content in the BME-3 phase (water-rich) resulted in a substantial enlargement of the radial size of the deposited porous copper, with measurements ranging from 10 to 30 µm.In conclusion, increasing the amount of the aqueous phase in the flexible template may significantly enhance the density and ligament width of porous Cu.

TLP Bonding and Solder Joint Structures
As depicted in Fig. 4d, after soldering on a normal Cu using the same procedure, the result is the formation of scallop-like Cu6Sn5 structures, along with an extremely thin layer of Cu3Sn at both ends of the bondline.Notably, a significant amount of Sn remains within the solder joint.Subsequently, we performed TLP bonding using the three types of copper foams sequentially, and the cross-sectional images of their bondline are presented in Fig. 4(a-c), respectively.Notably, there was no discernible Sn residue within the cross-sections after refluxing, confirming that the copper foam structure obtained through BME electrodeposition indeed accelerates the Cu-Sn reaction rate.However, it is worth highlighting that the structures of the solder joints produced with the three types of foam Cu exhibited significant differences.In the case of foam Cu-1, characterized by substantial porosity, Cu6Sn5 was generated within the solder joints of samples prepared using the BME-1 phase.These joints featured only a thin coating of Cu3Sn at the solder joint ends.Conversely, the presence of Cu arrays in samples generated using the BME-2 phase resulted in a higher ratio of Cu within the bondline.This led to the formation of a more substantial amount of Cu3Sn.Nevertheless, no residual Cu was observed as it was transformed into IMCs.Upon reaching foam Cu-3, a remarkable transformation became evident.At this point, a Cu skeleton traversed the entire solder joint, enclosed by a layer of Cu3Sn and Cu6Sn5 on its outer surface.Finally, we were able to control the porosity of porous copper films by altering the W/O ratio during the BME flexible template.Consequently, it allows modulation of the Cu, Cu6Sn5, and Cu3Sn ratios in solder joints, giving a realistic strategy for improving solder joint qualities for specific applications.

Conclusion
The adoption of the bicontinuous microemulsion flexible template might be used to create porous copper films with customizable porosity and ligament width.The flexible nature of the bicontinuous microemulsion flexible template guarantees that the step of electrodeposition takes place entirely inside the template's aqueous domain, eliminating unwanted electrodeposition involving the non-conductive region (oil phase) and resulting in regulated and homogeneous development of porous copper films.The technology is now feasible to manufacture variable pores in porous copper films by varying the waterto-oil ratio in the flexible template.Furthermore, the composition of the bondlines can be changed accordingly.This technology brings out novel opportunities for the evolution of microelectronics, providing a cost-effective and dependable alternative to satisfy the needs of advanced packaging technologies.

Fig. 2
Fig. 2 SEM images of the top surface of porous Cu deposited in BME at different current densities for 10 min; (a)1 A/dm 2 ; (b)2 A/dm 2 ; (c)3 A/dm 2 .