Abstract The physical properties and thermal stability of surface modified methylsilsesquioxane (MSQ) were studied. Various post-treatments, such as thermal oxygen, thermal N 2O and oxygen plasma, were adopted on the cured MSQ film as the surface-modification process. The Cu/TaN/MSQ/Si metal-insulation-semiconductor capacitors with various surface modified MSQ films were prepared to measure the dielectric constant, capacitance–voltage and current–voltage characteristics. X-Ray photoelectron spectroscopy and Fourier transform infrared spectroscopy were performed in order to understand the chemical composition of the modified film. From the above measurements, we find the best surface treatment condition for MSQ in Cu metallization.