Abstract: Epoxy based resist systems such as the HARE SQ system are designed for MEMS and microfluidics applications as a permanent material designed to remain within the device. As such, the SQ film must function over the lifetime of the particular device. One of the more common failure modes of the epoxy materials are caused by cracking and adhesion loss due to excessive stress with in the resist film. This document will explain the causes, consequences, and solutions, associated with stress within the resist film.
Causes: Upon spin coating, the liquid epoxy film is evenly spread across the desired substrate in a uniform coat at a nominal thickness ranging from 2 to 100+ microns. Immediately upon completion of the coating, the epoxy film, which has a low surface tension, attempts to pullback on the surface of the substrate. These forces lead to a buildup of stress within the film. The softbake process removes solvent from the film which also increases the stress within the now dry film.