Solid-Liquid Interdiffusion Bonding Research Articles

Microstructural and mechanical characterization of Cu/Sn SLID bonding utilizing Co as contact metallization layer

Most micro-electro-mechanical systems (MEMS) devices contain fragile moving parts, which poses challenges in process integration of interconnection methods requiring wet-chemistry, such as solid-liquid interdiffusion bonding (SLID). These sensitive MEMS structures can be protected from either the wet-chemistry or plated metals during chemical/electro-chemical plating of SLID interconnection materials; however, this is a complex process. Hence, our previous research has investigated employing a physically deposited contact metallization on the wafers containing functional devices instead of chemically deposited layers (such as electrochemical Cu). Co is a plausible contact metallization layer for Cu-Sn SLID bonding, as it is chemically compatible with Cu–Sn systems. Furthermore, it can positively impact the mechanical reliability of the intermetallic compounds (IMCs) due to the stabilizing of the HT-hexagonal Cu6Sn5 phase down to room temperature and suppressing the Cu3Sn phase formation and subsequent void formation. However, it is critical to control Co thickness to achieve a stable bond based on our previous research on Co bulk in contact with Cu-Sn electroplated silicon chips. To utilize Co as a contact metallization layer for wafer-level Cu-Sn SLID bonding, it is necessary to define appropriate metal layers in the contact metallization stack. Consequently, the present study investigated four different contact metallization stacks including A) 40nmTi/100 nm Co, B) 40 nm Ti/200 nm Mo/100 nm Co, C) 40 nm Ti/500 nm Co, and D) 40 nm Ti/200 nm Mo/500 nm Co. More specifically, we evaluated the microstructural formation and evolution and mechanical performance of the joints. Our study revealed that the Ti/Mo/100 nm Co contact metallization stack for (4 µm)Cu/(2 µm)Sn SLID bonding is composed of IMCs (Cu,2.5at%Co)6Sn5 and Cu3Sn without remaining Sn. Moreover, the joint contained a negligible number of voids even after long-time annealing at 150 °C. Our analysis of the mechanical properties of the joint showed that 1) the tensile fracture surface exhibited a mixture of ductile and brittle fractures, and 2) the Young’s modulus of (Cu,2.5at%Co)6Sn5 was higher than Cu6Sn5, while hardness of (Cu,2.5at%Co)6Sn5 and Cu6Sn5 were comparable. By employing a Ti/Mo/100 nm Co contact metallization stack, the current study was able to produce 25 µm and 10 µm void free (4 µm Cu/2 µm Sn) microbumps.

Solid-Liquid Interdiffusion Bonding between Ti/Ni/Ag/Sn Backside Metallized Si Chips and Cu/Al2O3 - DBC Substrates with Au/Pd/Ni Surface Finish

Solid-Liquid Interdiffusion Bonding between Ti/Ni/Ag/Sn Backside Metallized Si Chips and Cu/Al₂O₃ - DBC Substrates with Au/Pd/Ni Surface Finish

Low Temperature Cu/Ga Solid–Liquid Inter-Diffusion Bonding Used for Interfacial Heat Transfer in High-Power Devices

Interfacial heat transfer is essential for the development of high-power devices with high heat flux. The metallurgical bonding of Cu substrates is successfully realized by using a self-made interlayer at 10 °C, without any flux, by Cu/Ga solid-liquid inter-diffusion bonding (SLID), which can be used for the joining of heat sinks and power devices. The microstructure and properties of the joints were investigated, and the mechanism of Cu/Ga SLID bonding was discussed. The results show that the average shear strength of the joints is 7.9 MPa, the heat-resistant temperature is 200 °C, and the thermal contact conductance is 83,541 W/(m2·K) with a holding time of 30 h at the bonding temperature of 100 °C. The fracture occurs on one side of the copper wire mesh which is caused by the residual gallium. The microstructure is mainly composed of uniform θ-CuGa2 phase, in addition to a small amount of residual copper, residual gallium and γ3-Cu9Ga4 phase. The interaction product of Cu and Ga is mainly θ-CuGa2 phase, with only a small amount of γ3-Cu9Ga4 phase occurring at the temperature of 100 °C for 20 h. The process of Cu/Ga SLID bonding can be divided into three stages as follows: the pressurization stage, the reaction diffusion stage and the isothermal solidification stage. This technology can meet our requirements of low temperature bonding, high reliability service and interfacial heat transfer enhancement.

