For slow wet etching you should try a cold treatment, maybe at even at 5 C. If you try to prevent any damage of your structures (unfortunately its composition isn't specified) you can use a selective etchant with higly diluted chemicals, e.g. HF in DI water (1:100) or NH4OH:H2O2:H2O (1:1:50).  See for example http://www.cleanroom.byu.edu/wet_etch.phtml.