Study On Mechanical Properties Of Cu-TSV Film Based On Uniaxial Micro-tensile Test

With the miniaturization trend of microelectronic products, traditional chip packaging cannot meet the need of higher packaging density and lower power consuming, which gives birth to the concept of 3D stacking of chips. It is realized by use of TSV(through silicon via) technology. Copper, because of its high conductivity, matured fabrication process and high resistance to the electron migration, is chose to be the interconnection material in TSVs.However, the mechanical properties of micro-scale Cu are not only impacted by the size effect, but also by the fabrication process. On one hand, the size effect results in different mechanical performances of micro-scale materials compared to bulk ones, which means that it is not accurate if we still use the mechanical properties of bulk Cu to simulation and analyze the properties of micro scale Cu. On the other hand, the fabrication process also influences the mechanical properties of micro-scale Cu. For example, the fabrication method, additive types and electroplating process may have some effect on the microstructure and mechanical properties of micro-scale Cu.Therefore, it is necessary to characterize the mechanical properties of micro-scale copper in TSVs under various electroplating conditions. It will provide the mechanical data of Cu-TSV for guiding the structure design, simulation and analyze of TSV, which will be of significant meaning to the development of TSV technology.This thesis opens up detailed investigations of the mechanical properties of micro-tensile Cu-TSV film, including the design and fabrication of Cu-TSV film specimen, establishment of data collection & analysis system, the mechanical properties and micro structures of Cu-TSV electrodeposited in a methyl sulfonic acid copper plating bath with varying proportion of additives.Firstly, based on the uniaxial micro-tensile theory and the previous researches in our lab, this paper presents a novel micro-tensile Cu-TSV specimen with a reticular supporting frame. By FEA(finite element analysis) method, we conclude as follows: 1) Compared with solid supporting frame, the reticular one can not only protect Cu-TSV film from damaged during processing, but also effectively decline the deformation due to internal stress. 2) The stress concentrates in the area of cross connection of Cu-TSV film and supporting frame and is decreased by optimizing the arc radian on the tail end of Cu-TSV film. The optimized size of Cu-TSV film is a 200μm in length, a 50μm in width with an arc radius of 10 μm.Secondly, by use of MEMS microprocessing techniques based on sacrificial layer technology, a fabrication procedure of well-distributed specimen with low internal stress is developed in this study. This procedure is proved to have advantages of short processing cycle, easy operation and less damage to specimen. What’s more, high accuracy wire cutting technology is applied to fabricate the specialized grip equipment of micro-tensile system.Thirdly, we established an unaxial micro-tensile test platform, which consists of pressure sensor, step motor, image acquisition system and 3dimensional adjustable platform. The system measures real-time force and displacement of the specimen exactly. The minimum displacement is 20 nm. The tensile speed varies from 0.2μm/s to70μm/s. In order to verify the reliability of the data measured in our micro-tensile test platform, we have a contrasted experiment with commercial DMA system. It was found that the results from self-developed platform coincides with DMA tested ones in mechanical properties like yield strength and ultimate tensile strength.Lastly, the mechanical properties of micro-tensile Cu-TSV specimen deposited in methyl sulfonic acid copper plating bath with varying proportion of different additives are tested and analyzed. The surface morphologies and microstructures are characterized by SEM and XRD methods. The results are as follows: according to mechanical properties, compared to electroplating solution with standard composition, the ultimate tensile strength and yield strength of Cu-TSV specimen with varying proportion of additives are declined, while the Young’s modulus is increased. The ultimate tensile strength is mainly affected by suppressor which reaches a minimum value 330.9MPa. The yield strength is mainly impacted by leveler which reached a minimum value of 321.0MPa, respectively. With respect to texture of Cu-TSV, the preferred orientation on(220) is weakened, while(111) and(200) is increased. In addition, the outside surface and fracture surface microstructure of Cu-TSV specimen with varying proportion of additives are the same on the existence of shallow dimples and necking phenomenon. The fracture mechanism is inferred to be cleavage fracture.