Development of assembly techniques for connection of AlGaN/GaN/Si chips to DBC substrate

PurposeThis paper aims to investigate the sintering and solid liquid interdiffusion bonding (SLID) techniques to attach AlGaN/GaN-on-Si chips to direct bond copper (DBC) substrate. The influence of metal layers deposited on the backside of AlGaN/GaN-on-Si dies on the assembly process is also investigated.Design/methodology/approachThe authors assumed the value of the shear strength to be a basic parameter for evaluation of mechanical properties. Additionally, the surface condition after shearing was assessed by SEM photographs and the shear surface was studied by X-ray diffraction method. The SLID requires Sn-plated DBC substrate and can be carried out at temperature slightly higher than 250°C and pressure reduced to 4 MPa, while the sintering requires process temperature of 350°C and the pressure at least 7.5 MPa.FindingsAg-, Au-backside covered high electron mobility transistor (HEMT) chips can be assembled on Sn-plated DBC substrates by SLID technology. In case of sintering technology, Cu- or Ag-backside covered HEMT chips can be assembled on Ag- or Ni/Au-plated DBC substrates. The SLID process can be realized at lower temperature and decreased pressure than sintering process.Research limitations/implicationsFor SLID technology, the adhesion between Cu-backside covered HEMT die and DBC with Sn layer loses its operational properties after short-term ageing in air at temperature of 300°C.Originality/valueIn the SLID process, Sn-Cu and Sn-Ag intermetallic compounds and alloys are responsible for creation of the joint between Sn-plated DBC and micropowder Ag layer, while the sintered joint between the chip and Ag-based micropowder is formed in diffusion process.

Solid–Liquid Interdiffusion Bonding of Cu–Sn–Cu Interconnection and Sealing for High-Temperature Pressure Sensor Based on Graphene

Cu–Sn–Cu solid–liquid interdiffusion (SLID) bonding for high-temperature pressure sensors based on graphene has been investigated. Square bumps with a length of $160~\mu \text{m}$ and an 80- $\mu \text{m}$ -wide sealing ring were simultaneously fabricated by the evaporation process. The die surface was treated with Ar (5% H2) plasma before bonding. Chip-level bonding was performed at 260 °C for 15 min with a pressure of 9.13 MPa under N2 atmosphere. Pure Sn and Cu6Sn5 have been exhausted completely, and the as-bonded interface only contains three layers of Cu–Cu3Sn–Cu, with no serious Sn overflow. The average shear strength of 50.6 MPa and an excellent leak rate of around 2.64 $\times 10^{-4}$ Pa $\cdot $ cm3/s are achieved. The resistance measurements are similar to the theoretical estimation. After high-temperature storage (HTS) at 300 °C for 10 h, the average shear strength slightly decreases to 46.7 MPa and the leak rate increases to 1.01 $\times 10^{-3}$ Pa $\cdot $ cm3/s. More importantly, there is no degeneration of the graphene nanofilm during electrical measurement in the bonding process and HTS test. It is concluded that Cu–Sn–Cu SLID bonding is appropriate for high-temperature pressure sensors based on graphene.

Intermetallic Reactions during the Solid-Liquid Interdiffusion Bonding of Bi2Te2.55Se0.45 Thermoelectric Material with Cu Electrodes Using a Sn Interlayer

The intermetallic compounds formed during the diffusion soldering of a Bi2Te2.55Se0.45 thermoelectric material with a Cu electrode are investigated. For this bonding process, Bi2Te2.55Se0.45 was pre-coated with a 1 μm Sn thin film on the thermoelectric element and pre-heated at 250 °C for 3 min before being electroplated with a Ni barrier layer and a Ag reaction layer. The pre-treated thermoelectric element was bonded with a Ag-coated Cu electrode using a 4 μm Sn interlayer at temperatures between 250 and 325 °C. The results indicated that a multi-layer of Bi–Te–Se/Sn–Te–Se–Bi/Ni3Sn4 phases formed at the Bi2Te2.55Se0.45/Ni interface, ensuring sound cohesion between the Bi2Te2.55Se0.45 thermoelectric material and Ni barrier. The molten Sn interlayer reacted rapidly with both Ag reaction layers to form an Ag3Sn intermetallic layer until it was completely exhausted and the Ag/Sn/Ag sandwich transformed into a Ag/Ag3Sn/Ag joint. Satisfactory shear strengths ranging from 19.3 and 21.8 MPa were achieved in Bi2Te2.55Se0.45/Cu joints bonded at 250 to 300 °C for 5 to 30 min, dropping to values of about 11 MPa for 60 min, bonding at 275 and 300 °C. In addition, poor strengths of about 7 MPa resulted from bonding at a higher temperature of 325 °C for 5 to 60 min.

