Microstructure Evolution Mechanism Of Cu-filled Through Silicon Via Under Thermal And Electrical Environments

TSV technology is one of the most promising technologies to keep Moore's law rolling in microelectronics and semiconductor industries.As an indispensable part in three-dimensional integrated circuit packaging,TSVs could improve the performance of the electronic devices by reducing the device volume and decreasing the response time.Thus,as the development of TSV,the reliability of this type of interconnect is attracting more and more attention.TSV is a typical multiple interfaces structure including filling material,barrier layer,surrounding SiO₂ dielectric layer and Si substrate.For Cu-filled TSV,the CTE mismatch is large between consistent materials,thus free expansion and shrinkage of Cu will be restrained by surrounding materials during temperature changes.The induced thermal stresses can lead to TSV-Cu protrusion and void foramtion in TSV structure.Cu protrusion can further impose forces on the structure connected to TSV,resulting in their distortion and fracture.Void formation at Cu/barrier-layer interfaces and at Cu grain boundaries may degrade device performance,even result in open failure of TSVs.Therefore it is essential to comprehensively study on the Cu protrusion mechanism and voiding mechanism during temperature changes in TSV structure.This thesis also tries to investigate the electromigration in TSV–metal line structure,and study the hillocks growth and void formation mechanism during electromigration.The main contents and conclusions are shown as follows:1.The TSV-Cu pumping mechanism was studied by analyzing the microstructure evolution of TSV under different thermal loads.The microstructure evolution of TSV structure under different thermal loading conditions was observed by peak temperatures of 250°C,350°C and 450°C,and the thermal loading forms of entering furnace at peak temperature and heating with furnace.The pumping height of TSV-Cu,and the residual stress in TSV-Si after thermal load were measured.The results showed that the pumping of TSV-Cu coule be divided into two types:global pumping and local pumping,and the global or local pumping height of TSV-Cu increased with the increase of thermal stress.In addition,the growth model of TSV-Cu surface grains under thermal stress based on grain boundary sliding and Cobel creep theory was established.It was found that under certain thermal load conditions,only surface grains with certain grain boundary property and grain morphology could exhibit obvious local pumping phenomenon.The Cu pumping behavior was related to thermal stress,stress relaxation,grain boundary property and grain morphology of surface grains.TSV-Cu was subjected to the thermal axial compressive stress caused by the thermal expansion coefficient mismatch between materials during the heating process.Under this compressive stress,the elastic deformation would occur in TSV-Cu.During the stress relaxation process at high temperature,part of the elastic deformation would change into plastic deformation.Surface grains with certain grain boundary property and grain morphology would occur grain boundary sliding and further deformation,which could eventually result in local pumping of TSV-Cu.Otherwise,TSV-Cu global pumping would happen in TSV after cooling.2.By analyzing the microstructure of interfaces around void location and the stress distribution in void region,combining with the stress-induced void theory,the void formation mechanism of TSV under thermal loads was studied.Though the finite element analysis of the void regions at twin boundaries,grain boundary junctions and phase boundaries,it was found that there were stress concentrations at the 90 degree corner between?111?and?112?twin boundaries in TSV-Cu,at the junctions of Cu grain boundary and the junctions of Cu grain boundaries and Cu/Ti phase boundaries.According to stress-induced void theory,vacancies would migrate along the tensile stress gradient to the stress concentration position and gathered at the stress concentration position.Moreover,larger stress concentration and higher grain boundary energy would reduce the nucleation barrier and increase the void formation probability at these locations.Stress concentration and smaller nucleation barrier resulted in voids generated at 90 degree corner between?111?and?112?twin boundaries,grain boundary junctions and the junctions of Cu grain boundaries and Cu/Ti phase boundaries.3.By ex-situ observation of TSV–metal line structure,the mechanism of TSV electromigration was studied,and the formation of electromigration hillocks and voids in TSV–metal line structure were analyzed.The microstructure evolution of TSV–metal line structure was observed after 10 days of electromigration experiment with current of 0.4 A.It was found that the electromigration in TSV structure was influenced by TiN barrier layer.On one side,the Cu atoms migrated to the barrier layer under electronic wind force,but the further migration was blocked by the barrier layer,which resulted in the accumulation of Cu atoms and the formation of hillocks.On the other side,Cu atoms migrated along the electron flow to other places,however,the atomic depletion could not be supplemented due to the existence of barrier layer,resulting in the generation of voids.By establishing the growth model of surface grains under electric stress,it was found that the extrusion of surface grains was related to temperature,electric stress,grain boundary properties and grain morphology of surface grains.With the increase of temperature,the increase of electric stress,the decrease of surface grain size and the decrease of the angle between grain boundary and surface,the growth rate of surface grain would increase.