Undercut is most likely linked with stiction problem of the resist.
According to my experience SU-8 has sticking problem with glass substrate...
You better use an adhesion promoter and a very long hard bake (20 min- 40
min) before dipping the device in the etchant (or RIE?), or you may use a
Cr-Au mask instead of photoresist (but for 7 mic, resist should be OK).

Hope it helps,
Franck Chollet

        Franck CHOLLET, PhD          Tel: +65 874-8185
        Institute of Material Research and Engineering
                         Optical MEMS
        Blk S7 Lvl 3  National University of Singapore
        Singapore 119260             FAX: +65 872-0785
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