Reducing the Coefficient of Thermal Expansion of Polyimide Films in Microelectronics Processing Using ZnS Particles at Low Concentrations

We report a reduction in the coefficient of thermal expansion (CTE) of polyimide (PI) film in microelectronics processing by using ZnS particles as nanofillers. To prevent agglomeration of ZnS particles, the surfaces of ZnS particles were modified with the (3-mercaptopropyl)-trimethoxysilane, creating surface hydroxyl groups. For means of comparison, SiO2 and ZrW2O8 particles that have widely been studied as fillers for various polymer films were also synthesized. The CTE measurements showed that the ZnS particles produced PI nanocomposite film with a much lower CTE than either SiO2 or ZrW2O8 particles at the same concentration. In particular, the surface-modified ZnS particles showed the lowest CTE (13 ppm/K) at 15 wt %, which is comparable to the largest percentage decrease (70%) in CTE from the bare-PI film to date at a much lower particle concentration. To rationalize the significant reduction in CTE with the surface-modified ZnS particles, we considered the intrinsic CTE and thermal conductivity, thermoluminescence property, interfacial area, and dispersion state of ZnS particles, and found that the intrinsic thermal conductivity and dispersion state of ZnS particles were mainly responsible for the reduction in CTE at low particle concentration. Finally, we demonstrated that the optical and mechanical properties of the PI nanocomposite films containing surface-modified ZnS particles at 15 wt % were comparable to those of the bare-PI film.