Reduction of mechanical stress in micromachined components caused by humidity-induced volume expansion of polymer layers

Polymer layers frequently used in microsystem technology tend to absorb water from ambient humid air which typically results in swelling of the polymer layers. Depending on the specific component design, this volume expansion may cause changes in the mechanical stress, or even displacement of device structures. In most applications, these effects limit performance and reliability, and should therefore be minimized. The present paper describes an approach for the investigation of swelling phenomena in thin polymer layers. Polyimide PI2540 was chosen as a model polymer. After extensive characterization of the polymer layer properties, static and dynamic sorption behavior was investigated by different means. Based on our experimental results, we have developed linearized models to accurately describe the swelling behavior. Several polymers were characterized. Although the micro-physical mechanisms responsible for the humidity-induced swelling are still poorly understood, we are - based on our data - able to effectively reduce humidity induced stress by selecting suitable polymers and optimizing processing procedures.