Residual Stress Evolution in Dielectric Thin Films Prepared from Poly(methylsilsesquioxane) Precursor

Abstract

Soluble poly(methylsilsesquioxane) (PMSSQ) precursor is a potential dielectric candidate for advanced microelectronic devices based on a multilayer structure. Thermogravimetric and FT-IR spectroscopic analyses showed that the precursor undergoes a curing reaction between 100 and 450°C. Time-resolved stress analysis of the PMSSQ films was used to measure the residual stress, which is critical to the reliability of films in the multilayer devices. The film stress fluctuates with temperature over the range 0–100 MPa during the curing process, but increases almost linearly with temperature during the subsequent cooling run. The final stress at room temperature ranged from 30 to 120 MPa, and was found to depend on factors such as the number of coatings, thickness, heating rate and steps, final cure temperature, and degree of curing. The residual stress is caused by polymer chain immobilization and volume shrinkage due to the curing reaction. This effect competes with the reduction in stress due to the relaxation of polymer chains by thermal heat. In particular, the residual stress was found to induce cracks in films of thickness greater than 3 000 nm during the cooling run after curing. In addition, the structure, refractive index, and dielectric constant of the cured films were determined using X-ray diffraction and ellipsometry. It is recommended that PMSSQ films be processed at temperatures in the range 300–450°C and limited in thickness to less than 3 000 nm.