Long-term creep behavior of SU-8 membranes: Application of the time–stress superposition principle to determine the master creep compliance curve

We employ the concept of time–stress superposition on short-term creep tests to determine the master creep compliance curve of glassy SU-8. For the determination of the creep curves, the bulge test method is used. Rectangular membranes with a thickness of 2.25 μm, widths from 830 μm to 941 μm and a length of ≥6 mm are fabricated and measured. The chosen geometrical dimensions allow for the assumption of plane strain conditions. SU-8 exhibits rather low creep (≤10% at t = 1010 s) under constant equivalent stress of 13.2 MPa, where the initial value D0 and transient component De of the creep compliance are found to be 0.23 GPa−1 and 0.29 GPa−1, respectively. The fracture strength σf of the fully cured epoxy resin is measured to be 73.3 MPa. We report on modeling, fabrication, error analysis and experimental results.