A morphology of thin polyimide films prepared from polyamic acid solut
ion has been investigated by scanning electron microscopy. A strong de
pendence between the conditions of post-preparation temperature treatm
ent (imidization) and the structure and optical properties of the film
s has been found. The films exhibit a large optical anisotropy and wel
l-defined columnar structure. This structure is supposed to explain th
e ''polyimide grass'' observed after reactive ion etching in an O2 atm
osphere. Some techniques are proposed to avoid this effect in high yie
ld microfabrication.