The current limits of the laser-acoustic test method to characterize low-k films

The intention to make isolator films with dielectric constants <2.2 has initiated the development of porous siloxane-based films like silica xerogel and silsesquioxane-type materials. Although, the dielectric properties achieved are promising, introducing the technology still requires adapting their mechanical stability to the subsequent chemo-mechanical polishing (CMP). Porosity up to 50% causes the mechanical resistance to reduce drastically. According to several investigations, a value of more than 2 GPa seems to be required for the elastic modulus quantifying the stiffness of the film material. Efforts are currently undertaken to make high porous low-k films with an elastic modulus as high as possible. This requires the elastic modulus of thin soft films to be measured reliably. Surface acoustic waves have been shown to be very sensitive to thin surface films down to thickness of few nano-meters. This technique has been used to study the properties of low-k silica xerogel SiCOH-films. Until now it is necessary to prepare thicker films about 1000 nm for any elastic measurement in order to ensure, that elastic measurement is reliable. The goal of the investigation was to evaluate the extensibility of laser-acoustic based elasticity analysis to films with thickness below 500 nm.