X-ray absorption spectromicroscopy study of UV-photoinduced surface modification and anisotropy in polyimide films

X-ray absorption spectromicroscopy with an X-ray photoelectron emission microscope (X-PEEM) was applied to study the modification of polyimide films by linear polarized deep ultraviolet (LPDUV) light, using LPDUV-patterned (0.4-1 J/cm 2 ) CBDA-ODA samples as a test system. Stacks of successive X-PEEM images were collected in the total electron yield acquisition mode over the photon energy ranges containing the relevant C, N, and O K-edges. The LPDUV-induced patterns could be clearly distinguished in the images acquired at the excitation energies of characteristic absorption resonances. The analysis of these images and the derived NEXAFS microspectra suggests that the LPDUV irradiation causes an anisotropic decomposition of the PI backbone with the cleavage of the cyclobutane ring to form a maleimide structure as the main decomposition channel. Whereas the UV-induced anisotropy was not directly perceptible in the X-PEEM images acquired at the excitation energies of characteristic absorption resonances, it could be clearly seen and evaluated in the corresponding microspectra. In particular, the dichroic ratio of the π(C=O*) resonance of imide was found to be 6%, in good agreement with previous spectroscopic results.