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Robert Cook, Principal Investigator

Deformation of Solidifying Coatings and Prediction of Structure-Property Relationships for Silsesquioxane Spin on Glasses

The mechanical reliability of thin films for use in integrated circuit devices is dependent on the stress field generated during the solidification process. Tensile stresses are generated athermally during curing and thermally due to coefficient of thermal expansion mismatch between the film and substrate. Athermal stress generation may occur at an elevated temperature that drives the reaction. Thermal stresses are dependent on the conditions during cooling and the resulting temperature gradient in the film. These tensile stresses can lead to cracking and film delamination, degrading the film properties. Elucidation of a curing cycle to minimize the tensile stress is complicated by modulus, hardness, and fracture resistance dependence on curing stage and temperature during curing. This project incorporates shrinkage or expansion reaction during curing into existing stress models, investigates perturbations such as impurity defects and the resulting deformation or fracture, confirms constitutive relations based on experimental analysis, and develops models for mechanical reliability of thin films.

A second project involves the prediction and interpretation of structure-property relationships for silsesquioxane spin on glasses using molecular computation. The drive for higher density micro-electronic circuits has created a need for low dielectric constant materials to replace SiO2 as the dielectric in the interconnect structure for advanced ultra large-scale integrated circuits. Silsesquioxane (SSQ) spin on glass is one of the materials being considered as a replacement for CVD SiO2.

Two processing steps are involved in making spin on glasses. A solution of oligomers is initially spun onto a substrate. A curing process is then used, in which the individual SSQ oligomers are converted via a polymerization reaction to an interconnected three-dimensional network. The low dielectric constant of SSQ materials derives in part from the "nano" porosity incorporated during the curing stage. The nature and existence of the porosity depends critically on the conformation of the oligomers and their site-specific reactivity.

Molecular computation provides a means to follow the development of the spin on glass structure at the lowest level; from the basic unit to the oligomer development, through the curing process to the final interconnected structure. The results obtained are used to understand and define SSQ oligomer production. The defined oligomer structure helps provide insight into the bulk material structure used to understand and predict the dielectric constant, thermal expansion, and elastic and fracture behavior of spin on glass SSQ materials.

This information is available in alternative formats upon request by individuals with disabilities. Please send email to alt-format@msi.umn.edu or call 612-624-0528.

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