Characterization of thin polymer films with the focus on lateral stress and mechanical properties and their relevance to microelectronics

Thin film polymers play an essential role in system integration. The mechanical properties of the polymers are crucial for 3-D-Integration and advanced WLP because with the thinning of the silicon wafers, i.e. chips to less than 150 μm, the influence of the polymer layers gets an increasing impact on the mechanical stability of the electronic device. Next generation polymers have entered the market which are tailored to reach the further optimized mechanical property parameter set. This paper will give a guideline for the choice of the optimal polymer based on the demands of the application in relation to the material properties. The main material properties for high reliability are the Young's modulus, tensile strength, elongation at break and coefficient of thermal expansion. Aside of the material properties of the polymer the interaction of the polymer layer with the substrate is important. The material mismatch causes for example warpage and material cracking; the main impact factor being the residual stress in the layers in relation to the fracture toughness of the material and the interface. The warpage is an issue for the processing and the assembly process. The focus of our investigation is on properties of polymers on silicon substrates. The development of stresses in the polymer layers is measured and analyzed for different polymers (BCB, PI, PBO). The residual stress in a thin polymer film is measured by the warpage of the substrate in relation to different temperatures depending on the application. The estimation of stress-temperature behavior allows to develop processing concepts for a stress reduction being essential for 3-D integration. The generated stress drives cracking which leads to the effect that the impact of the forces should be taken into account for the quantification of the fracture toughness. The relation between the stress as the driving force and the fracture toughness are further discussed in details. A comprehensive study of the mechanical polymer properties is essential for high reliable devices.