Viscoelasticity and creep recovery of polyimide thin films

Creep and recovery of a polyimide thin film well below its glass transition temperature is demonstrated through use of a circular membrane bulge test. Extensive use is made of a recently developed mechanical CAD system linked to the fabrication process to model the structure. A creep power law is used in a nonlinear finite-element analysis to fit the experimental results, thereby measuring the viscoelastic properties. The use of circular membranes in measuring the viscoelastic and plastic material properties of thin films has some advantages over the uniaxial techniques. The membranes are edge free and therefore less susceptible to cracks which can prohibit observation of yield in glassy materials such as PI2525. The maximum stress region (center point) is also identified in advance; hence, the viscoelastic properties of thin films can be observed without having to yield or creep the entire sample. However, the analysis to determine the viscoelastic properties is more cumbersome. An attempt at such data analysis is presented. It has been determined that the creep compliance of PI2525 is a very nonlinear function of the stress (n=8) and also requires a threshold strain before creep begins.<>