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SU8 photoresist is a high aspect ratio epoxy-based negative photoresist commonly used as structural material in lithographic fabrication1. It has some remarkable properties, such as a wide range of coating thicknesses, almost straight sidewalls, high chemical resistance, and good mechanical properties1.

The difference between out of plane and in plane of SU8 photoresist lies in the way the material behaves when subjected to stress. In the out-of-plane direction, the SU8 layer is unconstrained and shrinking can occur freely. A shrinkage of approximately 4.5% (std 0.25%) was measured over a post-exposure bake (PEB) of 1 h at 65°C1. In-plane, although some relaxation occurs, an internal stress is built up (~4 MPa for a PEB temperature of 95°C in a standard process1).

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