Effect of Zinc Addition on Void Formation in Solid-Liquid Interdiffusion Bonding of Copper

Effect of Zinc Addition on Void Formation in Solid-Liquid Interdiffusion Bonding of Copper

Thermo-electric finite element analysis and characteristic of thermoelectric generator with intermetallic compound

Energy recycling is an important research topic worldwide. Thermoelectric generators (TEGs) are used to recycle energy. TEGs are suitable for small and medium-sized applications and industrial waste heat harvesting in medium to high temperatures. Traditional lead-free soldering cannot be applied at temperatures above 200°C. The joint may melt and fail at exceedingly high temperatures. Therefore, determining the suitable bonding material to apply during TEG assembly is necessary. Ni/Sn/Ag was adopted as the joint material for TEG packaging in this study. The joints were bonded and transformed into a full intermetallic compound (IMC) through solid–liquid interdiffusion bonding (SLID) and solid–solid interdiffusion reaction. The IMC has a high melting point and can be utilized at temperatures above 200°C. The mechanical strength of the joints was examined through shear test at different thermal treatment times. Shear strength declined with the increase in thermal treatment time. The TEG was assembled with 12 pairs of thermoelectric (TE) pillars, and its performance was measured through the slope method. The TE characteristics do not significantly vary before and after thermal treatment. With the established finite element (FE) model, temperature and output power were estimated at specific temperature loading through thermo-electrical FE analysis.

Assessment of Fluxless Solid Liquid Interdiffusion Bonding by Compressive Force of Au-PbSn and Au-SAC for Flip Chip Packaging

Flip chip packaging faces two primary bonding-process obstacles: flux use and geometry mismatch between die and substrate pad pitch. These obstacles motivated the development of a fluxless bonding method called solid-liquid interdiffusion bonding by compressive force (SLICF). SLICF utilizes a mechanical force to form the bond through solid-liquid interdiffusion with a joint-in-via (JIV) architecture for flip chip packaging. SLICF bonding (also known as thermo-mechanical (TM) bonding) forms an instantaneous bond and eliminates the need for reflow infrastructure. Both Au-PbSn and Au-SAC interconnect systems were studied for the SLICF bonding on the JIV architecture at a 130- mum pitch. The morphologies of Au-PbSn and Au-SAC in solid-liquid interdiffusion were studied with their kinetics measured by the Au consumption rate. The SLIFC bonds for Au-PbSn and Au-SAC were compared and assessed by mechanical shear tests and thermomechanical stresses. Au with PbSn was found to perform marginally better due to its joint geometry and slower kinetics.

Solid–liquid interdiffusion bonding between In-coated silver thick films

Solid–liquid interdiffusion (SLID) bonding between two pieces of In-coated silver thick films was investigated. The silver thick film was prepared by screen-printing of silver paste on an aluminum oxide substrate. Indium was coated in thickness ranging from 3 to 12 μm onto the silver thick films using thermal deposition (in vacuum). Two pieces of the In-coated thick films were placed in intimate contact (with a compressive stress of 0.04 MPa) and heated (at 180–250 °C) for various durations (600–3600 s) to undergo SLID bonding. Measurement of the bonding strength indicated that the SLID bond in 3-μm-In-coated thick-film couples is stronger than that in 8-μm-In-coated couples. X-Ray diffraction, scanning electron microscopy and electron probe microanalysis were used to analyze the bond. The stronger bond comprises a central zone of γ-Ag 2In sandwiched by two Ag films; the weaker bond comprises a central zone of AgIn 2 sandwiched by two layers of γ-Ag 2In. In the latter case, diffusion of excess indium from the coat into the interface between the silver film and alumina substrates is responsible for bond weakening. The development of microstructures in the SLID process for both strong and weak bonds is proposed.