Numerical Analysis Of Hygroscopic And Thermal Issues For Through Silicon Vias Structure

With the increasing demand for the electronic products’multi-function, high speed,low-power consumption, et al, the most advanced and the most widely usedinterconnect technology, Through Silicon Vias （TSVs）, expose thermal problems, signaldelay and crosstalk problems more and more obviously. It was discovered that using thematerial with a low dielectric constant （k） to replace the traditional silica （SiO₂）dielectric layer in TSV structures may be effective in improving the signal delay andcrosstalk, and alleviating the problems caused by the heat. Meanwhile, due to thehygroscopicity of the low-k materials, the new problems caused by the moistureemerged.Firstly, SiO₂-based TSV structure’s and Hydrogen Silsesquioxane （HSQ）-basedTSV structure’s two dimensional finite element model were built in ABAQUS. Thethermal expansion stresses due to the local thermal expansion mismatch, thehygroscopic expansion stresses due to the local hygroscopic expansion mismatch, andthe thermal-hygroscopic stress due to the local thermal and hygroscopic expansionmismatch were analyzed. And both the two TSV structures were compared. The resultsshow that: the use of HSQ, which belongs to low-k material and instead of SiO₂, coulddecreases the thermal expansion stresses in TSV structure. Although the absorption ofmoisture will induce the hygroscopic expansion stresses, the maximumthermal-hygroscopic stress in the HSQ-based TSV structure is less than the maximumthermal expansion stress in SiO₂-based TSV structure, even when in the maximumhumidity environment. The hygroscopic stresses are mainly distributed in the silicon（Si）, instead of copper （Cu） interconnecting wire. Compared with SiO₂-based TSVstructure, HSQ-based TSV structure has better thermal stability and equivalent stressesin Cu are significantly improved. Therefore, reasonable designing of the processparameters of the HSQ and Si layer can lead to better prospects for application.Secondly, hygroscopic characters of HSQ were investigated by MD methods. Itwas very important for the reliability of interconnecting structure. Due to the higher cost,complex process and unpredictable errors in the experimental determination of the hygroscopic characters, HSQ hygroscopic parameters were discussed using MolecularDynamics （MD） ways. MD models under different humidity were established. Then theMD simulations were run under NPT. Moisture diffusion coefficients at differenttemperatures of HSQ were calculated after MD simulations. It is found that the moisturediffusion coefficients are obviously linearly related to temperature, and the linearrelationship was1/1×10^-12y=0.00881x+7.42328. The swelling strain was calculated byobtaining the lattice length as a function of moisture concentration and temperature. Theswelling strain rates，i.e. the coefficients of moisture expansion （CME） were calculatedaccording to the slope of the functions of swelling strain and temperature. CME of theHSQ is5.63×10^-3,7.78×10^-3and9.79×10^-3under25,225and345℃. The resultswere similar compared with that of close material.Additionally， MD method was used again. the glass transition temperature ofHSQ is calculated for309.6℃by obtaining the break point of the curve of the functionof volume and temperature. And tensile modulus, Poisson’s ratio, bulk modulus andshear modulus are investigated related to temperature （0-400）℃.